- Email: [email protected]
Effect of substrate temperature on transport and optical properties of sprayed Cd1−xZnxS films
~
Solid State Communications, Vol.46,No.12, pp.847-850, 1983 Printed in Great Britain.
0038-1098/83 $3.00 + .OQ Pergamon Press Ltd.
EFFECT OF 5UBSTRATE TEMPERATURE ON TRANSPORT AND OPTICAL PROPERTIES OF SPRAYED Cdl_xZnxS FILMS M.D. Uplane and S.H.Pawar Energy Conversion Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, INDIA (Accepted for publication by S. Amelinckx 14.4.83)
Polycrystalline Cdl_xZnxS films are prepared by the spray pyrolysis technique on amorphous substrate, at different substrate temperatures. The dark conductivity and thermoelectric power are measured and applied to calculate the electron density and mobility. The electron density decreases with increase in substrate temperature. However, mobility is higher for the films prepared at 400°C substrate temperature. The variation of electron density and mobility with substrate temperature is explained in terms of crystallanity of the film and chlorine concentration in the films. The optical absorption of the films are studied and revealed that there is not much change in band gap with substrate temperature.
INTRODUCTION
sprayed through a specially designed glass nozzle. Air is used to atomise the spray. The substrate temperature is varied from 300°C to 500°C with the inter~al of 50°C. Fast cooling is used at the termination of spray, as slow cooling produces film with higher resistivity, possibly because of the reaction with oxygen in air over a longer time used in cooling. The films prepared at different substrafe temperatures are denoted by 5300, ~35 ~ 5 , ~ and S where the subscript ~ o t e ~ 5 ~ h e s u b ~ a t e temperature. The electrical properties namely dark conductivity and thermoelectric power are studied. The optical absorption of the film is recorded with the help of a monochromator (Carl Zeiss Jena), in the wavelength range between 400 nm to 700n~
THERE has been growing interest during the past few years in sprayed Cdl-xZn~$ films because of their potential a~pllcations in low cost solar cells~12], In the spray pyrolysis technique t~e ~ilms properties depend on various parameters such as, spray rate, substrate temperature, cooling rate etc. Bube et al [3] have studied the effect of these preparative parameters on the properties of Cd5 films. However, little work has been published to a date on the properties of solution sprayed alloy sulphide films. Our studies show that the PEC cells formed with Cd 0 8Zn 0 2S sprayed films give better performance [4]. Hence the films of this composition are studied for their properties. In the present investigation Cd 0 8Zn0 25 films are prepared by spray pyrolysis technique at different substrate temperatures. The electrical and optical properties of these films are studied. HXPERIN~NTAL The Cd N sZn N 9~ films were deposited on ultraso615alI~cleaned glass substrates, employing the spray pyrolzsis technique, described elsewhere [5]. The basic ingredients used are cadmfum chloride (CdCIg) , zinc chloride (ZnCIg) and thioure~. The starting solution is prepared by adding 320 cc of 0.2 M CdCl 2 solution into 80 cc of 0.2 M ZnCI 9 solution in order to maintain the~ Cd:Zn ratio 4:1. Further, 400 cc of 0.2 thiourea solution is added to the above solution so as to obtain the CdZn:& ratio i:i. This starting solution is
RESULTS AND DI~L~U~SlON The Cd 0 8Zn 0 2S films prepared by spray pyrolysis technique at different substrate temperatures are found to be uniform and adhere tightly to the substrate. However, the thickness of the films is found to d@crease with increase in the substrate temperature. The variation of thickness (t) with substrate to temperature is shown in figure i. The decrease in the film thickness with the substrate temperature is attributed to the increase in evaporation rate of the initial products with increase in the substrate temperature [6]. The dark conductivity (6) is measured at room temperature for all the films. It is found that6increases with rise in substrate temperature,attains a maximum value at 400°C and for further higher substrate temperatures it decre847
848
TRANSPORT AND OPTICAL PROPERTIES OF SPRAYED Cdl_xZnxS FILMS
o
Vol. 46, No. 12
distinct regions corresponding to low and high temperatures. The activation energies are estimated from these regions and listed in table i. It is seen that the activation energies in low temperature regions are of the order of 0.03 eV which corresponds to the ionization energies of Cl- donors [7] in the films. The activation energ 9 in the low temperature region is low for 5400 films. This can be understood from the study of intergranular barrier height as discussed later on.
2
u v ~n (n hl z
Table i.
Activation energies in high and low temperature regions
,u -r
300
350
400
450
500
SUBSTRATE TEMPERATURE(OC )
Fig 1
ases. The higher conductivity at 400°C may be due to the better crystallanity of the films.
The dark conductivities for all films were measured in the temperature range between 300°K to 550°K. The plots of log 6 versus I/T are shown in fig.2 for typical films, namely, $300, $400 and $450. It is seen that there a r e % w o
\
io
.T.o
• S4oo
°%~o .~'.0 7 ~
~.o
._
~.o
50
~.0
i~
° ~ 1.7
'zO
2.~
3.0
o - - - - - - g, _
3.5
Ji_ xlo3 (K4 ) T
Fig 2 -
Temperature dependence of dark conductivity (6) for films.
0.037 0,031 0.025 0.036 0.036
0.82 0.91 0.78 0.97 0.95
-
~[a
+
i~
2( 2~
m~KT)3/2
nh3
xs,
"~ ..__[o~ ~
$300 S350 5400 $450 5500
:
T.o
\
Activation energies(eV) Barrier Low temp. High temp. height region region ~ (eV)
0.61 0.56 0.18 0.36 0.47
The thermoelectric power (6) was measured in dark for all the films, in the temperature range between 300°K and 450°K. It is seen that ~ is higher for the Szn n film than other films. From the thermd~[ectric power measurement, the electron density(n) was determined by employing the relation
Plot of thickness of the films versus substrate temperature.
-
&le No.
~
]
(l)
where, A i s the t h e r m o e l e c t r i c f a c t o r , taken equal to 2 . 5 , me = 0.2m~ e f f e c t i v e e l e c t r o n mass and o t h e r symbols have t h e i r usual meanings. The e l e c t r o n dens i t i e s are calculated for all the films and are plotted as a function of the substrate temperature in fig.3. It is observed that the electron density(n) decreases slightly with increase in the substrate temperature. This is attributed to the decrease in the chlorine donor density with increase in substrate temperature [43. The electron mobility (~), was determined from the conductivity (6) and electron density (n) by using the relation = ne (2) The v a r i a t i o n of ~ w i t h s u b s t r a t e temp e r a t u r e i s a l s o shown in f i g . 3. I t i s found t h a t m o b i l i t y i n c r e a s e s upto 400°G s u b s t r a t e t e m p e r a t u r e and f o r f u r t h e r higher substrate temperatures it decreases. The h i g h e r m o b i l i t y at 400°C may be due to the b e t t e r c r y s t a Z l a n i t y of the f i l m s than t h a t of the f i l m s prepared at o t h e r s u b s t r a t e temperatures. The variation of the mobility with substrate temperature is further supported by the measurement of the intergra-
Vol. 46, No. 12
TRANSPORT A N D OPTICAL PROPERTIES
OF SPRAYED Cdl_xZnxS FILMS
849
g.s 16 z Z=
4.5
=::=, O4,
o
I
,6 4
'Eu 4.0
0.5
.~e"~, °~o--...O~o ~
166
5>.
.o O
To
E
x ¢:
o
S4o o
•
5450
o
S350
Tm
T.O'>,
E
o,
a..
s
u
0
--
g.5
3.5 300
400
500
SUBSTRATE TEMPERATURE ( ° C )
Fig 3 -
nular barrier height (@). The values of are determined by studying the variation of mobility with temperature. For thermally activated mobility one can write the relation in the following form
~.o
\
EO
Variation o f electron density (n) and mobility (~) with substrate temperature. The values are calculated at 150°C.
2,2
2.5
1000
3.5
(K'I) 3"0
T
Fig 4 -
Plots of log M versus I/T for the films.
[8,93
= Bo exp (- O/kT)
15
4,0
(3)
o
where all the terms have their usual meaning. Fig.4 shows the plots of log versus I/T for three typical samples. The plots are found to be linear and slopes of these plots give the intergranular barrier height ~ in electron volts (eV). The values of @ for all films are given in table I. It is observed that ~ decreases upto 400°C and for further higher substrate temper a t u r e it increases. It is seen that the smaller the barrier height (~) the higher the mobility, giving rise to large conductivity. The variation of electron density (n) and mobility (~) with temperature is shown in fig.5 for film S4QO. It is observed that the plots are linear, and n increase with temperature. These results are similar to the findings reported by other [i0]. Optical absorption was m e a s u r e d as function of the wavelength for all films and is shown in fig.6 for typical films. It is seen that there is not much change in absorption edge for the films 5300, 5350, $400 and $450 having
v;
~o
\7
3.5
10
uE v
To
x 3.0
5
c
E
o% \ 25
•
2.1
2.5 x10
Fig 5 -
\°\
3.0 ( K -I )
~.5
Temperature dependence of the electron density (n) and mobility (~) for the film $4OO.
850
TRANSPORT AND OPTICAL PROPERTIES OF SPRAYED Cdl_xZnxS FILMS
wavelength cut off at 490 nm. However the absorption coefficient is higher for the films $800 at longer wavelength side. The large absorption for the $8OO films at longer wavelength is attributed to the higher concentration of chlorine donor levels present in Cdl_xZnxS films and of trap levels by structural defects.
~15.
\\~[I
~-
$300
e
g
Vol. 46, No. 12
s
450
Fig 6 -
500
550
A(nm)
Acknowledgements - Authors are grateful to Mr.C.D. Lokhande and Mrs. S.J.Kshirsagar for the experimental help. One of the authors (MDU) expresses his deep sense of gratitude to C~IR Delhi, for the award of Junior Research Fellowship. Thanks are also extended to the referee for his valuable comments.
600
Variation o f the absorption coefficient (~) with wavelength (A) for films.
RSFHRENCES
i. 2. 3. 4. 5.
W.B.Hsu and L.C. Burton, J. of Hlectronic Mat. i0, 703 (1981). M.R. RajeBhonsale, Ph.D. Thesis, $hivaji Univ.Kolhapur,India, 1981. Y.Y.Ma and R.H. Bube, J.Electrochem. Soc. 124, 1430 (1977). M.D.Uplane and $.H. Pawar, Bulletin of Material Science, accepted. S.H. Pawar, M.D.Uplane, $.K. Pawar and J.S.Desai, J. Shivaji Univ. (submitted).
6.
Ya,A.Ugai, V.N.Semenov and E.M. Averbakh, Izvestiya Akademi Nauk SSSR, Neorgani Cheskie Materialy 14, 1529 (1978). 7. C.~u and R.H.Bube, J.Appl. Phys. 45, 648 (1974). 8. F.B.Micheletti and P.Mark, Appl. Phys Letters iO, 186 (1967). 9. R. Kessing and W. Bax, Proc. 6th Internl. Vacuum Congr. 1974, Japan. lO. C.D. Lokhande and S.H.Pawar, Solid State Commun. 44, 1187 (1982).
Solid State Communications, Vol.46,No.12, pp.847-850, 1983 Printed in Great Britain.
0038-1098/83 $3.00 + .OQ Pergamon Press Ltd.
EFFECT OF 5UBSTRATE TEMPERATURE ON TRANSPORT AND OPTICAL PROPERTIES OF SPRAYED Cdl_xZnxS FILMS M.D. Uplane and S.H.Pawar Energy Conversion Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, INDIA (Accepted for publication by S. Amelinckx 14.4.83)
Polycrystalline Cdl_xZnxS films are prepared by the spray pyrolysis technique on amorphous substrate, at different substrate temperatures. The dark conductivity and thermoelectric power are measured and applied to calculate the electron density and mobility. The electron density decreases with increase in substrate temperature. However, mobility is higher for the films prepared at 400°C substrate temperature. The variation of electron density and mobility with substrate temperature is explained in terms of crystallanity of the film and chlorine concentration in the films. The optical absorption of the films are studied and revealed that there is not much change in band gap with substrate temperature.
INTRODUCTION
sprayed through a specially designed glass nozzle. Air is used to atomise the spray. The substrate temperature is varied from 300°C to 500°C with the inter~al of 50°C. Fast cooling is used at the termination of spray, as slow cooling produces film with higher resistivity, possibly because of the reaction with oxygen in air over a longer time used in cooling. The films prepared at different substrafe temperatures are denoted by 5300, ~35 ~ 5 , ~ and S where the subscript ~ o t e ~ 5 ~ h e s u b ~ a t e temperature. The electrical properties namely dark conductivity and thermoelectric power are studied. The optical absorption of the film is recorded with the help of a monochromator (Carl Zeiss Jena), in the wavelength range between 400 nm to 700n~
THERE has been growing interest during the past few years in sprayed Cdl-xZn~$ films because of their potential a~pllcations in low cost solar cells~12], In the spray pyrolysis technique t~e ~ilms properties depend on various parameters such as, spray rate, substrate temperature, cooling rate etc. Bube et al [3] have studied the effect of these preparative parameters on the properties of Cd5 films. However, little work has been published to a date on the properties of solution sprayed alloy sulphide films. Our studies show that the PEC cells formed with Cd 0 8Zn 0 2S sprayed films give better performance [4]. Hence the films of this composition are studied for their properties. In the present investigation Cd 0 8Zn0 25 films are prepared by spray pyrolysis technique at different substrate temperatures. The electrical and optical properties of these films are studied. HXPERIN~NTAL The Cd N sZn N 9~ films were deposited on ultraso615alI~cleaned glass substrates, employing the spray pyrolzsis technique, described elsewhere [5]. The basic ingredients used are cadmfum chloride (CdCIg) , zinc chloride (ZnCIg) and thioure~. The starting solution is prepared by adding 320 cc of 0.2 M CdCl 2 solution into 80 cc of 0.2 M ZnCI 9 solution in order to maintain the~ Cd:Zn ratio 4:1. Further, 400 cc of 0.2 thiourea solution is added to the above solution so as to obtain the CdZn:& ratio i:i. This starting solution is
RESULTS AND DI~L~U~SlON The Cd 0 8Zn 0 2S films prepared by spray pyrolysis technique at different substrate temperatures are found to be uniform and adhere tightly to the substrate. However, the thickness of the films is found to d@crease with increase in the substrate temperature. The variation of thickness (t) with substrate to temperature is shown in figure i. The decrease in the film thickness with the substrate temperature is attributed to the increase in evaporation rate of the initial products with increase in the substrate temperature [6]. The dark conductivity (6) is measured at room temperature for all the films. It is found that6increases with rise in substrate temperature,attains a maximum value at 400°C and for further higher substrate temperatures it decre847
848
TRANSPORT AND OPTICAL PROPERTIES OF SPRAYED Cdl_xZnxS FILMS
o
Vol. 46, No. 12
distinct regions corresponding to low and high temperatures. The activation energies are estimated from these regions and listed in table i. It is seen that the activation energies in low temperature regions are of the order of 0.03 eV which corresponds to the ionization energies of Cl- donors [7] in the films. The activation energ 9 in the low temperature region is low for 5400 films. This can be understood from the study of intergranular barrier height as discussed later on.
2
u v ~n (n hl z
Table i.
Activation energies in high and low temperature regions
,u -r
300
350
400
450
500
SUBSTRATE TEMPERATURE(OC )
Fig 1
ases. The higher conductivity at 400°C may be due to the better crystallanity of the films.
The dark conductivities for all films were measured in the temperature range between 300°K to 550°K. The plots of log 6 versus I/T are shown in fig.2 for typical films, namely, $300, $400 and $450. It is seen that there a r e % w o
\
io
.T.o
• S4oo
°%~o .~'.0 7 ~
~.o
._
~.o
50
~.0
i~
° ~ 1.7
'zO
2.~
3.0
o - - - - - - g, _
3.5
Ji_ xlo3 (K4 ) T
Fig 2 -
Temperature dependence of dark conductivity (6) for films.
0.037 0,031 0.025 0.036 0.036
0.82 0.91 0.78 0.97 0.95
-
~[a
+
i~
2( 2~
m~KT)3/2
nh3
xs,
"~ ..__[o~ ~
$300 S350 5400 $450 5500
:
T.o
\
Activation energies(eV) Barrier Low temp. High temp. height region region ~ (eV)
0.61 0.56 0.18 0.36 0.47
The thermoelectric power (6) was measured in dark for all the films, in the temperature range between 300°K and 450°K. It is seen that ~ is higher for the Szn n film than other films. From the thermd~[ectric power measurement, the electron density(n) was determined by employing the relation
Plot of thickness of the films versus substrate temperature.
-
&le No.
~
]
(l)
where, A i s the t h e r m o e l e c t r i c f a c t o r , taken equal to 2 . 5 , me = 0.2m~ e f f e c t i v e e l e c t r o n mass and o t h e r symbols have t h e i r usual meanings. The e l e c t r o n dens i t i e s are calculated for all the films and are plotted as a function of the substrate temperature in fig.3. It is observed that the electron density(n) decreases slightly with increase in the substrate temperature. This is attributed to the decrease in the chlorine donor density with increase in substrate temperature [43. The electron mobility (~), was determined from the conductivity (6) and electron density (n) by using the relation = ne (2) The v a r i a t i o n of ~ w i t h s u b s t r a t e temp e r a t u r e i s a l s o shown in f i g . 3. I t i s found t h a t m o b i l i t y i n c r e a s e s upto 400°G s u b s t r a t e t e m p e r a t u r e and f o r f u r t h e r higher substrate temperatures it decreases. The h i g h e r m o b i l i t y at 400°C may be due to the b e t t e r c r y s t a Z l a n i t y of the f i l m s than t h a t of the f i l m s prepared at o t h e r s u b s t r a t e temperatures. The variation of the mobility with substrate temperature is further supported by the measurement of the intergra-
Vol. 46, No. 12
TRANSPORT A N D OPTICAL PROPERTIES
OF SPRAYED Cdl_xZnxS FILMS
849
g.s 16 z Z=
4.5
=::=, O4,
o
I
,6 4
'Eu 4.0
0.5
.~e"~, °~o--...O~o ~
166
5>.
.o O
To
E
x ¢:
o
S4o o
•
5450
o
S350
Tm
T.O'>,
E
o,
a..
s
u
0
--
g.5
3.5 300
400
500
SUBSTRATE TEMPERATURE ( ° C )
Fig 3 -
nular barrier height (@). The values of are determined by studying the variation of mobility with temperature. For thermally activated mobility one can write the relation in the following form
~.o
\
EO
Variation o f electron density (n) and mobility (~) with substrate temperature. The values are calculated at 150°C.
2,2
2.5
1000
3.5
(K'I) 3"0
T
Fig 4 -
Plots of log M versus I/T for the films.
[8,93
= Bo exp (- O/kT)
15
4,0
(3)
o
where all the terms have their usual meaning. Fig.4 shows the plots of log versus I/T for three typical samples. The plots are found to be linear and slopes of these plots give the intergranular barrier height ~ in electron volts (eV). The values of @ for all films are given in table I. It is observed that ~ decreases upto 400°C and for further higher substrate temper a t u r e it increases. It is seen that the smaller the barrier height (~) the higher the mobility, giving rise to large conductivity. The variation of electron density (n) and mobility (~) with temperature is shown in fig.5 for film S4QO. It is observed that the plots are linear, and n increase with temperature. These results are similar to the findings reported by other [i0]. Optical absorption was m e a s u r e d as function of the wavelength for all films and is shown in fig.6 for typical films. It is seen that there is not much change in absorption edge for the films 5300, 5350, $400 and $450 having
v;
~o
\7
3.5
10
uE v
To
x 3.0
5
c
E
o% \ 25
•
2.1
2.5 x10
Fig 5 -
\°\
3.0 ( K -I )
~.5
Temperature dependence of the electron density (n) and mobility (~) for the film $4OO.
850
TRANSPORT AND OPTICAL PROPERTIES OF SPRAYED Cdl_xZnxS FILMS
wavelength cut off at 490 nm. However the absorption coefficient is higher for the films $800 at longer wavelength side. The large absorption for the $8OO films at longer wavelength is attributed to the higher concentration of chlorine donor levels present in Cdl_xZnxS films and of trap levels by structural defects.
~15.
\\~[I
~-
$300
e
g
Vol. 46, No. 12
s
450
Fig 6 -
500
550
A(nm)
Acknowledgements - Authors are grateful to Mr.C.D. Lokhande and Mrs. S.J.Kshirsagar for the experimental help. One of the authors (MDU) expresses his deep sense of gratitude to C~IR Delhi, for the award of Junior Research Fellowship. Thanks are also extended to the referee for his valuable comments.
600
Variation o f the absorption coefficient (~) with wavelength (A) for films.
RSFHRENCES
i. 2. 3. 4. 5.
W.B.Hsu and L.C. Burton, J. of Hlectronic Mat. i0, 703 (1981). M.R. RajeBhonsale, Ph.D. Thesis, $hivaji Univ.Kolhapur,India, 1981. Y.Y.Ma and R.H. Bube, J.Electrochem. Soc. 124, 1430 (1977). M.D.Uplane and $.H. Pawar, Bulletin of Material Science, accepted. S.H. Pawar, M.D.Uplane, $.K. Pawar and J.S.Desai, J. Shivaji Univ. (submitted).
6.
Ya,A.Ugai, V.N.Semenov and E.M. Averbakh, Izvestiya Akademi Nauk SSSR, Neorgani Cheskie Materialy 14, 1529 (1978). 7. C.~u and R.H.Bube, J.Appl. Phys. 45, 648 (1974). 8. F.B.Micheletti and P.Mark, Appl. Phys Letters iO, 186 (1967). 9. R. Kessing and W. Bax, Proc. 6th Internl. Vacuum Congr. 1974, Japan. lO. C.D. Lokhande and S.H.Pawar, Solid State Commun. 44, 1187 (1982).