- Email: [email protected]
Structural and optical properties of La-doped ZnO films prepared by magnetron sputtering
Materials Letters 61 (2007) 2262 – 2265 www.elsevier.com/locate/matlet
Structural and optical properties of La-doped ZnO films prepared by magnetron sputtering Wei Lan a,b , Yanping Liu a , Ming Zhang b,⁎, Bo Wang b , Hui Yan b , Yinyue Wang a a
Department of Physics, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China b The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, PR China Received 29 June 2006; accepted 3 August 2006 Available online 15 September 2006
Abstract La-doped ZnO films were prepared by RF magnetron sputtering using different composition powder compacted targets (0, 1, 2, 3 and 5 at.%). All films show a preferred c-axis growth orientation. Furthermore, the (002) diffraction peak shifts to a small angle and the full-width at halfmaximum augments with increasing La concentration up to 2 at.%, which indicate that a small quantity of La atoms are incorporated into the ZnO lattice. The average transmittance in the visible range is over 80%, and a blue shift of the absorption edge is observed. With increasing La concentration, the band gap of ZnO films evaluated by the linear fitting linearly increases from 3.270 to 3.326 eV. In the photoluminescence spectra, a strong violet emission peak and a weak green emission band can be observed. The former is due to the electron transition between the defect energy levels, associated with the interfacial traps existing at the ZnO grain boundaries, and valence band. The latter could be ascribed to crystal defects related to oxygen vacancies. © 2006 Elsevier B.V. All rights reserved. Keywords: La-doped ZnO films; Sputtering; Band gap; Photoluminescence
1. Introduction Zinc oxide (ZnO) has been intensively studied due to its potential applications in short-wavelength light-emitting devices such as light-emitting diodes and diode lasers [1], as well as due its wide band gap of 3.37 eV at room temperature and high exciton binding energy (60 meV). It has already been reported that the green photoluminescence emitted from ZnO is due to the point defects such as oxygen vacancies in the crystals [2,3]. Jin et al. [4] found that the visible violet photoluminescence from ZnO films was deposited on the sapphire (001) substrate and believed that the emission was related to the defect level in the grain boundaries. However, there are few reports about the violet and green luminescence simultaneously present in ZnO films. La-doped ZnO film (ZnO:La) exhibits excellent violet and green light-emitting properties, and the band gap can be modulated by varying the doping concentrations.
⁎ Corresponding author. Tel.: +86 10 67392733; fax: +86 10 67392412. E-mail address: [email protected] (Ming Zhang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.061
Here, we report the structural and optical properties of ZnO: La films. The band gap engineering and the mechanism of visible photoluminescence are investigated. 2. Experimental ZnO:La films were deposited on quartz substrate using RF magnetron sputtering. La2O3 and ZnO powders were mixed thoroughly with the different ratios of La/Zn = 0, 1, 2, 3 and 5 at.%. The mixture was then pressed in an aluminium holder to the target used for sputtering. The substrate was cleaned ultrasonically by methylbenzene, acetone and ethanol in turn. Prior to deposition, the chamber was pumped up to 6 × 10− 3 Pa and the target was pre-sputtered for 120 min to remove contamination from the surface. The sputtering power, the substrate temperature and the deposition time were 50 W, 400 °C and 60 min, respectively. The sputtering gas was a mixture of Ar (4 N) and O2 (4 N) with a ratio of 4:1, at a total pressure of 1.0 Pa. The distance between the substrate and the target was 40 mm. After the deposition, the samples were in-situ annealed in vacuum at 600 °C for 60 min.
Wei Lan et al. / Materials Letters 61 (2007) 2262–2265
2263
Fig. 1. XRD patterns of ZnO:La films with different doping concentrations (0, 1, 2, 3 and 5 at.%).
The crystalline structure was identified using a BRUKERAXS D8 X-ray diffractometer (Cu kα, λ = 0.154056 nm). The optical transmittance and reflectance were measured by a double-beam spectrophotometer (SHIMADZU, UV-3101) within the UV–visible range. The photoluminescence spectra were carried out using a RF-540 spectrophotometer with an excitation wavelength of 325 nm (He–Cd laser). The film thicknesses were determined by the surface roughness analyzer (Surfcom 480A). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of ZnO:La films with different doping concentrations (0–5 at.%). Only the (002) diffraction peak of the hexagonal wurtzite ZnO structure is presented, indicating that all the films are grown along the preferential orientation of [002], which is perpendicular to the substrate surface. The intensity of the (002) peak increases followed by a sharp decrease and then hardly changes with the enhancement of La doping concentration. No new diffraction peaks are observed even when the concentration of La increases up to 5 at.%, which means that the impurities do not change the wurtzite structure of the ZnO films. As seen from the inset of Fig. 1, the (002) peaks for all doped films shift to the small angles compared to that of the undoped ZnO film. It means that the ZnO lattice is expanded along the c-axis for all doped samples due to the lattice mismatch between La3+ and ZnO lattice ( the ionic radii of La3+, Zn2+ and O2− are 0.106 nm, 0.074 nm and 0.132 nm, respectively).
According to the full-width at half-maximum (FWHM) of the (002) peak, the grain size of ZnO:La films can be evaluated by the Scherrer's formula [5]. D¼
0:9k W d cosh
ð1Þ
where λ, W and θ are the X-ray wavelength, FWHM and Bragg diffraction angle, respectively. The grain sizes for all the films are presented in Table 1. As is shown, the grain size decreases with the increase of La concentration up to 2 at.% and then hardly changes for further doping. It indicates that a small amount of La atoms (about 1– 2 at.%, based on the comprehensive analysis of the structure and the later photoluminescence properties) have been incorporated into the ZnO matrix as impurities, which may be associated with the solid solubility of La in ZnO film. Moreover, the incorporated La atoms probably represent obstructions to grain boundary movement and limit the grain growth.
Table 1 Grain size, film thickness, and band gap of ZnO:La films with different doping concentrations La concentration (at.%)
0
1
2
3
5
Grain size (nm) Film thickness (nm) Band gap (eV)
34.5 240 3.270
21.6 300 3.275
16.5 200 3.287
17.9 110 3.307
17.3 200 3.326
Fig. 2. Transmittance and reflectance spectra of ZnO:La films with varying dopant concentrations.
2264
Wei Lan et al. / Materials Letters 61 (2007) 2262–2265
Fig. 3. Photoluminescence spectra of ZnO:La films with different La concentrations.
The optical transmittance (T) and reflectance (R) spectra of ZnO:La films with different concentrations are shown in Fig. 2. All films show relatively high transparency (N80%) in the visible light range. Sharp absorption edges can be clearly observed and shift to the short wavelength with the increase of La concentration. The absorption coefficient α of the ZnO:La film can be calculated by using Eq. (2) (neglecting the interference and multiple reflections within the films as well as at the film–substrate interface and also assuming near normal incidence of radiation [6,7]) 1 1−R a ¼ ln d T
ð2Þ
where d is the film thickness that is given in Table 1. For the direct band gap semiconductor, the relationship of the absorption coefficient and the band gap energy can be described by the following equation [8] ðahmÞ2 ¼ Aðhm−Eg Þ
ð3Þ
where hν is the incident photon energy, Eg is the band gap energy and A is a constant. A plot of (αhν)2 vs. hv is made to determine Eg using the linear fit process. As shown in Table 1, the Eg, derived from the crosspoint at the axis of hm with the tangent to (αhm)2, increases from 3.270 to 3.326 eV with increasing doping concentration, which may be due to the Burstein–Moss effect [9,10]. On the other hand, the transmittance of the film in the UV region is also enhanced with increasing La concentration. We presume that the increase of the transmittance in the UV region might be ascribed to the fragment of Lanthanum oxide formed in ZnO films, because the band gap energies of Lanthanum oxide compounds are higher than that of ZnO, such as La2O3 5.5 eV. Meanwhile, it further approves that the excessive La impurities are excluded from the ZnO lattice and exist as oxides mainly in the grain boundaries. The photoluminescence (PL) spectra of ZnO:La films with varying dopant concentrations are characterized in the range of 400–600 nm. It is shown in Fig. 3 that a violet peak located at about 420 nm (2.95 eV) and the intensity of emission are found to be strongly dependent on the La concentration. The sample with 2 at.% exhibits excellent violet emission and presents a weak green emission band positioned at 510– 530 nm. When the doping concentration is above 2 at.%, the intensity of the violet and green emission decreases gradually. As the large lattice mismatch between La and ZnO matrix, ZnO grain size gradually
decreases with increasing La concentrations from 0 to 2 at.%, and the traps on the grain surface per unit volume increase with the increase of the specific surface area. Cordaro et al. [11] believed that interface traps lie in the depletion regions and locate at the ZnO–ZnO grain boundaries when a polycrystalline varistor forms, and the level of the interface trap was found to be about 0.33 eV below the conduction band edge. So the violet emission is probably attributed to the recombination centers connected with the interface traps existing at the grain boundaries, and the radiative transition happens between the level of the interface traps and the valence band. Jin et al. [4] also found that violet photoluminescence from the ZnO film prepared at low oxygen pressure originated from the defect levels in the grain boundaries of oxygen-deficient ZnO x crystals, and green-yellow luminescence appeared instead of the violet peak when the oxygen pressure increased. It is well known that the green light emitted is related to the ZnO lattice defect such as oxygen vacancies [2,3,12–14]. Therefore, the weak green emission, present in the ZnO:La film with 2 at.% concentration, indicates that the incorporation of La atoms in the ZnO lattice might lead to the escapement of oxygen atoms from the lattice site when the film were post-annealed in vacuum at 600 °C. When further doping, excessive La impurities and its oxide are separated at the grain boundaries, which might induce the diminution of the radiative defects to decrease the violet emission. Due to the larger difference of electronegativities between La (1.2) and O (3.5), compared with Zn (1.65)–O, it is reasonably estimated that the escapement of oxygen atoms from the La–O bond could be much more difficult than from the Zn–O bond in the ZnO lattice when the doping concentration is above 2 at.%. So the decrease of oxygen vacancies weakens the green emission.
4. Conclusion ZnO:La films (0–5 at.%) on quartz substrates were prepared by RF magnetron sputtering. All the films grow along the preferred orientation of [002]. According to the shift of the (002) diffraction peak and the increase of the FWHM, a small amount of La atoms about 1–2 at.% are incorporated into ZnO lattice. The band gap of ZnO:La films, with the average transmittance above 80%, linearly increases from 3.270 to 3.326 eV with the increase of the doping concentrations. The strong violet emission peak and the weak green emission band are obtained for the ZnO:La films, which are associated with the interface traps existing at the ZnO grain boundaries and oxygen vacancies in the ZnO lattice, respectively. Acknowledgements This work is supported by the Beijing Specific Project to Foster Elitist (No. 20041D0501513) and the National Natural Science Foundation of China (No. 60576012). References [1] E.M. Wong, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2939. [2] T. Yamamoto, Y. Wada, H. Miyamoto, S. Yanagida, Chem. Lett. 33 (2004) 246. [3] Y. Lin, J. Xie, H. Wang, Y. Li, et al., Thin Solid Films 492 (2005) 101. [4] B.J. Jin, S. Im, S.Y. Lee, Thin Solid Films 366 (2000) 107. [5] D. Song, P. Windenborg, et al., Sol. Energy Mater. Sol. Cells 73 (2002) 269.
Wei Lan et al. / Materials Letters 61 (2007) 2262–2265 [6] F. Demichelis, G. Kaniadakis, A. Tagliaferro, E. Tresso, Appl. Opt. 26 (1987) 1737. [7] A.N. Banerjee, K.K. Chattopadhyay, J. Appl. Phys. 97 (2005) 084308. [8] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, Englewood Cliffs, NJ, 1971. [9] E. Burstein, Phys. Rev. 93 (1954) 632.
[10] [11] [12] [13] [14]
2265
T.S. Moss, Proc. Phys. Soc. Lond. B 67 (1954) 775. J.F. Cordaro, Y. Shim, J.E. May, J. Appl. Phys. 60 (1986) 4186. K. Vanheusden, W.L. Warren, et al., J. Appl. Phys. 79 (1996) 7983. K. Vanheusden, C.H. Seager, et al., J. Lumin. 75 (1997) 11. S.A. Studenikin, N. Golego, M. Colivera, J. Appl. Phys. 84 (1998) 2287.
Structural and optical properties of La-doped ZnO films prepared by magnetron sputtering Wei Lan a,b , Yanping Liu a , Ming Zhang b,⁎, Bo Wang b , Hui Yan b , Yinyue Wang a a
Department of Physics, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China b The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, PR China Received 29 June 2006; accepted 3 August 2006 Available online 15 September 2006
Abstract La-doped ZnO films were prepared by RF magnetron sputtering using different composition powder compacted targets (0, 1, 2, 3 and 5 at.%). All films show a preferred c-axis growth orientation. Furthermore, the (002) diffraction peak shifts to a small angle and the full-width at halfmaximum augments with increasing La concentration up to 2 at.%, which indicate that a small quantity of La atoms are incorporated into the ZnO lattice. The average transmittance in the visible range is over 80%, and a blue shift of the absorption edge is observed. With increasing La concentration, the band gap of ZnO films evaluated by the linear fitting linearly increases from 3.270 to 3.326 eV. In the photoluminescence spectra, a strong violet emission peak and a weak green emission band can be observed. The former is due to the electron transition between the defect energy levels, associated with the interfacial traps existing at the ZnO grain boundaries, and valence band. The latter could be ascribed to crystal defects related to oxygen vacancies. © 2006 Elsevier B.V. All rights reserved. Keywords: La-doped ZnO films; Sputtering; Band gap; Photoluminescence
1. Introduction Zinc oxide (ZnO) has been intensively studied due to its potential applications in short-wavelength light-emitting devices such as light-emitting diodes and diode lasers [1], as well as due its wide band gap of 3.37 eV at room temperature and high exciton binding energy (60 meV). It has already been reported that the green photoluminescence emitted from ZnO is due to the point defects such as oxygen vacancies in the crystals [2,3]. Jin et al. [4] found that the visible violet photoluminescence from ZnO films was deposited on the sapphire (001) substrate and believed that the emission was related to the defect level in the grain boundaries. However, there are few reports about the violet and green luminescence simultaneously present in ZnO films. La-doped ZnO film (ZnO:La) exhibits excellent violet and green light-emitting properties, and the band gap can be modulated by varying the doping concentrations.
⁎ Corresponding author. Tel.: +86 10 67392733; fax: +86 10 67392412. E-mail address: [email protected] (Ming Zhang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.061
Here, we report the structural and optical properties of ZnO: La films. The band gap engineering and the mechanism of visible photoluminescence are investigated. 2. Experimental ZnO:La films were deposited on quartz substrate using RF magnetron sputtering. La2O3 and ZnO powders were mixed thoroughly with the different ratios of La/Zn = 0, 1, 2, 3 and 5 at.%. The mixture was then pressed in an aluminium holder to the target used for sputtering. The substrate was cleaned ultrasonically by methylbenzene, acetone and ethanol in turn. Prior to deposition, the chamber was pumped up to 6 × 10− 3 Pa and the target was pre-sputtered for 120 min to remove contamination from the surface. The sputtering power, the substrate temperature and the deposition time were 50 W, 400 °C and 60 min, respectively. The sputtering gas was a mixture of Ar (4 N) and O2 (4 N) with a ratio of 4:1, at a total pressure of 1.0 Pa. The distance between the substrate and the target was 40 mm. After the deposition, the samples were in-situ annealed in vacuum at 600 °C for 60 min.
Wei Lan et al. / Materials Letters 61 (2007) 2262–2265
2263
Fig. 1. XRD patterns of ZnO:La films with different doping concentrations (0, 1, 2, 3 and 5 at.%).
The crystalline structure was identified using a BRUKERAXS D8 X-ray diffractometer (Cu kα, λ = 0.154056 nm). The optical transmittance and reflectance were measured by a double-beam spectrophotometer (SHIMADZU, UV-3101) within the UV–visible range. The photoluminescence spectra were carried out using a RF-540 spectrophotometer with an excitation wavelength of 325 nm (He–Cd laser). The film thicknesses were determined by the surface roughness analyzer (Surfcom 480A). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of ZnO:La films with different doping concentrations (0–5 at.%). Only the (002) diffraction peak of the hexagonal wurtzite ZnO structure is presented, indicating that all the films are grown along the preferential orientation of [002], which is perpendicular to the substrate surface. The intensity of the (002) peak increases followed by a sharp decrease and then hardly changes with the enhancement of La doping concentration. No new diffraction peaks are observed even when the concentration of La increases up to 5 at.%, which means that the impurities do not change the wurtzite structure of the ZnO films. As seen from the inset of Fig. 1, the (002) peaks for all doped films shift to the small angles compared to that of the undoped ZnO film. It means that the ZnO lattice is expanded along the c-axis for all doped samples due to the lattice mismatch between La3+ and ZnO lattice ( the ionic radii of La3+, Zn2+ and O2− are 0.106 nm, 0.074 nm and 0.132 nm, respectively).
According to the full-width at half-maximum (FWHM) of the (002) peak, the grain size of ZnO:La films can be evaluated by the Scherrer's formula [5]. D¼
0:9k W d cosh
ð1Þ
where λ, W and θ are the X-ray wavelength, FWHM and Bragg diffraction angle, respectively. The grain sizes for all the films are presented in Table 1. As is shown, the grain size decreases with the increase of La concentration up to 2 at.% and then hardly changes for further doping. It indicates that a small amount of La atoms (about 1– 2 at.%, based on the comprehensive analysis of the structure and the later photoluminescence properties) have been incorporated into the ZnO matrix as impurities, which may be associated with the solid solubility of La in ZnO film. Moreover, the incorporated La atoms probably represent obstructions to grain boundary movement and limit the grain growth.
Table 1 Grain size, film thickness, and band gap of ZnO:La films with different doping concentrations La concentration (at.%)
0
1
2
3
5
Grain size (nm) Film thickness (nm) Band gap (eV)
34.5 240 3.270
21.6 300 3.275
16.5 200 3.287
17.9 110 3.307
17.3 200 3.326
Fig. 2. Transmittance and reflectance spectra of ZnO:La films with varying dopant concentrations.
2264
Wei Lan et al. / Materials Letters 61 (2007) 2262–2265
Fig. 3. Photoluminescence spectra of ZnO:La films with different La concentrations.
The optical transmittance (T) and reflectance (R) spectra of ZnO:La films with different concentrations are shown in Fig. 2. All films show relatively high transparency (N80%) in the visible light range. Sharp absorption edges can be clearly observed and shift to the short wavelength with the increase of La concentration. The absorption coefficient α of the ZnO:La film can be calculated by using Eq. (2) (neglecting the interference and multiple reflections within the films as well as at the film–substrate interface and also assuming near normal incidence of radiation [6,7]) 1 1−R a ¼ ln d T
ð2Þ
where d is the film thickness that is given in Table 1. For the direct band gap semiconductor, the relationship of the absorption coefficient and the band gap energy can be described by the following equation [8] ðahmÞ2 ¼ Aðhm−Eg Þ
ð3Þ
where hν is the incident photon energy, Eg is the band gap energy and A is a constant. A plot of (αhν)2 vs. hv is made to determine Eg using the linear fit process. As shown in Table 1, the Eg, derived from the crosspoint at the axis of hm with the tangent to (αhm)2, increases from 3.270 to 3.326 eV with increasing doping concentration, which may be due to the Burstein–Moss effect [9,10]. On the other hand, the transmittance of the film in the UV region is also enhanced with increasing La concentration. We presume that the increase of the transmittance in the UV region might be ascribed to the fragment of Lanthanum oxide formed in ZnO films, because the band gap energies of Lanthanum oxide compounds are higher than that of ZnO, such as La2O3 5.5 eV. Meanwhile, it further approves that the excessive La impurities are excluded from the ZnO lattice and exist as oxides mainly in the grain boundaries. The photoluminescence (PL) spectra of ZnO:La films with varying dopant concentrations are characterized in the range of 400–600 nm. It is shown in Fig. 3 that a violet peak located at about 420 nm (2.95 eV) and the intensity of emission are found to be strongly dependent on the La concentration. The sample with 2 at.% exhibits excellent violet emission and presents a weak green emission band positioned at 510– 530 nm. When the doping concentration is above 2 at.%, the intensity of the violet and green emission decreases gradually. As the large lattice mismatch between La and ZnO matrix, ZnO grain size gradually
decreases with increasing La concentrations from 0 to 2 at.%, and the traps on the grain surface per unit volume increase with the increase of the specific surface area. Cordaro et al. [11] believed that interface traps lie in the depletion regions and locate at the ZnO–ZnO grain boundaries when a polycrystalline varistor forms, and the level of the interface trap was found to be about 0.33 eV below the conduction band edge. So the violet emission is probably attributed to the recombination centers connected with the interface traps existing at the grain boundaries, and the radiative transition happens between the level of the interface traps and the valence band. Jin et al. [4] also found that violet photoluminescence from the ZnO film prepared at low oxygen pressure originated from the defect levels in the grain boundaries of oxygen-deficient ZnO x crystals, and green-yellow luminescence appeared instead of the violet peak when the oxygen pressure increased. It is well known that the green light emitted is related to the ZnO lattice defect such as oxygen vacancies [2,3,12–14]. Therefore, the weak green emission, present in the ZnO:La film with 2 at.% concentration, indicates that the incorporation of La atoms in the ZnO lattice might lead to the escapement of oxygen atoms from the lattice site when the film were post-annealed in vacuum at 600 °C. When further doping, excessive La impurities and its oxide are separated at the grain boundaries, which might induce the diminution of the radiative defects to decrease the violet emission. Due to the larger difference of electronegativities between La (1.2) and O (3.5), compared with Zn (1.65)–O, it is reasonably estimated that the escapement of oxygen atoms from the La–O bond could be much more difficult than from the Zn–O bond in the ZnO lattice when the doping concentration is above 2 at.%. So the decrease of oxygen vacancies weakens the green emission.
4. Conclusion ZnO:La films (0–5 at.%) on quartz substrates were prepared by RF magnetron sputtering. All the films grow along the preferred orientation of [002]. According to the shift of the (002) diffraction peak and the increase of the FWHM, a small amount of La atoms about 1–2 at.% are incorporated into ZnO lattice. The band gap of ZnO:La films, with the average transmittance above 80%, linearly increases from 3.270 to 3.326 eV with the increase of the doping concentrations. The strong violet emission peak and the weak green emission band are obtained for the ZnO:La films, which are associated with the interface traps existing at the ZnO grain boundaries and oxygen vacancies in the ZnO lattice, respectively. Acknowledgements This work is supported by the Beijing Specific Project to Foster Elitist (No. 20041D0501513) and the National Natural Science Foundation of China (No. 60576012). References [1] E.M. Wong, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2939. [2] T. Yamamoto, Y. Wada, H. Miyamoto, S. Yanagida, Chem. Lett. 33 (2004) 246. [3] Y. Lin, J. Xie, H. Wang, Y. Li, et al., Thin Solid Films 492 (2005) 101. [4] B.J. Jin, S. Im, S.Y. Lee, Thin Solid Films 366 (2000) 107. [5] D. Song, P. Windenborg, et al., Sol. Energy Mater. Sol. Cells 73 (2002) 269.
Wei Lan et al. / Materials Letters 61 (2007) 2262–2265 [6] F. Demichelis, G. Kaniadakis, A. Tagliaferro, E. Tresso, Appl. Opt. 26 (1987) 1737. [7] A.N. Banerjee, K.K. Chattopadhyay, J. Appl. Phys. 97 (2005) 084308. [8] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, Englewood Cliffs, NJ, 1971. [9] E. Burstein, Phys. Rev. 93 (1954) 632.
[10] [11] [12] [13] [14]
2265
T.S. Moss, Proc. Phys. Soc. Lond. B 67 (1954) 775. J.F. Cordaro, Y. Shim, J.E. May, J. Appl. Phys. 60 (1986) 4186. K. Vanheusden, W.L. Warren, et al., J. Appl. Phys. 79 (1996) 7983. K. Vanheusden, C.H. Seager, et al., J. Lumin. 75 (1997) 11. S.A. Studenikin, N. Golego, M. Colivera, J. Appl. Phys. 84 (1998) 2287.