Growth mode and strain evolution during InN growth on GaN(0001) by molecular-beam epitaxy

Epitaxial growth of InN on GaN(0001) by plasma-assisted molecular-beam epitaxy is investigated over a range of growth parameters including source flux and substrate temperature. Combining reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM), we establish a relationship between film growth mode and the deposition condition. Both two-dimensional (2D) and three-dimensional (3D) growth modes of the film are observed. For 2D growth, sustained RHEED intensity oscillations are recorded while STM reveals 2D nucleation islands. For 3D growth, less than three oscillation periods are observed indicating the Stranski-Krastanov (SK) growth mode of the film. Simultaneous measurements of (reciprocal) lattice constant by RHEED suggest a gradual relaxation of the strain in film, which commences during the first bilayer (BL) deposition and almost completes after 2-4 BLs. For SK growth, 3D islanding initiates after the strain has mostly been relieved, presumably by dislocations, so the islands are likely strain free. (C) 2002 American Institute of Physics.

GaN/GaAs(001)与GaN/Al_2O_3(0001)外延层光辅助湿法腐蚀行为的比较

研究了用金属有机物气相外延（MOVPE）方法在GaAs(001)衬底上生长的立方相GaN(c-GaN)外延层的光辅助湿法腐蚀特性，并和生长在蓝宝石（0001）衬底上的六方相GaN(h-GaN)外延层的光辅助湿法座蚀特性进行了比较。实验发现c-GaN膜的暗态电流和光电流的变化不同于h-GaN膜的腐蚀电流的变化规律。对引起上述差异的原因进行了简单的讨论。

Anisotropic crystallographic properties, strain, and their effects on band structure of m -plane GaN on LiAlO2 (100)

The m-plane GaN films grown on LiAlO2(100) by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson-Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN[0001] in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress epsilon(xx)=-0.79% and epsilon(zz)=-0.14% with an out-of-plane dilatation epsilon(yy)=0.38%. This anisotropic strain further separates the energy levels of top valence band at Gamma point. The energy splitting as 37 meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature. (c) 2008 American Institute of Physics.

Comparison of the properties of GaN grown on complex Si-based structures

With the aim of investigating the possible integration of optoelectronic devices, epitaxial GaN layers have been grown on Si(Ill) semiconductor-on-insulator (SOI) and on Si/CoSi2/Si(111) using metalorganic chemical vapor deposition. The samples are found to possess a highly oriented wurtzite structure, a uniform thickness, and abrupt interfaces. The epitaxial orientation is determined as GaN(0001)//Si(111), GaN[1120]//Si[110], and GaN[1010]//Si[112], and the GaN layer is tensilely strained in the direction parallel to the interface. According to Rutherford backscattering/channeling spectrometry and (0002) rocking curves, the crystalline quality of GaN on Si(111) SOI is better than that of GaN on silicide. Room-temperature photoluminescence of GaN/SOI reveals a strong near-band-edge emission at 368 nm (3.37 eV) with a full width at half-maximum of 59 meV. (c) 2005 American Institute of Physics.

Depth distribution of the strain in the GaN layer with low-temperature AlN interlayer on Si(111) substrate studied by Rutherford backscattering/channeling

The depth distribution of the strain-related tetragonal distortion e(T) in the GaN epilayer with low-temperature AlN interlayer (LT-AlN IL) on Si(111) substrate is investigated by Rutherford backscattering and channeling. The samples with the LT-AlN IL of 8 and 16 nm thickness are studied, which are also compared with the sample without the LT-AlN IL. For the sample with 16-nm-thick LT-AlN IL, it is found that there exists a step-down of e(T) of about 0.1% in the strain distribution. Meanwhile, the angular scan around the normal GaN <0001> axis shows a tilt difference about 0.01degrees between the two parts of GaN separated by the LT-AlN IL, which means that these two GaN layers are partially decoupled by the AlN interlayer. However, for the sample with 8-nm-thick LT-AlN IL, neither step-down of e(T) nor the decoupling phenomenon is found. The 0.01degrees decoupled angle in the sample with 16-nm-thick LT-AlN IL confirms the relaxation of the LT-AlN IL. Thus the step-down of e(T) should result from the compressive strain compensation brought by the relaxed AlN interlayer. It is concluded that the strain compensation effect will occur only when the thickness of the LT-AlN IL is beyond a critical thickness. (C) 2004 American Institute of Physics.

Current-induced migration of surface adatoms during GaN growth by molecular beam epitaxy

The influence of electric fields on surface migration of Gallium (Ga) and Nitrogen (N) adatoms is studied during GaN growth by molecular beam epitaxy (MBE). When a direct current (DC) is used to heat the sample, long distance migration of Ga adatoms and diffusion asymmetry of N adatoms at steps are observed. On the other hand, if an alternating current (AC) is used, no such preferential adatom migration is found. This effect is attributed to the effective positive charges of surface adatoms. representing an effect of electro-migration. The implications of such current-induced surface migration to GaN epitaxy are subsequently investigated. It is seen to firstly change the distribution of Ga adatoms on a growing surface, and thus make the growth to be Ga-limited at one side of the sample but N-limited at the other side. This leads to different optical qualities of the film and different morphologies of the surface. (C) 2001 Elsevier Science B.V. All rights reserved.

Observation of "ghost" islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

We observe "ghost" islands formed on terraces during homoepitaxial nucleation of GaN. We attribute the ghost islands to intermediate nucleation states, which can be driven into "normal" islands by scanning tunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface. The excess Ga also affect island number density: by increasing Ga coverage, the island density first decreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coverages above 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.

以金属为缓冲层在Si（111）上分子束外延GaN及其表征

用电子束蒸发方法在Si（111）衬底上蒸发了Au/Cr和Au/Ti/Al/Ti两种金属缓冲层,然后在金属缓冲层上用气源分子束外延（GSMBE）生长GaN.两种缓冲层的表面都比较平整和均匀,都是具有Au（111）面择优取向的立方相Au层.在Au/Cr/Si（111）上MBE生长的GaN,生长结束后出现剥离.在Au/Ti/Al/Ti/Si（111）上无AlN缓冲层直接生长GaN,得到的是多晶GaN;先在800℃生长一层AlN缓冲层,然后在710℃生长GaN,得到的是沿GaN（0001）面择优取向的六方相GaN.将Au/Ti/Al/Ti/Si（111）在800℃下退火20min,金属层收缩为网状结构,并且成为多晶,不再具有Au（111）方向择优取向.

带有A1N插入层的GaN薄膜的结构及应变研究

利用金属有机化学汽相沉积(MOCVD)法在硅衬底上生长具有AIN插入层的GaN外延膜，采用高分辨X射线衍射(HRXRD)和卢瑟福背散射/沟道(RBS/Channeling)技术研究分析其结构和应变性质。从RBS＜0001＞沟道谱可知，该外延膜具有良好的结晶品质，χ_(min)＝2.5％。利用不同方位角上XRD摇摆曲线测量，可得出GaN(0001)面与Si(111)面之间的夹角β＝1.379°。通过对GaN(0002)和GaN(10(1-bar)5)衍射面的θ-2θ扫描，可以得出GaN外延膜在垂直方向和水平方向的平均弹性应变分别为-0.10%±0.02％和0.69％±0.09％。通过对{10(1-bar)0}面内非对称<1(2-bar)13>轴RBS角扫描可得出由弹性应变引起的四方畸变e_T在近表面处为0.35％±0.02％。外延膜弹性性质表明GaN膜在水平方向具有张应力(e~〃＞0)、在垂直方向具有压应力(e~⊥＜0)，印证了XRD的结果。四方畸变是深度敏感的，通过对不同深度的四方畸变计算可知，A1N插入层下面的GaN外延膜弹性应变释放速度比A1N层上面的GaN层弹性应变释放快，说明A1N层的插入缓解了应变释放速度。

掺铒GaN薄膜的背散射/沟道分析和光致发光研究

采用背散射(RBS)/沟道(channeling)分析和傅里叶变换红外光谱(FT-IR)研究了掺铒GaN薄膜的晶体结构和光致发光(PL)特性.背散射/沟道分析结果表明:随退火温度的升高,薄膜中辐照损伤减少;但当退火温度达到1000℃,薄膜中的缺陷又明显增加.Er浓度随注入深度呈现高斯分布.通过沿GaN的<0001>轴方向的沟道分析,对于900℃,30min退火的GaN:Er样品,Er在晶格中的替位率约76%.光谱研究表明:随退火温度的升高,室温下样品的红外PL峰强度增加;但是当退火温度达到1000℃,样品的PL峰强度明显下降;测量温度从15K变化到300K时,样品(900℃,30min退火的GaN:Er)的1540nm处PL温度猝灭为30%.

横向外延GaN结晶质量的同步辐射研究

采用同步辐射X光衍射技术研究了α-Al_2O_3(0001)衬底上横向外延GaN的结构特征。发现横向生长区的GaN（0001）晶面与窗口区的GaN(0001)晶面在垂直掩模方向上存在取向差。ω/2θ联动扫描发现横向生长区的GaN的衍射峰半高宽约为窗口区GaN的一半，这表明横向外延生长技术在降低GaN穿透位错密度的同时，还能大幅度提高GaN的晶粒尺寸。

(111)Si上外延生长六方GaN的TEM观察

利用衍衬、SAED、HRTEM对在（111）Si上外延生长的六方GaN进行了观察分析。GaN外延层与缓冲层和基底的取向关系为(0001)_(GaN)∥(0001)_(AlN)∥(111)_(Si)，[11(2-bar)0]_(GaN)∥[11(2-bar)0]_(AlN)∥[110]_(Si)。GaN外延层中存在倒反畴。GaN中位错以刃型位错为主。In_(0.1) Ga_(0.9) N/GaN的多重量子阱结构（MQW）具有阻挡穿透位错，降低位错密度的作用。

High-mobility Ga-polarity GaN achieved by NH3-MBE

GaN epilayers were grown on (0001) sapphire substrates by NH3-MBE and RF-MBE (radio frequency plasma). The polarities of the epilayers were investigated by in-situ RHEED, chemical solution etching and AFM surface examination. By using a RF-MBE grown GaN layer as template to deposit GaN epilayer by NH3-MBE method, we found that not only Ga-polarity GaN films were repeatedly obtained, but also the electron mobility of these Ga-polarity films was significantly improved with a best value of 290 cm(2)/V.s at room temperature. Experimental results show it is an easy and stable way for growth of high quality Ga-polarity GaN films.

Current-induced migration of surface adatoms during GaN growth by molecular beam epitaxy

The influence of electric fields on surface migration of Gallium (Ga) and Nitrogen (N) adatoms is studied during GaN growth by molecular beam epitaxy (MBE). When a direct current (DC) is used to heat the sample, long distance migration of Ga adatoms and diffusion asymmetry of N adatoms at steps are observed. On the other hand, if an alternating current (AC) is used, no such preferential adatom migration is found. This effect is attributed to the effective positive charges of surface adatoms. representing an effect of electro-migration. The implications of such current-induced surface migration to GaN epitaxy are subsequently investigated. It is seen to firstly change the distribution of Ga adatoms on a growing surface, and thus make the growth to be Ga-limited at one side of the sample but N-limited at the other side. This leads to different optical qualities of the film and different morphologies of the surface. (C) 2001 Elsevier Science B.V. All rights reserved.

R面、C面蓝宝石上生长的InGaN多层量子点的发光性质研究

采用表面钝化和MOCVD低温生长在蓝宝石（0001）面（即C面）和蓝宝石(1(1-bar)02)面（即R面）上形成了InGaN量子点，并构成了该量子点的多层结构。原子力显微镜测试的结果表明单层InGaN量子点平均宽约40nm，高约15nm；而多层量子点上层的量子点则比单层的InGaN量子点大。R面蓝宝石衬底上生长的InGaN量子点和C面蓝宝石衬底上生长的InGaN量子点相比，其PL谱不仅强度高，而且没有多峰结构。这是由于在C面蓝宝石衬底上生长的InGaN/GaN多层量子点沿生长方向[0001]存在较强的内建电场，而在R面蓝宝石衬底上得到的多层量子点沿着生长方向[11(2-bar)0]没有内建电场。InGaN量子点变温光致发光（PL）谱研究发现量子点相关的峰有快速红移现象，这是量子点系统所特有的PL谱特征。用在R面蓝宝石上生长的InGaN量子点作有源层有望避免内建电场的影响，得到高量子效率且发光波长稳定的发光器件。