S uc u al and op ical cha ac e iza ion o
nanos uc u ed ZnO g own on alumina empla es
M Yus e
1
, R Escoba Galindo
1
, O Ma ínez Sac is án
3
, I Mínguez-Bacho
1
,
Sonia Rod iguez
2
, M He nández-Vélez
2
and O Sánchez
1,4
1
Ins i u o de Ciencia de Ma e iales de Mad id (CSIC), Spain
2
Uni e sidad Au ónoma de Mad id, Spain
3
UGdS-Op onlab G oup, Dp o. Física Ma e ia Condensada, Uni e sidad de Valladolid, Paseo
de Belén 11, 47011—Valladolid, Spain
E-mail: [email p o ec ed]
Recei ed 29 July 2014, e ised 3 Oc obe 2014
Accep ed o publica ion 24 Oc obe 2014
Published 12 No embe 2014
Ma e ials Resea ch Exp ess 1(2014) 045028
doi:10.1088/2053-1591/1/4/045028
Abs ac
The g ow h o nanos uc u ed ZnO hin films on nanopo ous anodic alumina
subs a es (NAAF) by dc eac i e magne on spu e ing using a pu e Zn me al
a ge is epo ed. ZnO nanos uc u es ep oduce he po e a ays o he NAAF
subs a es used as empla es mimicking hei hexagonal long ange o de .
S oichiome ic nanos uc u ed ZnO samples we e g own wi h wu zi e ype
s uc u e, highly ex u ed and o ien ed in he (002) di ec ion. The s udy o he
NAAF po e size e ec in he final mo phology and op ical p ope ies o he
nanos uc u ed ZnO is p esen ed o di e en ZnO hicknesses. The po e size o
he nanos uc u ed ZnO films was con olled in he ange o 15–65nm by
choosing app op ia e NAAF and he spu e ing deposi ion condi ions. The b oad
emission band obse ed in PL spec a o he samples should be associa ed wi h
colo cen e s ansi ions (F and H cen e s) appea ing in he alumina empla es.
Keywo ds: nanos uc u ed ZnO, nanopo ous anodic alumina, magne on
spu e ing
1. In oduc ion
Zinc oxide (ZnO), wi h band gap o 3.37 eV a oom empe a u e (RT) and la ge exci onic
binding ene gy, a ound 60 meV, has a ac ed inc easingly wo ldwide a en ion in he pas ew
decades due o i s unique p ope ies and a la ge numbe o po en ial applica ions such as
anspa en conduc i e con ac s [1], sola cells [2], lase diodes [3], ul a iole lase s [4], hin
4
Au ho o whom any co espondence should be add essed.
Ma e ials Resea ch Exp ess 1(2014) 045028
2053-1591/14/045028+16$33.00 © 2014 IOP Publishing L d
film ansis o s [5], hin-film gas senso s [6] and op oelec onic and piezoelec ic applica ions o
su ace acous ic wa e de ices [7–9]. ZnO films g own on Si subs a es ha e been widely
epo ed, howe e , ZnO nanos uc u ed hin films show e y in e es ing addi ional p ope ies in
compa ison o con inuous hin films [10,11]. Those p ope ies a e likely o be al e ed and
con olled by he ZnO g ow h p ocesses [12] such ha many esea ch g oups ha e epo ed he
syn hesis o ZnO nanos uc u es by means o bo h bo om-up and op-down echniques, among
hem, chemical apou deposi ion (CVD) [13], molecula beam epi axy (MBE) [14],
elec ochemical deposi ion [15], e apo a ion and RF spu e ing [16].
Magne on spu e ing echniques ha e been success ully used in he deposi ion o
unc ional ma e ials such as, TiN [17] and TiO
2
[18] on nanos uc u ed empla es achie ing a
con o mal ep oduc ion o he main ea u es o he subs a e su aces. This echnique gi es
di ec ly he equi ed nanos uc u ed ma e ial wi hou addi ional ea men a e he deposi ion
p ocess. In addi ion, his echnique allows de eloping deposi ion p ocesses wi h high s abili y
and pu i y con ol o he final p oduc s. Mo eo e , he nanos uc u es a e ab ica ed using a
acuum-based echnique o e ing a highe le el o p ocess s abili y and pu i y con ol han in
he case o solu ion based me hods.
In his wo k, we epo he g ow h o nanos uc u ed ZnO hin films on nanopo ous anodic
alumina films (NAAFs) used as empla es, by dc magne on spu e ing. The su ace
mo phology and s uc u al p ope ies o he ZnO nanos uc u es we e cha ac e ized by means
o field-emission scanning elec on mic oscopy (FESEM), a omic o ce mic oscopy (AFM) and
x- ay di ac ion (XRD). The chemical composi ion was de e mined by RBS (Ru he o d
backsca e ing spec ome y). The op ical p ope ies we e de e mined by UV- isible spec o-
pho ome y and pho oluminescence (PL) measu emen s.
The use o dc magne on spu e ing echnique allows us o p oduce ma e ial deposi s wi h
g ain sizes smalle han 30–40 nm which is an ad an age wi h espec o o he physical
deposi ion echniques. In he ea ly s ages o he deposi ion, he complex s uc u es o he elie
pa e n a e pe ec ly co e ed by small pa icles o ma e ial o be deposi ed, g owing a laye o
ma e ial on ‘con o mal con ac ’wi h he pa e n whose su ace is ep oduced wi h high
e ficiency [17]. The combina ion o dc magne on spu e ing echnique and anodiza ion
echniques o p oduce ZnO nanos uc u es p o ides an economical s a egy o ob ain ZnO
nanoma e ials.
2. Expe imen al de ails
ZnO films we e g own by using a home-made high acuum dc magne on spu e ing sys em on
NAAF empla es. The spu e ing ca hode consis s o a s anda d ci cula plana magne on wi h
a high pu i y comme cial Zinc a ge (99.99%), 3-in. diame e and 0.125-in. hickness. All
expe imen s we e pe o med in oducing a mix u e o A (99.999%) and O
2
(99.992%) in he
acuum chambe a a cons an o al gas flow (A + O
2
) o 30 sccm. The chambe was pumped
ou o a base p essu e o 2 · 10
−6
mba and he wo king p essu e was app oxima ely o
3·10
−3
mba . Du ing he deposi ion p ocess he ela i e O
2
gas concen a ion in he eac i e
a mosphe e (p
O
2
) was main ained a 50%, he a ge powe a 50 W and he a ge -subs a e
dis ance a 20 cm.
NAAF empla es we e p epa ed by ollowing he wo-s ep anodiza ion p ocess desc ibed
elsewhe e [19]. Sul u ic and oxalic acid solu ions we e used as elec oly es in he anodiza ion
2
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al
p ocesses. Table 1summa izes he po e sizes, Φ, and in e -po e dis ances, D
in
. o he NAAF
used as subs a es. In o de o pe o m he op ical cha ac e iza ion o he samples (in
ansmi ance mode) he aluminum back subs a es we e emo ed by an e ching p ocess using
an acid solu ion (CuCl
2
/HCl). These s uc u es exhibi high op ical anspa ency in he UV-
isible anges and high chemical and he mal s abili y which makes hem e y sui able o use
as subs a es-pa e ns, as will be shown la e .
RBS expe imen s we e ca ied ou o elucida e he chemical composi ion o he deposi ed
ZnO films. RBS we e pe o med using he 5MeV HVEE Tande on accele a o loca ed a he
‘Cen o de Mic o-Análisis de Ma e iales o Uni e sidad Au ónoma de Mad id’. The samples
we e i adia ed wi h 3.035 MeV He
+
ions o make use o he c oss sec ion esonance
16
O(α,α)
a ha pa icula ene gy and, he e o e, o imp o e he sensi i i y o oxygen nucleus. The
chemical composi ion o he ZnO films has been ex ac ed using he RBX so wa e [20].
C ys alline s uc u e o he films was de e mined by XRD using a Cu anode (Cu
K
α
= 1.54 Å) egis e ed unde g azing incidence condi ions wi h an incidence angle o 0.7° a
oom empe a u e. The XRD measu emen s we e pe o med on a Siemens D-5000
di ac ome e . The su ace and ac u e c oss-sec ion mo phologies o he hin films we e
examined by a No a NanoSEM 230 FEG scanning elec on mic oscope (HRSEM). An AFM
sys em om Nano ec Elec ónica S. L. wo king in non-con ac mode a ambien condi ions was
also used o cha ac e ize he su ace mo phology o he samples.
Op ical cha ac e iza ion o NAAFs is no abundan in he cu en li e a u e and hei
influence on he op ical p ope ies o unc ional hin films deposi ed on hem has been sca cely
epo ed. Ce ainly i is no an easy ask. The e o e, as a fi s app oach, we made a s udy o he
op ical beha iou o he NAAF empla es used in his wo k o comple eness a e he op ical
cha ac e iza ion o he ZnO/NAAF samples. T ansmi ance measu emen s we e acqui ed in a
SolidSpec-3700 nea UV- isible and nea in a ed ange (190–1500 nm) spec opho ome e . PL
spec a we e ca ied ou by means o a LabRAM sys em om Ho iba Jobin-Y on, model
HR800 UV, wi h an He–Cd lase (325 nm) as he exci a ion ligh sou ce.
3. Resul s and discussion
3.1. ZnO nanos uc u es
P io o he g ow h o ZnO on NAAF empla es we pe o med a s udy o op imize he
expe imen al condi ions o g owing his compound o ming well defined nanos uc u es on he
selec ed empla es displayed in able 1.
Table 1. Po e size and in e -po e dis ance o he NAAF subs a es.
NAAF Φ(nm) D
in
(nm)
M0 15 44
M1 15.5 45
M2 18 51
M3 21.5 57
M4 25 65
M5 35 105
M6 50 105
3
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al
In ag eemen wi h ou esul s p e iously epo ed [21], he expe imen al spu e ing
condi ions chosen in his wo k led o he g ow h o nea ly s oichiome ic ZnO films a a low
deposi ion a e (8 nm min
−1
) which enhanced he con o mal cha ac e o he spu e ing
echnique. Hence, unde hese condi ions, 150 nm o ZnO we e g own on M4 and M5 samples
wi h o de pa ame e s alues (Φand D
in
) in he in e media e ange o he p e iously ab ica ed
NAAFs (see able 1).
Figu e 1shows ypical sec ional HRSEM images o M4 (a) and M5 (b) NAAFs a e
spu e ing o ZnO films. The g ow h o nanos uc u ed ZnO films is clea ly shown ep oducing
he po e a angemen o NAAFs pe pendicula ly o he Al subs a es. The x- ay di ac og ams
co esponding o M4 and M5 samples coa ed wi h ZnO a e displayed in figu e 2. In bo h
spec a, signals o cubic aluminum phase [22] (JCPDS ca d No. 40–787) coming om he
aluminum disk used o g ow he NAAF empla es a e p esen . Fo compa a i e pu poses
figu e 2also shows he spec um o a ZnO film 150nm hick g own in he same condi ions on
Silicon 〈100〉. Acco ding o ou p e ious esul s [21], ZnO film on silicon p esen s wu zi e
ype s uc u e, highly ex u ed and o ien ed in he 〈002〉di ec ion. The mean c ys al size alue
in ha p e e ed di ec ion was es ima ed in 22 ± 5 nm by using Sche e ’s o mula [23]. When
ZnO is deposi ed on he NAAF empla es, he 〈002〉p e e ed o ien a ion emains o bo h
coa ings bu wi h a g ea e FWHM han hose exhibi ed by he hin film on silicon. This ac
esul s in lowe c ys al sizes achie ing alues o 16 ± 5 nm in bo h samples. O he di ac ion
peaks co esponding o ZnO hexagonal c ys alline s uc u e could also be ound in hese
Figu e 1. Sec ional iew o samples M4 (a) and M5 (b) (25 and 35 nm po e size,
espec i ely), a e deposi ing ZnO on i s su ace.
4
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al
di ac og ams. The dispe sion in he p e e ed o ien a ions is a ibu ed o he su ace
oughness o he g own ZnO nanos uc u es mimicking he su ace ea u es o he NAAF
empla es. I is known ha subs a es wi h small oughness p omo e be e c ys alline quali y o
ZnO films [24].
Figu e 3(a) shows he expe imen al and simula ed RBS spec a o he NAAFs M4 (a) and
M5 (b) be o e and a e being coa ed wi h ZnO. F om RBS fi ings we de i ed Zn/O a ios o
49/51 and 48/52 o he films deposi ed on M4 and M5, espec i ely i.e. a ZnO compound
almos s oichiome ic). In he case o M4 a signal has been de ec ed loca ed a 1850 keV,
co esponding o he sul u con amina ion coming om he sul u ic acid used in he NAAF
g ow h. Elemen al concen a ions ob ained om he NAAF fi s we e Al/O/S = 38/59/3 o M4
and Al/O = 39/61 o M5, hese alues being e y close o he s oichiome ic composi ion o
alumina (Al
2
O
3
). In he case o M4 empla e, a signal a 1850 keV has been de ec ed,
co esponding o he sul u con amina ion coming om he sul u ic acid used in he NAAF
g ow h. The main di e ences be ween bo h RBS analyses a ise when s udying he ZnO/NAAF
in e ace (see do ed lines in figu e 3(a)). Fo he case o he ZnO film deposi ed on M4, he
ZnO/NAAF in e ace is ab up and he ise o he aluminum signal was ound o esemble he
one o he uncoa ed M4 subs a e. By con as , o he sample deposi ed on M5 he e a e
e idences o di usion o he ZnO in o he subs a e. In o de ob ain a mo e accu a e
de e mina ion o he ZnO/NAAF in e ace we ha e calcula ed he in-dep h concen a ion
p ofiles. Figu e 3(b) shows he p ofiles o samples M4 (a) and M5 (b) a e he deposi ion o
ZnO. The RBX fi made in figu e 2(a) was ob ained assuming a single ZnO laye on Al
2
O
3
subs a e. This p ofile p esen ed an ab up in e ace down o he esolu ion limi o RBS
(app oxima ely 1 nm [25]) wi h he composi ion o bo h Al
2
O
3
and ZnO, being homogeneous
h oughou he en i e hickness. On he o he hand, o M5 sample, in o de o ob aining a good
RBX fi o he spec um (figu e 2(b)) i was necessa y o conside he p esence o in e media e
laye s wi h a a iable composi ion Zn
x
Al
y
O
z
(x+y+z= 1) be ween he ZnO and he NAAF
subs a e. The e o e, in his sample he in- dep h concen a ion p ofile e eals ha he ZnO/
NAAF in e ace is no comple ely ab up ha ing app oxima ely 50 nm in hickness. These
esul s sugges ha he e is some g ow h o ZnO in o he alumina bu only in he NAAFs wi h
Figu e 2. XRD pa e ns o samples M4 and M5 coa ed wi h ZnO.
5
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al

highe po e size, i.e. M5 empla e. This di usi e e ec in he g ow h o unc ional ma e ials on
NAAFs has been epo ed elsewhe e [18].
On he o he hand, one o he c ucial pa ame e s o ob ain well defined nanos uc u es on
po ous su aces is he hickness o he unc ional ma e ial o be spu e ed on hem, in o de o
a oid he comple ed co e age o he po ous in he NAAF empla e. In o de o s udy he
co e age o he NAAF su aces, MO empla es (wi h he smalles po e size i.e. 15 nm) we e
coa ed wi h h ee di e en ZnO hicknesses: 29, 43 and 108 nm. HRSEM images o hese
samples (no shown) e ealed ha only o he hinnes coa ing (29 nm) was a po ous ZnO
su ace de eloped. Fo a ZnO hickness a ound 40nm he po e su ace s uc u e collapse and
only a small pe cen age (∼20%) o po es could be dis inguished. Besides, mos obse ed
po ous su aces lose he long ange o de wi h hexagonal symme y and he ypical g ain
g ow h o ZnO hin films on fla subs a es [21] was obse ed. Finally, in he case o coa ings
hicke han 108 nm, no e idence o po e g ow h was de ec ed. Taking in o accoun hese
esul s, ZnO films wi h cons an hickness o 30 nm we e g own on a NAAF se ies (samples
Figu e 3. (a) RBS spec a (expe imen al and simula ed) o samples M4 and M5 be o e
and a e he deposi ion o ZnO, (b) concen a ion dep h p ofiles ex ac ed om RBS
measu emen s o M4 (a) and M5 (b) a e he deposi ion o ZnO.
6
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al
Figu e 4. SEM images o NAAF su aces be o e and a e deposi ing 30 nm o ZnO.
7
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al
M1–M6) wi h inc easing po e diame e sizes, om 15 o 50 nm, o s udy he po e diame e
e ec s in he mo phology and op ical p ope ies o nanos uc u ed ZnO films.
Figu e 4shows HRSEM images o he s a ing NAAF and a e he deposi ion o 30nm o
ZnO on hem. The e is a change in he con as o he HRSEM images a e he ZnO deposi ion.
This e ec is ela ed o he pa ial educ ion o po e diame e s being mo e p onounced o
samples wi h smalle po e diame e sizes, i.e., samples M1 and M2. In o de o ob ain a
quan i a i e analysis o he po e diame e educ ion a e 30nm o ZnO deposi ion he HRSEM
images we e p ocessed by using he Image J p og am [26].
In able 2, he nominal and final po e diame e s, as well as he es ima ed po e diame e
educ ion a e shown. In his way i was possible o modula e he po e diame e size and also
hei la ice pa ame e o he nanos uc u ed ZnO films by choosing he app op ia e NAAF
empla e o ailo ing o he s no epo ed in his wo k.
Figu e 5(a) shows a ypical AFM image o he su ace o he deposi ed samples along wi h
i s Fou ie as ans o m (FFT) pa e n acqui ed om image p ocessing so wa e [27]. The FFT
image p o ides he high hexagonal o de ing deg ee o nanos uc u ed ZnO laye , e ealing he
exis ence o a sel -o de ed po es s uc u e (up o 3 d neighbou s) in a single domain. This ac
allows us o confi m he guided g ow h o ZnO nanos uc u e on he NAAF empla e su aces.
In figu e 5(b) he su ace p ofile along he pa h eflec s an accu a e measu emen o he po e size
and in e po e dis ance a e he ZnO deposi ion. The wid h and heigh o he peaks in he
su ace p ofile ga e place o an a e age po e diame e (Φ) and in e po e dis ance (D
in
) alues o
45 ± 4 nm and 112 ± 5 nm, espec i ely acco ding o he expec ed da a indica ed in able 1.
3.2. Op ical cha ac e iza ion o ZnO/NAAF nanos uc u es
The cha ac e iza ion o samples o med o mo e han one phase is no an easy ask. In ou case
we ha e ab ica ed nanos uc u es compound by wo di e en solid phases, i.e., on one side he
nanopo ous anodic alumina wi h insula o p ope ies and on he o he hand, zinc oxide, which is
a well-known II–VI semiconduc o . The e o e, we ha e s udied he op ical p ope ies o hese
nanos uc u es in wo s eps, fi s ly hose o he NAAF and la e on he join e ec on such
p ope ies o he nanos uc u ed ZnO/NAAF.
Table 2. Po e diame e s ob ained p ocessing HRSEM images o samples wi h and
wi hou ZnO deposi .
Sample heo e ical Φ(nm) calcula ed Φ(nm) % Reduc ion
M1 15.5 15 31
M1+ZnO —10.5 —
M2 18 19 16
M2+ZnO —16 —
M3 21.5 21 11
M3+ZnO —19 —
M4 25 27 8
M4+ZnO —25 —
M5 35 39 2.5
M5+ZnO —38 —
M6 50 57 7
M6+ZnO —53 —
8
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al
Figu es 6and 7show he ansmi ance spec a o he uncoa ed NAAF subs a es, samples
M1–M6, and he ones co esponding o 30nm o ZnO spu e ed on hose NAAFs, espec i ely.
Figu e 7has shi ed e ically o clea pu poses. Bo h spec a g oups e eal he high
anspa ency (>80%) o all samples in he UV-Visible ange. The obse ed Fab i–Pe o
in e e ence inges o he longes wa eleng hs a e associa ed wi h in e e ences among
mul iple eflec ions coming om he ai –film in e aces, and also p obably om in e nal
in e aces exis ing in he bulk NAAFs due o hei complex composi ional s uc u e, as RBS
esul s sugges .
3.2.1. NAAF abso p ion coe icien s and band gaps. In his wo k, he abso p ion coe ficien s
o he all samples we e de i ed om he ansmi ance spec a and by using he Bee –Lambe
Law [28] gi en by:
Figu e 5. (a) AFM image o sample M6 co e ed wi h 30 nm o ZnO wi h he
co esponding FFT, (b) opog aphic su ace p ofile along he line shown in (a).
9
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al
[33] Miyazaki S 2001 J. Vac. Sci. Technol. B19 2212
[34] Eale B, ElyakhloufiM H, Gille E and Ricci M 1994 Thin Solid Films 250 92
[35] Ci aci S and Ba a I P 1983 Phys. Re . B28 982
[36] Hong S, Kim E, Kim D W, Sung T H and No K J 1997 J. Non-C ys . Solids 221 245
[37] Li G H, Zhang Y, Wu Y C and Zhang L D 2003 J. Phys.: Condens. Ma e 15 8663
[38] Momida H, Nigo S, Kido G and Ohno T 2011 App. Phys. Le . 98 042102
[39] Xu W L, Chen H, Zheng M J, Ding G Q and Shen W Z 2006 Op . Ma e . 28 1160
[40] Minseok C, Ande son J and an de Walle C G 2013 J. Appl. Phys. 113 044501
[41] Ca dona M 1993 Sp inge Se ies in Solid S a e Sciences ol 105 (Be lin: Sp inge )
[42] Lushchik A, Ki m M, Lushchik C, Ma inson I and Zimme e G 2000 J. Luminesc. 87-89 232
16
Ma e . Res. Exp ess 1(2014) 045028 M Yus e e al