Structural and optical characterization of nanostructured ZnO grown on alumina templates

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. 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