[1] Sundqvist P A, Zhao Q X and Willander M 2003 Phys.
Rev. B 68 155334

[2]Pearton S J, Norton D P, Ip K, Heo Y W and Steiner T 2004
J. Vac. Sci. Technol. B 22 932

[3]\"Ozg\"ur \"U,Alivov Ya I, Liu C, Teke A, Reshchikov M A,
Do\v{gan S, Cho S J, Avrutin V and Morkoc H 2005 J. Appl.
Phys. 98 041301

[4]Uedono A, Koida T, Tsukazaki A, Kawasaki M, Chen Z Q,
Chichibu SF and Koinuma H 2003 J. Appl. Phys. 93 2481

[5]Joseph M, Tabata H, Saeki H, Ueda K and Kawai T 2001
Physica B 302/303 140

[6]Minegishi K, Koiwai Y, Kikuchi Y, Yano K, Kasuga M and
Shimizu A 1997 Jpn J. Appl. Phys. 36 L1453

[7]Ryu Y R, Zhu S, Look D C, Wrobel J M, Jeong H M and White H
W 2000 J. Crys. Growth 216 330

[8]Park C H, Zhang S B and Wei S H 2002 Phys. Rev. B
66 073202

[9]Lee W J, Kang J and Chang K J 2006 73 024117

[10] Lu J G, Ye Z Z, Zhuge F, Zeng Y J, Zhao B H and Zhu L P
2004 Appl. Phys. Lett. 85 3134

[11] Lee E C, Kim Y S, Jin Y G and Chang K J 2001 Phys.
Rev. B 64 085120

[12]Yan Y F, Al-Jassim M. M. and Wei S H 2006 Appl. Phys.
Lett. 89 181912

[13]Kang H S, Ahn B D, Kim J H, Kim G H and Lim S H 2006
Appl. Phys. Lett. 88 202108

[14]Ahn B D, Kang H S, Kim J H, Kim G H, Chang H W and Lee S Y
2006 J. Appl. Phys. 100 093701

[15]Duan L, Lin B X, Zhang W Y, Zhong S and Fu Z X 2006
Appl. Phys. Lett. 88 232110

[16]Sernelius B E, Berggren K F, Jin Z C, Hamberg I and
Granqvist C G 1988 Phys. Rev. B 37 10244

[17]Ma J and Feng J 1996 Thin Solid Films 279 213

[18]Minami T, Suzuki S and Miyata T 2001 Thin Solid
Films 398 53

[19]Bamiduro O, Mustafa H, Mundle R, Konda R B and Pradhan A K
2007 Appl. Phys. Lett. 90 252108

[20]Oba F, Nishitani S R, Isotani S and Adachi H 2001 J.
Appl. Phys. 90 824

[21]Saarinen K 2000 III--V Nitride Semiconductors:
Electrical, Structural and Defects Properties
edited by Manasreh M (Amsterdam: Elsevier) p 109

[22]Krause-Rehberg R and Leipner H S 1998 (Berlin:Springer)

[23] Tuomisto F, Ranki V, Saarinen K and Look D C 2003
Phys. Rev. Lett. 91 205502

[24]Kawasuso A, Redmann F, Krause-Rehberg R, Yoshikawa M,
Kojima K and Itoh H 2001 Phys. Stat. Sol. (b) 223 R8

[25]Xiong G, Wilkinson J, Mischuck B, T\"uzemen S, Ucer K B
and Williams R T 2002 Appl. Phys. Lett. 80 1195

[26]Zhang S B, Wei S H and Zunger A 2001 Phys. Rev. B
63 075205

[27]Peng C X, Weng H M, Yang X J, Ye B J, Cheng B, Zhou X Y
and Han R D 2006 Chin. Phys. Lett. 23 489

[28]Wang Q X, Xiong Z H, Rao J P, Dai J N, Yue S P, Wang G P
and Jiang F Y 2007 Chin. J. Semiconduct. 28 696 (in
Chinese)