Influence of process parameters on the electrical transport mechanism in sprayed CdS films

Solar Energy Materials 15 (1987) 189-208 North-Holland, Amsterdam

189

INFLUENCE OF PROCESS PARAMETERS ON THE ELECTRICAL T R A N S P O R T M E C H A N I S M I N SPRAYED CdS F I L M S Shailaja K O L H E 1), S.K. K U L K A R N I 2), M.G. T A K W A L E B.R. M A R A T H E 1) and V.G. B H I D E 1)

1),

1) School of Energy Studies, University of Poona, Pune - 411 007, India 2) Department of Physics, University of Poona, Pune - 411 007, India

Received 17 April 1986; in revised form 12 September 1986 This report deals with electrical and structural properties of sprayed CdS films produced under varying process parameters such as pyrolysis temperature, solution concentration and film thickness. The number of traps appears to depend on the structural quality factor of the CdS films. However, electrical properties are dependent on the presence of oxygen or cadmium in the grain boundaries, which significantly alters the grain boundary potential barrier and hence the mobility.

1. Introduction Polycrystalline semiconductor thin films are of considerable interest due to their potential for use in solar cells and other semiconductor devices. However, realization of such devices requires reproducible production of thin films with desired electrical parameters. This can be achieved by the establishment of a clear correlation between growth parameters and electrical properties. In this paper, we report the effects of process parameters on the electrical properties of sprayed CdS films. The effect of adsorbed oxygen on electrical properties of sprayed CdS films is well known [1-3]. The effect of other influencing factors like composition and structural quality is less understood [4-6]. However, there is an indication that these physical properties affect the electrical conduction mechanism in vacuum deposited CdS films [7-10]. The present work indicates that the effect of structural quality of the film is to vary the number of traps N t in the grain boundary region. The electronic mobility ~t appears to be a sensitive function of excess cadmium in the film. The correlation between structure of the films and mobility is rather complicated. However, the amount of cadmium incorporated in the film and the structural parameters such as preferred crystalline orientation and hexagonality ( H (%)) are interrelated factors. It appears that this excess cadmium goes preferentially to the grain boundaries. Being a donor in CdS, this excess cadmium at the grain boundaries reduces the grain boundary potential barrier. 0165-1633/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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S. Kolhe et al. / Sprayed CdS films

2. Experimental details CdS films were deposited using the spray pyrolysis technique. The details are given elsewhere [11-13]. The standard set of process parameters is listed in table 1. The effect of process parameters was investigated by varying only one of the process parameters at a time, keeping all others constant. The pyrolysis temperature was varied from 350 to 430°C. Solution concentration was changed from 0.05 to 0.2M. The thickness of the film was varied in the range 1-10 ~tm. Each of these films was subjected to various investigations. The structural parameters such as grain size, crystalline orientation and hexagonality H expressed in percentage were evaluated using SEM and X R D [4]. SEM studies were carried out using a Cambridge Stereoscan 150 Scanning Electron Microscope. X-ray diffraction patterns were taken using a Philips PW 1730 automatic scanning X-ray diffractometer and using Cu Kot incident radiation. The electrical properties were evaluated from conductivity measurements and thermoelectric power (TEP) measurements. Vacuum evaporated indium contacts in planar configuration served as ohmic contacts. The ohmic nature of the contacts was initially tested for several samples. Conductivity and TEP measurements were carried out in the dark after stabilizing the dark resistance for about 12-15 h. A typical variation of the resistance of the film with time is given in fig. 1. TEP measurements were carried out using a copper constantan thermocouple fixed to the two electrodes of the sample. The sample was mounted on a copper block which was initially cooled to liquid nitrogen temperature. A miniheater kept in thermal contact with one end of the sample was responsible for maintaining a temperature gradient along the sample. The electrical parameters were evaluated in the temperature range 100-300 K. In order to eliminate the effect of oxygen, the films were subjected to anneahng treatment in hydrogen at 400°C for a duration of 10 min. The electrical parameters of the films in as-deposited condition and after hydrogen annealing were evaluated. It m a y be mentioned here, that most of the earlier workers have reported Hall effect measurements for determining the electrical properties of films. In the present investigations, electrical properties of the films have been evaluated by using thermoelectric power measurements. Comparison of some of the results in the present work where conditions of growth are similar to those in the earlier work suggests that both methods give similar results [14,15].

Table 1 Standard set of process parameters Pyrolysis temperature t s Solution concentration Rate of deposition Thickness of the film Cd/S ratio in the solution

400 ° C 0.1M 2 I~m/h 4-5 ~m 1/1

191

S. Kolhe et aL / Sprayed CdS films

5"30I 5.20F

0

O

G

I 10

I 12

1 l&

o

E-- 5.10

x n,,"

5.00

4.30 rr

4.80

4.70

I

I

I

2

4

6 Time

I 8

in

hours

16

=-

Fig. 1. Typical curve showing stabilization of dark resistance R with time in hours for sprayed CdS films.

The variation in film stoichiometry was estimated using X-ray photoelectron spectroscopy and the details are given elsewhere [16]. XPS analysis was carried out in a VG Scientific ESCALAB Mark II system, and using a Mg Kot (hp = 1253.6 eV) source and a concentric hemispherical analyser with a resolution of 0.8 eV. The variation in film stoichiometry was estimated from the ratio of the count rate of the Cd 3d5/2 level to the S 2p level and using sensitivity factors published for practical analysis by Briggs et al. [17]. Similarly the oxygen content was estimated using C d 3d5/2 and O ls peak intensities. These ratios represent the trend in variation of C d / S and C d / O content in the films and are not absolute measures of the composition. Most of the experiments were repeated two or three times and the trends of the results were always the same.

3. Results and discussion

The structural misfit in the intergrain region causes scattering of charge carriers by virtue of (a) the faults and defects, (b) trapped charge carriers, and (c) impurities [18]. The effects of faults, trapped carriers and impurities are overridden by oxygen

192

S. Kolhe et aL / Sprayed CdS films 0.20'

ro 0.15

0 "8-

0

o~

0.10

_c

0,05"

0

0

I 1

I 2

I~ ' ~ ' ~ ' 1 3 4

/ "

~ 5

t G ---

Fig. 2. Grain boundary potential barrier height against XPS peak height ratio Cd 3d5/2/O ls for sprayed CdS films deposited under various conditions.

adsorption in sprayed CdS films [1,15]. The effect of oxygen adsorption is an increase in the grain boundary potential barrier height [1,4]. Our results are in agreement with this observation. The plot of grain boundary potential barrier height against the ratio of XPS peak heights of Cd 3ds/2 to O ls is shown in fig. 2. Annealing of CdS films in a reducing hydrogen atmosphere causes a decrease in resistivity by 2-6 orders of magnitude [4,14,19,22]. This could be due to [13] (1) removal of oxygen, (2) removal of sulphur, (3) partial grain growth, and (4) passivation of dangling bonds in the intergrain region. SEM observations do not indicate appreciable grain growth in our sprayed CdS films. Our XPS analysis indicates that the major change is removal of oxygen from

S. Kolhe et aL

/ Sprayed CdS films

193

103

10 2

/

£ u i

o,>. _>

B

rw

i0 C

10-1 330

~ 350

I 370

Pyrolysis

I 330

I 410

temperature

I 430

*C

450

-~

Fig. 3. Dark resistivity against pyrolysis temperature for CdS films (A) in as-deposited conditions, (B) after hydrogen annealing. (Cd/S in solution: 1/1; Spray solution molarity: O.1M; Film thickness 5 ~m.)

the films [16]. The results are: for as-deposited film: C d / O = 1.8 and for hydrogen annealed film: C d / O = 3.2 (a small decrease in sulphur was also observed); for as-deposited film: C d / S = 0.86 and for hydrogen annealed film: C d / S = 0.90. The variation in resistivity before and after hydrogen annealing is shown in figs. 3-5. In fig. 3, at a temperature of deposition around 430°C, the resistivity values before and after annealing in hydrogen do not show much change. This is because less oxygen is retained in the films which are deposited at a substrate temperature of about 430°C. This has been confirmed by XPS results [16], which are: film deposited at 350°C: C d / S = 1.0 and film deposited at 430°C: C d / S = 0.6.

194

S. Kolhe et aL / Sprayed CdS films 103

A m

A 102

E u I

E

-g 101 o--, ,>, :>

t.2_ cY

100

B

O

10-10

I

0.04

,

I

0.08 SoLution

,

(D

I

0.12

I

"-""""~"D""-'-'--'--.-.---

I

0.16

i

I

0.20

Concentration (Motarity)

Fig. 4. Dark resistivity for CdS films deposited by spraying solutions with varying concentration in (A) as-deposited condition, (B) after hydrogen annealing treatment. (Cd/S in solution: 1/1; Pyrolysis temperature: 400°C; Film thickness: 5 ~m.)

All these measurements indicate reduction in resistivity by 2 - 3 orders of magnitude due to hydrogen annealing. Similar results have been obtained by earlier workers [4,19-21]. Two causes for such a reduction in resistivity are p r e s e n c e / a b sence of oxygen as indicated by XPS analysis and passivation of grain boundaries. Removal of oxygen from the films is supported by XPS data. It is difficult to estimate the effect of grain boundary passivation. It was also noticed that the variation in resistivity is mainly due to the variation in mobility and this is discussed in the following sections.

195

S. Kolhe et aL / Sprayed CdS films

1°'I lo3

102

E u t

E

o~

>.

101

.>_

I0C

lo-ll

1

t 3

, 5

Fi[m

I 7

thickness

I 9

t 11

I 13

in N m

Fig. 5. Dark resistivity of the CdS films of varying thickness in (A) as-deposited condition, (B) after hydrogen annealing treatment (Cd/S in solution: 1/1; Pyrolysis temperature: 400°C; Solution concentration: 0.1M.)

196

S. Kolhe et al. / Sprayed CdS films

The processing parameters such as pyrolysis temperature, solution concentration and film thickness etc. affect the electrical transport mechanism. The results obtained are explained on the basis of Seto's grain boundary trapping theory [15,18,23-24]. The various scattering factors such as phonons, charged impurities, grain boundaries etc. govern the electrical transport in polycrystalline films. The effective mobility of the charge carriers is given by [25,26]. (1//*) = (1//*g) + (1//*gb),

(1)

where /,g is the part that takes into account bulk scattering factors and /~gb represents the part played by grain boundaries. Grain boundaries contain a fairly high density of traps. These traps originate from the structural disorder in the intergrain region. These interface states trap free electrons from the hulk of the grain and form a potential barrier. The density of charge carrier traps Nt (per cm2), the doping level N (per cm3) and the crystallite size a are important factors in determining the electrical properties. These parameters determine the height of the intergrain potential barrier and the depletion width w which influence the electronic transport. We now discuss briefly the salient features of Seto's grain boundary trapping model [23]. According to Seto, the variation in bulk doping level N does not linearly change the carrier concentration in the sample. The mobility is also a complicated function of the doping level N. Depending on the values of Nt, N, L D (Debye length) and a, Seto explains the results assuming three regimes: Regime I: Condition Nt < Na. In this regime, the mobility is thermally activated and the carrier concentration is independent of temperature. The experimentally observed charge carrier concentration is equal to the bulk doping level N. Regime II: Nt > N a , L D < a / 2 holds true. In this regime, the whole grain is just depleted. The grain boundary potential q~gb becomes maximum. This regime is cbaracterised by thermally activated mobility and carrier concentration. Regime III: The basic condition for this region is Nt > N a and Debye length L D > a / 2 . Here approximately flat band conditions exist. Mobility is not thermally activated (not governed by the grain boundary barrier). The carrier concentration n is smaller than the bulk doping level N and is thermally activated. We now discuss the results of our studies on the influence of pyrolysis temperature, solution concentration and film thickness on electrical properties. 3.1. Pyrolysis temperature

A high pyrolysis temperature is known to be responsible for an improvement in the structural quality and chemical purity of the films [4,14,27]. Fig. 6 depicts crystalline orientation and hexagonality (H (%)) against pyrolysis temperature. Both these factors tend to increase sharply with pyrolysis temperature. On the other hand, the grain size decreases with increasing pyrolysis temperature. The variation in the

S. Kolhe et aL / Sprayed CdS films

197

65

1.0

3.9 60 -0.8

0.7

I 0.6

55 0 C3~

o x

0.5

t,N

0.4 50 0.3

0.2

45 300

/'1

I

350

Pyrolysis

1 10.1

400

temperature

*C

~---

Fig. 6. Hexagonality H % and preferred orientation of crystallites in sprayed CdS films deposited at varying pyrolysis temperature (other deposition conditions are the same as those indicated in fig. 3).

grain size (in the range 0.4-0.7 I~m) is, however, minor and hence is less likely to affect electrical properties. The room temperature values of mobility and carrier concentration of CdS films deposited at various pyrolysis temperatures are given in fig. 7. The high mobility # seems to indicate a negligible effect of the grain boundaries (which is otherwise a predominant scattering factor in the case of polycrystalline materials). The temperature dependence of mobility/~(T) is attributable to bulk type scattering phenomena, substantiating further the assumption of negligible contribution due to grain boundaries (see fig. 8).

S. Kolhe et al. / Sprayed CdS films

198

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i0L,

o

m

1017 'E

103

u

o~ x.

u

o~ v

•

102

1016

•

~r

101 330

I 350

I 370

I 330

Pyrotysis temperature

I 410 *C

I 430

i015

~---

Fig. 7. Mobility ~ and carrier concentration n at 300 K of sprayed CdS films deposited at various pyrolysis temperatures (other deposition conditions are the same as those given in fig. 3).

The values of # for the films deposited at lower pyrolysis temperature are indicative of a still lower influence of the grain boundaries. These films are different from those deposited at high temperature in two respects. (1) The cadmium content in the film is high in low temperature deposited films as determined by XPS [16]. (The C d / S ratio varies from 1 to 0.6 as t s increases from 350°C to 430°C.) (2) These films are characterised by a higher defect density which may lead to a higher number of traps and higher accommodation of cadmium in the grain boundaries. These traps scatter the charge carriers and affect the mobility. The effective mobility will depend additionally on trapping of impurities such as excess cadmium which acts as an electron donor in CdS. Although an increase in trap density (as a result of the poor structure of the film) reduces the mobility of charge carriers in polycrystaUine films, it appears that the most predominant factor influencing the mobility is the grain boundary barrier which in turn depends on the trapped charge carriers (electrons in the present case of CdS). In addition the amount of trapped

199

S. Kolhe et al. / Sprayed CdS films

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..

o

~u

"

T

o

% 1oI

olS

v

100-

10-1

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I

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10-3

IO~T Fig. 8. A typical curve of mobility against 103/T indicating phonon scattering as a dominant phenomenon in sprayed CdS film.

charge depends on the segregation of atoms like cadmium or oxygen at the grain boundaries. While the former reduces the number of trapped electrons and hence the grain boundary potential barrier (Cd being a donor in CdS), the latter assists trapping of electrons at the grain boundary, increasing the grain boundary potential barrier. A similar effect has been observed by Joshi et al. [28] who have reported that indium in CdS films causes a lowering of the grain boundary barrier with a consequent increase in the mobility of charge carders. It therefore appears that

200

S. Kolhe et aL / Sprayed CdS films

cadmium in the grain boundary region predominantly determines the mobility value. The contribution of structural quality factors is, however, secondary. The films deposited under given conditions of solution concentration, rate of deposition etc. but with varying pyrolysis temperature have shown thermally activated carrier concentration and a single-crystal-type variation of mobility # with temperature. These characteristics correspond to regime III of Seto's model, i.e. where conditions N t > N a and L D > a / 2 are satisfied. In the present work the average grain size measured by SEM is about 0.4 # m and the carrier concentration n is about 1017 (cm -3) which leads to L D = 30 nm. This implies that the second condition L D > a / 2 in regime III of Seto's model is not satisfied in our case. However, the possibility of the existence of small grains in between two large grains exists. SEM measurements have not been able to detect these small grains which may lie at different depths from the surface. These small grains may satisfy the condition L D > a / 2 . According to Orton et al. [15] the carrier concentration in this regime is given as n ( N N c a / N t ) e x p ( e t / k T ), (2) where et is the charge carrier activation energy. =

0.07

0.06

0.05

40

0"04

30

u x

0

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v

003

0.0~

20

0

O,01~1~-...-----.d330 350

370

330

Pyrolysis

410

temperature

430

_•10 450

0

in °C

Fig. 9. Variation in activation energy for charge carriers e t and N / N t deduced from Seto's model for films deposited at various pyrolysis temperature (Conditions of deposition are the same as those given in fig. 3).

S. Kolhe et al. / Sprayed CdS films

201

Fig. 9 gives electrical parameters and values of N / N t deduced from eq. (2) and values of e t deduced from n against 103//T curves. The lower value of N / N t for films deposited at low pyrolysis temperature indicate higher number of traps N t since cadmium incorporation is also higher in the films deposited at low pyrolysis temperature. Effect of N on N / N t , if any, should increase N / N t since the film is deposited at low pyrolysis temperature when the cadmium content is higher [16]. However, since the ratio N / N t is decreasing, it must be due to an increase in N r The lower values of N / N t for films deposited at low pyrolysis temperature indicate higher number of traps present in the films. Thus, these observations quantitatively indicate that lower hexagonality and crystalline orientation leads to higher intergrain trap density. 3.2. Solution concentration

The variation in solution concentration leads to a variation in film deposition rate. It is well known that a higher deposition rate leads to the formation of faults

60 -0.8

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-

55

3.6

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o

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3.~-~

c

o 50 "I-

~

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0.2

45

I 0.04

I 0.06

I 0.08 Solution

I 0.10

I 0.12

Concentration

I 0.14

I 0.16

I

0.18

0.1 0.20

(Molarity)

Fig. 10. Hexagonality and crystallite orientation as a function of soludon concentration (Other process parameters are the same as those given in fig. 4).

202

S. Kolhe et aL / Sprayed CdS films 103

1019

lO2

1018 u

% u

1017 ._~

101 0

100 02

0

0

I

I

0.06

0.10 Solution

I

0.14

0

0.18

Concentration (Motar ity)

1016

I

0-22 .-~

Fig. 11. Variation of mobility/, and carrier concentration n at 300 K of CdS films deposited by spraying solution with varying concentration (Other process parameters are the same as those given in fig. 4).

[4]. We have also observed similar results (fig. 10). The variation of mobility and carrier concentration against solution concentration are shown in fig. 11. The mobility # increases slightly with increase in spray solution concentration and mobility values are also higher. Thus, as argued in the previous section, higher values of the mobility reflect lower influence of grain boundaries on electrical transport. A better structure of the film deposited at lower deposition rate leads to a decrease in the number of traps N t. However, in this case also, the films deposited at a lower solution concentration (with better structure) have a lower cadmium [16] content: C d / S = 0.85 for a film deposited with a 0.075M solution; and C d / S = 1 for a film deposited with a 0.1M solution. Thus, these results support the arguments in section 3.1, where lower cadmium content resulted in lower mobility on the ground that the grain boundary potential is higher in the absence of cadmium. The film deposited under these conditions have thermally activated carrier concentration and bulk type behaviour of mobility with temperature. These results fit in regime III of Seto's model. The ratio N / N t is calculated in this case using eq.

S. Kolhe et al. / Sprayed CdS films

203

1

6O

0"07t.......(3,_~

60

0"06I

"7E U

~.o

0'05F

x

30

0-0•

zl~

~7

0-0.:

-20

0.02

10

0.01 ~ 0.04

0-06

I 0.08

I 0.10 Solution

I 0.12 Concentration

I 0-14

I 0.16

(Motarity)

I 0.18

I 0.20 ~,~

Fig. 12. Variation in activation energy for charge carriers et and ratio N / N t deduced from Seto's model for films deposited by spraying solution with various concentration (Other process parameters are the same as those given in fig. 4).

(2) and the trend of variation in N / N t is shown in fig. 12. The higher concentration leads to defect structure and consequently higher values of N t. 3.3. Film thickness

The dependence of electrical characteristics and structural properties of the films on film thickness is more interesting. Fig. 13 illustrates the variation in crystalline orientation and hexagonality ( H (%)) caused due to variation in film thickness. The decrease in hexagonality with increasing thickness suggests the presence of a large number of cubic crystallites. Cubic crystallites may be located between large hexagonal columns. The increased number of cubic crystallites indicates an increase in fault density, since cubic phase is formed due to faults in the hexagonal phase [7]. The mobility /~ and carrier concentration n are plotted as a function of film thickness (fig. 14). This films exhibit lower mobilities. This lower value of mobility appears to be caused by higher susceptibility of thin films for oxygen intake in addition to relatively higher scattering from the surface. It is likely that thin films retain oxygen due to a high surface to volume ratio. Our observation that the mobility did not change after hydrogenation suggests that thin films pick up oxygen

S.

204

Kolhe et

al. / Sprayed CdS filrm'

60

0.7

55

0.6

A

A

0.5

T >.

3.4 ×

-r-

50 3.3

3.2

45

I 3

I 5

Fi[rn

I 7

thickness

I 3

in

~

n

jum

I II

I 13

0.~

~-~

Fig. 13. Hexagonality and preferred orientation against film thickness (Deposition conditions are the same as those given in fig. 5).

after hydrogenation. This has been supported by XPS results. The effect of such oxygen at the surface on the electrical properties of thin films appears to dominate changes due to other factors. The higher oxygen content on the surface of hydrogenated films than that in the bulk material can be seen from table 2 in which the XPS peak height ratio of Cd 3d5/2 to O ls is given. In the case of thick films, since the surface to volume ratio is smaller, the effect of oxygen at the surface is smaller.

Table 2 Oxygen content on the surface and in bulk of CdS films as determined by XPS

Annealing treatment

Ar ion b o m b a r d m e n t (min)

H(Cd 3d 5 / 2 ) / H ( O l s)

As deposited H 2 annealed as received Ar b o m b a r d m e n t 4 kV, 10 ~A Ar b o m b a r d m e n t 4 kV, 10 ~A Ar b o m b r a d m e n t 4 kV, 10 I~A

10 20 40

1.8 3.2 3.8 4.7 5.1

S. Kolhe et aL / Sprayed CdS films

205

~o3

1019

102

1018

% u u ta i >

101

o

f~

1017 .~

0

% u

o v

1016 cJ

100

10-1

I

3

I

I

5

7

Film

thickness

I

9

I

11

1015 13

in ~rn

Fig. 14. Effect of film thickness on mobility and carrier concentration at 300 K (Deposition conditions are the same as those given in fig. 5).

Thin films are characterised by thermally activated mobility and temperature independent carrier concentration. These characteristics indicate that thin films satisfy conditions in regime I, i.e. Nt < N a . T h e value of N t is calculated from the relationship [15]: q~gb = e 2 N t Z / 4 e , a

and

n = N.

These values of N / N t are given in fig. 15.

(3)

S. Kolhe et aL / Sprayed CdS films

206

80 ~70

0,07 0.06[ 0

~

50 qE 40 ~ou

0.05[

0.031-

\

0.021-

0-01.1

•

3

"

-120

/

",x"

5 7 9 w 11 Film thickness in ~um

-110

13

.0

Fig. 15. Influence of film thickness on charge carrier activation energy e t and ratio N / N t deduced from Seto's model (Deposition conditions are the same as those given in fig. 5).

4. Conclusions

The structural and electrical properties of sprayed CdS films depend upon growth parameters. Better structural properties are obtained when the pyrolysis temperature is high, the film growth rate is low and the thickness of the film is less. Between the two factors, viz. hexagonality and crystalline orientation, the former appears to affect the structural properties of the films and hence the trap density more than the latter: the number of traps N t is closely related to the structure of the films (see figs. 3-5). The electrical properties, especially the mobility of charge carriers, however, depend more on the presence of oxygen and cadmium at the grain boundaries. The presence of oxygen at the grain boundaries, as is observed in the thin films, increases the grain boundary potential barrier thus reducing the mobility appreciably. Hydrogenation of thin films is not able to remove oxygen in thin films because of the higher surface to bulk ratio. XPS results support this statement.

S. Kolhe et al. / Sprayed CdS films

207

The effect of cadmium at the grain boundaries is exactly opposite. Being a donor in CdS, cadmium inhibits collection of electrons at the grain boundaries. This reduces the grain boundary potential which results in a higher mobility of charge carriers. The presence of cadmium at the grain boundaries is more predominant in films with poorer structure. In the absence of cadmium, films with poor structure should result in a lower mobility. However, since poor structure results in a greater amount of cadmium at the grain boundaries, the mobility of these films is higher.

Acknowledgements The authors wish to thank Dr. A.P.B. Sinha and Dr. S.K. Date, National Chemical Laboratory, Pune for making the X-ray diffractometer and scanning electron microscope available. This work was supported by the Department of Non-Conventional Energy Sources and Department of Science and Technology, New Delhi, India.

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