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Preparation and properties of laser evaporated CuGaSe2 thin films
Journal of Crystal Growth 108 (1991) 765 769 North-Holland
765
Preparation and properties of laser evaporated CuGaSe2 thin films K.T. Ramakrishna Reddy and P. Jayarama Reddy Department of Physic5, Sri Venkateswara University, Tirupati-51 7 502, India
Received 29 June 1990; manuscript received in final form 14 September 1990
Bulk copper gallium diselenide material was prepared by melting the pure constituent elements in stoichiometric proportion. Thin films of CuGaSe2 were deposited by a laser evaporation technique. Both the bulk material and thin films were charactensed by XRD, SEM and AES. The effect of substrate temperature on the structure and composition, electrical and optical properties of the films has been studied. Polycrystalline stoichiometric films of CuGaSe2 can be deposited at substrate temperatures in the range 350 400 ° C and the films prepared at 370°C have a resistivity of about iO~ fJ cm. The energy band gap was found to be 1.68 eV at stoichiometric composition.
1. Introduction
2. Experimental
Among the available alternative materials to Si, the ternary chalcopyntes of the type Cu III V12 appear to be the most promising as candidates for use in photovoltaic devices [1,2]. CuGaSe2 is a member of this group which is a direct band gap semiconductor crystallising in the chalcopyrite structure [3,4]. Though studies on the growth and properties of this compound have been reported, both in bulk and in thin film form, detailed investigations are still required [5,6]. Thin films of this compound which is a prerequisite to the development of cost effective device structures have been fabricated by many techniques [7 10]. The laser evaporation is a relatively new technique for the deposition of CuGaSe2 thin films. In this technique, the kinetic energy of the incident particles in the vapour flux are found to be larger than the experimental thermal energy distribution which enhances ad-atom mobility and improves the crystallimty of the films [11,12]. The present paper reports the preparation and properties of CuGaSe2 thin films.
The starting material of CuGaSe2 was synthesised from a stoichiometric mixture of its constituent elements. Copper, gallium and selenium of 5N purity were taken in a sealed quartz ampoule evacuated to a pressure of 10 ~ Torr. The mixture was heated in a vertical furnace initially to a temperature of 200 °Cand kept for 24 h to ensure better homogeneity and to avoid the risk of explosion due to the exothermic reaction of gallium and selenium [13]. Then the temperature was slowly increased to 1200°Cand was maintained for 40 h. Finally, the ampoule was quenched to room temperature in cold water. The ingot obtained was pulvensed and made into small pellets for evaporation. CuGaSe2 thin films were prepared using a CO2 laser of 60 W power. The continuous laser beam entered the vacuum chamber through a vacuum tight ZnSe window. A gold coated concave mirror was used to reflect and concentrate the laser beam onto the source material kept in a carbon crucible. The experimental arrangement of the technique is
0022-0248/91/$03.50 © 1991
Elsevier Science Publishers B.V. (North-Holland)
766
K I Ramakrishna Reddy, P Jayarama Reddy
_______
A
SH
~i 2LASE
CuGaSe, thin films
The composition analysis of the films shows that the elemental atomic percentages depend critically on the substrate temperature. The AES composition data of the films formed at three different
S
CO
/ Laser evaporated
2
substrate are shownat inthefig.substrate 2. The films are temperatures nearly stoichiometric temperatures T3 in the range 350 400°C. For T,, < 350 °C, gallium was found to be in excess
M
~
and the films formed at T~>400°C were found to be copper rich. This behaviour may be due to the slight decomposition of the compound and the C
ABC H M
ALUMINIUM DISCS ALUMINIUM BASE CARBON CRUCIBLE HEATER GOLD COATED MIRROR
B
large difference in the vapour pressures of the constituent elements, as VP5e>> VPGO> VP~. The substrate temperature was found to have a significant influence on the structure of the films.
VA C UUM S SUBSTRATE (lxi cm°) Z-ZnSe WINDOW SH-SUBSTRATE HOLDER SP-SLIDINGPOST S1S2S3-SCREWS
T
300°C
Fig. 1. Schematic diagram of the laser beam focussing system. Go
shown in fig. 1. The films were prepared on Corning 7059 glass substrates at temperatures in the range 250 450°C in a vacuum better than 2 x 10 6 Torr with a deposition rate of 25 A/s. The film thickness was in the range, 0.4—0.8 ~.Lm.The composition of the bulk and the thin films was analysed by AES. The X-ray diffraction studies were carried out on both bulk and thin films using Cu Ka radiation. The standard van der Pauw technique was employed to study the electrical properties and the optical studies were measured by employing an Hitachi U:3400 UV VIS NIR spectrophotometer. Evaporated nickel was used as ohmic contacts to the films.
Go
Go
Go
Ts
370
C
Go
Go
~
3. Results and discussion The composition analysis made on the bulk material at different parts of the ingot showed no significant variation in the stoichiometry of the constituent elements, suggesting the homogeneous nature of the ingot. The XRD data obtained on the material are in agreement with the reported data of Mandel et al. [14].
Co
Go
Cu
I I
600
800
1100
1200
IL~W
1GIXJ
Energy (eV)
Fig. 2. AES profiles of laser evaporated CuGaSe2 films.
K. T. Ramakrishna Reddy, P. Jayarama Reddy
The XRD spectra of CuGaSe2 films formed at three different substrate temperatures and the corresponding SEM micrographs are shown in figs. 3 and 4, respectively. The films formed at temperatures below 350°C showed GaSe and Ga2Se3 as additional phases along with a CuGaSe2 phase. For deposition temperatures above 400°C, the films showed a Cu2 ~Se phase in addition to a CuGaSe2 phase. The reason for this is probably that the reaction velocities of the elements are such that RV~u+se>RVGa+Se> RVCu+Ga+5e [10]. This was also supported by the AES composition analysis of the films where excess of gallium was seen at 7 <350°C and excess of copper at T3>
/ Laser evaporated CuGaSe,
15 :
______
______________________________________________
11-
~JJJ
_________
.1
3
I
3
3
Fig. 4. Scanning electron micrographs of CuGaSe2 films: (a) copper deficient; (b) near stoichiometnc; (c) copper rich. Markers represent gm.
4
20
~
.~t. . . . . . . . . ~
42 5 C 3
767
____
~---
T~ 370C
thin films
30
40
50
60
2 0 Cd eg re e s) Fig. 3. XRD spectra of CuGaSe2 films formed at various substrate temperatures: (1) Ga2Se3 (2) GaSe; (3) CuGaSe2 (4) Cu2 ~Se.
400°C (fig. 2). With an increase of substrate temperature, the diffraction lines became sharp and intense, which revealed the improvement in crystallinity with substrate temperature. However, films prepared at 370°C showed single phase chalcopyrite structure with a grain size of about
768
K. I Ramakrishna Reddy, P. Jayarama Reddy
Laser evaporated CuGaSe, thin films
0.6 0.8 ~tm depending on the film thickness calculated using the Scherrer formula [15]. The variation of electrical resistivity with the substrate temperature of CuGaSe2 films is shown in fig. 5. At temperatures below 350°C there is not much variation in the resistivity and it decreases almost sharply above this temperature. The higher resistivity at the temperatures below 350°C is due to additional phases of higher resistivity such as GaSe and Ga2Se3 [6]. As the temperature increases the effect of these phases is not found to be significant and at about 400°C the further decrease in resistivity is due to the additional high conductivity phase of Cu2 ~Se [6]. This is in accordance with the XRD studies which indicate the different phases at different substrate temperatures. The optical absorption coefficient was calculated from the transmittance versus wavelength spectrum. Near the absorption edge the absorption coefficient is given by [16] A(hv
—
io~
\
10
\ \
10
~ 25 C
~
20
b
a
E
,,.‘°~
15
10
,“ 0 15
16
1 7 h~
18
19
20
(ev)
Eg)~
los
—
a 300 C
Fig. 6 (cihi’).’ versus he for CuGaSe2 films formed at different substrate temperatures.
2/hv, Cs
30
\ id
\ 0
10
~
it?
Substrate temperature ~c)
Fig. 5. Variation of resistivity with substrate temperature in CuGaSe2 films.
where A is an energy independent constant and hv is the photon energy. For direct allowed transitions n 1 and a plot of (ahv)2 versus hv will have an intercept with the hv axis which gives the band gap, Eg. Fig. 6 shows the variation of (ahv)2 versus hi.’ for the films formed at three different substrate temperatures. It is seen from the figure that as the substrate temperature increases, the band gap decreases due to the change in composition of the films. This may be caused by the superposition of two or three absorption curves due to different phases with different optical properties and impurity to band transitions in GaSe and Ga 2Se3, which can shift the absorption curve [6]. The change of band gap on the copper rich side is correlated with the existence of “d” bonding character of the Cu atoms where the copper d-orbitals push the valence band up, thereby reducing the band gap. Further, the band gap variation in ternary chalcopyrites is supported by the theosetical calculations of Jaffe and Zun~er [17]. The films formed at T~ 370° C showed an optical band gap of 1.68 eV, which is in good agreement with the reported value [18]. A detailed —
K. T. Ramakrishna Reddy, P. Jayarama Reddy
analysis of the optical properties was reported in an earlier paper [19].
4. Conclusions Polycrystalline, single phase and stoichiometric CuGaSe2 films can be prepared by a laser evaporation techniq’üe at substrate temperatures in the range 350 400°C. The substrate temperature plays a dominant role in the physical properties of the films. The CuGaSe2 films formed at a substrate temperature of 370°C showed single phase chalcopyrite structure resistivity of about 3 Q cm. The optical and bandagap was found to be i0 1.68 eV.
Acknowledgement
/ Laser evaporated
CuGaSe, thin films
769
[2] K. Zweibel, R. Mitchell and A. Hermann, in: Proc. 18th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 1985, p. 1393. [3] B. Tell and P.M. Bridenbaugh, Phys. Rev. B12 (1075) 3330. [4] J.L. Shay, B. Tell, H.M. Kasper and L.M. Schiavone, Phys. Rev. B5 (1972) 5003. [5] J. Stankiewicz, W. Giriat, J. Ramos and M.P. Vecchi, Solar Energy Mater. 1 (1979) 369. [6] W. Arndt, H. Dittrich and H.W. Schock, Thin Solid Films 130 (1985) 209. [7] N. Hong, H. Neumann, B. Schumann and G. Kuhn, Phys. Status Solidi (b) 85 (1978) K57. [8] KR. Murali. B.S.V. Gopalam and J. Sobhanadn, J. Mater Sci. Letters 5 (1986) 421. [9] R. Noufi. R. Powell. C. Herrington and T. Coutts. Solar 17 (1986) 303. [10] Cells H. Dittnch, B. Dimmier, R. Menner and H.W. Schock, in: Proc. 7th Intern. Conf. on Ternary and Multinary Compounds, Snowmass, CO, 1986, p. 161. [11] J.F. Frichtenicht, Rev. Sci. Instr. 45 (1974) 51. [12] T. Ishida, S. Wako and U. Ushino, Thin Solid Films 39 (1976) 227. [13] L.L. Lerner, J. Phys. Chem. Solids 27 (1966) 1.
One of the authors, K.T. Ramakrishna Reddy, wishes to thank the Council of Scientific and Industrial Research, New Delhi, for providing financial support.
[14] L. Mandel, RD. Tomlinson and M.J. Hampshire, J. Appl. Cryst. 10 (1977) 130. [15] H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures (Wiley, London, 1954) p. 491. [16] J.l. Pankove, Optical Pwccsses in Seiiiicoriductors (Dovei, New York, 1971) p. 34. [17] J.E. Jaffe and A. Zunger, Phys. Rev. B29 (1984) 1882.
References
[18] J.L. and K.V. Reddy, Indian J. Pure Appl. Phys.Annapurna 24 (1986) 283. [19] K.T. Ramakrishna Reddy and P. Jayarama Reddy, J. Mater. Sci. Letters 8 (1989) 110.
[1] L.L. Kazmerski and S. Wagner, in: Current Trends in Photovoltaics, Eds. T.J. Coutts and J.D. Meakin (Academic Press, London, 1985) p. 41.
765
Preparation and properties of laser evaporated CuGaSe2 thin films K.T. Ramakrishna Reddy and P. Jayarama Reddy Department of Physic5, Sri Venkateswara University, Tirupati-51 7 502, India
Received 29 June 1990; manuscript received in final form 14 September 1990
Bulk copper gallium diselenide material was prepared by melting the pure constituent elements in stoichiometric proportion. Thin films of CuGaSe2 were deposited by a laser evaporation technique. Both the bulk material and thin films were charactensed by XRD, SEM and AES. The effect of substrate temperature on the structure and composition, electrical and optical properties of the films has been studied. Polycrystalline stoichiometric films of CuGaSe2 can be deposited at substrate temperatures in the range 350 400 ° C and the films prepared at 370°C have a resistivity of about iO~ fJ cm. The energy band gap was found to be 1.68 eV at stoichiometric composition.
1. Introduction
2. Experimental
Among the available alternative materials to Si, the ternary chalcopyntes of the type Cu III V12 appear to be the most promising as candidates for use in photovoltaic devices [1,2]. CuGaSe2 is a member of this group which is a direct band gap semiconductor crystallising in the chalcopyrite structure [3,4]. Though studies on the growth and properties of this compound have been reported, both in bulk and in thin film form, detailed investigations are still required [5,6]. Thin films of this compound which is a prerequisite to the development of cost effective device structures have been fabricated by many techniques [7 10]. The laser evaporation is a relatively new technique for the deposition of CuGaSe2 thin films. In this technique, the kinetic energy of the incident particles in the vapour flux are found to be larger than the experimental thermal energy distribution which enhances ad-atom mobility and improves the crystallimty of the films [11,12]. The present paper reports the preparation and properties of CuGaSe2 thin films.
The starting material of CuGaSe2 was synthesised from a stoichiometric mixture of its constituent elements. Copper, gallium and selenium of 5N purity were taken in a sealed quartz ampoule evacuated to a pressure of 10 ~ Torr. The mixture was heated in a vertical furnace initially to a temperature of 200 °Cand kept for 24 h to ensure better homogeneity and to avoid the risk of explosion due to the exothermic reaction of gallium and selenium [13]. Then the temperature was slowly increased to 1200°Cand was maintained for 40 h. Finally, the ampoule was quenched to room temperature in cold water. The ingot obtained was pulvensed and made into small pellets for evaporation. CuGaSe2 thin films were prepared using a CO2 laser of 60 W power. The continuous laser beam entered the vacuum chamber through a vacuum tight ZnSe window. A gold coated concave mirror was used to reflect and concentrate the laser beam onto the source material kept in a carbon crucible. The experimental arrangement of the technique is
0022-0248/91/$03.50 © 1991
Elsevier Science Publishers B.V. (North-Holland)
766
K I Ramakrishna Reddy, P Jayarama Reddy
_______
A
SH
~i 2LASE
CuGaSe, thin films
The composition analysis of the films shows that the elemental atomic percentages depend critically on the substrate temperature. The AES composition data of the films formed at three different
S
CO
/ Laser evaporated
2
substrate are shownat inthefig.substrate 2. The films are temperatures nearly stoichiometric temperatures T3 in the range 350 400°C. For T,, < 350 °C, gallium was found to be in excess
M
~
and the films formed at T~>400°C were found to be copper rich. This behaviour may be due to the slight decomposition of the compound and the C
ABC H M
ALUMINIUM DISCS ALUMINIUM BASE CARBON CRUCIBLE HEATER GOLD COATED MIRROR
B
large difference in the vapour pressures of the constituent elements, as VP5e>> VPGO> VP~. The substrate temperature was found to have a significant influence on the structure of the films.
VA C UUM S SUBSTRATE (lxi cm°) Z-ZnSe WINDOW SH-SUBSTRATE HOLDER SP-SLIDINGPOST S1S2S3-SCREWS
T
300°C
Fig. 1. Schematic diagram of the laser beam focussing system. Go
shown in fig. 1. The films were prepared on Corning 7059 glass substrates at temperatures in the range 250 450°C in a vacuum better than 2 x 10 6 Torr with a deposition rate of 25 A/s. The film thickness was in the range, 0.4—0.8 ~.Lm.The composition of the bulk and the thin films was analysed by AES. The X-ray diffraction studies were carried out on both bulk and thin films using Cu Ka radiation. The standard van der Pauw technique was employed to study the electrical properties and the optical studies were measured by employing an Hitachi U:3400 UV VIS NIR spectrophotometer. Evaporated nickel was used as ohmic contacts to the films.
Go
Go
Go
Ts
370
C
Go
Go
~
3. Results and discussion The composition analysis made on the bulk material at different parts of the ingot showed no significant variation in the stoichiometry of the constituent elements, suggesting the homogeneous nature of the ingot. The XRD data obtained on the material are in agreement with the reported data of Mandel et al. [14].
Co
Go
Cu
I I
600
800
1100
1200
IL~W
1GIXJ
Energy (eV)
Fig. 2. AES profiles of laser evaporated CuGaSe2 films.
K. T. Ramakrishna Reddy, P. Jayarama Reddy
The XRD spectra of CuGaSe2 films formed at three different substrate temperatures and the corresponding SEM micrographs are shown in figs. 3 and 4, respectively. The films formed at temperatures below 350°C showed GaSe and Ga2Se3 as additional phases along with a CuGaSe2 phase. For deposition temperatures above 400°C, the films showed a Cu2 ~Se phase in addition to a CuGaSe2 phase. The reason for this is probably that the reaction velocities of the elements are such that RV~u+se>RVGa+Se> RVCu+Ga+5e [10]. This was also supported by the AES composition analysis of the films where excess of gallium was seen at 7 <350°C and excess of copper at T3>
/ Laser evaporated CuGaSe,
15 :
______
______________________________________________
11-
~JJJ
_________
.1
3
I
3
3
Fig. 4. Scanning electron micrographs of CuGaSe2 films: (a) copper deficient; (b) near stoichiometnc; (c) copper rich. Markers represent gm.
4
20
~
.~t. . . . . . . . . ~
42 5 C 3
767
____
~---
T~ 370C
thin films
30
40
50
60
2 0 Cd eg re e s) Fig. 3. XRD spectra of CuGaSe2 films formed at various substrate temperatures: (1) Ga2Se3 (2) GaSe; (3) CuGaSe2 (4) Cu2 ~Se.
400°C (fig. 2). With an increase of substrate temperature, the diffraction lines became sharp and intense, which revealed the improvement in crystallinity with substrate temperature. However, films prepared at 370°C showed single phase chalcopyrite structure with a grain size of about
768
K. I Ramakrishna Reddy, P. Jayarama Reddy
Laser evaporated CuGaSe, thin films
0.6 0.8 ~tm depending on the film thickness calculated using the Scherrer formula [15]. The variation of electrical resistivity with the substrate temperature of CuGaSe2 films is shown in fig. 5. At temperatures below 350°C there is not much variation in the resistivity and it decreases almost sharply above this temperature. The higher resistivity at the temperatures below 350°C is due to additional phases of higher resistivity such as GaSe and Ga2Se3 [6]. As the temperature increases the effect of these phases is not found to be significant and at about 400°C the further decrease in resistivity is due to the additional high conductivity phase of Cu2 ~Se [6]. This is in accordance with the XRD studies which indicate the different phases at different substrate temperatures. The optical absorption coefficient was calculated from the transmittance versus wavelength spectrum. Near the absorption edge the absorption coefficient is given by [16] A(hv
—
io~
\
10
\ \
10
~ 25 C
~
20
b
a
E
,,.‘°~
15
10
,“ 0 15
16
1 7 h~
18
19
20
(ev)
Eg)~
los
—
a 300 C
Fig. 6 (cihi’).’ versus he for CuGaSe2 films formed at different substrate temperatures.
2/hv, Cs
30
\ id
\ 0
10
~
it?
Substrate temperature ~c)
Fig. 5. Variation of resistivity with substrate temperature in CuGaSe2 films.
where A is an energy independent constant and hv is the photon energy. For direct allowed transitions n 1 and a plot of (ahv)2 versus hv will have an intercept with the hv axis which gives the band gap, Eg. Fig. 6 shows the variation of (ahv)2 versus hi.’ for the films formed at three different substrate temperatures. It is seen from the figure that as the substrate temperature increases, the band gap decreases due to the change in composition of the films. This may be caused by the superposition of two or three absorption curves due to different phases with different optical properties and impurity to band transitions in GaSe and Ga 2Se3, which can shift the absorption curve [6]. The change of band gap on the copper rich side is correlated with the existence of “d” bonding character of the Cu atoms where the copper d-orbitals push the valence band up, thereby reducing the band gap. Further, the band gap variation in ternary chalcopyrites is supported by the theosetical calculations of Jaffe and Zun~er [17]. The films formed at T~ 370° C showed an optical band gap of 1.68 eV, which is in good agreement with the reported value [18]. A detailed —
K. T. Ramakrishna Reddy, P. Jayarama Reddy
analysis of the optical properties was reported in an earlier paper [19].
4. Conclusions Polycrystalline, single phase and stoichiometric CuGaSe2 films can be prepared by a laser evaporation techniq’üe at substrate temperatures in the range 350 400°C. The substrate temperature plays a dominant role in the physical properties of the films. The CuGaSe2 films formed at a substrate temperature of 370°C showed single phase chalcopyrite structure resistivity of about 3 Q cm. The optical and bandagap was found to be i0 1.68 eV.
Acknowledgement
/ Laser evaporated
CuGaSe, thin films
769
[2] K. Zweibel, R. Mitchell and A. Hermann, in: Proc. 18th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 1985, p. 1393. [3] B. Tell and P.M. Bridenbaugh, Phys. Rev. B12 (1075) 3330. [4] J.L. Shay, B. Tell, H.M. Kasper and L.M. Schiavone, Phys. Rev. B5 (1972) 5003. [5] J. Stankiewicz, W. Giriat, J. Ramos and M.P. Vecchi, Solar Energy Mater. 1 (1979) 369. [6] W. Arndt, H. Dittrich and H.W. Schock, Thin Solid Films 130 (1985) 209. [7] N. Hong, H. Neumann, B. Schumann and G. Kuhn, Phys. Status Solidi (b) 85 (1978) K57. [8] KR. Murali. B.S.V. Gopalam and J. Sobhanadn, J. Mater Sci. Letters 5 (1986) 421. [9] R. Noufi. R. Powell. C. Herrington and T. Coutts. Solar 17 (1986) 303. [10] Cells H. Dittnch, B. Dimmier, R. Menner and H.W. Schock, in: Proc. 7th Intern. Conf. on Ternary and Multinary Compounds, Snowmass, CO, 1986, p. 161. [11] J.F. Frichtenicht, Rev. Sci. Instr. 45 (1974) 51. [12] T. Ishida, S. Wako and U. Ushino, Thin Solid Films 39 (1976) 227. [13] L.L. Lerner, J. Phys. Chem. Solids 27 (1966) 1.
One of the authors, K.T. Ramakrishna Reddy, wishes to thank the Council of Scientific and Industrial Research, New Delhi, for providing financial support.
[14] L. Mandel, RD. Tomlinson and M.J. Hampshire, J. Appl. Cryst. 10 (1977) 130. [15] H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures (Wiley, London, 1954) p. 491. [16] J.l. Pankove, Optical Pwccsses in Seiiiicoriductors (Dovei, New York, 1971) p. 34. [17] J.E. Jaffe and A. Zunger, Phys. Rev. B29 (1984) 1882.
References
[18] J.L. and K.V. Reddy, Indian J. Pure Appl. Phys.Annapurna 24 (1986) 283. [19] K.T. Ramakrishna Reddy and P. Jayarama Reddy, J. Mater. Sci. Letters 8 (1989) 110.
[1] L.L. Kazmerski and S. Wagner, in: Current Trends in Photovoltaics, Eds. T.J. Coutts and J.D. Meakin (Academic Press, London, 1985) p. 41.