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TiO2 thin film
Materials Letters 61 (2007) 4735 – 4737 www.elsevier.com/locate/matlet
Co-emission of UV, violet and green photoluminescence of ZnO/TiO2 thin film Linxing Shi a,⁎, Hua Shen b , Liyong Jiang a , Xiangyin Li a a
b
Department of Physics, Nanjing University of Science and Technology, Nanjing 210094, PR China Institute of Electronic Engineering and Photo-electric Technology, Nanjing University of Science and Technology, Nanjing 210094, PR China Received 20 January 2007; accepted 4 March 2007 Available online 14 March 2007
Abstract ZnO/TiO2 thin films were fabricated on quartz glass substrates by E-beam evaporation. The structural and optical properties were investigated by X-ray diffraction (XRD), Raman spectra, optical transmittance and photoluminescence. XRD analysis indicates that the TiO2 buffer layer can increase the preferential orientation along the (002) plane of the ZnO film. PL measurements suggest that co-emission of strong UV peak at 378 nm, violet peak at 423 nm and weak green luminescence at 544 nm is observed in the ZnO/TiO2 thin film. The violet luminescence emission at 423 nm is attributed to the interface trap in the ZnO film grain boundaries. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; TiO2 buffer layer; Co-emission; Luminescence; Thin films
1. Introduction Zinc oxide (ZnO) with a wurtzite crystal structure has a wide direct bandgap (3.37 eV) and a high free exciton binding energy (60 meV) at room temperature. ZnO thin films have attracted considerable attention because of their potential applications in fields such as short wavelength light emitting diodes [1], thin-film solar cells [2] and photodetectors [3]. Multilayer thin films have been studied in order to change the electrical and optical properties of ZnO. Films deposited on a buffer layer are better than those grown directly on a substrate. Wang et al. [4] fabricated a high temperature multilayer ZnO/ MgO film by pulsed laser deposition and compared its photoluminescence (PL) spectra with those of the bare ZnO film. Similarly, Sun et al. [5] fabricated ZnO/MgO multiple quantum wells grown on Si (001) substrates using RF reactive magnetron sputtering, and found a blueshift of the UV PL emission at room temperature. Hong et al. [6,7] investigated the PL emissions of ZnO/ITO/ZnO and ZnO/MgF2/ZnO sandwich structures and enhanced the near-band-edge photo⁎ Corresponding author. Tel.: +86 25 84315592. E-mail address: [email protected] (L. Shi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.019
luminescence. However, to the best of our knowledge, there is no report on using TiO2 film as a buffer layer to enhance the photoluminescence emission or change the emission wavelength of ZnO thin film. 2. Experimental ZnO, TiO2, ZnO/TiO2, and TiO2/ZnO films were grown on Φ30 mm quartz glass substrates by E-beam evaporation (PMC90S, Protech Korea Ltd). TiO2 was deposited at 200 °C and ZnO at 300 °C. The chamber was evacuated to a base pressure of 2.0 × 10− 5 Torr, then Ar (purity: 99.9999%) at 18 sccm (cubic centimeter per minute at STP) was used to etch the substrates for 5 min. Finally, a flow of O2 (purity: 99.9999%) at ∼ 35 sccm for TiO2 and ∼ 60 sccm for ZnO was turned on (working pressure 1.0 × 10− 4 Torr for TiO2 and 2.4 × 10− 4 Torr for ZnO). The electron gun voltage was 7.11 kV for depositing both ZnO and TiO2. The current was 78 mA for ZnO and 246 mA for TiO2. The deposited sources of ZnO and TiO2 are 99.999% pure. The distance between the substrates and sources is about 1.5 m. The substrates were rotated at 40 rpm to make the films uniform. Both films were deposited at the rate of 5.0 Å/s. Each layer is ∼ 200 nm thick.
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L. Shi et al. / Materials Letters 61 (2007) 4735–4737
Fig. 1. XRD patterns of thin films.
Fig. 3. Transmittance spectra of ZnO thin films.
Structures of the films were characterized by a Bruker D8advance X-ray diffractometer with Cu-Kα (λ = 1.5406 Å) radiation. The θ range used in the measurement was 20–70° in steps of 0.05° s− 1. Optical transmittance spectra were taken using a Lambda 950 UV/VIS spectrometer (Perkin Elmer, USA). Raman spectra were collected on a Renishaw Invia Raman microscope excited by Ar+ 514.5 nm laser beam (20 mW). PL spectra were acquired in a FluoroMax-2 fluorescence spectrometer (JOBIN YVON-SPEX) at 350– 600 nm, with a 325-nm (Xe lamp) excitation light. All measurements were performed in air at room temperature.
Fig. 1 depicts the XRD patterns of these different structural samples. It shows that the TiO2 thin film is non-crystalline when directly deposited on the quartz glass at 200 °C. Only a weak (002) peak at 34.45° is observed for the ZnO film, indicating that it has a hexagonal wurtzite crystal structure with its c-axis perpendicular to the substrate surface. The average size of ZnO in our experiment is about 15.15 nm according to the Scherrer formula. When capped-layer TiO2 film is used, the full width at half maximum (FWHM) decreases and
the size of ZnO increases, therefore the crystalline quality of the ZnO film is slightly improved. However, the average size of ZnO changes from 19.45 nm to 25.31 nm when TiO2 film is used as a buffer layer. The crystalline quality of the ZnO film is drastically improved. ZnO has a hexagonal wurtzite structure with the space group P63mc. The phonon modes belong to the 2E2, 2E1, 2A1 and 2B1 symmetries. The two B1 symmetry modes are not Raman-active. A1 and E1 are infrared-active and split into longitudinal (LO) and transverse optical (TO) components. Fig. 2 shows the Raman spectra obtained at room temperature. Since TiO2 directly deposited on the substrate at 200 °C is non-crystalline and bare ZnO film has poor crystallization, their Raman spectra are omitted here. For ZnO/TiO2 thin film, the characteristic ZnO peaks are at 384, 435.6 and 573 cm− 1. The Raman band at 384 cm− 1 is attributed to A1 (TO) mode. The peak at 435.6 cm− 1 is due to E2 (H) mode. Raman shift of 573 cm− 1 is assigned to A1 (LO) mode. For TiO2/ZnO thin film, the characteristic ZnO peaks are at 384 (A1 (TO)) and 437 cm− 1 (E2 (H)). The Raman peak at 573 cm− 1 disappears, indicating the decrease in defect concentration in TiO2/ZnO multilayer film due to annealing of the ZnO film before depositing the TiO2 film. The peak at ∼ 435.6 cm− 1 in ZnO/ TiO2 film is stronger than that in TiO2/ZnO film, showing that the crystalline quality of ZnO is improved by using TiO2 as a buffer layer, consistent with the XRD result.
Fig. 2. Room temperature Raman spectra of ZnO/TiO2 and TiO2/ZnO films.
Fig. 4. Room temperature PL spectra of samples excited with a 325-nm Xe lamp.
3. Results and discussion
L. Shi et al. / Materials Letters 61 (2007) 4735–4737
Fig. 3 illustrates the transmittance spectra of four samples. It shows that all films have high transmittance (∼ 80%) in the 400–800 nm range. The oscillations in the visible region are caused by optical interference at the interface between the thin film and substrate. However, transmission, falling very sharply in the UV region, is attributed to strong absorption by the ZnO film. The optical bandgap energy (Eg) is calculated from the α2 vs Eg plot, assuming α2 ∝ (hν − Eg), where hν is the photon energy and α is the optical absorption coefficient. The Eg's of ZnO for bare ZnO, TiO2/ZnO and ZnO/TiO2 samples are 3.38, 3.35 and 3.32 eV, respectively. However, the Eg of the bare TiO2 sample is ∼ 3.76 eV, larger than ∼ 3.2 eV of the crystal TiO2. Non-crystalline TiO2 is of Eg ∼ 4.0 eV [8]. So the TiO2 film directly deposited on the quartz substrate at 200 °C is noncrystalline, consistent with the XRD result. Fig. 4 shows the room temperature PL spectra at 350–600 nm of four samples excited by the Xe lamp. For the bare ZnO film, a weak UV peak at 378 nm with a wide FWHM of 40 nm is observed, and there is a negligible luminescence in the visible region. It possibly arises from the poor crystalline quality of ZnO film directly deposited on substrate at low temperature. The co-emission of sharp UV luminescence at 378 nm, strong violet emission at 423 nm and weak green emission at 544 nm is found from the ZnO/TiO2 thin film. Compared with the PL of bare ZnO film, the intensity of UV luminescence has increased about six-fold and its FWHM has decreased from 40 nm to 16 nm. Generally, the PL spectra from ZnO consist of the UV emission band and the visible emission broadband. UV emission is attributed to exciton recombination. The visible luminescence is mainly due to the structural defects which are related to deep-level emissions, such as zinc vacancy, oxygen vacancy, interstitial zinc and interstitial oxygen [9]. It is not surprising that UV and green luminescence are found in our ZnO film. It is interesting that a strong violet emission at 423 nm, even stronger than UV luminescence, is also observed. The co-emission of strong UV and violet luminescence has hardly been reported so far. For the purpose of excluding the origin of the luminescence at 423 nm from the TiO2 film, we measured the PL of bare TiO2 film and TiO2/ZnO thin film at identical excitation intensity. Violet luminescence at about 423 nm was not found (see Fig. 4). So, we believe that the violet emission is not ascribed to amorphous or crystal TiO2 films. There are many explanations for violet emission. A violet luminescence at 420 nm was reported [10] as electron-hole plasma state at high excitation, while 414 nm emission was attributed to shallow defects at weak excitation intensity [11]. Cao et al. [12] reported that the violet emission was produced from exciton recombination between the electrons localized at the Zni-shallow donor levels and holes in the valence band at 10 K. However, the violet luminescence band was inconspicuous at room temperature in their experiment. These explanations do not agree with ours. ZnO films deposited on the TiO2 buffer layer may have more grain boundary defects. So the violet luminescence is probably due to radiative defects related to the interface traps existing at the grain boundaries and emitted from the
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radiative transition between this level and the valence band. Meanwhile, the preferential orientation is a necessary condition, since grain boundaries may produce different defects such as non-radiative defects. Therefore, violet emission at 423 nm is probably due to both the preferential orientation and abundant grain boundaries in the ZnO film.
4. Conclusions ZnO, TiO2, TiO2/ZnO and ZnO/TiO2 films on quartz glass substrates were fabricated by E-beam evaporation. XRD and Raman spectra demonstrate that the crystallinity of ZnO film is considerably improved by using a TiO2 film buffer layer. PL spectroscopy shows that the co-emission of UV, violet and green luminescence is observed from the ZnO/TiO2 thin film. Compared with the PL of ZnO single-layer film deposited at 300 °C, UV intensity of ZnO/TiO2 multilayer film has increased about six-fold and its FWHM has decreased from 40 nm to 16 nm. We observed violet luminescence at 423 nm, rarely reported before. It is much stronger than similar luminescence reported at 420 nm. It is attributed to the interface trap in the ZnO film grain boundaries. Acknowledgment This work was financially supported by the Doctoral Program Foundation of the Institution of High Education of China (Grant 20020288026). References [1] N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I. Sakaguchi, K. Koumoto, Adv. Mater. 14 (2002) 418. [2] O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, Thin Solid Films 442 (2003) 80. [3] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225 (2001) 110. [4] Z. Wang, L. Hu, J. Zhao, H. Zhang, Z. Wang, Vacuum 80 (2006) 977. [5] C.W. Sun, P. Xin, Z.W. Liu, Q.Y. Zhang, Appl. Phys. Lett. 88 (2006) 221914. [6] R. Hong, J. Shao, H. He, Z. Fan, J. Appl. Phys. 99 (2006) 093520. [7] R. Hong, J. Shao, H. He, Z. Fan, J. Cryst. Growth 290 (2006) 334. [8] S.K. Deb, Solid State Commun. 11 (1972) 713. [9] Y.G. Wang, S.P. Lau, H.W. Lee, S.F. Yu, B.K. Tay, X.H. Zhang, H.H. Hng, J. Appl. Phys. 94 (2003) 354. [10] G. Tobin, E. McGlynn, M.O. Henry, J.P. Mosnier, E. de Posada, J.G. Lunney, Appl. Phys. Lett. 88 (2006) 071919. [11] X.P. Shen, A.H. Yuan, Y. Jiang, Z. Xu, Z. Hu, Nanotechnology 16 (2005) 2039. [12] B. Cao, W. Cai, H. Zeng, Appl. Phys. Lett. 88 (2006) 161101.
Co-emission of UV, violet and green photoluminescence of ZnO/TiO2 thin film Linxing Shi a,⁎, Hua Shen b , Liyong Jiang a , Xiangyin Li a a
b
Department of Physics, Nanjing University of Science and Technology, Nanjing 210094, PR China Institute of Electronic Engineering and Photo-electric Technology, Nanjing University of Science and Technology, Nanjing 210094, PR China Received 20 January 2007; accepted 4 March 2007 Available online 14 March 2007
Abstract ZnO/TiO2 thin films were fabricated on quartz glass substrates by E-beam evaporation. The structural and optical properties were investigated by X-ray diffraction (XRD), Raman spectra, optical transmittance and photoluminescence. XRD analysis indicates that the TiO2 buffer layer can increase the preferential orientation along the (002) plane of the ZnO film. PL measurements suggest that co-emission of strong UV peak at 378 nm, violet peak at 423 nm and weak green luminescence at 544 nm is observed in the ZnO/TiO2 thin film. The violet luminescence emission at 423 nm is attributed to the interface trap in the ZnO film grain boundaries. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; TiO2 buffer layer; Co-emission; Luminescence; Thin films
1. Introduction Zinc oxide (ZnO) with a wurtzite crystal structure has a wide direct bandgap (3.37 eV) and a high free exciton binding energy (60 meV) at room temperature. ZnO thin films have attracted considerable attention because of their potential applications in fields such as short wavelength light emitting diodes [1], thin-film solar cells [2] and photodetectors [3]. Multilayer thin films have been studied in order to change the electrical and optical properties of ZnO. Films deposited on a buffer layer are better than those grown directly on a substrate. Wang et al. [4] fabricated a high temperature multilayer ZnO/ MgO film by pulsed laser deposition and compared its photoluminescence (PL) spectra with those of the bare ZnO film. Similarly, Sun et al. [5] fabricated ZnO/MgO multiple quantum wells grown on Si (001) substrates using RF reactive magnetron sputtering, and found a blueshift of the UV PL emission at room temperature. Hong et al. [6,7] investigated the PL emissions of ZnO/ITO/ZnO and ZnO/MgF2/ZnO sandwich structures and enhanced the near-band-edge photo⁎ Corresponding author. Tel.: +86 25 84315592. E-mail address: [email protected] (L. Shi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.019
luminescence. However, to the best of our knowledge, there is no report on using TiO2 film as a buffer layer to enhance the photoluminescence emission or change the emission wavelength of ZnO thin film. 2. Experimental ZnO, TiO2, ZnO/TiO2, and TiO2/ZnO films were grown on Φ30 mm quartz glass substrates by E-beam evaporation (PMC90S, Protech Korea Ltd). TiO2 was deposited at 200 °C and ZnO at 300 °C. The chamber was evacuated to a base pressure of 2.0 × 10− 5 Torr, then Ar (purity: 99.9999%) at 18 sccm (cubic centimeter per minute at STP) was used to etch the substrates for 5 min. Finally, a flow of O2 (purity: 99.9999%) at ∼ 35 sccm for TiO2 and ∼ 60 sccm for ZnO was turned on (working pressure 1.0 × 10− 4 Torr for TiO2 and 2.4 × 10− 4 Torr for ZnO). The electron gun voltage was 7.11 kV for depositing both ZnO and TiO2. The current was 78 mA for ZnO and 246 mA for TiO2. The deposited sources of ZnO and TiO2 are 99.999% pure. The distance between the substrates and sources is about 1.5 m. The substrates were rotated at 40 rpm to make the films uniform. Both films were deposited at the rate of 5.0 Å/s. Each layer is ∼ 200 nm thick.
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Fig. 1. XRD patterns of thin films.
Fig. 3. Transmittance spectra of ZnO thin films.
Structures of the films were characterized by a Bruker D8advance X-ray diffractometer with Cu-Kα (λ = 1.5406 Å) radiation. The θ range used in the measurement was 20–70° in steps of 0.05° s− 1. Optical transmittance spectra were taken using a Lambda 950 UV/VIS spectrometer (Perkin Elmer, USA). Raman spectra were collected on a Renishaw Invia Raman microscope excited by Ar+ 514.5 nm laser beam (20 mW). PL spectra were acquired in a FluoroMax-2 fluorescence spectrometer (JOBIN YVON-SPEX) at 350– 600 nm, with a 325-nm (Xe lamp) excitation light. All measurements were performed in air at room temperature.
Fig. 1 depicts the XRD patterns of these different structural samples. It shows that the TiO2 thin film is non-crystalline when directly deposited on the quartz glass at 200 °C. Only a weak (002) peak at 34.45° is observed for the ZnO film, indicating that it has a hexagonal wurtzite crystal structure with its c-axis perpendicular to the substrate surface. The average size of ZnO in our experiment is about 15.15 nm according to the Scherrer formula. When capped-layer TiO2 film is used, the full width at half maximum (FWHM) decreases and
the size of ZnO increases, therefore the crystalline quality of the ZnO film is slightly improved. However, the average size of ZnO changes from 19.45 nm to 25.31 nm when TiO2 film is used as a buffer layer. The crystalline quality of the ZnO film is drastically improved. ZnO has a hexagonal wurtzite structure with the space group P63mc. The phonon modes belong to the 2E2, 2E1, 2A1 and 2B1 symmetries. The two B1 symmetry modes are not Raman-active. A1 and E1 are infrared-active and split into longitudinal (LO) and transverse optical (TO) components. Fig. 2 shows the Raman spectra obtained at room temperature. Since TiO2 directly deposited on the substrate at 200 °C is non-crystalline and bare ZnO film has poor crystallization, their Raman spectra are omitted here. For ZnO/TiO2 thin film, the characteristic ZnO peaks are at 384, 435.6 and 573 cm− 1. The Raman band at 384 cm− 1 is attributed to A1 (TO) mode. The peak at 435.6 cm− 1 is due to E2 (H) mode. Raman shift of 573 cm− 1 is assigned to A1 (LO) mode. For TiO2/ZnO thin film, the characteristic ZnO peaks are at 384 (A1 (TO)) and 437 cm− 1 (E2 (H)). The Raman peak at 573 cm− 1 disappears, indicating the decrease in defect concentration in TiO2/ZnO multilayer film due to annealing of the ZnO film before depositing the TiO2 film. The peak at ∼ 435.6 cm− 1 in ZnO/ TiO2 film is stronger than that in TiO2/ZnO film, showing that the crystalline quality of ZnO is improved by using TiO2 as a buffer layer, consistent with the XRD result.
Fig. 2. Room temperature Raman spectra of ZnO/TiO2 and TiO2/ZnO films.
Fig. 4. Room temperature PL spectra of samples excited with a 325-nm Xe lamp.
3. Results and discussion
L. Shi et al. / Materials Letters 61 (2007) 4735–4737
Fig. 3 illustrates the transmittance spectra of four samples. It shows that all films have high transmittance (∼ 80%) in the 400–800 nm range. The oscillations in the visible region are caused by optical interference at the interface between the thin film and substrate. However, transmission, falling very sharply in the UV region, is attributed to strong absorption by the ZnO film. The optical bandgap energy (Eg) is calculated from the α2 vs Eg plot, assuming α2 ∝ (hν − Eg), where hν is the photon energy and α is the optical absorption coefficient. The Eg's of ZnO for bare ZnO, TiO2/ZnO and ZnO/TiO2 samples are 3.38, 3.35 and 3.32 eV, respectively. However, the Eg of the bare TiO2 sample is ∼ 3.76 eV, larger than ∼ 3.2 eV of the crystal TiO2. Non-crystalline TiO2 is of Eg ∼ 4.0 eV [8]. So the TiO2 film directly deposited on the quartz substrate at 200 °C is noncrystalline, consistent with the XRD result. Fig. 4 shows the room temperature PL spectra at 350–600 nm of four samples excited by the Xe lamp. For the bare ZnO film, a weak UV peak at 378 nm with a wide FWHM of 40 nm is observed, and there is a negligible luminescence in the visible region. It possibly arises from the poor crystalline quality of ZnO film directly deposited on substrate at low temperature. The co-emission of sharp UV luminescence at 378 nm, strong violet emission at 423 nm and weak green emission at 544 nm is found from the ZnO/TiO2 thin film. Compared with the PL of bare ZnO film, the intensity of UV luminescence has increased about six-fold and its FWHM has decreased from 40 nm to 16 nm. Generally, the PL spectra from ZnO consist of the UV emission band and the visible emission broadband. UV emission is attributed to exciton recombination. The visible luminescence is mainly due to the structural defects which are related to deep-level emissions, such as zinc vacancy, oxygen vacancy, interstitial zinc and interstitial oxygen [9]. It is not surprising that UV and green luminescence are found in our ZnO film. It is interesting that a strong violet emission at 423 nm, even stronger than UV luminescence, is also observed. The co-emission of strong UV and violet luminescence has hardly been reported so far. For the purpose of excluding the origin of the luminescence at 423 nm from the TiO2 film, we measured the PL of bare TiO2 film and TiO2/ZnO thin film at identical excitation intensity. Violet luminescence at about 423 nm was not found (see Fig. 4). So, we believe that the violet emission is not ascribed to amorphous or crystal TiO2 films. There are many explanations for violet emission. A violet luminescence at 420 nm was reported [10] as electron-hole plasma state at high excitation, while 414 nm emission was attributed to shallow defects at weak excitation intensity [11]. Cao et al. [12] reported that the violet emission was produced from exciton recombination between the electrons localized at the Zni-shallow donor levels and holes in the valence band at 10 K. However, the violet luminescence band was inconspicuous at room temperature in their experiment. These explanations do not agree with ours. ZnO films deposited on the TiO2 buffer layer may have more grain boundary defects. So the violet luminescence is probably due to radiative defects related to the interface traps existing at the grain boundaries and emitted from the
4737
radiative transition between this level and the valence band. Meanwhile, the preferential orientation is a necessary condition, since grain boundaries may produce different defects such as non-radiative defects. Therefore, violet emission at 423 nm is probably due to both the preferential orientation and abundant grain boundaries in the ZnO film.
4. Conclusions ZnO, TiO2, TiO2/ZnO and ZnO/TiO2 films on quartz glass substrates were fabricated by E-beam evaporation. XRD and Raman spectra demonstrate that the crystallinity of ZnO film is considerably improved by using a TiO2 film buffer layer. PL spectroscopy shows that the co-emission of UV, violet and green luminescence is observed from the ZnO/TiO2 thin film. Compared with the PL of ZnO single-layer film deposited at 300 °C, UV intensity of ZnO/TiO2 multilayer film has increased about six-fold and its FWHM has decreased from 40 nm to 16 nm. We observed violet luminescence at 423 nm, rarely reported before. It is much stronger than similar luminescence reported at 420 nm. It is attributed to the interface trap in the ZnO film grain boundaries. Acknowledgment This work was financially supported by the Doctoral Program Foundation of the Institution of High Education of China (Grant 20020288026). References [1] N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I. Sakaguchi, K. Koumoto, Adv. Mater. 14 (2002) 418. [2] O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, Thin Solid Films 442 (2003) 80. [3] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225 (2001) 110. [4] Z. Wang, L. Hu, J. Zhao, H. Zhang, Z. Wang, Vacuum 80 (2006) 977. [5] C.W. Sun, P. Xin, Z.W. Liu, Q.Y. Zhang, Appl. Phys. Lett. 88 (2006) 221914. [6] R. Hong, J. Shao, H. He, Z. Fan, J. Appl. Phys. 99 (2006) 093520. [7] R. Hong, J. Shao, H. He, Z. Fan, J. Cryst. Growth 290 (2006) 334. [8] S.K. Deb, Solid State Commun. 11 (1972) 713. [9] Y.G. Wang, S.P. Lau, H.W. Lee, S.F. Yu, B.K. Tay, X.H. Zhang, H.H. Hng, J. Appl. Phys. 94 (2003) 354. [10] G. Tobin, E. McGlynn, M.O. Henry, J.P. Mosnier, E. de Posada, J.G. Lunney, Appl. Phys. Lett. 88 (2006) 071919. [11] X.P. Shen, A.H. Yuan, Y. Jiang, Z. Xu, Z. Hu, Nanotechnology 16 (2005) 2039. [12] B. Cao, W. Cai, H. Zeng, Appl. Phys. Lett. 88 (2006) 161101.