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Effect of heat treatments in vacuum on CdS thin films prepared by the spray deposition technique
Solar Cells, 11 0 9 8 4 ) 211 - 220
211
EFFECT OF HEAT TREATMENTS IN VACUUM ON CdS THIN FILMS PREPARED BY THE SPRAY DEPOSITION TECHNIQUE
L. ESCOSURA, E. GARCIA-CAMARERO, F. ARJONA and F. RUEDA
Departamento de Fisica Aplicada, Madrid 34 (Spain)
Universidad Aut6noma de Madrid, Cantoblanco,
(Received April 13, 1983;accepted September 14, 1983)
Summary The effects of short heat treatments in vacuum (10 min at 200 - 400 °C) on the electrical, structural and optical properties of hexagonal polycrystalline CdS thin films prepared by the spray deposition technique were studied. It was found that the electrical and structural properties change remarkably under these treatments. However, the optical properties do not vary significantly. The resistivity, in particular, decreased from about 500 ~ cm to less than 1 ~ cm for a 300 °C annealing while the electron mobility increased by two orders of magnitude, a result similar to that reported by Martinuzzi and coworkers. A hexagonal phase was obtained regardless of deposition temperature. The preferential orientation changed when samples were subjected to vacuum annealing at 300 °C or more, indicating that a recrystallization had taken place in the films.
1. Introduction A detailed knowledge of the electrical and structural changes in CdS films is important in relation to the manufacture of Cu2S/CdS solar cells. Ma and Bube [1 ] have studied the properties of CdS prepared by the pyrolytic spray technique and found cubic and hexagonal phases and a preferential orientation of {112) in the substrate temperature range 325475 °C. They evaluated the changes in the electrical properties as a function of the temperature of the substrate and the effects of heat treatments in various atmospheres and showed that annealing in hydrogen at 400 °C produces an increase in electron density and mobility. This is attributed to the removal of chlorine and oxygen from the grain boundaries. More recently, Bougnot e t al. [2] have confirmed this decrease in resistivity when studying annealing in hydrogen. 0379-6787/84/$3.00
© Elsevier Sequoia]Printed in The Netherlands
212
Wu and Bube [3] have studied the effects of heat treatments in vacuum and in air but only at relatively low temperatures (120 °C). Vacuum heat treatments do not seem to affect the conductivity markedly while air treatments decrease it by two orders of magnitude. Martinuzzi et al. [ 4], however, reported a decrease in resistivity by four orders of magnitude when samples were subjected to annealing at 260 + 20 °C in a high vacuum; this is attributed to oxygen desorption. Since the early work o f Gilles and van Cakenberghe [ 5] on silver-doped CdS the recrystallization of CdS films has received attention but the recrystallization temperature did not drop much below 500 °C. Stimulated by the results reported by Martinuzzi et al. [4] which showed a reduction in resistivity by a factor of 10 4 when an annealing treatment at 260 °C in a vacuum of 10 -.6 Torr was used, we made a preliminary study of the correlation between the structural and electrical changes caused by heat treatments in vacuum.
2. Experimental details Samples of CdS were obtained by a technique similar to that described by Oudeacoumar [6]; the work of Chamberlin and Skarman [7] was taken as the starting point. The ratio of the concentration of S 2- ions to the concentration of Cd 2+ ions is adjusted by the concentration of the solution used which is 0.1 M thiourea and 0.09 M CdC12, which gives the anionic concentration ratio of Cd 2+ to S 2- as 0.9. This does n o t correspond to that given by other researchers [1, 4, 7] who used an ionic concentration ratio of unity. The reason for this change was that, when a concentration ratio of unity was used, we obtained a peak in the X-ray diffraction diagrams corresponding to metallic cadmium. This peak is placed at 20 = 38.5 ° and corresponds to the cadmium crystallographic indices (101}. To provide a pressure to the sprayer we used nitrogen and the deposition was made onto Pyrex substrates. The spray rate varied from 5 to 10 ml min -1, and the temperature of the substrate from 230 to 400 °C. Various deposition rates were obtained by varying the spray flow and the temperature of the substrate {Fig. 1). The thickness of the samples was about 4 #m. Heat treatments were carried out in a vacuum system at 10 -s Torr for 10 min and the temperature was varied from 200 to 400 °C. Higher temperatures were not used because of the appearance of a yellow deposit which indicated film decomposition. Standard X-ray diffraction measurements were made in a Philips PW 1050/25 rotating sample diffractometer and the preferential orientation was estimated using the peak heights. The incident and diffracted beams are symmetrical to the film substrate. The ratio of the rotation speed of the detector to t h a t of the film was 2 to 1, i.e. the rotation speed of the detector was 2 ° min -1 and t h a t of the film 1 ° min -1. The film was n o t subjected to spin. To assess the preferential orientation of a film, the diffractogram of the
213
ts 230°C /Ts=300°C
i
4
~Ts:3 20oC
3 S
i
I
I
I
5
10 RATE OF SPRAY(ml min -1) Fig. 1. Growth velocity of sprayed CdS films as a function of solution'flow for various substrate temperatures. film in its present state was compared with the diffractogram obtained when the film had been ground to p o w d e r and the peak height ratios of the film had been compared with the ratios o f intensity data given in ref. 8. The optical properties were studied by transmittance and reflectance using a Cary 17D spectrophotometer. We t o o k into account the multiple incoherent reflections b o t h in the film and in the substrate to determine the refractive index and the absorption coefficient [9 - 11 ]. The film thickness measured with a Talystep stylus instrument was a b o u t 4/~m. The Van der Pauw technique [12, 13] was used to evaluate the film resistivity, the carrier concentration and the mobility .at room temperature in the dark. Evaporated aluminium dots (0.8 mm in diameter) were used to form ohmic contacts with the silver paste.
3. Results
3.1. Structure of the samples All the samples obtained in this work b y spray pyrolysis have a hexagonal structure regardless of the substrate temperature. Their preferential growing planes (101), (002) and (112) parallel to the substrate depend on the temperature of deposition. The presence of the zinc blende t y p e of CdS was discounted because of the absence of the following reflections in both grown and ground films (Fig. 2): (200)4 at 20 = 30.8 °, (400)2 at 20 = 64.1 ° and (331)3 at 20 = 70.4 °, where the subscripts 4, 2 and 3 indicate the relative peak intensities of the (200), (400) and (331) planes.
214
B
z
cr ~J Q_
u~ o
Z
A
~)
II Ir
Fig. 2. X-ray diffractometer spectra of sprayed CdS films (spectrum A) deposited at 350 °C and the ground powder (spectrum B) after the film had been detached from the substrate. Only hexagonal phase peaks are observed and the preferential orientations for the planes are (002), (101) and (112) parallel to the substrate.
U3 ~- lOq0 LJ 8 "-F .< 13..
[002]
6
{o 2
~112]
250
300
350
400
~(°C)
Fig. 3. The peak heights of (101), (112) and (002) reflections as a function of substrate temperature normalized with respect to powder intensities.
The samples fabricated at temperatures lower than 250 °C have (101) and (112) as the preferential planes and are of lower grade. At higher temperatures the {002) reflection prevails (Fig. 3).
215
I
200
15C~
loot z
LI.J
a_ 50
10021
;
^,
260 350 450 T(°C) Fig. 4. Variation in the X-ray diffraction peak heights for different reflections as a function of temperature for 24 h step annealings in a sample grown at 260 °C. These results confirm those of Oudeacoumar [6] and differ from the results reported by Ma and Bube [1] and by Martinuzzi et al. [4] for temperatures in this range in which the (112) orientation prevails. A series of successive vacuum annealings for 24 h were carried out on a sample, and the resistivity and X-ray diffractograms were measured at room temperature. Samples annealed in vacuum at temperatures higher than the deposition temperature change the preferential orientation of their grains as indicated in Fig. 4 where the evolution of the height of the peaks for successive 24 h annealings in a sample grown at 260 °C is shown. The intensity produced by the (002) reflection is reduced at first but recovers partly on successive annealings.
3.2. Optical properties We observed that, as the temperature of the substrate increases, the transmittance of the samples increases (Fig. 5) and no interference peaks appear even in samples grown at high temperatures. At high deposition temperatures (Ts > 300 °C) the transmittance versus wavelength plot is similar to that of evaporated samples; however, it does differ in t h a t interference maxima are absent. The values of the refractive index vary from one sample to another in the range 2.4 - 2.7 at 500 nm.
216 It%)
EVAPORAT ED
70 /
/
Ts=320oC
/
60
S
~
Ts :230°C
50 40 30 20 10
s
I
I
I
I
I
*
I
]
I
0.5
1 X (nm) ~.5 Fig. 5. Spectral optical transmittance as a function of substrate temperature during spraying. 4×102
~'-~
\ 10 2
\
+
\ \
20
,
I
i
i
\ I
200 2BO 360 Ts(°C) Fig. 6. Sample resistivity as a function of substrate temperature during spraying. The vacuum heat treatments do not substantially change the transmitt a n c e o f t h e samples.
3.3. Electrical properties T h e resistivity o f as-grown s a m p l e s as a f u n c t i o n o f t h e t e m p e r a t u r e o f t h e s u b s t r a t e is s h o w n in Fig. 6 a n d t h e m o b i l i t y values w e r e n e a r l y c o n s t a n t , a b o u t 10 -2 c m 2 V -1 s -1.
217
1000 10( lC o
°'°'2o~V260
2~,o
3~o
3.~o
4~o
T(°C)
Fig. 7. Resistivity of CdS samples v s . temperature of annealing (10 min) for samples grown at various temperatures: z~, T s = 230 °C; o, T S = 260 °C; D, T S = 300 °C; o, T s = 320 °C.
2 I 1
F
/'--"---"-'--"
1000]
t2xlOZO
F"
/
I
41o ~°
5" "T >
n 1(
r~ ',0
s.. lo-' i- I
C
"4
I
41o ,9
0.1
2 xl OI'~L. ~
200
300
T~C;
/-,00
2X1018
Fig. 8. Carrier mobility, resistivity and carrier concentration v s . temperature of annealing (t0 rain) for a CdS sample grown at 320 °C.
218
The vacuum heat treatments for 10 min made the room temperature resistivity decrease abruptly as shown in Fig. 7. At temperatures above 300 °C the resistivity rises and the effect is largest for the lowest fabrication temperature. The vacuum annealings at a b o u t 200 °C produce a marked increase in the mobility and carrier concentration at room temperature, as shown in Fig. 8 for a sample grown at 320 °C. These results are in general agreement with those of ref. 4.
4. Discussion and conclusions We found that hexagonal phase CdS thin films with promising properties for photovoltaic devices can be prepared b y the spray deposition technique, when the samples are subjected to short heat treatments in vacuum (10 -s Torr). The preferential orientation, however, is not as marked as in vacuumevaporated films where the only planes parallel to the substrate have a {002) orientation. For an ion concentration ratio of 0.9 (lower than usual) we always obtained hexagonal structures in the CdS samples even at deposition temperatures as low as 230 °C. At temperatures above 300 °C the predominant preferential growing plane was (002) parallel to the substrate. These results agree fully with those of Oudeacoumar [6] and differ from the observations of Ma and Bube [1] who found that in this temperature range the preferential plane was (112) parallel to ~he substrate. Perhaps the difference in the prevailing orientation is caused by the use of nitrogen instead of air as the pressurizing gas. When the temperature of the substrate is 320 °C, the optical properties of the films obtained are good enough to be used in photovoltaic devices and they do not change significantly with further heat treatments. As is usual for sprayed CdS films, no interference maxima and minima were observed; the lack of coherence might be caused b y the misorientation of grains in the {002) direction and by internal stresses between other grains. The low temperature heat treatments in vacuum reduce the resistivity by several orders of magnitude, and the final resistivity of the samples could be controned by choosing the temperature of the heat treatment carefully to be between 200 and 350 °C. The resistivity then ranged between 10 and 0.1 $2 cm. These results are similar to those obtained using heat treatments in a hydrogen atmosphere [2, 4] which are more effective in reducing the resistivity than those in other atmospheres (nitrogen, oxygen and argon) [7]. The reduction in resistivity has been attributed principally to an increase in the electron density produced b y oxygen desorption. Similarly, in our case the heat treatment in vacuum may have the effect of producing a high desorption o f oxygen, thereby markedly increasing the carrier concentration. However, the electron mobility in the films increases b y t w o orders of magnitude with these heat treatments in vacuum up to 250 °C.
219
A high sensitivity of CdS to the oxygen absorbed in the grain boundaries has been reported [1, 3, 4], the absorbed oxygen acting as an acceptor impurity. On the one hand, this causes a decrease in the electron density and, on the other hand, acts as a trap for the carriers that increase the potential barrier at the grain boundaries, which in turn affects the mobility according to the relation [14, 15]
The reduction in resistivity observed when samples were annealed in vacuum (200 - 300 °C) [4] and also for our samples indicates the evaporation of oxygen and chlorine from grain boundaries with a consequent reduction in ¢, the effect being similar to the resistivity reduction obtained in a hydrogen atmosphere. The increase in resistivity that we observed when the temperature of the heat t r e a t m e n t for 10 min in vacuum was above 300 °C might be due to the secondary recrystaUization observed at this temperature as a change in preferential orientation. A recrystallization temperature at 300 °C is low compared with the value of 600 °C reported by Kahle and Berger [16] for pure CdS deposited in vacuum. However, the presence of impurities other than oxygen, which are known as inhibitors of recrystallization, may produce this effect. During recrystallization the grain boundary movement due to crystallite growth would sweep the impurities into the bulk and incorporate them into the boundaries. This would increase the potential barrier at the boundaries. The grain size after recrystallization is still low (about 1000 A) as the K s and K~ lines are hardly resolved in the peaks and therefore the boundaries would be effective scatterers, causing the high resistivity increase observed. Thus at present it appears that, in order to reduce the resistivity, 300 °C should be the m a x i m u m temperature for heat treatments in vacuum.
Acknowledgments This work was supported by the Centro para el Desarrollo TecnolAgico e Industrial, Ministerio de Industria, Spain, and International Telephone and Telegraph Standard El~ctrica.
References 1 Y. Ma and R. H. Bube, J. Electrochem. Soc., 1 (24) (1977) 1430. 2 J. Bougnot, M. Perotin, J. Marucchi, M. Sirkis and M. Savelli, Proc. 12th Photovoltaic Specialists' Conf., Baton Rouge, LA, November 15- 18, 1976, IEEE, New York, 1976, p. 519. 3 C. Wu and R. H. Bube, J. Appl. Phys., 45 (1974) 648.
220 4 S. Martinuzzi, F. Cabane-Brouty, J. Qualid, J. Gervais, A. Mostavan and J. L. Granier, in A. Strub (ed.), Proc. 1st Commission o f the European Communities Conf. on Photovoltaic Solar Energy, Luxembourg, September 27 - 30, 1977, Reidel, Dordrecht, 1978, p. 581. 5 J. M. Gilles and J. van Cakenberghe, Solid State Physics in Electronics and Telecommunications, Vol. 2, Part 2, Academic Press, New York, 1960, p. 900. 6 O. Oudeacoumar, Th~se, Universit~ des Sciences et Techniques du Languedoc, 1979. 7 R. R. Chamberlin and R. S. Skarman, J. Electrochem. Soc., 113 (1966) 86. 8 NBS Circ. 539, Vol. IV, 1955, pp. 15 - 16, Card 6-0314 (National Bureau of Standards, U.S. Department of Commerce). 9 J. M. Benet, J. L. Stanford and E. J. Ashley, J. Opt. Soc. Am., 60 (1970) 224. 10 J. M. Benet and M. J. Booty, J. Appl. Opt., 5 (1966) 41. 11 F. Arjona, Tesis Doctoral, Universidad AutSnoma de Madrid, 1979. 12 L. J. Van der Pauw, Philips Tech. Rev., 20 ( 8 ) ( 1 9 5 8 - 1 9 5 9 ) 220;PhilipsRes. Rep., 13 (1958) 1. 13 F. B. Micheletti and P. Mark, Appl. Phys. Lett., 10 (1967) 136. 14 L. L. Kazmerski, W. B. Berry and C. W. Allen, J. Appl. Phys., 43 (1972) 3515. 15 R. L. Petritz, Phys. Rev., 104 (1956) 1508. 16 W. Kahle and H. Berger, Phys. Status Solidi A, 2 (1970) 717.
211
EFFECT OF HEAT TREATMENTS IN VACUUM ON CdS THIN FILMS PREPARED BY THE SPRAY DEPOSITION TECHNIQUE
L. ESCOSURA, E. GARCIA-CAMARERO, F. ARJONA and F. RUEDA
Departamento de Fisica Aplicada, Madrid 34 (Spain)
Universidad Aut6noma de Madrid, Cantoblanco,
(Received April 13, 1983;accepted September 14, 1983)
Summary The effects of short heat treatments in vacuum (10 min at 200 - 400 °C) on the electrical, structural and optical properties of hexagonal polycrystalline CdS thin films prepared by the spray deposition technique were studied. It was found that the electrical and structural properties change remarkably under these treatments. However, the optical properties do not vary significantly. The resistivity, in particular, decreased from about 500 ~ cm to less than 1 ~ cm for a 300 °C annealing while the electron mobility increased by two orders of magnitude, a result similar to that reported by Martinuzzi and coworkers. A hexagonal phase was obtained regardless of deposition temperature. The preferential orientation changed when samples were subjected to vacuum annealing at 300 °C or more, indicating that a recrystallization had taken place in the films.
1. Introduction A detailed knowledge of the electrical and structural changes in CdS films is important in relation to the manufacture of Cu2S/CdS solar cells. Ma and Bube [1 ] have studied the properties of CdS prepared by the pyrolytic spray technique and found cubic and hexagonal phases and a preferential orientation of {112) in the substrate temperature range 325475 °C. They evaluated the changes in the electrical properties as a function of the temperature of the substrate and the effects of heat treatments in various atmospheres and showed that annealing in hydrogen at 400 °C produces an increase in electron density and mobility. This is attributed to the removal of chlorine and oxygen from the grain boundaries. More recently, Bougnot e t al. [2] have confirmed this decrease in resistivity when studying annealing in hydrogen. 0379-6787/84/$3.00
© Elsevier Sequoia]Printed in The Netherlands
212
Wu and Bube [3] have studied the effects of heat treatments in vacuum and in air but only at relatively low temperatures (120 °C). Vacuum heat treatments do not seem to affect the conductivity markedly while air treatments decrease it by two orders of magnitude. Martinuzzi et al. [ 4], however, reported a decrease in resistivity by four orders of magnitude when samples were subjected to annealing at 260 + 20 °C in a high vacuum; this is attributed to oxygen desorption. Since the early work o f Gilles and van Cakenberghe [ 5] on silver-doped CdS the recrystallization of CdS films has received attention but the recrystallization temperature did not drop much below 500 °C. Stimulated by the results reported by Martinuzzi et al. [4] which showed a reduction in resistivity by a factor of 10 4 when an annealing treatment at 260 °C in a vacuum of 10 -.6 Torr was used, we made a preliminary study of the correlation between the structural and electrical changes caused by heat treatments in vacuum.
2. Experimental details Samples of CdS were obtained by a technique similar to that described by Oudeacoumar [6]; the work of Chamberlin and Skarman [7] was taken as the starting point. The ratio of the concentration of S 2- ions to the concentration of Cd 2+ ions is adjusted by the concentration of the solution used which is 0.1 M thiourea and 0.09 M CdC12, which gives the anionic concentration ratio of Cd 2+ to S 2- as 0.9. This does n o t correspond to that given by other researchers [1, 4, 7] who used an ionic concentration ratio of unity. The reason for this change was that, when a concentration ratio of unity was used, we obtained a peak in the X-ray diffraction diagrams corresponding to metallic cadmium. This peak is placed at 20 = 38.5 ° and corresponds to the cadmium crystallographic indices (101}. To provide a pressure to the sprayer we used nitrogen and the deposition was made onto Pyrex substrates. The spray rate varied from 5 to 10 ml min -1, and the temperature of the substrate from 230 to 400 °C. Various deposition rates were obtained by varying the spray flow and the temperature of the substrate {Fig. 1). The thickness of the samples was about 4 #m. Heat treatments were carried out in a vacuum system at 10 -s Torr for 10 min and the temperature was varied from 200 to 400 °C. Higher temperatures were not used because of the appearance of a yellow deposit which indicated film decomposition. Standard X-ray diffraction measurements were made in a Philips PW 1050/25 rotating sample diffractometer and the preferential orientation was estimated using the peak heights. The incident and diffracted beams are symmetrical to the film substrate. The ratio of the rotation speed of the detector to t h a t of the film was 2 to 1, i.e. the rotation speed of the detector was 2 ° min -1 and t h a t of the film 1 ° min -1. The film was n o t subjected to spin. To assess the preferential orientation of a film, the diffractogram of the
213
ts 230°C /Ts=300°C
i
4
~Ts:3 20oC
3 S
i
I
I
I
5
10 RATE OF SPRAY(ml min -1) Fig. 1. Growth velocity of sprayed CdS films as a function of solution'flow for various substrate temperatures. film in its present state was compared with the diffractogram obtained when the film had been ground to p o w d e r and the peak height ratios of the film had been compared with the ratios o f intensity data given in ref. 8. The optical properties were studied by transmittance and reflectance using a Cary 17D spectrophotometer. We t o o k into account the multiple incoherent reflections b o t h in the film and in the substrate to determine the refractive index and the absorption coefficient [9 - 11 ]. The film thickness measured with a Talystep stylus instrument was a b o u t 4/~m. The Van der Pauw technique [12, 13] was used to evaluate the film resistivity, the carrier concentration and the mobility .at room temperature in the dark. Evaporated aluminium dots (0.8 mm in diameter) were used to form ohmic contacts with the silver paste.
3. Results
3.1. Structure of the samples All the samples obtained in this work b y spray pyrolysis have a hexagonal structure regardless of the substrate temperature. Their preferential growing planes (101), (002) and (112) parallel to the substrate depend on the temperature of deposition. The presence of the zinc blende t y p e of CdS was discounted because of the absence of the following reflections in both grown and ground films (Fig. 2): (200)4 at 20 = 30.8 °, (400)2 at 20 = 64.1 ° and (331)3 at 20 = 70.4 °, where the subscripts 4, 2 and 3 indicate the relative peak intensities of the (200), (400) and (331) planes.
214
B
z
cr ~J Q_
u~ o
Z
A
~)
II Ir
Fig. 2. X-ray diffractometer spectra of sprayed CdS films (spectrum A) deposited at 350 °C and the ground powder (spectrum B) after the film had been detached from the substrate. Only hexagonal phase peaks are observed and the preferential orientations for the planes are (002), (101) and (112) parallel to the substrate.
U3 ~- lOq0 LJ 8 "-F .< 13..
[002]
6
{o 2
~112]
250
300
350
400
~(°C)
Fig. 3. The peak heights of (101), (112) and (002) reflections as a function of substrate temperature normalized with respect to powder intensities.
The samples fabricated at temperatures lower than 250 °C have (101) and (112) as the preferential planes and are of lower grade. At higher temperatures the {002) reflection prevails (Fig. 3).
215
I
200
15C~
loot z
LI.J
a_ 50
10021
;
^,
260 350 450 T(°C) Fig. 4. Variation in the X-ray diffraction peak heights for different reflections as a function of temperature for 24 h step annealings in a sample grown at 260 °C. These results confirm those of Oudeacoumar [6] and differ from the results reported by Ma and Bube [1] and by Martinuzzi et al. [4] for temperatures in this range in which the (112) orientation prevails. A series of successive vacuum annealings for 24 h were carried out on a sample, and the resistivity and X-ray diffractograms were measured at room temperature. Samples annealed in vacuum at temperatures higher than the deposition temperature change the preferential orientation of their grains as indicated in Fig. 4 where the evolution of the height of the peaks for successive 24 h annealings in a sample grown at 260 °C is shown. The intensity produced by the (002) reflection is reduced at first but recovers partly on successive annealings.
3.2. Optical properties We observed that, as the temperature of the substrate increases, the transmittance of the samples increases (Fig. 5) and no interference peaks appear even in samples grown at high temperatures. At high deposition temperatures (Ts > 300 °C) the transmittance versus wavelength plot is similar to that of evaporated samples; however, it does differ in t h a t interference maxima are absent. The values of the refractive index vary from one sample to another in the range 2.4 - 2.7 at 500 nm.
216 It%)
EVAPORAT ED
70 /
/
Ts=320oC
/
60
S
~
Ts :230°C
50 40 30 20 10
s
I
I
I
I
I
*
I
]
I
0.5
1 X (nm) ~.5 Fig. 5. Spectral optical transmittance as a function of substrate temperature during spraying. 4×102
~'-~
\ 10 2
\
+
\ \
20
,
I
i
i
\ I
200 2BO 360 Ts(°C) Fig. 6. Sample resistivity as a function of substrate temperature during spraying. The vacuum heat treatments do not substantially change the transmitt a n c e o f t h e samples.
3.3. Electrical properties T h e resistivity o f as-grown s a m p l e s as a f u n c t i o n o f t h e t e m p e r a t u r e o f t h e s u b s t r a t e is s h o w n in Fig. 6 a n d t h e m o b i l i t y values w e r e n e a r l y c o n s t a n t , a b o u t 10 -2 c m 2 V -1 s -1.
217
1000 10( lC o
°'°'2o~V260
2~,o
3~o
3.~o
4~o
T(°C)
Fig. 7. Resistivity of CdS samples v s . temperature of annealing (10 min) for samples grown at various temperatures: z~, T s = 230 °C; o, T S = 260 °C; D, T S = 300 °C; o, T s = 320 °C.
2 I 1
F
/'--"---"-'--"
1000]
t2xlOZO
F"
/
I
41o ~°
5" "T >
n 1(
r~ ',0
s.. lo-' i- I
C
"4
I
41o ,9
0.1
2 xl OI'~L. ~
200
300
T~C;
/-,00
2X1018
Fig. 8. Carrier mobility, resistivity and carrier concentration v s . temperature of annealing (t0 rain) for a CdS sample grown at 320 °C.
218
The vacuum heat treatments for 10 min made the room temperature resistivity decrease abruptly as shown in Fig. 7. At temperatures above 300 °C the resistivity rises and the effect is largest for the lowest fabrication temperature. The vacuum annealings at a b o u t 200 °C produce a marked increase in the mobility and carrier concentration at room temperature, as shown in Fig. 8 for a sample grown at 320 °C. These results are in general agreement with those of ref. 4.
4. Discussion and conclusions We found that hexagonal phase CdS thin films with promising properties for photovoltaic devices can be prepared b y the spray deposition technique, when the samples are subjected to short heat treatments in vacuum (10 -s Torr). The preferential orientation, however, is not as marked as in vacuumevaporated films where the only planes parallel to the substrate have a {002) orientation. For an ion concentration ratio of 0.9 (lower than usual) we always obtained hexagonal structures in the CdS samples even at deposition temperatures as low as 230 °C. At temperatures above 300 °C the predominant preferential growing plane was (002) parallel to the substrate. These results agree fully with those of Oudeacoumar [6] and differ from the observations of Ma and Bube [1] who found that in this temperature range the preferential plane was (112) parallel to ~he substrate. Perhaps the difference in the prevailing orientation is caused by the use of nitrogen instead of air as the pressurizing gas. When the temperature of the substrate is 320 °C, the optical properties of the films obtained are good enough to be used in photovoltaic devices and they do not change significantly with further heat treatments. As is usual for sprayed CdS films, no interference maxima and minima were observed; the lack of coherence might be caused b y the misorientation of grains in the {002) direction and by internal stresses between other grains. The low temperature heat treatments in vacuum reduce the resistivity by several orders of magnitude, and the final resistivity of the samples could be controned by choosing the temperature of the heat treatment carefully to be between 200 and 350 °C. The resistivity then ranged between 10 and 0.1 $2 cm. These results are similar to those obtained using heat treatments in a hydrogen atmosphere [2, 4] which are more effective in reducing the resistivity than those in other atmospheres (nitrogen, oxygen and argon) [7]. The reduction in resistivity has been attributed principally to an increase in the electron density produced b y oxygen desorption. Similarly, in our case the heat treatment in vacuum may have the effect of producing a high desorption o f oxygen, thereby markedly increasing the carrier concentration. However, the electron mobility in the films increases b y t w o orders of magnitude with these heat treatments in vacuum up to 250 °C.
219
A high sensitivity of CdS to the oxygen absorbed in the grain boundaries has been reported [1, 3, 4], the absorbed oxygen acting as an acceptor impurity. On the one hand, this causes a decrease in the electron density and, on the other hand, acts as a trap for the carriers that increase the potential barrier at the grain boundaries, which in turn affects the mobility according to the relation [14, 15]
The reduction in resistivity observed when samples were annealed in vacuum (200 - 300 °C) [4] and also for our samples indicates the evaporation of oxygen and chlorine from grain boundaries with a consequent reduction in ¢, the effect being similar to the resistivity reduction obtained in a hydrogen atmosphere. The increase in resistivity that we observed when the temperature of the heat t r e a t m e n t for 10 min in vacuum was above 300 °C might be due to the secondary recrystaUization observed at this temperature as a change in preferential orientation. A recrystallization temperature at 300 °C is low compared with the value of 600 °C reported by Kahle and Berger [16] for pure CdS deposited in vacuum. However, the presence of impurities other than oxygen, which are known as inhibitors of recrystallization, may produce this effect. During recrystallization the grain boundary movement due to crystallite growth would sweep the impurities into the bulk and incorporate them into the boundaries. This would increase the potential barrier at the boundaries. The grain size after recrystallization is still low (about 1000 A) as the K s and K~ lines are hardly resolved in the peaks and therefore the boundaries would be effective scatterers, causing the high resistivity increase observed. Thus at present it appears that, in order to reduce the resistivity, 300 °C should be the m a x i m u m temperature for heat treatments in vacuum.
Acknowledgments This work was supported by the Centro para el Desarrollo TecnolAgico e Industrial, Ministerio de Industria, Spain, and International Telephone and Telegraph Standard El~ctrica.
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