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p-Type conduction in Al–N co-doped ZnO films
Materials Letters 58 (2004) 3741 – 3744 www.elsevier.com/locate/matlet
p-Type conduction in Al–N co-doped ZnO films Guodong Yuan, Zhizhen Ye*, Liping Zhu, Yujia Zeng, Jingyun Huang, Qing Qian, Jianguo Lu State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, P. R. China Received 21 October 2003; received in revised form 12 February 2004; accepted 12 February 2004 Available online 19 August 2004
Abstract p-Type conduction in ZnO thin films was realized by Al–N co-doping method. The ZnO films were prepared on a-Al2O3 (0001) substrates by DC reactive magnetron sputtering technique. Alx Zn1x (x=0.1) target and ammonia gas were used as the source of Al and N, respectively. The growth temperature was varied from 400 to 600 8C. The ZnO films deposited at 450 8C showed p-type conduction, with a resistivity of 278 V cm, a Hall mobility of 1.32 cm2/V s and a carrier density of 1.71016 cm3. X-ray diffraction (XRD) measurements showed that all of the Al–N co-doped ZnO films grown at different temperature were of completely (002)preferred orientation. Second ion mass spectroscopy (SIMS) tests proved that the presence of Al facilitated the incorporation of N into ZnO. The p-type ZnO thin films possess a transmittance of about 90% in the visible region and a band gap of 3.28 eV at room temperature. D 2004 Elsevier B.V. All rights reserved. PACS: 61. 72. Ss; 73.61.Ga; 81. 15.Cd Keywords: Semiconductors; Epitaxial growth; Al and N co-doping method; Zinc oxide thin films; p-Type conduction; Magnetron sputtering
1. Introduction ZnO is a semiconductor with a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV. Since the recent discovery of its ultraviolet (UV) excitonic emission at room temperature, ZnO has been expected for a material of light emitting diode (LED) and laser diode (LD) in UV or blue spectral region, various studies aimed at optical devices have been stimulated [1,2]. To obtain p-type ZnO film, which is indispensable for realizing ZnO film based optoelectronic device, there have been a few fundamental studies focused on the synthesis of p-type ZnO doped with N [3–5]. Yamamoto and Yoshida [6] proposed a donor– acceptor co-doping method based upon first principle calculations and prepared p-type ZnO film with this method successfully [7]. The proposed choices of donors are Al, Ga or In, while N being the acceptor. By now, many groups
* Corresponding author. Tel.: +86 0571 87953139; fax: +86 571 87952625. E-mail address: [email protected] (Z. Ye). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.056
have been engaged in the investigations on Ga–N co-doping method [6–10]. However, there is few or no investigation on Al–N and In–N co-doping in the growth of p-type ZnO film. Here, we have succeeded in preparing p-type ZnO films using Al–N co-doping method using N as the acceptor and Al as the reactive co-dopant. This method was found to be very good in reproducibility. In this letter, the electrical properties, the structural characteristics and the optical transmission spectra of the p-type ZnO film were discussed. Secondary ion mass spectroscopy (SIMS) of the co-doped ZnO films was given.
2. Experiment procedure The ZnO films were prepared by the DC reactive magnetron sputtering technique. The vacuum chamber was evacuated to 103 Pa, then NH3 and O2 gases, both with the purity of 99.99%, were introduced through the separate mass flow controllers. The alloy of Alx Zn1x (x=0.1) was used as the target. The total pressure was set to 3 Pa, and the ammonia content NH3/(NH3+O2) was 0.6. The sputtering
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time was 20 min, and the sputtering current and sputtering voltage were 300 mA and 200 V. In order to study the effects of substrate temperature, ZnO films were deposited at various substrate temperatures, while other growth parameters being kept constant. The electrical properties were investigated by means of a HL5500 system. The crystal structure of the films was analysed by X-ray diffraction (XRD) (Philip. CuKa=0.1542 nm). The depth profiles of Al, N, Zn and O were measured by the secondary ion mass spectroscopy (SIMS, IMS 6F; CAMECA, Courberoie, France). Optical transmission spectra were measured with a Lambda20 spectrometer.
3. Results and discussion The XRD patterns of the as-grown ZnO films deposited at different substrate temperatures are shown in Fig. 1. Only one peak corresponding to the (002) plane of ZnO is observed in the XRD patterns, and no Zn3N2, AlN and Zn peaks are detected. It suggests that the obtained ZnO films exhibit (002) preferential orientation with the c-axis perpendicular to the substrate. We have subtracted the aAl2O3 (0006) peak (occurred at around 41.68) [11] corresponding to substrate. When the substrate temperature is varied, the intensity of the (002) peak initially increases, and reaches the maximum value at 550 8C, then decreases. It can be inferred that the ZnO film growth at 550 8C possesses the best crystallinity. As the substrate temperature increases, c-axis is lengthened. And this is likely due to the onset of the in-plane compressive stress in the films [12]. It is obvious that the growth of the ZnO films is very dependent on the substrate temperature. The substrate temperature exert a remarkable influence on the structure characterization, this is because that the substrate temperature restricts diffusion behaviour of nucleation particle on substrate surface. According to the nucleation theory, for a perfect heteroepitaxy nucleation, it
Fig. 1. XRD patterns of the ZnO films co-doped with Al and N at various temperatures.
is necessary to satisfy surface diffusion condition [13], the substrate temperature must be higher than the critical value. Below this value, the sputtering molecules or atoms with higher energy can also be bcooledQ quickly, their surface diffusion length is greatly reduced, and they cannot migrate to nucleation position. The surface of the film obtained under the critical temperature would be rough, and with polycrystal or amorphous structure. When the growth temperature is too high, the absorption life length of ZnO molecule will be shorten, and the decomposition of ZnO molecule is faster than the combination of Zn and O, even under high partial pressure of O2. This will introduce local Zn-rich region and result in a large number of point defect (Vo or Zno), even leading to amorphous state, with a bad compactness in the ZnO film. Hall effect results for the ZnO films grown at different temperatures were measured by the four-probe van der Pauw method at room temperature. Data were compiled employing both positive and negative currents and magnetic fields, and the results were averaged in order to compensate for various electromagnetic effects and get a more accurate data. The Hall coefficient was positive for several samples prepared at the same conditions, which gives reassurance that the samples are truly p-type. Based on these measurements, ZnO films grown with substrate temperature of 450 and 600 8C show p-type characteristics. The p-type ZnO films grown at 450 8C has better electrical properties, such as, hole carrier concentration of 1.71016 cm3, Hall mobility of 1.32 cm2/V s, and resistivity of 278 V cm. The co-doping effect, which can lead to the formation of the p-type conduction, is very dependent on the growth temperature [14,15]. Beside the co-doping effect, hydrogen passivation, Al–N interaction and N–O desorption are effective to the properties of the films [16,17]. Because of all the influences described above, p-type conduction can be realized only at an appropriate temperature. More studies are needed to understand the mechanism. SIMS depth profiles for the co-doped ZnO films are shown in Fig. 2. Fig. 2a are the depth profile of Al, N, Zn and O in the co-doped ZnO films by the two-step growth, which consists of the first step of growing at 600 8C for 5 min, and the second step of growing at 500 8C for 20 min. Fig. 2b is based on the sample deposited at 600 8C. As can be seen in Fig. 2a, the content of Al in the ZnO film increasing, the content of N in the ZnO films has the same trend of increasing. From Fig. 2b, it can be also observed that the contents of Al and N have similar trend of depth profile near the surface of the ZnO films (the decrease of Al content on surface may due to Al absorption), and Zn, O have a uniform distribution across the ZnO films. These findings suggest that the presence of Al facilitates the incorporation of N into the ZnO, which are consistent with the prediction of Ref. [6], namely that the presence of the donor (Al) enhances N incorporation. Calculations in Ref. [6] revealed that the repulsive interactions between the N acceptors lead to low solubility and instability of N atoms in
G. Yuan et al. / Materials Letters 58 (2004) 3741–3744
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Fig. 4. E g of the n- and p-type ZnO thin films derived from transmittance spectra.
Fig. 3 shows the typical optical transmission spectra of ntype and p-type ZnO thin films measured at room temperature. The fundamental absorption starts at about 370 nm. The transmission of the p-type ZnO thin film is almost identical to that of the n-type thin ZnO film. A large transmittance greater than 90% is obtained in visible wavelength regions. The weak fluctuation in the spectrum is principally due to interference phenomenon. The absorption coefficient (a) of the ZnO thin films was determined from the transmittance data obtained at normal incidence, using the following relation [18]: Fig. 2. SIMS depth profiles of Al–N co-doped ZnO films: (a) for a twostep growth (600 8C/5min, 500 8C/20min) film and (b) for a film grown at 600 8C).
p-type ZnO:N films, however, the formation of acceptor– donor–acceptor complexes which occupy nearest-neighbor sites is energetically favorable.
a ¼ ½2:303logð1=T Þ=d where T is the transmittance and d is the film thickness. As a direct band gap semiconductor, ZnO thin film has an absorption coefficient (a) obeying the following relation for high photon energies (hc): ðahcÞ2 ¼ A hc Eg where E g is the optical band gap of thin film, and A is a constant. The variations of (ahc) 2 versus hc in the fundamental adsorption region are plotted in Fig. 4, and E g can be evaluated by extrapolation of the linear part. The optical band gap is determined to be about 3.28 eV for the Al–N co-doped p-type ZnO thin films and 3.31 eV for the Al–N co-doped n-type ZnO thin film. The electron negativity of O (3.5) is larger than that of N (3.0), so the Zn–O band has larger ionicity than the Zn–N band. Thus, the decrease in E g is probably ascribed to the decrease in ionicity due to the formation of Zn–N bonds within the p-type ZnO thin films.
4. Conclusions
Fig. 3. Optical transmittance spectra of the n- and p-type ZnO thin films.
In summary, Al–N co-doped p-type ZnO films have been synthesized by DC reactive magnetron sputtering technique.
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The properties of the ZnO films are substantially dependent on the growth temperature. The as-grown ZnO film deposited at 450 8C shows p-type conduction with better properties, such as resistivity of 278 V cm, Hall mobility of 1.32 cm2/V s and carrier density of 1.71016 cm3. The presence of Al facilitates the incorporation of N into ZnO films. The p-type ZnO thin films possess a transmittance of about 90% in the visible region and a band gap of 3.28 eV at room temperature.
Acknowledgements Supported by Chinese Special Funds for Major State Basic Research Project G20000683-06 and National Natural Science Foundation of China under Contract No. 90201038.
References [1] Z.K. Tang, G.K.L. Wong, P. Yu, et al., Appl. Phys. Lett. 72 (1998) 3270. [2] D.M. Bagnell, Y.F. Chen, Z. Zhu, et al., Appl. Phys. Lett. 70 (1997) 2230.
[3] K. Minegishi, Y. Koiwai, Y. Kikuchi, et al., Jpn. J. Appl. Phys. 36 (1997) L1453. [4] X.L. Guo, H. Tabata, T. kawai, J. Cryst. Growth 233 (2001) 135. [5] Y.R. Ryu, S. Zhu, D.C. Look, et al., J. Cryst. Growth 216 (2000) 330. [6] T. Yamamoto, H. Yoshida, Jpn. J. Appl. Phys., Part 2 38 (1999) L166. [7] M. Joseph, H. Tabata, et al., Jpn. J. Appl. Phys. 38 (1999) L1205 – L1207. [8] M. Komatsu, N. Ohashi, I.S. guchi, Appl. Surf. Sci. 189 (2002) 349 – 352. [9] K. Nakahara, H. Takasu, Appl. Phys. Lett. 79 (2001) 25. [10] A. Tsukazaki, H. Saito, K. Tamura, et al., Appl. Phys. Lett. 81 (2002) 2. [11] A. Tiwari, C. Jin, A. Kvit, D. Kumar, J.F. Muth, J. Narayan, Solid State Commun. 121 (2002) 371 – 374. [12] De-Wei Ma, Zhi-Zhen Ye, Jing-Yun Huang, et al., Chin. Phys. Lett. 20 (6) (2003) 942. [13] Dongjiang Qiu, Huizhen Wu, Ailing Yang, Xiaoling Xu, Chin. J. Mater. Res. 14 (5) (2000) 485 (in Chinese). [14] K. Minegishi, Y. Koiwai, Y. Kikuchi, et al., Jpn. J. Appl. Phys. 36 (1997) L1453. [15] A. Kamata, H. Mitsuhashi, et al., Appl. Phys. Lett. 63 (24) (1993) 3353 – 3354. [16] M. Joseph, H. Tabata, et al., Physica. B 140–148 (2001) 302 – 303. [17] Zhi-Zhen Ye, Jian-Guo Lu, et al., J. Cryst. Growth 253 (2003) 258. [18] D.M. Carballeda-Galicia, R. Castanedo-Perez, Thin Solid Films 371 (2000) 105 – 108.
p-Type conduction in Al–N co-doped ZnO films Guodong Yuan, Zhizhen Ye*, Liping Zhu, Yujia Zeng, Jingyun Huang, Qing Qian, Jianguo Lu State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, P. R. China Received 21 October 2003; received in revised form 12 February 2004; accepted 12 February 2004 Available online 19 August 2004
Abstract p-Type conduction in ZnO thin films was realized by Al–N co-doping method. The ZnO films were prepared on a-Al2O3 (0001) substrates by DC reactive magnetron sputtering technique. Alx Zn1x (x=0.1) target and ammonia gas were used as the source of Al and N, respectively. The growth temperature was varied from 400 to 600 8C. The ZnO films deposited at 450 8C showed p-type conduction, with a resistivity of 278 V cm, a Hall mobility of 1.32 cm2/V s and a carrier density of 1.71016 cm3. X-ray diffraction (XRD) measurements showed that all of the Al–N co-doped ZnO films grown at different temperature were of completely (002)preferred orientation. Second ion mass spectroscopy (SIMS) tests proved that the presence of Al facilitated the incorporation of N into ZnO. The p-type ZnO thin films possess a transmittance of about 90% in the visible region and a band gap of 3.28 eV at room temperature. D 2004 Elsevier B.V. All rights reserved. PACS: 61. 72. Ss; 73.61.Ga; 81. 15.Cd Keywords: Semiconductors; Epitaxial growth; Al and N co-doping method; Zinc oxide thin films; p-Type conduction; Magnetron sputtering
1. Introduction ZnO is a semiconductor with a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV. Since the recent discovery of its ultraviolet (UV) excitonic emission at room temperature, ZnO has been expected for a material of light emitting diode (LED) and laser diode (LD) in UV or blue spectral region, various studies aimed at optical devices have been stimulated [1,2]. To obtain p-type ZnO film, which is indispensable for realizing ZnO film based optoelectronic device, there have been a few fundamental studies focused on the synthesis of p-type ZnO doped with N [3–5]. Yamamoto and Yoshida [6] proposed a donor– acceptor co-doping method based upon first principle calculations and prepared p-type ZnO film with this method successfully [7]. The proposed choices of donors are Al, Ga or In, while N being the acceptor. By now, many groups
* Corresponding author. Tel.: +86 0571 87953139; fax: +86 571 87952625. E-mail address: [email protected] (Z. Ye). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.056
have been engaged in the investigations on Ga–N co-doping method [6–10]. However, there is few or no investigation on Al–N and In–N co-doping in the growth of p-type ZnO film. Here, we have succeeded in preparing p-type ZnO films using Al–N co-doping method using N as the acceptor and Al as the reactive co-dopant. This method was found to be very good in reproducibility. In this letter, the electrical properties, the structural characteristics and the optical transmission spectra of the p-type ZnO film were discussed. Secondary ion mass spectroscopy (SIMS) of the co-doped ZnO films was given.
2. Experiment procedure The ZnO films were prepared by the DC reactive magnetron sputtering technique. The vacuum chamber was evacuated to 103 Pa, then NH3 and O2 gases, both with the purity of 99.99%, were introduced through the separate mass flow controllers. The alloy of Alx Zn1x (x=0.1) was used as the target. The total pressure was set to 3 Pa, and the ammonia content NH3/(NH3+O2) was 0.6. The sputtering
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time was 20 min, and the sputtering current and sputtering voltage were 300 mA and 200 V. In order to study the effects of substrate temperature, ZnO films were deposited at various substrate temperatures, while other growth parameters being kept constant. The electrical properties were investigated by means of a HL5500 system. The crystal structure of the films was analysed by X-ray diffraction (XRD) (Philip. CuKa=0.1542 nm). The depth profiles of Al, N, Zn and O were measured by the secondary ion mass spectroscopy (SIMS, IMS 6F; CAMECA, Courberoie, France). Optical transmission spectra were measured with a Lambda20 spectrometer.
3. Results and discussion The XRD patterns of the as-grown ZnO films deposited at different substrate temperatures are shown in Fig. 1. Only one peak corresponding to the (002) plane of ZnO is observed in the XRD patterns, and no Zn3N2, AlN and Zn peaks are detected. It suggests that the obtained ZnO films exhibit (002) preferential orientation with the c-axis perpendicular to the substrate. We have subtracted the aAl2O3 (0006) peak (occurred at around 41.68) [11] corresponding to substrate. When the substrate temperature is varied, the intensity of the (002) peak initially increases, and reaches the maximum value at 550 8C, then decreases. It can be inferred that the ZnO film growth at 550 8C possesses the best crystallinity. As the substrate temperature increases, c-axis is lengthened. And this is likely due to the onset of the in-plane compressive stress in the films [12]. It is obvious that the growth of the ZnO films is very dependent on the substrate temperature. The substrate temperature exert a remarkable influence on the structure characterization, this is because that the substrate temperature restricts diffusion behaviour of nucleation particle on substrate surface. According to the nucleation theory, for a perfect heteroepitaxy nucleation, it
Fig. 1. XRD patterns of the ZnO films co-doped with Al and N at various temperatures.
is necessary to satisfy surface diffusion condition [13], the substrate temperature must be higher than the critical value. Below this value, the sputtering molecules or atoms with higher energy can also be bcooledQ quickly, their surface diffusion length is greatly reduced, and they cannot migrate to nucleation position. The surface of the film obtained under the critical temperature would be rough, and with polycrystal or amorphous structure. When the growth temperature is too high, the absorption life length of ZnO molecule will be shorten, and the decomposition of ZnO molecule is faster than the combination of Zn and O, even under high partial pressure of O2. This will introduce local Zn-rich region and result in a large number of point defect (Vo or Zno), even leading to amorphous state, with a bad compactness in the ZnO film. Hall effect results for the ZnO films grown at different temperatures were measured by the four-probe van der Pauw method at room temperature. Data were compiled employing both positive and negative currents and magnetic fields, and the results were averaged in order to compensate for various electromagnetic effects and get a more accurate data. The Hall coefficient was positive for several samples prepared at the same conditions, which gives reassurance that the samples are truly p-type. Based on these measurements, ZnO films grown with substrate temperature of 450 and 600 8C show p-type characteristics. The p-type ZnO films grown at 450 8C has better electrical properties, such as, hole carrier concentration of 1.71016 cm3, Hall mobility of 1.32 cm2/V s, and resistivity of 278 V cm. The co-doping effect, which can lead to the formation of the p-type conduction, is very dependent on the growth temperature [14,15]. Beside the co-doping effect, hydrogen passivation, Al–N interaction and N–O desorption are effective to the properties of the films [16,17]. Because of all the influences described above, p-type conduction can be realized only at an appropriate temperature. More studies are needed to understand the mechanism. SIMS depth profiles for the co-doped ZnO films are shown in Fig. 2. Fig. 2a are the depth profile of Al, N, Zn and O in the co-doped ZnO films by the two-step growth, which consists of the first step of growing at 600 8C for 5 min, and the second step of growing at 500 8C for 20 min. Fig. 2b is based on the sample deposited at 600 8C. As can be seen in Fig. 2a, the content of Al in the ZnO film increasing, the content of N in the ZnO films has the same trend of increasing. From Fig. 2b, it can be also observed that the contents of Al and N have similar trend of depth profile near the surface of the ZnO films (the decrease of Al content on surface may due to Al absorption), and Zn, O have a uniform distribution across the ZnO films. These findings suggest that the presence of Al facilitates the incorporation of N into the ZnO, which are consistent with the prediction of Ref. [6], namely that the presence of the donor (Al) enhances N incorporation. Calculations in Ref. [6] revealed that the repulsive interactions between the N acceptors lead to low solubility and instability of N atoms in
G. Yuan et al. / Materials Letters 58 (2004) 3741–3744
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Fig. 4. E g of the n- and p-type ZnO thin films derived from transmittance spectra.
Fig. 3 shows the typical optical transmission spectra of ntype and p-type ZnO thin films measured at room temperature. The fundamental absorption starts at about 370 nm. The transmission of the p-type ZnO thin film is almost identical to that of the n-type thin ZnO film. A large transmittance greater than 90% is obtained in visible wavelength regions. The weak fluctuation in the spectrum is principally due to interference phenomenon. The absorption coefficient (a) of the ZnO thin films was determined from the transmittance data obtained at normal incidence, using the following relation [18]: Fig. 2. SIMS depth profiles of Al–N co-doped ZnO films: (a) for a twostep growth (600 8C/5min, 500 8C/20min) film and (b) for a film grown at 600 8C).
p-type ZnO:N films, however, the formation of acceptor– donor–acceptor complexes which occupy nearest-neighbor sites is energetically favorable.
a ¼ ½2:303logð1=T Þ=d where T is the transmittance and d is the film thickness. As a direct band gap semiconductor, ZnO thin film has an absorption coefficient (a) obeying the following relation for high photon energies (hc): ðahcÞ2 ¼ A hc Eg where E g is the optical band gap of thin film, and A is a constant. The variations of (ahc) 2 versus hc in the fundamental adsorption region are plotted in Fig. 4, and E g can be evaluated by extrapolation of the linear part. The optical band gap is determined to be about 3.28 eV for the Al–N co-doped p-type ZnO thin films and 3.31 eV for the Al–N co-doped n-type ZnO thin film. The electron negativity of O (3.5) is larger than that of N (3.0), so the Zn–O band has larger ionicity than the Zn–N band. Thus, the decrease in E g is probably ascribed to the decrease in ionicity due to the formation of Zn–N bonds within the p-type ZnO thin films.
4. Conclusions
Fig. 3. Optical transmittance spectra of the n- and p-type ZnO thin films.
In summary, Al–N co-doped p-type ZnO films have been synthesized by DC reactive magnetron sputtering technique.
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The properties of the ZnO films are substantially dependent on the growth temperature. The as-grown ZnO film deposited at 450 8C shows p-type conduction with better properties, such as resistivity of 278 V cm, Hall mobility of 1.32 cm2/V s and carrier density of 1.71016 cm3. The presence of Al facilitates the incorporation of N into ZnO films. The p-type ZnO thin films possess a transmittance of about 90% in the visible region and a band gap of 3.28 eV at room temperature.
Acknowledgements Supported by Chinese Special Funds for Major State Basic Research Project G20000683-06 and National Natural Science Foundation of China under Contract No. 90201038.
References [1] Z.K. Tang, G.K.L. Wong, P. Yu, et al., Appl. Phys. Lett. 72 (1998) 3270. [2] D.M. Bagnell, Y.F. Chen, Z. Zhu, et al., Appl. Phys. Lett. 70 (1997) 2230.
[3] K. Minegishi, Y. Koiwai, Y. Kikuchi, et al., Jpn. J. Appl. Phys. 36 (1997) L1453. [4] X.L. Guo, H. Tabata, T. kawai, J. Cryst. Growth 233 (2001) 135. [5] Y.R. Ryu, S. Zhu, D.C. Look, et al., J. Cryst. Growth 216 (2000) 330. [6] T. Yamamoto, H. Yoshida, Jpn. J. Appl. Phys., Part 2 38 (1999) L166. [7] M. Joseph, H. Tabata, et al., Jpn. J. Appl. Phys. 38 (1999) L1205 – L1207. [8] M. Komatsu, N. Ohashi, I.S. guchi, Appl. Surf. Sci. 189 (2002) 349 – 352. [9] K. Nakahara, H. Takasu, Appl. Phys. Lett. 79 (2001) 25. [10] A. Tsukazaki, H. Saito, K. Tamura, et al., Appl. Phys. Lett. 81 (2002) 2. [11] A. Tiwari, C. Jin, A. Kvit, D. Kumar, J.F. Muth, J. Narayan, Solid State Commun. 121 (2002) 371 – 374. [12] De-Wei Ma, Zhi-Zhen Ye, Jing-Yun Huang, et al., Chin. Phys. Lett. 20 (6) (2003) 942. [13] Dongjiang Qiu, Huizhen Wu, Ailing Yang, Xiaoling Xu, Chin. J. Mater. Res. 14 (5) (2000) 485 (in Chinese). [14] K. Minegishi, Y. Koiwai, Y. Kikuchi, et al., Jpn. J. Appl. Phys. 36 (1997) L1453. [15] A. Kamata, H. Mitsuhashi, et al., Appl. Phys. Lett. 63 (24) (1993) 3353 – 3354. [16] M. Joseph, H. Tabata, et al., Physica. B 140–148 (2001) 302 – 303. [17] Zhi-Zhen Ye, Jian-Guo Lu, et al., J. Cryst. Growth 253 (2003) 258. [18] D.M. Carballeda-Galicia, R. Castanedo-Perez, Thin Solid Films 371 (2000) 105 – 108.