Bargale, B B

Pawar, S H

Shikalgar, A G

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Effect of annealing time on dark and photo-currents of CdS : Na thin filmsB  B .  B A R G A L E , a  G  iS llIK A L G A R  A N D  S . H .  P A W A RMaterials Research Laboratory, Department of Physics, Shwaji University, Kolhapur-AhtSiMH{Received 23 June 1977)Indian J .  Phya. 52A, 306-310 (10‘^ S)Gluimuiallj ckij)OMt(Ml CdS Diin films havo boon jjroparod with tind wii))out sodium dopant Tlio films li.aV(‘ Ijoen anji(‘,alr(l in air at; 150‘ C for vaj‘i<jus poriods up to 40 h(u>i‘H Effoct- ol annoalmg tim(‘ on dark and p}i,oio-curront.s of tho films has boon iiivoStigatod. Tint rolo oJ adsorption o ! oxygon and tho formation of Bulhir vaoaiuoos au- dis- cnssod Tho offoct of liglit inioiisity on phutocnrront has also boon roportod1 IN T R O D U O T IO Nx\s a part of oontinuing progiam (Pawar et al 1977, Sliikalgar et at 1977) tJio offoot of sodium dopant as woll as aiinoaling timo on plu»toourront and dark tmrront in ohomioally dojiositod CdtS thin films is boing invostigaWl Until now vory littlo work has boon dono on tho photooomlnoting iiropi'rtioS of sodium dopod CdS A fow workors (Homy cl al 1970, Yosliizava 1970 and Toll 1971) havo rooontly roportod tl\at tho sodium and litlnnm aro tho good shallow aoooiitors ro-iponsiblo for photoconductanoc and photolnminosooiico Howovor, thoir main intoroSt was to»»btain p typo matoiial, but so fai that was uiisuooossful ThoS(‘ studios woro inado on siiiglo crystals and tlio doxjing of sodium in ohomioally d(‘,lDO'-'ltcd CdS thin films is fow to dato as far as tho authors aro awar<\ olIn tho xuosont nivostigation sodium dojiod CdS thin films havo Tioon ]>ro- Iian'x) by ohomioal bath doposition todiiiiquo Th.o iTloot of sodium doxiant and annoaling <1^ 1 k and xduito-currents of thoso films has booninvostigatod Tho variation in xdiotociirnmt witli light intensity has also boon studied.2 E xperimentalCdS . Na thin films liavo lioon x^upared by ohomioal bath deposition techni­que following clofloly the method reported earlier (Pawer et al 1977) Briefly, for each deposition 20 c.c of 1 mol. CdS04 Solution and 100 c.c. of 2 mol. ammonia solution were mixed in a fioaker to form a oomxilcx compound. To this complex 20 c.c. of 1 mol. tliiourea and O-Ol wt percent of Na were added. The reaction vessel was heated to 90“C for 30 minutes Four glass slides were mounted as tho substrates on a samiilo holder which was kept rotating in the reaction mixture305at a uoriHiant wpootl nf 125 rovolutioim jK;r miiiuto. Tliia Servod the piirpoKe of Htirriu  ^ the eliemieal mixture, a,s well aa exposing the HuhKtratoH to fresh reactants which is important for uniform deposition of the films. TJio films wen> washed Sev^ eral times with distilled water, dried and preServiHl in a dark dessicatorThe films wore annealed at 150'^ C in air for various peritids witli an int(u-val of four hours and upto loi-ty hours For photoconductivity studies, a special holder was diiSigiuMl for giving the pressure contacts to the films wliich was mount(‘,d in a light tight box The photocUrrent was measured with Aplab TVM 13 FE"J’ iiaiioamraetc'r A 500 watt tungstim filament lamp operated at 2800'"K was used as the souri;e ol liglit. A water filter Avas used in between film and lamp to avoid tlie heating of the film.3. R esults and 'Discussion ^Figure J gives tln^  nature of variation of dark current /dAvith tlie anneal­ing time both for doped and undoped films 3’be increase in 7  ^ Avitli annealing time IS attributed to the better crystalinity as well as to tlie formation of sulphii; vacancies giving rise to the additional donor levels (Pavaskar 1970).The other feature noted from figure 1 is that the 7 for doped films is smallc‘,r than that for undoped films This is attributed to the folloAving reasons :1) Tn th e  p ie se n t  c lu m iica l h a th  d e p o s it io n  te ch n iq u e  th ere  is an eX(;ess o f  sn lp ln ii (P a v a sk a r  &. Monzi^s 1968) Avhieli m a x im izes  t lie  d e n s ity  o f  ca d m iu m  vacaiici(»s A s tlui S odium  io n s  an» rax iid ly  d iffu s in g  a n d  h ig h ly  soluble', th ese  can  en tre  e a s ily  a t th e  su b s t itu tio n a l s ites  g iv in g  rise t o  a c i;ep tor  leve ls  . T h is  Avill diHjreaSe th e  d e n s ity  o f  e le ctron s2) The doping of sodium may reduce the (*,li5ctron mobility eitlu-r throngli mtrodneing additional scattering conties f)v tlirougb altering the barrier lusights at the iiitm-grain liariers. (Wu & Bubo 1974)3) The suljihur atmosphere minimizes tlie density of simple intrinsic donor defects (suli)hiir vacjaiicics oi cadmium interstitials) wliich, could comijonsate the Sodium4) As observed from X-ray studios, the films are found to be loss crystalline after sodium dojiiiigFrom the slopes of the plots in figure 1, it is seiui that the rate of increase of la with annealing time for doped films is smaller than that for undoped films This can be understood as follows : As described above (reason 1) there is a small number of cadmium vacancies in doped films than that of undoped films. At the annealing tomptu-aturo of ISO^C, there starts the jirocesa of formation of sulphur vacancies Due to the comparatively less number of cadmium vacan­cies in doped films the rate of formation of sulphur vacancies is smaller than306 B .  B .  B a rg a le , A .  G .  S h ik a lg a r  a n d  S . H .  P a w a rtJiai of undopod films This roflocls in tho rato of inoroaso of Ja with annciiling time.Tho variation of photocun’ont A /, with annealing time is shown in figure 2— for both doped and nndoped films. TJie gom)ral behaviour of tl\e plots is that A / increases with annealing tune, attains a maximum valui' at the annealingDark and photo^currents of CdS : N a thin films 307t'lg 1. Dark current U  at room temperature with tho annealing time m hours for doped and undopod films,period ol 28 hours and then decreases. The increase of A / is attributed to tho crystallization of the fibn during the first few hours of annealing and to tho formation of suliihur vacancies The eryatallizalion holjis in increasing tho mobility while tlie sulphur vacancies help in increasing tho recombination centres.308 B . B .  B a rg a le , A .  G . S h ik a lg a r  a n d  S . H .  P a w a rTho rlooroafie in pliotocuirront after the 28 hours of annealing, can ho understood an followsVor the long(ir annoalod films, an oxygon plays an important role in decreas­ing photoeurront Oxygiui molecules an^  known to he first j)hysi(,ally adsorhtHlTIME (hourt)l?ig. 2 Variation of photoouvrent A/ at room tomperaturo with tho annealing time in hours for doped and undopod films.on the CdS surface, and a transition from physical to chemical absorption then takers place hy capture of conduction hand electrons At longoi annealing period,this procotss expands insid(i Uk  ^ fUni and cjxygon moloenlos ai(‘ adsojhed at tlu^  grain bmmdarios, whieli alters llio barrjc'i’ lioight and baiier ^^ 'ldt]l Under photo-oxciiation the adsorbiMl oxygc^n at tbe liJru snrfat o and a1 the gram bonn- daiios act as additional recombniatjoii ci^ntres and nnlucu'S tJio pliotooxcitod electron life time (Wu & Bubc^  1974) TJu* adsorbe,d oxygen also decreases the tunneling probability for ekMitron transi>ort tjirough tin? intiirgrain barriers. This gives rise to decrease in photocuirent of CdS films annealed lor more than 28 hours.Erom figure 2, it is se(Oi that sodium dojied films an^  more photosensitive It IS likely that .sodium lon.s ael. as aticeptors entiMMiig at llui snbstiintional sites This is .similar to that ol I'.opjier dojiing in GdSDark and photo-currents of CdS : Na thin films 309Fig. 3 Plot of photocurronfc A7 vortuis meidoiit light intonsity in lux for dopod and undopod films at room temperaturo.Th« olToct/ f>f lig]it inlt>iiHJty on undopod and Hodmm dopod Cd^ S thin films aniicfal^d for 28 liours has boon studi(«l Tho variation tif Ai with light intensity IS shown in figure 3 Tlie lincai- nature of th(^  plots mdioatos that tho density of oonduetion t^ Ioctrons is dir^otly p/oporticmal to tho intonsity of illumination and in turn rovoals that tho oonoontration of availablo rootuubinatic)ii centres is higlnw than th(^  oonoontration of free electronsA oKNO WLEDGMBN TTho aiithois wish to thank Dr. R. N. Patif for his constant encouragement aiul comments One of the authors, A G >S exproHSeS his diH'.p sense of gratitude to C iS r Pi, , D(dhi tor Junior Rosi^arch Fellowship. This work was sujipoi tod by grants for support oi rosoaroli in Universities from U.G 0 , Dellii,310 B .  B .  B a rg a le , A .  G .  S h ik a lg a r  a n d  S . H .  P a w a rR e f e r e n c e sHenry t!. H. Nassau 1C. iSr, Rhiovoi J. AV 1970 Physical linnnv Letleis 24, 820.Pavaskar N, 1970 l^ h D. Thesis Poona Uiiivorsity.I'avaskar N &; Menozos C. A 190S Ja p  J . A pp l. Phys 7, 7411I'awar >S. H. Shikalgai A. O & Bargain B. B. 1977 Thm Solid  (Hubmiilocl),Shikalgar A. CJ. Bargalrs H. B iSt Pawai S. H. 1977 hid J. Pmv and Appl Phys (Subiniiied). Toll B 1971 Journal o f  Applied Phys 42, 2919.Wu C. & Biihe Bi. H 1974 J A p p l Phys 45, 048.YoHlii'/awa Miehio 1970 Jajt. J A ppl. Phys. 15, 2143.

Effect of Annealing Time on Dark and Photo-Currents of CdS : Na Thin Films

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Effect of Annealing Time on Dark and Photo-Currents of CdS : Na Thin Films

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