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Influence of annealing treatment on the optical properties and structure of Cd2SnO4 thin films
Surface and Coatings Technology 167 (2003) 284–287
Influence of annealing treatment on the optical properties and structure of Cd2SnO4 thin films W.L. Wang*, K.J. Liao, C.Z. Cai, G.B. Liu, Y. Ma Department of Applied Physics, Chongqing University, Chongqing 400044, PR China
Abstract The optical properties of Cd2SnO4 films were investigated after annealing treatment. Cd2 SnO4 thin films on glass substrates were prepared by r.f. reactive sputtering from Cd–Sn alloy target in an Ar–O2 atmosphere. The films obtained were characterized by scanning electron microscopy, X-ray diffraction and optical transmission spectra. The experimental results showed that the post-deposition annealing treatment has a significant influence on the properties and structure of the films. The transmittance and optical gap energy were increased with increasing annealing temperature. These results can be explained by the changes in the oxygen vacancies and the Burstein shift effect of Cd2SnO4 films. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cd2SnO4 films; Optical transmittance; Oxygen vacancies
1. Introduction Cd2SnO4 thin films are an n-type defect semiconductor materials. The films of ternary oxides show extremely promising properties such as low metal-like electrical resistivity (10y6 V m), high transmissivity (090%) in the visible range of the light spectrum and high reflectivity in the IR range w1–4x. They can be used similarly to the well known In2O3:Sn films, as transparent electrodes, antistatic layers, heat mirrors in optoelectronic and solar energy conversion technology w5–7x. The optical and electrical properties of these films were found to be sensitive to deposition conditions. In this paper, the effect of annealing treatment on the optical properties and energy band structure of Cd2SnO4 films was investigated. 2. Experimental details Cd2SnO4 thin films in this study were deposited on glass substrates by radio frequency reactive sputtering from a Cd–Sn alloy target in an Ar–O2 mixture atmosphere. Cd–Sn alloy target at atomic ratio of 2:1, 100 mm in diameter made of metal Cd and Sn of purity *Corresponding author. Tel.: q86-23-65103490; fax: q86-2365106704. E-mail address: [email protected] (W.L. Wang).
99.99% was used. The r.f. power was in the range of 100–400 W. The pressure of the ArqO2 mixture was approximately 6 Pa. The substrates were mounted onto the water-cooled sample holder equipped with a heater. The water-cooled target-to-substrate distance was kept at 60 mm. The substrates were heated at 473 K for 15 min in vacuum before sputtering. The deposition time was 30 min. The post-deposition heat treatment of the films was carried out in Ar at 400 8C for 40 min. The films obtained were characterized by SEM, X-ray diffraction and optical and electrical measurement. Fig. 1 shows the X-ray diffraction patterns of Cd2SnO4 thin films with an oxygen concentration of 10% in Ar gas. The X-ray diffraction analysis shows that the Cd2SnO4 thin films were mostly polycrystalline with the major part of the films in the cubic spinal phase, with only a small SnO2 phase. The lattice constant was approximately as0.9182"0.0005 nm, which was determined by X-ray diffraction data. Fig. 2 shows the SEM images of a Cd2SnO4 thin film. Fig. 2a and b are SEM micrographs without and with annealing treatment. Cd2SnO4 films consisted of nanograins with no any metallurgical faces. The grains are slightly expanded after annealing treatment at 400 8C. 3. Results and discussion Fig. 3 shows the changes of the optical transmission
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(02)00920-9
W.L. Wang et al. / Surface and Coatings Technology 167 (2003) 284–287
285
Fig. 1. X-ray diffraction patterns of Cd2 SnO4 films with oxygen concentration of 10% in Ar.
spectra of Cd2SnO4 films. Fig. 3a and b are the spectrum without and with annealing treatment at 400 8C for 40 min in a argon atmosphere. From Fig. 3a, it can be seen that the optical transmittance in the visible region of the light spectrum is greater than 85% with an electrical conductivity of the order of 10y5 Vy1 my1 measured by electrical four-probe method. The shifting of the fundamental optical absorption edge towards shorter wavelengths is clearly seen after-annealed films. The relationship between the transmittance and annealing temperature was shown in Fig. 4. The transmittance maximum of 85% was used as measurement of transmittance changes with annealing temperature. The annealing treatment was performed in an Ar atmosphere. It was kept for 40 min at every annealing temperature. The transmittance of the films was increased with increasing annealing temperature. The transmittance increased from 85 to 90% when the annealing temperature increased from 400 to 510 8C. Similarly, the optical transmittance of the films was also depended on the
Fig. 2. SEM images of Cd2SnO4 films; (a) without and (b) with annealing treatment.
Fig. 3. Changes of optical transmission spectra after annealing treatment at 400 8C, (a) not annealing treatment; (b) annealing treatment.
oxygen concentration in Ar and substrate temperature w8,9x, i.e. the transmittance to be increased with the decreasing the oxygen concentration and increasing substrate temperature. The optical transmittance of the films can be expressed by w10x
Ts
Ž1yR1.Ž1yR2.Ž1yR3.exp(yat) Ž1yR1R2.µ1yR2R3exp(y2at)y2yR2R3expwcosf(yat)x∂
(1) where a is optical absorption coefficient, R1sw(nsy1)y (nsq1)x2, R2sw(nfyns)y(nf qns)x2 , R3sw(nfy1)y(nfq 1)x2, fs4pnftyl, ns and nf are refractive index of glass
Fig. 4. Relationship between the optical transmittance and annealing temperature.
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W.L. Wang et al. / Surface and Coatings Technology 167 (2003) 284–287
talline Cd2SnO4 films, and that only the conduction band has high curvature w13x, then 3y2 nsŽ8py3h3.Ž2mU e DE.
Fig. 5. Changes of optical gap energy with annealing temperature.
substrate and Cd2SnO4 films, respectively, t is thickness of Cd2SnO4 films and l is wavelength. The refractive index of Cd2SnO4 films was calculated by w11x nfsl1l2y2tŽl1yl2.
(2)
where l1 and l2 are the wavelength of two neighboring interference fringes (maximal or minimal) in the visible region of transmission spectrum for Cd2SnO4 films. From Eqs. (1) and (2), the values of a in the region of the absorption edge could be obtained, and the optical gap energy Eopt could be given by w12x (ahn)1y2sBŽhnyEopt.
(3)
where B is a constant. A plot of (ahn)1y2 vs. hn should be linear, and the intercept of the line on the abscissa at (ahn)1y2s0 yields the optical gap energy Eopt of Cd2SnO4 films. The changes in the optical gap energy with annealing temperature are shown in Fig. 5. The optical gap energy is approximately 2.64 eV after annealing treatment at 500 8C, which is increased by 6.5% as compared with untreated films. Cd2SnO4 thin films with spinal structure are an ntype defect semiconductor in which oxygen vacancies provide the donor states. The extra electronic charges in Cd2SnO4 films are trapped only at the oxygen vacancies. The oxygen vacancies can be increased by the annealing treatment, and led to increasing carrier concentration. Our Hall effect measurements showed that the carrier concentration was increased from 1=1026 to 4.46=1026 ym3 after annealing treatment at 500 8C. If one assumes spherical energy surface for the crys-
(4)
where n is the free carrier concentration, DE is the Burstein shift w14x, namely DEsEoptyEg, and mU e is the effective mass of conduction electron. The intrinsic optical gap energy (Eg) of Cd2SnO4 films was approximately 2.156 eV by measuring photoluminescence spectra. From Eq. (4), it is quite apparent that the conduction band generates a shift towards high energy level after annealing treatment. In fact, our experimental results have demonstrated that the optical gap energy values are all larger than Eg before and after heat treatment. This means that the Cd2SnO4 films are degenerate semiconductor. The value of mU e can be calculated from Eq. (4), and the ratio of mU e yme can also be obtained. me is free electron mass. The results showed that the effective mass of the conduction electron is very small, and increases with increasing carrier concentration. The electron effective mass in Cd2SnO4 films is max 0.5 me, which is in good agreement with that obtained by Pisarkiewicz using measuring thermoelectric power w15x. 4. Conclusion Cd2SnO4 thin films belong to a group of broadband gap semiconductors that show remarkable properties of being highly conductive while still transparent in the visible region of the light spectra. The experimental results showed the post-deposition annealing treatment has an important effect on the optical properties and structure of Cd2SnO4 films. The transmittance and optical gap energy were increased after annealing treatment of 400 8C in Ar atmosphere for 40 min. This is attributed to the Burstein shift effect. Acknowledgments This work was supported by the National High Technology Foundation of China under Grant No. 2001AA513011. References w1x R.R. Mehta, S.F. Vogel, J. Electrochem. Soc. 119 (1972) 752. w2x J.C.C. Fan, F.J. Bachner, Appl. Opt. 15 (1976) 1012. w3x J.C. Manifacier, L. Szepessy, J.F. Bresse, et al., Mater. Res. Bull. 14 (1979) 163. w4x D.L. Peng, S.R. Jiang, L. Xie, Phys. Stat. Sol. 12 (1985) 57. w5x Y.F. Dong, W.L. Wang, K.J. Kiao, Sensors Actuators B 67 (2000) 254. w6x X. Wu, R. Sheldon, et al., J. Appl. Phys. 89 (2001) 4564. w7x X. Wu, J.J. Coutts, US Pattent No. 6221495, 2001.
W.L. Wang et al. / Surface and Coatings Technology 167 (2003) 284–287 w8x D.L. Peng, S.R. Jiang, W.L. Wang, Acta Energiae Solaris Sin. 13 (1992) 291. w9x S.R. Jiang, D.L. Peng, W.L. Wang, et al., Acta Energiae Solaris Sin. 13 (1992) 118. w10x W.L. Wang, K.J. Liao, et al., Acta Energiae Solaris Sin. 11 (1990) 335.
w11x w12x w13x w14x w15x
A.J. Nozik, Phys. Rev. B 6 (1972) 453. N. Miyata, et al., J. Electrochem. Soc. 127 (1980) 918. T.S. Moss, Proc. Phys. Soc. Lond. B 67 (1954) 775. E. Burstein, Phys. Rev. 93 (1954) 632. T. Pisarkiewicz, Thin Solid Films 153 (1987) 479.
287
Influence of annealing treatment on the optical properties and structure of Cd2SnO4 thin films W.L. Wang*, K.J. Liao, C.Z. Cai, G.B. Liu, Y. Ma Department of Applied Physics, Chongqing University, Chongqing 400044, PR China
Abstract The optical properties of Cd2SnO4 films were investigated after annealing treatment. Cd2 SnO4 thin films on glass substrates were prepared by r.f. reactive sputtering from Cd–Sn alloy target in an Ar–O2 atmosphere. The films obtained were characterized by scanning electron microscopy, X-ray diffraction and optical transmission spectra. The experimental results showed that the post-deposition annealing treatment has a significant influence on the properties and structure of the films. The transmittance and optical gap energy were increased with increasing annealing temperature. These results can be explained by the changes in the oxygen vacancies and the Burstein shift effect of Cd2SnO4 films. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cd2SnO4 films; Optical transmittance; Oxygen vacancies
1. Introduction Cd2SnO4 thin films are an n-type defect semiconductor materials. The films of ternary oxides show extremely promising properties such as low metal-like electrical resistivity (10y6 V m), high transmissivity (090%) in the visible range of the light spectrum and high reflectivity in the IR range w1–4x. They can be used similarly to the well known In2O3:Sn films, as transparent electrodes, antistatic layers, heat mirrors in optoelectronic and solar energy conversion technology w5–7x. The optical and electrical properties of these films were found to be sensitive to deposition conditions. In this paper, the effect of annealing treatment on the optical properties and energy band structure of Cd2SnO4 films was investigated. 2. Experimental details Cd2SnO4 thin films in this study were deposited on glass substrates by radio frequency reactive sputtering from a Cd–Sn alloy target in an Ar–O2 mixture atmosphere. Cd–Sn alloy target at atomic ratio of 2:1, 100 mm in diameter made of metal Cd and Sn of purity *Corresponding author. Tel.: q86-23-65103490; fax: q86-2365106704. E-mail address: [email protected] (W.L. Wang).
99.99% was used. The r.f. power was in the range of 100–400 W. The pressure of the ArqO2 mixture was approximately 6 Pa. The substrates were mounted onto the water-cooled sample holder equipped with a heater. The water-cooled target-to-substrate distance was kept at 60 mm. The substrates were heated at 473 K for 15 min in vacuum before sputtering. The deposition time was 30 min. The post-deposition heat treatment of the films was carried out in Ar at 400 8C for 40 min. The films obtained were characterized by SEM, X-ray diffraction and optical and electrical measurement. Fig. 1 shows the X-ray diffraction patterns of Cd2SnO4 thin films with an oxygen concentration of 10% in Ar gas. The X-ray diffraction analysis shows that the Cd2SnO4 thin films were mostly polycrystalline with the major part of the films in the cubic spinal phase, with only a small SnO2 phase. The lattice constant was approximately as0.9182"0.0005 nm, which was determined by X-ray diffraction data. Fig. 2 shows the SEM images of a Cd2SnO4 thin film. Fig. 2a and b are SEM micrographs without and with annealing treatment. Cd2SnO4 films consisted of nanograins with no any metallurgical faces. The grains are slightly expanded after annealing treatment at 400 8C. 3. Results and discussion Fig. 3 shows the changes of the optical transmission
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(02)00920-9
W.L. Wang et al. / Surface and Coatings Technology 167 (2003) 284–287
285
Fig. 1. X-ray diffraction patterns of Cd2 SnO4 films with oxygen concentration of 10% in Ar.
spectra of Cd2SnO4 films. Fig. 3a and b are the spectrum without and with annealing treatment at 400 8C for 40 min in a argon atmosphere. From Fig. 3a, it can be seen that the optical transmittance in the visible region of the light spectrum is greater than 85% with an electrical conductivity of the order of 10y5 Vy1 my1 measured by electrical four-probe method. The shifting of the fundamental optical absorption edge towards shorter wavelengths is clearly seen after-annealed films. The relationship between the transmittance and annealing temperature was shown in Fig. 4. The transmittance maximum of 85% was used as measurement of transmittance changes with annealing temperature. The annealing treatment was performed in an Ar atmosphere. It was kept for 40 min at every annealing temperature. The transmittance of the films was increased with increasing annealing temperature. The transmittance increased from 85 to 90% when the annealing temperature increased from 400 to 510 8C. Similarly, the optical transmittance of the films was also depended on the
Fig. 2. SEM images of Cd2SnO4 films; (a) without and (b) with annealing treatment.
Fig. 3. Changes of optical transmission spectra after annealing treatment at 400 8C, (a) not annealing treatment; (b) annealing treatment.
oxygen concentration in Ar and substrate temperature w8,9x, i.e. the transmittance to be increased with the decreasing the oxygen concentration and increasing substrate temperature. The optical transmittance of the films can be expressed by w10x
Ts
Ž1yR1.Ž1yR2.Ž1yR3.exp(yat) Ž1yR1R2.µ1yR2R3exp(y2at)y2yR2R3expwcosf(yat)x∂
(1) where a is optical absorption coefficient, R1sw(nsy1)y (nsq1)x2, R2sw(nfyns)y(nf qns)x2 , R3sw(nfy1)y(nfq 1)x2, fs4pnftyl, ns and nf are refractive index of glass
Fig. 4. Relationship between the optical transmittance and annealing temperature.
286
W.L. Wang et al. / Surface and Coatings Technology 167 (2003) 284–287
talline Cd2SnO4 films, and that only the conduction band has high curvature w13x, then 3y2 nsŽ8py3h3.Ž2mU e DE.
Fig. 5. Changes of optical gap energy with annealing temperature.
substrate and Cd2SnO4 films, respectively, t is thickness of Cd2SnO4 films and l is wavelength. The refractive index of Cd2SnO4 films was calculated by w11x nfsl1l2y2tŽl1yl2.
(2)
where l1 and l2 are the wavelength of two neighboring interference fringes (maximal or minimal) in the visible region of transmission spectrum for Cd2SnO4 films. From Eqs. (1) and (2), the values of a in the region of the absorption edge could be obtained, and the optical gap energy Eopt could be given by w12x (ahn)1y2sBŽhnyEopt.
(3)
where B is a constant. A plot of (ahn)1y2 vs. hn should be linear, and the intercept of the line on the abscissa at (ahn)1y2s0 yields the optical gap energy Eopt of Cd2SnO4 films. The changes in the optical gap energy with annealing temperature are shown in Fig. 5. The optical gap energy is approximately 2.64 eV after annealing treatment at 500 8C, which is increased by 6.5% as compared with untreated films. Cd2SnO4 thin films with spinal structure are an ntype defect semiconductor in which oxygen vacancies provide the donor states. The extra electronic charges in Cd2SnO4 films are trapped only at the oxygen vacancies. The oxygen vacancies can be increased by the annealing treatment, and led to increasing carrier concentration. Our Hall effect measurements showed that the carrier concentration was increased from 1=1026 to 4.46=1026 ym3 after annealing treatment at 500 8C. If one assumes spherical energy surface for the crys-
(4)
where n is the free carrier concentration, DE is the Burstein shift w14x, namely DEsEoptyEg, and mU e is the effective mass of conduction electron. The intrinsic optical gap energy (Eg) of Cd2SnO4 films was approximately 2.156 eV by measuring photoluminescence spectra. From Eq. (4), it is quite apparent that the conduction band generates a shift towards high energy level after annealing treatment. In fact, our experimental results have demonstrated that the optical gap energy values are all larger than Eg before and after heat treatment. This means that the Cd2SnO4 films are degenerate semiconductor. The value of mU e can be calculated from Eq. (4), and the ratio of mU e yme can also be obtained. me is free electron mass. The results showed that the effective mass of the conduction electron is very small, and increases with increasing carrier concentration. The electron effective mass in Cd2SnO4 films is max 0.5 me, which is in good agreement with that obtained by Pisarkiewicz using measuring thermoelectric power w15x. 4. Conclusion Cd2SnO4 thin films belong to a group of broadband gap semiconductors that show remarkable properties of being highly conductive while still transparent in the visible region of the light spectra. The experimental results showed the post-deposition annealing treatment has an important effect on the optical properties and structure of Cd2SnO4 films. The transmittance and optical gap energy were increased after annealing treatment of 400 8C in Ar atmosphere for 40 min. This is attributed to the Burstein shift effect. Acknowledgments This work was supported by the National High Technology Foundation of China under Grant No. 2001AA513011. References w1x R.R. Mehta, S.F. Vogel, J. Electrochem. Soc. 119 (1972) 752. w2x J.C.C. Fan, F.J. Bachner, Appl. Opt. 15 (1976) 1012. w3x J.C. Manifacier, L. Szepessy, J.F. Bresse, et al., Mater. Res. Bull. 14 (1979) 163. w4x D.L. Peng, S.R. Jiang, L. Xie, Phys. Stat. Sol. 12 (1985) 57. w5x Y.F. Dong, W.L. Wang, K.J. Kiao, Sensors Actuators B 67 (2000) 254. w6x X. Wu, R. Sheldon, et al., J. Appl. Phys. 89 (2001) 4564. w7x X. Wu, J.J. Coutts, US Pattent No. 6221495, 2001.
W.L. Wang et al. / Surface and Coatings Technology 167 (2003) 284–287 w8x D.L. Peng, S.R. Jiang, W.L. Wang, Acta Energiae Solaris Sin. 13 (1992) 291. w9x S.R. Jiang, D.L. Peng, W.L. Wang, et al., Acta Energiae Solaris Sin. 13 (1992) 118. w10x W.L. Wang, K.J. Liao, et al., Acta Energiae Solaris Sin. 11 (1990) 335.
w11x w12x w13x w14x w15x
A.J. Nozik, Phys. Rev. B 6 (1972) 453. N. Miyata, et al., J. Electrochem. Soc. 127 (1980) 918. T.S. Moss, Proc. Phys. Soc. Lond. B 67 (1954) 775. E. Burstein, Phys. Rev. 93 (1954) 632. T. Pisarkiewicz, Thin Solid Films 153 (1987) 479.
287