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 a o , K h u c Q u a n g D a t, N g u y e n T h i e n H u o n g College o f N a tu ra l Sciences, Vietnam N ational 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.
1. I n t r o d u c t io n
Doped and undoped ZnO are of g rea t in te r e s t for m any potential applications in optoelectronic devices due to th e ir physical p roperties, low cost and th e various fabrication techniques (See Ref. [1, 2] for reviews). D epending on the specific applications, ZnO a n d doped ZnO films can be p re p a re d by a variety of deposition tech n iq u es such as s p u tte rin g [3], chem ical vapor deposition [4], spray pyrolysis [5 6], pulsed la s e r ablation [7], charge liquid c lu s te r beam [8, 9], sol-gel process [1 0-12].
From a p ractical stan d p o in t, the sol-gel process is an a ttra c tiv e technique and h as the a d v a n ta g e s of easily controlling the film composition and easily fabricating a larg e -are a film w ith low cost. Most of the stu d ies recently reported have been focusing on ZnO films doped w ith A1 or In. In a previous p a p er [13], we proposed the p re p a ra tio n of ZnO:Al films by sol-gel process.
The sol-gel m ethod, however, h as the d isa d v an tag e of re q u irin g high h eatin g te m p e r a tu re s for crystallization. Searching for an a p p ro p ria te h e a t tr e a tm e n t
process is one of re s e a rc h topics on th is deposition technique.
The aim of th is work is to develop a n o th e r sol-gel-derived doped ZnO/ZnO: In in order to get b e tte r u n d e rs ta n d in g how th e p re p a ra tio n procedure governs the crystalline s tr u c tu r e a n d influences the optical p roperties of th e obtained films.
Special a tte n tio n h a s b een paid to the XRD an aly sis a n d photolum inescence spectra of th is m aterial.
2. E x p e r i m e n t a l d e t a i l s
The s ta r tin g m a te ria l used in the p resen t study w as zinc acetate 2-hydrate (ZnAc 99 9% purity). The solvent was ethylene glycol. The sol stabilizer was
7
8 P h u n g Quoc Bao
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Khuc Qua ng Dat, Nguyen Thien Huon g trie th y la m in e . The ZnAc was first dispersed into eth y len e glycol and th e n th e in d iu m n itr a te 9-h y d rate was added. The m ix tu re was m agnetically stirred , a n d m oderately h e ated a t 1 0 0 °c to 115°c till to obtain a uniform t r a n s p a r e n t solution.This p recu rso r solution was th e n kept a t 115°c of ab o u t 30 m in to avoid th e re- c ry stallisa tio n of ZnAc. Once p repared, the solution was p reserv ed in a herm etic flask to keep stable a n d clear for a long period. The final concentration of th e p recursor solution was 1,0 mol/1 corresponding to th e m olar ra tio n In: n Zn= 3%.
The p recu rso r solution was readily gels on addition of a few drops of trie th y la m in e (1 mol equivalent) to obtain th e coating solution. The adding of trie th y la m in e is to a ss is t hydrolysis of zinc acetate. Excessive addition of trieth y lam in e, however, produced sizeable gel particles. The a s-tre a te d solution became milky and useless. It was needed to remove sm all air bubble from the coating solution by ultraso n icatio n .
As a s u b s tr a te m ateria l, we used a well-polished borosilicate glass plate 20x20x1 mm in size. After cleaning the su b sta te w ith distilled w a te r and acetone successively in an u ltraso n ic bath , it was dried w ith hot air and preserved in a desiccator before use.
The spin coating procedure was as follows: dropping the coating solution (approx. 0,2ml) onto th e su b s tra te , w aiting for a uniform sticky expansion, spin- coating w ith 3000 rev.m in 1 for 30s in air. The as-deposited film was dried in air a t room te m p e r a tu re for 15h and th e n an n ealed in a ir in a L enton electric furnace at various te m p e r a tu re s for 2h. This h e a t tr e a tm e n t decomposed and oxidized th e as- deposited films so as to produce ZnO:In films. We have re p e a te d several cycles of the spin-coating - drying - an n ea lin g process to obtain m u ltila y ere d ZnO:In films with various thickness. The films p rep ared by re p e a tin g th e spin- coating and postheating more th a n 3 tim es were considerably opaque a n d th u s not app ro p riate to n e asu re m en ts.
The crystalline s tr u c tu r e of th e films u n d e r in v estig atio n was confirmed by X- r a v diffraction (XRD) a n aly sis usin g CuKa rad ia tio n w ith a B rucker Siem ens 5000 diffractometer. Photolum inescence spectra were recorded a t room te m p e r a tu re with a Jobin-Yvon spectrofluorom eter FL3-22 u sing a XFOR-450 Xe lam p as a u v light source.
3. R e s u lts a n d d i s c u s s i o n
F ig .l shows XRD p a tte r n s of single-layered ZnO:In films after th e h e a t treatm ent a t 220-520°C. A very broad peak aro u n d 29 = 22° is due to the glass substrate. No o th er p h a ses are detected u n d e r the p re s e n t ex p erim e n tal condition.
T\eve is no evidence of any resid u al In compound in the films. Typical peaks caused by crystalline ZnO are observed for th e films t h a t were a n n e a le d a t more than
Sol - gel spin-coating prepa ra tion of ZnO:In films
.
9 2 0 0°c. The h ig h e st p eak a p p ea rs aro u n d 29 = 34° corresponding to th e diffraction by a (002) plane of ZnO crystal. In te n s ity of the (002) p e ak is in cre as ed with increasing th e a n n e a lin g te m p e ra tu re , w h ereas th e (1 0 0) and (1 01) p e a k s £et sm aller but still a p p e a r in th e XRD p a tte r n of the film t h a t w as an n ea le d a t 520°c.Relative in te n sitie s of th e th re e XRD peaks a t various a n n e a lin g te m p e r a tu r e s are ta b u la te d in T a b .l.
Fig. 1. XRD p a tte rn s of single-layer ZnO:In films at
520°c
(a);420°c
(b);320°c
(c);220°c
(d) a n n e a lin g te m p e ra tu re sTab. 1. Relative intensities of the (100), (002) and (101) XRD peaks a t various annealing tem p eratu res
10 P h u n g Quoc B a o
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K h u c Q u a n g Dat, N g u y e n Thien Huon g XRD p a t t e 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 diffractio n p e ak s a p p e a r in th e 3-layerecl film ’s p a t t e r n except the (002). H ig h e r degree of th e c-axis o rie n ta tio n is found for t h e film s w ith several coatings. It is well k n o w n t h a t th e o r ie n ta tio n of c ry s ta ls in th e film is strongly d e p e n d e n t on th e c ry s ta llo g ra p h y of th e s u b s tr a te . In th e sin g le -la y e re d films, the p re fe re n tia l o rie n ta tio n m ay occur via n u c le atio n a t th e film/ s u b s t r a t e interface and th e r a n d o m atom ic a r r a n g e m e n t of th e glass s u b s t r a t e p ro b ab ly d istu rb s the orien ted c ry s ta l grow th. T h erefo re, th e (002) p e ak in te n s ity w a s r a t h e r weak. In the second (third) layer, th e o rie n te d c ry s ta l grow th m ay easily ta k e place onto the slightly o rie n te d c ry s ta ls of th e fir s t (second) layer. As a r e s u lt, th e r e le v a n t (0 0 2) p eak in te n s ity in c re a s e s . The (100) a n d (101) p e a k i n te n s i t ie s for th e 2-layered films a re also w e a k e r t h a n th o se for th e sin g le-lay ered ones.Fig. 2. XRD p a tte r n s of 2-layered (a) and single-layered (b) ZnO:In films a n n ealed a t 5200C
Zinc oxide e x h ib its two k in d s of PL em issions: one is a n u ltr a v io le t near-band- edge em issio n a t a p p ro x im a tiv e ly 380 nm a n d th e o th e r in a visible deep-level em ission w ith a p e a k a n y w h e re in th e ra n g e 450-730 n m (VisPL). The visible e m issio n s a re r e la te d to in tr in s ic defects or d o p a n ts in ZnO c ry s ta l a n d depend g rea tly on th e p r e p a r a t i o n m e th o d s a n d conditions.
Sol - gel spin-coating p r e p a r a t i o n of ZnO:In films. 11 The p h o to lu m in es ce n ce s p e c tra a t room te m p e r a t u r e ex cited w ith th e 325-nm line of XFOR Xe la m p for th e sin g le-lay ered film s a n n e a le d a t 220°-520°C are displayed in F ig .3. T h e com m on fe a tu re is t h a t PL s p e c tr u m c o n sists of 2 d istin c t bands: a n e a r u v e m issio n a t ~ 380 nm a n d a g reen e m issio n a t ~ 508 nm. The former called excitonic b a n d is probably c au se d by th e decay of an exciton lying about 40 meV below th e conduction b a n d edge in ZnO [14, 15]. The l a t t e r is originated from th e oxygen vacancy [16]. Both excitonic a n d g ree n b a n d s were observed m ostly in c re a s in g w ith th e a n n e a lin g t e m p e r a t u r e for all sam ples.
Fig.3. Photolum inescence spectra for th e single-layered films a t various an n ealin g te m p e ra tu re s
It should be n o ted t h a t excitonic p h o to lu m in es ce n ce is v ery se n sitiv e to th e qu ality of c ry s ta l s t r u c t u r e s a n d to th e p res en c e of defects. T h e m ore perfect is th e cry stal a n d th e m ore s u b s ti t u t io n a l is th e im p u r ity a sso c ia tio n in th e m ain lattice, the more p ro b ab le is th e o b se rv atio n of excitonic PL. F ig .4 i ll u s t r a te s two PL spectra: one for th e sin g le -la y e re d film, th e o th e r for th e 2 -lay ered film. The excitonic PL em iss io n i n te n s ity in c re a se s a b ru p tly for th e 2 -lay ered films. All th e above-m entioned sh o w s t h a t th e d iscussion on PL s p e c tr a is co m p a tib le w ith t h a t on XRD a n a ly sis for th e a s - p r e p a r e d ZnO:In films.
12 P h u n g Quoc Bao
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Khuc Q uang D a t,
N g u y e n Thien Hu on gX 104
Fig.4. Photoluminescence spectra for 2-layered (a) and single-layered (b) films annealed at 520°c
4. C o n c l u s i o n
In-doped ZnO films have been deposited by th e sol-gel sp in - coating m ethod onto glass su b s tra te s . The XRD p a tt e r n analysis h a s been s tu d ie d u n d e r different post-deposition h e a t t r e a tm e n t conditions. PL sp ectra w ith a n excitonic emission and a b ro ad b an d green em ission u n d e r u v excitation com patibly reflect th e change of the film m ic ro stru ctu re and cry stallite o rien tatio n observed in XRD experim ents.
It a p p ea rs t h a t th e p referen tially oriented cry stallites of th e films plays an im p o rtan t role in th e ir optical spectroscopic properties. F u r t h e r in v estig atio n s on electrical p ro p erties of th e films are necessary to confirm th e u sed p re p a ra tio n procedure and will soon be reported.
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