Optical and transport properties of chemical bath deposited CdS: Al films

Solid State Communications, Vol.44,No.8, pp. II37-1139, 1982. Printed in Great Britain.

0038-I098/8~/441137-03503.00/0 Pergamon Press Ltd.

OPTICAL AND TRANSPORT PROPERTIF_S OF CHEMICAL BATH DEPOSITED CdS:AI FILMS

C.D.Lokhande and S.H.Pawar, Energy Conversion L a b o r a t o r y , Department of P h y s i c s , S h t v a J t U n i v e r s i t y , Kolhapur 416 004 ( I n d i a ) (Received 24 August 1982 by So Amelinckx) Aluminium doped and undoped CdS films are deposited on the

g l a s s s u b s t r a t e s by chemical bath d e p o s i t i o n technique. T h e i r o p t i c a l and t r a n s p o r t p r o p e r t i e s are s t u d i e d and the e f f e c t o f dopant c o n c e n t r a t i o n on these p r o p e r t i e s i s discussed at 1 ength.

i. INTRODUCTION Polycrystalline CdS films have received considerable attention during recent years because of their proven and potential applications in the photoconductorsl,2 as well as in photovoltaic

d e v i c e s 3 , 4 . Studies have been made on f i l m s prepared by v a r i o u s methods such as vacuum d e p o s i t i o n , s i n t e r i n g , spray p y r o l y s i s and chemical d e p o s i t i o n . V a r i ous dopants have been t r i e d according to the film applications5,6. Jatar et al 7 have prepared A1 doped CdS films by vacuum deposition technique and discussed their utility in liquid crystal display as the r e s i s t a n c e can be matched

by a d d i t i o n of AI and g i v i n g proper heat treatments. In CdS/Cu2S s o l a r c e l l s , Cu from Cu2S surface l a y e r d i f f u s e s i n t o t h e n type base l a y e r d u r i n g c e l l f a b r i c a t i o n , g i v i n g r i s e to Cu-compensated p h o t o c o n d u c t i v e CdS l a y e r . This r e f l e c t s i n the s p e c t r a l dependence of observed enhancement, quenching e f f e c t of b i a s i n g l i g h t and observed time dependence of s p e c t r a l response of the ce112,8. Chopra et a l have prepared AI doped CdS f i l m s by spray p y r o l y s i s technique to f a b r i c a t e CdS/Cu~S s o l a r c e l l s where A1 i s added t o i n h i S t t the columnar growth and f i l l the g r a i n boundary space so as t o p r e v e n t t h e deep p e n e t r a t i o n of Cu2S f i n g e r s i n t o CdS. In the p r e s e n t s t u d y , A1 doped CdS films are prepared by chemical bath d e p o s i t i o n technique and t h e i r p r o p e r t i e s such as c o n d u c t i v i t y , t h e m o e l e c t t i c power and o p t i c a l a b s o r p t i o n have been s t u d i e d and discussed.

s u b s t r a t e s were kept r o t a t i n g in the r e a c t i o n bath w i t h the speed of 60 r . p . m. 20 C.C. of 0 . 5 mole t h i o u r e a s o l u t i o n was added drop by drop i n h a l f an hour. The d e p o s i t e d f i l m s were washed f o r sevv e r a I times w i t h d i s t i l l e d water and were preserved in dark d e s s i c a t o r . A l u minium sulphate s o I u t i o n was added in the r e a c t i o n bath f o r A1 doped CdS films. 3. RESULTS AND DISCUSSION The CdS fiIms were prepared by slow growth from a cadmium compiex in the soIutlon. The process is simpIe and entireIy reproducibie and adhesive fiIms with Iarge area are obtained. Fiim thickness is found to depend upon deposition time, moiar concentration of reactants, temperature of bath and speed of rotationIO, II. In the present study, aII parameters were kept fixed and AI doping concentration in the bath was varied. It is interesting to note that film thickness increases with AI doping as shown in fig. l. This can be understood as follows: When the glass substrates are dipped into the chemical

T.8.

2.2

2. E X P E R I ~ N T A L

o

CdS films were deposited on the ultrasonically cleaned g l a s s s u b s t r a t e s by

q-

0

LOG. CONC. OF At

u s i n g chemical d e p o s i t i o n technique. 20 C.C. of 0 . 5 mole CdSO4 s o l u t i o n and 100 C.C. of i mole ammonia s o l u t i o n were mixed in a glass beaker at room temp. (27°C) and then h~ated t o 85oc. Glass

Fig 1.

Thickness v a r i a t i o n of CdS f i l m w i t h l o g c o n c e n t r a t i o n o£ Aluminitn doping.

1137

1138

CHEMICAL BATH DEPOSITED CdS: A1 FILMS

b a t h , a suspension of Cd (CH)~ form a l a y e r on i t which s t i m u l a t e s the decomp o s i t i o n of thtourea and r e s u l t s in a CdS l a y e r on the s u b s t r a t e . Further film growth takes place by 'ion by ion growth'. On our case, Cd(OH)o suspension w i l l contain A1 i n the fbrm of AI(OH)~ which w i l l c o n t r i b u t e in f o r ~ ing a I a y e r on the glass substrates. T h e s t i c k i n g c o e f f i c i e n t of A1 i s more than sulphur ion7 and hence a d d i t i o n of A1 f u r t h e r gives r i s e to large thickness. V a r i a t i o n of absorption c o e f f i c i ent (~) w i t h wave l e n g t h f o r three different AI dopings is shown in fig 2.

Vol. 44, No. 8

doping level (I wt ~ AI). the CdS prop e r t i e s have altered remarkably. Dark resistance for three samples was measured between 30OOK. to 450o1(. Fig 3 shows log 6 versus I/T plots for these samples. Activation energies for

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3

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t

Fig 3.

420

500

%OO

700

P l o t of hog c o n d u c t i v i t y ( ~ ) versus l i T f o r three f i l m s , 0 - pure CdS, • 0.1 wt % A1 CdS.

~(.m)

Fig 2e

V a r i a t i o n of absorption c o e f f i c i e n t (~) with wavelength (k) f o r three f i l m s . O - Pure CdS, • - O.1 wt % A1 CdS, and - 1 wit AI CdS.

I t i s seen t h a t pure CdS has some red s h i f t and higher cut o f f wavelength at 5 3 0 0 @ , w h i l e 0.1 wt ~ A1 doped CdS f i l m has higher absorption c o e f f i c i e n t and higher cut o f f wavelength at 5~OOAq 1 wt % A1 doping has low absorption c o l l . than p u r e CdS and f l a t response t o the wavelength. From these data, o p t i c a l b@nd g@ps were determined by p l o t t i n g (oq~)z versus h~ p l o t s and i t was found t h a t o p t i c a l bandgap decreases w i t h a d d i t i o n of A1 in CdS. Above o b s e r v a t i o n s can be explained as f o l l o w s : undoped CdS f i l m s are Cd r i c h and sulphur deficient. Excess of Cd gives rise to donar levels in the bandgap of CdS. A large concentration of Cd in the lattice makes these donar levels degenerate and merge into the conduction band of CdS. This i s reflected in the increased red response and the higher cut off wavelength. In AI doped films, sulphur deficiency increases further 7 and gives rise to the higher red response and higher cut off of wavelength. However, at higher

two regions were c a l c u l a t e d . FOr low temperature region, 0.1 wt %A1CdS has a c t i v a t i o n energy as 0.013 eV which i s lower than other two, while in h i g her temperature r e g i o n , 0.1 wt % A1CdS has a c t i v a t i o n energy 0.72 eV which i s h i g h e r than other two. Pure CdS has lower conductivity than 0. I wt % A 1 C d $ but higher than 1 wt % A 1 C d S which can be understood as f o l l o w s : Since A1 goes i n the t r i v a l e n t s t a t e and replaces d i v a l e n t cadmium, there i s a p o s s i b i l i t y of Cd+ ions and/or Cd vacancies being formed to compensate the charge. The existance of such Cd~ ions in the doped f i l m s has been corroborated by the presence of donor centers in such films whereas Cd vacancies is indicated by the observation of accepter centers. Since Cd and AI are deposited simultaneously there is less chance for forming Cd vacancies. The decrease in resistivity upto 0. I wt % A1 is attributed to this mechanism. After that, as A1 doping ~Dcreases there is a possibility that AI~3(O. 51A °) would go to the inte~ stitial position rather than vacancy position of Cd+2(O.97A °) and probably this may be the reason why conductivity decreases with 1 wt ~ A1 doping in CdS films. Thermoelectric power (TEP) measurement was carried out in the temperature range 3OO°K to 450OK. All films are

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CHEMICAL BATH DEPOSITED CdS: A1 FILMS

1139

found to be o f n t y p e . It is seen t h a t TEP i s h i g h e r f o r 0.1 w t ~ A1 than pure CdS and 1 w t % At CdS. From t h i s data e l e c t r o n d e n s i t y was determined by u s i n g the r e l a t i o n ,

¢ = -~ [A+In 2( 2~m~ KT

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) 312] . . ( 1 )

e nh 3 where A - t h e r m o e l e c t r i c f a c t o r , 2 . 5 , for CdS, and m e = O. 2 m, effective electron mass. Other terms have their usual meanings. Hlectron densities are c a l c u l a t e d and found to be of the order of I019 cm-3, fig 4(a) shows their variations with temperature. The mobility, ~, was calculated from the conductivity, 6 ,and using the relation 6 . . . (2) b=n--~ F i g ~ b ) shows m o b i l i t y v a r i a t i o n w i t h t e m p e r a t u r e . ~ i s higher for 0.I wt % AI CdS than pure CcLS and i w t % AI CdS. Acknowledgement - The authors are g r a t e f u l t o Shrl M.D.Uplane and Mrs.J.S. Desai f o r e x p e r i m e n t a l help and d i s c u s s i o n s . One of the authors (CDL) i s i n d e b t e d to t h e C o u n c i l of S c i e n t i f i c and I n d u s t r i a l Research ( C S I R ) , D e l h i f o r t h e award o f a J u n i o r Research Fellowship.

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Fig 4(a) V a r i a t i o n of carrier d e n s i t y (n) w i t h I / T and (b) v a r i a t i o n of mobility (~) with l / T , for t h r e e f i l m s 0 - pure CdS, 0 - 0.1 w t % A1CdS, A 1 wt % A1CdS.

REFERENCES 1. D . P . A m a l n e r k a r , M . S . S e t t y , ( M i s s ) N . R . Pavaskar and A . P . B . S t n h a , B u l l . M a t e r . S c t . ~ , 1251,(1980). 2. Chen.ho.wu and R.H.Bube, J . A p p l . P h y s . 45, 648, (1974). 3. K.W.Mitchell,A.i.Fahrenbruch and R.H.Bube, J.Appl.Phys.48, 4365,(1977). 4. P.F.Lindqulst and R.H.Bube, J.Electrochem.Soc. 119, 936, (1972) 5. N.Nakpyama, Jpn J.Appl.Phys.,8,450, (1969). 6. A.G.Shikalgar and S.H~Pawa;, Thin S o l i d Fllms, 61, 313,(1979). 7. S.Jatar, A.C.Rastogl and V.G.Bhlde, Pramana, 16, 477, (1978).

8. A.E.Vanaerschot, J . J . C a p a r t , K.H. David, M.F.Fabhricottt, K.H.Heffels, J.J.Loferskl and K.K.Reinhartz, ' S o l a r c e l l s ' e d i t e d by C.E.Backus, IEEE Press,New York, P 239, (1976). 9. D.K.Pandya and K.L.Chopra, 'Vacuums u r f a c e s - Thin films' edited by K.L.Chopra add T.C.Goel, Vanity Books, D e l h i ( I n d i a ) , P 258,(1981). 10. A . G . S h i k a l g a r and S.H.Pawar, ' P h i l o s o p h i c a l Magazine' B , 4 0 , 1 3 9 , ( 1 9 7 9 ) . 11. I n d e r J e e t Kaur,D.K.Pandya and K.Lo Chopra, J.Electrochem.Soc. 127,943, (Z980).