共溅法Mg掺杂氮化镓纳米结构的制备与研究
|
摘要
氮化镓(GaN)作为一种优良的Ⅲ-Ⅴ族宽禁带半导体材料,被广泛地用来制作各种蓝、绿光发光二极管和激光器,及各种抗辐射、高频、高温和高密度集成的电子器件。近年来发现一维GaN纳米材料具有许多新奇的物理特性,其纳米线、纳米带和纳米棒作为新颖的低维材料越来越多引起了人们的研究兴趣。同时随着GaN基器件的发展,为了更好地实现其光电子特性,适当的掺杂是非常有必要的。Be、Mg、Zn、C等多种p型化杂质都曾被研究过,而目前以Mg使用最广泛。于是在实现GaN一维纳米结构生长的基础上,进一步实现GaN纳米结构的P型掺杂是一个非常值得研究的热门课题。
本文介绍了采用共溅法制备Mg掺杂GaN纳米结构的方法。用X射线衍射(XRD)、扫描电子显微镜(SEM)、高分辨透射电镜(HRTEM)、傅里叶红外吸收谱(FTIR)、X射线光电子能谱(XPS)和光致发光谱(PL)等测试手段详细分析了Mg掺杂GaN纳米材料的结构、组分、形貌和光致发光特性。研究了不同的氨化温度、不同的氨化时间和不同Mg层厚度对GaN纳米结构的影响,初步提出并探讨了此方法合成GaN纳米结构的生长机制。所取得的主要研究结果如下:
1.用共溅射和氨化制备Mg掺杂GaN纳米结构
利用磁控溅射法在Si衬底上溅射Mg:Ga_2O_3层状结构薄膜,然后对溅射的Mg:Ga_2O_3层状薄膜在氨气气氛下退火制备GaN纳米结构。通过改变退火时间、退火温度及Mg层的厚度研究其对合成的GaN纳米结构的影响。研究结果表明:不同的退火温度、退火时间和Mg层的厚度对合成GaN纳米结构都有很大影响,合成的一维纳米结构为六方纤锌矿结构的单晶GaN。其中制备高质量Mg掺杂氮化镓纳米线的最佳条件是共溅射过程包含25个循环,Mg:Ga_2O_3薄膜总的溅射厚度为600nm左右,溅射总时间为100分钟。单个循环过程如下:先生长20nm左右的非故意掺杂Ga_2O_3,保持工作气压不变,同时停止溅射Ga_2O_3,如此保持一段时间(5s)再开始溅射Mg 2s。最后,将溅射好的Mg:Ga_2O_3薄膜样品放入管式炉的恒温区进行氨化。氨化温度和氨化时间分别为900℃和15分钟。
2.GaN纳米结构的光学特性
室温下,用波长为298nm光激发样品表面,出现359nm、384nm、425nm和442nm四处发光峰,改变实验条件发光峰位置基本没有随之发生移动,只是发光强度发生了变化。对于位于359nm处的发光峰对应本征GaN常温370nm的发光峰表现了明显的蓝移,归因于Burstein-Moss效应;其余三个发光峰都是由于Mg的掺杂引起的不同的杂质能级的跃迁;
3.对GaN纳米结构生长机制的探索
高温下氨气逐步分解成NH_2、NH、H_2、N_2等产物,固态Ga_2O_3与H_2反应生成中间产物气态的Ga_2O,在衬底处与体系中氨气发生催化反应得到GaN晶核,这些晶核在衬底合适的能量位置生长,成为下一个晶核生长的依托点,随着氨化过程的进行GaN晶核继续长成GaN微晶,当微晶的生长方向沿着相同的方向生长,就形成了单晶GaN纳米线、纳米线、纳米颗粒。同时氨化层状结构的Mg:Ga_2O_3薄膜,能使得在微晶生长过程中,会有更多的Mg离子进驻GaN晶体内部,实现了Mg的有效掺杂。但更深刻的原因仍在进一步的研究之中。
GaN is an excellentⅢ-Ⅴwide-band gap semiconductor material,commonly used in blue or green light-emitting diodes or laser diodes,and all kinds of radioresistance, high-frequency,high-temperature and high-density integrated devices.Nanowires, nanobelts,and nanorods are a new class of one-dimensional materials that have been attracting a great deal of interest in research in the last few years.In order to improve its performances in both electronic and optoelectronic devices,however,appropriate doping is necessary.Be,Mg Zn,C have been used to study the P-doping of GaN,Mg has been widely usded.So on the base of the growth of one-dimension GaN nanostructures,It is very interesting to investigate the growth of Mg-doped GaN nanostructures.
In this paper,Mg-doped GaN one-dimensional nanosmaterials were synthesized through magnetron sputtering and ammoniating progress.The structure,elemental composition,morphology and photoluminescence properties of the GaN nanomaterials were determined by X-ray diffraction(XRD),Scanning electronic microscope(SEM), High-resolution transmission electronic microscope(HRTEM),Fourier transformed infrared spectroscopy(FTIR),X-ray photoelectron energy spectroscopy(XPS) and Photoluminescence spectroscopy(PL).The growth mechanism of these GaN nanomaterials were proposed and discussed based on the investigation of the influence of the ammoniating temperature,ammoniating time and the thickness of Mg layer on the properties of Mg-doped GaN nanostructure.
1.Synthesis of one-dimensional Mg-doped GaN nanostructures through co-sputtering and ammonification method
One-dimension Mg-doped GaN nanomaterials were fabricated through ammoniating Mg:Ga_2O_3 thin films deposited by direct current(DC) magnetron sputtering system and radio frequency(RF) magnetron sputtering system respectively.The influences on the growth of GaN nanostructures were investigated by changing the ammoniating temperature,ammoniating time or the thickness of Mg layer.The results reveal that different anneal temperature,different ammoniating time of Mg:Ga_2O_3 thin films and different thickness of Mg layer have a great influence on the synthesis of GaN nanostructures.The synthesized nanostructures were of hexagonal wurtzite single-crystal GaN.There were 25 cycles in this process,the total thickness of the Mg: Ga_2O_3 thin films was about 600nm and the total sputtering time was 100 min.The single sputtering cycle was that:first an un-doped Ga_2O_3 film was deposited to a thickness of about 20 nm,and then a Mg layer was deposited for 2 s.In the second step,as-deposited Mg:Ga_2O_3 thin films were ammoniated in a conventional tube furnace at 900℃for 15 min.
2.Optical properties of GaN nanostructures
The nanowires showed four emission peaks at 359 nm,384 nm,425 nm,and 442 nm, respectively.An obviously blueshift of the band gap emission occurs from the 370 nm of bulk GaN to 359 nm of Mg-doped GaN,which is attributed to Burstein-Moss effect. The other peaks are arisen from the transtion excition of different impurity levels from Mg.
3.Exploration of the growth mechanism for GaN nanostructures
In the ammoniating process,NH_3 decomposes into NH_2,NH,H_2,N_2 and N when the ammoniating temperature is above 800℃.The Ga_2O_3 particles are reduced to gaseous Ga_2O by H_2 and then GaN molecules are synthesized through the reaction between Ga_2O and NH_3.The formed GaN molecules diffuse and agglomerate into GaN crystalline nuclei,and then the very small GaN crystalline nuclei grow up gradually with the progress of the ammonification.When the microcrystalline grow along the same direction,they become nanowires,nanorods or nanograins.In the sputtering process, layered structure of the Ga_2O_3 films doped with Mg has been obtained.Thus,in the growth process,Mg has more opportunity to substitute the position of Ga.At that temperature,vapourized Mg was doped into the GaN particles to occupy the position of Ga vacancies.
本文介绍了采用共溅法制备Mg掺杂GaN纳米结构的方法。用X射线衍射(XRD)、扫描电子显微镜(SEM)、高分辨透射电镜(HRTEM)、傅里叶红外吸收谱(FTIR)、X射线光电子能谱(XPS)和光致发光谱(PL)等测试手段详细分析了Mg掺杂GaN纳米材料的结构、组分、形貌和光致发光特性。研究了不同的氨化温度、不同的氨化时间和不同Mg层厚度对GaN纳米结构的影响,初步提出并探讨了此方法合成GaN纳米结构的生长机制。所取得的主要研究结果如下:
1.用共溅射和氨化制备Mg掺杂GaN纳米结构
利用磁控溅射法在Si衬底上溅射Mg:Ga_2O_3层状结构薄膜,然后对溅射的Mg:Ga_2O_3层状薄膜在氨气气氛下退火制备GaN纳米结构。通过改变退火时间、退火温度及Mg层的厚度研究其对合成的GaN纳米结构的影响。研究结果表明:不同的退火温度、退火时间和Mg层的厚度对合成GaN纳米结构都有很大影响,合成的一维纳米结构为六方纤锌矿结构的单晶GaN。其中制备高质量Mg掺杂氮化镓纳米线的最佳条件是共溅射过程包含25个循环,Mg:Ga_2O_3薄膜总的溅射厚度为600nm左右,溅射总时间为100分钟。单个循环过程如下:先生长20nm左右的非故意掺杂Ga_2O_3,保持工作气压不变,同时停止溅射Ga_2O_3,如此保持一段时间(5s)再开始溅射Mg 2s。最后,将溅射好的Mg:Ga_2O_3薄膜样品放入管式炉的恒温区进行氨化。氨化温度和氨化时间分别为900℃和15分钟。
2.GaN纳米结构的光学特性
室温下,用波长为298nm光激发样品表面,出现359nm、384nm、425nm和442nm四处发光峰,改变实验条件发光峰位置基本没有随之发生移动,只是发光强度发生了变化。对于位于359nm处的发光峰对应本征GaN常温370nm的发光峰表现了明显的蓝移,归因于Burstein-Moss效应;其余三个发光峰都是由于Mg的掺杂引起的不同的杂质能级的跃迁;
3.对GaN纳米结构生长机制的探索
高温下氨气逐步分解成NH_2、NH、H_2、N_2等产物,固态Ga_2O_3与H_2反应生成中间产物气态的Ga_2O,在衬底处与体系中氨气发生催化反应得到GaN晶核,这些晶核在衬底合适的能量位置生长,成为下一个晶核生长的依托点,随着氨化过程的进行GaN晶核继续长成GaN微晶,当微晶的生长方向沿着相同的方向生长,就形成了单晶GaN纳米线、纳米线、纳米颗粒。同时氨化层状结构的Mg:Ga_2O_3薄膜,能使得在微晶生长过程中,会有更多的Mg离子进驻GaN晶体内部,实现了Mg的有效掺杂。但更深刻的原因仍在进一步的研究之中。
GaN is an excellentⅢ-Ⅴwide-band gap semiconductor material,commonly used in blue or green light-emitting diodes or laser diodes,and all kinds of radioresistance, high-frequency,high-temperature and high-density integrated devices.Nanowires, nanobelts,and nanorods are a new class of one-dimensional materials that have been attracting a great deal of interest in research in the last few years.In order to improve its performances in both electronic and optoelectronic devices,however,appropriate doping is necessary.Be,Mg Zn,C have been used to study the P-doping of GaN,Mg has been widely usded.So on the base of the growth of one-dimension GaN nanostructures,It is very interesting to investigate the growth of Mg-doped GaN nanostructures.
In this paper,Mg-doped GaN one-dimensional nanosmaterials were synthesized through magnetron sputtering and ammoniating progress.The structure,elemental composition,morphology and photoluminescence properties of the GaN nanomaterials were determined by X-ray diffraction(XRD),Scanning electronic microscope(SEM), High-resolution transmission electronic microscope(HRTEM),Fourier transformed infrared spectroscopy(FTIR),X-ray photoelectron energy spectroscopy(XPS) and Photoluminescence spectroscopy(PL).The growth mechanism of these GaN nanomaterials were proposed and discussed based on the investigation of the influence of the ammoniating temperature,ammoniating time and the thickness of Mg layer on the properties of Mg-doped GaN nanostructure.
1.Synthesis of one-dimensional Mg-doped GaN nanostructures through co-sputtering and ammonification method
One-dimension Mg-doped GaN nanomaterials were fabricated through ammoniating Mg:Ga_2O_3 thin films deposited by direct current(DC) magnetron sputtering system and radio frequency(RF) magnetron sputtering system respectively.The influences on the growth of GaN nanostructures were investigated by changing the ammoniating temperature,ammoniating time or the thickness of Mg layer.The results reveal that different anneal temperature,different ammoniating time of Mg:Ga_2O_3 thin films and different thickness of Mg layer have a great influence on the synthesis of GaN nanostructures.The synthesized nanostructures were of hexagonal wurtzite single-crystal GaN.There were 25 cycles in this process,the total thickness of the Mg: Ga_2O_3 thin films was about 600nm and the total sputtering time was 100 min.The single sputtering cycle was that:first an un-doped Ga_2O_3 film was deposited to a thickness of about 20 nm,and then a Mg layer was deposited for 2 s.In the second step,as-deposited Mg:Ga_2O_3 thin films were ammoniated in a conventional tube furnace at 900℃for 15 min.
2.Optical properties of GaN nanostructures
The nanowires showed four emission peaks at 359 nm,384 nm,425 nm,and 442 nm, respectively.An obviously blueshift of the band gap emission occurs from the 370 nm of bulk GaN to 359 nm of Mg-doped GaN,which is attributed to Burstein-Moss effect. The other peaks are arisen from the transtion excition of different impurity levels from Mg.
3.Exploration of the growth mechanism for GaN nanostructures
In the ammoniating process,NH_3 decomposes into NH_2,NH,H_2,N_2 and N when the ammoniating temperature is above 800℃.The Ga_2O_3 particles are reduced to gaseous Ga_2O by H_2 and then GaN molecules are synthesized through the reaction between Ga_2O and NH_3.The formed GaN molecules diffuse and agglomerate into GaN crystalline nuclei,and then the very small GaN crystalline nuclei grow up gradually with the progress of the ammonification.When the microcrystalline grow along the same direction,they become nanowires,nanorods or nanograins.In the sputtering process, layered structure of the Ga_2O_3 films doped with Mg has been obtained.Thus,in the growth process,Mg has more opportunity to substitute the position of Ga.At that temperature,vapourized Mg was doped into the GaN particles to occupy the position of Ga vacancies.
引文
[1]GPopvici.Raman scattering and photoluminescence of Mg doped GaN films grown by Molecular Beam Eitaxy.J.Appl.Phys,82 (8):4020 (1997).
[2]R.Hickman,J.M.Van Hove,P.P.Chow,et al.GaN PN junction issues and developments,Solid State Electronics,44:377 (2000).
[3]Takao Someya et.al.,Room Temperature Lasing at Blue Wavelengths in Gallium Nitride Microcavities,Science,285:1905 (1999).
[4]Huang M H,Mao S,Yang Peidong,et al.Room-temperature ultraviolet nanowire nanolasers.Science,292:1897 (2001).
[5]Wang J C,Feng S Q,Yu D P.High-quality GaN nanowires synthesized using a CVD approach.Appl Phys A,75:691 (2002).
[6]D.Appell.Wired for success.Nature,419:553-555 (2002).
[7]Zhonglin Wang.Nanowires and nanobelts-materials,properties and devices Vol Ⅱ:Nanowires and nanobelts of functional materials.清华大学出版社,3(2004)．
[8]J.I.Pankove,T.D.Moustakas,“Gallium Nitride(GaN)I”,Academic Press,(1998).
[9]R.D.Vispute,V.Talyansky,Z.Trajanovic,S.Choopun,M.Downes,R.P.Sharma,T.Venkatesan,M.C.Woods,R.T.Lareau,K.A.Jones,A.A.Iliadis.High quality crystalline ZnO buffer layers on sapphire (001)by pulsed laser deposition for Ⅲ-Ⅴ nitrides,Appl.Phys.Lett,70(20):2735 (1997).
[10]R.Fischer,A.Miehr,O.Ambacher,T.Metzger,E.Born.Novel single source precursors for MOCVD of A1N,GaN and InN.J.Cryst.Growth,170:139 (1997).
[11]M.Furtado and G Jacob,Study on the influence of annealing effects in GaN VPE,J.Cryst.Growth,64(2):257-267 (1983).
[12]I.Adesida,A.Mahajan,E.Andideh,et al.,Reactive ion etching of gallium nitride in silicon tetrachloride plasmas,Appl.Phys.Lett,63(20):2777-2779 (1993).
[13]S.Nakamura,T.Mukai,M.Senoh.In situ monitoring and Hall measurements of GaN grown with GaN buffer layers,J.Appl.Phys,71:5543 (1992).
[14]M.Asif Khan,J.N.Kuznia,J.M.Van Hove,et al.,Growth of high optical and electrical quality GaN layers using low-pressure metalorganic chemical vapor deposition,Appl.Phys.Lett,58:526 (1991).
[15]H.Amano,M.Kito,K.Hiramatsu,et al.,P-Type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam Irradiation (LEEBI),Jpn.J.Appl.Phys,28:2112(1989).
[16]S.Nakamura,T.Mukai,M.Senoh,et al.,Thermal Annealing Effects on P-Type Mg-Doped GaN Films Jpn.J.Appl.Phys,31:L 139 (1992).
[17]X.G Qiu,Y.Segawa,Q.K.Xue,Q.Z.Xue,and T.Sakurai.Influence of threading dislocations on the near-bandedge photoluminescence of wurtzite GaN thin films on SiC substrate.Appl.Phys.Lett,77:316-1318 (2000).
[18]V.Kirilyuk,A.R.A.Zauner,P.C.M.Christianen,J.L.Weyher,P.R.Hageman,P.K.Larsen.Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities.Appl.Phys.Lett,76:2355 (2000).
[19]J.I.Pankove,J.E.Berkeyheiser,H.P.Maruska,J.Wittke.Luminescent properties of GaN.Solid State Communications,8:1051 (1970).
[20]H.G Grimmeiss,B.Monemar.Low-temperature luminescence of GaN.J.Appl.Phys.41:4054 (1970).
[21]O.Lagerstedt and B.Monemar.Luminescence in epitaxial GaN:Cd,J.Appl.Phys.,45:2266(1974).
[22]S.Gu,R.Zhang,Y.Shi,et al.,The impact of initial growth and substrate nitridation on thick GaN growth on sapphire by hydride vapor phase epitaxy,J.Cryst.Growth,231(3):342-351 (2001).
[23]H.Amano,I.Akasaki,K.Hiramatsu,et al.,Effects of the buffer layer in metalorganic Vapour phase epitaxy of GaN on Sapphire substrate,Thin Solid Films,163:415-420(1988).
[24]S.Nakamura,M.Senoh,S.Nagahama,et al.,InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices.Jpn.J.Appl.Phys,36:L1518-L1571 (1997).
[25]W.Kim,Q.ktas,A.E.Botchkarev,et al.,Reactive molecular beam epitaxy of wurtzite GaN:Materials Characteristics and growth kinetics,J.Appl.Phys.79(10):7657-7666 (1996).
[26]Sung Hwan Cho,Uitsu Tanaka,Kazutaka Hata,et al.,Photoluminescence properties of GaN Grown under Ion Flux Reduced Condition by Plasma Enhanced Molecular Beam EpitaxyJpnJ.Appl.Phys.35:L644-L647 (1996).
[27]Q.Chen,M.A Khan,J.W.Yang,et al.,High transconductance heterostructure field-effect transistors based on AlGaN/GaN.Applied Physics Letters,69(6):794-796(1996).
[28]Y.Huang,X.F.Duan,C.M.Lieber,Gallium Nitride Nanowire Nanodevices.Nano Lerrer,2:101-104(2002).
[29]F.A.Ponce,D.P.Bour,Nitride-based semiconductors for blue and green light-emitting devices.Nature,386:351-359(1997).
[30]S Nakamura,M Senoh,S Nagahama,et al.,InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate.Applied Physics Letters,72(2):211-213(1998).
[31]张立德,牟季美.纳米材料和纳米结构[M].北京:科学出版社,6(2001).
[32]张立德,牟季美.纳米材料和纳米结构[M].北京:科学出版社,34(2001).
[33]张立德,牟季美等,纳米材料和纳米结构[M].北京:科学出版社,16-17(2002).
[34]Peyghambarian N et al.Nanomaterials:Synthesis,Properties and applications,Eds.By A.S.Edelstein,R.C.Cammarata,Bristol:IOP Publishing,(1996).
[35]Goronkim H et al.Nanostructure Science and Technology,a worldwide study.Eds.By Siegel R W,Hu E and Rocco MC,NSTC,(1999).
[36]F.A.Ponce,D.P.Bour,Nitride-based semiconductors for blue and green light-emitting devices,Nature(London),386:351-359(1997).
[37]Jae-Ryoung Kim,Hye Mi So,Jong Wan Park,Ju-Jin Kim,Jinhee Kim,Cheol Jin Lee,Seung Chul Lyu,Electrical transport properties of individual gallium nitride nanowires by chemical-vapor-deposition,Appl.Phys.Lett.,80(19):3548-3550(2002).
[38]Han Weiqiang,Fan Shoushan,Li Qunqing,Synthesis of gallium nitride nanorods through a carbon nanotube-confined reaction.Science,277:1287-1289(1997).
[39]Cheng G.S.,Zhang L.D.,Zhu Y.,et al.,Large-scale synthesis of single crystalline gallium nitride nanowires.Appl.Phy.Lett.,75(16):2455-2457(1999).
[40]Cheng G.S.,Chen S.H.,Zhu X.G.,Mao Y.Q.,Zhang L.D.,Highly ordered nanostructures of sigle crystalline GaN nanowires.Mater Sci & Eng A.286:165-168(2000).
[41]Goldberger Joshua,He Rongrui,Zhang Yanfeng,Single-crystal gallium nitride nanotubes.Nature,422:599-602(2003).
[42]X.F.Duang,C.M.Lieber,Laser-assisted catalytic growth of single crystal GaN nanowires,J.Am.Chem.Soc,122:188(2000).
[43]He M.Q.,Minus I.Zhou P.Z.,Mohammed S.N.et al.,Growth of large-scale GaN nanowires and tubes by direct reaction of Ga with NH3.Appl.Phys,Lett.77:3731-3733(2000).
[44]He M.Q.,Zhou P.Z.,Mohammed S.N.et al.,Growth of GaN nanowires by direct reaction of Ga with NH3.J.Crystal.Growth.231:357-365 (2001).
[45]W.S.Shi,Y.F.Zheng,N.Wang,C.S.Lee,S.T.Lee,Microstructure of gallium nitride nanowires synthesized by oxide-assisted method,Chem.Phys.Lett,345:377-380(2001).
[46]Y.J.Li,Z.Y.Qiao,L.X.Chen,GY.Cao,C.Y.Lan,Y.C.Wang,Y.G Chen,Y.C.Cao,C.Y.Lan,Wang.Morphologies of GaN one-dimensional materials,Appl.Phys.A,71:587(2000).
[47]J.Y.Li,X.L.Chen,Z.Y.Qiao,Y.G Cao,M.He,T.Xu,Synthesis of aligned gallium nitride quasi-arrays,Appl.Phys.A,71:349 (2000).
[48]Y.U.Peng,T.X.Zhou,N.Wang,F.Y.Zheng,S.L.Liao,S.W.Shi,S.C.Lee,T.S.Lee,Bulk-quantity GaN nanowires synthesized from hot filament chemical vapor deposition,Chem.Phys.Lett,327:263 (2000).
[49]S.N akamura and G Faso 1,The Blue L aser D iode,Sp ringer,Berlin,1997.
[50]Xie Shiyong,Zheng Youdou,Chen Peng,et al.p-type doping of GaN.Research &Progress of SSE,21 (2):204 (2001).
[51]Pankove J I,Hutchby J A.J Appl Phys,47(12):5387 (1976).
[52]H.Amano,M.Kito,K.H iramatsu and I.A kasak i,Jpn.J.App 1.Phys.28:2112(1989).
[53]S.N akamura,N.Iwasa,M.Senoh and T.M ukai,Jpn.J.App 1.Phys.31:191(1992).
[54]Nakamura S,Mukai T,Senoh M,et al.Jpn J Appl Phys,31(2B):L139 (1992).
[55]Liu Xianglin,Wang Chengxin,Han Peide,et al.Growth of P-type GaN and AlGaN epitaxial films.High Technology Letters,8:26 (2000).
[56]S.N akamura,N.Iwasa,M.Senoh and T.M ukai,Jpn.J.App 1.Phys.31:191 (1992).
[57]H.Tokunaga,I.W ak i,A.Yamaguch i,N.A kutsu and K.M atsumo to,J.Cryst.Growth,519:189-190 (1998).
[58]Li Shuti,Wang Li,Peng Xuexin,et al.Doping Mg dose in MOCVD growth of P-GaN films.Chinese Journal of Semiconductors,21:365 (2000).
[59]Mao Xiangjun,Yang Zhijianjing Sixuan,et al.PL characteristics of rapidly annealed Mg:GaN.Chinese Journal of Semiconductors,20:857 (1999).
[60]Eckey L,Von Gfug U,Holst J,et al.J Cryst Growth,523:189-190 (1998).
[61]Oh E,Park M,Kang S,et al.J Cryst Growth,537:189-190 (1998).
[62]Xue shoubin,Zhuang Huizhao,Xue Chengshan and Hu Lijun,Synthesis of GaN nanorods by ammoniating Ga203 films,Chinese Physics Letters,23(11):3055-3057 (2006).
[63]Han,D.S.;Park,J.;Rhie,K.W.;Kim,S.;Chang,J.Appl.Phys.Lett.86:320-506(2005).
[64]Ying-Ge Yang,Hong-Lei Ma,Cheng-Shan Xue,et al.,Preparation and structural properties for GaN films grown on Si (111)by annealing.Applied Surface Science,193:254-260(2002).
[65]Jin-Hyo Boo,Carsten Rohr and Wilson Ho,MOCVD of BN and GaN thin films on silcon:new attempt of GaN growth with BN buffer layer,J.Cryst.Growth.190:439-444 (1998).
[66]Yong Sun,Tatsuro Miyasato and Nobuo Snoda,Outdiffusion of the excess carbon in SiC films into Si substrate during film growth,J.Appl.Phys.84:6451-6453 (1998).
[67]W.K.Choi,T.Y.Ong,L.S.Tan,F.C.Loh and K.L.Tan,Infrared and X-ray photoelectron studies of as-prepared and furnace-annealed radio-frequency sputtered amorphous silicon carbide films,J.Appl.Phys.83(9):4968 (1998).
[68]Choi W.F.,Song T.Y,Tan L.S.,Infrared and x-ray photoelectron studies of as-prepared and furnace-annealed radio-frequency sputtered amorphous silicon carbide films,J.Appl.Phys,83(9):4968-4973 (1998).
[69]Li D,Sumiys M,Fuke S.Selective etching of GaN polar surface in potassium hydroxide solution studied by x-ray photoelectron spectroscopy.J.Appl.Phys.,90(8):4219-4223 (2001).
[70]Veal T D,Mahboob I,Piper L F J,McConville C F.Core-level photoemission spectroscopy of nitrogen bonding in GaNxAsl-X alloys.Appl.Phys.Lett.,85(9):1550-1552(2004).
[71]Elkashef N.,Srinivasa R.S.,Major S.,Sabharwal S.C,Muthe K.P.,Sputter deposition of gallium nitride films using a GaAs target.Thin Solid Films,333:9-12(1998).
[72]Kingsley C.R.,Whitaker T.J.,Wee A.T.S.et al.,Development of chemical beam epitaxy for the deposition of gallium nitride.Mater.Sci.Eng,B,29:78-82 (1995).
[73]Sasaki T.,Matsuoka T.,Substrate-polarity dependence of metal-organic-vaporphase epitaxy-grown GaN on SiC.J.App.Phys,64:4531-4535 (1998).
[74]Xue C,Wu Y,Zhuang H,et al.Growth and characterization of high-quality GaN nanowires by ammonification technique.Physica E,30:179-181 (2005).
[75]T.J.Ghuang,C.R.Brundle,D.W.Rice,Interpretation of the x-ray photoemission spectra of cobalt oxides and cobalt oxide surfaces,Surf.Sci.59:413 (1979).
[76]K.Ueda,H.Yamamoto and M.Naito:Precise Measurement of B Meson Lifetimes with Hadronic Decay Final States.Phys.Rev.88,171 (2002).
[77]Cai,X.M.;Djuri?i?,A.B.;Xie,M.H.;Chiu,C.S.;Gwo,S.Appl.Phys.Lett.,87:183103(2005).
[78]Monemar B.Fundamental energy gap of GaN from photoluminescence excitations pectra.Phys.Rev,B.10:676-681 (1974).
[79]S.M.Zhou,X.H.Zhang,X.Meng,S.T.Lee,Nanotechnology,15:1152 (2004).
[80]J.C.Zolper,M.H.Crawford,A.J.Howard,J.Ramer,S.D.Hersee:Appl.Phys.Lett,68:200 (1996).
[81]Cheng GS.,Zhang L.D.,Zhu Y,et al.,Large-scale synthesis of single crystalline gallium nitride nanowires.Appl.Phy.Lett.,75(16):2455-2457 (1999).
[82]Cheng GS.,Chen S.H.,Zhu X.G,Mao Y.Q.,Zhang L.D.,Highly ordered nanostructures of single crystalline GaN nanowires.Mater Sci & Eng A,286:165-168(2000).
[83]Goldberger Joshua,He Rongrui,Zhang Yanfeng,Single-crystal gallium nitride nanotubes.Nature,422:599-602 (2003).
[84]Wagner R.S.,Ellis WC.Vapor-liquid-solid mechanism of single crystal growth.Appl.Phys.Lett.,4:89-91 (1964).
[85]Y.Xia,P.et al.One-dimensional nanostructures:synthesis,characterization,and applications Adv.Mater,15:353-389 (2003).
[86]Chen X.L.,Li J.Y.,Cao Y.G,Lan Y.C.,Qiao Z.Y.,Straight and smooth GaN nanowires.Adv,Mater.,12:1432-1434(2000).
[87]Li J.Y,Chen X.L.,Cao Y.G,Qiao Z.Y.,Lan Y.C.,Raman scattering spectrum of GaN straight nanowires.Appl.Phys.A,71:345-346 (2000).
[88]Chen C.C,Yeh C.C,Large-scale catalytic synthesis of crystalline gallium nitride nanowires.Adv.Mater.,12:738-741 (2000).
[89]Chen C.C,Yeh C.H.,Chen C.H.,Yu M.Y,Liu H.L.,Wu J.J.,Chen K.H.,Chen L.C,Peng J.Y,Chen Y.F.,Catalytic Growth and Characterization of Gallium Nitride Nanowires.J.Am.Chem.Soc,123:2791-2798(2001).
[90]A.M.Morales,C.M.Liber.A laser ablation method for the synthesis of crystalline semiconductor nanowires,Science,279:208-211 1998().
[91]梁长浩.[D].中国科学院固体物理研究所博士学位论文,(2001).
[92]J.Hu,M.Ouyang,P.Yang,C.M.Lieber.Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires,Nature,399:48-51(1999).
[93]C.M.Balkas and R.F.Davis,Synthesis Routes and Characterization of High-purity,Single-phase Gallium Nitride Powders,J.Am.Ceram.Soc.79:2309 -2312(1996).
[94]W.-S.Jung,Reaction Intermediate(s) in the Conversion of b-gallium Oxide to Gallium Nitride Under a Flow of Ammonia,Mater.Lett.57:110-114(2002).
[95]Y.Ai,C.Xue,C.Sun,L.Sun,H.Zhuang,F.Wang,Z.Yang,L.Qin,Fabrication of needle-shaped GaN nanowires by ammoniating Ga2O3 films on MgO layers deposited on Si(111) substrates.Mater.Lett.,61:2833-2836(2007).
[2]R.Hickman,J.M.Van Hove,P.P.Chow,et al.GaN PN junction issues and developments,Solid State Electronics,44:377 (2000).
[3]Takao Someya et.al.,Room Temperature Lasing at Blue Wavelengths in Gallium Nitride Microcavities,Science,285:1905 (1999).
[4]Huang M H,Mao S,Yang Peidong,et al.Room-temperature ultraviolet nanowire nanolasers.Science,292:1897 (2001).
[5]Wang J C,Feng S Q,Yu D P.High-quality GaN nanowires synthesized using a CVD approach.Appl Phys A,75:691 (2002).
[6]D.Appell.Wired for success.Nature,419:553-555 (2002).
[7]Zhonglin Wang.Nanowires and nanobelts-materials,properties and devices Vol Ⅱ:Nanowires and nanobelts of functional materials.清华大学出版社,3(2004)．
[8]J.I.Pankove,T.D.Moustakas,“Gallium Nitride(GaN)I”,Academic Press,(1998).
[9]R.D.Vispute,V.Talyansky,Z.Trajanovic,S.Choopun,M.Downes,R.P.Sharma,T.Venkatesan,M.C.Woods,R.T.Lareau,K.A.Jones,A.A.Iliadis.High quality crystalline ZnO buffer layers on sapphire (001)by pulsed laser deposition for Ⅲ-Ⅴ nitrides,Appl.Phys.Lett,70(20):2735 (1997).
[10]R.Fischer,A.Miehr,O.Ambacher,T.Metzger,E.Born.Novel single source precursors for MOCVD of A1N,GaN and InN.J.Cryst.Growth,170:139 (1997).
[11]M.Furtado and G Jacob,Study on the influence of annealing effects in GaN VPE,J.Cryst.Growth,64(2):257-267 (1983).
[12]I.Adesida,A.Mahajan,E.Andideh,et al.,Reactive ion etching of gallium nitride in silicon tetrachloride plasmas,Appl.Phys.Lett,63(20):2777-2779 (1993).
[13]S.Nakamura,T.Mukai,M.Senoh.In situ monitoring and Hall measurements of GaN grown with GaN buffer layers,J.Appl.Phys,71:5543 (1992).
[14]M.Asif Khan,J.N.Kuznia,J.M.Van Hove,et al.,Growth of high optical and electrical quality GaN layers using low-pressure metalorganic chemical vapor deposition,Appl.Phys.Lett,58:526 (1991).
[15]H.Amano,M.Kito,K.Hiramatsu,et al.,P-Type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam Irradiation (LEEBI),Jpn.J.Appl.Phys,28:2112(1989).
[16]S.Nakamura,T.Mukai,M.Senoh,et al.,Thermal Annealing Effects on P-Type Mg-Doped GaN Films Jpn.J.Appl.Phys,31:L 139 (1992).
[17]X.G Qiu,Y.Segawa,Q.K.Xue,Q.Z.Xue,and T.Sakurai.Influence of threading dislocations on the near-bandedge photoluminescence of wurtzite GaN thin films on SiC substrate.Appl.Phys.Lett,77:316-1318 (2000).
[18]V.Kirilyuk,A.R.A.Zauner,P.C.M.Christianen,J.L.Weyher,P.R.Hageman,P.K.Larsen.Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities.Appl.Phys.Lett,76:2355 (2000).
[19]J.I.Pankove,J.E.Berkeyheiser,H.P.Maruska,J.Wittke.Luminescent properties of GaN.Solid State Communications,8:1051 (1970).
[20]H.G Grimmeiss,B.Monemar.Low-temperature luminescence of GaN.J.Appl.Phys.41:4054 (1970).
[21]O.Lagerstedt and B.Monemar.Luminescence in epitaxial GaN:Cd,J.Appl.Phys.,45:2266(1974).
[22]S.Gu,R.Zhang,Y.Shi,et al.,The impact of initial growth and substrate nitridation on thick GaN growth on sapphire by hydride vapor phase epitaxy,J.Cryst.Growth,231(3):342-351 (2001).
[23]H.Amano,I.Akasaki,K.Hiramatsu,et al.,Effects of the buffer layer in metalorganic Vapour phase epitaxy of GaN on Sapphire substrate,Thin Solid Films,163:415-420(1988).
[24]S.Nakamura,M.Senoh,S.Nagahama,et al.,InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices.Jpn.J.Appl.Phys,36:L1518-L1571 (1997).
[25]W.Kim,Q.ktas,A.E.Botchkarev,et al.,Reactive molecular beam epitaxy of wurtzite GaN:Materials Characteristics and growth kinetics,J.Appl.Phys.79(10):7657-7666 (1996).
[26]Sung Hwan Cho,Uitsu Tanaka,Kazutaka Hata,et al.,Photoluminescence properties of GaN Grown under Ion Flux Reduced Condition by Plasma Enhanced Molecular Beam EpitaxyJpnJ.Appl.Phys.35:L644-L647 (1996).
[27]Q.Chen,M.A Khan,J.W.Yang,et al.,High transconductance heterostructure field-effect transistors based on AlGaN/GaN.Applied Physics Letters,69(6):794-796(1996).
[28]Y.Huang,X.F.Duan,C.M.Lieber,Gallium Nitride Nanowire Nanodevices.Nano Lerrer,2:101-104(2002).
[29]F.A.Ponce,D.P.Bour,Nitride-based semiconductors for blue and green light-emitting devices.Nature,386:351-359(1997).
[30]S Nakamura,M Senoh,S Nagahama,et al.,InGaN/GaN/AlGaN-based laser diodes with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate.Applied Physics Letters,72(2):211-213(1998).
[31]张立德,牟季美.纳米材料和纳米结构[M].北京:科学出版社,6(2001).
[32]张立德,牟季美.纳米材料和纳米结构[M].北京:科学出版社,34(2001).
[33]张立德,牟季美等,纳米材料和纳米结构[M].北京:科学出版社,16-17(2002).
[34]Peyghambarian N et al.Nanomaterials:Synthesis,Properties and applications,Eds.By A.S.Edelstein,R.C.Cammarata,Bristol:IOP Publishing,(1996).
[35]Goronkim H et al.Nanostructure Science and Technology,a worldwide study.Eds.By Siegel R W,Hu E and Rocco MC,NSTC,(1999).
[36]F.A.Ponce,D.P.Bour,Nitride-based semiconductors for blue and green light-emitting devices,Nature(London),386:351-359(1997).
[37]Jae-Ryoung Kim,Hye Mi So,Jong Wan Park,Ju-Jin Kim,Jinhee Kim,Cheol Jin Lee,Seung Chul Lyu,Electrical transport properties of individual gallium nitride nanowires by chemical-vapor-deposition,Appl.Phys.Lett.,80(19):3548-3550(2002).
[38]Han Weiqiang,Fan Shoushan,Li Qunqing,Synthesis of gallium nitride nanorods through a carbon nanotube-confined reaction.Science,277:1287-1289(1997).
[39]Cheng G.S.,Zhang L.D.,Zhu Y.,et al.,Large-scale synthesis of single crystalline gallium nitride nanowires.Appl.Phy.Lett.,75(16):2455-2457(1999).
[40]Cheng G.S.,Chen S.H.,Zhu X.G.,Mao Y.Q.,Zhang L.D.,Highly ordered nanostructures of sigle crystalline GaN nanowires.Mater Sci & Eng A.286:165-168(2000).
[41]Goldberger Joshua,He Rongrui,Zhang Yanfeng,Single-crystal gallium nitride nanotubes.Nature,422:599-602(2003).
[42]X.F.Duang,C.M.Lieber,Laser-assisted catalytic growth of single crystal GaN nanowires,J.Am.Chem.Soc,122:188(2000).
[43]He M.Q.,Minus I.Zhou P.Z.,Mohammed S.N.et al.,Growth of large-scale GaN nanowires and tubes by direct reaction of Ga with NH3.Appl.Phys,Lett.77:3731-3733(2000).
[44]He M.Q.,Zhou P.Z.,Mohammed S.N.et al.,Growth of GaN nanowires by direct reaction of Ga with NH3.J.Crystal.Growth.231:357-365 (2001).
[45]W.S.Shi,Y.F.Zheng,N.Wang,C.S.Lee,S.T.Lee,Microstructure of gallium nitride nanowires synthesized by oxide-assisted method,Chem.Phys.Lett,345:377-380(2001).
[46]Y.J.Li,Z.Y.Qiao,L.X.Chen,GY.Cao,C.Y.Lan,Y.C.Wang,Y.G Chen,Y.C.Cao,C.Y.Lan,Wang.Morphologies of GaN one-dimensional materials,Appl.Phys.A,71:587(2000).
[47]J.Y.Li,X.L.Chen,Z.Y.Qiao,Y.G Cao,M.He,T.Xu,Synthesis of aligned gallium nitride quasi-arrays,Appl.Phys.A,71:349 (2000).
[48]Y.U.Peng,T.X.Zhou,N.Wang,F.Y.Zheng,S.L.Liao,S.W.Shi,S.C.Lee,T.S.Lee,Bulk-quantity GaN nanowires synthesized from hot filament chemical vapor deposition,Chem.Phys.Lett,327:263 (2000).
[49]S.N akamura and G Faso 1,The Blue L aser D iode,Sp ringer,Berlin,1997.
[50]Xie Shiyong,Zheng Youdou,Chen Peng,et al.p-type doping of GaN.Research &Progress of SSE,21 (2):204 (2001).
[51]Pankove J I,Hutchby J A.J Appl Phys,47(12):5387 (1976).
[52]H.Amano,M.Kito,K.H iramatsu and I.A kasak i,Jpn.J.App 1.Phys.28:2112(1989).
[53]S.N akamura,N.Iwasa,M.Senoh and T.M ukai,Jpn.J.App 1.Phys.31:191(1992).
[54]Nakamura S,Mukai T,Senoh M,et al.Jpn J Appl Phys,31(2B):L139 (1992).
[55]Liu Xianglin,Wang Chengxin,Han Peide,et al.Growth of P-type GaN and AlGaN epitaxial films.High Technology Letters,8:26 (2000).
[56]S.N akamura,N.Iwasa,M.Senoh and T.M ukai,Jpn.J.App 1.Phys.31:191 (1992).
[57]H.Tokunaga,I.W ak i,A.Yamaguch i,N.A kutsu and K.M atsumo to,J.Cryst.Growth,519:189-190 (1998).
[58]Li Shuti,Wang Li,Peng Xuexin,et al.Doping Mg dose in MOCVD growth of P-GaN films.Chinese Journal of Semiconductors,21:365 (2000).
[59]Mao Xiangjun,Yang Zhijianjing Sixuan,et al.PL characteristics of rapidly annealed Mg:GaN.Chinese Journal of Semiconductors,20:857 (1999).
[60]Eckey L,Von Gfug U,Holst J,et al.J Cryst Growth,523:189-190 (1998).
[61]Oh E,Park M,Kang S,et al.J Cryst Growth,537:189-190 (1998).
[62]Xue shoubin,Zhuang Huizhao,Xue Chengshan and Hu Lijun,Synthesis of GaN nanorods by ammoniating Ga203 films,Chinese Physics Letters,23(11):3055-3057 (2006).
[63]Han,D.S.;Park,J.;Rhie,K.W.;Kim,S.;Chang,J.Appl.Phys.Lett.86:320-506(2005).
[64]Ying-Ge Yang,Hong-Lei Ma,Cheng-Shan Xue,et al.,Preparation and structural properties for GaN films grown on Si (111)by annealing.Applied Surface Science,193:254-260(2002).
[65]Jin-Hyo Boo,Carsten Rohr and Wilson Ho,MOCVD of BN and GaN thin films on silcon:new attempt of GaN growth with BN buffer layer,J.Cryst.Growth.190:439-444 (1998).
[66]Yong Sun,Tatsuro Miyasato and Nobuo Snoda,Outdiffusion of the excess carbon in SiC films into Si substrate during film growth,J.Appl.Phys.84:6451-6453 (1998).
[67]W.K.Choi,T.Y.Ong,L.S.Tan,F.C.Loh and K.L.Tan,Infrared and X-ray photoelectron studies of as-prepared and furnace-annealed radio-frequency sputtered amorphous silicon carbide films,J.Appl.Phys.83(9):4968 (1998).
[68]Choi W.F.,Song T.Y,Tan L.S.,Infrared and x-ray photoelectron studies of as-prepared and furnace-annealed radio-frequency sputtered amorphous silicon carbide films,J.Appl.Phys,83(9):4968-4973 (1998).
[69]Li D,Sumiys M,Fuke S.Selective etching of GaN polar surface in potassium hydroxide solution studied by x-ray photoelectron spectroscopy.J.Appl.Phys.,90(8):4219-4223 (2001).
[70]Veal T D,Mahboob I,Piper L F J,McConville C F.Core-level photoemission spectroscopy of nitrogen bonding in GaNxAsl-X alloys.Appl.Phys.Lett.,85(9):1550-1552(2004).
[71]Elkashef N.,Srinivasa R.S.,Major S.,Sabharwal S.C,Muthe K.P.,Sputter deposition of gallium nitride films using a GaAs target.Thin Solid Films,333:9-12(1998).
[72]Kingsley C.R.,Whitaker T.J.,Wee A.T.S.et al.,Development of chemical beam epitaxy for the deposition of gallium nitride.Mater.Sci.Eng,B,29:78-82 (1995).
[73]Sasaki T.,Matsuoka T.,Substrate-polarity dependence of metal-organic-vaporphase epitaxy-grown GaN on SiC.J.App.Phys,64:4531-4535 (1998).
[74]Xue C,Wu Y,Zhuang H,et al.Growth and characterization of high-quality GaN nanowires by ammonification technique.Physica E,30:179-181 (2005).
[75]T.J.Ghuang,C.R.Brundle,D.W.Rice,Interpretation of the x-ray photoemission spectra of cobalt oxides and cobalt oxide surfaces,Surf.Sci.59:413 (1979).
[76]K.Ueda,H.Yamamoto and M.Naito:Precise Measurement of B Meson Lifetimes with Hadronic Decay Final States.Phys.Rev.88,171 (2002).
[77]Cai,X.M.;Djuri?i?,A.B.;Xie,M.H.;Chiu,C.S.;Gwo,S.Appl.Phys.Lett.,87:183103(2005).
[78]Monemar B.Fundamental energy gap of GaN from photoluminescence excitations pectra.Phys.Rev,B.10:676-681 (1974).
[79]S.M.Zhou,X.H.Zhang,X.Meng,S.T.Lee,Nanotechnology,15:1152 (2004).
[80]J.C.Zolper,M.H.Crawford,A.J.Howard,J.Ramer,S.D.Hersee:Appl.Phys.Lett,68:200 (1996).
[81]Cheng GS.,Zhang L.D.,Zhu Y,et al.,Large-scale synthesis of single crystalline gallium nitride nanowires.Appl.Phy.Lett.,75(16):2455-2457 (1999).
[82]Cheng GS.,Chen S.H.,Zhu X.G,Mao Y.Q.,Zhang L.D.,Highly ordered nanostructures of single crystalline GaN nanowires.Mater Sci & Eng A,286:165-168(2000).
[83]Goldberger Joshua,He Rongrui,Zhang Yanfeng,Single-crystal gallium nitride nanotubes.Nature,422:599-602 (2003).
[84]Wagner R.S.,Ellis WC.Vapor-liquid-solid mechanism of single crystal growth.Appl.Phys.Lett.,4:89-91 (1964).
[85]Y.Xia,P.et al.One-dimensional nanostructures:synthesis,characterization,and applications Adv.Mater,15:353-389 (2003).
[86]Chen X.L.,Li J.Y.,Cao Y.G,Lan Y.C.,Qiao Z.Y.,Straight and smooth GaN nanowires.Adv,Mater.,12:1432-1434(2000).
[87]Li J.Y,Chen X.L.,Cao Y.G,Qiao Z.Y.,Lan Y.C.,Raman scattering spectrum of GaN straight nanowires.Appl.Phys.A,71:345-346 (2000).
[88]Chen C.C,Yeh C.C,Large-scale catalytic synthesis of crystalline gallium nitride nanowires.Adv.Mater.,12:738-741 (2000).
[89]Chen C.C,Yeh C.H.,Chen C.H.,Yu M.Y,Liu H.L.,Wu J.J.,Chen K.H.,Chen L.C,Peng J.Y,Chen Y.F.,Catalytic Growth and Characterization of Gallium Nitride Nanowires.J.Am.Chem.Soc,123:2791-2798(2001).
[90]A.M.Morales,C.M.Liber.A laser ablation method for the synthesis of crystalline semiconductor nanowires,Science,279:208-211 1998().
[91]梁长浩.[D].中国科学院固体物理研究所博士学位论文,(2001).
[92]J.Hu,M.Ouyang,P.Yang,C.M.Lieber.Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires,Nature,399:48-51(1999).
[93]C.M.Balkas and R.F.Davis,Synthesis Routes and Characterization of High-purity,Single-phase Gallium Nitride Powders,J.Am.Ceram.Soc.79:2309 -2312(1996).
[94]W.-S.Jung,Reaction Intermediate(s) in the Conversion of b-gallium Oxide to Gallium Nitride Under a Flow of Ammonia,Mater.Lett.57:110-114(2002).
[95]Y.Ai,C.Xue,C.Sun,L.Sun,H.Zhuang,F.Wang,Z.Yang,L.Qin,Fabrication of needle-shaped GaN nanowires by ammoniating Ga2O3 films on MgO layers deposited on Si(111) substrates.Mater.Lett.,61:2833-2836(2007).