VNU JOURNAL OF SCIENCE.Mathematics - Physics T.xx, N01 - 2004 SOL-GEL SPIN-COATING PREPARATION OF ZNO:IN FILMS WITH HIGHLY C-AXIS ORIENTATION P h u n g Q u o c B ao, K h u c Q u a n g D at, N g u y e n T h i e n H u o n g College o f N atura l Sciences, Vietnam National University, Hanoi A bstract: The c-axis oriented ZnO:In films were prepared by the sol-gel spin-coating process using zinc acetate 2-hydrate, ethylene glycol, indium nitrate 9-hydrate, triethylamine as starting materials The preferred orientation of the films increased with heat treatment temperature Near u v and green photoluminescent emisions from the films was observed at room temperature The PL intensity was found to be greatly dependent on post-deposition heat treatment Influence of the multilayer deposition on XRD analysis and PL characterisation was also discussed I n t r o d u c tio n Doped and undoped ZnO are of great in te re s t for many potential applications in optoelectronic devices due to th eir physical properties, low cost and the various fabrication techniques (See Ref [1, 2] for reviews) Depending on the specific applications, ZnO and doped ZnO films can be p rep ared by a variety of deposition techniques such as sp u tterin g [3], chemical vapor deposition [4], spray pyrolysis [5 ], pulsed laser ablation [7], charge liquid clu ste r beam [8 , 9], sol-gel process [ - 12 ] From a practical standpoint, the sol-gel process is an attrac tiv e technique and has the ad v an tag e s of easily controlling the film composition and easily fabricating a large-area film with low cost Most of the stud ies recently reported have been focusing on ZnO films doped with A1 or In In a previous p aper [13], we proposed the p rep a ratio n of ZnO:Al films by sol-gel process The sol-gel method, however, has the disadvantage of req u irin g high heating te m p e tu re s for crystallization Searching for an ap p ro p riate h e a t tre a tm e n t process is one of resea rch topics on this deposition technique The aim of th is work is to develop a n o th er sol-gel-derived doped ZnO/ZnO: In in order to get b e tte r u n d e rstan d in g how the p rep aratio n procedure governs the crystalline s tru c tu re and influences the optical properties of the obtained films Special a tten tio n h a s been paid to the XRD analysis and photolum inescence spectra of this m aterial E x p e r i m e n t a l d e t a i l s The sta rtin g m ateria l used in the present study was zinc acetate 2-hydrate (ZnAc 99 9% purity) The solvent was ethylene glycol The sol stabilizer was P Quoc Bao , Khuc Quang Dat, Nguyen Thien Huong trie th y la m in e The ZnAc was first dispersed into ethylene glycol and th en the indium n itr a te 9-hydrate was added The m ixture was m agnetically stirred, and m oderately h eated a t 100°c to 115°c till to obtain a uniform tra n s p a r e n t solution This precursor solution was th en kept at 115°c of about 30 to avoid the rec rystallisation of ZnAc Once prepared, the solution was preserved in a herm etic flask to keep stable and clear for a long period The final concentration of the precursor solution was 1,0 mol/1 corresponding to the m olar ratio n In: nZn= 3% The precursor solution was readily gels on addition of a few drops of triethylam ine (1 mol equivalent) to obtain the coating solution The adding of triethylam ine is to a ssist hydrolysis of zinc acetate Excessive addition of triethylam ine, however, produced sizeable gel particles The a s-tre ate d solution became milky and useless It was needed to remove sm all air bubble from the coating solution by ultrasonication As a s u b s tra te m aterial, we used a well-polished borosilicate glass plate 20x20x1 mm in size After cleaning the sub state with distilled w ater and acetone successively in an ultrasonic bath, it was dried with hot air and preserved in a desiccator before use The spin coating procedure was as follows: dropping the coating solution (approx , ml) onto the su b stra te, w aiting for a uniform sticky expansion, spincoating w ith 3000 rev.m in for 30s in air The as-deposited film was dried in air at room te m p e tu re for 15h and th en annealed in a ir in a Lenton electric furnace at various te m p e tu re s for 2h This h e a t tre a tm e n t decomposed and oxidized the asdeposited films so as to produce ZnO:In films We have rep e ate d several cycles of the spin-coating - drying - an nealin g process to obtain m ultilayered ZnO:In films with various thickness The films prepared by rep eatin g the spin- coating and postheating more th a n tim es were considerably opaque a n d th u s not appropriate to neasurem ents The crystalline stru c tu re of the films u n d er investigation was confirmed by Xr a v diffraction (XRD) analysis using CuKa radiatio n with a Brucker Siemens 5000 diffractometer Photolum inescence spectra were recorded a t room te m p e tu re with a Jobin-Yvon spectrofluorom eter FL3-22 using a XFOR-450 Xe lam p as a u v light source R e su lts a n d d i s c u s s i o n Fig.l shows XRD p a tte rn s of single-layered ZnO:In films after the h eat treatm ent at 220-520°C A very broad peak around 29 = 22° is due to the glass substrate No o ther phases are detected und er the p resen t experim ental condition T\eve is no evidence of any residual In compound in the films Typical peaks caused by crystalline ZnO are observed for the films th a t were ann ealed at more than Sol - gel spin-coating preparation of ZnO:In films 0 °c The h ig hest peak appears around 29 = 34° corresponding to the diffraction by a (002) plane of ZnO increasing the a n n ea lin g sm aller but still a p p ea r in Relative intensities of the tab u late d in T ab l crystal In tensity of the (002) peak is increased with te m p era tu re , w hereas the ( 100 ) and ( 101 ) p eak s £et the XRD p a tte rn of the film t h a t was an nealed a t 520°c th ree XRD peaks at various a n n ealin g te m p e tu re s are Fig XRD pa ttern s of single-layer ZnO:In films at 220°c 520°c (a); 420°c (b); 320°c (c); (d) a n n ealin g tem p era tu re s Tab Relative intensities of the (100), (002) and (101) XRD peaks at various annealing tem peratures 10 P h u n g Quoc B ao , K huc Q u a n g Dat, N guyen Thien Huong XRD p a tte r n s of single- a n d double-layered film s a n n e a le d a t 520°c are shown in Fig.2 No o th e r diffraction peak s a p p e a r in th e 3-layerecl film ’s p a tt e r n except the (002) H igh er degree of th e c-axis o rien tatio n is found for th e films with several coatings It is well know n t h a t the o rien tatio n of c ry stals in th e film is strongly d ep en d en t on th e c ry stallo g p h y of the su b s tra te In th e single-layered films, the p refe ren tial o rie n ta tio n m ay occur via nucleation a t th e film/ s u b s tr a te interface and th e ran d o m atom ic a rr a n g e m e n t of the glass s u b s tr a te pro bably d istu rb s the oriented c ry stal growth T herefore, the (002) peak in te n s ity w as r a t h e r weak In the second (third) layer, th e o rien ted c ry stal grow th m ay easily ta k e place onto the slightly o rie n te d c ry stals of th e first (second) layer As a re s u lt, th e re le v a n t (0 ) peak in te n sity in creases The (100) an d (101) peak in te n s itie s for th e 2-layered films a re also w e a k e r t h a n those for the single-layered ones Fig XRD p a tte rn s of 2-layered (a) and single-layered (b) ZnO:In films annealed at 5200C Zinc oxide ex h ib its two k in d s of PL emissions: one is a n u ltra v io le t near-bandedge em ission a t a p p ro x im ativ ely 380 nm a n d the o th e r in a visible deep-level emission w ith a peak a n y w h ere in the range 450-730 nm (VisPL) The visible em issions a re re la te d to in trin sic defects or d o p a n ts in ZnO c ry s ta l and depend greatly on the p r e p a r a tio n m ethod s a n d conditions Sol - gel spin-coating preparation of ZnO:In films 11 The ph otolum inescence sp ectra at room te m p e r a tu re excited w ith th e 325-nm line of XFOR Xe lam p for the single-layered films a n n e a le d a t 220°-520°C are displayed in Fig.3 T he common fe a tu re is t h a t PL sp e c tru m consists of distinct bands: a n e a r u v em ission a t ~ 380 nm and a green em ission a t ~ 508 nm The former called excitonic b a n d is probably caused by th e decay of an exciton lying about 40 meV below th e conduction b an d edge in ZnO [14, 15] The l a tte r is originated from th e oxygen vacancy [16] Both excitonic a n d green b a n d s were observed mostly in c re a sin g w ith th e a n n e a lin g te m p e r a tu r e for all sam ples Fig.3 Photolum inescence spectra for the single-layered films a t various annealing tem p era tu re s It should be noted t h a t excitonic ph otolum inescence is very sensitive to the quality of c ry stal s tr u c tu r e s a n d to th e presence of defects T he m ore perfect is the crystal and th e m ore s u b s titu tio n a l is the im p u rity a sso ciatio n in th e m ain lattice, the more probable is th e o bserv atio n of excitonic PL F ig illu s tr a te s two PL spectra: one for th e single-layered film, th e o th e r for th e 2-layered film The excitonic PL em issio n in te n s ity increases a b ru p tly for th e 2-layered films All the above-m entioned show s t h a t th e discussion on PL sp e c tra is com patible w ith t h a t on XRD a n aly sis for th e a s-p re p a re d ZnO:In films P h u n g Quoc Bao , Khuc Quang Dat , Nguyen Thien Huong 12 X 104 Fig.4 Photoluminescence spectra for 2-layered (a) and single-layered (b) films annealed at 520°c C o n c lu s io n In-doped ZnO films have been deposited by the sol-gel spin - coating m ethod onto glass su b stra tes The XRD p a tte rn analysis has been stu d ied u n d e r different post-deposition h e a t tre a tm e n t conditions PL spectra with a n excitonic emission and a broadband green emission u n d e r u v excitation compatibly reflect the change of the film m icrostructu re and crystallite o rientation observed in XRD experim ents It app ears th a t the preferentially oriented crystallites of th e films plays an im portant role in th e ir optical spectroscopic properties F u r th e r investigations on electrical properties of the films are necessary to confirm th e used p rep aratio n procedure and will soon be reported i• References S A Studenikin, N Golego and M Cocivera, J App Phys., 84, (1998), 2287 Sol - gel spin-coating preparation of ZnO:In films 13 D c Look, M aterials Science and Engineering B80(2001), 383 M Tzolov, N Tzenov, D Dimova-Malinovska, M Kalitzova, c Pizzuto, Vitali, G Zollo and I Ivanov, T hin Solid F ilm s , 379(2000), 28 H Sato, T Minami, T Miyata, s Takata and M Ishii, T hin Solid Films 246(1994), 65 M Krunks A.S Riad, s A M ahm oud and A A Ibrahim , Physica B Conden Mat., 296(2001), 319 K L Narasimhan, s p Pai, V R Palkar and p Pinto, T h in So lid F ilm s , 295(1997), 104 c K Ryu a n d K Kim, App Phys Lett., 67(22) (1995), 3337 M Koyano, P h u n g QuocBao, Le Thi T hanhB inh, Le HongHa, Nguyen NgocLong and S K atay am a, Phys Stat Sol (a) 193, No 1(2002), 125 G and E Melikov, T hin Solid F ilm s, 270(1995), 33 10 Y N atsum e a n d H S ak ata, T hin Solid Films, 372(2000), 30 11 S F u jihara, H Naito and T Kim ura, Thin Solid F ilm s 389(2001), 227 12 M ikrajuddin, F Isk a n d ar, K O kuyam a and F G Shi, J Appl P h y s 89(2001), 6431 13 P Quoc Bao, Khuc Quang D at and Nguyen Trong Nghia, VNU J Sci M a th -P h y s., T XIX, No 1(2003), 16 14 A.A Gladyschuk, J Lumin.y 42(1988), 49 15 T v Butkhuzi, T G Chelidze, A N Georgobiani, D L Jashiashvili, T G K hulordava and B E c Tsekvava, Phys Rev B58(1998), 10692 16 F H Leiter, H R Alves, A Hofstaetter, D M Hofmann and B K Meyer, Phys Stat Sol (b) 226(2001), R4-R5  ... asdeposited films so as to produce ZnO:In films We have rep e ate d several cycles of the spin-coating - drying - an nealin g process to obtain m ultilayered ZnO:In films with various thickness The films. .. evidence of any residual In compound in the films Typical peaks caused by crystalline ZnO are observed for the films th a t were ann ealed at more than Sol - gel spin-coating preparation of ZnO:In films. .. ry s ta l and depend greatly on the p r e p a r a tio n m ethod s a n d conditions Sol - gel spin-coating preparation of ZnO:In films 11 The ph otolum inescence sp ectra at room te m p e r a tu