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Preparation of p-type transparent conducting CuAlO2 thin films by reactive DC sputtering
Materials Letters 58 (2003) 10 – 13 www.elsevier.com/locate/matlet
Preparation of p-type transparent conducting CuAlO2 thin films by reactive DC sputtering A.N. Banerjee, R. Maity, K.K. Chattopadhyay * Department of Physics, Jadavpur University, Calcutta 700 032, India Received 27 January 2003; accepted 6 May 2003
Abstract P-type transparent conducting thin films of copper aluminate were prepared by reactive DC sputtering of a prefabricated target having 1:1 atomic ratio of Cu and Al. Films of CuAlO2 were deposited on Si (400) and glass substrates. The sputtering was performed in Ar + O2 (40 vol.%) atmosphere and the substrate temperature was 475 K. X-ray diffraction (XRD) spectra of the films showed the peaks which could be assigned with those of the crystalline CuAlO2. UV – Visible spectrophotometric measurement showed high transparency of the films in the visible region. Both direct and indirect band gaps were found to exist and their corresponding estimated values were 3.75 and 1.85 eV, respectively. The room temperature conductivity of the film was fairly high and was of the order of 0.22 S cm 1 while the activation energy was f 0.25 eV. Seebeck coefficient at room temperature gave a value of + 115 AV/K confirming the p-type conductivity. Room temperature Hall effect measurement also indicated positive value of Hall coefficient with a value RH = 14.1 cm3/C. D 2003 Elsevier Science B.V. All rights reserved. Keywords: P-type conductivity; CuAlO2 thin films; DC sputtering
1. Introduction Most of the known transparent conducting oxides (TCOs) and their doped versions such as ZnO 1 x , In1 xSnxO3, SnO2:F, etc. are all n-type materials. P-type TCOs have recently attracted much attention around the globe after Kawazoe et al. [1] reported p-type conductivity in a thin film of CuAlO2. This has opened up a new field in device technology, the so-called ‘transparent electronics’ [2] in which a junctional device can be fabricated which would transmit the visible portion of solar radiation, yet generates electricity from the UV part of it. Due to this tremendous potential of the p-type TCOs to revolutionize the optoelectronic device technology, various research groups around the globe are working on the synthesis of several p-type TCO thin films and also on device fabrications using them [3 –8]. Also infrared transparent spinel films such as NiCo2O4 with p-type conductivity were reported by Windisch et al. [9,10].
* Corresponding author. E-mail address: [email protected] (K.K. Chattopadhyay).
Defect chemistry plays an important role in making the metal oxides either p- or n-type. The p-type conductivity results due to the metal deficit (or excess oxygen) within the crystallite sites. Nonstoichiometric defect reaction for a metal deficient oxide may be represented by the following equation [11], þ 3 O2 ðgÞ ¼ 2OX O þ VCu þ VAl þ 4h
ð1Þ
where OO, VCu, VAl and h denote lattice oxygen, Cu vacancy, Al vacancy and hole, respectively. Superscripts X, and + denote effective neutral, negative and positive charge states, respectively. Fabrication of the material in an excess oxygen atmosphere can introduce p-type conductivity in it. Subsequent improvement in the conductivity can be obtained by regulating the preparation conditions. For the synthesis of CuAlO2 thin film, Kawazoe et al. [1] and Yanagi et al. [12] used pulsed laser deposition (PLD) of CuAlO2 target, which was fabricated by solid state reaction between Cu2O and Al2O3 powder and then by sintering. However, there are some limitations of the PLD method, such as costly equipment is involved and scaling-up of the technology is difficult. Hence it is of utmost importance to develop alternative deposition route for this important
0167-577X/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00395-1
A.N. Banerjee et al. / Materials Letters 58 (2003) 10–13
11
material. Recently, we have been able to synthesize [13] ptype CuAlO2 thin film by DC sputtering technique using the target of CuAlO2 fabricated by sintering Cu2O and Al2O3. In this letter, we have reported the preparation of CuAlO2 thin film, directly from Cu and Al metal powder through reactive DC sputtering in an excess oxygen atmosphere and at an elevated substrate temperature. This method not only removed the intermediate procedure of CuAlO2 powder preparation, but also, due to higher conductivity of metal powders than that of CuAlO2 powder, DC sputtering technique can more adequately be applied.
2. Experimental
Fig. 1. XRD pattern of CuAlO2 thin film deposited on Si (400) substrate.
Ultra pure powder of Cu (99.999%) and Al (99.999%) were taken at 1:1 atomic ratio and mixed thoroughly for 1.5 h. The mixture was then pressed with a hydrostatic pressure of 150 kgf/cm2 to form pellets. The pellets were then used as targets for sputtering. Aluminium holder of diameter f 5 cm with appropriate arrangements was used to hold the target. Negative terminal of the DC generator was connected with the target and the substrates were placed on the grounded electrode. Si (400) and glass were used as substrates for film deposition. The substrates were cleaned ultrasonically in 20% HF, acetone and methanol for Si while in acetone and methanol for glass substrates. Before the deposition, the chamber was evacuated by standard rotary and diffusion pumping arrangements to a base pressure of 10 6 mbar. Subsequently, the chamber was flushed with Ar several times and then the target was pre-sputtered at 0.05 mbar in Ar atmoshphere for 10 min to remove the contaminations, if any, present on the target surface. The summary of the deposition conditions is shown in Table 1. After every 2 h of deposition, the films were post annealed in the same vacuum chamber at 493 K for 1 h (at pressure 0.2 mbar) maintaining the oxygen flow. The deposited thin films were characterized by X-ray diffraction (XRD, Philips PW 1730/PW 1710, by CuKa line) to study the structural properties. UV – Vis spectrophotometer (HITACHI—U3410) was used to determine the optical properties. Thermoelectric power (Seebeck coefficient) and Hall effect study were used to determine the type of conduction taking place in the deposited films. Temperature dependence of electrical conductivity of the films was studied by standard two-probe method using Kiethley multiTable 1 Summary of deposition parameters Electrode distance Sputtering voltage Current density Substrates Sputtering gasses Deposition pressure Substrate temperature Deposition time
1.8 cm 1.0 kV 12 mA/cm2 Si (400), glass Ar and O2 (3:2 volume ratio) 0.2 mbar 475 K 4h
meter (Model—6514) from 300 to 550 K. The contact was made with silver paint which showed linear I– V characteristic over a wide range of applied voltage.
3. Results and discussion Fig. 1 shows the XRD spectrum of the film deposited on Si substrate without background correction. It shows a strong (006) orientation. Previous workers [1,12] also obtained similar orientation for CuAlO2 films deposited by PLD method on sapphire substrate. Along with the above peak, other peaks were also observed in the XRD spectrum which could be assigned for (003), (101), (012), (104) and (018) reflections of crystalline CuAlO2. Also no peaks corresponding to starting materials, for example, Cu and Al metal powders as well as their oxides were found in the pattern. This conclusively indicated that the reactants were stoichiometrically mixed to give the proper phase of the copper aluminium oxide and no residual metal oxides remained in the film. The information on strain and the particle size of the deposited films was obtained from the full widths at half maximum (FWHM) of the diffraction peaks. The FWHM (b) can be expressed as a linear combination of the contributions from the strain (e) and particle size (L) through the following relation [14], bcosh=k ¼ 1=L þ esinh=k
ð2Þ
Fig. 2 gives the plot of b cosh/k vs. sinh/k. From the slope and intercepts, the strain and the particle size were determined. The particle size and strain were found to be f32 nm and 2.7 10 2, respectively. Fig. 3 shows the optical transmittance spectrum of the film deposited on glass substrate. It depicts that the film is fairly transparent in the visible region. From the transmittance data, using Manifacier model [15], we have calculated absorption coefficients (a) at the region of strong absorption. The fundamental absorption, which corresponds to
12
A.N. Banerjee et al. / Materials Letters 58 (2003) 10–13
Fig. 2. Plot to determine strain and particle size of CuAlO2 thin film deposited on Si substrate.
Fig. 4. Test for direct allowed band gap transition for CuAlO2 thin film. Inset: Determination of indirect band gap.
electron excitation from the valance band to conduction band, can be used to determine the nature and value of the optical band gap. The relation between the absorption coefficients (a) and the incident photon energy (hm) can be written as [16],
thin films was studied below room temperature by previous authors [1,12], but no study on high temperature conduction was reported. The straight line nature of the Arhenius plot indicated the thermally activated conduction as often found in semiconductors. Room temperature conductivity of the film was obtained as 0.22 S cm 1, which is comparable with that obtained by Kawazoe et al. for their PLD deposited films [1]. From the slope of the graph, we get the value of activation energy (Ea) which corresponds to the minimum energy required to transfer carriers from acceptor level to the valence band and the value of Ea comes out as 0.25 eV. Seebeck coefficient of the film was determined at room temperature by measuring the thermoelectric power which gave a value of + 115 AV/K. Hall coefficient of the films was determined to be RH = 14.1 cm 3 C 1, corresponding to carrier density 4.4 1017 cm 3. Positive value of the Seebeck coeficient and Hall coefficient confirmed the p-type conductivity of the film.
ðahmÞ1=n ¼ Aðhm Eg Þ
ð3Þ
where A is a constant and Eg is the band gap of the material and exponent n depends on the type of transition. For indirect allowed transition, n = 2; for direct allowed, n = 1/ 2. The (ahm)2 vs. hm plot is shown in Fig. 4. Extrapolating the linear portion of the graph to the hm axis, we have obtained the direct allowed band gap from the intercept on hm axis which comes to be f 3.75 eV. The inset of Fig. 4 shows (ahm)1/2 vs. hm plot and the indirect band gap comes out to be f 1.85 eV. These values agree well with the values reported by Kawazoe et al. [1] for their PLD deposited films from CuAlO2 pellet targets. Fig. 5 represents lnr vs. 1/T plot of the CuAlO2 film on glass substrate from room temperature (300 K) to 550 K. The temperature variation of conductivity of the CuAlO2
Fig. 3. Optical transmission spectrum of CuAlO2 thin film deposited on glass substrate.
Fig. 5. Temperature dependence of electrical conductivity of CuAlO2 thin films.
A.N. Banerjee et al. / Materials Letters 58 (2003) 10–13
4. Conclusions To summarize, we have successfully deposited polycrystalline p-type semiconducting CuAlO2 thin films on glass and Si (400) substrates by DC sputtering of a target, fabricated from a stoichiometric mixture of Cu and Al metal powders. XRD spectrum confirmed the polycrystalline nature of the films with small grain size ( f 32 nm). The films were transparent in the visible region. Both allowed direct and indirect band gaps were found to exist and their corresponding values were 3.75 and 1.85 eV, respectively. The p-type conductivity was confirmed by positive value of the Seebeck and Hall coefficient. The film showed fairly high room temperature conductivity of the order of 0.22 S cm 1. This may be due to nonstoichiometric defect attributed from excess oxygen atmosphere introduced into the system during deposition.
Acknowledgements Two of us (A.N.B.) and (R.M.) wish to thank CSIR, Governement of India for awarding them junior research fellowship (JRF) during the work.
References [1] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939.
13
[2] G. Thomas, Nature 389 (1997) 907. [3] H. Kawazoe, H. Yanagi, K. Ueda, H. Hosono, MRS Bull. 25 (2000) 28. [4] R. Nagarajan, A.D. Draeseke, A.W. Sleight, J. Tate, J. Appl. Phys. 89 (2001) 8022. [5] M.K. Jayaraj, A.D. Draeseke, J. Tate, A.W. Sleight, Thin Solid Films 397 (2001) 244. [6] N. Duan, A.W. Sleight, M.K. Jayaraj, J. Tate, Appl. Phys. Lett. 77 (2000) 1325. [7] A. Kudo, H. Yanagi, H. Hosono, H. Kawazoe, Appl. Phys. Lett. 73 (1998) 220. [8] A. Kudo, H. Yanagi, K. Ueda, H. Hosona, H. Kawazoe, Y. Yano, Appl. Phys. Lett. 75 (1999) 2851. [9] C.F. Windisch Jr., K.F. Ferris, G.J. Exarhos, J. Vac. Sci. Technol., A 19 (2001) 1647. [10] C.F. Windisch Jr., G.J. Exarhos, K.F. Ferris, M.H. Engelhard, D.C. Stewart, Thin Solid Films 45 (2001) 398 – 399. [11] P. Kofstad, Nonstoichiometry, Diffusion, and Electrical Conductivity in Binary Metal Oxides, Wiley-Interscience, New York, 1972, p. 19. [12] H. Yanagi, S. Inoue, K. Ueda, H. Kawazoe, H. Hosono, N. Hamada, J Appl. Phys. 88 (2000) 4159. [13] A.N. Banerjee, S Kundoo, K.K. Chattopadhyay, Thin Solid Films, in press. [14] S.B. Quadri, E.F. Skelton, D. Hsu, A.D. Dinsmore, J. Yang, H.F. Gray, B.R. Ratna, Phys. Rev., B 60 (1999) 9191. [15] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9 (1976) 1002. [16] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, Englewood Cliffs, 1971, p. 34.
Preparation of p-type transparent conducting CuAlO2 thin films by reactive DC sputtering A.N. Banerjee, R. Maity, K.K. Chattopadhyay * Department of Physics, Jadavpur University, Calcutta 700 032, India Received 27 January 2003; accepted 6 May 2003
Abstract P-type transparent conducting thin films of copper aluminate were prepared by reactive DC sputtering of a prefabricated target having 1:1 atomic ratio of Cu and Al. Films of CuAlO2 were deposited on Si (400) and glass substrates. The sputtering was performed in Ar + O2 (40 vol.%) atmosphere and the substrate temperature was 475 K. X-ray diffraction (XRD) spectra of the films showed the peaks which could be assigned with those of the crystalline CuAlO2. UV – Visible spectrophotometric measurement showed high transparency of the films in the visible region. Both direct and indirect band gaps were found to exist and their corresponding estimated values were 3.75 and 1.85 eV, respectively. The room temperature conductivity of the film was fairly high and was of the order of 0.22 S cm 1 while the activation energy was f 0.25 eV. Seebeck coefficient at room temperature gave a value of + 115 AV/K confirming the p-type conductivity. Room temperature Hall effect measurement also indicated positive value of Hall coefficient with a value RH = 14.1 cm3/C. D 2003 Elsevier Science B.V. All rights reserved. Keywords: P-type conductivity; CuAlO2 thin films; DC sputtering
1. Introduction Most of the known transparent conducting oxides (TCOs) and their doped versions such as ZnO 1 x , In1 xSnxO3, SnO2:F, etc. are all n-type materials. P-type TCOs have recently attracted much attention around the globe after Kawazoe et al. [1] reported p-type conductivity in a thin film of CuAlO2. This has opened up a new field in device technology, the so-called ‘transparent electronics’ [2] in which a junctional device can be fabricated which would transmit the visible portion of solar radiation, yet generates electricity from the UV part of it. Due to this tremendous potential of the p-type TCOs to revolutionize the optoelectronic device technology, various research groups around the globe are working on the synthesis of several p-type TCO thin films and also on device fabrications using them [3 –8]. Also infrared transparent spinel films such as NiCo2O4 with p-type conductivity were reported by Windisch et al. [9,10].
* Corresponding author. E-mail address: [email protected] (K.K. Chattopadhyay).
Defect chemistry plays an important role in making the metal oxides either p- or n-type. The p-type conductivity results due to the metal deficit (or excess oxygen) within the crystallite sites. Nonstoichiometric defect reaction for a metal deficient oxide may be represented by the following equation [11], þ 3 O2 ðgÞ ¼ 2OX O þ VCu þ VAl þ 4h
ð1Þ
where OO, VCu, VAl and h denote lattice oxygen, Cu vacancy, Al vacancy and hole, respectively. Superscripts X, and + denote effective neutral, negative and positive charge states, respectively. Fabrication of the material in an excess oxygen atmosphere can introduce p-type conductivity in it. Subsequent improvement in the conductivity can be obtained by regulating the preparation conditions. For the synthesis of CuAlO2 thin film, Kawazoe et al. [1] and Yanagi et al. [12] used pulsed laser deposition (PLD) of CuAlO2 target, which was fabricated by solid state reaction between Cu2O and Al2O3 powder and then by sintering. However, there are some limitations of the PLD method, such as costly equipment is involved and scaling-up of the technology is difficult. Hence it is of utmost importance to develop alternative deposition route for this important
0167-577X/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00395-1
A.N. Banerjee et al. / Materials Letters 58 (2003) 10–13
11
material. Recently, we have been able to synthesize [13] ptype CuAlO2 thin film by DC sputtering technique using the target of CuAlO2 fabricated by sintering Cu2O and Al2O3. In this letter, we have reported the preparation of CuAlO2 thin film, directly from Cu and Al metal powder through reactive DC sputtering in an excess oxygen atmosphere and at an elevated substrate temperature. This method not only removed the intermediate procedure of CuAlO2 powder preparation, but also, due to higher conductivity of metal powders than that of CuAlO2 powder, DC sputtering technique can more adequately be applied.
2. Experimental
Fig. 1. XRD pattern of CuAlO2 thin film deposited on Si (400) substrate.
Ultra pure powder of Cu (99.999%) and Al (99.999%) were taken at 1:1 atomic ratio and mixed thoroughly for 1.5 h. The mixture was then pressed with a hydrostatic pressure of 150 kgf/cm2 to form pellets. The pellets were then used as targets for sputtering. Aluminium holder of diameter f 5 cm with appropriate arrangements was used to hold the target. Negative terminal of the DC generator was connected with the target and the substrates were placed on the grounded electrode. Si (400) and glass were used as substrates for film deposition. The substrates were cleaned ultrasonically in 20% HF, acetone and methanol for Si while in acetone and methanol for glass substrates. Before the deposition, the chamber was evacuated by standard rotary and diffusion pumping arrangements to a base pressure of 10 6 mbar. Subsequently, the chamber was flushed with Ar several times and then the target was pre-sputtered at 0.05 mbar in Ar atmoshphere for 10 min to remove the contaminations, if any, present on the target surface. The summary of the deposition conditions is shown in Table 1. After every 2 h of deposition, the films were post annealed in the same vacuum chamber at 493 K for 1 h (at pressure 0.2 mbar) maintaining the oxygen flow. The deposited thin films were characterized by X-ray diffraction (XRD, Philips PW 1730/PW 1710, by CuKa line) to study the structural properties. UV – Vis spectrophotometer (HITACHI—U3410) was used to determine the optical properties. Thermoelectric power (Seebeck coefficient) and Hall effect study were used to determine the type of conduction taking place in the deposited films. Temperature dependence of electrical conductivity of the films was studied by standard two-probe method using Kiethley multiTable 1 Summary of deposition parameters Electrode distance Sputtering voltage Current density Substrates Sputtering gasses Deposition pressure Substrate temperature Deposition time
1.8 cm 1.0 kV 12 mA/cm2 Si (400), glass Ar and O2 (3:2 volume ratio) 0.2 mbar 475 K 4h
meter (Model—6514) from 300 to 550 K. The contact was made with silver paint which showed linear I– V characteristic over a wide range of applied voltage.
3. Results and discussion Fig. 1 shows the XRD spectrum of the film deposited on Si substrate without background correction. It shows a strong (006) orientation. Previous workers [1,12] also obtained similar orientation for CuAlO2 films deposited by PLD method on sapphire substrate. Along with the above peak, other peaks were also observed in the XRD spectrum which could be assigned for (003), (101), (012), (104) and (018) reflections of crystalline CuAlO2. Also no peaks corresponding to starting materials, for example, Cu and Al metal powders as well as their oxides were found in the pattern. This conclusively indicated that the reactants were stoichiometrically mixed to give the proper phase of the copper aluminium oxide and no residual metal oxides remained in the film. The information on strain and the particle size of the deposited films was obtained from the full widths at half maximum (FWHM) of the diffraction peaks. The FWHM (b) can be expressed as a linear combination of the contributions from the strain (e) and particle size (L) through the following relation [14], bcosh=k ¼ 1=L þ esinh=k
ð2Þ
Fig. 2 gives the plot of b cosh/k vs. sinh/k. From the slope and intercepts, the strain and the particle size were determined. The particle size and strain were found to be f32 nm and 2.7 10 2, respectively. Fig. 3 shows the optical transmittance spectrum of the film deposited on glass substrate. It depicts that the film is fairly transparent in the visible region. From the transmittance data, using Manifacier model [15], we have calculated absorption coefficients (a) at the region of strong absorption. The fundamental absorption, which corresponds to
12
A.N. Banerjee et al. / Materials Letters 58 (2003) 10–13
Fig. 2. Plot to determine strain and particle size of CuAlO2 thin film deposited on Si substrate.
Fig. 4. Test for direct allowed band gap transition for CuAlO2 thin film. Inset: Determination of indirect band gap.
electron excitation from the valance band to conduction band, can be used to determine the nature and value of the optical band gap. The relation between the absorption coefficients (a) and the incident photon energy (hm) can be written as [16],
thin films was studied below room temperature by previous authors [1,12], but no study on high temperature conduction was reported. The straight line nature of the Arhenius plot indicated the thermally activated conduction as often found in semiconductors. Room temperature conductivity of the film was obtained as 0.22 S cm 1, which is comparable with that obtained by Kawazoe et al. for their PLD deposited films [1]. From the slope of the graph, we get the value of activation energy (Ea) which corresponds to the minimum energy required to transfer carriers from acceptor level to the valence band and the value of Ea comes out as 0.25 eV. Seebeck coefficient of the film was determined at room temperature by measuring the thermoelectric power which gave a value of + 115 AV/K. Hall coefficient of the films was determined to be RH = 14.1 cm 3 C 1, corresponding to carrier density 4.4 1017 cm 3. Positive value of the Seebeck coeficient and Hall coefficient confirmed the p-type conductivity of the film.
ðahmÞ1=n ¼ Aðhm Eg Þ
ð3Þ
where A is a constant and Eg is the band gap of the material and exponent n depends on the type of transition. For indirect allowed transition, n = 2; for direct allowed, n = 1/ 2. The (ahm)2 vs. hm plot is shown in Fig. 4. Extrapolating the linear portion of the graph to the hm axis, we have obtained the direct allowed band gap from the intercept on hm axis which comes to be f 3.75 eV. The inset of Fig. 4 shows (ahm)1/2 vs. hm plot and the indirect band gap comes out to be f 1.85 eV. These values agree well with the values reported by Kawazoe et al. [1] for their PLD deposited films from CuAlO2 pellet targets. Fig. 5 represents lnr vs. 1/T plot of the CuAlO2 film on glass substrate from room temperature (300 K) to 550 K. The temperature variation of conductivity of the CuAlO2
Fig. 3. Optical transmission spectrum of CuAlO2 thin film deposited on glass substrate.
Fig. 5. Temperature dependence of electrical conductivity of CuAlO2 thin films.
A.N. Banerjee et al. / Materials Letters 58 (2003) 10–13
4. Conclusions To summarize, we have successfully deposited polycrystalline p-type semiconducting CuAlO2 thin films on glass and Si (400) substrates by DC sputtering of a target, fabricated from a stoichiometric mixture of Cu and Al metal powders. XRD spectrum confirmed the polycrystalline nature of the films with small grain size ( f 32 nm). The films were transparent in the visible region. Both allowed direct and indirect band gaps were found to exist and their corresponding values were 3.75 and 1.85 eV, respectively. The p-type conductivity was confirmed by positive value of the Seebeck and Hall coefficient. The film showed fairly high room temperature conductivity of the order of 0.22 S cm 1. This may be due to nonstoichiometric defect attributed from excess oxygen atmosphere introduced into the system during deposition.
Acknowledgements Two of us (A.N.B.) and (R.M.) wish to thank CSIR, Governement of India for awarding them junior research fellowship (JRF) during the work.
References [1] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939.
13
[2] G. Thomas, Nature 389 (1997) 907. [3] H. Kawazoe, H. Yanagi, K. Ueda, H. Hosono, MRS Bull. 25 (2000) 28. [4] R. Nagarajan, A.D. Draeseke, A.W. Sleight, J. Tate, J. Appl. Phys. 89 (2001) 8022. [5] M.K. Jayaraj, A.D. Draeseke, J. Tate, A.W. Sleight, Thin Solid Films 397 (2001) 244. [6] N. Duan, A.W. Sleight, M.K. Jayaraj, J. Tate, Appl. Phys. Lett. 77 (2000) 1325. [7] A. Kudo, H. Yanagi, H. Hosono, H. Kawazoe, Appl. Phys. Lett. 73 (1998) 220. [8] A. Kudo, H. Yanagi, K. Ueda, H. Hosona, H. Kawazoe, Y. Yano, Appl. Phys. Lett. 75 (1999) 2851. [9] C.F. Windisch Jr., K.F. Ferris, G.J. Exarhos, J. Vac. Sci. Technol., A 19 (2001) 1647. [10] C.F. Windisch Jr., G.J. Exarhos, K.F. Ferris, M.H. Engelhard, D.C. Stewart, Thin Solid Films 45 (2001) 398 – 399. [11] P. Kofstad, Nonstoichiometry, Diffusion, and Electrical Conductivity in Binary Metal Oxides, Wiley-Interscience, New York, 1972, p. 19. [12] H. Yanagi, S. Inoue, K. Ueda, H. Kawazoe, H. Hosono, N. Hamada, J Appl. Phys. 88 (2000) 4159. [13] A.N. Banerjee, S Kundoo, K.K. Chattopadhyay, Thin Solid Films, in press. [14] S.B. Quadri, E.F. Skelton, D. Hsu, A.D. Dinsmore, J. Yang, H.F. Gray, B.R. Ratna, Phys. Rev., B 60 (1999) 9191. [15] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9 (1976) 1002. [16] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, Englewood Cliffs, 1971, p. 34.