Ind i; 1Il Journal or Pure & Applied Phy~i c~ Yol. :lX. ;\ugu~t 2()OO. pp. 000-61 ()

Studies on deposition of triselenide thin films by chemical method: SILAR

B R Sankapal , V Ganesan * & CD LokhanLie

Thin Film Ph y~ i c, Llhoratory. Department orphy~ic~. Sl li v; lji Uni ve rsity. Kolhapur 4 1() !)()4

Rece ived 7 Fehruary 2()OO: rev i ~e d 16 M ;IY 200(): accepted 17 Jilly 2()()!)

Thin rillll ~ or Sh ~ S C . l ha ve hec n d epo~ it cd using a simple and Ics s in vcsti ga ted chemical mClhod namcly. ~ u ccc~s i vc ioni c 1;l ycr adsorption ;I nd reaclion (SILAR). The prcparat ive paramcters such as co nccntration. numhcr Dr inullcrsiollS . illllll c r ~ i()n

lime ctc. arc oplimized to get good qllalily andwcll adherent Sh2Sc, th in rilms. Thc rilm ~ arc char;lctcrized hy Illea ns or X-ray dillr;lclion. ~ca nn i n g elcclron micro ~copy (SEM). atomic rorce Illicro ~co p y (A FM ). optical ah~orp ti nn aIII I elec tri ca l measll rc­ lrIenl ~. XRD st ud y ~ ho lVs Ih at rilm ~ ;Irc or Sh ~ S Cl with orthorhomhic crys t;tI ~ I ructure . SEM and AFr", inl;lgcs show th;1I ri lm, arc n ; lIlo cry~ l;tllill e . The optic:ti h;l nd g; lp i~ e~ tilllatcd 10 he I .X eY . The room tcmperatu l'c d;lIt elecl ri c; tI res isti \, ily is or Iltl' order or I 0) ~l Cill.

Introduction chaicogen s. The ri Im format ion takcs place \\' hen The of antimony is a good pOlential absorb­ ioni c product exceeds so luhility prod uc t. Thi s al so re­ in g material in photoacti ve conve rsion device of solar sul ts into precipitatc forillati on into th e solution and energy with an opt ical ba ndgap inlhe range or 1.06-1 .88 cont ro l over the process is losl. In order to avo id sucil l eV in crys tal s and polycrys tallinc thin rilms -" with difficu lti es, SILAR meth od was deve loped. It is based V ,- VI , compositi ons. According to prince Loferski ef­ on th e illlmersion of th e suhstrate into selxl rat e ly pbced fici cncy bandgap diagranyl a conver1i on crrici ency or cationic and anionic precursors and rin sin g hetween > 20 0'r is possible in solar cell s cmploying absorber or every immcrsion with ion exchanged water to ;Ivo id thi s bandgap. homogeneous precipitation in solution. It i .~ modifi ed A numher or mcth ods have heen employee! ror th e vcrsion or chemical bath deposition

~. The se lenide ions reacted with pre-adsorbed antimony T ahlc I - Optimizcd co nditions for th c dcposition of ions to form one layer of Sb,Se, material. The unreacted Sh2SC, thin fi lm Sc c- ions or powdery Sb c S~\ t ~ateria l ha ve been sepa­ Prcc ursors rated out by rinsin g th e subs trate again in hi ghly purified Dcpos ition cationic an ionic flow in g water for 3() s. This compl etes one SILAR-cycle conditions for th e depos iti on of Sb Sc, thin fi I ms. By repeating such 1 Sourccs an timony sodium SILAR cycl es for 350 times, Sb2Se, film of thi ck ness potassium sc lcnos ulphite O.Ot) ~lm under th e optimi zed depos iti on conditions has tartratc been obtained. Optimized depositi on conditi ons are Conccntr,ltion (M dm- ' ) 0.2 O. I give n in Table I for the depositi on of Sb1Se, thin film . pH - :I X.'i

3 Results and Discllssion Immcrsion timc (scc.) 40 40

Immcrsion cycles :I'i() ]'i() 3.1 Reaction mechanism Rinsing time (scc.) :10 :I() The form ati on of Sb1Se , involves following steps. In th c anio ns precursors soluti on, the hydrolys is of sodium T cmperaturc (oC) 27 27 selenosulphite takes place whi ch releases selenide ions as:

Na 2SeSO, + OW ~ Na1S04 + HSe­ ... ( I ) 2 HS c- + OW ~ H,O + HS e - ... (2) layer. Thus the optimized value of concentration is taken as 0.2 M dm-' . and cati oni c so luti on releases Sb'\+ ions from compl exed For the optimization of number of immersion. the Sb-'+ as: graph of film thick ness against number of immersion (Sb '+ I tartaric acid} ) ~ Sb-' + + Tartaric ac id ... (3) cyc les was pl otted. In thi s, the number of immersion When substrate is immersed in Sb'+ containing solu ­ cycles varied from 100 to 450 cycles. At 35() cyc les. a ti on. Sb \+ ion s are adsorbed on the substrate surface. maximum value of thickness of Sb Se, was obtained 2 1 Art er immersi on of such a substrate in Se - ions contain ­ which may be call ed as terminal thickness. After 35() in g so luti on foll owin g reacti on takes place: cycles, the thickness decreases du e to peelin g off the 2 - 2 Sb 1+ + 3Se ~ Sb2Sc, ... (4) film from the substrate surface. During thi s va riation concentration, immersion time and rinsing time were 3.2 Optimization of preparative pammeters kept constant. The nominal growth rate was calculated In th e present study, equivolumes of cationic precur­ up to terminal thickness only and was found to be 0.56 sor so luti on antimony potassium tartrate and ani oni c nm cycle-Ion glass substrate. Valk onen1o found growth prec ursQ r W.I M dm-:') sod ium selenosulphite were rate 0.13 nm cycl e- I for CdS thin film ont o FrO coated ta ke n in separately placed two beakers for the optimiza­ glass substrate. ti on or concentration of Sly'+ ions. The pH of cati onic Immersion time of Sb2Se, was optimi zed by keeping precursor was maintai ned as 3 by adding compl ex in g concentration and immersi on cycles constant. It wa:-­ age nt as tartari c acid . By keeping number of immersion, found that film attains the maximum thi ckn ess at immer­ i rtlmcrsion time, rinsi ng ti me constant, the concentration sion time of 40 s. of cati oni c precursor soluti on was va ri ed from 0.025 Mdm- ' to 0.4 M dm-' and respecti ve thickness was 3.3 Sample characterization measured and graph of thi ckness against concentration The X-ray diffrac ti on studi es were conducted in the of antilllony potassium tartrate was pl otted . It was found ran ge of scanning angles from 10 to 100° with Cu Ka

that thc Sb2Se, film formation starts from concentration radiation (A. = 1.5406 .A.). A Philips PW-3710, X-ray (l.()5 M c1m-\ of antimony pota ssium tartrate, when ioni c diffractometer was used. The surface morphology of the product exceeds th e solubility product and reaches a film was studied by means of scanning el ectron mi cro­ ma ximum valuc thi ckn ess up to 0.2 M dm-J At thi s scope I model LEICA s 44()iJ and atomi c force mi cro­ concentration, compac t la yer of Sb,Se, is formed. After scope (Di g ital instrume nt , make) . The optica l _1 . : . 0.2 M.c1m . concentratton . the tht ckn ess goes on de- absorption measurement was carri ed out within 35()-85() creasing which ma y be du e to formati on of out er porous nm wavelength ran ge by Hitachi-:n() (Japan) UV- VIS- AOX INDIAN J PURE & APPL PHYS, VOL 38, AUGUST 2000

NIR spectrophotomete r. The e lectrical resistivity of the the he ight and the diameter of the Sb2Se:; islands are in film was measured in the temperature range 330-560 K the same ord er of size. by employin g two po int probe method. S il ver paste was 3.6 Optical absorption studies used as a contact material. The working temperature was Study of material s by means of opti cal absorption for sensed by means of calibrated C r-AI thermocouple. explaining some Features concerning the band structure o f materials was studied. In the present paper, optical 3.4 XRD studies absorption of Sb Se, fi 1m on g la ss substrate was studied Fig. I. shows the X-ray diffractograms ofSb Se, thin 2 2 in the wavelength range 350-850 nm. The plot of absorb­ film deposited onto g lass substrate. The Sb Se, film on 2 ence against wave length spectra showed cl earl y the glass substrate shows orie ntation along (2 1 I) plane w ith absorption edge shift towards lower energy side for orth orh ombic . A comparison of ob­ Sb}Sc, sample. The nature of the transition in volved can served d va lues made w ith ASTM datal I. he determined on the basis of the dependcnce of absorp- Use o f XRD peak for estimation of the average grain size is well known. In case of thin films. direct observa­ tion of grain size by means o fTEM and estimated grain l U J s.i ze by XRD peaks agrees we 11 1-1. There fore in orde r to study th e average grain size of

SbcSc\ particles, thin film o f Sb2Sc\ was stud ied by taking a slow-scan X-ray diffraction patte rn around the (2 1 I) peak with a step w idth of the di ffractometer ofCl.0 2 (28) min- I. The grain size was calc ul ated by using the full width at half maximum meth od and by using Scher­ rer' s re lat ion : A r/ = --- ... (5) f) cns 8

where D is the full width at half maximum of the peak alld/c= 1.5406 Ai s the wave length w ith a C u- Ka target. The ave rage gra in size is found to be 3 nm. rig. 2(;1) - Scalln in g elec tron mi crograph or Sh.' SCl thill 3.5 Surface mOl'phology filill at m;lgn iricatioll 2().()()() x Fig. 2(a) shows scanning e lectron micrograph of

Sh2Se, thi n fi 1m at magnification 20,000 x. It is seen that the fillll is well covered to the substrate. Atolllic force Illicroscopy (AFM) has been proved to be a unique Illethod to analyze the surface Illorphology of thin filllls. Fig. 2(b) shows tapping mode (TM-AFM)

image of Sh2Sc\ thin film. The surface o f Sb2Sc, thin film is found to he rou!!h . It should he noted that both

....., 40 :J a ?: . iii 20 c c:'"

20 40 60 80 20(deg)

~ b !ie-s I J ' . 002 Fi g. I - XRD pallcrn or Sh2Se.\ thin rilill deposi ted on gl;lss suhstrat e rig. 2(h) - TM-AFM im age or ShlSC3 thin filill SAN KAPAL £' 1 al.: ANTJMO NY TRISELENIDE THI N FILM S 6 0 ~

tion coeffi cient on photon energy hv. For all owed direct transit ion a is given as: 5.0 ( h v - E r:) 1/ 2 a ce < ...(6) h v 4.6 where Eg is the energy ga p between the bottom of the E conducti on band and the top of the valence band at the u ~ sa me va lue of wave vector k. . Variati on of (ahv)] with - 4.2 Sb ~ S e , th e plot for fi 1m 1F ig. 31 is a straight line indicat- 0'> .lI1 g t Il e transIt...Ion lI1 VO I ve d ·I S d·I rect ga p1"1·· 1-1 . B y extrapo- o · lati ng the strai ght porti on ro the energy ax is at a = 0, the 3.8 hand gap was estimated to be 1.8 eV for Sb]Se, thin film whi ch is greater than the va lu e of Eg = 1.3 e V, reported earl ier for sin gle crystals 15 of Sb]Se, . The band gap reported by Pramani k and Bhattacharya') for chemi call y 2.0 2.4 2.8 3.2 deposited Sb]Se, thin film is 1.88 eV. The difference in 1000 ( K""1) ba ndgap obtain ed could be du e to impurities rSbO, T Sb(OH) etc. 1 andlor meth od of preparation of materi als Fig. 4 - PI OI or (log p) wilh ( I/T) ror Sh] Sc, thin rilms on lies in sin gle crystals or thin films. glass suhslralc

3.7 ElectJ'ical resistivity semi conductin g behav iour of Sb2Se, fi lm. The acti va­ To stu dy the electrica l resisti vit y of the Sb2Se, thin ti on energy is found to be 0.2 1 e V. films, the dark electri ca l resisti vity measurement was carried out in the temperature range 330-560 K using dc 4 Conclusions IWO point probe meth od . The room temperature res isti v­ A simple chemi cal meth od namely, successive ioni c it y is of the order of IO:i Q Clll whi ch is less th an the layer adsorpti on and reaction (SILAR) ca n be empl oyed earl ier reported va lue 107 Q cm for amorp hous Sb]Se, to deposit good quality and well adherent Sb]Se, thin film . Quali ty of the fi lm depends upon optimi zed prepa­ thin film by Rajpure ('I (1 /. -"' . The variation of log p with rative parameters. XRD studies reveal that films are of l i T is depi cted in Fig. 4. It is observed that res isti vity Sb]Se, with orth orhomb ic crystal structure. SEM illlage decreases with in crease in temoerature. su!!!!estin !! that shows that fi Iill S are we ll covered to the substrate surface 20 with some roughness as seen fro m AFM image. The optical bandgap is found to be 1.8 e V. The room tem­ N perature dark electri cal resistiv ity is of the order of ""IE 10:i cm. u n I > CLo Acknowledgement ,,- Authors are thankful to JNCASR, Banga lore. fo r 's? 10 / SEM facility. IUC- DAEF. Indore for AFM fLl cility and N~ Uni versity Grants Commi ss ion. ew Delhi , In dia for ~ .c the financial support under the project F: 10-7197 (S R-I) . 'd Refel'ences I Madelung O. (Ed) (Sprin gcr-Vcrlag. Bcrl in ). 1 ~ <) 2. pp. 4X -SI . 2 Garcia V M:Na ir M T S. Nair P K & Z in garo R A . S,./II icolIIl 2.0 3.0 Sci Tl'cI/lwl. 12 ( 1 9~7 ) 645. h'})( eV) Pri nce M B . .I A/'/I! Phn. 27 ( I ,)S6) 777. 4 Wood C. Gilhcrt L R. Mucllcr R. Ga rncr C M . .I V(/ e Sci Fig. 3 - Plot or (altv)2 vc rsu s ltv ror Sh2SC, lhin ril m T,.c//l w l. I () ( 1973) 5. 5 Raj purc K Y. Lokh,l ndc C D & Bhosalc C H. Tlt ill Solid ri/m.\". 33 1 ( 19')7) 114 . 610 INDIAN J PURE & APPL PHYS, VOL 3X, AUGUST 2000

() Nikalll P S & Pa war R R, /JL/I! Mo/er Sci. 13 (19<)0) 343 . 12 Lokhantle C D. Ennaou i A. Patil P S. Gicrsig M "/ II I .. Thill Solid Filll/s. 330 ( 199R) 70. 7 Torallc A P. RajplIrc K Y & Bho~al c C H. Ma/('/' Chell/ Pln·s. Cl l ( I ()<)<) 2 1<) 13 Ghosh C & Varma B P. Thill Solid FilllI.l. 60 ( 1979) hi . X DCSi li.l D & G,1I1agc K N. /Jill! U"C'/J'()Ch"III. IS ( 1<)<)<) 3 1X. 14 Nayak B B. Acharya 1-1 N. Chollclhari T K & Mitra G 13. Thill () Pr,lI11iln ik P & Bhallac h,lrY,l R N . .I Solid S/(I/ £' CO/llIIIIII I. 44 Solid Filll/s. 92 ( 19R2) 30<) ( I