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Structural, optical and electrical properties of indium doped CdS0.9Se0.1 thin films
MATERIALS CHEMISTRYAND PHYSICS ELSEVIER
Materials Chemistry and Physics 51 (1997) 246-251
Structural, optical and electrical properties of indium doped CdSo.9Seo. 1 thin films G.S. Shahane, K.M. Garadkar, L.P. Deshmukh * Thin Film and Solar Studies Research Laboratory, Deparonent of Physics (Appl. Electronics), Shivaji University, Centre for P.G. Studies, Solapur 413 003, M.S., India
Received 20 February I997; revised 28 April 1997; accepted 4 June 1997
Abstract
Indium doping concentration dependent structural, optical and electrical properties of chemically prepared CdSo 9Seo 1thin films have been studied and reported in this paper. Structural investigations on these films revealed the polycrystalline nature of the samples with the presence of hexagonal phase of CdSo.gSeo.l. The indium doping concentration up to 0.05 mot% increases the sample crystallinity. For higher concentrations of indium, material tends towards amorphous. No appreciable change in 20 value with doping concentration has been observed. Optical studies indicated displacement in the absorption edge from 565 to 640 nm with a band to band type of transition. The electrical transport properties showed two conduction mechanisms; a high temperature grain boundary limited and a low temperature variable range hopping. Both the carrier concentration (n) and mobility (/x) have been found to be higher at 0.05 tool% In doping concentration; mobility being a sensitive function of both temperature and doping concentration. The observed grain boundary potential (q~B) is minimum at 0.05 mo1% In concentration. © 1997 Elsevier Science S.A. Keywords: Structural properties: Optical properties; Electrical properties; Indium;Thin films
1. Introduction
There is a recent upsurge developed in synthesizing and characterisation of I I - V t semiconducting materials by virtue of their proven potential capabilities in a variety of electric and opto-electronic devices [1-6]. It is worth mentioning that cadmium sulphide and selenide have an important place in this respect [7,8] and we have already established that the solid solution/alloyed phases of the CdS~ _,Sex (0 < x < 1) type are most promising, especially in the photoelectrochemical solar cell applications [7-10]. It has also been found that file properties of these solid solution/alloyed film structures are extremely sensitive to their composition and the composition dependent structural, optical and electrical transport properties have already formed the basis of our earlier investigations [7-11 ]. Although a considerable improvement in the photoelectrochemical celt performance has been noted at x = 0 . 1 [10], the observed conversion efficiency is much smaller compared to the existing literature values. The lower conversion efficiency has been attributed partly to the higher resistivity of the photoelectrode material that could effectively be reduced by a suitable donor impurity concentration. * Corresponding author.
0254-0584/97/$17.00 © 1997Elsevier Science S.A. All rights reserved P I I S 0 2 5 4 - 0 5 8 4 ( 9 7 ) 01993-7
Indium, a third group element, has shown pronounced effects in a number of host lattices, and this work attempts to study the effect of an indium doping concentration on the structural, optical and electrical transport properties of chemically deposited CdSo.gSeod thin films.
2. Experimental
The CdSo.gS% i thin films with an indium doping concentration of 0-1 tool% were deposited using a chemical deposition process reported elsewhere [ 6,9,12]. For deposition of the samples, cadmium acetate, thiourea and sodium selenosulphate were mixed together in a volume stoichiometric proportion. An indium trichlofide was used as a source material for doping and was varied so as to achieve In concentration from 0.01 to 1 tool%. Triethanolamine was used as a complexing agent and sodium hydroxide and an aqueous ammonia were used to adjust pH ( ~ 10.4) of the resulting mixture. Deposition occurred for 75 rain at 55°C. The X-ray diffractograms were obtained for these samples using a Philips PW-1710, X-ray diffractometer with CuKoe line. The range of 20 angles was from 10° to 80 °. The optical
G.S, Shahane et al. / Materials Chemistry died Physics 51 (1997) 246-25]
density was measured in the 300-900 nm wavelength range using a Hitachi-330 (Japan), double beam spectrophotometer. The surface features of these samples were examined through a stereoscan, 250 MK III, (Cambridge Instruments, UK), scanning electron microscope. The dc electrical conductivity of these samples was measured in the 300-500 K temperature range. A two probe press contact technique was used for this purpose. A silver paint was applied to the sample as it offers ohmic contact to CdS and CdSe. Thermo e.m.f.s generated by the samples were noted in the range of working temperature from 300 to 550 KI The various film parameters have been calculated from these studies.
a
247
I0
a - (002)H b - I200 ) C
c -(II0)H d -(I 12)H d e ..,(l,00)C
(3%
O%.O25
3. Results and discussion
'd
e-
Both as-grown CdSo.gSeon and indium doped CdSo.gSeon samples are thin, uniform, tightly adherent with colour changing from orange-yellow to pale yellow with an increasing amount of indium doping concentration. The film thickness increased initially and then decreased with an increase in doping concentration. The behaviour can be understood; first, from the role of an indium atom as a nucleation centre that enhances the growth process, and therefore the thickness. Secondly, at higher doping levels, indium may occupy the interstitial sites causing an impurity scattering and thereby preventing the further film growth [ 13-15]. The X-ray diffractograms were obtained for these samples and are shown in Fig. 1. The diffractograms show that as-deposited films are crystalline in nature and the crystallinity increased with In doping concentration up to 0.05 mol% and then decreased for higher doping levels making the material amorphous; the amorphousity being attributed to the surface adsorption of indium atoms over a growing film preventing further growth of the microcrystals. Reports are available wherein the effect of trace addition of cations such as Cu, Ag, A1, In, Sb in the host structure is explained more or less on similar lines [ 1416]. The scanning electron micrographs of these samples (Fig. 2) also support the above observations. From Fig. 2 it is clearly seen that the grain size is improved up to 0.05 mol% In doping concentration and decreased thereafter. For zero In doping concentration, the strongest reflection is observed at d=3.364,~, which can be indexed as (002) plane of the hexagonal CdSo.gSeo.~ solid solution [9]. In the case of Indoped films, the (002) diffraction peak occurs at the same position (indicating no appreciable shift in 20 values) with modifications in both the intensity and width of the peak. We therefore conclude from the above observations that indium acts as a dopant in the host CdSo9Seo, l structure. The other peaks corresponding to cubic CdS, which were present initially in the host structure, are also present at the same 20 value (a = 5.693 A) [9]. The lattice parameters have been determined for all the samples and are listed in Table 1. In order to evaluate the grain size, X-ray diffraction patterns (magnified) were used, and the relation
.A
L. <
~
0,25°/,
02 I
20
I
40 20degree
f
60
80
Fig. i. X-ray d~ffractogramsfor eight representativeIn-dopedCdSogSeo.l thin films. d=
0.9 3. B cos 0
(1)
where, 3. is the X-ray wavelength, B is the angular line width at half maximum intensity and 0 is the Bragg angle, is used to calculate the actual grain size. The grain sizes for various film structures are cited in Table 1. It is seen that the film with 0.05 mol% In doping concentration showed higher sized grains. The grain size for the cubic phase has also been calculated and is of the order of 150 A for all the compositions. The optical densities were measured for all the films in the 300-900 nm wavelength range, and the absorption coefficient was also calculated. Fig. 3 shows a sketch of the absorption coefficient (oe) versus wavelength (A) for five representative film compositions. For all the film compositions, the magnltude of oe is high ( = 10 4 to 10 5 c m - t) and a shift in absorption edge, typically from 565 to 640 nm has been observed for the change of indium doping concentration from 0-0.05 mol%. The optical gaps of the various indium doped films have been determined by plotting (ahv)2 versus hv curves. Typically, the energy gap decreased from 2.27 to 2.13 eV as indium content in the film is increased from 0 to
248
G.S. Shahane et al. /Materials Chemistry and Physics 51 (1997) 246-251
Fig. 2, SEM images showing surface morphology of six different thin film compositions: (a) 0% In; (b) 0.025% In; (c) 0.05% In; (d) 0.1% In; (e) 0.5% In; and (f) 1% In. Table 1 Effect of In doping concentration on various properties of CdSo.~Sev.~thin films In (mol%)
0 0.01 0.025 0,05 0,075 0.1 0.25 0.5 0.75 1
Thickness ( rxm)
1 1.12 1.3 1.39 1 0.85 0.66 0.5 0.38 0.35
Lattice parameters c (~,)
a (A)
6.72 6.72 6.75 6.78 6.78 6.78 6.77 6.78
4.12 4.12 4.14 4.16 4.t6 4. t 6 4.15 4.16
0.05 m o l % , and for higher concentrations of indium, the energy band gap increased. W e may attribute the decrease in the band gap to the i m p r o v e d grain structure o f the film o w i n g to segregation o f the impurity atoms along the grain boundaries [16,17], whereas the increase in band gap at higher
Grain size (~)
270 290 315 350 290 223 180 90
Power factor (m)
0.4 t 0.48 0.5 0.46 0.42 0.45 0.44 0.45 0.45 0.5
Activation energy H.T. (eV)
L.T. (eV)
0.703 0.728 0.711 0.662 0.762 0,854 0.894 0.894 0,901 0.911
o. 132 0.092 0.085 0.082 0,149 o. 144 o. 163 0.163 0.198 0.182
doping levels can be ascribed to the increased amount of disorder caused by the impurity addition. Fig. 4 represents dependence of an optical energy gap (Eg) as a function o f the indium doping concentration. As suggested by the value of absorption coefficient ( a ) and the straight line nature of
249
G.S. Shahane et al. / Materials Chemi~ir3" and Physics 51 (1997) 2462251
o-C~01%In ,','-.,' 0.05%In
12.0
• -~ 0.1%In 4
~'-" 0.5 %
o
~ O0 %In
• --. 0'025%tn & ""~ 0'0 5
In
"-*0"75% In
% In
• ~ 0'25'/, ~0.5 %
//./
In In
/"///"
./~/
.
7o" o-
•
>
"75 X2
7 10-4
0
t
-2
900
600
300
Wavelingth, nm Fig. 3. Optical absorption spectra for various thin film structures.
i/
/
~
I
-i [n (h-)-Eg)
0
Fig. 5. Plots ofln(c&t ) vs. ln(ha,-Eg) for the evaluationof mode of optical transition. -5
iI
2'4
/
/
•
/ I /t i
2.2
i/ •
/
-6
.¢
r'"
",,
i
"
"-.
•
•
-7 243
I
-1 0 Log(In-conc.) Fig. 4. Variation of an optical energy gap with In doping concentration. -2
"8
the (c~h/.!)2 versus hi, plots, especially on the high energy side, the mode of optical transitions in these films has also been confimaed by computing ln(oehu) versus l n ( h u - E g ) . For a direct allowed type of transition, this variati~on should yield a straight line with slope (power factor) equal to 0.5 [ 18 ]. We have also plotted these variations for five representative film compositions ( Fig. 5) ; the straight line nature with slopes of nearly 0.5 confirmed the direct type of transitions. The values of the power factor (m) for various films are listed in Table 1. The dark electrical conductivity at room temperature was measured for all the films. It was found that the room temperature conductivity is increased as the indium doping concentration is increased from 0 to 0.05 tool% and then decreased for higher doping concentrations (Fig. 6). Similar results have been reported earlier for doped and mixed thin films [13,15,19-21]. The indium doping concentration dependent conductivity can be explained as follows: Incorporation of indium in the host structure (CdS09Seo. l) has two possibilities; ( 1 ) substitution of a divalent Cd by a trivalent In and (2) possibility of formation of Cd-vacancies. Since Cd 2+ and In ~+ are deposited simultaneously, there is less
I
-1 log (In--cone,) Fig. 6. Variation of an electrical conductivitywith In-dopingconcentration. chance for forming the Cd-vacancies [ 13,15,18]. Thus substitution of a divalent cadmium by a trivalent indium is more predominant which makes the indium atom act as a donor [ 13,15,18]. This causes electrical conductivity to increase up to 0.05 tool% In concentration. For an In concentration in excess of 0.05 m0i%, the grain structure becomes more and more disordered, reducing the conductivity. The temperature dependence of an electrical conductivity was also studied for all the samples. The variation shows a usual Arrhenius behaviour consisting of high and low temperature regions. The activation energies of electrical conduction have been determined from these plots (not shown) for all the film structures in both the conduction regions and are listed in Table 1. To check for the type of conduction mechanism, the plots of log c r T ~/2 versus I 0 3 / T have been plotted [22,23]. These are Shown in Fig. 7(a). Obviously, in the high temperature region, grain boundary limited conduction mechanism is the dominant conduction mechanism. It has further been noted that the low temperature region is characterised by a variable
250
G.S. Shahane et al. /Materials Chemisto' and Physics 51 (1997) 246-251
-t ~ , , ~
a~O-O *(.In b-00~ ~. ~
k".."-..'..."-~ l "~ ",(~"~".~T, ~
C "0"02~I. In
I "<'-.~ I
d-.o.os.,. Io e.-* 0.25o/. In
"..,,,"-~-...'-~._
g, 3
ia) -4
4 2.2
~ 1"8
2"0
"~
1000/T, k I _3 ¸
a-O'O '/,In
"
~
-G
?8
24
q
t
2.2
2'4
looo/T,~ t Fig. 9. Variation of log (,aT w2) vs. 1/T for six representative film compositions.
b--0.01% In c .-,,0,025'/ol n d - 0"05 % Ir e - 0.25% Ir f .~ 0,'75 % Ir
_
I
2"0
0.9
4 m
,' '~0'7
s
¢t /
m _
,
,
,
0.5 Fig. 7. Plots to determine the conduction mechanisms: (a) grain boundary limited; and (b) variable range hopping.
-2
-I [og(In-conc )
0
Fig, t0, Dependence of grain boundary potential (O~) on tn doping concentration.
•,•
3'6
<--x, u
X
coo'~ X
100 ~ 3'2
/
', E
]
2.8
50 ",2
~\
~J
[
-I log (In-conc) Fig. 8, The variation ofn and/x with In concentration at 358 K. -2
range hopping conduction mechanism (Fig. 7(b) ) [24]. The localized states for such a hopping conduction are a direct consequence of imperfections associated with the polycrystalline films. The thermopower measurements showed n-type
conduction for all the samples. This data, coupled with an electrical conductivity data, were used to compute the charge cartier concentration, n and mobility,/z. It was found that carrier concentration increases slightly up to 0.05 mol% doping concentration and decreased afterwards. The carrier mobility is found to be a sensitive function of both temperature and In doping concentration. At 0.05 mol% doping concentration, the mobility is found to be maximum and can be attributed to the improved grain structure of the film. The variation of n and p, with log-In concentration at 358 K is shown in Fig. 8. The height of the potential barrier (OB) at the grain is determined from these observations. The plots of log (/xT ~/2) versus 103/T for six typical film compositions are shown in Fig. 9, and the magnitudes of qbB are calculated from their slopes [23]. It is seen that cPB decreases with In doping concentration, attains minimum at 0.05 tool% In and then increases with further increase in In doping concentration. The doping concentration dependent grain boundary potential is shown in Fig. i0.
G.S. Shahane et a l . / Materials Chemistry and Physics 5J (1997) 246-251
4. Conclusions An attempt is made, in these investigations, to understand the effect of indium doping concentration on the structural, optical and electrical transport properties of CdSo.9Seo,l thin films. X-ray diffraction studies revealed improvement in the crystallinity without any appreciable change in the dominant peak position after doping with indium up to 0.05 tool%. Higher doping concentrations made material more amorphous. The optical gap is found to be decreased initially and then increased for higher doping levels. The electrical transport studies showed two conduction mechanisms; the high temperature grain boundary limited and a variable range hopping conduction mechanism at low temperatures. An indium doping concentration is found to enhance the electrical conductivity (up to 0.05 tool% indium), which can be ascribed to the increased car~ier concentration due to the substitution of Cd by In, increased mobility because of the reduced grain boundary potential and decreased band gap.
Acknowledgements One of the authors (L.P.D.) thankfully acknowledges the Department of Science and Technology, Govt. of India, New Delhi for the contract SERC/SP/S-2/M-11B/91.
References [ I ] R.C. Kainthla, J.F. McCann and D. Haneman, Sol. Ener. Mater., 7 (1983) 491. [2] P.J. Sebastin, Thin Solid Films, 245 (I994) 132.
251
[3] M.A. Kenawy, H.AI Zayed and A.M. Ibrahim, Ind. J. Pure and Appl. Phys., 29 (1991) 624. [4] A. Mondal, T.K. Chaudhuri and P. Pramanik, Sol. Ener. Mater., 7 (1983) 431. [5] S.H. Pawar and L.P. Deshmukh, Mater. Chem. Phys., i0 (1984) 83. [6] L.P. Deshmukh, S.G. Holikatti and P.P. Hankare, J. Phys. D: Appl. Phys., 27 (1994) 1786. [7] R.N. Noufi, P.A. Kohl and AJ. Bard, J. Electrochem. Soc., 125 (1978) 375. [8] P.K. Mahapatra and A.R. Dubey, Sol. Ener. Mater. Sol. Cells, 32 (1994) 29. [9] L.P. Deshmukh and G.S. Shahane, Ind. J. Pure and Appl. Phys., 34 (1996) 989. [ I0] L.P. Deshmukh and G.S. Shahane, Int. J. Electronics, in press. [I1 ] G.S. Shahane, B.M. More, C.B. Rotti and L.P. Deshmukh, Mater. Chem. Phys., 47 (1997) 263. [I2] L.P. Deshmukh, A.B. Palwe and V.S. Sawant, Sol. Cells, 28 (1990) 1; SoL Ener. Mater., 20 (1990) 341. [ I3] S. Jatar, A.C. Rastogi and V.G. Bhide, Pramana, 16 (1978) 477. [ i4] N.R. Pavaskar, C.A. Menezes and A.P.B. Sinha, J. Electrochem. Soc., I24 (1977) 743. [ I5] S.H. Pawar and L.P. Deshmukh, Ind. J. Pure Appl. Phys., 22 (1984) 315. [16] L.P. Deshmukh, S.G. Holikatti and B.M. More, Mater. Chem. Phys., 39 (1995) 743. [17] U. Pal, S. Saha, A.K. Chaudhuri, V.V. Rao and H.D. Banerjee, J. Phys. D: Appl. Phys., 22 (I989) 965. l 183 D. Bhattacharya, S. Chaudhuri and A.K. Pal, Vacuum, 43 (1992) 313. K. Subbaramaiah and V. Sundara Raja, Sol. Ener. Mater. Sol. Cells, [191 32 (1994) 1. [20] G.K. Padam, G.L. Malhotra and S.U.M. Rao, J. AppI. Phys., 63 (1988) 770. [ 21 ] J.C. Joshi and B.K. Sachar, Thin Solid Films, 88 (1982) 189. [22] J.Y.W. Seto, J. Appl. Phys., 46 (1975) 5247. [23] R.L. Petritz, Phys. Rev., 104 (1956) 1508. [24] Y. Natsume, H. Sakata and T. Hiryama, Phys. Status Solidi A, 148 (1995) 485.
Materials Chemistry and Physics 51 (1997) 246-251
Structural, optical and electrical properties of indium doped CdSo.9Seo. 1 thin films G.S. Shahane, K.M. Garadkar, L.P. Deshmukh * Thin Film and Solar Studies Research Laboratory, Deparonent of Physics (Appl. Electronics), Shivaji University, Centre for P.G. Studies, Solapur 413 003, M.S., India
Received 20 February I997; revised 28 April 1997; accepted 4 June 1997
Abstract
Indium doping concentration dependent structural, optical and electrical properties of chemically prepared CdSo 9Seo 1thin films have been studied and reported in this paper. Structural investigations on these films revealed the polycrystalline nature of the samples with the presence of hexagonal phase of CdSo.gSeo.l. The indium doping concentration up to 0.05 mot% increases the sample crystallinity. For higher concentrations of indium, material tends towards amorphous. No appreciable change in 20 value with doping concentration has been observed. Optical studies indicated displacement in the absorption edge from 565 to 640 nm with a band to band type of transition. The electrical transport properties showed two conduction mechanisms; a high temperature grain boundary limited and a low temperature variable range hopping. Both the carrier concentration (n) and mobility (/x) have been found to be higher at 0.05 tool% In doping concentration; mobility being a sensitive function of both temperature and doping concentration. The observed grain boundary potential (q~B) is minimum at 0.05 mo1% In concentration. © 1997 Elsevier Science S.A. Keywords: Structural properties: Optical properties; Electrical properties; Indium;Thin films
1. Introduction
There is a recent upsurge developed in synthesizing and characterisation of I I - V t semiconducting materials by virtue of their proven potential capabilities in a variety of electric and opto-electronic devices [1-6]. It is worth mentioning that cadmium sulphide and selenide have an important place in this respect [7,8] and we have already established that the solid solution/alloyed phases of the CdS~ _,Sex (0 < x < 1) type are most promising, especially in the photoelectrochemical solar cell applications [7-10]. It has also been found that file properties of these solid solution/alloyed film structures are extremely sensitive to their composition and the composition dependent structural, optical and electrical transport properties have already formed the basis of our earlier investigations [7-11 ]. Although a considerable improvement in the photoelectrochemical celt performance has been noted at x = 0 . 1 [10], the observed conversion efficiency is much smaller compared to the existing literature values. The lower conversion efficiency has been attributed partly to the higher resistivity of the photoelectrode material that could effectively be reduced by a suitable donor impurity concentration. * Corresponding author.
0254-0584/97/$17.00 © 1997Elsevier Science S.A. All rights reserved P I I S 0 2 5 4 - 0 5 8 4 ( 9 7 ) 01993-7
Indium, a third group element, has shown pronounced effects in a number of host lattices, and this work attempts to study the effect of an indium doping concentration on the structural, optical and electrical transport properties of chemically deposited CdSo.gSeod thin films.
2. Experimental
The CdSo.gS% i thin films with an indium doping concentration of 0-1 tool% were deposited using a chemical deposition process reported elsewhere [ 6,9,12]. For deposition of the samples, cadmium acetate, thiourea and sodium selenosulphate were mixed together in a volume stoichiometric proportion. An indium trichlofide was used as a source material for doping and was varied so as to achieve In concentration from 0.01 to 1 tool%. Triethanolamine was used as a complexing agent and sodium hydroxide and an aqueous ammonia were used to adjust pH ( ~ 10.4) of the resulting mixture. Deposition occurred for 75 rain at 55°C. The X-ray diffractograms were obtained for these samples using a Philips PW-1710, X-ray diffractometer with CuKoe line. The range of 20 angles was from 10° to 80 °. The optical
G.S, Shahane et al. / Materials Chemistry died Physics 51 (1997) 246-25]
density was measured in the 300-900 nm wavelength range using a Hitachi-330 (Japan), double beam spectrophotometer. The surface features of these samples were examined through a stereoscan, 250 MK III, (Cambridge Instruments, UK), scanning electron microscope. The dc electrical conductivity of these samples was measured in the 300-500 K temperature range. A two probe press contact technique was used for this purpose. A silver paint was applied to the sample as it offers ohmic contact to CdS and CdSe. Thermo e.m.f.s generated by the samples were noted in the range of working temperature from 300 to 550 KI The various film parameters have been calculated from these studies.
a
247
I0
a - (002)H b - I200 ) C
c -(II0)H d -(I 12)H d e ..,(l,00)C
(3%
O%.O25
3. Results and discussion
'd
e-
Both as-grown CdSo.gSeon and indium doped CdSo.gSeon samples are thin, uniform, tightly adherent with colour changing from orange-yellow to pale yellow with an increasing amount of indium doping concentration. The film thickness increased initially and then decreased with an increase in doping concentration. The behaviour can be understood; first, from the role of an indium atom as a nucleation centre that enhances the growth process, and therefore the thickness. Secondly, at higher doping levels, indium may occupy the interstitial sites causing an impurity scattering and thereby preventing the further film growth [ 13-15]. The X-ray diffractograms were obtained for these samples and are shown in Fig. 1. The diffractograms show that as-deposited films are crystalline in nature and the crystallinity increased with In doping concentration up to 0.05 mol% and then decreased for higher doping levels making the material amorphous; the amorphousity being attributed to the surface adsorption of indium atoms over a growing film preventing further growth of the microcrystals. Reports are available wherein the effect of trace addition of cations such as Cu, Ag, A1, In, Sb in the host structure is explained more or less on similar lines [ 1416]. The scanning electron micrographs of these samples (Fig. 2) also support the above observations. From Fig. 2 it is clearly seen that the grain size is improved up to 0.05 mol% In doping concentration and decreased thereafter. For zero In doping concentration, the strongest reflection is observed at d=3.364,~, which can be indexed as (002) plane of the hexagonal CdSo.gSeo.~ solid solution [9]. In the case of Indoped films, the (002) diffraction peak occurs at the same position (indicating no appreciable shift in 20 values) with modifications in both the intensity and width of the peak. We therefore conclude from the above observations that indium acts as a dopant in the host CdSo9Seo, l structure. The other peaks corresponding to cubic CdS, which were present initially in the host structure, are also present at the same 20 value (a = 5.693 A) [9]. The lattice parameters have been determined for all the samples and are listed in Table 1. In order to evaluate the grain size, X-ray diffraction patterns (magnified) were used, and the relation
.A
L. <
~
0,25°/,
02 I
20
I
40 20degree
f
60
80
Fig. i. X-ray d~ffractogramsfor eight representativeIn-dopedCdSogSeo.l thin films. d=
0.9 3. B cos 0
(1)
where, 3. is the X-ray wavelength, B is the angular line width at half maximum intensity and 0 is the Bragg angle, is used to calculate the actual grain size. The grain sizes for various film structures are cited in Table 1. It is seen that the film with 0.05 mol% In doping concentration showed higher sized grains. The grain size for the cubic phase has also been calculated and is of the order of 150 A for all the compositions. The optical densities were measured for all the films in the 300-900 nm wavelength range, and the absorption coefficient was also calculated. Fig. 3 shows a sketch of the absorption coefficient (oe) versus wavelength (A) for five representative film compositions. For all the film compositions, the magnltude of oe is high ( = 10 4 to 10 5 c m - t) and a shift in absorption edge, typically from 565 to 640 nm has been observed for the change of indium doping concentration from 0-0.05 mol%. The optical gaps of the various indium doped films have been determined by plotting (ahv)2 versus hv curves. Typically, the energy gap decreased from 2.27 to 2.13 eV as indium content in the film is increased from 0 to
248
G.S. Shahane et al. /Materials Chemistry and Physics 51 (1997) 246-251
Fig. 2, SEM images showing surface morphology of six different thin film compositions: (a) 0% In; (b) 0.025% In; (c) 0.05% In; (d) 0.1% In; (e) 0.5% In; and (f) 1% In. Table 1 Effect of In doping concentration on various properties of CdSo.~Sev.~thin films In (mol%)
0 0.01 0.025 0,05 0,075 0.1 0.25 0.5 0.75 1
Thickness ( rxm)
1 1.12 1.3 1.39 1 0.85 0.66 0.5 0.38 0.35
Lattice parameters c (~,)
a (A)
6.72 6.72 6.75 6.78 6.78 6.78 6.77 6.78
4.12 4.12 4.14 4.16 4.t6 4. t 6 4.15 4.16
0.05 m o l % , and for higher concentrations of indium, the energy band gap increased. W e may attribute the decrease in the band gap to the i m p r o v e d grain structure o f the film o w i n g to segregation o f the impurity atoms along the grain boundaries [16,17], whereas the increase in band gap at higher
Grain size (~)
270 290 315 350 290 223 180 90
Power factor (m)
0.4 t 0.48 0.5 0.46 0.42 0.45 0.44 0.45 0.45 0.5
Activation energy H.T. (eV)
L.T. (eV)
0.703 0.728 0.711 0.662 0.762 0,854 0.894 0.894 0,901 0.911
o. 132 0.092 0.085 0.082 0,149 o. 144 o. 163 0.163 0.198 0.182
doping levels can be ascribed to the increased amount of disorder caused by the impurity addition. Fig. 4 represents dependence of an optical energy gap (Eg) as a function o f the indium doping concentration. As suggested by the value of absorption coefficient ( a ) and the straight line nature of
249
G.S. Shahane et al. / Materials Chemi~ir3" and Physics 51 (1997) 2462251
o-C~01%In ,','-.,' 0.05%In
12.0
• -~ 0.1%In 4
~'-" 0.5 %
o
~ O0 %In
• --. 0'025%tn & ""~ 0'0 5
In
"-*0"75% In
% In
• ~ 0'25'/, ~0.5 %
//./
In In
/"///"
./~/
.
7o" o-
•
>
"75 X2
7 10-4
0
t
-2
900
600
300
Wavelingth, nm Fig. 3. Optical absorption spectra for various thin film structures.
i/
/
~
I
-i [n (h-)-Eg)
0
Fig. 5. Plots ofln(c&t ) vs. ln(ha,-Eg) for the evaluationof mode of optical transition. -5
iI
2'4
/
/
•
/ I /t i
2.2
i/ •
/
-6
.¢
r'"
",,
i
"
"-.
•
•
-7 243
I
-1 0 Log(In-conc.) Fig. 4. Variation of an optical energy gap with In doping concentration. -2
"8
the (c~h/.!)2 versus hi, plots, especially on the high energy side, the mode of optical transitions in these films has also been confimaed by computing ln(oehu) versus l n ( h u - E g ) . For a direct allowed type of transition, this variati~on should yield a straight line with slope (power factor) equal to 0.5 [ 18 ]. We have also plotted these variations for five representative film compositions ( Fig. 5) ; the straight line nature with slopes of nearly 0.5 confirmed the direct type of transitions. The values of the power factor (m) for various films are listed in Table 1. The dark electrical conductivity at room temperature was measured for all the films. It was found that the room temperature conductivity is increased as the indium doping concentration is increased from 0 to 0.05 tool% and then decreased for higher doping concentrations (Fig. 6). Similar results have been reported earlier for doped and mixed thin films [13,15,19-21]. The indium doping concentration dependent conductivity can be explained as follows: Incorporation of indium in the host structure (CdS09Seo. l) has two possibilities; ( 1 ) substitution of a divalent Cd by a trivalent In and (2) possibility of formation of Cd-vacancies. Since Cd 2+ and In ~+ are deposited simultaneously, there is less
I
-1 log (In--cone,) Fig. 6. Variation of an electrical conductivitywith In-dopingconcentration. chance for forming the Cd-vacancies [ 13,15,18]. Thus substitution of a divalent cadmium by a trivalent indium is more predominant which makes the indium atom act as a donor [ 13,15,18]. This causes electrical conductivity to increase up to 0.05 tool% In concentration. For an In concentration in excess of 0.05 m0i%, the grain structure becomes more and more disordered, reducing the conductivity. The temperature dependence of an electrical conductivity was also studied for all the samples. The variation shows a usual Arrhenius behaviour consisting of high and low temperature regions. The activation energies of electrical conduction have been determined from these plots (not shown) for all the film structures in both the conduction regions and are listed in Table 1. To check for the type of conduction mechanism, the plots of log c r T ~/2 versus I 0 3 / T have been plotted [22,23]. These are Shown in Fig. 7(a). Obviously, in the high temperature region, grain boundary limited conduction mechanism is the dominant conduction mechanism. It has further been noted that the low temperature region is characterised by a variable
250
G.S. Shahane et al. /Materials Chemisto' and Physics 51 (1997) 246-251
-t ~ , , ~
a~O-O *(.In b-00~ ~. ~
k".."-..'..."-~ l "~ ",(~"~".~T, ~
C "0"02~I. In
I "<'-.~ I
d-.o.os.,. Io e.-* 0.25o/. In
"..,,,"-~-...'-~._
g, 3
ia) -4
4 2.2
~ 1"8
2"0
"~
1000/T, k I _3 ¸
a-O'O '/,In
"
~
-G
?8
24
q
t
2.2
2'4
looo/T,~ t Fig. 9. Variation of log (,aT w2) vs. 1/T for six representative film compositions.
b--0.01% In c .-,,0,025'/ol n d - 0"05 % Ir e - 0.25% Ir f .~ 0,'75 % Ir
_
I
2"0
0.9
4 m
,' '~0'7
s
¢t /
m _
,
,
,
0.5 Fig. 7. Plots to determine the conduction mechanisms: (a) grain boundary limited; and (b) variable range hopping.
-2
-I [og(In-conc )
0
Fig, t0, Dependence of grain boundary potential (O~) on tn doping concentration.
•,•
3'6
<--x, u
X
coo'~ X
100 ~ 3'2
/
', E
]
2.8
50 ",2
~\
~J
[
-I log (In-conc) Fig. 8, The variation ofn and/x with In concentration at 358 K. -2
range hopping conduction mechanism (Fig. 7(b) ) [24]. The localized states for such a hopping conduction are a direct consequence of imperfections associated with the polycrystalline films. The thermopower measurements showed n-type
conduction for all the samples. This data, coupled with an electrical conductivity data, were used to compute the charge cartier concentration, n and mobility,/z. It was found that carrier concentration increases slightly up to 0.05 mol% doping concentration and decreased afterwards. The carrier mobility is found to be a sensitive function of both temperature and In doping concentration. At 0.05 mol% doping concentration, the mobility is found to be maximum and can be attributed to the improved grain structure of the film. The variation of n and p, with log-In concentration at 358 K is shown in Fig. 8. The height of the potential barrier (OB) at the grain is determined from these observations. The plots of log (/xT ~/2) versus 103/T for six typical film compositions are shown in Fig. 9, and the magnitudes of qbB are calculated from their slopes [23]. It is seen that cPB decreases with In doping concentration, attains minimum at 0.05 tool% In and then increases with further increase in In doping concentration. The doping concentration dependent grain boundary potential is shown in Fig. i0.
G.S. Shahane et a l . / Materials Chemistry and Physics 5J (1997) 246-251
4. Conclusions An attempt is made, in these investigations, to understand the effect of indium doping concentration on the structural, optical and electrical transport properties of CdSo.9Seo,l thin films. X-ray diffraction studies revealed improvement in the crystallinity without any appreciable change in the dominant peak position after doping with indium up to 0.05 tool%. Higher doping concentrations made material more amorphous. The optical gap is found to be decreased initially and then increased for higher doping levels. The electrical transport studies showed two conduction mechanisms; the high temperature grain boundary limited and a variable range hopping conduction mechanism at low temperatures. An indium doping concentration is found to enhance the electrical conductivity (up to 0.05 tool% indium), which can be ascribed to the increased car~ier concentration due to the substitution of Cd by In, increased mobility because of the reduced grain boundary potential and decreased band gap.
Acknowledgements One of the authors (L.P.D.) thankfully acknowledges the Department of Science and Technology, Govt. of India, New Delhi for the contract SERC/SP/S-2/M-11B/91.
References [ I ] R.C. Kainthla, J.F. McCann and D. Haneman, Sol. Ener. Mater., 7 (1983) 491. [2] P.J. Sebastin, Thin Solid Films, 245 (I994) 132.
251
[3] M.A. Kenawy, H.AI Zayed and A.M. Ibrahim, Ind. J. Pure and Appl. Phys., 29 (1991) 624. [4] A. Mondal, T.K. Chaudhuri and P. Pramanik, Sol. Ener. Mater., 7 (1983) 431. [5] S.H. Pawar and L.P. Deshmukh, Mater. Chem. Phys., i0 (1984) 83. [6] L.P. Deshmukh, S.G. Holikatti and P.P. Hankare, J. Phys. D: Appl. Phys., 27 (1994) 1786. [7] R.N. Noufi, P.A. Kohl and AJ. Bard, J. Electrochem. Soc., 125 (1978) 375. [8] P.K. Mahapatra and A.R. Dubey, Sol. Ener. Mater. Sol. Cells, 32 (1994) 29. [9] L.P. Deshmukh and G.S. Shahane, Ind. J. Pure and Appl. Phys., 34 (1996) 989. [ I0] L.P. Deshmukh and G.S. Shahane, Int. J. Electronics, in press. [I1 ] G.S. Shahane, B.M. More, C.B. Rotti and L.P. Deshmukh, Mater. Chem. Phys., 47 (1997) 263. [I2] L.P. Deshmukh, A.B. Palwe and V.S. Sawant, Sol. Cells, 28 (1990) 1; SoL Ener. Mater., 20 (1990) 341. [ I3] S. Jatar, A.C. Rastogi and V.G. Bhide, Pramana, 16 (1978) 477. [ i4] N.R. Pavaskar, C.A. Menezes and A.P.B. Sinha, J. Electrochem. Soc., I24 (1977) 743. [ I5] S.H. Pawar and L.P. Deshmukh, Ind. J. Pure Appl. Phys., 22 (1984) 315. [16] L.P. Deshmukh, S.G. Holikatti and B.M. More, Mater. Chem. Phys., 39 (1995) 743. [17] U. Pal, S. Saha, A.K. Chaudhuri, V.V. Rao and H.D. Banerjee, J. Phys. D: Appl. Phys., 22 (I989) 965. l 183 D. Bhattacharya, S. Chaudhuri and A.K. Pal, Vacuum, 43 (1992) 313. K. Subbaramaiah and V. Sundara Raja, Sol. Ener. Mater. Sol. Cells, [191 32 (1994) 1. [20] G.K. Padam, G.L. Malhotra and S.U.M. Rao, J. AppI. Phys., 63 (1988) 770. [ 21 ] J.C. Joshi and B.K. Sachar, Thin Solid Films, 88 (1982) 189. [22] J.Y.W. Seto, J. Appl. Phys., 46 (1975) 5247. [23] R.L. Petritz, Phys. Rev., 104 (1956) 1508. [24] Y. Natsume, H. Sakata and T. Hiryama, Phys. Status Solidi A, 148 (1995) 485.