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Effect of oxygen partial pressure ratios on the properties of Al–N co-doped ZnO thin films
ARTICLE IN PRESS
Journal of Crystal Growth 274 (2005) 178–182 www.elsevier.com/locate/jcrysgro
Effect of oxygen partial pressure ratios on the properties of Al–N co-doped ZnO thin films Zhi-zhen Ye, Qing Qian, Guo-Dong Yuan, Bing-Hui Zhao, De-Wei Ma State Key Laboratory of Silicon Materials, Materials Department, Zhejiang University, Hangzhou 310027, China Received 8 June 2004; accepted 7 October 2004 Communicated by D.P. Norton Available online 14 November 2004
Abstract p-Type zinc oxide (ZnO) thin films with C-axis orientations were realized using the DC reactive magnetron sputtering by Al and N co-doping method. Second ion mass spectroscopy (SIMS) tests proved that both Al and N were doped in the ZnO films and the incorporation of Al facilitated the N solution into ZnO, thus promoted the formation of p-type conduction. When oxygen partial pressure ratios was 40% or 85%, the as-grown ZnO thin films showed p-type conduction, and the latter had better electrical properties. The obtained p-type ZnO films showed a resistivity of 157 O cm, a hole concentration of 5.59 1017 cm 3, and a Hall mobility of 0.0711 cm2/V s at room temperature. X-ray diffraction (XRD) patterns showed that the ZnO film prepared in 60% of oxygen partial pressure ratio had the best Caxis orientation. The as-grown ZnO films possessed transmittance of about 90% in the visible region. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Cd; 61.72.Vv; 73.61.Ga; 61.10.Nz Keywords: A1. Co-doping; A1. p-Type conduction; A3. DC magnetron reactive sputtering; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide (ZnO) is a II–VI compound semiconductor with a wide direct band gap of 3.37 eV at room temperature [1]. It has an exciton binding energy of 60 meV larger than that of GaN and Corresponding author. Tel.:+86 571 879 53139; +86 571 879 52625. E-mail addresses: [email protected] (Z.-z. Ye), [email protected] (Q. Qian).
fax:
high exciton emission efficiency. Due to these features, ZnO has become a promising candidate for applications in blue and ultraviolet (UV) light sources and as a UV detector [2–5]. Its practical applications in these fields depend on the fabrication of ZnO p–n homo-junctions. Unfortunately, ZnO occurs naturally as an n-type conduction due to its intrinsic donor defects (Zni or Vo). It is difficult to achieve p-type conduction because of the low solubility of the dopant and high selfcompensating process upon doping [6].
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.10.018
ARTICLE IN PRESS Z.-z. Ye et al. / Journal of Crystal Growth 274 (2005) 178–182
2. Experimental procedure ZnO films were prepared on sapphire substrates by the DC reactive magnetron sputtering method. A 1 wt% Al-doped Zn metal was used as the sputtering target. The vacuum chamber was evacuated to a base pressure of 10 3 Pa, and the NH3 (99.99%) and O2 (99.99%) gases were introduced through separate mass flow controllers. The total pressure was set to 8–10 Pa and the oxygen partial pressure ratios were varied from 25% to 85%. The substrate temperature (Ts) was 450 1C. The sputtering power was 80 W(200 V 0.4 A), and the sputtering time was 30 min. The depth profiles of Al, N, Zn and O were measured by the secondary ion mass spectroscopy (SIMS, IMS 6 F, CAMECA, courberoie, France). The electrical properties were studied by means of a room temperature HL5500 system, adopting the van der Pauw method. The crystal orientation was examined by X-ray diffraction (XRD) (Thermo ARL SCINTAG X’TRA, CuKa=0.154056 nm) measurement. The optical transmission spectra were measured with a Lambda20 spectrometer.
3. Results and discussion 3.1. Electrical properties Fig. 1 exhibits the typical SIMS profile of the asgrown p-type ZnO thin film at sapphire substrate, aiming to investigate the content trend of N as a function of Al in the Al–N co-doped ZnO thin films. As can be seen in Fig. 1, with the content of Al in the ZnO thin films increasing, the content of N in the ZnO films has the same trend of increasing. These findings suggest that the presence of Al facilitates the incorporation of N into the ZnO, which is consistent with the prediction of Ref. [10], namely that the presence of the donor (Al) enhances N incorporation. The electrical properties of Al–N co-doped ZnO thin films as a function of oxygen partial pressure ratios are shown in Table 1 and Fig. 2. The conduction type of films deposited in 25% of oxygen partial pressure ratio is not clear. The films deposited in 50%, 60% and 75% of oxygen partial pressure ratios showed n-type conduction, but those deposited in 40% and 85% ratios showed p-type conduction of the hole concentration of 4.93 1016 and 5.59 1017 cm 3, respectively. These observations may be understood as follows. (1) As oxygen partial pressure ratio is as low as 25%, the crystallinity of ZnO film is so poor that the conduction type of films can not be verified. However, when oxygen partial Al
Zn 105 Ion intensity (cps)
Great efforts were made to obtain p-type ZnO, but only a few successes were reported [7–9]. Yamamoto [10–13] predicted theoretically that ptype ZnO could be obtained by simultaneous codoping using reactive donor co-dopants (e.g. Al, Ga and In) and acceptor co-dopants (e.g. N) because the former could enhance the incorporation of N acceptors and give rise to shallower N-acceptor levels in the band gap of p-type co-doped ZnO crystal as well. Joseph [14,15] and Ye [16] had successfully fabricated p-type ZnO crystal by Ga–N and Al–N co-doping methods, respectively. Ye [16] prepared ZnO thin films by Al–N codoping method using N2O gas as the acceptor dopant source. In this paper, we attempted to grow p-type ZnO films using NH3 gas as the acceptor dopant source. The effect of oxygen partial pressure ratios on electrical, structural and optical properties of films was examined.
179
O 104
O
Al
Al Zn N O
N
103
0.0
0.1
0.2
0.3 0.4 Depth (µm)
0.5
0.6
Fig. 1. Typical SIMS depth profile of Al–N co-doped ZnO thin film.
ARTICLE IN PRESS Z.-z. Ye et al. / Journal of Crystal Growth 274 (2005) 178–182
180
Table 1 Electrical properties of Al–N co-doped ZnO thin films prepared in different oxygen partial pressure ratios Carrier concentration n(cm 3)
Resistivity r (O cm)
Hall mobility m(cm2/V s)
Carrier type
1# 2# 3# 4# 5# 6#
25 40 50 60 75 85
–– 4.93 1016 1.18 1018 7.65 1017 2.17 1018 5.59 1017
–– 665 10.4 25.8 4.21 157
–– 0.19 0.507 0.316 0.683 0.0711
–– p n n n p
1019
3.0 ne
1017
2.5 2.0
nh
1.5 p
1016
1.0 µ
1015
0.5 0.0
1014 40
75 85 60 50 Oxygen partial pressure ratios (%)
Mobility u (cm2/V.s)
1018
700 600 500 400 300 200 100 0 -100 -200 -300
Resisitivity p (O.cm)
Oxygen partial pressure ratios (%)
Carrier concentrations n (cm-3)
Samples
-400
Fig. 2. Effect of oxygen partial pressure ratios on the conduction type, carrier concentrations, mobility and resisitivity of ZnO thin films.
pressure ratio increases to 40%, nitrogen atoms have been doped into the ZnO films as acceptor dopants, which substitute for oxygen in ZnO crystal lattice, and finally result in p-type conduction of the ZnO films. Minegishi [9] reported p-type conduction in ZnO films deposited in an excess of Zn in N ambient due to the possibility of formation of N–Zn–N complexes. Such a possibility in our case may occur. (2) With oxygen partial pressure ratios sequentially increasing, the concentration of native defects, such as oxygen vacancies (Vo) or interstitials (Zni) was more than that of active acceptor. Hence, n-type conduction was observed. (3) As the oxygen ratio further increases in the sputtering ambience during film deposition, such as our case of 85% oxygen partial
pressure ratio, the concentration of oxygen vacancies decreased and the concentration of Zn vacancies increased. Enhancement of VZn may facilitate the substitution of Al for the sites of Zn atoms in the ZnO crystal lattice, which possibly leads to the formation of the bonds of Al–N [12,13]. This is probably the main reason for the better p-type electrical properties of this stage. Except that, the reduction of Vo may arouse the decreasing of Zn:O ratio [17,18], which may also result in p-type conduction in Al–N co-doped ZnO thin films. In addition, the resistivity of 2# and 6# samples are, respectively, 665 and 157 O cm, far greater than the values of that of as-grown n-type ZnO films, which may result from the restrictive function of the incorporated nitrogen in 2# sample
ARTICLE IN PRESS Z.-z. Ye et al. / Journal of Crystal Growth 274 (2005) 178–182
3.2. Structural properties Fig. 3 shows the XRD profiles of the ZnO films prepared on sapphire substrate in atmosphere of various oxygen partial pressure ratios. Only one peak corresponding to the (0 0 2) plane of ZnO is observed in the profile, and no Zn3N2 or Zn peaks are detected. It suggests that ZnO films exhibit (0 0 2) preferential orientation with the C-axis perpendicular to the substrate. When oxygen partial pressure ratio is 25%, the (0 0 2) plane is not obviously observed, probably because the oxygen gas is so scarce that the ZnO thin film of good crystallinity is difficult to be deposited. As the oxygen partial pressure ratio increases to 40%, the crystallinity of ZnO film becomes better, but not the best, which is because nitrogen atoms have substituted oxygen atoms in ZnO crystal lattice so that p-type ZnO has been obtained. With the ratio of oxygen gas in the sputtering ambience further increasing, the (0 0 2) diffraction peak intensity increases to the maximum when the oxygen partial pressure ratios is 60% and subsequently decreases,
counts/s
50000 40000 30000
100 90 80 Transmittance (%)
and the adsorbed oxygen atoms in the grain boundary of 6# sample. Accordingly, the values of Hall mobility in the Al–N co-doped p-ZnO thin films are relatively small, namely 0.19 and 0.0711 cm2/V s, respectively.
181
70 60 50
25% 40% 50% 60% 75% 85%
40 30 20 10 0 300
400
500 600 wavelength (nm)
700
Fig. 4. Transmittance spectra of the ZnO films prepared in atmosphere of different oxygen partial pressure ratios.
which may be due to the adsorbed oxygen atoms in the grain boundary and the substitution function of Al for Zn atoms, consequently leading to the worse crystallinity. 3.3. Optical properties Fig. 4 shows the transmittance spectra of the ZnO films prepared in atmosphere of various oxygen partial pressure ratios. The transmittance of the most films in the visible region is more than 90% except for that deposited in atmosphere of 25% of oxygen partial pressure ratio. From the XRD spectra of Fig. 2 above, we can conclude that the ZnO thin film deposited in atmosphere of 25% of oxygen partial pressure ratio has the worst crystallinity. Accordingly, the ZnO thin film prepared at the same condition also has the lowest transmittance in the visible region, only 85% or so.
20000 10000 30
60 40 2T het 50 a
25 60
40
75
85 l tia ar (%) p n os ge rati y Ox ure s es pr
50
Fig. 3. XRD profiles of the ZnO films prepared in different oxygen partial pressure ratios.
4. Conclusions In conclusion, p-type ZnO thin films with C-axis orientations were obtained by Al and N co-doping method. The ZnO film prepared in the atmosphere of 60% of oxygen partial pressure ratio has the best C-axis orientation. When oxygen partial pressure ratio is 85%, the as-grown ZnO thin film shows better p-type electrical properties, such as a
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hole concentration of 5.59 1017 cm 3, a resistivity of 157 O cm, and a Hall mobility of 0.0711 cm2/ V s. The as-grown ZnO thin films possess transmittance of about 90% in the visible region.
Acknowledgements The research is supported by Special Funds for Major State Basic Research Project G20000683-06 and National Natural Science Foundation of China for Key Project 90201038, and Doctoral Foundation of State Education Ministry of China 20020335010. References [1] C. Klingshirn, Phys. Stat. Sol. B 71 (1975) 547. [2] Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 (25) (1998) 3270. [3] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, Appl. Phys. Lett. 73 (8) (1998) 1038. [4] R.K. Thareja, A. Mitra, Appl. Phys. B 71 (2000) 181.
[5] T. Makino, C.H. Chia, T.T. Nguen, Y. Segawa, Appl. Phys. Lett. 77 (11) (2000) 1632. [6] A. Kobayshi, O.F. Sankey, J.D. Dow, Phys. Rev. B 28 (1983) 946. [7] X.L. Guo, H. Tabata, T. Kawai, J. Crystal Growth 223 (2001) 135. [8] Z.Z. Ye, J.G. Lv, H.H. Chen, J. Crystal Growth 253 (2003) 258. [9] K. Minegish, Y. Koiwai, Y. Kikuchi, K. Yano, M. Kasuga, A. Shimizu, Jpn J. Appl. Phys. 36 (1997) L1453. [10] T. Yamamoto, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 38 (1999) 166. [11] T. Yamamoto, H. Katayama-Yoshida, J. Crystal Growth 214 (2000) 552. [12] T. Yamamoto, Thin Solid Film 420 (2002) 100. [13] T. Yamamoto, Phys. Stat. Sol. (a) 193 (3) (2002) 423. [14] M. Joseph, H.K. Tabata, Jpn J. Appl. Phys. 38 (1999) L1205. [15] M. Joseph, H. Tabata, H. Saeki, K. Ueda, T. Kawai, Physica B 302–303 (2001) 140. [16] Z.Z. Ye, F. Zhu-Ge, J. Crystal Growth 265 (2004) 127. [17] B.J. Jin, S.H. Bae, S.Y. Lee, S. Im, Mater. Sci. Eng. B 71 (2000) 301. [18] S.H. Bae, S.Y. Lee, B.J. Jin, S. Im, Appl. Surf. Sci. 458 (2000) 154.
Journal of Crystal Growth 274 (2005) 178–182 www.elsevier.com/locate/jcrysgro
Effect of oxygen partial pressure ratios on the properties of Al–N co-doped ZnO thin films Zhi-zhen Ye, Qing Qian, Guo-Dong Yuan, Bing-Hui Zhao, De-Wei Ma State Key Laboratory of Silicon Materials, Materials Department, Zhejiang University, Hangzhou 310027, China Received 8 June 2004; accepted 7 October 2004 Communicated by D.P. Norton Available online 14 November 2004
Abstract p-Type zinc oxide (ZnO) thin films with C-axis orientations were realized using the DC reactive magnetron sputtering by Al and N co-doping method. Second ion mass spectroscopy (SIMS) tests proved that both Al and N were doped in the ZnO films and the incorporation of Al facilitated the N solution into ZnO, thus promoted the formation of p-type conduction. When oxygen partial pressure ratios was 40% or 85%, the as-grown ZnO thin films showed p-type conduction, and the latter had better electrical properties. The obtained p-type ZnO films showed a resistivity of 157 O cm, a hole concentration of 5.59 1017 cm 3, and a Hall mobility of 0.0711 cm2/V s at room temperature. X-ray diffraction (XRD) patterns showed that the ZnO film prepared in 60% of oxygen partial pressure ratio had the best Caxis orientation. The as-grown ZnO films possessed transmittance of about 90% in the visible region. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Cd; 61.72.Vv; 73.61.Ga; 61.10.Nz Keywords: A1. Co-doping; A1. p-Type conduction; A3. DC magnetron reactive sputtering; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide (ZnO) is a II–VI compound semiconductor with a wide direct band gap of 3.37 eV at room temperature [1]. It has an exciton binding energy of 60 meV larger than that of GaN and Corresponding author. Tel.:+86 571 879 53139; +86 571 879 52625. E-mail addresses: [email protected] (Z.-z. Ye), [email protected] (Q. Qian).
fax:
high exciton emission efficiency. Due to these features, ZnO has become a promising candidate for applications in blue and ultraviolet (UV) light sources and as a UV detector [2–5]. Its practical applications in these fields depend on the fabrication of ZnO p–n homo-junctions. Unfortunately, ZnO occurs naturally as an n-type conduction due to its intrinsic donor defects (Zni or Vo). It is difficult to achieve p-type conduction because of the low solubility of the dopant and high selfcompensating process upon doping [6].
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.10.018
ARTICLE IN PRESS Z.-z. Ye et al. / Journal of Crystal Growth 274 (2005) 178–182
2. Experimental procedure ZnO films were prepared on sapphire substrates by the DC reactive magnetron sputtering method. A 1 wt% Al-doped Zn metal was used as the sputtering target. The vacuum chamber was evacuated to a base pressure of 10 3 Pa, and the NH3 (99.99%) and O2 (99.99%) gases were introduced through separate mass flow controllers. The total pressure was set to 8–10 Pa and the oxygen partial pressure ratios were varied from 25% to 85%. The substrate temperature (Ts) was 450 1C. The sputtering power was 80 W(200 V 0.4 A), and the sputtering time was 30 min. The depth profiles of Al, N, Zn and O were measured by the secondary ion mass spectroscopy (SIMS, IMS 6 F, CAMECA, courberoie, France). The electrical properties were studied by means of a room temperature HL5500 system, adopting the van der Pauw method. The crystal orientation was examined by X-ray diffraction (XRD) (Thermo ARL SCINTAG X’TRA, CuKa=0.154056 nm) measurement. The optical transmission spectra were measured with a Lambda20 spectrometer.
3. Results and discussion 3.1. Electrical properties Fig. 1 exhibits the typical SIMS profile of the asgrown p-type ZnO thin film at sapphire substrate, aiming to investigate the content trend of N as a function of Al in the Al–N co-doped ZnO thin films. As can be seen in Fig. 1, with the content of Al in the ZnO thin films increasing, the content of N in the ZnO films has the same trend of increasing. These findings suggest that the presence of Al facilitates the incorporation of N into the ZnO, which is consistent with the prediction of Ref. [10], namely that the presence of the donor (Al) enhances N incorporation. The electrical properties of Al–N co-doped ZnO thin films as a function of oxygen partial pressure ratios are shown in Table 1 and Fig. 2. The conduction type of films deposited in 25% of oxygen partial pressure ratio is not clear. The films deposited in 50%, 60% and 75% of oxygen partial pressure ratios showed n-type conduction, but those deposited in 40% and 85% ratios showed p-type conduction of the hole concentration of 4.93 1016 and 5.59 1017 cm 3, respectively. These observations may be understood as follows. (1) As oxygen partial pressure ratio is as low as 25%, the crystallinity of ZnO film is so poor that the conduction type of films can not be verified. However, when oxygen partial Al
Zn 105 Ion intensity (cps)
Great efforts were made to obtain p-type ZnO, but only a few successes were reported [7–9]. Yamamoto [10–13] predicted theoretically that ptype ZnO could be obtained by simultaneous codoping using reactive donor co-dopants (e.g. Al, Ga and In) and acceptor co-dopants (e.g. N) because the former could enhance the incorporation of N acceptors and give rise to shallower N-acceptor levels in the band gap of p-type co-doped ZnO crystal as well. Joseph [14,15] and Ye [16] had successfully fabricated p-type ZnO crystal by Ga–N and Al–N co-doping methods, respectively. Ye [16] prepared ZnO thin films by Al–N codoping method using N2O gas as the acceptor dopant source. In this paper, we attempted to grow p-type ZnO films using NH3 gas as the acceptor dopant source. The effect of oxygen partial pressure ratios on electrical, structural and optical properties of films was examined.
179
O 104
O
Al
Al Zn N O
N
103
0.0
0.1
0.2
0.3 0.4 Depth (µm)
0.5
0.6
Fig. 1. Typical SIMS depth profile of Al–N co-doped ZnO thin film.
ARTICLE IN PRESS Z.-z. Ye et al. / Journal of Crystal Growth 274 (2005) 178–182
180
Table 1 Electrical properties of Al–N co-doped ZnO thin films prepared in different oxygen partial pressure ratios Carrier concentration n(cm 3)
Resistivity r (O cm)
Hall mobility m(cm2/V s)
Carrier type
1# 2# 3# 4# 5# 6#
25 40 50 60 75 85
–– 4.93 1016 1.18 1018 7.65 1017 2.17 1018 5.59 1017
–– 665 10.4 25.8 4.21 157
–– 0.19 0.507 0.316 0.683 0.0711
–– p n n n p
1019
3.0 ne
1017
2.5 2.0
nh
1.5 p
1016
1.0 µ
1015
0.5 0.0
1014 40
75 85 60 50 Oxygen partial pressure ratios (%)
Mobility u (cm2/V.s)
1018
700 600 500 400 300 200 100 0 -100 -200 -300
Resisitivity p (O.cm)
Oxygen partial pressure ratios (%)
Carrier concentrations n (cm-3)
Samples
-400
Fig. 2. Effect of oxygen partial pressure ratios on the conduction type, carrier concentrations, mobility and resisitivity of ZnO thin films.
pressure ratio increases to 40%, nitrogen atoms have been doped into the ZnO films as acceptor dopants, which substitute for oxygen in ZnO crystal lattice, and finally result in p-type conduction of the ZnO films. Minegishi [9] reported p-type conduction in ZnO films deposited in an excess of Zn in N ambient due to the possibility of formation of N–Zn–N complexes. Such a possibility in our case may occur. (2) With oxygen partial pressure ratios sequentially increasing, the concentration of native defects, such as oxygen vacancies (Vo) or interstitials (Zni) was more than that of active acceptor. Hence, n-type conduction was observed. (3) As the oxygen ratio further increases in the sputtering ambience during film deposition, such as our case of 85% oxygen partial
pressure ratio, the concentration of oxygen vacancies decreased and the concentration of Zn vacancies increased. Enhancement of VZn may facilitate the substitution of Al for the sites of Zn atoms in the ZnO crystal lattice, which possibly leads to the formation of the bonds of Al–N [12,13]. This is probably the main reason for the better p-type electrical properties of this stage. Except that, the reduction of Vo may arouse the decreasing of Zn:O ratio [17,18], which may also result in p-type conduction in Al–N co-doped ZnO thin films. In addition, the resistivity of 2# and 6# samples are, respectively, 665 and 157 O cm, far greater than the values of that of as-grown n-type ZnO films, which may result from the restrictive function of the incorporated nitrogen in 2# sample
ARTICLE IN PRESS Z.-z. Ye et al. / Journal of Crystal Growth 274 (2005) 178–182
3.2. Structural properties Fig. 3 shows the XRD profiles of the ZnO films prepared on sapphire substrate in atmosphere of various oxygen partial pressure ratios. Only one peak corresponding to the (0 0 2) plane of ZnO is observed in the profile, and no Zn3N2 or Zn peaks are detected. It suggests that ZnO films exhibit (0 0 2) preferential orientation with the C-axis perpendicular to the substrate. When oxygen partial pressure ratio is 25%, the (0 0 2) plane is not obviously observed, probably because the oxygen gas is so scarce that the ZnO thin film of good crystallinity is difficult to be deposited. As the oxygen partial pressure ratio increases to 40%, the crystallinity of ZnO film becomes better, but not the best, which is because nitrogen atoms have substituted oxygen atoms in ZnO crystal lattice so that p-type ZnO has been obtained. With the ratio of oxygen gas in the sputtering ambience further increasing, the (0 0 2) diffraction peak intensity increases to the maximum when the oxygen partial pressure ratios is 60% and subsequently decreases,
counts/s
50000 40000 30000
100 90 80 Transmittance (%)
and the adsorbed oxygen atoms in the grain boundary of 6# sample. Accordingly, the values of Hall mobility in the Al–N co-doped p-ZnO thin films are relatively small, namely 0.19 and 0.0711 cm2/V s, respectively.
181
70 60 50
25% 40% 50% 60% 75% 85%
40 30 20 10 0 300
400
500 600 wavelength (nm)
700
Fig. 4. Transmittance spectra of the ZnO films prepared in atmosphere of different oxygen partial pressure ratios.
which may be due to the adsorbed oxygen atoms in the grain boundary and the substitution function of Al for Zn atoms, consequently leading to the worse crystallinity. 3.3. Optical properties Fig. 4 shows the transmittance spectra of the ZnO films prepared in atmosphere of various oxygen partial pressure ratios. The transmittance of the most films in the visible region is more than 90% except for that deposited in atmosphere of 25% of oxygen partial pressure ratio. From the XRD spectra of Fig. 2 above, we can conclude that the ZnO thin film deposited in atmosphere of 25% of oxygen partial pressure ratio has the worst crystallinity. Accordingly, the ZnO thin film prepared at the same condition also has the lowest transmittance in the visible region, only 85% or so.
20000 10000 30
60 40 2T het 50 a
25 60
40
75
85 l tia ar (%) p n os ge rati y Ox ure s es pr
50
Fig. 3. XRD profiles of the ZnO films prepared in different oxygen partial pressure ratios.
4. Conclusions In conclusion, p-type ZnO thin films with C-axis orientations were obtained by Al and N co-doping method. The ZnO film prepared in the atmosphere of 60% of oxygen partial pressure ratio has the best C-axis orientation. When oxygen partial pressure ratio is 85%, the as-grown ZnO thin film shows better p-type electrical properties, such as a
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hole concentration of 5.59 1017 cm 3, a resistivity of 157 O cm, and a Hall mobility of 0.0711 cm2/ V s. The as-grown ZnO thin films possess transmittance of about 90% in the visible region.
Acknowledgements The research is supported by Special Funds for Major State Basic Research Project G20000683-06 and National Natural Science Foundation of China for Key Project 90201038, and Doctoral Foundation of State Education Ministry of China 20020335010. References [1] C. Klingshirn, Phys. Stat. Sol. B 71 (1975) 547. [2] Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 (25) (1998) 3270. [3] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, Appl. Phys. Lett. 73 (8) (1998) 1038. [4] R.K. Thareja, A. Mitra, Appl. Phys. B 71 (2000) 181.
[5] T. Makino, C.H. Chia, T.T. Nguen, Y. Segawa, Appl. Phys. Lett. 77 (11) (2000) 1632. [6] A. Kobayshi, O.F. Sankey, J.D. Dow, Phys. Rev. B 28 (1983) 946. [7] X.L. Guo, H. Tabata, T. Kawai, J. Crystal Growth 223 (2001) 135. [8] Z.Z. Ye, J.G. Lv, H.H. Chen, J. Crystal Growth 253 (2003) 258. [9] K. Minegish, Y. Koiwai, Y. Kikuchi, K. Yano, M. Kasuga, A. Shimizu, Jpn J. Appl. Phys. 36 (1997) L1453. [10] T. Yamamoto, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 38 (1999) 166. [11] T. Yamamoto, H. Katayama-Yoshida, J. Crystal Growth 214 (2000) 552. [12] T. Yamamoto, Thin Solid Film 420 (2002) 100. [13] T. Yamamoto, Phys. Stat. Sol. (a) 193 (3) (2002) 423. [14] M. Joseph, H.K. Tabata, Jpn J. Appl. Phys. 38 (1999) L1205. [15] M. Joseph, H. Tabata, H. Saeki, K. Ueda, T. Kawai, Physica B 302–303 (2001) 140. [16] Z.Z. Ye, F. Zhu-Ge, J. Crystal Growth 265 (2004) 127. [17] B.J. Jin, S.H. Bae, S.Y. Lee, S. Im, Mater. Sci. Eng. B 71 (2000) 301. [18] S.H. Bae, S.Y. Lee, B.J. Jin, S. Im, Appl. Surf. Sci. 458 (2000) 154.