[1] Ng D K T, Hong M H, Tan L S. Field emission enhancement from patterned gallium nitride nano- wires[J]. Nanotechnol, 18, 375707(2007).

[2] Li E, Cui Z, Dai Y. Synthesis and field emission properties of GaN nanowires[J]. Appl. Surf. Sci, 257, 10850(2011).

[3] Nabi G, Cao C B, Husain S. Synthesis， photoluminescence and field emission properties of well aligned/well patterned conical shape GaN nanorods[J]. Cryst. Eng. Comm, 14, 8492(2012).

[4] Cui Z, Li E. . Growth and field emission properties of GaN nanopencils[J]. Ceram. Int, 41, 6074(2015).

[5] Cui Z, Ke X, Li E. Electronic and optical properties of titanium-doped GaN nanowires[J]. Mater. Des, 96, 409(2016).

[6] Li B, Chang B. . Research and development of GaN photocathode[J]. Acta Phys. Sin, 60, 088503(2011).

[7] Li Y H, Pan H H, Xu P S. First-principle study on GaN（1010） surface structure[J]. Acta Phys. Sin, 54, 317(2005).

[8] Du Y J, Chang B K, Fu X Q. Electronic structure and optical properties of zinc-blende GaN[J]. Optik, 123, 2208-2212(2012).

[9] Wang Y. J， Lu W. AlGaN/GaN FET for DNA hybridization detection[J]. Phys. Status Solidi, 208, 1623-1625(2011).

[10] Pang L, Kim K. Bimodal gate-dielectric deposition for improved performance of AlGaN/GaN metal–oxide–semiconductor high-electron-mobility transistors[J]. . Phys, 45, 045105(2012).

[11] Hsu C S, Chen H I, Chang C F. On the hydrogen sensing characteristics of a Pd/AlGaN/GaN heterostructure field-effect transistor （HFET）[J]. Sens. Actuators B, 165, 19-23(2012).

[12] Bar-Llan A H, Zamir S, Katz O. GaN layer growth optimization for high power devices[J]. Mater. Sci. Eng, 302, 14-17(2001).

[13] Nguyen T Q, Shih H A, Kudo M. Appl[J]. Phys, 51, 19-23(2012).

[14] Kim D S, Im K S, Kang H S. Appl[J]. Phys, 51, 4101(2012).

[15] Wu C I, Kahn A J. Electronic states and effective negative electron affinity at cesiated p-GaN surfaces[J]. Appl. Phys, 86, 3209-3212(1999).

[16] Machuca F, Sun Y, Liu Z. Role of oxygen in semiconductor negative electron affinity photocathodes[J]. . Vac. Sci. Technol, 20, 2721-2725(2002).

[17] Maruyama T, Brachmann A, Clendenin J E. Nucl. Instrum. A very high charge， high polarization gradient-doped strained GaAs photocathode[J]. Methods Phys. Res. Sect, 492, 199-211(2002).

[18] Siegmund O, Vallerga J, Mcphate J. Development of GaN photocathodes for UV detectors[J]. Nucl. Instrum. Methods Phys. Res. Sect, 567, 89-92(2006).

[19] Gao C X, Yu F C, Choi A R. A comparative study on Be and Mg doping in GaN films grown using a single GaN precursor via molecular beam epitaxy[J]. . Cryst. Growth, 291, 60-65(2006).

[20] Kawaharazuka A, Tanimoto T, Nagai K. Be and Mg co-doping in GaN[J]. . Cryst. Growth, 301-302, 414-416(2007).

[21] Li S, Mo C, Wang L. The influence of Si-doping to the growth rate and yellow luminescence of GaN grown by MOCVD[J]. J. Funct. Mater. Devices, 93, 321-326(2001).

[22] Furuhashi Y, Yoshida S, Ozaki D. Electrical properties of n-type layers formed in GaN by Si implantation[J]. Nucl. Instrum. Methods Phys. Res. Sect, 242, 633-636(2006).

[23] Li D, Ma B, Miyagawa R. Photoluminescence study of Si-doped a-plane GaN grown by MOVPE[J]. . Cryst. Growth, 311, 2906-2909(2009).

[24] Ji Y J, Du Y J, Wang M H. First-principles studies of electronic structure and optical properties of GaN surface doped with Si[J]. Optik, 125, 2234-2238(2014).

[25] Svane A, Christensen N E, Petit L. Electronic structure of rare-earth impurities in GaAs and GaN[J]. Phys. Rev, 165204(74).

[26] Sanna S, Schmidt W. . Rare-earth defect pairs in GaN： LDA+ U calculations[J]. Phys. Rev, 80, 104120(2009).

[27] Tom H W K, Mate C M, Zhu X D. Studies of alkali adsorption on Rh（111） using optical second-harmonic generation[J]. Surf. Sci, 172, 466-476(1986).

[28] Kiejna A, Ossowski T, Wachowicz E. Alkali metals adsorption on the Mg（0001） surface[J]. Surf. Sci, 548, 22-28(2004).

[29] Jin K H, Choi S M, Jhi S. H. Crossover in the adsorption properties of alkali metals on graphene[J]. ： Condens. Matter, 82, 033414(2010).

[30] Sahin H, Peeters F M. Adsorption of alkali， alkaline-earth， and 3d transition metal atoms on silicene[J]. ： Condens. Matter, 87, 218-224(2013).

[31] Sun M, Tang W, Ren Q. First-principles study of the alkali earth metal atoms adsorption on graphene[J]. Appl. Surf. Sci, 356, 668-673(2015).

[32] Xia S. . Study of Cs adsorption on （100） surface of ［001］-oriented GaN nanowires： A first principle research[J]. Appl. Surf. Sci, 387, 1110-1115(2016).

[33] Benemanskaya G V, Vikhnin V S, Shmidt N M. Electron accumulation layer at the Cs-covered GaN（0001） n-type surface[J]. Appl. Phys. Lett, 85, 1365-1367(2004).

[34] Du Y J, Chang B K, Wang X H. Theoretical study of Cs adsorption on GaN （0 0 0 1） surface[J]. Appl. Surf. Sci, 258, 7425(2012).

[35] Wang Z G, Zhang C L, Li J B. Weber. First principles study of electronic properties of gallium nitride nanowires grown along different crystal directions[J]. Comput. Mater. Sci, 50, 344-348(2010).

[36] Kampen T U, Eyckelerr M, Monch W. Electronic properties of cesium-covered GaN（0001） surfaces[J]. Appl. Surf. Sci, 123-124, 28-32(1998).

[37] Gonzalez-Hernandez R, Lopez-Perez W. Vanadium adsorption and incorporation at the GaN（0001） surface： A first-principles study[J]. Rev, 195407(81).