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Structural and optical properties of annealed CdO thin films prepared by spray pyrolysis
Materials Chemistry and Physics 68 (2001) 249–252
Structural and optical properties of annealed CdO thin films prepared by spray pyrolysis O. Vigil a,1 , F. Cruz a , A. Morales-Acevedo b,∗ , G. Contreras-Puente a , L. Vaillant c , G. Santana b,2 a
Escuela Superior de F´ısica y Matemáticas — Instituto Politécnico Nacional (IPN), Edif. 9, U.P. ALM, Lindavista, Mexico, D.F., C.P. 07738, Mexico b Sección de Electrónica del Estado Sólido, Departamento de Ingenier´ıa Eléctrica, CINVESTAV-IPN, 07360 Mexico, D.F., Mexico c Facultad de F´ısica — IMRE, Universidad de La Habana, 43100 Havana, Cuba Received 21 February 2000; accepted 19 June 2000
Abstract CdO films were prepared on glass substrates by the spray pyrolysis technique. Results on structural, optical and electrical properties of the layers as a function of the thermal annealing are reported. XRD data indicates that samples show microstructural perfection improvement as a function of annealing time. The optical band-gap shows a dependence with the inverse of the squared crystallite size, suggesting that electron confinement is an important effect. The lattice parameter and band-gap energy of the samples annealed at 450◦ C for 120 min correspond to the reported values of bulk CdO crystals. In addition, the electrical resistivity measurement shows a slight decrease when annealing time is increased up to 40 min but it saturates for larger times. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Optical properties; CdO thin films; Spray pyrolysis; XRD analysis
1. Introduction
2. Experimental
The use of transparent conducting oxides (TCO) in optoelectronic and photovoltaic devices has stimulated research on this field in recent years. In particular, cadmium oxide is a promising material for solar cell application [1–3], but also for photodiodes [4] and gas sensors [5]. A variety of techniques have been reported to make CdO thin films [1,6–8], but published work on the preparation and characterization of CdO thin films grown by spray pyrolysis is still very limited. For example, recently we have reported some results on the structural, electrical and optical properties of CdO:F thin films deposited by spray pyrolysis, as a function of the concentration of [NH4 F] in the initial solution and the substrate temperature [9]. In this work we report the variation of the structural properties and the optical band-gap of CdO thin films obtained by spray pyrolysis, as a function of the annealing time in an air ambient.
CdO thin films approximately 300 nm thick were prepared by spray pyrolysis. The experimental set-up used is similar to that described in Ref. [10]. CdO was obtained from a solution containing cadmium acetate (0.1 M) dissolved in a mixture of methanol and deionized water (1:1), and sprayed onto soda-lime glass substrates at 250±5◦ C. The solution and carrier gas flows were kept constant at 5 ml min−1 and 6 l min−1 , respectively. The nozzle-to-substrate distance was approximately 25 cm, and the spraying time was around 20 min. Post-deposition thermal treatments of the samples were performed in an air atmosphere at a fixed temperature of 450◦ C for periods in the 15–120 min range. For these thermal treatment we proceeded as follows: the samples were divided into two parts, each with the same size. One of them was annealed during 15 min (called first annealing); a second annealing in the same sample was made during 30 min (called second annealing). For the second part, a third and fourth annealing were made during 60 and 120 min, respectively. After each annealing the structural, optical and electrical properties were measured. The layer thickness was measured before and after the annealing by using a step profiler (Sloan Dektak II). No change of the sample thickness was observed within the resolution of the equipment. Optical transmission data were obtained
∗ Corresponding author. E-mail address: [email protected] (A. Morales-Acevedo). 1 Permanent address: Facultad de F´ısica-IMRE, Universidad de La Habana, 43100 La Habana, Cuba. 2 Permanent address: Facultad de F´ısica-IMRE, Universidad de La Habana, 43100 La Habana, Cuba.
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with an UV/VIS Shimadzu 3101 PC spectrometer in the 300–900 nm range. The step size of the monochromator was 1 nm, corresponding to an energy resolution of ±5 meV in the range of 2.3–2.6 eV. The crystalline structure of the films was analyzed using a D-500 Siemens X-ray diffractometer using the Cu K␣ line. The diffractograms were smoothed using the DIFRAC-AT program [11]. The maximum was fitted by means of the FULLPROF program [12] using a pseudo-Voigt function [13,14]. From these fittings, the integral width (β) and the full width at half maximum (FWHM) were obtained. Then, with the aim of determining the tendencies of the crystallite size and microstrain, a Willamson Hall graphics [14–17] was built. The crystallite size and the microstrain were determined using the method of the single line [18], averaging the best quality measured lines. Corrections due to instrumental effects were made according to Langford et al. [17,18], by deconvoluting the pseudo-Voigt function in their Gaussian and Cauchy (Lorentzian) parts. These corrections were carried out for both the CdO samples as well as for the substance used in the determination of these instrumental parameters [17]. For the determination of these parameters a powder of ZnO maintained during 4 days at 910◦ C was used.
3. Results and discussion Fig. 1 shows the X-ray diffraction profiles of CdO films with different annealing times. For the as-deposited films the peaks associated to planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) of the cubic structure are observed. When the samples are annealed, the results can be divided into two categories: (a) annealing times for which there is an increase of the intensity of the peaks (crystallites are oriented with the [1 1 1] direction perpendicular to the substrate, with a slight shift of the maximum towards higher 2θ values), and (b)
Fig. 1. X-ray diffraction patterns of CdO thin films, Ts =250◦ C, and deposition time=20 min. The annealing was carried out at 450◦ C in air for different times.
Table 1 Values of the crystallite size (D), the lattice parameter (a), the microstrain (hei), and the band-gap (Eg ) for as-deposited and annealed CdO samples at 450◦ C in air Annealing time (min)
D (nm)
a (Å)
hei(␦d/d) (×10−3 )
Eg (eV)
0a 15 30 60 120
18 26 34 51 63
4.706 4.696 4.694 4.693 4.689
7.4 5.6 4.6 3.5 3.1
2.53 2.43 2.39 2.36 2.29
a
As-grown.
increasing crystallite size, D and decreasing microstrain, hei values when the annealing time is further increased. Table 1 shows the crystallite size, the microstrain associated to the (1 1 1) peak and the lattice parameter. From these results, we can see that the fundamental effect of the annealing is related to an increase in the size of the crystallites and a decrease in the lattice parameter and the microstrain. Fig. 2 shows the optical transmittance for as-deposited and annealed films. This figure clearly indicates two effects: (a) a shift in the absorption edge as a function of annealing time and (b) a decrease in the optical transmittance of the films. To determine the values of the band-gap energy (BGE) a plot of (αhν)2 as a function of the energy of the incident radiation was made. In Table 1 we show the BGE values. This behavior is similar to that observed for other thin film materials annealed at different temperatures and atmospheres [19–21]. We assume that the most important influence of the annealing process on the optical characteristics is related to recrystallization of the CdO crystallites. Changes in the BGE with annealing time can be associated to changes in the lattice parameter, induced strains due to thermal expansion and to size growth of the crystallites. The BGE should increase when the lattice parameter decreases, but it should decrease when the size of the crystallites increases [22].
Fig. 2. Optical transmission characteristics of CdO films grown at Ts =250◦ C and annealed for different times.
O. Vigil et al. / Materials Chemistry and Physics 68 (2001) 249–252
251
Fig. 3. BGE shift vs. the inverse of the crystallite size squared. Fig. 4. Crystallite size dependence as a function of the cubic root of annealing time for CdO thin films annealed at 450◦ C.
From X-ray diffraction, it can be seen that the diffraction peaks are symmetric and wide, and there is a slight shift regarding their normal position. This means that there are crystallites with compressed lattice planes and other with expanded lattice planes distributed in equal proportions (macrostrains are not apparent in our results, and therefore they are discarded). The compression and expansion effects induce opposite contributions to the optical properties of the annealed samples, i.e. some crystallites contribute to increase the BGE while others cause a decrease of the BGE. Therefore, we expect that in average the compression–expansion effects on the BGE are negligible. However, a decrease of the BGE was observed when the diameter of CdO crystallites becomes larger. For the as-grown samples a blue shift by about 0.24 eV compared to
that of bulk CdO (2.29 eV) is observed. For these samples the diameter of the crystallites was estimated to be around 18 nm by XRD measurements. For films annealed at 450◦ C during 120 min, the diameter of the crystallites was estimated to be 63 nm, and the value of the BGE approaches that of bulk CdO. Then, post-thermal annealing causes grain size growth with increasing time. A similar behavior has been observed for microcrystalline CdS films [23]. Since this material is degenerate (low resistivity), the large BGE value for the non-annealed samples could be due to either the Burstein effect or electron localization within the crystallites, in addition to other phenomena such as disorder and defects at the grain-boundaries. In Fig. 3, the BGE shift as a function of the inverse crystallite size squared is
Fig. 5. Variation of resistivity with annealing time for CdO films grown at Ts =250◦ C.
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O. Vigil et al. / Materials Chemistry and Physics 68 (2001) 249–252
shown. The observed linear behavior suggests some effect of electron confinement upon the optical band-gap, but further investigation should be made in order to establish this correlation with more certitude. We should also notice another interesting relationship between the crystallite size of CdO crystallites and the cubic root of the annealing time. As shown in Fig. 4, this is also linear. A similar relationship has been observed for CdS glass composites, explained by the authors of Ref. [24] by a diffusion-limited structure-coercing process. In order to explain the drop of the optical transmittance as a function of annealing time we suggest a better packing of the CdO grains in the layer, with a reduction of voids in it. Then, the optical transmittance should decrease in a monotonous way, as a function of the annealing time, due to re-crystallization of the crystallites in the films. Finally, the resistivity as a function of annealing time is shown in Fig. 5. This figure shows that the electrical resistivity decreases for annealing times up to 30 min, but it saturates for larger times. As explained before, the structural quality of the films increases with annealing time, and hence a reduction of the grain-boundary scattering is obtained in addition to the grain size growth. Therefore, the resistivity reduction can be explained as very likely due to the increase of the electron mobility as a consequence of the crystallite size growth for annealing times of the order of 30 min. In summary, we have shown that annealing (at 450◦ C in air) CdO films grown by spray pyrolysis, improves the structural quality of the films giving the material electrical and optical properties close to the bulk CdO crystalline values.
4. Conclusions In this work we have reported the influence of post-thermal annealing in the structural, optical and electrical characteristics of CdO thin films as a function of annealing time. When annealing in air is provided to samples, the microstructural and electrical properties are improved. A decrease for the BGE and the lattice parameter is observed approaching the reported CdO bulk values at room temperature (2.29 eV and 4.695 Å, respectively, [25]). We have found that there could be some electron confinement within the grains for the non-annealed samples. Also, a small reduction of the resistivity possibly due to an increase of the electron mobility has been observed for annealing in air at 450◦ C during times up to 30 min.
Acknowledgements This work was partially supported by the Mexican Agency for Science and Technology (CONACyT), México. References [1] T.L. Chu, S.S. Chu, J. Elect. Mater. 19 (1990) 1003. [2] C.H. Champness, C.H. Chan, Solar Energy Mater. Solar Cells 37 (1995) 75. [3] A.A. Al-Qurani, C.H. Champness, in: Proceedings of the 26th IEEE Photovoltaic Specialists Conference, Anaheim, CA, 1997, p. 415. [4] R. Kondo, H. Okhimura, Y. Sakai, Jpn. J. Appl. Phys. 10 (1971) 1547. [5] A. Shiori, Japanese Patent 7,909,995 (1979). [6] C. Sravani, K.T. Ramakrishna Reddy, P. Jayarama Reddy, Mater. Lett. 15 (1993) 356. [7] M. Ocampo, P.J. Sebastian, J. Campos, Phys. Stat. Sol. (a) 143 (1994) K29. [8] G. Phatk, R. Lal, Thin Solid Films 209 (1992) 240. [9] R. Ferro, J.A. Rodr´ıguez, O. Vigil, A. Morales-Acevedo, G. Contreras-Puente, F-doped CdO thin films deposited by spray pyrolysis, Phys. Stat. Sol. (a) 177 (2) (2000) 477. [10] O. Vigil, L. Vaillant, F. Cruz, G. Santana, A. Morales-Acevedo, G. Contreras-Puente, Spray pyrolysis deposition of cadmium–zinc oxide thin films, Thin Solid Films 361–362 (2000) 53. [11] Diffract-AT, Socabim, Bruker, Siemens, 1996. [12] Fullprof ver. 99 beta, J. Rodriguez–Carvajal, T. Rosinel, Laboratoire L. Brillouin (CEA-CNRS), Saclay, France. [13] R.A. Young, The Rietveld Method, International Union of Crystallography, Oxford University Press, Oxford, 1993. [14] Techniques D’Analyses des Diagrammes de Diffraction des Rayons X et des Neutrons par les Poudres, J. Pannetier. Nantes (Formation Permanate du CNRS) (1995). [15] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [16] A. Taylor, X-ray Metallography, Wiley, New York, 1961. [17] J.I. Langford, D. Louër, E.J. Sonneveld, J.W. Visser, Powder Diffraction 1 (3) (1986) 211. [18] J.L. Langford, R. Delhez, Th.H. de Keijser, E.J. Mittemeijer, Aust. J. Phys. 41 (1988) 173. [19] S.A. Tomá, O. Vigil, J.J. Alvarado-Gil, R. Lozada-Morales, O. Zelaya-Angel, H. Vargas, A. Ferreira de Silva, J. Appl. Phys. 78 (1995) 2204. [20] S.A. Studenikin, N. Galego, M. Cocivera, J. Appl. Phys. 83 (1998) 2104. [21] D.H. Zhang, R.W. Gao, H.L. Ma, Thin Solid Films 295 (1997) 83. [22] V. Srikant, D.R. Clarke, J. Appl. Phys. 81 (1997) 6357. [23] I. Tanihashi, A. Tsuyimuru, T. Mitsuyu, A. Nishino, Jpn. J. Appl. Phys. 81 (1990) 2111. [24] Landolt-Bornstein, in: O. Madelung, M. Schulzand, H. Weiss (Eds.), Physics of II–VI and I–VII Compounds, Vol. 17, Springer/Heidelberg, Berlin/New York, 1982. [25] B.G. Potter Jr., J.H. Simmons, Phys. Rev. B 37 (1988) 10838.
Structural and optical properties of annealed CdO thin films prepared by spray pyrolysis O. Vigil a,1 , F. Cruz a , A. Morales-Acevedo b,∗ , G. Contreras-Puente a , L. Vaillant c , G. Santana b,2 a
Escuela Superior de F´ısica y Matemáticas — Instituto Politécnico Nacional (IPN), Edif. 9, U.P. ALM, Lindavista, Mexico, D.F., C.P. 07738, Mexico b Sección de Electrónica del Estado Sólido, Departamento de Ingenier´ıa Eléctrica, CINVESTAV-IPN, 07360 Mexico, D.F., Mexico c Facultad de F´ısica — IMRE, Universidad de La Habana, 43100 Havana, Cuba Received 21 February 2000; accepted 19 June 2000
Abstract CdO films were prepared on glass substrates by the spray pyrolysis technique. Results on structural, optical and electrical properties of the layers as a function of the thermal annealing are reported. XRD data indicates that samples show microstructural perfection improvement as a function of annealing time. The optical band-gap shows a dependence with the inverse of the squared crystallite size, suggesting that electron confinement is an important effect. The lattice parameter and band-gap energy of the samples annealed at 450◦ C for 120 min correspond to the reported values of bulk CdO crystals. In addition, the electrical resistivity measurement shows a slight decrease when annealing time is increased up to 40 min but it saturates for larger times. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Optical properties; CdO thin films; Spray pyrolysis; XRD analysis
1. Introduction
2. Experimental
The use of transparent conducting oxides (TCO) in optoelectronic and photovoltaic devices has stimulated research on this field in recent years. In particular, cadmium oxide is a promising material for solar cell application [1–3], but also for photodiodes [4] and gas sensors [5]. A variety of techniques have been reported to make CdO thin films [1,6–8], but published work on the preparation and characterization of CdO thin films grown by spray pyrolysis is still very limited. For example, recently we have reported some results on the structural, electrical and optical properties of CdO:F thin films deposited by spray pyrolysis, as a function of the concentration of [NH4 F] in the initial solution and the substrate temperature [9]. In this work we report the variation of the structural properties and the optical band-gap of CdO thin films obtained by spray pyrolysis, as a function of the annealing time in an air ambient.
CdO thin films approximately 300 nm thick were prepared by spray pyrolysis. The experimental set-up used is similar to that described in Ref. [10]. CdO was obtained from a solution containing cadmium acetate (0.1 M) dissolved in a mixture of methanol and deionized water (1:1), and sprayed onto soda-lime glass substrates at 250±5◦ C. The solution and carrier gas flows were kept constant at 5 ml min−1 and 6 l min−1 , respectively. The nozzle-to-substrate distance was approximately 25 cm, and the spraying time was around 20 min. Post-deposition thermal treatments of the samples were performed in an air atmosphere at a fixed temperature of 450◦ C for periods in the 15–120 min range. For these thermal treatment we proceeded as follows: the samples were divided into two parts, each with the same size. One of them was annealed during 15 min (called first annealing); a second annealing in the same sample was made during 30 min (called second annealing). For the second part, a third and fourth annealing were made during 60 and 120 min, respectively. After each annealing the structural, optical and electrical properties were measured. The layer thickness was measured before and after the annealing by using a step profiler (Sloan Dektak II). No change of the sample thickness was observed within the resolution of the equipment. Optical transmission data were obtained
∗ Corresponding author. E-mail address: [email protected] (A. Morales-Acevedo). 1 Permanent address: Facultad de F´ısica-IMRE, Universidad de La Habana, 43100 La Habana, Cuba. 2 Permanent address: Facultad de F´ısica-IMRE, Universidad de La Habana, 43100 La Habana, Cuba.
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 5 8 - 8
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O. Vigil et al. / Materials Chemistry and Physics 68 (2001) 249–252
with an UV/VIS Shimadzu 3101 PC spectrometer in the 300–900 nm range. The step size of the monochromator was 1 nm, corresponding to an energy resolution of ±5 meV in the range of 2.3–2.6 eV. The crystalline structure of the films was analyzed using a D-500 Siemens X-ray diffractometer using the Cu K␣ line. The diffractograms were smoothed using the DIFRAC-AT program [11]. The maximum was fitted by means of the FULLPROF program [12] using a pseudo-Voigt function [13,14]. From these fittings, the integral width (β) and the full width at half maximum (FWHM) were obtained. Then, with the aim of determining the tendencies of the crystallite size and microstrain, a Willamson Hall graphics [14–17] was built. The crystallite size and the microstrain were determined using the method of the single line [18], averaging the best quality measured lines. Corrections due to instrumental effects were made according to Langford et al. [17,18], by deconvoluting the pseudo-Voigt function in their Gaussian and Cauchy (Lorentzian) parts. These corrections were carried out for both the CdO samples as well as for the substance used in the determination of these instrumental parameters [17]. For the determination of these parameters a powder of ZnO maintained during 4 days at 910◦ C was used.
3. Results and discussion Fig. 1 shows the X-ray diffraction profiles of CdO films with different annealing times. For the as-deposited films the peaks associated to planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) of the cubic structure are observed. When the samples are annealed, the results can be divided into two categories: (a) annealing times for which there is an increase of the intensity of the peaks (crystallites are oriented with the [1 1 1] direction perpendicular to the substrate, with a slight shift of the maximum towards higher 2θ values), and (b)
Fig. 1. X-ray diffraction patterns of CdO thin films, Ts =250◦ C, and deposition time=20 min. The annealing was carried out at 450◦ C in air for different times.
Table 1 Values of the crystallite size (D), the lattice parameter (a), the microstrain (hei), and the band-gap (Eg ) for as-deposited and annealed CdO samples at 450◦ C in air Annealing time (min)
D (nm)
a (Å)
hei(␦d/d) (×10−3 )
Eg (eV)
0a 15 30 60 120
18 26 34 51 63
4.706 4.696 4.694 4.693 4.689
7.4 5.6 4.6 3.5 3.1
2.53 2.43 2.39 2.36 2.29
a
As-grown.
increasing crystallite size, D and decreasing microstrain, hei values when the annealing time is further increased. Table 1 shows the crystallite size, the microstrain associated to the (1 1 1) peak and the lattice parameter. From these results, we can see that the fundamental effect of the annealing is related to an increase in the size of the crystallites and a decrease in the lattice parameter and the microstrain. Fig. 2 shows the optical transmittance for as-deposited and annealed films. This figure clearly indicates two effects: (a) a shift in the absorption edge as a function of annealing time and (b) a decrease in the optical transmittance of the films. To determine the values of the band-gap energy (BGE) a plot of (αhν)2 as a function of the energy of the incident radiation was made. In Table 1 we show the BGE values. This behavior is similar to that observed for other thin film materials annealed at different temperatures and atmospheres [19–21]. We assume that the most important influence of the annealing process on the optical characteristics is related to recrystallization of the CdO crystallites. Changes in the BGE with annealing time can be associated to changes in the lattice parameter, induced strains due to thermal expansion and to size growth of the crystallites. The BGE should increase when the lattice parameter decreases, but it should decrease when the size of the crystallites increases [22].
Fig. 2. Optical transmission characteristics of CdO films grown at Ts =250◦ C and annealed for different times.
O. Vigil et al. / Materials Chemistry and Physics 68 (2001) 249–252
251
Fig. 3. BGE shift vs. the inverse of the crystallite size squared. Fig. 4. Crystallite size dependence as a function of the cubic root of annealing time for CdO thin films annealed at 450◦ C.
From X-ray diffraction, it can be seen that the diffraction peaks are symmetric and wide, and there is a slight shift regarding their normal position. This means that there are crystallites with compressed lattice planes and other with expanded lattice planes distributed in equal proportions (macrostrains are not apparent in our results, and therefore they are discarded). The compression and expansion effects induce opposite contributions to the optical properties of the annealed samples, i.e. some crystallites contribute to increase the BGE while others cause a decrease of the BGE. Therefore, we expect that in average the compression–expansion effects on the BGE are negligible. However, a decrease of the BGE was observed when the diameter of CdO crystallites becomes larger. For the as-grown samples a blue shift by about 0.24 eV compared to
that of bulk CdO (2.29 eV) is observed. For these samples the diameter of the crystallites was estimated to be around 18 nm by XRD measurements. For films annealed at 450◦ C during 120 min, the diameter of the crystallites was estimated to be 63 nm, and the value of the BGE approaches that of bulk CdO. Then, post-thermal annealing causes grain size growth with increasing time. A similar behavior has been observed for microcrystalline CdS films [23]. Since this material is degenerate (low resistivity), the large BGE value for the non-annealed samples could be due to either the Burstein effect or electron localization within the crystallites, in addition to other phenomena such as disorder and defects at the grain-boundaries. In Fig. 3, the BGE shift as a function of the inverse crystallite size squared is
Fig. 5. Variation of resistivity with annealing time for CdO films grown at Ts =250◦ C.
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O. Vigil et al. / Materials Chemistry and Physics 68 (2001) 249–252
shown. The observed linear behavior suggests some effect of electron confinement upon the optical band-gap, but further investigation should be made in order to establish this correlation with more certitude. We should also notice another interesting relationship between the crystallite size of CdO crystallites and the cubic root of the annealing time. As shown in Fig. 4, this is also linear. A similar relationship has been observed for CdS glass composites, explained by the authors of Ref. [24] by a diffusion-limited structure-coercing process. In order to explain the drop of the optical transmittance as a function of annealing time we suggest a better packing of the CdO grains in the layer, with a reduction of voids in it. Then, the optical transmittance should decrease in a monotonous way, as a function of the annealing time, due to re-crystallization of the crystallites in the films. Finally, the resistivity as a function of annealing time is shown in Fig. 5. This figure shows that the electrical resistivity decreases for annealing times up to 30 min, but it saturates for larger times. As explained before, the structural quality of the films increases with annealing time, and hence a reduction of the grain-boundary scattering is obtained in addition to the grain size growth. Therefore, the resistivity reduction can be explained as very likely due to the increase of the electron mobility as a consequence of the crystallite size growth for annealing times of the order of 30 min. In summary, we have shown that annealing (at 450◦ C in air) CdO films grown by spray pyrolysis, improves the structural quality of the films giving the material electrical and optical properties close to the bulk CdO crystalline values.
4. Conclusions In this work we have reported the influence of post-thermal annealing in the structural, optical and electrical characteristics of CdO thin films as a function of annealing time. When annealing in air is provided to samples, the microstructural and electrical properties are improved. A decrease for the BGE and the lattice parameter is observed approaching the reported CdO bulk values at room temperature (2.29 eV and 4.695 Å, respectively, [25]). We have found that there could be some electron confinement within the grains for the non-annealed samples. Also, a small reduction of the resistivity possibly due to an increase of the electron mobility has been observed for annealing in air at 450◦ C during times up to 30 min.
Acknowledgements This work was partially supported by the Mexican Agency for Science and Technology (CONACyT), México. References [1] T.L. Chu, S.S. Chu, J. Elect. Mater. 19 (1990) 1003. [2] C.H. Champness, C.H. Chan, Solar Energy Mater. Solar Cells 37 (1995) 75. [3] A.A. Al-Qurani, C.H. Champness, in: Proceedings of the 26th IEEE Photovoltaic Specialists Conference, Anaheim, CA, 1997, p. 415. [4] R. Kondo, H. Okhimura, Y. Sakai, Jpn. J. Appl. Phys. 10 (1971) 1547. [5] A. Shiori, Japanese Patent 7,909,995 (1979). [6] C. Sravani, K.T. Ramakrishna Reddy, P. Jayarama Reddy, Mater. Lett. 15 (1993) 356. [7] M. Ocampo, P.J. Sebastian, J. Campos, Phys. Stat. Sol. (a) 143 (1994) K29. [8] G. Phatk, R. Lal, Thin Solid Films 209 (1992) 240. [9] R. Ferro, J.A. Rodr´ıguez, O. Vigil, A. Morales-Acevedo, G. Contreras-Puente, F-doped CdO thin films deposited by spray pyrolysis, Phys. Stat. Sol. (a) 177 (2) (2000) 477. [10] O. Vigil, L. Vaillant, F. Cruz, G. Santana, A. Morales-Acevedo, G. Contreras-Puente, Spray pyrolysis deposition of cadmium–zinc oxide thin films, Thin Solid Films 361–362 (2000) 53. [11] Diffract-AT, Socabim, Bruker, Siemens, 1996. [12] Fullprof ver. 99 beta, J. Rodriguez–Carvajal, T. Rosinel, Laboratoire L. Brillouin (CEA-CNRS), Saclay, France. [13] R.A. Young, The Rietveld Method, International Union of Crystallography, Oxford University Press, Oxford, 1993. [14] Techniques D’Analyses des Diagrammes de Diffraction des Rayons X et des Neutrons par les Poudres, J. Pannetier. Nantes (Formation Permanate du CNRS) (1995). [15] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [16] A. Taylor, X-ray Metallography, Wiley, New York, 1961. [17] J.I. Langford, D. Louër, E.J. Sonneveld, J.W. Visser, Powder Diffraction 1 (3) (1986) 211. [18] J.L. Langford, R. Delhez, Th.H. de Keijser, E.J. Mittemeijer, Aust. J. Phys. 41 (1988) 173. [19] S.A. Tomá, O. Vigil, J.J. Alvarado-Gil, R. Lozada-Morales, O. Zelaya-Angel, H. Vargas, A. Ferreira de Silva, J. Appl. Phys. 78 (1995) 2204. [20] S.A. Studenikin, N. Galego, M. Cocivera, J. Appl. Phys. 83 (1998) 2104. [21] D.H. Zhang, R.W. Gao, H.L. Ma, Thin Solid Films 295 (1997) 83. [22] V. Srikant, D.R. Clarke, J. Appl. Phys. 81 (1997) 6357. [23] I. Tanihashi, A. Tsuyimuru, T. Mitsuyu, A. Nishino, Jpn. J. Appl. Phys. 81 (1990) 2111. [24] Landolt-Bornstein, in: O. Madelung, M. Schulzand, H. Weiss (Eds.), Physics of II–VI and I–VII Compounds, Vol. 17, Springer/Heidelberg, Berlin/New York, 1982. [25] B.G. Potter Jr., J.H. Simmons, Phys. Rev. B 37 (1988) 10838.