RF graphene oscillators for biomedical applications

Uni e sidade do Minho
Escola de Engenha ia
Vi o Filipe Hen iques da Sil a
RF g aphene oscilla o s
o biomedical applica ions
Janei o de 2025
Uni e sidade do Minho
Escola de Engenha ia
Vi o Filipe Hen iques da Sil a
RF g aphene oscilla o s o biomedical
applica ions
Tese de Dou o amen o
Engenha ia Biomédica
T abalho e e uado sob a o ien ação do(a)
P o esso Dou o Paulo M. Mendes
P o esso Dou o João Ped o S. H. A. Alpuim
Janei o de 2025
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Di ei os de au o e condições de u ilização do abalho po e cei os
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Ag adecimen os
O alcança des a e apa é u o de um longo caminho de ap endizagem, dedicação e supe ação.
No en an o, esse caminho não e ia sido possí el sem o apoio, a o ien ação e o incen i o de mui as
pessoas que es i e am ao meu lado du an e es a jo nada.
Não podia inicia es es ag adecimen os sem ealça a insubs i uí el impo ância da minha amília
na minha ida. Aos meus pais, Angelina e Hen ique, ag adeço- os po odo o amo que me dedica am e
po oda a educação que me p opo ciona am. Ob igado po me apoia em em odas as ases da minha
ida. Se sou o que sou hoje, a ocês o de o. Ao meu i mão, ag adeço po me e ajudado e apoiado
semp e que p ecisei. Ag adeço ambém à Inês, po e caminhado ao meu lado, pela sua paciência,
comp eensão e apoio nos momen os mais di íceis. O meu pe cu so a é aqui oi conseguido a a és de
odo o osso supo e.
Ao P o esso Dou o Paulo Ma eus Mendes, o ien ado des a ese, o meu mui o ob igado pelo
e e encial humano e p o issional, po oda a sua disponibilidade, paciência, cuidado e conhecimen o
ansmi ido ao longo des es anos. Ob igado po me e desa iado de o ma cons an e, po me ques iona
das opções omadas, po odas as discussões, pa ilha de conhecimen o e expe iência, que me ajuda am
a e olui , a e ina o meu pensamen o c í ico e a desen ol e es a ese. A sua o ien ação oi essencial
pa a o meu c escimen o académico e pessoal.
Ao P o esso Dou o João Ped o dos San os Hall Ago e a de Alpuim, co-o ien ado des a ese,
ob igado po se uma on e de mo i ação e incen i o ao longo dos úl imos anos. A sua dedicação,
o ien ação, p o issionalismo e colabo ação ize am com que odo o abalho deco esse de o ma
ha moniosa, o que me pe mi iu in es iga de o ma ap o undada a emá ica des e abalho e possibili ou
a melho ia cons an e dos esul ados.
Ao Dou o Jé ôme Bo me, pela disponibilidade cons an e em discu i e apoia nos p ocessos de
ab ico de sala limpa, pela sua amizade, p o issionalismo e expe iência, o meu since o ag adecimen o
po odas as ho as dedicadas de apoio a es e abalho.
Aos meus amigos e colegas de labo a ó io de Gual a e do INL (Labo a ó io Ibé ico In e nacional
de Nano ecnologia), ag adeço o bom ambien e de abalho e odos os momen os de pa ilha, discussão
e en eajuda. Ag adeço ambém de o ma especial a odos que, de alguma o ma, con ibuí am com o
seu conhecimen o pa a a ealização des e abalho.

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Ao Hugo e I o, o meu especial ag adecimen o po odos os momen os de discussão, pela ossa
amizade e companhei ismo. Ob igado po e em sido igu as p esen es que me incen i a am,
enco aja am e apoia am desde o início.
A odos os demais amigos e amilia es, que me apoia am e acompanha am ao longo dos úl imos
anos, sei que es ão mui o sa is ei os po me e em chega à me a e, po se em as pessoas an ás icas
que são, exp esso aqui ambém a minha g a idão.
À Uni e sidade do Minho e ao INL, po odo o supo e ins i ucional ao desen ol imen o des e
abalho, pela disponibilização de ecu sos ma e iais e logís icos, pelo apoio de odos aqueles que azem
pa e des as ins i uições, po me acolhe em e p opo ciona em odas as condições necessá ias ao
desen ol imen o e conclusão des e abalho, o meu p o undo e since o ag adecimen o.
Po im, ag adeço à Fundação pa a a Ciência e Tecnologia (FCT), pelo inanciamen o a a és das
bolsas SFRH/BD/137529/2018 e COVID/BD/153232/2023, co inanciadas pelo FSE a a és do
P og ama Ope acional Regional.
Finalizo es e abalho con ic o de que o caminho académico não é apenas sob e a ob enção de
um g au académico, mas sob e o c escimen o pessoal e in elec ual ao longo desse caminho. Que es e
abalho seja uma pequena con ibuição de conhecimen o e que as ap endizagens que me p opo cionou
con inuem a inspi a -me pa a desa ios u u os.
i
S a emen o in eg i y
I he eby decla e ha ing conduc ed his academic wo k wi h in eg i y. I con i m ha I ha e no
used plagia ism o any o m o undue use o in o ma ion o alsi ica ion o esul s along he p ocess
leading o i s elabo a ion.
I u he decla e ha I ha e ully acknowledged he Code o E hical Conduc o he Uni e si y o
Minho.
Resumo
O g a eno, um ma e ial com es u u a a ómica e com p op iedades excecionais, em despe ado
in e esse na comunidade académica. Com uma ele ada mobilidade de po ado es, excelen e
condu i idade é mica e ca ac e ís icas ele ónicas no á eis, o g a eno ap esen a um eno me po encial
pa a aplicações em ele ónica de adio equência (RF). Con udo, a in eg ação des e ma e ial em
disposi i os ele ónicos ap esen a desa ios, especialmen e na adap ação dos p ocessos con encionais
de ab ico CMOS (
Complemen a y Me al-Oxide-Semiconduc o
). Es a ese abo da esses desa ios,
a ançando no desen ol imen o de mé odos de desenho, ab ico e ca ac e ização de ansís o es e
oscilado es de RF baseados em g a eno, com especial en oque em aplicações biomédicas. Pa a melho a
o desempenho dos ansís o es de RF ab icados, o am implemen adas á ias es a égias, como a
u ilização de
ga es
en e adas pa a eduzi a esis ência da
ga e
, a minimização da esis ência de acesso
a a és de um p ocesso
Damascene
adap ado, o uso de subs a os de silício de al a esis i idade com
camadas de SiO2 pa a mi iga e ei os pa asi as e a in eg ação de camadas inas de óxidos com ele ada
cons an e dielé ica (alumina) deposi ados po deposição de camada a ómica (ALD).
Duas me odologias de ab ico dis in as o am desen ol idas: um p ocesso de
ga e
asei a sem
plana ização da supe ície e um p ocesso de
ga e
en e ada com plana ização, u ilizando
ion milling
an es da ans e ência do g a eno. Es as me odologias o am adap adas às condições da sala limpa do
Labo a ó io Ibé ico In e nacional de Nano ecnologia (INL), pa a aumen a a epe ibilidade dos p ocessos,
uma ez que são baseados em p ocessos pad ão. Adicionalmen e, os p ocessos de sín ese e
ans e ência do g a eno o am es udados.
Nes a ese, ab ica am-se ansís o es com equências de co e e equências máximas de
oscilação de 80 GHz e 14 GHz, espe i amen e. Es es disposi i os o am modelados a a és de um
modelo semi-empí ico. Embo a não enha sido ab icado um oscilado de anel de RF baseado em g a eno
o almen e uncional, as simulações ealizadas com base no modelo pe mi i am es uda e o imiza os
pa âme os necessá ios pa a o desen ol imen o de disposi i os uncionais no u u o, demos ando a
e e i idade do p ocesso de ab ico pa a a sua u ilização em biossenso es de g a eno. Assim, es a ese
es abelece um p ocesso de ab ico epe í el e adap á el a ou os disposi i os baseados em g a eno,
lançando as bases pa a a in eg ação des e ma e ial em senso es biomédicos e ecnologias de
adio equência.
Pala as-cha e: g a eno, oscilado es em anel de g a eno, adio equência, ansís o es de
adio equência de g a eno
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Abs ac
G aphene, a single-a om- hick ca bon ma e ial a anged in a honeycomb la ice, has gained
signi ican a en ion due o i s excep ional elec onic, he mal, and mechanical p ope ies. Wi h high
ca ie mobili y, supe io he mal conduc i i y, and ema kable elec onic cha ac e is ics, g aphene has
immense po en ial o elec onic and adio equency (RF) applica ions. Howe e , in eg a ing g aphene
in o p ac ical de ices p esen s subs an ial challenges, pa icula ly in adap ing con en ional
Complemen a y Me al-Oxide-Semiconduc o (CMOS) ab ica ion p ocesses o accommoda e i s unique
p ope ies. This hesis add esses hese challenges by ad ancing he design, ab ica ion, and
cha ac e iza ion o g aphene-based RF ansis o s and oscilla o s, wi h a ocus on biomedical applica ions.
S a egies o enhance he pe o mance o he ab ica ed RF g aphene ansis o s included implemen ing
bu ied ga es o educe ga e esis ance, minimizing access esis ance using an adap ed damascene
p ocess, using high- esis i i y silicon subs a es wi h SiO2 laye s o mi iga e pa asi ic e ec s, and
in eg a ing hin high-k oxide (alumina) laye s g own ia a omic laye deposi ion (ALD) o imp o ed ga e
con ol.
Two dis inc ab ica ion me hodologies we e de eloped: a bo om-ga e p ocess wi hou su ace
plana iza ion and a plana ized bu ied bo om-ga e p ocess using ion milling p io g aphene ans e . These
me hodologies we e adap ed o he clean oom condi ions o he Ibe ian In e na ional Nano echnology
Labo a o y (INL) o enhance p ocess epea abili y, as hey a e based on s anda d p ocesses. Addi ionally,
he g aphene syn hesis and ans e p ocesses we e op imized, p io i izing con inuous g aphene ilms
o e la ge lakes o ensu e scalabili y and uni o m de ice pe o mance.
In his wo k in insic cu o equencies o up o 80 GHz and maximum oscilla ion equencies o
14 GHz using plana bu ied ga e de ices wi h a 5 nm oxide laye we e achie ed. These ab ica ed de ices
we e modelled by a semi-empi ical model. Al hough a ully ope a ional g aphene-based RF ing oscilla o
was no ab ica ed, simula ions suppo ed ha u he op imiza ion o pa ame e s could lead o designs
ha sus ain oscilla ion in he u u e, demons a ing he e ec i eness o he ab ica ion p ocess o i s
applica ion in g aphene biosenso s. In sho , his hesis p o ides a epea able ab ica ion p ocess
adap able o o he g aphene-based de ices, laying he basis o in eg a ing g aphene in o ad anced
biomedical senso s and RF echnologies.
Keywo ds: g aphene, g aphene ing oscilla o s, RF g aphene ansis o s, adio equency
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ga e in ai o o m an Al2O3 laye a e he PMMA esis emo al. (d) Sel -aligned sou ce and d ain con ac s
ab ica ion by he deposi ion o Pd. e) Diag am o he inal GFET. ) C oss-sec ion o he inal de ice.
Figu e 26 – a) Silicon wa e wi h ans e ed g aphene on op. b) Pho og aph o ab ica ed g aphene
de ices. c) A i icial colo SEM image o a sel -aligned g aphene ansis o wi h mul i-ga e con igu a ion.
d) SEM image o he ansis o channel [53].
Figu e 27 – Fab ica ion low o a sel -aligned g aphene FET, epo ed in [60]. In a) is shown he deposi ion
o gold on op o g aphene, b) shows he pa e ning o he esis o a T-shaped ga e, c) shows he e ching
o he gold unde he ga e, d) shows he deposi ion o 2 nm o Al, which is sel -oxidized o Al2O3, o ming
he ga e oxide, e) shows he deposi ion o Al o o m he ga e con ac , and ) shows he c oss sec ion o
he inal de ice.
Figu e 28 – Mul iple-ga ed GFET wi h na u al oxide o aluminum [61]. a) 3-D schema ic o he ab ica ed
de ice. b) SEM image o he mul i-ga ed ansis o .
Figu e 29 – SEM image o he ab ica ed mul iple-ga e ansis o on op, and c oss sec ion o he de ice
on he bo om, epo ed in [62].
Figu e 30 – a) C oss sec ion o dual-ga e GFET s uc u e. b) Op ical mic oscope image o he ab ica ed
GFET [115].
Figu e 31 – Schema ic diag am o he ab ica ion p ocess epo ed in [63]. a) E ching o he bu ied ga e.
b) Ti/Au deposi ion. c) ALD deposi ion o Al2O3 and g aphene ans e . d) Sou ce and d ain con ac s
de ini ion. e) G ow h o he op-ga e dielec ic by ALD. ) Top-ga e ab ica ion [63].
Figu e 32 – Schema ic o he ab ica ion p ocess epo ed in [64]: a) RIE o bo om pa o he T-ga e. b)
Filling he enches wi h Cu and plana iza ion wi h CMP. c) and d) Simila p ocess o a) and b) o o m
he op ga e. e) G ow h o 10 nm o Al2O3 by ALD o o m he ga e oxide. ) Monolaye g aphene ans e .
g) Deposi ion o he me al con ac s and g aphene pa e ning.
Figu e 33 – Images o he g aphene ansis o s epo ed in [64]: a) SEM image o he c oss-sec ion o he
embedded T-ga e. b) Op ical mic og aph o he de ice.
Figu e 34 – Schema ic o he ab ica ion low o he ab ica ed de ices: a) 30 nm Au/g aphene on Si
subs a e. b) ilaye pho o esis o T-ga es pa e ned by E-beam li hog aphy. c) Gold ilm unde he T-
ga es e ched away. d) Ti/Au deposi ed on op as he ga e me al a e he deposi ion o he ga e dielec ic
by ALD (Al2O3). e) Li -o o ab ica e he inal de ices. ) Pho og aph o he ans e ed g aphene wi h
Au. g) pho o image o he pa e ned silicon wa e wi h he pa e ned GFETs [65].
Figu e 35 – Images o he GFETs epo ed in [65]: a) SEM image o a dual-ga e GFET. b) FIB (Focused
Ion Beam) image o he c oss-sec ion o he GFET.

xi
Figu e 36 – Back-ga ed GFET wi h h-BN ga e dielec ic: a) Schema ic o he a omic s uc u e o he
g aphene and h-BN. b) op ical image o an ex olia ed h-BN lake. c) AFM image o h-BN wi h di e en
laye hickness. d) op ical image o GFET. e) C oss-sec ion schema ic o he back-ga e de ice s uc u e
[90].
Figu e 37 – a) o d) Fab ica ion s eps o he BN/g aphene/BN FETs. e) Op ical image o a bilaye
g aphene lake ex olia ed and ans e ed on op o he h-BN subs a e. ) Op ical image o he inal de ice.
g) Op ical mic og aph o he inal de ice. h) SEM image o he inal de ice [66].
Figu e 38 – Schema ic illus a ion o he ab ica ion s eps o he op-ga ed g aphene ansis o [67].
Figu e 39 – Sel -aligned g aphene ansis o s epo ed in [67]. a) Op ical image o he assembled
nanowi e a ay. b) Op ical image o GFETs and c) Zoom in o b) in one GFET. d) SEM image o GFETs
showing he channel and he ga e nanowi e.
Figu e 40 – Illus a ion o he ab ica ion o sel -aligned g aphene ansis o s wi h ans e ed ga e s acks
epo ed in [49].
Figu e 41 – The sel -aligned GFETs: a) Pho o image o he inal de ices on a glass subs a e. b) Op ical
image o he GFETs. c) SEM image o he channel o he GFET. d) TEM image o he c oss sec ion o he
inal de ice [49].
Figu e 42 – Diag am o he wo main p ocesses epo ed in his hesis wi h and wi hou plana iza ion, a)
and b) espec i ely. In a) i) HR silicon wa e wi h ch omium + gold and alumina on op; ii) e-beam
li hog aphy o de ine sou ce, d ain and ga e con ac s by ion milling; iii) de ice a e ion milling; i ) e-beam
li hog aphy o de ine he ga e oxide a e he g ow h o esh alumina by ALD (a e he esis emo al by
O2 plasma, and alumina by we e ch); ) esul a e he pa e ning o he ga e oxide by ion milling; i)
inal de ice wi h g aphene. b) i) de ice a e e-beam li hog aphy and ICP RIE; ii) a e he ch omium gold
deposi ion; iii) a e he li -o , showing he ea s o be emo ed by ion milling, o plana ize he de ice; i )
a e he g aphene ans e ; ) a e he e-beam li hog aphy and coppe + gold deposi ion o de ine he
sou ce/d ain elec odes by li -o ; i) inal de ice
Figu e 43 – Fi s s eps o he ab ica ion p ocess wi h a) and b) showing an op ical image o he li hog aphy
and c) and d) showing he op ical mic og aph a e he ion milling and he esis emo al by an O2 plasma.
Figu e 44 – a) and b) show an op ical image o he li hog aphy pe o med o pa e n he ga e dielec ic
(alumina) and c) and d) shows he op ical image a e he ion milling. To no e, since he ga e is e y hin,
some e-beam esis was le o ac as ancho o he e-beam esis esponsible o he pa e ning o he
ga e dielec ic.
Figu e 45 – a) and b) Op ical pho og aph o he de ice a e he esis emo al wi h he oxygen plasma.
Figu e 46 – Op ical image o he g aphene laying in he channel o he de ice.
x
Figu e 47 – Mic oscope image o he pa e ned subs a e wi h he g aphene lakes (a) and Vgs s IDS
and 𝑔𝑚 cha ac e is ic cu es o de ices p esen ed in ha subs a e wi h W = 35.6 µm and L = 1.19 µm
(b) and (c).
Figu e 48 – SEM images o g aphene a e he ans e . a) a e he ans e wi hou he pa e ning, b)
a e he pa e ning wi h g aphene ollowing he opog aphy o he con ac s, c) and d) suspended g aphene
pa e ned and d ied on ai e idencing he c acks and e) and ) a e he pa e ning and d ied wi h c i ical
poin d ye .
Figu e 49 – SEM images o he ga e s uc u es showing he me al ea s a) and a e he plana iza ion b).
c) P o ilome e measu emen o he ga e a e he plana iza ion.
Figu e 50 – a) and b) op ical image o di e en g aphene pa e ns used in g aphene o educe he con ac
esis ance, c) op ical image o he inal de ice and d) SEM image o he inal de ice showing g aphene
wi hou c acks.
Figu e 51 – Di e en ypes o he ab ica ion s uc u es, a) single ga e ansis o , b) in e e , c) ing
oscilla o wi h a iable g aphene capaci o s h ough back ga e and compac ing oscilla o , e) SEM image
o an in e e a e he elec ical measu emen showing he con ac pads sc a ched o damaged, ) SEM
image o he compac ing oscilla o , g) SEM image o he channel o he in e e and h) mic oscope
image o he clean g aphene laying in a ga e o a de ice p io he deposi ion o he emaining con ac s (in
his case an in e e ).
Figu e 52 – Raman spec a o he monoc ys alline g aphene used in he ab ica ed de ices, a) Raman
spec a si es and b) Raman spec a a 532 nm in ensi y lase . I is possible o e alua e he co ec
pa e ning since ou o he g aphene channel he e is no g aphene signa u e.
Figu e 53 – a) Gene al ab ica ion p ocess o he single laye g aphen con inuous ilm. b) g aphene on
coppe and c) ca bon chunk on op o he g aphene a ising om con amina ions in he g aphene u nace.
Figu e 54 – Pic u e o he sapphi e moun ed o e he ea ed coppe oil.
Figu e 55 – Gene al scheme o he g aphene lake g ow h.
Figu e 56 – E olu ion o g aphene lakes wi h he inc easing o Hyd ogen (𝐻2) low.
Figu e 57 – E olu ion o g aphene lakes wi h he inc easing o me hane (𝐶𝐻4) low.
Figu e 58 – Images o g aphene lakes on coppe a e he oxida ion on a ho pla e a) op ical mic oscope
and b) pho og aph o he coppe oil.
Figu e 59 – Di e en Raman g aphene peaks o a lase exci a ion o 532 nm [129].
x i
Figu e 60 – Raman spec a o single laye g aphene and bulk g aphi e a a 532 nm lase [131].
Figu e 61 – A linea inc ease in G band in ensi y occu s as he numbe o g aphene laye s inc eases,
wi h a 532 nm lase . Adap ed om [132].
Figu e 62 – Rela ionship be ween 2D peak and G peak o single laye g aphene. Adap ed om [132].
Figu e 63 – a) Raman spec a a g aphene lake on a coppe oil and b) Raman spec a a 532 nm o he
lake.
Figu e 64 – Con amina ed CVD u nace. The black po ions a e ca bon chunks ha esul ed om CNT
g ow h.
Figu e 65 – G aphene c acks and PMMA esidues [144].
Figu e 66 – La ge-signal model equi alen ci cui . CPGS, CPDS, LG, LD, and LS a e pa asi ic capaci ance
alues and induc ances. RG is he ga e esis ance, and RS and RD a e he sou ce and d ain esis ances
including con ac and access esis ances. Adap ed om [146].
Figu e 67 – Majo ca ie ype in he g aphene channel a di e en bias ol ages.
Figu e 68 – Small-signal model a e de-embedding he pa asi ic elemen s a 𝑉𝐺𝑆=𝑉𝐷𝑖𝑟𝑎𝑐. Adap ed
om [147].
Figu e 69 – Two-po sys em equi alen ci cui om he Y-pa ame e s (le ) and Z-pa ame e s ( igh ).
Figu e 70 – De-embedding equi alen ci cui s. De ice unde es (le ), Sho (cen e ) and Open ( igh ).
Figu e 71 – Measu emen se up used o ex ac he FOMs o he GFETs.
Figu e 72 – VGS s IDS and 𝑔𝑚 cha ac e is ic cu es o a de ice wi h W = 39 µm and L = 1.100 µm
(0.95 µm o ga e channel and 0.075 µm o d ain/ga e and sou ce/ga e o e lap) measu ed a a VDS o
100 mV.
Figu e 73 – a) S-pa ame e s o a de ice wi h W = 39 µm and L = 1.100 µm and a La (access leng h) o
85 nm and espec i ely T and max b). c) RF measu emen scheme used o measu e he de ices.
Figu e 74 – E olu ion o he T and max wi h he a ia ion on access leng h.
Figu e 75 – T ans e cu es o a de ice wi h L = 1.102 μm and W = 33.82μm. a) VGS s IDS cu e o
a VDS o 10 mV and b) Cu es o VGS s IDS o di e en VDS and c) cu es o VDS s IDS o di e en
VGS.
Figu e 76 – RF pe o mance o he de-embedded de ices. H21 and MUG o a de ice wi h an L = 1.101
µm a) and c) o a de ice wi h a L = 1.136 µm and a La o 54.35 nm. b) and d) SEM images o he
espec i e de ices showing he measu emen s o he channel leng h and access leng h. De ice d) shows
x ii
a small misalignmen in be ween he d ain and ga e. The max and T o a) a e 11 GHz and 44 GHz
espec i ely and in c) a e 14 GHz and 80 GHz espec i ely.
Figu e 77 – a) In e e schema ic wi h GFET. b) Equi alence o he ab ica ed de ice wi h he schema ic
shown in a) c) Measu emen o a g aphene in e e wi h a VDD o 8V showing a Vou o 500 mV o
ampli ude swing (channel 2) o a Vin o 1V swing cen e ed in 5.7 V.
Figu e 78 – RF g aphene mixe and equency double . a) ci cui o he g aphene mixe and b) expec ed
ou pu spec um. c) Measu ed RF spec um o he g aphene mixe and d) ou pu spec um o he
equency double o an inpu signal wi h a equency o 3 GHz a 0 dBm.
Figu e 79 – Simula ed s measu ed IDS s VGS o he ansis o .
Figu e 80 – Simula ion o an in e e wi h he ex ac ed pa ame e s. a) W shape cu e o he in e e ; b)
Vou s Vin and Gain o he in e e showing a maximum gain o 0.7 and c) esponse o he in e e o a
10 kHz squa e wa e.
Figu e 81 – Simula ion o he in e e by educing he Rex 0 o 10. a) W shape cu e o he in e e ; b)
Vou s Vin and Gain o he in e e showing a maximum gain o 0.8.
Figu e 82 – Simula ion o he in e e by educing he Ro o 40. a) W shape cu e o he in e e ; b) Vou
s Vin and Gain o he in e e showing a maximum gain o 2.7.
Figu e 83 – Simula ion o he in e e by educing shi ing he VDi ac o VDi ac+0.3. a) W shape cu e o
he in e e ; b) Vou s Vin and Gain o he in e e showing a maximum gain o 2.8.
Figu e 84 – Ring oscilla o assessmen . Simula ion o he op imized in e e (a) and ing oscilla o
simula ion (b).
x iii
LIST OF TABLES
Table 1- Compa ison be ween he pe o mances o he epo ed ansis o s. ...................................... 26
Table 2- Con ac esis i i y (Adap ed om [104]) ............................................................................... 34
Table 3- Equa ions o in insic and ex insic d ain-sou ce esis ances ................................................. 77
Table 4 - Ex ac ed pa ame e s om he model by using he p e ious ansis o . ................................ 91
Table 5- Pa ame e s used o he in e e and in he ing oscilla o simula ion.................................... 95

1
1 INTRODUCTION
G aphene is a 2D ma e ial wi h a one-a om- hick laye o ca bon in a honeycomb la ice. I s
disco e y has caused an inc ease in he numbe o s udies whe e i s syn hesis, physical p ope ies, and
elec ical p ope ies we e analysed. The ad ances in hose a eas allowed o he exploi a ion o g aphene’s
p ope ies o i s in eg a ion and imp o emen o elec ical ci cui s [1]. Mo eo e , he de elopmen o
la ge-scale syn hesis me hods such as Chemical Vapou Deposi ion (CVD) has allowed o an inc ease in
he numbe o esea ch indings and has made comme cial applica ions possible. G aphene’s excep ional
elec ical and mechanical p ope ies make i pa icula ly sui able o biomedical applica ions. I also
exhibi s excellen adso p ion capabili ies, enabling he de elopmen o biosenso s ha quan i y adso bed
molecules by de ec ing changes in g aphene’s p ope ies [2]. Gi en g aphene's excep ional mechanical
p ope ies, i is possible o ab ica e lexible senso s, as will be discussed la e . Despi e hese ad an ages,
in eg a ing g aphene-based de ices wi h CMOS (Complemen a y Me al-Oxide-Semiconduc o ) echnology
emains challenging due o i s dis inc i e ma e ial p ope ies. Many con en ional CMOS ab ica ion
p ocesses canno be di ec ly applied o g aphene, complica ing e o s o inco po a e ea u es such as RF
communica ion o analy e quan i ica ion h ough equency changes in de ices.
Gi en he p e ious conside a ions, his hesis ocuses on de eloping a g aphene-based oscilla o
o biomedical applica ions. This could ange om a ing oscilla o (RO), a esonan oscilla o o a MEMS
s uc u e. The p oposed design should be ully compa ible wi h he In e na ional Ibe ian Nano echnology
Labo a o y (INL) clean oom, aligning wi h he s anda dized p ocesses employed in mos esea ch
labo a o ies wo ldwide.
1.1 Biomedical de ices
The in es iga ion and applica ion o biomedical de ices ha e indeed wi nessed emendous g ow h
o e he pas yea s, pa icula ly wi h he ad en o a ious echnologies aimed a enhancing pa ien ca e
and moni o ing. Wea able de ices, lexible elec onics, on-body sys ems, and minia u e de ices, along
wi h emo e cha ging, wi eless powe ans e sys ems, ha e eme ged as pi o al inno a ions in his ield.
Wea able de ices ha e gained signi ican ac ion due o hei abili y o moni o a ious physiological
indica o s con inuously and non-in asi ely, allowing use s o ack hei heal h wi hou dis up ing hei
2
daily ac i i ies. Fo ins ance, Wang e al [3] de eloped a wea able de ice capable o simul aneously
moni o ing elec oca diog am (ECG) signals and coun ing physical ac i i ies in ca dio ascula pa ien s.
This de ice has p o en e ec i eness in clinical p ac ice, acili a ing ea ly de ec ion o a hy hmias and
enabling imely in e en ions o mi iga e complica ions associa ed wi h se e e ca dio ascula diseases
[3]. This de ice as well as he example o applica ion can be seen in Figu e 1 a). Simila ly, Susana e al
[4] in oduced a non-in asi e de ice ha uses pho ople hysmog aphy senso s o moni o blood glucose
le els, signi ican ly imp o ing he quali y o li e o diabe ic pa ien s by elimina ing he need o equen
inge p icking. The con inuous moni o ing capabili ies o hese de ices unde sco e hei impo ance in
mode n heal hca e, pa icula ly o ch onic disease managemen [4]. The e olu ion o lexible biomedical
de ices has u he enhanced he com o and sus ainabili y o wea able echnologies. Recen
ad ancemen s in ma e ials science ha e led o he de elopmen o lexible elec onics using biocompa ible
polyme s and conduc i e ma e ials such as doped g aphene. As epo ed in [5] he g aphene could be
used as a s e chable ma e ial o de ec empe a u e, p essu e, and me aboli es in swea , demons a ing
i s po en ial o his ma e ial o pe sonalized heal h moni o ing. In [6], he au ho s epo ed he
de elopmen o a wi eless and lexible biosenso pa ch designed o con inuous, longi udinal moni o ing
o mul iple physiological signals, including body empe a u e, blood p essu e, and elec oca diog aphy.
The pa ch was enginee ed o be highly compa ible wi h he skin, ea u ing op imized lexibili y and
mechanical s e chabili y o ensu e du able and com o able a achmen , e en on cu ed su aces as can
be seen in Figu e 1b). The lexibili y o hese de ices is c ucial o ensu ing accu a e skin con ac and
com o du ing use, which is essen ial o e ec i e heal h moni o ing. On-body de ices ha e e olu ionized
con inuous, non-in asi e pa ien moni o ing by in eg a ing seamlessly wi h he human body. The end
owa ds minia u iza ion has enabled he de elopmen o smalle , ligh e , and mo e com o able de ices
ha main ain high accu acy in moni o ing unc ions. Fo ins ance, he pacemake , an implan able medical
de ice, has been minia u ized o less han he size o a coin, allowing o di ec implan a ion in o hea
issue [7], as can be seen in Figu e 1 c). Hassan e al [8] u he exempli ied his end wi h an implan able
glucose senso (LC ank esona o ) ha p o ides con inuous moni o ing o diabe ic pa ien s, ansmi ing
da a wi elessly [8]. This minia u iza ion no only enhances pa ien com o bu also inc eases adhe ence
o ea men egimens. One o he signi ican challenges in he ealm o implan able de ices is he need
o ba e y eplacemen s, which can need in asi e p ocedu es. Inno a ions in wi eless powe ans e
echnologies ha e add essed his issue. Laughne e al [9] epo ed a wi eless implan able pacemake in
mouses which could pa e he way o elimina ing he need o su gical ba e y eplacemen s and allowing
o con inuous moni o ing om wi hin he body [9]. Addi ionally, Heo e al [10] de eloped a wi eless deep
3
b ain s imula ion de ice o Pa kinson’s disease oden s, powe ed h ough induc i e coupling, which
educes he need o su gical e isions and enhances pa ien com o [10] . The de ice could be seen in
Figu e 1 d). These ad ancemen s highligh he ans o ma i e po en ial o wi eless echnologies in
imp o ing he unc ionali y and usabili y o biomedical de ices. Gi en ha , he ecen ad ancemen s in
biomedical de ices, pa icula ly in he a eas o wea ables, lexibili y, minia u iza ion, and wi eless powe ,
signi y a ema kable e olu ion in heal hca e echnology. These inno a ions a e no only enhancing he
accu acy and pe sonaliza ion o heal h moni o ing solu ions bu a e also pa ing he way o p oac i e
heal hca e managemen . Ongoing esea ch in o new ma e ials, minia u iza ion echniques, and wi eless
powe echnologies p omises o u he e olu ionize he ield, ul ima ely leading o imp o ed pa ien
ou comes and a mo e in eg a ed app oach o heal h managemen .
4
Figu e 1 – Examples o biomedical de ices a) Wea able in elligen ECG moni o ing sys em. Adap ed om [3]. b) Design and
mechanical p ope ies o lexible biosenso pa ch. Adap ed om [6] c) pacemake de ice. Adap ed om [7] d) wi eless deep
b ain s imula ion de ice o Pa kinson’s disease oden s. Adap ed om [10].
1.2 Oscilla o s and g aphene echnology
The in eg a ion o g aphene in o senso echnology has ans o med esea ch ac oss a ious
ields, including heal hca e, en i onmen al moni o ing, and sma ma e ials. G aphene has become a
e y a ac i e ma e ial due o i s p ope ies. I is a single shee o 𝑠𝑝2-bonded ca bon, ze o-gap
semiconduc o wi h a high conduc i i y. When compa ed o coppe wi h he same hickness ( hough
achie ing such hinness wi h coppe may no be easible), i has go a lowe esis ance and can sus ain
a highe cu en densi y han any o he known ma e ial [1]. Fo elec onic applica ions, i s key p ope y
is i s high ca ie mobili y, wi h suspended g aphene eaching mobili ies up o 200000 cm2 V-1 s-1 [11],
while silicon has go an elec on mobili y o 1400 cm2 V-1 s-1 [12]. This p ope y makes he g aphene a
sui able ma e ial o adio equency (RF) applica ions [13] The epo ed de ices ange om ansis o s
wi h measu ed in insic cu -o equencies ( T) o up o 300 GHz, and wi h applica ions on low-loss
in e connec s and passi es [14][15]. These unique elec ical p ope ies, along wi h mechanical s eng h,
and lexibili y, make i an ideal candida e o senso applica ions [16], including p essu e sensing, s ain
de ec ion, and physiological signal moni o ing. Fo ins ance,
Yang e al.
[17] demons a ed a wea able
g aphene s ain senso capable o p ecise pulse wa e moni o ing [17]. Simila ly,
Xie e al.
[18] epo ed
on he de ec ion o physiological signals using g aphene, emphasizing i s po en ial in heal h moni o ing
sys ems [18]. Howe e , while g aphene-based senso s ely on he g aphene as he sensing ma e ial, hey
o en ely on CMOS chips o RF communica ion o da a ansmission as can be seen in he de ice
epo ed in [19]. This need e eals a signi ican gap in cu en echnology, emphasizing he impo ance
o de eloping RF g aphene de ices, pa icula ly RF g aphene oscilla o s, o sensing o communica ion.
Achie ing a ully g aphene-based senso equi es a uni ied and compa ible ab ica ion p ocess. Relying
on CMOS in eg a ion wi h g aphene, which is being done by he esea ch communi y [20], can inc ease
de ice complexi y and cos s, and i may e en deg ade pe o mance since he s anda d RF componen s
used in CMOS migh no be op imized o g aphene's unique cha ac e is ics. By using he esonan
11
Figu e 4 – a) Ou pu signals o bu e ed g aphene ROs epo ed in [25], a VDD = 3.5V b) Powe spec um o he medium and
small ROs.
In [27], ano he simila ing oscilla o was p esen ed. In ha wo k, nine di e en ing oscilla o s
we e made, wi h nine di e en ga e leng hs, om 0.5 µm o 3.3 µm while a ying he channel wid h,
access leng h, and sou ce and d ain con ac hickness a he same ime. The sho es achie ed ga e delay
was 31 ps, a a ga e leng h o 0.9 µm. This alue is smalle han he CMOS ansis o s wi h he same
ga e leng h. This delay allowed he au ho s o ab ica e he as es g aphene ing oscilla o epo ed o
da e, unning a 4.3 GHz. I was possible o ob ain his alue since a high ol age gain (A » 5) was achie ed.
Fo ha wo k, g aphene monolaye s we e g own by CVD on coppe oil. A 4 nm hick ilm o aluminium
was e apo a ed on op o g aphene o be oxidized, o o m he ga e dielec ic. The ing oscilla o s, which
equi e low o ze o back-ga e ol age, we e ope a ed a oom empe a u e while he ones ha equi e
la ge back ga e ol ages (VBG > 50 V) we e ope a ed on ai unde an N2 low ( o s abilize he posi ion o he
Di ac poin a 0).
In ha wo k, he ROs made wi h in e e s wi h L < 0.8 µm do no oscilla e. The au ho s ound ha
he gain o he s ages dec eased om 5 o app oxima ely 2. This occu ed due o he con ac and access
esis ance, which do no scale wi h he ga e leng h. A sho ga e leng hs, hese esis ances become
compa able o he channel esis ance and he e o e supp ess he ol age gain. The ab ica ed de ices
can be seen in Figu e 5.

12
Figu e 5 - Fab ica ed ing oscilla o s (ROs) in [27]. a) 3D iew o a bu e ed h ee-s age g aphene RO. b) Ci cui diag am o he
h ee-s age ing oscilla o . c) and d) SEM image o he ab ica ed RO. e) SEM image o an GFET in a RO [27].
Ano he wo k ha p esen s a g aphene ing oscilla o is epo ed in [28]. In ha publica ion, he
au ho s no only made a ully g aphene ing oscilla o bu also ied o inco po a e some CMOS ci cui s
o p o e he compa ibili y o he wo echnologies. They p oposed a no el class o g aphene-Si CMOS
ci cui s ha exploi he ambipola i y o g aphene, o simpli y he ci cui and p o ide addi ional unc ionali y
o hem. The au ho s implemen ed a ing oscilla o ab ica ed only wi h g aphene, a Si CMOS D la ch,
and a iming RC ci cui . The objec i e o he D la ch is o p o ide swi ching and la ge ol age swing o
con ol he GFET, while he RC ci cui is used o se he oscilla ion equency. The maximum ob ained
oscilla ion equency was 4.2 MHz. The au ho s claimed ha he ambipola i y o he g aphene allows o
he accomplishmen o pulse-wid h modula o s (PWMs) and ol age-con olled oscilla o s (VCOs) om he
same ci cui s, i.e., wi hou any ci cui modi ica ion. The op ga ed GFETs we e made wi h CVD g own
g aphene ans e ed o a SiO2/Si subs a e on which h-BN was p e iously ex olia ed. The h-BN was used
since he au ho s belie e ha he GFETs ab ica ed on op o his subs a e a e mo e s able (wi h he
same elec ical p ope ies o e ime), han he ones ab ica ed di ec ly on op o he SiO2 subs a es. A 4
nm hick ga e dielec ic made o Al2O3 was ob ained a e oxidizing an e apo a ed Al ilm di ec ly on op o
g aphene. To pe o m he s udy epo ed ea lie , his de ice was connec ed o he silicon ci cui s ha a e
assembled on a b eadboa d o in p in ed ci cui boa ds. These ci cui s will no be add essed in his epo ,
since only he layou o he ing oscilla o and i s pe o mance a e impo an o his wo k. The
implemen ed g aphene RO can be seen in Figu e 6.
13
Figu e 6 – G aphene oscilla o ab ica ed on op o an h-BN lake, wi h i e GFETs epo ed in [28].
In [29] a g aphene ing oscilla o based on high pe o mance in e e s we e epo ed. The
oscilla ion equency o such de ices was in he ange o dozens o MHz, demons a ing an e ec i e
me hod o ab ica ing RF g aphene ansis o s. Howe e , his app oach may no be sui able o ce ain
applica ions due o he low oscilla ion equency. The op ical mic og aph o he ing oscilla o can be seen
in Figu e 7.
Figu e 7 – Op ical mic og aph o a ing oscilla o wi h alse colo anno a ions highligh ing he a ious ci cui elemen s. The
scale ba ep esen s 10 mm. Sou ce and d ain con ac s a e shown in blue, he local back ga e.
2.2 Resonan oscilla o s and MEMs oscilla o s
Ring oscilla o s a e no he only kind o oscilla o s whe e g aphene could be impo an . G aphene
is also impo an in some MEMS (Mic oelec omechanical Sys ems) s uc u es o LC anks. In [30], a
an alum disul ide-bo on ni ide-g aphene de ice was de eloped. In ha oscilla o , he bo on ni ide was
used o p o ec he de ice agains oxidiza ion, main aining he phase o he gene a ed wa e. A GFET was
ab ica ed o con ol he oscilla ion equency o he oscilla o , which in his case is a ol age-con olled
14
oscilla o (VCO). The oscilla ion equency o he de ice could be adjus ed by changing he ga e ol age
o he de ice. The illus a ion and he SEM image o he ab ica ed de ice can be seen in Figu e 8.
Figu e 8 – SEM image o he VCO ab ica ed wi h g aphene and 1T-TaS2, highligh ed in he igu e, and epo ed in [31].
In [32] ano he GFET based oscilla o was p esen ed. This de ice is based on a esona o nanopla e
o aluminium ni ide. A g aphene memb ane was placed on op o his s uc u e ob aining an oscilla ion
equency o 245 MHz. The wo king p inciple o his de ice is simila o he epo ed be o e. The
illus a ion and he SEM image o he ab ica ed de ice a e shown in Figu e 9.
Figu e 9 – Aluminium ni ide nanopla e esona o wi h g aphene: a) schema ic illus a ion o he s uc u e o he p oposed
de ice, and b) SEM image o he de ice epo ed in [32].
I is impo an o no e ha RF ansis o s can also be used in LC anks o compensa e o losses
du ing oscilla ions. In his con ex , academia has been ocusing on his ma e ial, as demons a ed in [33],
whe e he au ho s p oposed he i s algo i hm o analysing such ci cui s. This de elopmen could pa e
he way o he in eg a ion o his echnology in o LC anks and senso s. In Figu e 10 i is possible o
obse e he schema ic o he p oposed GFET based LC-VCO.
15
Figu e 10 – Schema ic o he p oposed GFET based LC-VCO on he le , and c oss-sec ion o he GFETs on he igh [33].
2.3 O he ypes o oscilla o s
T ansis o -based oscilla o s a e no he only ype o oscilla o s ha can be ab ica ed. G aphene
MEMS-oscilla o s ha e been also epo ed.
Simila ly o ansis o s, o e alua e he pe o mance o MEMS oscilla o s, i is necessa y o es ablish
he FOMs. The mos used FOM o MEMS oscilla o s s uc u es a e he quali y ac o (Q) and he
esonance equency ( ) [34]. The quali y ac o is he a io be ween he s o ed ene gy in one de ice and
he dissipa ed ene gy pe esonance cycle, and is gi en by 𝑄:
𝑄= 𝑓𝑟
𝑓𝑢−𝑓𝑜
(1)
whe e 𝑓𝑟 is he esonan equency, 𝑓𝑢 is he uppe oscilla ion equency and 𝑓𝑜 is he lowe oscilla ion
equency, being 𝑓𝑢−𝑓𝑜, also known as Δ .
The esonan equency o an oscilla ing de ice gene ally e e s o a pa icula equency whe e he
s o ed kine ic ene gy and he po en ial ene gy a e in esonance. G aphene is a sui able ma e ial o
esona o s uc u es due o i s high young modulus and i s a omic hickness. These p ope ies enable
g aphene o achie e esonan na u al equencies o hund eds o MHz and o ole a e high s ains wi hou
damage. The g aphene memb ane could be ac ua ed he mally, mechanically o elec ically o achie e
high oscilla ion equencies. In he nex ew pages, se e al MEMS de ices will be p esen ed. In [35], he
au ho s used g aphene oils ob ained by mechanical ex olia ion and placed hem di ec ly on op o
enches p e iously opened on SiO2 wi h gold con ac s. A gene al illus a ion o he ab ica ed de ice can
be seen Figu e 11.
16
Figu e 11 – On he le is ep esen ed he schema ic o he oscilla o , and on he igh he SEM image o he de ice, as epo ed
in [35].
These memb anes we e ac ua ed in 2 o ms: elec ically and op ically. The oscilla ion equency o
he oscilla o s a ied om 1 MHz o 170 MHz, and he Q changed om 20 o 850. A SEM image o his
de ice is shown Figu e 11 on he igh .
In [36] an on-chip hea e o uning o g aphene nanod ums is p esen ed. This hea e adjus s he
nanod ums p ope ies h ough he Joules e ec . Using a ing s uc u e submi ed o he mal expansion,
he au ho s demons a ed ha he applied nanod ums ol age con ol unes he oscilla ion equency.
The oscilla ion equency o he de ice is 11.5 MHz. The SEM image o he de ice and he gene al
ope a ion schema ic o he de ice can be seen in Figu e 12.
Figu e 12 – SEM image o he epo ed oscilla o nanod ums on he le , and schema ic diag am o he de ice on he igh
[36].
O he non-con en ional op ions o ob ain oscilla o s could consis in using spin- o que nano-
oscilla o s. This kind o oscilla o s a e ypically mo e complex, bu he g ea ad an age o hese s uc u es
is he absence o cu en lowing h ough he de ice’s ac i e a ea. In [37], he au ho s p oposed a me hod
o make spin-o bi o que nano-oscilla o based on a single pe malloy laye wi h he hickness in he 15-
20 nm ange, g own on an alumina subs a e and caped wi h silicon dioxide. Wi h his me hod, he nano-

17
oscilla o s could achie e equencies o se e al GHz. The ab ica ed de ice is p esen ed below, in Figu e
13.
Figu e 13 – SEM image o a pemalloy nano-cons ic ion, wid h o 30 nm [37].
In [38], a 3- e minal spin o que nano-oscilla o is p esen ed (STNO), which is based on a magne ic
unnel junc ion exci ed by a spin injec ion mechanism. The combina ion o he wo mechanisms
ou pe o ms he use o only one o exci e he ee laye in o dynamic egimes. As a esul , high ou pu
powe s a e achie ed. The ab ica ed de ice is shown in Figu e 14.
Figu e 14 – Op ical mic oscope o he spin- o que nano-oscilla o de ice, showing he MTJ and he spin Hall mic o-s ipe
ab ica ed in [38].
In [39], a de ice ha combines local injec ion o a pu e spin cu en wi h enhanced spin-wa e
adia ion losses is p esen ed. The au ho s demons a ed cohe en au o-oscilla ions exci ed by a pu e spin
cu en in magne ic nanode ices. Thei indings pa ed he way o he implemen a ion o magne ic nano-
oscilla o s based on conduc ing o insula ing ma e ials, wi hou he use o cu en lowing h ough he
ac i e a ea o he de ice. In Figu e 15 i is possible o see he de ice unde es in [39].
18
Figu e 15 – SEM image o he de ice showing i s s uc u e and composi ion, and he measu emen scheme, applying a DC
cu en be ween bo h con ac s, and p obing i wi h a lase ligh [39].
A sky mion-based spin- o que nano-oscilla o is p esen ed in [40]. The sys em in ol es a ci cula
nanopilla wi h an ul a hin ilm ee magne ic laye wi h s ong Dzyaloshinkii-Mo iya in e ac ion and a
pola ize laye wi h a o ex-like spin con igu a ion. The au ho s showed he appea ance o a sky mion
gy a ion which leads o oscilla ions o he ma e ial. The gene al schema o he p oposed de ice can be
seen in Figu e 16.
Figu e 16 – Sky mion-based spin- o que nano-oscilla o , designed wi h a nanopilla , wi h he sky mion in he ee laye , and
wo di e en possibili ies o he dis ibu ion o he magne ic ield in he e e ence laye [40].
19
2.4 RF G aphene ansis o s
As s a ed below; o ab ica e g aphene ing oscilla o s, i is necessa y o ab ica e g aphene
ansis o s wi h RF capabili ies, since hey a e he building block o hese ci cui s. In ecen yea s, he e
has been signi ican p og ess in he speed and in eg a ion o in eg a ed ci cui s, p ima ily due o e o s
in downscaling silicon ansis o s. Howe e , silicon ansis o s a e now app oaching hei echnological
limi s. To gua an ee ha elec onics echnologies a e e ol ing and o ensu e ha he imp o emen o he
cu en de ices is possible, u he esea ch on new ma e ials o eplace silicon needs o be unde aken.
Fo his pu pose, esea che s a e pu ing a big e o in o he s udy o g aphene due o i s amazing
elec ical p ope ies such as high sa u a ion eloci y and high ca ie mobili y, which a e impo an o RF
elec onics. The e a e se e al RF g aphene ac i e de ices epo ed in he exis ing li e a u e. Due o
g aphene’s ambipola conduc ion, i is possible o de elop non-linea elec onics which can be swi ched
om n- ype o p- ype by adjus ing he Fe mi´s le el h ough uning he ga e bias. The high ca ie mobili y
and high cu en sa u a ion eloci y o g aphene can enable he use o g aphene in RF ci cui s. The
di icul y o u n o hese de ices is a p oblem o logic applica ions due o inc eased powe consump ion
[41], howe e , in RF applica ions i does no ha e a big e ec [25]. RF g aphene ansis o s ha e
oscilla ion equencies and cu o equencies in he ens o GHz ange (concep s ha will be u he
add essed la e in his documen ), wi h some o he epo ed de ices achie ing in insic cu o equencies
highe han 300 GHz [15]. Connec ing hese ansis o s o pola izing hem in pa icula s a es enable he
design o se e al RF ci cui s. One simple example o hese can be he ambipola mixe epo ed in [42].
Mo e complex RF ci cui s can be ab ica ed wi h g aphene, such as ing oscilla o s. These de ices can
wo k abo e 4 GHz [27] and can be used in biosenso s o RF communica ions. In he ield o RF
communica ions, a g aphene adio equency ecei e was de eloped in [43]. Despi e he low a ailabili y
o g aphene o p o ide powe a high equencies (because o i s low sa u a ion cu en [44]), some RF
g aphene ampli ie s we e epo ed in [45] and [46]. Al hough g aphene isn’ a well-es ablished
echnology, i is a good candida e o de elop RF de ices due o i s p ope ies. G aphene has a highe
elec on mobili y han silicon, and al hough CMOS has a highe T and max, i is achie ed a wice he
cu en consump ion [47]. Ne e heless, GFETs only ha e a na ow IDS ope a ion window, hei
pe o mance is limi ed by i s lowe 𝑔𝑚 and pa asi ics, and o exceed he GFETs pe o mance, i is
p edic ed ha a mobili y highe han 3000 cm2V-1s-1 is equi ed [47].
The pe o mance o RF g aphene FETs is commonly e alua ed by he cu -o equency ( T) and he
maximum oscilla ion equency ( max). The cu -o equency ( T), he wides igu e o me i o RF g aphene
20
ansis o s, is de ined as he equency a which he magni ude o he small-signal cu en gain o he
ansis o s is educed o uni y. In gene al, he cu -o equency can be de ined as desc ibed in equa ion
2 [48]:
𝑓𝑇=𝑔𝑚
2𝜋[(𝐶𝑔𝑠+𝐶𝑔𝑑)(1+(𝑅𝑑+𝑅𝑠)𝑔𝑑𝑠)+𝐶𝑔𝑑𝑔𝑚(𝑅𝑑+𝑅𝑠)+𝐶𝑝𝑔]
(2)
whe e 𝑔𝑚 is he ansconduc ance, 𝐶𝑔𝑠 is he ga e o sou ce capaci ance, 𝐶𝑔𝑑 is he ga e o
d ain capaci ance, 𝐶𝑝𝑔is he ga e pa asi ic capaci ance, 𝑅𝑠 and 𝑅𝑑 a e he sou ce and d ain esis ance,
and 𝑔𝑑𝑠 is he ou pu conduc ance (conduc ance be ween d ain and sou ce). The ansconduc ance, 𝑔𝑚,
is he slope o he cu e 𝐼𝐷𝑆 s 𝑉𝐺𝑆 wi h 𝑉𝐷𝑆 cons an , as shown in sec ion 4.1.4.
The max which e e s o he equency when he maximum a ailable powe gain becomes uni a y.
This e e s o he abili y o he ansis o in p o iding gain, impo an o example in ampli ie s and ing
oscilla o s. Achie ing a high max is challenging because GFETs a e s ongly a ec ed by sho -channel
e ec s. This is p ima ily due o he ga e capaci ance app oaching he same o de o magni ude as he
g aphene quan um capaci ance, which educes he abili y o modula e he g aphene channel e ec i ely.
Addi ionally, he high con ac esis ance in g aphene de ices u he impai s pe o mance, leading o a
loss o con ol o e he channel. Typically, his alue is one magni ude lowe han he T GFETs [49]. The
max can be ob ained by:
𝑓𝑚𝑎𝑥=𝑓𝑇
2√𝑔𝑑𝑠(𝑅𝑔+𝑅𝑠)+2𝜋𝑓𝑇𝑅𝑔𝐶𝑔𝑑
(3)
whe e 𝑅𝑔 co esponds o ga e esis ance, and he emaining a iables a e he same as in
equa ion 2.
By analysing equa ion 2, o inc ease he T i is necessa y o inc ease 𝑔𝑚 o dec ease 𝑔𝑑𝑠 and all
he pa asi ic capaci ances, by dec easing he ga e oxide hickness, o example. Since he 𝑔𝑚 is
p opo ional o he ca ie mobili y and all 𝑔𝑚, 𝐶𝑔𝑠, and 𝐶𝑔𝑑 a e in e sely p opo ional o he channel
leng h, o achie e a big cu -o equency, T, i is essen ial o ha e a high ca ie mobili y and a small
channel leng h o he de ice. Howe e , his is no so simple, since he GFETs a e highly a ec ed by he
sho channel e ec due o he quan um capaci ance o he ma e ial [50].
27
In Table 1 i is possible o obse e he mos ele an GFETs and hei pe o mances. The silicon
MOSFETs we e e iewed in [68], and he epo ed de ices can be used o compa e he MOSFET’s
pe o mance wi h GFET’s. In [62], he GFET wi h he highes ansconduc ance is epo ed, 2 mS/μm in
con as wi h he highes epo ed alues o n-channel MOSFET o 1.84 mS/μm, o 1.6 mS/μm o p-
channel MOSFET [69]. Howe e , a highe ansconduc ance was achie ed in 3.45 mS/μm in III-V FETs
[70]. When analysing he 𝑓𝑇, he maximum alue o 427 GHz was achie ed. When compa ed wi h he
MOSFET de ices, his alue is nea he s a e o he a , wi h he highes epo ed alue o 485 GHz [69].
Rega ding 𝑓𝑚𝑎𝑥, he highes alue was achie ed in [65], 200 GHz. When compa ed wi h he epo ed
de ices in [68] he 450 GHz o an n-channel MOSFET was achie ed. This highligh s ha , despi e
g aphene's po en ial, u he de elopmen is equi ed o imp o e his igu e o me i in g aphene
echnology.
[66]
-
22
-
5
hund e
ds
o mA
0.25
Top ga e
Ex olia ed
g aphene
Low
High
[67]
-
75
-
55
mA
0.36
Top ga e
CVD
g aphene
High
High
[49]
29
427
-
-
mA
1.33
Top ga e
CVD
g aphene
High
High

28
3 DESIGN AND FABRICATION
As discussed in he p e ious chap e s, he design and ab ica ion o g aphene ansis o s is
challenging and equi es ca e ul conside a ion o bo h he design and ab ica ion p ocesses. Since
ansis o s se e as he undamen al uni s o ing oscilla o s, he oscilla o ype selec ed in his hesis,
gi en i s sui abili y o in eg a ion wi h biosenso s and o he biomedical de ices, i s ab ica ion is de ailed
in his chap e , wi h a p ima y ocus on he ab ica ion o RF g aphene ansis o s. Due o he limi a ions
o he INL clean oom, a ade-o be ween pe o mance and ab ica ion capabili ies mus be conside ed.
This chap e p o ides a comp ehensi e o e iew o he design app oaches employed o mee he
speci ica ions equi ed o ab ica ing he building blocks o he ing oscilla o s, speci ically he ansis o s,
while conside ing he a ailable ab ica ion p ocesses. Two ab ica ion p ocesses a e p oposed and
ab ica ed. I also ou lines he s a egies used o add ess he challenges encoun e ed du ing he
ab ica ion s eps. The chap e begins wi h an explo a ion o how a ious design echniques a e applied o
op imize key pa ame e s o achie e high-pe o mance de ices. Addi ionally, he ab ica ion o CVD-g own
g aphene is discussed, along wi h he challenges and op imiza ion s a egies in ol ed in he ma e ial's
p oduc ion, since he ab ica ed g aphene we e used in he ab ica ion o he RF g aphene de ices
p esen ed in his hesis.
3.1 Ma e ials and ab ica ion me hods
As seen p e iously, only a ew wo ks wi h g aphene oscilla o s we e epo ed. This is due o he
single a omic hickness o g aphene which p esen s signi ican challenges, since many elec onic
ab ica ion p ocesses adi ionally used in silicon echnology canno be di ec ly applied o g aphene. These
con en ional p ocesses o en damage he g aphene la ice, leading o pe o mance deg ada ion. The mos
c i ical and challenging s ep in he ab ica ion o RF g aphene de ices is he in eg a ion o a high-k
dielec ic on he g aphene su ace. S anda d ab ica ion echniques ypically damage he g aphene la ice,
al e ing i s in insic p ope ies o , in some cases, des oying i . Fu he mo e, he ab ica ion o g aphene
in a la ge scale is also challenging. The inco po a ion o a high-k dielec ic is c ucial o p e en excessi e
cu en leakage due o di ec elec on unnelling. High-k dielec ic ma e ials enable an inc ease in he
physical hickness o he g aphene ansis o ’s ga e dielec ics while p ese ing he pe o mance
cha ac e is ics ypical o hin SiO₂ laye s. To add ess his challenge, se e al esea ch g oups a e explo ing
inno a i e solu ions, such as he physical ga e ans e using co e-shell nanowi es [71] o he applica ion
29
o he mal ape [72] o ans e p e-pa e ned s uc u es. Addi ionally, sel -aligned p ocesses a e being
de eloped o minimize access esis ance and pa asi ic capaci ances, bo h o which a e de imen al o RF
pe o mance.
The de ices ab ica ed o his hesis we e designed o be compa ible wi h he INL clean oom, as all
ab ica ion s eps we e pe o med wi hin hese acili ies, whe e pa ame e unabili y is cons ained by he
equi emen s o o he p ocesses unning in pa allel in his clean oom. INL’s clean oom ( ha a e based
on s anda d la ge scale ab ica ion p ocesses), classi ied as ISO 5 class and spanning an a ea o 1,200
m², is equipped wi h cu ing-edge ools o mic o- and nano ab ica ion mainly op imized o 200 mm
wa e s, suppo ing applica ions ac oss ields such as elec onics, medical de ices, mic o luidics, ene gy,
and senso echnologies. Despi e he ex ensi e capabili ies, adjus men s we e necessa y o g aphene
p ocessing, as ce ain clean oom p ocesses a e no ully op imized o g aphene applica ions. Fo
g aphene g ow h, he acili y includes an Easy ube 3000 CVD sys em, while ma e ial deposi ion op ions
include se e al specialized sys ems. The Kenosis ec KS1000 PVD sys em is equipped wi h bo h RF and
DC (Di ec Cu en ) capabili ies, ea u ing a UHV mul i- a ge se up wi h 11 magne ons. This includes 3
RF magne ons o deposi ing non-conduc i e ma e ials and he emaining DC magne ons o me al
deposi ion, enabling he spu e ing o hin ilms. The SPTS MPX CVD Module is designa ed o PECVD
silicon oxide and silicon ni ide deposi ions. Addi ionally, hin ilms such as Al2O3 (alumina) o AlSiCu can
be deposi ed wi h he Tima is FTM sys em om Singulus. This machine has he a ailabili y o pe o m RF
and DC spu e ing (oxide and me als) wi h a good uni o mi y (70%) and con o mali y. The dielec ic and
amo phous silicon ilms a e deposi ed using an SPTS PECVD machine, which can p oduce ilms in he
nanome e o mic on hickness ange. Fo dielec ic deposi ion, a omic laye deposi ion (ALD) is a ailable,
al hough only Al2O3 can be deposi ed. The sys em is he ALD Beneq TSF 200, which ega ding i s modula
a chi ec u e is ideal o R&D ac i i ies, suppo ing he mal ALD, emo e plasma ALD and di ec plasma
ALD. Li hog aphy p ocesses a INL include mask aligne s and a di ec -w i ing li hog aphy (DWL) sys em
om Heidelbe g, wi h a ailable lase beams a λ = 405 nm and λ = 375 nm, which p o ides signi ican
lexibili y o i e a i e R&D p ocesses by allowing CAD layou exposu es. Fo high-p ecision nanome e -
scale wo k, an elec on beam sys em o Vis ec is a ailable, he EBPG 5200 wi h an ope a ion ol age o
100 kV and 50 kV; howe e , due o i s cos and complexi y, i is used spa ingly. Suppo ing li hog aphy,
INL o e s a p iming o en and au oma ic coa e s by Suss Mic oTec. A c i ical poin d ye is also accessible
wi hin he acili y.
While INL is compa ible o a ange o CMOS p ocesses, some limi a ions o g aphene emain,
pa icula ly he lack o a gold e apo a o , which is commonly needed o c ea ing con ac s in g aphene
30
de ices. None heless, he acili y’s equipmen suppo s many aspec s o g aphene-based RF de ice
ab ica ion. In he ollowing sec ions, key conside a ions in he de elopmen o RF g aphene de ices will
be examined in de ail wi h ocus in g aphene ansis o s since hey a e he building block o he ing
oscilla o s which was he selec ed kind o oscilla o o his hesis due o hei small size and since he
inco po a ion o g aphene ansis o s wi h RF capabili ies in g aphene biosenso s could be aluable o
implemen o he kind o RF ci cui y (RF mixe s, equency double s o ec i ie s).
3.1.1
Subs a e o g aphene ansis o s ab ica ion
The co ec choice o he subs a e o he g aphene ansis o s ab ica ion impac s he pe o mance
and/o ab ica ion s eps, as well as in eg a ion po en ial. The subs a e may be imposed by he ab ica ion
acili ies, by he in eg a ion me hodology equi ed, o by he g aphene ab ica ion and p ocessing s eps.
G aphene ansis o s can be ab ica ed on se e al subs a es, wi h he mos common being Si/SiO2
[73], SiC [74], glass and qua z [75]. G aphene ansis o s a e also epo ed in lexible subs a es such
as polye hylene naph hala e (PEN) [76] o polye hylene e eph hala e (PET) [77]. These a e e y use ul
in nonplana applica ions (wea ables o example), wi h hei low inse ion loss being a good cha ac e is ic
o THz applica ions [78]. The mos used subs a e is Si/SiO2, due o hei compa ibili y wi h he s anda d
ab ica ion p ocesses and he easy op ical iden i ica ion o he g aphene on SiO2. This p ope y makes
hese he mos sui able subs a es o he ab ica ion o high-quali y CVD-g own g aphene de ices a e
g aphene ans e . The p esence o a hick laye o SiO2 on op o he Si subs a e makes his subs a e
also e y use ul in RF applica ions, dec easing he pa asi ic subs a e capaci ances. SiC subs a es a e
o en used o he epi axial g ow h o g aphene o elec onics applica ions, wi h he ad an ages o he
use o he con en ional op-down li hog aphy echniques well es ablished in nano echnology [76]. In
con as o CVD-g own g aphene, g aphene g own on SiC subs a es does no equi e a ans e . Howe e ,
SiC-g own g aphene emains cos -ine ec i e. Insula o subs a es such as glass o qua z a e e y use ul
in g aphene FET ab ica ion. These subs a es b ing ad an ages o RF applica ions due o hei abili y o
educe he pa asi ic subs a e capaci ance, educing he RF losses associa ed wi h he subs a es [79].
3.1.2
Oxide deposi ion on g aphene
To ab ica e RF de ices, i is necessa y o g ow an oxide on op o he g aphene, o do a ga e oxide
o a GFET o he cen e con ac o a coil, o example. The i s app oach o g ow a high-k dielec ic on
op o he g aphene is he ALD, howe e , he lack o eac ion si es on g aphene u ns his me hod mo e
31
complex han usual, being he deposi ion o con inuous hin oxides ha d. The in en ional la ice damage,
ha imp o es he oxide laye s adhesion, is no desi able [80]. In he case o he GFETs (and o he
ansis o s), a big Cox is desi able, and consequen ly a hinne oxide, since i inc eases he
ansconduc ance and consequen ly he equency pe o mance o he de ice. Howe e , hin oxides ha e
high leak cu en and poo long- ime eliabili y [81]. The app op ia e choice o he ga e oxide is e y
ad an ageous o he GFET pe o mance, as epo ed in [82], whe e, despi e he high-k o TiO₂, he H O₂
showed o be a be e choice o inc ease he GFET pe o mance. The deposi ion o a me al seed laye
p io o he ALD showed o be e ec i e in he ALD p ocess, howe e , he deposi ion o he seed laye
migh cause g aphene doping du ing he ALD p ocess. The use o a me al as a seed laye a ec s he
oxide k dielec ic cons an and he mo phology o he oxide [83]. O he app oach could be he g ow h o
an induced laye o amo phous ca bon by elec on beam scanning as a seed laye o oxide deposi ion
by ALD. In [84] a hin H O₂ hin laye wi h 1.3 nm o EOT (equi alen oxide hickness) was g own,
achie ing a ga e capaci ance o 2.63 µF/cm2. The unc ionaliza ion o g aphene p io o he ALD
deposi ion could be done by an ozone p e- ea men which allows a con o mal deposi ion o he oxide
wi h low mobili y deg ada ion and minimal g aphene doping [85]. In his wo k, a i-me hylaluminium
(TMA) and O3 a 25ºC ALD was pe o med on a comme cial ALD eac o o p e- ea he g aphene p io
o he Al2O3 deposi ion. O he su ace ea men s could be pe o med such as exposu e o NO2 [86]. Spin
coa ed polyme s p io o ALD we e epo ed also, such as in [87], whe e a PVA polyme was adso bed o
g aphene su ace as seed laye o ALD due o i s high dielec ic cons an (~6) leading o a ga e
capaci ance educ ion. Molecula bu e laye s could be also chosen p io o he ALD deposi ion, such as
he nonco alen unc ionaliza ion o g aphene by ca boxyla e- e mina ed pe ylene molecules in [88].
Bo on Ni ide could also ac as a bu e laye o plasma enhanced a omic laye deposi ion (PE-ALD). Wi h
his echnique, hin dielec ics o 4 nm could be g own wi hou unc ionaliza ion o need o a seed laye
[89]. The use o hexagonal Bo on Ni ide (h-BN) as ga e dielec ic on op o g aphene is also epo ed in
[66] and [90]. This app oach p ese es he g aphene mobili y wi h a ela i ely high dielec ic cons an
(k~4), howe e , he wa e -scale de ice ab ica ion is no possible. New eme gen ab ica ion p ocesses
a e appea ing based on op-down echniques, emo ing he need o he deposi ion/g ow h o an oxide on
op o he g aphene [49], [91], [92]. In sec ion 3.2.5, his opic will be u he explo ed, p o iding li e a u e
examples o g aphene ansis o s whe e a ious ab ica ion app oaches ha e been employed o de elop
RF g aphene ansis o s.
32
3.2 Design echniques o imp o e pe o mance o g aphene de ices
Despi e he challenges associa ed wi h he ab ica ion me hods discussed in subsec ion 3.1,
pa icula ly in selec ing he app op ia e subs a e o g aphene deposi ion and he op imal app oach o
g owing he ga e oxide o ab ica e RF g aphene ansis o s and, consequen ly, ing oscilla o s, he e a e
se e al echniques ha can be employed o enhance he pe o mance o hese de ices. While hese
echniques inhe en ly in ol e ab ica ion p ocesses, hei p ima y ocus is on op imizing and imp o ing
he pe o mance o he ansis o s. These echniques will be explo ed in he ollowing subsec ions.
3.2.1
G aphene con ac s and con ac esis ance
To achie e high pe o mance in RF de ices, mainly in RF ansis o s, i is necessa y o achie e good
con ac be ween g aphene and he me al. Nume ous s udies ha e explo ed me hods o educe con ac
esis ance in g aphene. This issue s ems om he injec ion o cha ge ca ie s a he in e ace be ween a
h ee-dimensional ma e ial (me al elec ode) and he wo-dimensional g aphene laye . In sho -channel
GFETs, con ac esis ance can signi ican ly impac pe o mance, as i domina es he o al esis ance,
ende ing he modula ed esis ance o he channel negligible. To minimize he con ac esis ance (Rc), he
selec ion o he me al con ac mus be ca e ully selec ed o achie e lowe con ac esis ance. Fo example,
he Pd p o ides a lowe Rc when compa ed wi h Au [93]. To educe he con ac esis ance se e al
app oaches ha e been s udied such as he wo k unc ion enginee ing o g aphene wo k unc ion unde
me al elec odes [94], he annealing o he con ac s p io he g aphene ans e ( o emo e he polyme
esidues ha a e s icked o he su ace o he me al elec ode) [95], cleaning he g aphene su ace p io
he me al con ac s deposi ion wi h UV/ozone plasma [96], edge con ac o g aphene wi h me al by he
ab ica ion o an ido s [97] o pa e ning g aphene by in oducing g aphene cu s in he con ac a ea [98],
c ea ing de ec s on g aphene by he mal annealing in hyd ogen o Ni con ac s [99], adding a MoOx
in e laye on con ac be ween g aphene and he me al elec ode [100] o a clean ans e o g aphene
using gold as suppo laye [101]. Sandwiched con ac s uc u es ha e been epo ed also [102].
Rega ding o he chosen me al and wo k unc ion enginee ing, se e al s udies a e being ca ied ou
o unde s and how he con ac esis ance in luences he de ice´s pe o mance and how his e ec can
be mi iga ed. The o al esis ance be ween wo con ac s is he sum o he semiconduc o esis ance, he
con ac esis ance, and he me al esis ance. Some imes he me al esis ance is neglec ed due o i s low
alue when compa ed wi h he o he wo. So, by sub ac ing he g aphene esis ance om he o al

33
esis ance i is possible o ob ain he con ac esis ance be ween he me al and he g aphene. The con ac
esis ance o each con ac is hal o he o al con ac esis ance (Equa ion 5).
𝑅𝑐=𝑅𝑡𝑜𝑡𝑎𝑙−𝑅𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒
2
(5)
Whe e 𝑅𝑐 is he con ac esis ance be ween g aphene and me al, assumed o be he same o bo h
con ac s (sou ce and d ain), 𝑅𝑡𝑜𝑡𝑎𝑙 is he o al esis ance measu ed be ween sou ce and d ain and
𝑅𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 is g aphene channel esis ance.
In [103] is possible o obse e he in luence o he chosen me hod o con ac deposi ion in he
con ac esis ance. Fo RF applica ions low con ac esis ances a e p e e able. In Figu e 21 i is possible
o see he measu emen esul s o di e en me hods o con ac deposi ion.
Figu e 21 – a) The I2p – V2p plo s o Ti/SLG de ices o EBM (elec on beam e apo a ion) and spu e p ocesses, espec i ely.
The op ical image o measu ed de ice is shown in inse o a); (b) The g aphene esis ance Rg aphene (R4p) be ween wo p obes as
a unc ion o o ce cu en . The schema ic o ou -p obe measu emen is shown in inse o b. Adap ed om [104].
A e he measu emen s, he au ho s epo ed a con ac esis ance o 0.78 kΩ and 4.1kΩ o EBM
(elec on-beam e apo a ion) and spu e deposi ion, espec i ely. I is impo an o no e ha in GFETs
whe e he con ac s we e deposi ed by E-Beam e apo a o , he ρc (con ac esis i i y) is insensi i e o he
laye hickness, howe e , in he de ices whe e he con ac s we e spu e ed, he ρc exhibi s laye
dependence and inc eases wi h he dec easing o he laye hickness. I is impo an o no e ha , in
mul ilaye g aphene, his e ec is no obse ed. In such cases, he ρc is compa ed wi h ha o EBM
de ices. This is because, in mul ilaye g aphene, he me al a oms p ima ily in e ac wi h he su ace laye s,
c ea ing acancies whe e he me al pene a es, con ac ing he unde lying laye s. This in e ac ion di e s
om single-laye g aphene, whe e he con ac esis ance e ec is mo e p onounced. Ano he esis ance
ha some imes is no conside ed when he GFET models a e de eloped is he esis ance be ween he
34
g aphene and he subs a e ( ypically SiO2 o H O2). The e a e se e al echniques o educe his esis ance,
bu in some cases i should be conside ed such as in applica ions ha equi e low leakage cu en s.
The G aphene-me al con ac esis ance, Rc, has p o en o be he main esponsible o deg ading he
ansis o ´s pe o mance. The con ac esis ance plays an impo an ole on he ansis o ´s
pe o mance, mainly when he ga e leng h dec eases, since his does no scale down wi h he ga e leng h,
playing an impo an ole in he cu -o equency, ex insic ansconduc ance, he maximum equency o
oscilla ion, he 𝐼𝐷𝑆-𝑉𝐺𝑆 linea i y and he limi o he on-cu en . Se e al con ac esis ances o he
li e a u e epo ed de ices could be obse ed in he nex able [104].
Me al/G aphene
ρc Ω µm
Re e ence
Ni
500
[105]
Ti
>1000
[106]
Ti/P /Au s ack
100
[107]
C /Pd/Au s ack (1D edge con ac )
100
[108]
Cu (Cu s pa e ned)
125
[109]
Pd (Cu s pa e ned)
457
[109]
Pd
185-900
[110]
Typically, g aphene ansis o s ha e op and back ga e, he back ga e is used o dope he g aphene
educing he con ac esis ance. The global back ga e is used o dope he g aphene unde he con ac s
which signi ican ly dec eases he o e all se ies pa asi ic esis ance.
3.2.2
Access esis ance in RF g aphene ansis o s
Despi e he impo ance o he con ac esis ance be ween he g aphene and he me al, ano he
esis ance plays a big ole in he RF GFET ab ica ion, he access esis ance (Ra). This esis ance is caused
by he unga ed g aphene egion, a ec ing he RF pe o mance o he de ice. In [111] he impac o he
Rc and Ra in GFET on qua z subs a es was in es iga ed sepa a ely. To educe his esis ance se e al
wo ks using sel -aligned s uc u es we e epo ed. In [60] a sel -aligned s uc u e based on T-shape ga e
s uc u e was implemen ed achie ing an max o 105 GHz, in [15] a high-speed ansis o wi h a sel -aligned
nanowi e ga e was epo ed wi h a T o 300 GHz wi h an ex insic cu o equency bigge han 50 GHz in
a qua z subs a e [67]. In [112], a p ocess o sel -aligned ga e g aphene FETs was epo ed. In [113]
Table 2- Con ac esis i i y (Adap ed om [104])
35
he access esis ance was educed by he chemical doping using polye hyleneimine wi h a 2.5 x inc ease
in elec ical pe o mance, such as he ansconduc ance.
3.2.3
Designs o imp o e he max o he de ices
One o he main d awbacks o he GFET ansis o s is hei low max. As p e iously desc ibed, i is
one o he mos impo an FOM o he ansis o s since i indica es he ansis o ’s powe ampli ica ion
abili y. This low alue is caused by he lack o a bandgap o g aphene. Lowe ing he ga e esis ance
(bu ying he ga es, o example) could be a solu ion o imp o e he max [51]. In [59], a high-quali y g own
g aphene was used o imp o e he quali y o he de ice. The g aphene was g own on a sapphi e subs a e
ha enables a clean ans e due o he absence o a me al o be e ched, which could lea e some
impu i ies on g aphene. In he abo emen ioned wo k, he au ho s ab ica ed a mush oom-shaped co e-
shell AlOx/Al op-ga e wi h 200 nm (Figu e 22). A sel -aligned p ocess was employed o dec ease he
access esis ance. The 10.1 GHz and 5.6 GHz we e he ex insic alues o T and max o he de ice,
espec i ely.
Figu e 22 - Schema ic illus a ion o he ab ica ion p ocesses o he op ga e and sel -aligned p ocess epo ed in [59]. a)
Fo ma ion o he unde cu using an e-beam exposu e in an MMA/PMMA wo-laye esis . b) Deposi ion o aluminium o
ab ica e he T-ga e. c) Fo ma ion o he na u al aluminium oxide laye by oxidizing he aluminium d) Me alliza ion o sel -
aligned sou ce/d ain con ac s. e) SEM image o he inal de ice.
In [51], a max o 50 GHz was achie ed wi h a bu ied-ga e ansis o (leng h 200 nm). De ices wi h
100 nm ga e leng h we e also ab ica ed, bu he esul s we e no sa is ac o y since he pa asi ic e ec s
36
ha e an impo an ole in sho e channel ansis o s. The gene al ab ica ion p ocess low is shown in
Figu e 23.
Figu e 23 – P ocess low o he de ices epo ed in [51]. a) High- esis i i y silicon subs a e wi h SiO2 on op. b) Ha d mask
sac i icial laye o he de ini ion o he ga e. c) E ching o he ga e. d) Sou ce/d ain bo om con ac s de ini ion and e ching. e)
Tungs en (W) deposi ion and plana iza ion (CMP). ( ) Deposi ion o H O2 ga e dielec ic. g) G aphene ans e . h) Li -o o he
sou ce/d ain op con ac s.
These de ices we e ab ica ed in high esis i i y silicon subs a es ollowing he Damascene
p ocess. The ga es we e pa e ned by e-beam li hog aphy, ollowed by RIE (Reac i e Ion E ching) o open
he enches. The me al was deposi ed by PECVD (plasma-enhanced chemical apo deposi ion) and CMP
(chemical mechanical polishing) was used o plana ize he su ace. A e ha , H O2 was deposi ed by ALD
and he excess o H O2 was emo ed by ICP (induc i e coupling plasma). The g aphene used in his de ice
was ob ained by CVD g ow h on P oils. In Figu e 24, i is possible o see he ab ica ed de ices.
Figu e 24 – a) Pho og aph o he ab ica ed de ices on a 200 mm Si wa e . b) Bu ied ga e enches. c) SEM image o he
c oss-sec ion o a 100-nm-ga e-leng h GFET [114].
Ano he wo k ha was de eloped wi h a iew o inc easing he max/ T a io was accomplished in
[53]. In his wo k, a T-shaped ga e was ab ica ed. This s uc u e is simila o he one epo ed in [59],
whe e a sel -aligned p ocess was used o minimize he access esis ance. This echnique allows small
ga e dioxide hicknesses o 2.3 nm, ga e leng hs down o 100 nm, and ansconduc ance o 0.5 mS/mm.
The bes pe o mance was ob ained o a de ice wi h a ga e leng h o 110 nm, wi h a T o 23 GHz, and
a maximum oscilla ion equency o 10 GHz ( max). The g aphene used o his de ice was g own by using
43
3.2.5
O he solu ions o ansis o ga e dielec ics
In he p e iously men ioned publica ions, i is possible o no e a g ea e o made by he
esea che s o place high-k dielec ics di ec ly on op o g aphene. ALD is he mos used echnique and
he leas agg essi e o he g aphene la ice. Howe e , ano he way o place a dielec ic on op o g aphene
could be by ans e ing h-BN ilm. In [90], a h-BN ilm was used as he ga e dielec ic and suppo
subs a e. Despi e he low dielec ic cons an o he h-BN ilm (~3.4), he au ho s showed ha his
echnique was able o p ese e he high mobili y o g aphene, imp o ing he cu en cha ac e is ics o he
de ice. The h-BN ilm was ob ained by di ec ex olia ion o h-BN single c ys als and he hickness o he
ga e dielec ic was app oxima ely 8.5 nm. The ad an age o using his ma e ial is i s la ness, which is 3
imes less ough han he silicon oxide, commonly used on GFET ansis o s. Back-ga ed de ices wi h
3.4 µm o wid h and 2.8 µm o ga e leng h we e ab ica ed. Finally, cu en s o mo e han 1 mA/mm
we e achie ed. The ab ica ed de ice can be obse ed in Figu e 36.
Figu e 36 – Back-ga ed GFET wi h h-BN ga e dielec ic: a) Schema ic o he a omic s uc u e o he g aphene and h-BN. b)
op ical image o an ex olia ed h-BN lake. c) AFM image o h-BN wi h di e en laye hickness. d) op ical image o GFET. e)
C oss-sec ion schema ic o he back-ga e de ice s uc u e [90]
In [66], h-BN was also used as he ga e dielec ic. In ha wo k, wo laye s o h-BN we e used, one
o ac as he passi a ion laye o he silicon, and ano he one o ac as he ga e dielec ic. This echnique
allows o he p ese a ion o he g aphene quali y and consequen ly o he g aphene ca ie mobili y. The
achie ed T a e de-embedding was 22 GHz on de ices wi h 450 nm o ga e leng h and 3 µm o ga e
wid h. A high cu en densi y o 1.2A/mm was achie ed. The ab ica ed de ice is shown in Figu e 37.

44
Figu e 37 – a) o d) Fab ica ion s eps o he BN/g aphene/BN FETs. e) Op ical image o a bilaye g aphene lake ex olia ed
and ans e ed on op o he h-BN subs a e. ) Op ical image o he inal de ice. g) Op ical mic og aph o he inal de ice. h)
SEM image o he inal de ice [66].
In wo ks [15] and [67], he au ho s p oposed he use o a co e-shell nanowi e as he ga e o he
GFET. In [67], he used subs a e was glass o a oid he powe loss h ough he subs a e, hus minimizing
he ga e pa asi ic capaci ance. The p ocess consis s o he p ecise alignmen o he sou ce-ga e-d ain
elec odes, minimizing he access esis ance. The physical assembly o a nanowi e on op o he g aphene
ma e ial p e en s he damage o he g aphene la ice o p ese e i s p ope ies. The nanowi e was
assembled by dielec opho esis on op o CVD g own g aphene. The shell (Al2O3) o he nanowi e was
deposi ed by ALD and he co e o he nanowi e was cons i u ed by Co2Si. The gene al low o he
ab ica ion p ocess and he ab ica ed de ice a e shown in Figu e 38 and Figu e 39, espec i ely.
45
Figu e 38 – Schema ic illus a ion o he ab ica ion s eps o he op-ga ed g aphene ansis o [67].
Figu e 39 – Sel -aligned g aphene ansis o s epo ed in [67]. a) Op ical image o he assembled nanowi e a ay. b) Op ical
image o GFETs and c) Zoom in o b) in one GFET. d) SEM image o GFETs showing he channel and he ga e nanowi e.
The p e iously epo ed ansis o s we e able o achie e high ex insic cu -o equency, highe
han 50 GHz, wi h a 170 nm ga e leng h (diame e o he nanowi e). Despi e he good esul s achie ed,
his is a ha d p ocess o de elop and eplica e since posi ioning he nanowi es is challenging. Fu he
wo k unde aken o p ese e he g aphene la ice cha ac e is ics using a physical ga e ans e is epo ed
in [49]. In ha publica ion, he access esis ance is educed due o he sel -aligned echnique ha was
employed. The au ho s epo ha a high in insic T and max o 427 GHz and 29 GHz we e achie ed.
Fu he mo e, i is also epo ed ha a p e-pa e ned s uc u e was ans e ed wi h he help o a he mal
elease ape di ec ly on he op o he glass and Si/SiO2 subs a es, and CVD g own g aphene was
p e iously ans e ed o he op o his. The ga e s ack was cons i u ed by Al2O3/Ti/Au. In addi ion, he
ga e s acks we e easily peeled o by he mal elease ape due o he low adhesion be ween SiO2 and Au.
46
Finally, he sou ce-d ain elec odes we e pa e ned a e he ga e s ack ans e . The gene al p ocess can
be seen in Figu e 40.
Figu e 40 – Illus a ion o he ab ica ion o sel -aligned g aphene ansis o s wi h ans e ed ga e s acks epo ed in [49].
The inal aspec o he ab ica ed de ices is shown in Figu e 41.
Figu e 41 – The sel -aligned GFETs: a) Pho o image o he inal de ices on a glass subs a e. b) Op ical image o he GFETs.
c) SEM image o he channel o he GFET. d) TEM image o he c oss sec ion o he inal de ice [49].
47
3.2.6
Selec ed app oaches o RF ope a ion
As p e iously discussed, se e al c i ical ac o s mus be conside ed o he ab ica ion o RF
g aphene ansis o s, he undamen al componen o ing oscilla o s. These include he educ ion o ga e
esis ance, achie ed by employing bu ied ga es, and he use o high-quali y g aphene. Minimizing access
esis ance is also essen ial o enhance he pe o mance o RF g aphene ansis o s, as men ioned ea lie .
The use o a high- esis i i y silicon (HR-Si) subs a e, wi h a SiO2 laye on op, is c ucial o mi iga e pa asi ic
subs a e e ec s, especially when compa ed o doped silicon wi h SiO2.
Fo imp o ed RF pe o mance, he in eg a ion o hin ga e oxides wi h high-k dielec ics is
essen ial. Such ma e ials enable be e elec os a ic con ol o he ga e, which is c i ical o achie ing a
ol age gain g ea e han 1. A omic laye deposi ion (ALD) is he p e e ed echnique o oxide deposi ion
due o i s minimal impac on he g aphene su ace. Al e na i ely, he ans e o p e-pa e ned physical
ga es (me al/oxide s acks) o oxide ilms such as h-BN has been explo ed, al hough hese app oaches
pose addi ional challenges o scalabili y.
While educing he channel leng h is bene icial o as e ansis o swi ching, i equi es a ade-
o due o sho -channel e ec s inhe en in g aphene ansis o s. These e ec s can comp omise ga e
con ol o e he channel. The implemen a ion o mul i-ga e ansis o a chi ec u es mi iga es his issue by
imp o ing channel con ol. As highligh ed ea lie , chemical apo deposi ion (CVD) g aphene is he mos
employed ma e ial due o i s scalabili y and unabili y, ensu ing consis en de ice pe o mance.
Based on hese conside a ions, he ollowing s a egies we e selec ed o maximize he
pe o mance o he ab ica ed ansis o s:
1. Implemen a ion o bu ied ga es o educe ga e esis ance.
2. Minimiza ion o access esis ance by deposi ing sou ce/d ain elec odes a he end o he p ocess
( ia a damascene p ocess) o align ga e, sou ce, and d ain elec odes.
3. Use o a high- esis i i y silicon subs a e wi h a SiO2 laye o minimize pa asi ic e ec s be ween
he de ice and subs a e.
4. Inco po a ion o a hin oxide laye g own ia ALD using a high-k dielec ic (e.g., alumina) o
enhance ga e con ol o e he channel.
5. Adop ion o CVD g aphene o enable la ge-scale p ocess scalabili y and op imize g aphene quali y
o de ice pe o mance.
I is impo an o no e ha hese app oaches we e selec ed no only based on hei echnical
ad an ages bu also o ensu e ull compa ibili y wi h he ab ica ion p ocesses a ailable a he In e na ional
Ibe ian Nano echnology Labo a o y (INL), one o he key objec i es o his hesis. Consequen ly, al e na i e
48
me hods such as physical ga e ans e o h-BN as a ga e oxide we e excluded due o hei scalabili y
limi a ions. Despi e he ab ica ion o mul i-ga e ansis o s, hey do no exhibi be e pe o mance
compa ed o single-ga e de ices and, he e o e, will no be discussed u he in his hesis.
3.3 Fab ica ion o RF g aphene de ices
The main goal o his wo k is o ab ica e RF g aphene ing oscilla o s o biomedical applica ions.
As p e iously discussed, he undamen al building block o a g aphene ing oscilla o is a g aphene
in e e , which is composed o se e al g aphene ansis o s (as i will be add essed in sec ion 4).
The e o e, he p ocesses equi ed o ab ica e g aphene ansis o s capable o ope a ing unde RF
condi ions ha e been ho oughly esea ched. To achie e scalabili y in g aphene echnology, CVD
g aphene is being used. The ans e o g aphene om he g ow h subs a e o he inal subs a e poses
signi ican scalabili y challenges and in oduces de ec s ha can ad e sely impac he pe o mance o
g aphene de ices. Fo he ab ica ion o RF g aphene de ices, he use o a high-k ga e dielec ic is equi ed
o maximize he RF pe o mance. This is due o he inc eased oxide capaci ance, which leads o a highe
o al capaci ance and consequen ly a highe cu o equency (𝑓𝑇). Ne e heless, deposi ing o g owing
such ma e ials on op o g aphene is challenging due o g aphene's 2D na u e, i s hyd ophobici y, and
he lack o eac i e si es, pa icula ly when using a omic laye deposi ion (ALD). This o en esul s in he
island- ype g ow h o 𝐴𝑙2𝑂3 on he g aphene su ace. To a oid his issue, esea che s ha e been
ans e ing physical ga es, bu his app oach is di icul o scale.
Bu ied ga e opologies ha e been epo ed in he li e a u e as a iable me hod o ab ica ing
g aphene de ices due o hei ela i ely s aigh o wa d ab ica ion p ocess. Howe e , he use o global
back ga es is no pe ec because hey pola ize he en i e g aphene laye , which modula es he g aphene
unde bo h he sou ce and d ain elec odes, leading o a ia ions in esis ance du ing de ice ope a ion.
When a hick oxide is used, a ela i ely la su ace is ob ained o he g aphene ans e , minimizing
issues du ing he ans e p ocess. Howe e , challenges a ise when a ga e con ac needs o be pa e ned
on a hin oxide laye , which is p e e ed o RF applica ions as i educes pa asi ic esis ances compa ed
o op-ga e con igu a ions. The use o a hin oxide can lead o signi ican su ace opog aphy, complica ing
he g aphene ans e p ocess. Al hough di ec ly ans e ing g aphene on o a ough su ace migh esul
in small c acks, i simpli ies he ab ica ion p ocess.
Gi en his, wo app oaches will be p esen ed in his hesis. Fi s , a bo om-ga e g aphene de ice
ab ica ion p ocess wi hou su ace plana iza ion, along wi h a de ailed discussion o he associa ed

49
challenges. To achie e a c ack- ee g aphene ans e , a c i ical poin d ying p ocess was de eloped and
employed o d y he samples, p ese ing he g aphene la ice cha ac e is ics. In he o he app oach, a
su ace plana iza ion echnique using ion milling p io o g aphene ans e was applied, esul ing in he
success ul ab ica ion o a bu ied ga e RF g aphene FET, demons a ing he e ec i eness o he
plana iza ion p ocess.
The diag am o he wo ab ica ion p ocesses epo ed in his hesis can be seen in Figu e 42.
Figu e 42 – Diag am o he wo main p ocesses epo ed in his hesis wi h and wi hou plana iza ion, a) and b) espec i ely.
In a) i) HR silicon wa e wi h ch omium + gold and alumina on op; ii) e-beam li hog aphy o de ine sou ce, d ain and ga e
con ac s by ion milling; iii) de ice a e ion milling; i ) e-beam li hog aphy o de ine he ga e oxide a e he g ow h o esh
alumina by ALD (a e he esis emo al by O2 plasma, and alumina by we e ch); ) esul a e he pa e ning o he ga e oxide
by ion milling; i) inal de ice wi h g aphene. b) i) de ice a e e-beam li hog aphy and ICP RIE; ii) a e he ch omium gold
deposi ion; iii) a e he li -o , showing he ea s o be emo ed by ion milling, o plana ize he de ice; i ) a e he g aphene
ans e ; ) a e he e-beam li hog aphy and coppe + gold deposi ion o de ine he sou ce/d ain elec odes by li -o ; i) inal
de ice.
3.3.1
Bo om-ga e g aphene de ice ab ica ion p ocess wi hou su ace plana iza ion
The ab ica ion o he i s s uc u e (bo om-ga e wi hou plana iza ion) began wi h a p-doped
silicon wa e , on which 500 nm o SiO₂ was g own ia PECVD. This choice o subs a e was d i en by
cos conside a ions, as he p-doped wa e s we e used as a “dummy'” op ion du ing he p ocess alida ion
phase. Since he p ocess was success ully alida ed, high- esis i i y (HR) silicon wa e s we e used o he
inal s uc u es. Subsequen ly, 3 nm o ch omium and 97 nm o gold we e deposi ed by spu e ing. To
de ine he con ac s using ion milling, 10 nm o alumina was spu e ed on op o he gold o acili a e he
emo al o he pho o esis . A e alumina deposi ion, he sample was p epa ed o elec on beam (e-
beam) li hog aphy. The wa e was coa ed wi h AR-N 7520.18 1+1, and e-beam li hog aphy was pe o med
o pa e n he sou ce, d ain, and ga e con ac s. The con ac s we e hen pa e ned using ion milling a an
angle o 130 deg ees, ollowed by 165 deg ees o emo e edeposi ed me al "ea s." The pho o esis was
s ipped using an oxygen plasma, and an aluminum e chan (Fuji ilm AES) was employed o emo e he
alumina, esul ing in a clean su ace de oid o esis esidues. These s eps a e shown in Figu e 43.
50
Figu e 43 – Fi s s eps o he ab ica ion p ocess wi h a) and b) showing an op ical image o he li hog aphy and c) and d)
showing he op ical mic og aph a e he ion milling and he esis emo al by an O2 plasma.
Nex , 10 nm o Al₂O₃ was deposi ed by ALD o se e as he ga e dielec ic. The dielec ic was
pa e ned using ion milling a e an e-beam li hog aphy wi h AR-N 7520.18 1+1. The pho o esis was hen
emo ed using an oxygen plasma, as illus a ed in he Figu e 44 and Figu e 45.
Figu e 44 – a) and b) show an op ical image o he li hog aphy pe o med o pa e n he ga e dielec ic (alumina) and c) and
d) shows he op ical image a e he ion milling. To no e, since he ga e is e y hin, some e-beam esis was le o ac as
ancho o he e-beam esis esponsible o he pa e ning o he ga e dielec ic.
51
Figu e 45 – a) and b) Op ical pho og aph o he de ice a e he esis emo al wi h he oxygen plasma.
CVD g aphene g own on coppe oil was ans e ed on o he op o he s uc u es using a PMMA-
assis ed we ans e me hod (as de ailed in he nex chap e ). Sho ly, p io o he ans e , an O₂ plasma
ea men was applied o he back side o he coppe oil (which con ains he g aphene p o ec ed wi h he
PMMA) o emo e he g aphene om ha side. The coppe oil was hen e ched in an i on chlo ide
solu ion. Subsequen ly, he g aphene-PMMA s ack was ans e ed o an HCl solu ion o emo e i on
chlo ide con aminan s and hen o wa e be o e he inal ans e o he subs a e. P io o he ans e o
he inal subs a e he subs a e su ace was dehyd a ed and p imed using a apo p ime o en o p omo e
he adhesion o he g aphene o he inal subs a e. A e he ans e , he g aphene was d ied a oom
empe a u e The PMMA was hen emo ed using ace one. Finally, he g aphene was pa e ned using an
O₂ plasma beam ollowing li hog aphy wi h AZ1505. The esis was emo ed using ace one. In Figu e 46
i is possible o obse e he g aphene laying in he g aphene channel.
Figu e 46 – Op ical image o he g aphene laying in he channel o he de ice.
To e alua e he easibili y o using g aphene lakes in GFET ab ica ion, de ices we e cons uc ed
using his kind o g aphene. The g ow h o such lakes will be add essed la e in his documen . Howe e ,
he subs anda d quali y o he lakes, likely esul ing om inadequa e handling du ing pos - ab ica ion,
led o unsa is ac o y de ice pe o mance. The Figu e 47 p esen s he pe o mance o hese ab ica ed
de ices. Addi ionally, as his p ocess was implemen ed on p-doped silicon wa e s, he de ices did no
exhibi RF pe o mance.
52
Figu e 47 – Mic oscope image o he pa e ned subs a e wi h he g aphene lakes (a) and Vgs s IDS and 𝑔𝑚 cha ac e is ic cu es
o de ices p esen ed in ha subs a e wi h W = 35.6 µm and L = 1.19 µm (b) and (c).
Since he g aphene lakes did no yield sa is ac o y esul s and a la ge a ea o con inuous
g aphene is essen ial o achie ing high yield in de ice ab ica ion, con inuous g aphene ilms we e used
in a new sample eplica ing he same ab ica ion p ocess. The p-doped silicon wa e was eplaced by a
high- esis i i y silicon wa e o minimize pa asi ic capaci ances om he subs a e. Due o he small gaps
be ween he sou ce and d ain con ac s, he d ying p ocess is c i ically impo an . Two app oaches we e
es ed wi h he con inuous g aphene ilm a e pa e ning (using O₂ plasma, as i was disco e ed ha
g aphene only b eaks a e pa e ning, i.e., a e he emo al o he pho o esis , no a e he emo al o
he PMMA used o ans e ).
In he i s app oach, he sample was d ied on ai . In he second app oach, c i ical poin d ying
was used. In he i s app oach, due o he su ace ension o ace one (du ing he d ying p ocess), esul ed
in damage o he g aphene a some si es, making i unsui able o RF de ice ab ica ion (see Figu e 48
c) and d)). Fo hicknesses abo e app oxima ely 500 nm, he g aphene ends o con o m o he su ace
o he con ac s a he han emaining suspended (see Figu e 48 b)), which mi iga es he impac o he
d ying p ocess. This obse a ion suppo s he use o coplana s uc u es in senso ab ica ion, whe e
g aphene on he sidewalls enhances senso sensi i i y [118]. The pe o mance o he ab ica ed de ices
is shown in he nex sec ion.
59
unde a p essu e o 4.5 To du ing 30 minu es wi hin he qua z ube u nace. Fo he g aphene g ow h
p ocess, a gas mix u e o 300 sccm A , 100 sccm H2, and 1.5 sccm CH4 was in oduced in o he qua z
chambe , main aining he 4.5 To p essu e o 60 minu es. A e g ow h, he samples we e cooled using
500 sccm A un il he u nace eached ambien empe a u e. As documen ed in he li e a u e, apid
cooling is essen ial o minimize de ec s wi hin he g aphene la ice, he eby p ese ing i s high c ys alline
quali y [123]. The gene al p ocess o he g aphene con inuous ilm ab ica ion could be seen in he Figu e
53.
Figu e 53 – a) Gene al ab ica ion p ocess o he single laye g aphen con inuous ilm. b) g aphene on coppe and c) ca bon
chunk on op o he g aphene a ising om con amina ions in he g aphene u nace.
3.4.2
G aphene lakes
Despi e he o e all quali y o con inuous CVD-g own g aphene ilms, a signi ican d awback is he
p esence o g ain bounda ies, which occu when g aphene lakes coalesce du ing g ow h. These g ain
bounda ies in oduce de ec s in he ilm due o he p esence o dangling bonds ha can ancho
con aminan s, ac ing as sca e ing cen es and he eby deg ading c i ical p ope ies o g aphene, such as
i s ca ie mobili y—an essen ial pa ame e in elec onic applica ions. Inc easing he g ain size in la ge-
a ea con inuous g aphene ilms educes he numbe o g ain bounda ies pe uni a ea, which in u n
enhances he mobili y o g aphene, leading o imp o ed pe o mance in g aphene-based de ices.
To mi iga e he de imen al e ec s o g ain bounda ies, he g ow h o isola ed CVD g aphene lakes
is essen ial, since i acili a es he selec ion o he bes egions isually. Achie ing high-quali y g aphene
lakes equi es p ecise con ol o e he low a es o he gases in ol ed in he g ow h p ocess. This hesis
p esen s a comp ehensi e s udy on he CVD g ow h o g aphene lakes, wi h a pa icula ocus on he
in luence o key gas low pa ame e s, including hyd ogen (H₂), me hane (CH₄), and a gon (A ).

60
Hyd ogen plays a c ucial ole in g aphene g ow h, unc ioning as a co-ca alys ha aids in he
ac i a ion o he me al su ace, he eby p omo ing he g ow h o g aphene. Addi ionally, H₂ signi ican ly
in luences he shape o he g aphene g ains by e ching weak ca bon-ca bon bonds, hus con olling he
mo phology o he g aphene lakes [124]. The low o me hane is equally c i ical, as i se es as he
ca bon sou ce, di ec ly impac ing he g ow h a e and quali y o he g aphene laye .
To op imize he g ow h p ocess, a con inemen g aphi e box con aining an oxygen sou ce (sapphi e
wa e ) was employed o manage he na i e oxide laye p io o annealing. This app oach was aimed a
con olling nuclea ion on he coppe su ace. The coppe oil was delibe a ely oxidized on a ho pla e
wi hin a clean oom en i onmen o inco po a e addi ional oxygen in o he p ocess, which helped dec ease
he nuclea ion si es on he coppe oil. The p esence o oxidized coppe supp esses ca bon nuclea ion,
educing he likelihood o nuclea ion and inc easing he spacing be ween nuclea ion si es. This allows o
g ea e la e al g ow h o g aphene lakes and pe mi s he use o a highe me hane low a e du ing g ow h,
wi hou sa u a ing he coppe oil wi h unwan ed g aphene sa elli es.
Subs a e p epa a ion
The oughness o he coppe oil signi ican ly in luences he quali y o he ans e ed g aphene.
To p e en he collapse o g aphene ollowing ans e , i is essen ial o minimize he oughness o he
coppe su ace, as any i egula i ies on he coppe oil will be ans e ed o he PMMA and subsequen ly
o he inal subs a e. To achie e a smoo h coppe su ace, chemical polishing was pe o med using an
ul asonic ba h. The polishing solu ion consis ed o 10 ml o 0.5 M FeCl₃, 10 ml o 37% HCl, and 280 ml
o deionized (DI) wa e . A e polishing, he samples we e insed ho oughly wi h DI wa e and hen d ied
using a ni ogen (N₂) gun. A e he polishing p ocess, he coppe oils we e oxidized in a ho pla e se a
250°C o 30 minu es wi hin an ISO 5 class clean oom en i onmen . This oxida ion s ep is c ucial o
p epa ing he coppe su ace o subsequen g aphene ans e .
Ca alys oxida ion s a e con ol
To main ain he oxida ion s a e o he coppe oil du ing he g aphene g ow h p ocess, a sapphi e
disk (3-inch Al₂O₃ wa e ) was posi ioned o e he subs a e using wo g aphi e space s as shown in Figu e
54. This se up was hen placed inside a g aphi e box. The g aphi e box se es mul iple c i ical unc ions:
i aps gases eleased om bo h he sapphi e disk and he oxidized coppe oil du ing g ow h, he eby
inc easing he local oxygen concen a ion. Addi ionally, he g aphi e box p o ec s he sample om
con amina ion by SiO₂ pa icles ha may be eleased om he walls o he qua z ube.
61
Figu e 54 – Pic u e o he sapphi e moun ed o e he ea ed coppe oil.
G aphene lakes g ow h
The g aphene lakes we e g own using he same CVD sys em (EasyTube 3000 om Fi s Nano) as
employed o con inuous g aphene ilms, wi h he se up p e iously desc ibed: coppe subs a es wi h a
g aphene cap enclosed in a g aphi e box. The g ow h p ocedu e in ol ed he ollowing s eps:
1. Hea ing: The eac o and he sample a e hea ed o 1040 °C wi h an A low (which las s a ound
30 minu es).
2. Annealing: The sample was annealed in an a gon (A ) a mosphe e o p epa e he su ace o
g aphene deposi ion o 30 minu es.
3. G aphene Deposi ion: G aphene g ow h was achie ed by in oducing 1.5 sccm o me hane (CH₄)
and 350 sccm o hyd ogen (H₂) in o he eac o o 90 minu es a a p essu e o 4.5 To .
Bo h he annealing and g ow h p ocesses we e conduc ed a he same p essu e. A e he g ow h
p ocess, he g aphene lakes we e cha ac e ized using op ical mic oscopy. This was pe o med
immedia ely a e cooling he sample o oom empe a u e. The p esence o g aphene was con i med by
obse ing a eas o he oxidized coppe whe e g aphene was absen . The oxidized coppe was clea ly
isible in egions wi hou g aphene. The wo k low o g aphene lakes g ow h can be seen in Figu e 55.
Figu e 55 – Gene al scheme o he g aphene lake g ow h.
62
3.4.2.1
Con ol o g aphene lake mo phology h ough gas low a ios
Tuning he low a es o H2, CH4 and A gases du ing he CVD p ocess allows o p ecise con ol
o e he nuclea ion densi y, size, and shape o he g aphene lakes.
E ec o he H2 in he lake’s g ow h
In he chemical apo deposi ion (CVD) p ocess, H2 se es bo h as a su ace ac i a o and an
e chan . By adjus ing he low a e o H2, i is possible o in luence he shape o he g aphene c ys als.
Speci ically, he a io o H₂ o CH₄ plays a c i ical ole in de e mining he mo phology o he g aphene
lakes. A low H₂/CH₄ a io ends o p oduce dend i ic shapes, whe eas a high H₂/CH₄ a io esul s in
hexagonal and ci cula shapes, wi h excess H₂ acili a ing he o ma ion o hese la e geome ies.
Hexagonal g aphene lakes a e pa icula ly desi able because hey help o minimize g ain bounda ies
when o ming con inuous g aphene ilms. S udies ha e shown ha con inuous ilms composed o
andomly o ien ed, i egula ly shaped, mic ome e -sized lakes end o ha e a high densi y o g ain
bounda ies, which can ad e sely a ec he ilm’s p ope ies. In con as , hexagonal lakes wi h zigzag
edges a e bene icial o he o ien ed g ow h o con inuous g aphene ilms, educing he numbe o g ain
bounda ies and he eby enhancing he ilm’s p ope ies [125].
In a s udy conduc ed o in es iga e hese e ec s, he low a es o a gon and me hane we e kep
cons an a 300 sccm and 1.5 sccm, espec i ely, while he H₂ low a e was a ied om 50 sccm o
300 sccm a a p essu e o 4.5 To , wi h a g ow h du a ion o 90 minu es. The esul s, as depic ed in he
Figu e 56, align wi h exis ing li e a u e: inc easing he H₂ low leads o a mo e ounded lake mo phology
due o he e ching o he lake edges by hyd ogen.
Challenges we e encoun e ed in iden i ying an op imal g aphene ecipe o RF de ice ab ica ion
due o con amina ion issues om ca bon nano ube g ow h, as he same eac o was used o bo h
p ocesses. This con amina ion a ec ed he cleanliness o he eac o and complica ed he de elopmen
o a sui able g aphene syn hesis p o ocol.
Figu e 56 – E olu ion o g aphene lakes wi h he inc easing o Hyd ogen (𝐻2) low.
63
E ec o he CH4 in he lake’s g ow h
Me hane plays a c ucial ole in he g ow h o g aphene as he ca bon sou ce in his molecule is
essen ial o g aphene o ma ion. To conduc his, an H₂ low a e o 350 sccm and 300 sccm o A a
4.5 To we e used, wi h a ying CH₄ low a es o s udy hei e ec s ( om 1.5 sccm o 2.95 sccm).
Consequen ly, he me hane low a e was ound o di ec ly in luence he size o he g aphene lakes.
Howe e , simply inc easing he CH4 low is no always ad an ageous. Ele a ed CH4 le els can lead o an
inc eased numbe o nuclea ion si es, esul ing in a highe densi y o coalesced g aphene lakes. This
coalescence o en leads o mul ilaye g aphene o ming a he cen e s o lakes and an inc ease in g ain
bounda ies.
While inc easing he CH4 low enhances he a ailabili y o eac i e ca bon species, which p omo es
he g ow h o la ge g aphene lakes, i also in oduces complica ions. Speci ically, excessi e me hane
can lead o a highe nuclea ion densi y, which may cause mo e lakes o me ge. This me ging can c ea e
mul ilaye egions and inc ease he o e all numbe o g ain bounda ies wi hin he g aphene ilm. The
e olu ion o he g aphene lakes wi h he inc ease o CH4 can be seen in Figu e 57.
.
Figu e 57 – E olu ion o g aphene lakes wi h he inc easing o me hane (𝐶𝐻4) low.
The use o he same eac o o bo h g aphene and ca bon nano ube syn hesis posed signi ican
challenges in op imizing g aphene ecipes o RF de ice ab ica ion.
3.4.2.2
Samples cha ac e iza ion
Following he g ow h p ocess, he g aphene lakes we e cha ac e ized using op ical mic oscopy (OM),
scanning elec on mic oscopy (SEM), and Raman spec oscopy. Ini ially, he samples we e p o ec ed om
oxida ion by he g aphene laye . To e eal he unde lying coppe in a eas wi hou g aphene, he samples
we e oxidized in ai by placing hem on a ho pla e a 180°C o app oxima ely 5 minu es. This oxida ion
ea men highligh ed he egions whe e g aphene lakes we e absen , acili a ing he iden i ica ion o
g aphene co e age as shown in Figu e 58.
64
Figu e 58 – Images o g aphene lakes on coppe a e he oxida ion on a ho pla e a) op ical mic oscope and b) pho og aph o
he coppe oil.
Op ical mic oscopy and scanning elec on mic oscopy
Op ical mic oscopy was used o obse e and e alua e he size and shape o he g aphene lakes
immedia ely a e he oxida ion p ocess. Fo a mo e de ailed analysis, scanning elec on mic oscopy
(SEM) was pe o med using a FEI No a NanoSEM 650 a elec on beam ol ages o 5 kV and 10 kV. This
p o ided high- esolu ion images o assess he mo phology and quali y o he g aphene lakes
Raman spec oscopy
Raman spec oscopy was employed o u he cha ac e ize he g aphene lakes. The measu emen s
we e conduc ed using a Wi ec Alpha300R con ocal Raman mic oscope equipped wi h a 532 nm
equency-doubled Nd lase . Raman spec oscopy is a powe ul echnique o assessing g aphene quali y
and p ope ies. While elas ic ligh sca e ing (Rayleigh spec oscopy) can de e mine he numbe o
g aphene laye s, i is p ima ily e ec i e o ex olia ed samples on op imized subs a es and does no
p o ide addi ional s uc u al o elec onic in o ma ion [126]. In con as , Raman spec oscopy is e sa ile
and applicable o all ypes o g aphene samples, p o iding comp ehensi e insigh s in o g aphene quali y,
including he p esence o con aminan s, doping, and de ec s, and enabling e ec i e compa ison among
di e en samples [127]. In g aphene, he Raman spec um exhibi s cha ac e is ic peaks due o phonon
ene gy shi s caused by lase exci a ion. Fo a 532 nm lase exci a ion, hese peaks a e p ima ily he G
peak (a app oxima ely 1580 cm⁻¹), he 2D peak (a a ound 2690 cm⁻¹), and he D peak (a
app oxima ely 1350 cm⁻¹) [128] . The posi ions o hese peaks can a y depending on he wa eleng h o
he exci a ion lase used. The di e en Raman g aphene peaks can be seen in Figu e 59.

65
Figu e 59 – Di e en Raman g aphene peaks o a lase exci a ion o 532 nm[129].
The Raman spec um o single-laye g aphene di e s sligh ly om ha o mul ilaye g aphene due o
in e ac ions be ween he g aphene laye s. Speci ically, he 2D peak in single-laye g aphene is na owe
and sha pe compa ed o ha in mul ilaye g aphene [130]. Addi ionally, he G peak exhibi s a ed shi
wi h an inc ease in he numbe o g aphene laye s [131]. The numbe o g aphene laye s can be de i ed
om he a io o he peak in ensi ies, I₂D/IG, as well as om he posi ion and shape o he peaks as shown
in Figu e 60 and Figu e 62. The I₂D/IG a io is pa icula ly use ul o dis inguishing be ween monolaye and
mul ilaye g aphene. The G and 2D peaks' posi ions and shapes p o ide addi ional in o ma ion on he
laye numbe and he p esence o any de ec s o s ain in he g aphene s uc u e [130].
Figu e 60 – Raman spec a o single laye g aphene and bulk g aphi e a a 532 nm lase [132].
The posi ion and in ensi y o he G band can be in luenced by a ious ac o s such as doping and
s ain wi hin he g aphene. Consequen ly, shi s in he G band posi ion can p o ide aluable in o ma ion
abou hese aspec s o he g aphene sample. The in ensi y o he G band also ollows a p edic able
beha iou ha aids in p edic ing he hickness o he g aphene laye s as shown in Figu e 61.
66
Figu e 61 – A linea inc ease in G band in ensi y occu s as he numbe o g aphene laye s inc eases, wi h a 532 nm lase .
Adap ed om [133].
In addi ion o he G band, ano he p ominen ea u e in he Raman spec um o g aphene is he D
band, also e e ed o as he de ec band. The D band appea s a ound 1350 cm⁻¹ and is indica i e o
de ec s wi hin he g aphene s uc u e. The in ensi y o he D band is di ec ly p opo ional o he le el o
de ec s p esen in he g aphene sample. In high-quali y g aphene o g aphi e, he D band is ypically
e y weak o absen [133].
This ela ionship be ween D band in ensi y and de ec densi y p o ides a aluable ool o
assessing he quali y o g aphene, wi h lowe D band in ensi y signi ying ewe de ec s and highe
s uc u al in eg i y.
Figu e 62 – Rela ionship be ween 2D peak and G peak o single laye g aphene. Adap ed om [133].
The 2D band in he Raman spec um, which appea s a ound 2690 cm⁻¹, is he second-o de
peak o he D band and is c ucial o cha ac e izing g aphene. This band is a s ong and consis en ea u e
o he Raman spec um, e en in he absence o he D band. I plays a signi ican ole in de e mining he
numbe o g aphene laye s p esen in a sample. The posi ion and shape o he 2D band a e c i ical
indica o s; as he numbe o g aphene laye s inc eases, he peak becomes less symme ic due o he
67
o e lapping spec a o he indi idual laye s. The 2D band can be used o dis inguish be ween single-laye
and mul ilaye g aphene, wi h he capabili y o iden i y up o ou laye s. Fo single-laye g aphene, he
in ensi y a io be ween he 2D and G peaks (I₂D/IG) is a key pa ame e . In high-quali y g aphene, his a io
is ypically g ea e han o equal o 2.
Gi en hese cha ac e is ics, Raman spec oscopy is an e icien me hod o e alua ing he quali y
o g aphene. Ini ially, Raman measu emen s we e conduc ed immedia ely a e he g aphene g ow h, as
shown in Figu e 63. Subsequen Raman analyses we e pe o med a e ans e ing he g aphene o he
inal subs a e, which is c i ical o nanode ice ab ica ion.
Figu e 63 – a) Raman spec a a g aphene lake on a coppe oil and b) Raman spec a a 532 nm o he lake.
3.4.3
G aphene g ow h con aminan s
As p e iously men ioned, he clean oom acili ies a INL a e sha ed among a ious esea ch
p ojec s. The Easy ube sys em used o g aphene g ow h was also used o ca bon nano ube syn hesis.
The nano ube g ow h p ocess used much highe low a es o me hane and e hylene (no used in
g aphene) compa ed o hose equi ed o g aphene g ow h, which led o signi ican con amina ion o he
eac o chambe and associa ed plumbing. Despi e ho ough cleaning e o s, la ge ca bon deposi s
emained in he sys em. The high sensi i i y o g aphene g ow h pa ame e s o en i onmen al condi ions
made i challenging o achie e op imal esul s wi h his le el o con amina ion. The esidual ca bon om
nano ube syn hesis (shown in Figu e 64) ad e sely a ec ed he abili y o inely une he g ow h condi ions
necessa y o p oducing high-quali y g aphene. Consequen ly, he con amina ion impeded he
de elopmen o g aphene sui able o s a e-o - he-a RF de ices.
68
Figu e 64 – Con amina ed CVD u nace. The black po ions a e ca bon chunks ha esul ed om CNT g ow h.
A cleaning p o ocol in ol ing high- empe a u e oxida ion wi h oxygen was de eloped o add ess
con amina ion wi hin he eac o chambe . Howe e , his me hod p o ed insu icien o comple ely emo e
all esidual ca bon deposi s om he pipes and chambe . Addi ionally, manual cleaning e o s we e
employed o u he add ess con amina ion, bu hese measu es s ill did no achie e he desi ed le el o
cleanliness.
The pe sis en con amina ion comp omised he e ec i eness o he cleaning p ocedu es and
ad e sely a ec ed he ep oducibili y o he g aphene g ow h p ocess. As a esul , main aining consis en
and high-quali y g aphene p oduc ion o RF de ice applica ions became challenging.
3.4.4
G aphene ans e
PMMA is he mos used polyme o suppo ing g aphene du ing he ans e p ocess. This polyme
has excellen solubili y in o ganic sol en s such as ace one and oluene. A e PMMA emo al, ollowing
he g aphene ans e o he inal subs a e, i is common o obse e PMMA esidues on op o he
g aphene memb ane. Despi e hese esidues, he PMMA ans e me hod emains he mos eliable and
widely used o g aphene ans e . PMMA ac s as a suppo ing laye o p ese e he in eg i y o he
g aphene du ing he we e ching o he coppe subs a e and p o ides mechanical s abili y h oughou
he ans e p ocess. Howe e , hese esidues can dope he g aphene, po en ially deg ading i s he mal
and elec ical p ope ies. The g aphene ca ie ’s mobili y can dec ease one o de o magni ude a e he
ans e [134][135]. The g aphene ans e is also a ec ed by he appea ance o c acks in he memb ane
as shown in he Figu e 65. Se e al me hods ha e been epo ed o emo e he PMMA om he op o he
g aphene su ace. Ho ace one was used o emo e he PMMA, since he empe a u e enhances he
sol en ex ac ion [136]. O he sol en s could be used o dissol e PMMA such as anisole, chlo obenzene
and chlo o o m. Despi e he PMMA sol en s being used o emo e he PMMA om he g aphene su aces
75
whe e µ𝑒 and µℎ a e elec on and hole mobili y. 𝑛0 is he esidual ca ie densi y due o diso de and
e mal exci a ion, C is he ga e capaci ance pe a ea, W and L a e he channel wid h and leng h, q is he
elec on cha ge, 𝛽 ela es he phonon wa eleng h o he dominan sca e ing phonon and
m
is a i ing
𝐼𝑑𝑠1=µ𝑒𝑉0𝑄0
√1+(µ𝑒|𝑉𝑔𝑠−𝑉𝑔𝑑|
L 𝜐sa )𝑚
𝑚𝑊
𝐿𝑓 (𝑉𝑔𝑠,𝑉𝑔𝑑)
(6)
𝐼𝑑𝑠2=µ𝑒𝑉0𝑄0
√1+(𝑢𝑒|𝑉𝑔𝑠−𝑉𝑔𝑑|
L 𝜐sa )𝑚
𝑚𝑊
𝐿𝑓 (𝑉𝑔𝑠,0) + µh𝑉0𝑄0
√1+(µℎ|𝑉𝑔𝑠−𝑉𝑔𝑑|
L 𝜐sa )𝑚
𝑚𝑊
𝐿𝑓 (0,𝑉𝑔𝑑)
(7)
𝐼𝑑𝑠3=µh𝑉0𝑄0
√1+(µℎ|𝑉𝑔𝑠−𝑉𝑔𝑑|
L 𝜐sa )𝑚
𝑚𝑊
𝐿𝑓 (𝑉𝑔𝑠,0) + µe𝑉0𝑄0
√1+(µℎ|𝑉𝑔𝑠−𝑉𝑔𝑑|
L 𝜐sa )𝑚
𝑚𝑊
𝐿𝑓 (0,𝑉𝑔𝑑)
(8)
𝐼𝑑𝑠4=µh𝑉0𝑄0
√1+(µℎ|𝑉𝑔𝑠−𝑉𝑔𝑑|
L 𝜐sa )𝑚
𝑚𝑊
𝐿𝑓 (𝑉𝑔𝑠,𝑉𝑔𝑑)
(9)
𝑓 (𝑥,𝑦)=𝑥√1+𝑥2−𝑦√1+𝑦2+𝑙𝑛( √1+𝑥2 + 𝑥
√1+𝑦2+𝑦)
(10)
𝜐𝑠𝑎𝑡= 𝜐𝐹𝛽
√𝑛𝑜2+ (𝐶(𝑉𝑔𝑠+𝑉𝑔𝑑
2𝑞 )2
4
(11)
𝑉𝑔𝑠=𝑉𝑔𝑠
𝑉𝑜
(12)
𝑉𝑔𝑑= 𝑉𝑔𝑑
𝑉𝑜
(13)
𝑉𝑜=𝑄𝑜
𝐶
(14)
𝐶= 𝐶𝑔𝑑+𝐶𝑔𝑠
𝑊𝐿
(15)
𝑄𝑜=𝑞𝑛𝑜
(16)

76
pa ame e . These unc ions can be me ged by using he ope a o Θ (x), i.e., using he combina ion o he
s ep unc ions whe e he a iables a e 𝑉𝑔𝑠 and 𝑉𝑔𝑑. Gi en ha , he single 𝐼𝑑𝑠 and Θ (x) equa ions a e:
𝐼𝑑𝑠=𝐼𝑑𝑠1Θ (𝑉𝑔𝑠)Θ(𝑉𝑔𝑑) + 𝐼𝑑𝑠2Θ (𝑉𝑔𝑠)Θ(−𝑉𝑔𝑑) × 𝐼𝑑𝑠3Θ (−𝑉𝑔𝑠)Θ(𝑉𝑔𝑑) + 𝐼𝑑𝑠4Θ (−𝑉𝑔𝑠)Θ(−𝑉𝑔𝑑)
(17)
Θ(x)= 1+ anh(𝑉1𝑥)
2
(18)
whe e 𝑉1 is a i ing pa ame e . The 𝑉𝑜 pa ame e is in oduced o add he unin en ional cha ging e ec
in he channel a low 𝑉𝑑𝑠, (𝑉𝑔𝑠 = 𝑉𝑔𝑠-𝑉𝑜 and 𝑉𝑑𝑠 = 𝑉𝑑𝑠-𝑉𝑜).
Despi e he dielec ic hickness being less han 10 nm in ha wo k [147], he au ho s o ha
model did no include quan um capaci ance. Al hough c i ical, o simplici y, i was omi ed because
quan um capaci ance appea s in se ies wi h he geome ic capaci ance, and ypically he smalles
capaci ance domina es. ( he geome ic capaci ance).
As p e iously men ioned, he con ac esis ance a ies based on he ype o ca ie wi hin he
channel. To accoun o his, he model uses a se ies esis ance ha depends on he ca ie ype o he
con ac esis ance. Addi ionally, he con ac esis ance a bo h he sou ce and d ain is assumed o be
iden ical, as i is in luenced by he channel wid h a he han he con ac a ea [147]. Consequen ly, he
con ac esis ance is gi en by he ollowing equa ion:
𝑅𝑠=𝑅𝑑=𝑅𝑜+𝑅𝑒𝑥𝑡(𝑉𝑔𝑠,𝑉𝑔𝑑)
(19)
𝑅𝑒𝑥𝑡(𝑥,𝑦)=𝑅𝑒𝑥𝑡𝑜1+ anh(𝑉2𝑥)
21+ anh(𝑉2𝑦)
2
(20)
whe e 𝑅𝑜 is he esis ance when 𝑉𝑔𝑠<0 and 𝑉𝑔𝑑<0, 𝑅𝑒𝑥𝑡𝑜 is he esis ance added o accoun o he
di e ence in he esis ance due o di e en majo i y ca ie s, and 𝑉2 is a i ing pa ame e .
77
4.1.2
T ansis o model pa ame e s ex ac ion
Fo he ex ac ion o he ex insic pa ame e s (LS, LG, LD, CPGS, CPDS, and RG), excluding Rs and 𝑅D,
he S-pa ame e s o he de-embedding s uc u es we e used. A mo e de ailed explana ion o his app oach
can be shown below in he sec ion 4.1.3. The RS and RD pa ame e s aim o accoun o a ia ions in
con ac esis ance depending on he ca ie ype and can be ob ained om DC measu emen s o 𝐼𝐷𝑆 −𝑉𝐺𝑆
cu es a low d ain ol ages. This app oach is necessa y because, a low d ain ol ages, Equa ion 6 and
9, can simpli y he in insic and ex insic d ain-sou ce esis ance o he equa ions p esen ed in Table 3.
Holes
Elec on
In insic
𝑅𝑑𝑠=𝑉𝑑𝑠
𝐼𝑑𝑠
1
𝛼ℎ√1 + ( 𝑉𝐺𝑆
𝑉0 )2
1
𝛼𝑒√1 +( 𝑉𝐺𝑆
𝑉0 )2
Ex insic
𝑅𝐷𝑆=𝑉𝐷𝑆
𝐼𝐷𝑆=𝑅𝑆+𝑅𝐷+𝑅𝑑𝑠
2𝑅𝑜+ 𝛼ℎ
√1 +( 𝑉𝐺𝑆
𝑉0 )2
2𝑅𝑜+2𝑅𝑒𝑥𝑡𝑜 𝛼𝑒
√1 + ( 𝑉𝐺𝑆
𝑉0 )2
𝛼𝑒,ℎ=𝐿
𝑊𝜇𝑒,ℎ𝑄0
By i ing he a o emen ioned equa ions (in Table 3) o model he 𝑅DS p o ile, one can de e mine
he alues o 𝑅S, 𝑅D, (𝑅𝑜 𝑎𝑛𝑑 𝑅𝑒𝑥𝑡𝑜), 𝑉0, and 𝛼ℎ,𝑒. Ex ac ing in insic capaci ances in ol es measu ing
he de ice S-pa ame e s when biased a he Di ac poin (Cgd and Cds). This speci ic bias poin is c ucial
because, unde hese condi ions, he 𝑔𝑚 is equal o 0, and he pa asi ic pa ame e s can be ex ac ed
ollowing he de-embedding p ocess in (sec ion 4.1.3.) and he small-signal model shown in Figu e 68,
he ollowing pa ame e s can be ob ained:
𝐶𝑔𝑑=𝐼𝑚𝑎𝑔[−𝑌12]
𝑗𝑤
(21)
𝐶𝑔𝑠=𝐼𝑚𝑎𝑔[𝑌11+𝑌12]
𝑗𝑤
(22)
𝐶𝑑𝑠=𝐼𝑚𝑎𝑔[𝑌22+𝑌12]
𝑤
(23)
Table 3- Equa ions o in insic and ex insic d ain-sou ce esis ances
78
And he emaining pa ame e s a e calcula ed as ollows:
𝐶=𝐶𝑔𝑠+𝐶𝑔𝑑
𝐿𝑊
(24)
𝑄𝑜=𝐶𝑉𝑜
(25)
𝜇𝑒,ℎ=𝐿
𝑊𝛼𝑒,ℎ𝑄𝑜
(26)
Figu e 68 – Small-signal model a e de-embedding he pa asi ic elemen s a 𝑉𝐺𝑆=𝑉𝐷𝑖𝑟𝑎𝑐. Adap ed om
[147]
.
4.1.3
T ansis o de-embedding
Fo RF measu emen s, a mo e complex se up is equi ed. The de ice mus be se a an ope a ing
poin using a DC ol age, necessi a ing he use o a bias ee (Figu e 71). The bias ee se es o combine
he DC and RF signals. The ou pu o he bias ee, ca ying bo h RF and DC componen s, is hen connec ed
o he ga e and d ain e minals o bias he GFET and enabling he RF measu emen s. The S-pa ame e s
we e acqui ed using a Vec o Ne wo k Analyze (VNA). The gene al ma ix o he S-pa ame e s is as
ollows:
[𝑏1
𝑏2]=[𝑆11 𝑆12
𝑆21 𝑆22 ] ∙ [𝑎1
𝑎2]
(27)
whe e a is he inpu signal, b is he ou pu signal,
𝑆11 is he inpu po ol age e lec ion coe icien ,
𝑆12 is
he e e se ol age gain,
𝑆21
is he o wa d ol age gain, and 𝑆22 is he ou pu po ol age e lec ion
coe icien .
79
The S-pa ame e s a e aluable o ex ac ing he H-pa ame e s (such as 𝑓𝑇), as well as he
admi ance pa ame e s (Y) and impedance pa ame e s (Z). These pa ame e s a e c ucial o de e mining
he de ice’s capaci ances, esis ances, and induc ances. I is impo an o no e ha Y = 𝑍−1.
The admi ance pa ame e s (Y) can be ob ained om he S-pa ame e s using he ollowing
ela ionships:
𝑌11=(1−𝑆11)(1+𝑆22)+𝑆12𝑆21
(1+𝑆11)(1+𝑆22)−𝑆12𝑆21𝑌0
(28)
𝑌12=−2𝑆12
(1+𝑆11)(1+𝑆22)−𝑆12𝑆21𝑌0
(29)
𝑌21=−2𝑆21
(1+𝑆11)(1+𝑆22)−𝑆12𝑆21𝑌0
(30)
𝑌22=(1+𝑆11)(1−𝑆22)+𝑆12𝑆21
(1+𝑆11)(1+𝑆22)−𝑆12𝑆21𝑌0
(31)
whe e Y0 and Z0 a e he po impedance ha ypically Z0 = 50 ohm. The wo-po sys em equi alen ci cui
om he Y-pa ame e s and Z-pa ame e s is shown in Figu e 69.
Figu e 69 – Two-po sys em equi alen ci cui om he Y-pa ame e s (le ) and Z-pa ame e s ( igh ).
As p e iously desc ibed, i is possible o ex ac he in insic cha ac e is ics o a de ice by de-
embedding i . Fo ha pu pose, open and sho s uc u es should be measu ed o pe o m an open-sho
de-embedding. The igu e o he comple e de ice, along wi h he open and sho s uc u es, can be seen
in Figu e 70.
Figu e 70 – De-embedding equi alen ci cui s. De ice unde es (le ), Sho (cen e ) and Open ( igh ).
80
Compa ing he 2 p e ious igu es (Figu e 69 and Figu e 70) and knowing ha he Yc=j
w
C, he
ollowing equa ions can be ex ac ed:
𝐶𝑃𝐺𝐷=Imag[−𝑌𝑂12]
𝑤
(32)
𝐶𝑃𝐺𝑆=Imag[𝑌𝑂11+𝑌𝑂12]
𝑤
(33)
𝐶𝑃𝐷𝑆=Imag[𝑌𝑂12+𝑌𝑂22]
𝑤
(34)
whe e 𝑌𝑂 a e he Y-pa ame e s o he open s uc u e o he de-embedding.
F om he sho ci cui and knowing he 𝑌𝑂 and 𝑌𝑆 (Y-pa ame e s o he sho s uc u e) i is
possible o ex ac he Z pa ame e s o he blue s uc u e and knowing ha he impedance o a esis o
in se ies wi h an induc o is 𝑍𝑅𝐿 = R+
jw
L, he ollowing equa ions can be ex ac ed:
𝑅𝐺=Real [𝑍𝑆𝑂11− 𝑍𝑆𝑂12]
(35)
𝑅𝐷=Real [𝑍𝑆𝑂22− 𝑍𝑆𝑂12]
(36)
𝑅𝑆=Real [𝑍𝑆𝑂12]
(37)
𝐿𝐺=Imag [𝑍𝑆𝑂11− 𝑍𝑆𝑂12]
𝑤
(38)
𝐿𝐷=Imag [𝑍𝑆𝑂22− 𝑍𝑆𝑂12]
𝑤
(39)
𝐿𝑆=Imag [𝑍𝑆𝑂12]
𝑤
(40)
To de-embed he inal s uc u e, he ollowing o mula could be used [147].
𝑌𝑖𝑛𝑡𝑟𝑖𝑛𝑠𝑖𝑐=1
1
𝑌𝐷𝑈𝑇−𝑌𝑂−1
𝑌𝑆−𝑌𝑂
(41)
4.1.4
Ex ac ion o
𝑔𝑚
and g aphene mobili y om g aphene ansis o s
Since i is impo an o ex ac he g aphene p ope ies o conclude abou he g aphene quali y, he
g aphene mobili y and ansconduc ance (impo an pa ame e s used o compa e he pe o mance o DC

81
ansis o s) can be ex ac ed a e he 𝐼𝐷𝑆-𝑉𝐺𝑆 measu emen s. The ansconduc ance can be ex ac ed
by applying a nume ical de i a i e using he ollowing o mula:
𝑔𝑚[𝑖]=𝐼𝐷𝑆[𝑖+1]−𝐼𝐷𝑆[𝑖]
𝑉𝐺𝑆[𝑖+1]−𝑉𝐺𝑆[𝑖]
(42)
The mobili y o he g aphene in he ield-e ec ansis o can be ex ac ed by:
𝜇= 𝐿|𝑔𝑚|
𝑊𝐶𝑔𝑉𝐷𝑆
(43)
whe e L and W a e he channel leng h and wid h and 𝐶𝑔 is he ga e capaci ance pe a ea. Despi e his
me hod is used o simplici y, i unde es ima es he g aphene mobili y since i does no conside he
con ac esis ance o he g aphene and he unga ed egions. To mo e accu a ely ex ac g aphene
mobili y, Hall measu emen s should be conduc ed [147].
4.2 T ansis o pe o mance assessmen
In his sec ion, he pe o mance o he p e iously ab ica ed de ices, as desc ibed in sec ion 3.3,
will be e alua ed. The analysis will begin wi h he ex ac ion o he igu es o me i . I is no ewo hy ha
imp o emen s in he ab ica ion p ocesses, as highligh ed ea lie , ha e esul ed in enhanced RF
pe o mance. Speci ically, he de ices de ailed in Sec ion 3.3.3, ab ica ed wi h a ga e oxide hickness o
5 nm, demons a ed supe io pe o mance compa ed o hose p esen ed in Sec ion 3.3.1 and 3.3.2.
Howe e , he de ices discussed in Sec ion 3.3.1 we e no ully cha ac e ized, as he de-embedding
s uc u es did no achie e he expec ed pe o mance.
4.2.1
T ansis o igu es o me i ex ac ion
As p e iously epo ed in his wo k, and as ag eed by he academic communi y, he key igu es o
me i (FOM) o GFETs a e he cu o equency (𝑓𝑇) and he maximum oscilla ion equency (𝑓𝑚𝑎𝑥).
These FOMs a e c ucial o e alua ing he pe o mance o ansis o s in p ac ical applica ions, whe he in
DC o RF ci cui y. In b ie , he cu o equency (𝑓𝑇) is he equency a which he magni ude o he
small-signal cu en gain is uni y (𝐻21=0 𝑑𝐵). Rega ding he S-pa ame e s, 𝐻21 can be exp essed as:
𝐻21=−2𝑆21
(1−𝑆11)(1+𝑆22)+𝑆12𝑆21
(44)
82
To p edic he 𝑓𝑇, as shown be o e, he gene ally used exp ession is:
𝑓𝑇=𝑔𝑚
2𝜋 [(𝐶𝑔𝑠+𝐶𝑔𝑑)(1+(𝑅𝑑+𝑅𝑠)𝑔𝑑𝑠)+𝐶𝑔𝑑𝑔𝑚(𝑅𝑑+𝑅𝑆)+𝐶𝑝𝑔]
(45)
The 𝑓𝑚𝑎𝑥 is desc ibed as he equency a he maximum gain (Mason’s Gain (U)), Maximum
A ailable Gain (MAG) and Maximum S able Gain (MSG), o simply U/MAG/MSG), becomes uni a y
(U/MAG/MSG = 0 dB). This gain should sa is y some condi ions o be calcula ed [148]. To ex ac he
𝑓𝑇 and 𝑓𝑚𝑎𝑥 om he S-pa ams, he i s hing o e alua e is he s abili y ac o , ex ac ed om he
exp ession:
𝑘=1+|𝑆11𝑆22−𝑆12𝑆21|2−|𝑆11|2−|𝑆22|2
2|𝑆12||𝑆21|≡S abili y Fac o
(46)
I all k’s a e less han one (k<1 o all equencies), he U/MAG/MSG co esponds o Mason’s
Gain (U), and i can be calcula ed using he ollowing exp ession:
𝑈= |𝑆21
𝑆12−1|2
2𝑘|𝑆21
𝑆12|− 2 𝑅𝑒𝑎𝑙 [𝑆21
𝑆12]≡Mason′s Gain
(47)
I all k’s a e no less han one, he k should be e alua ed o e e equency. I k<1, he
U/MAG/MSG co esponds o Maximum S able Gain (MSG) and can be calcula ed using he ollowing
exp ession:
𝑀𝑆𝐺= |𝑆21|
|𝑆12| ≡Maximum S able Gain
(48)
I k>1, he U/MAG/MSG co esponds o Maximum A ailable Gain (MAG) and can be calcula ed
using he ollowing exp ession:
𝑀𝐴𝐺=𝑀𝑆𝐺 ∙(𝑘 − √𝑘2− 1)≡Maximum A ailable Gain
(49)
83
The gene al equa ion o max, as shown be o e, is:
𝑓𝑚𝑎𝑥=𝑓𝑇
2√𝑔𝑑𝑠(𝑅𝑔+𝑅𝑠)+2𝜋𝑓𝑇𝑅𝑔𝐶𝑔𝑑
(50)
The ex ac ion o he igu e o me i (FOM) o g aphene ield-e ec ansis o s (GFETs) equi es
he expe imen al se up illus a ed in Figu e 71. This se up inco po a es wo bias ees o sepa a e di ec
cu en (DC) and adio equency (RF) signals, enabling he simul aneous applica ion o a DC bias ol age
and an RF signal o he de ice. A DC powe supply is employed o p o ide he necessa y ga e and d ain
bias ol ages, he eby biasing he GFET in o he desi ed ope a ional egion. A ec o ne wo k analyse
(VNA) is used o gene a e and measu e high- equency RF signals, allowing o he de e mina ion o he
de ice's sca e ing pa ame e s (S-pa ame e s), which a e c i ical o he calcula ion o he ansis o 's
FOM. Addi ionally, a con ol compu e in e ace is employed o egula e he ga e's DC ol age and o
acili a e he ex ac ion and analysis o he S-pa ame e s.
Figu e 71 – Measu emen se up used o ex ac he FOMs o he GFETs.
4.2.2
Bo om-ga e g aphene de ice ab ica ion p ocess wi hou su ace plana iza ion (wi h CPD)
To e alua e he pe o mance o he de ices ab ica ed using he CPD me hod ( ab ica ed in
sec ion 3.3.1), ex insic RF cha ac e iza ion was pe o med. In insic RF cha ac e iza ion could no be
pe o med due o limi a ions in he ab ica ion p ocess since he de-embedding s uc u es did no exhibi
he expec ed pe o mance. No only RF measu emen s, bu also he DC measu emen s, we e pe o med
wi h he VGS s IDS and he espec i ely 𝑔𝑚 is shown in Figu e 72. Figu e 73 illus a es he RF
measu emen s, he S-pa ame e s (a) and he ex ac ed FOMs (b). In his de ice, an ex insic cu -o
84
equency ( T) o 5.5 GHz and a maximum oscilla ion equency ( max) o 0.7 GHz, measu ed a a VDS o 3 V
and a VGS o 5 V we e demons a ed. These alues we e ob ained wi hou employing a global back ga e,
sugges ing a high le el o p-doping in he g aphene memb ane. This ab ica ion me hod achie ed a high
yield o ope a ional de ices, wi h app oxima ely 70% exhibi ing RF unc ionali y.
Figu e 72 – VGS s IDS and 𝑔𝑚 cha ac e is ic cu es o a de ice wi h W = 39 µm and L = 1.100 µm (0.95 µm o ga e channel
and 0.075 µm o d ain/ga e and sou ce/ga e o e lap) measu ed a a VDS o 100 mV.
Figu e 73 – a) S-pa ame e s o a de ice wi h W = 39 µm and L = 1.100 µm and a La (access leng h) o 85 nm and espec i ely
T and max b). c) RF measu emen scheme used o measu e he de ices.
4.2.3
Plana bu ied bo om-ga e opology
The pe o mance o he ansis o s ab ica ed h ough he bu ied p ocess (p esen ed in sec ion 3.3.2)
was analysed. In he i s p ocess (𝑡𝑜𝑥 = 10 nm o alumina) a yield a ound 100% was achie ed. To
assess he success o he plana bu ied bo om-ga e opology agains he p ocess wi hou su ace
91
epo ed model accu a ely cap u es he ansis o 's beha iou . This le el o accu acy sugges s ha he
model is su icien ly obus o p edic he pe o mance o o he ansis o s, p o ided ha ele an
pa ame e s a e app op ia ely adjus ed. Analysing his de ice unde a ious condi ions o e s c i ical
insigh s in o i s ope a ional h esholds and pe o mance me ics, which a e essen ial o alida ing
heo e ical p edic ions agains p ac ical ou come. The Figu e 79 illus a es he model's i o he de ice,
along wi h he ex ac ed pa ame e s (Table 4), which will be used in subsequen sec ions o pe o m he
ci cui simula ions.
Figu e 79 – Simula ed s measu ed IDS s VGS o he ansis o .
W (μm)
L (μm)
ox (nm)
μh (cm2V-1s-1)
μe (cm2V-1s-1)
33.82
1.102
5
899.56
1124.45
Cgs ( F)
Cgd ( F)
Ro (Ω)
RG (Ω)
Rex 0(Ω)
22.5e-5
45e-15
70=(RD+RS)/2
10
20
4.4.2
In e e design
Like p e iously epo ed, he uni y elemen o he ing oscilla o is he in e e . Wi h ha pu pose,
a g aphene in e e was designed and simula ed wi h he p e ious model by using a ini e elemen
analysis so wa e. To no e ha all he nex simula ions we e pe o med using his ini e elemen analysis
so wa e. An in e e is c ea ed by connec ing wo GFETs in se ies, as shown in Figu e 77 a). This se up
mi o s he con igu a ion o CMOS in e e s, whe e he uppe ansis o unc ions like a p- ype and he
lowe one like an n- ype. The ope a ing p inciple o his in e e is ha when one ansis o is ully
conduc ing, he o he should be u ned o , o in he case o GFETs, ope a e nea he Di ac poin . Due o
Table 4 - Ex ac ed pa ame e s om he model by using he p e ious ansis o .

92
a sligh a ia ion in VDS, he e is a co esponding small di e ence in he VDi ac o he GFETs, allowing o he
cons uc ion o an in e e wi h closely ma ched GFETs. This a angemen p oduces a ans e cu e wi h
a dis inc "W" shape, and an in e e can be e ec i ely implemen ed by ope a ing he de ice wi h Vin se
be ween he lowes poin s o his “W-shaped” cu e. To conduc he simula ions wi h mo e p ac ical
alues sui able o use in ci cui s, he Di ac poin was adjus ed o 0 V. I 's impo an o no e ha due o
he shi in he Di ac ol age (VDi ac) caused by he d ain-sou ce ol age (VDS), he in e e 's maximum gain
is no a 0 V bu a he shi ed o app oxima ely 1.5 V. The VDS o he in e e is se a 3.3 V. As obse ed,
o an inpu signal wi h a peak- o-peak ol age o 1.3 V, he ou pu peak- o-peak ol age is a ound 0.62 V,
which closely ma ches he alues ( ega ding he in e e ’s gain) obse ed in he measu ed de ice
desc ibed Figu e 80. I is wo h men ioning ha some disc epancies be ween he simula ion and he
expe imen al measu emen s a e expec ed. These di e ences a ise om a ia ions in he modeled de ices,
which, despi e being simila , exhibi di e ences in g aphene quali y, his a iabili y is due o he inhe en
challenges o achie ing uni o m g aphene cha ac e is ics ac oss a sample. Addi ionally, small a ia ions
in he ab ica ion p ocess, such as li hog aphy misalignmen s o sligh changes in channel leng h o wid h,
can lead o no iceable di e ences in de ice beha io , especially a smalle dimensions. The cha ac e is ics
o he simula ed in e e could be seen in he nex igu e being he maximum in e e ’s gain (A max) o 0.7.
Gi en ha gain, and acco ding o Ba khausen's c i e ion, i is expec ed ha he ing oscilla o ab ica ed
wi h ha de ices will no oscilla e.
Figu e 80 – Simula ion o an in e e wi h he ex ac ed pa ame e s. a) W shape cu e o he in e e ; b) Vou s Vin and Gain o
he in e e showing a maximum gain o 0.7 and c) esponse o he in e e o a 10 kHz squa e wa e.
93
4.5 Oscilla o imming
As discussed be o e, gi en he low gain o he p e ious de ices, maximum gain (A max) lowe han 1
(0.7), and acco ding o Ba khausen's c i e ia, i is expec ed ha he de ice will no oscilla e. In he
measu emen s conduc ed in he p e ious sec ion, i was no possible o measu e all he ab ica ed de ices,
due o he ime cons ain s. Due o a ia ions in W/L a ios, La, and he di e en cha ac e is ics o he
g aphene (s emming om he a iabili y in g aphene quali y ac oss he sample), i is possible ha some
oscilla o s may ha e oscilla ed. To explo e he pa ame e s ha should be op imized o de eloping
g aphene in e e s wi h A max g ea e han 1, se e al pa ame e s will be a ied, and hei in luence on he
A max o he in e e will be analysed.
4.5.1
Reducing Rex 0 o 10
To in es iga e he impac o g aphene's cha ac e is ics on he in e e and achie e a maximum
gain (A max) g ea e han 1, he eby sa is ying he Ba khausen c i e ion, a ious pa ame e adjus men s
we e pe o med. Since g aphene mobili y in luences some o he o he pa ame e s, Rex 0 was ini ially
educed o 10, an achie able as epo ed in [147]. Since Rex 0 ep esen s he ex insic esis ance a ze o
ol age, including con ac esis ance ( he esis ance be ween g aphene and me al elec odes) and access
esis ances, his pa ame e can be minimized by op imizing g aphene-me al con ac s o by employing he
echniques p e iously discussed o educe he access leng h (La). The W-shaped ans e cu e and he
Vin s. Vou cha ac e is ics, along wi h he co esponding maximum gain, a e shown in he Figu e 81.
Al hough he gain inc eased, eaching a alue o 0.8, i emains insu icien o sa is y he Ba khausen
c i e ion, making his in e e unsui able o use in a ing oscilla o .
Figu e 81 – Simula ion o he in e e by educing he Rex 0 o 10. a) W shape cu e o he in e e ; b) Vou s Vin and Gain o he
in e e showing a maximum gain o 0.8.
94
4.5.2
Reducing Ro o 40
Since Ro ep esen s he in insic channel esis ance a he Di ac poin , de e mined by he inhe en
p ope ies o he g aphene ma e ial, i s alue was educed o 40, a alue ha is eadily achie able as
epo ed in he li e a u e [147]. To achie e his educ ion, highe quali y g aphene (wi h inc eased ca ie
mobili y) should be used, o a sui able subs a e, such as h-BN, should be employed o minimize he
in e ac ions be ween g aphene and he subs a e. Addi ionally, his pa ame e can be u he dec eased
by using a high-k ga e dielec ic wi h low hickness, which enhances ca ie con ol o e he channel,
he eby educing Ro. Wi h his educ ion, i was possible o achie e a g aphene in e e wi h a maximum
gain o 2.6, which is su icien o sa is y he Ba khausen c i e ion. The ob ained esul s a e p esen ed
Figu e 82.
Figu e 82 – Simula ion o he in e e by educing he Ro o 40. a) W shape cu e o he in e e ; b) Vou s Vin and Gain o he
in e e showing a maximum gain o 2.7.
To u he op imize his in e e , he Di ac poin o he op GFET (which unc ions as he p- ype
FET) was shi ed o VDi ac + 0.3 V.
4.5.3
Reducing Rex o 10, Ro o 40 and shi VDi ac o VDi ac+0.3 V
The VDi ac o he ansis o s was ini ially sligh ly unma ched, and as discussed ea lie , he in e e
ope a es due o he sligh di e ence in VDS and VDi ac be ween he ansis o s. To u he inc ease he
misma ch, he VDi ac o he op ansis o (p-mos) in he p e ious in e e was shi ed o VDi ac +0.3 V. A e
his adjus men , as obse ed in he W-shaped cu e, he di e ence be ween IDS max and IDS min inc eased,
leading o an imp o emen in he in e e 's gain om 2.6 o nea ly 2.7. This kind o shi can be achie ed
using a dual-ga e s uc u e, whe e one ga e speci ically adjus s he Di ac poin , o by chemical doping.
The esul ing cha ac e is ics o he in e e a e shown in Figu e 83.
95
Figu e 83 – Simula ion o he in e e by educing shi ing he VDi ac o VDi ac+0.3. a) W shape cu e o he in e e ; b) Vou s Vin
and Gain o he in e e showing a maximum gain o 2.8.
4.5.4
Oscilla o equency unning
To alida e he p e iously op imized in e e , a squa e wa e wi h a maximum ol age (Vmax) o 2.35
V wi h a minimum ol age (Vmin) o 1.15 V, and a equency o 10 kHz was applied o he in e e 's inpu .
This inpu esul ed in an ou pu squa e wa e wi h a Vmax o 2.5 V and a Vmin o 0.880 V, he eby con i ming
he calcula ed gain o app oxima ely 2.7. The esul s a e illus a ed in Figu e 84 (a), and he upda ed
pa ame e s o he op imized in e e a e lis ed in Table 5. The sui abili y o his op imized in e e o
use in a ing oscilla o was also assessed. Acco ding o he Ba khausen c i e ion, essen ial o es ablishing
oscilla ions in a eedback loop, he o al loop gain mus be equal o o g ea e han one, and he o al
phase shi a ound he loop mus be 360 deg ees. Wi h a gain o 2.7, he in e e mee s hese
equi emen s, ensu ing ha any ini ial noise o pe u ba ion is su icien ly ampli ied o ini ia e and sus ain
oscilla ions, while he app op ia e phase shi ensu es cons uc i e ein o cemen o he signal as i
a e ses he loop. The simula ion esul s alida e his, demons a ing ha he oscilla o ope a es a
1.08 GHz wi h a ol age swing o 2.318 V (Vmax = 3.263 V and Vmin = 0.945 V) as seen Figu e 84.
W (μm)
L (μm)
ox (nm)
μh (cm2 V-1 s-1)
μe (cm2 V-1 s-1)
33.82
1.102
5 nm
899.56
1124.45
Cgs (F)
Cgd (F)
Ro (Ω)
RG (Ω)
Rex 0 (Ω)
22.5e-5
45e-15
40 = (RD+RS)/2
20
10
Table 5- Pa ame e s used o he in e e and in he ing oscilla o simula ion.
96
Figu e 84 – Ring oscilla o assessmen . Simula ion o he op imized in e e (a) and ing oscilla o simula ion (b).
4.6 Discussion
Modelling he ab ica ed de ices plays a i al ole in enhancing hei pe o mance and p edic ing
he beha iou o oscilla o s, pa icula ly when di ec measu emen s a e no easible. The de ices
p oduced h ough a plana ab ica ion p ocess demons a ed in insic cu -o equencies o up o 80 GHz.
This equency, no ably high compa ed o o he alues epo ed in he li e a u e, highligh s a s ong
co ela ion be ween he achie ed 𝑓𝑇 and he simplici y o he ab ica ion p ocess, as shown in Table 1.
The p ac ical ealiza ion o a ing oscilla o using he op imized in e e con i ms he design's
iabili y and i s po en ial applica ions in iming ci cui s and signal gene a ion wi hin digi al sys ems. While
simula ions sugges he po en ial o oscilla ion, u he enhancemen s a e equi ed o achie e eliable
oscilla ion in he ab ica ed de ices. Po en ial imp o emen s include op imizing g aphene-me al con ac s,
educing he access leng h o dec ease Rex o, using highe -quali y g aphene wi h inc eased ca ie mobili y,
and enhancing subs a e quali y, such as by employing bo on ni ide. Addi ionally, he use o a high-k
ga e dielec ic wi h educed hickness could be ad an ageous. To add ess he misma ch in Di ac poin s
among ansis o s, a ious echniques can be conside ed. These include chemical doping [152], elec ical
ga ing con igu a ions [153], o induced s ain in g aphene [154]. Fu he mo e, e oelec ic pola iza ion
ields may also be used o shi he Di ac poin [155].
I is impo an o no e ha , gi en he ab ica ion o a ious g aphene ansis o s and oscilla o
opologies and conside ing he a iabili y in g aphene quali y ac oss he sample, some o he ab ica ed
ing oscilla o s a e likely o unc ion. Howe e , as p e iously men ioned in his hesis, ime domain
cons ains p e en ed he alida ion o hese oscilla o s.

97
5 CONCLUSIONS
G aphene has become a b eak h ough ma e ial wi h high in e es s because o i s unique
p ope ies and po en ials o dis up i e applica ions in a ious ields. G aphene's applica ion in elec onics,
especially o adio equency (RF) ci cui s, is qui e p omising wi hin i s a ious use cases. Ne e heless,
ans e ing his possibili y in o eal-wo ld g aphene-based RF de ices is a signi ican challenge equi ing
addi ional e o s and echnological de elopmen . This hesis aimed o ad ance he unde s anding o
g aphene's capabili ies, add ess he challenges associa ed wi h i s in eg a ion in o RF de ices, and
con ibu e o he de elopmen o s a e-o - he-a g aphene echnology.
This wo k is conce ned wi h design, ab ica ion, and cha ac e iza ion o g aphene-based RF
ansis o s and oscilla o s pa ing he way o i s in eg a ion in biosenso s. These we e de eloped using
s anda d ab ica ion p ocesses o be epea able and enable seamless echnology ans e o indus ial
applica ions. A comp ehensi e e iew o he li e a u e iden i ied key obs acles we e pe o med.
The ab ica ion p ocess was a c i ical aspec o his wo k, pa icula ly in sha ed acili ies, which
equi ed adap a ions o p e en equipmen con amina ion and accommoda e acili y limi a ions. Two
ab ica ion me hodologies we e de eloped: a bo om-ga e p ocess wi hou su ace plana iza ion and a
plana ized bu ied bo om-ga e p ocess using ion milling p io o g aphene ans e . The use o c i ical
poin d ying p o ed e ec i e in p e en ing g aphene om b eaking a e pa e ning. Despi e cons ain s,
hese app oaches p o ed e ec i e, enabling he success ul ab ica ion o g aphene-based de ices. The
g aphene's g ow h and ans e p ocesses we e also s udied. The PMMA-assis ed ans e me hod, while
in oducing esiduals, showed negligible impac on he pe o mance o g aphene FETs due o he de ice
dimensions. Challenges such as he p oduc ion o la ge g aphene lakes, which a e c i ical o high-yield
RF pe o mance, we e no ed, being he in luence o H2 and CH4 in he g aphene mo phology. The
complexi y o ine- uning g ow h pa ame e s and con amina ion om ca bon nano ubes hinde ed he
de elopmen o an op imized syn hesis p o ocol. Consequen ly, con inuous g aphene ilms we e
employed o de ice ab ica ion o p io i ize scalabili y and consis en pe o mance.
The modelling and cha ac e iza ion o he ab ica ed de ices highligh ed he p og ess achie ed.
In insic cu o equencies up o 80 GHz and a maximum oscilla ion equency o 14 GHz we e ob ained
using he plana bo om-ga e opology wi h a 5 nm oxide laye . While a unc ional ing oscilla o was no
ob ained wi h hese ansis o s, simula ions demons a ed ha imming in e e pa ame e s enable he
design o a RF g aphene ing oscilla o s wi h his ab ica ion p ocess. This unde sco es he po en ial o
he p oposed app oach, con ingen on u he op imiza ion. Despi e he a iabili y in g aphene quali y
ac oss samples, he hesis es ablished a epea able ab ica ion p ocess applicable o o he g aphene-
98
based de ices beyond he scope o his esea ch. P oposed imp o emen s include op imizing me al-
g aphene con ac s, educing access leng h o lowe ex e nal esis ance, and employing highe -quali y
g aphene wi h enhanced ca ie mobili y. The use o imp o ed dielec ics wi h high-k could also
signi ican ly ele a e de ice pe o mance.
In conclusion, his hesis has made signi ican con ibu ions o le e age g aphene echnology use
by add essing c i ical challenges in he design, ab ica ion, and cha ac e iza ion o g aphene-based RF
de ices. Al hough he ab ica ion o a g aphene oscilla o demons a ing RF pe o mance was no
achie ed, he indings o his esea ch lay he g oundwo k o u u e de elopmen s. In pa icula , he
po en ial applica ion o his echnology in biosenso s is highly p omising, hanks o he de eloped
s anda d-compa ible ab ica ion p ocess and he designed layou s. Since g aphene emains exposed
du ing ab ica ion, his opens oppo uni ies o de elop unc ionaliza ion me hods speci ically ailo ed o
biosensing applica ions. These esul s p o ide a ounda ion o in eg a ing g aphene's unique p ope ies
in o nex -gene a ion biomedical and RF echnologies, pa ing he way o u he explo a ion and inno a ion
in hese ields.
5.1 Fu u e wo k
Fu u e wo k should build on he p og ess achie ed in his hesis, which p o ided aluable insigh s
in o g aphene's RF p ope ies and de eloped a p ocess compa ible wi h he s anda d ab ica ion
p ocesses. To alida e he p oposed ing oscilla o design, ab ica ion using he op imized pa ame e s
and imp o emen s ou lined in his esea ch is essen ial. This p ac ical ealiza ion would con i m he
accu acy o he simula ions and enable u he pe o mance e inemen . Conside ing he in ended
applica ion o hese ing oscilla o s in biosenso s, hei alida ion should include unc ionaliza ion
s a egies. This could in ol e unc ionalizing g aphene in e connec s o using he capaci i e elemen s o
he p oposed g aphene-based s uc u es. Such unc ionaliza ion would allow o changes in he oscilla ion
equency in esponse o he binding o speci ic analy es, pa ing he way o highly sensi i e and selec i e
biosenso de elopmen . Addi ionally, he p ope ies o g aphene mus be u he enhanced o imp o e
de ice pe o mance. E o s should ocus on inc easing ca ie mobili y, minimizing de ec densi y, and
e ining syn hesis me hods o p oduce high-quali y, uni o m g aphene. A back ga e should be applied o
shi he Di ac poin o he ansis o s, enabling he use o he de ices a easonable ol ages. These
ad ancemen s would no only op imize he pe o mance o RF de ices bu also b oaden g aphene's
po en ial applica ions in cu ing-edge elec onic and sensing echnologies.
99
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