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Vacuum annealing e ec on physical p ope ies and
elec ical ci cui model o ZnO:Sn/SnO2:F bilaye
s uc u e
Sama Dabbabi, Mehdi Souli, Ta ek Ben Nas, An onio Ga cia-Lou ei o and
Najoua Kamoun
Ve sion: accep ed a icle
Sama Dabbabi, Mehdi Souli, Ta ek Ben Nas, An onio Ga cia-Lou ei o and Najoua Kamoun (2019)
Vacuum annealing e ec on physical p ope ies and elec ical ci cui model o ZnO:Sn/SnO2:F
bilaye s uc u e. Vacuum, 167, 416 - 420.
Doi: h ps://doi.o g/10.1016/j. acuum.2019.06.008
How o ci e:
Copy igh in o ma ion:
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Vacuum annealing e ec on physical p ope ies and elec ical ci cui model o
ZnO:Sn/SnO2:F bilaye s uc u e
Sama Dabbabi1, Mehdi Souli1, Ta ek Ben Nas 1, An onio Ga cia-Lou ei o2, Najoua Kamoun1
1. Uni e si é Tunis El Mana , Facul é des Sciences de Tunis, Dépa emen de Physique, LR99ES13 Labo a oi e de
Physique de la Ma iè e Condensée (LPMC), 2092 Tunis, Tunisia
2. Cen o Singula de In es igación en Tecnoloxías de In o mación (CiTIUS), Uni e sidad de San iago de
Compos ela, España.
Co esponding au ho : [email p o ec ed]
Abs ac :
Tin doped Zinc oxide/Fluo ine doped in dioxide bilaye ilms (ZnO:Sn/SnO2:F)
we e deposi ed on glass subs a es using sp ay py olysis echnique. The e ec o acuum
annealing a di e en empe a u e was in es iga ed. Bo h s uc u al and mo phological
analysis ha e shown ha he e is a signi ican modi ica ion in he bilaye ilm s uc u e
and su ace ollowing he acuum annealing p ocess a 450°C. Elec ical p ope ies ha e
been in es iga ed using he Hall E ec measu emen s as well as he impedance
spec oscopy a oom empe a u e. The ci cui pa ame e s we e de e mined using an
equi alen ci cui model i ed om he impedance spec a and sugges ing he p esence
o g ain and g ain bounda y conduc ions in he bilaye s uc u e. I was ound ha he
ilm annealed in acuum o 1h a 350°C is op imal in all espec s, as i possesses all he
desi able cha ac e is ics including he lowes esis i i y, high po osi y and be e g ain
bounda y conduc i i y.
Key wo ds: Sp ay py olysis, ZnO:Sn/SnO2:F ilm, Elec ical p ope ies, impedance
spec oscopy.
Fab ica ing undoped o doped oxides in hin ilm o m has long been a subjec o
many in es iga ions o applica ion in nanoelec onics [1], sola cells [2], acous ic de ices
2
[3-4] and gas senso s [5-8]. Two me al oxides, namely zinc oxide (ZnO) and in dioxide
(SnO2) a e excep ional choices o ab ica ion o elec onic de ices [4, 9-10] wi h high
pe o mances, low cos and mass p oduc ion [11-12]. The e iew o li e a u e e ealed
ha a double laye ed hin ilm composed o ZnO and SnO2 doped wi h me als elemen s
[11] and ollowed wi h annealing p ocess [12] could gi e bi h o he ideal elec onic
de ice whe e he c ys allini y, he elec ical conduc i i y and he mo phological
p ope ies o ZnO/SnO2 bilaye s a e imp o ed compa ed o co esponding single laye s
and all o he double laye ilms [1, 6]. Mo eo e , he ZnO/SnO2 bilaye s uc u e has
been in es iga ed o applica ions in he sola cells, pho oelec ochemical cells (PECs),
hin ilm ansis o s, gas senso s and nanogene a o s [13-15]. P. Ra ikuma e al. [15]
ab ica ed SnO2:Al/ZnO:F bi-laye wi h high ansmi ance, low esis i i y, and
homogenei y su ace by employing sp ay py olysis echnique and annealing p ocess. The
same esul was ob ained o ZnO:F/SnO2:Al bi-laye [16]. In his wo k, he
ZnO:Sn/SnO2:F bi-laye ilms we e ab ica ed by low cos sp ay py olysis and he e ec
o acuum annealing empe a u e on s uc u al, mo phological and elec ical we e
in es iga ed. To he bes o ou knowledge, his is he i s epo ollowing annealing
p ocess in acuum on he ZnO:Sn/SnO2:F double laye ed ilm. All bi-laye ed ilms we e
analyzed in e ms o X- ay di ac ion (XRD), Scanning elec on mic oscopy s udy
(SEM), and Hall E ec measu emen s. In o de o in es iga e he de ice, he e ec s o
acuum annealing empe a u e on he complex impedance beha io we e analyzed and
an elec ical model was p oposed.
Sp ay py olysis echnique was used o ob ain he ZnO:Sn/SnO2:F bilaye and i s
co esponding laye s deposi ed on glass subs a es. F-doped SnO2 solu ion was p epa ed
using 0.1 M o s annous chlo ide (SnCl2:2H2O) dilu ed in me hanol (CH3OH) and
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deionized wa e (1:1). The luo ine doping was achie ed using ammonium luo ine
(NH4F). On he o he hand, Sn-doped ZnO solu ion was p epa ed using 0.1 M o zinc
ace a e dehyd a e (Zn(CH3COO)2.2H2O) dilu ed in me hanol: deionized wa e (1:1)
mix u e. S annous chlo ide (SnCl2:2H2O) was in oduced as Sn dopan sou ce. The
addi ion o 6 a .% o F and 0.6 a .% o Sn ha e been op imized in SnO2 and ZnO,
espec i ely, achie ing op imal ma e ial pe o mance in e ms o esis i i y. Sui able
olumes o ZnO:Sn p ecu so solu ion we e sp ayed o e he SnO2:F laye s in o de o
ge ZnO:Sn/SnO2:F bilaye ilms. The samples we e g own a he no malized deposi ion
empe a u e o 450 °C. Du ing he deposi ion un, he a e sp ay and he dis ance be ween
he subs a e and he nozzle we e main ained cons an a 20 ml min-1 and 25 cm,
espec i ely. A e he deposi ion, he samples ha e been annealed in acuum be e han
10-5 mba a di e en empe a u es. The s uc u al cha ac e iza ion was pe o med by X-
ay di ac ion (XRD), using he coppe adia ion Kα (λ = 1.5418 A°) in he 2θ ange 20–
70° wi h a s ep size o 0.013° and a scan ime pe s ep o 2 s. The su ace mo phologies
o he hin ilms we e s udied using Scanning Elec on Mic oscopy (SEM, EVOLSIS
Zeiss). SEM c oss sec ion was pe o med o measu e he ilm hickness. Elec ical
pa ame e s o he ilms we e measu ed by ou p obe, in a Van de Pauw con igu a ion.
The impedance measu emen s we e pe o med using wo-elec odes pain ed on he wo
ex emi ies o he ilm using sil e pas e. The impedance da a was de e mined using
Agilen E4980A impedance analyze a oom empe a u e in he equency ange [20 Hz
- 2 MHz].
The X- ay pa e ns and he s uc u e o he ab ica ed ZnO:Sn/SnO2:F bi-laye
ilms a e shown in Fig.1a and he inle , espec i ely. Fig.1b shows XRD pa e ns o
bilaye ilm annealed a di e en empe a u es unde acuum o 1h. Fig.1a sugges s ha
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ZnO:Sn/SnO2:F ilm is polyc ys alline. All di ac ion peaks can be pe ec ly indexed as
he hexagonal wu zi e phase o ZnO:Sn (JCPDS 89-1397) as well as he e agonal u ile
s uc u e o SnO2:F phase (JCPDS 41-1445). Majo di ac ion peaks a angles 2θ a ound
26.32°, 33.62°, 37.72°, 51.44°, 64.57° and 65.62° belong o he planes {110},
{101},{200}, {211}, {310} and {301} o SnO2:F. The small e lec ion peaks a angles
a ound 31.58°, 34.23°, 36.03°, 47.33°, 56.47° and 62.62° co espond o {100}, {002},
{101}, {102},{110} and {103} planes o ZnO:Sn, espec i ely. The same esul was
ob ained by Hossein e al. [17]. As shown in Fig.1b, he c ys al s uc u e o
ZnO:Sn/SnO2:F ilm annealed a 250 °C was iden i ied o ha o as-g own ilm, whe eas
wi h he inc ease in he annealing empe a u e a 350°C, he posi ion o all di ac ions
peaks we e obse ed o dec ease d as ically. The shi in he di ac ion peaks posi ion is
due o he s ess changing in he ilm [18]. By inc easing annealing empe a u e a 450°C,
we obse ed he o ma ion o amo phous phase in he bi-laye ilm. Thus, he acuum
annealing p ocess a 450°C inhibi s he c ys alli e g ow h o ZnO:Sn on SnO2:F. C oss-
sec ional SEM image o he p epa ed ZnO:Sn/SnO2:F bi-laye ilm is shown in Fig. 2. As
indica ed in his image, he hickness alue is 918.2 nm o all he deposi ed bi-laye ilms.
The in luence o acuum annealing o a pe iod o 1h on he su ace mo phology is shown
in Fig.3. The as-deposi ed ilm (Fig.3a) exhibi ed a iny chilly like s uc u es wi h
homogeneous su ace. I is clea ha he su ace o he ilm annealed a 350°C (Fig.3c) is
much smoo he and mo e uni o m han as-deposi ed hin ilm. Howe e , hese iny chilly
like s uc u es become bigge and hey a e p o ube an a 450°C empe a u e (Fig.3d).
When he ilms a e annealed in acuum a high empe a u e, g ains ge agglome a ed o
o m small clus e s hen i esul s in an inc ease in he g ain size [19, 20] which could
explain he case o he ilm annealed a 450°C unde acuum a mosphe e. These esul s
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co obo a ed well wi h ou XRD indings, by which an amo phous s uc u e was obse ed
a 450°C annealing empe a u e.
The elec ical pa ame e s like he esis i i y (ρ), ca ie concen a ion (N), mobili y (μ)
and elec ical conduc i i y (σ) we e ob ained using Hall E ec measu emen s. Fig.4
p esen s he a ia ion o elec ical pa ame e s o he p epa ed ilms as unc ion o
annealing empe a u es used in acuum o 1h. Fi s ly, he esis i i y dec eased wi h
inc easing annealing empe a u e and hen i inc eased a high empe a u e o 450°C. A
minimum esis i i y o 0.05×10-2 Ω.cm is ob ained o ZnO:Sn/SnO2:F bilaye annealed
a 350 °C. The conduc i i y exhibi ed he opposi e beha io as e ealed in Fig.4. I is seen
ha he ca ie concen a ion exhibi ed 4×1024 cm-3 alue sa u a ed a 350 °C and 450 °C
empe a u es, which is lowe han ha a 250 °C (7×1024cm-3). While, he highes Hall
mobili y (μ0.7×10-8 cm2V-1s-1) is ob ained o he ilm annealed a 350 °C. The ilm
annealed a 450 °C exhibi s a low mobili y as compa ed o ha a 350 °C, possibly due o
hei being mo e diso de ed in e ms o c ys allini y [20]. I is ob ious om hese esul s
ha he acuum annealing a 350 °C empe a u e p omo e highe mobili y and less
esis i i y o he ZnO:Sn/SnO2:F bilaye .
Elec ochemical impedance spec oscopy (EIS) is a powe ul ool o s udying he de ice
in e ace in o ma ion [21]. Fig.5 (a-d) show he impedance spec a o he Nyquis diag am
o ZnO:Sn/SnO2:F based de ices wi h and wi hou acuum annealing empe a u e. I is
obse ed ha he impedance spec a o all hese ilms a e cha ac e ized by a single
semici cula a c. The cen e o each obse ed semi-ci cle is shi ed down o he eal axis
con i ming ha he impedance cu es o his bi-laye ilm obey he Cole-Cole o malism
[22]. The p esence o only one semici cle in he Cole-Cole plo ep esen s he Debye
elaxa ion p ocess [23]. In o de o iden i y he con ibu ion o each compound o he
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ma e ial (g ain, g ain bounda y, and elec odes), i is use ul o ha e an equi alen ci cui
model [24]. In his ega ds, he impedance cu es we e i ed by using Z iew so wa e
[24], o an equi alen ci cui consis ing o a se ies esis ance (R1) and a capaci ance
(CPE1) in pa allel wi h a esis ance (R2) as indica ed in he inse s o Fig.5 (a-d). The
g ains con ibu e a high equencies, he g ain bounda ies a in e media e equencies
and elec odes a low equencies [25]. The elec ical pa ame e s R1, CPE1, and R2
ep esen he con ibu ion o he elec odes, g ain and g ain bounda y, espec i ely. The
alues o hese pa ame e s a e illus a ed in Fig.5 (a-d). I is obse ed ha as annealing
empe a u e inc eases in he ange o [250-350°C], he g ain esis ances dec ease. The
lowes R2 alue is 0.078 ×103 Ω which was ob ained o he ilm annealed a 350°C.
Howe e , he inc ease annealing empe a u e a 450°C induce he educ ion o R2 alue
o he g ain esis ance. This enhancemen in conduc i i y a 350°C empe a u e is also
con i med om he Hall e ec analyzes. To he bes o ou knowledge, i was no ound
in he bibliog aphy a simila s udy o compa e ou ob ained alues which ma k he
o iginali y o ou wo k.
In his s udy, we ha e s udied he e ec o annealing unde acuum a mosphe e
a 250, 350 and 450 °C on s uc u al, mo phological and elec ical beha io o
ZnO:Sn/SnO2:F bilaye hin ilms. The X- ay di ac ion analysis e ealed ha he
acuum annealing p ocess a 250°C and 350°C esul ing in polyc ys alline ma e ials,
while he ilm annealed a 450°C leads o an amo phous phase. This signi ican
modi ica ion was s ongly suppo ed by he su ace mo phological esul s. Elec ical
p ope ies o his bilaye we e pe o med using Hall E ec measu emen s as well as
complex impedance spec oscopy. As a esul , a minimum esis i i y o 0.05×10-2 Ω.cm
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and an enhanced g ain bounda y conduc i i y we e achie ed o he ilm annealed a
350°C unde acuum gas condi ion.
Acknowledgemen s
The Au ho s would like o hank he use o RIAIDT-USC analy ical acili ies a he
Uni e si y o San iago de Compos ela. Au ho s a e also g a e ul o (F ancisco Ri adulla
and Vic o Lebo án, CIQUS-USC) o assis ing us wi h he Hall E ec measu emen s.
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Fig.5. Complex impedance spec a o ZnO:Sn/SnO
2
:F bilaye (a) as g own and
annealed o 1h in acuum a (b) 250°C, (c) 350°C and (d) 450°C.
