MOCVD侧向外延GaN的结构特性

侧向外延（EL0）GaN的原子力显微镜（AFM）表面形貌图像表明,ELO—GaN相当平整,而掩膜区中央有线状分布的小丘群,这是由ELO—GaN在掩膜区中央接合时晶向不一致而产生的缺陷造成的。观察ELO—GaN扫描电子显微镜（SEM）截面图,侧向接合处存在着三角空洞,并且侧向接合导致上述小丘群。X射线衍射（XRD）曲线表明,掩膜边界处的GaN晶面有倾斜。拉曼散射谱表明,掩膜区GaN质量相对于窗口区大为提高。ELO—GaN和普通外延GaN的拉曼散射谱比较表明,ELO—GaN中的应力较小,晶体质量较高。     

N极性GaN薄膜的MOCVD外延生长

采用MOCVD技术在c面蓝宝石衬底上外延制备了N极性GaN薄膜。通过KOH腐蚀的方法判定了GaN外延薄膜的极性。通过X射线双晶衍射(XRD)摇摆曲线和光致荧光(PL)谱测试研究了成核层生长时间对N极性GaN薄膜晶体质量和发光性能的影响。研究结果表明,成核层生长时间为300 s时,N极性GaN薄膜样品的位错密度最低,发光性能最好。采用拉曼(Raman)光谱对样品的应变状态进行了分析。     

侧向外延生长GaN的结构特性

研究了在刻有图形的GaN"衬底”上用HVPE方法侧向外延生长(ELO)GaN的结构特性.在SiO2衬底上侧向外延生长GaN已经实现,并得到了平面的ELO GaN薄膜.采用扫描电子显微镜、透射电微镜和原子力显微镜技术研究了这种ELO GaN材料的结构和表面形貌.原子力显微镜图像表明:在ELO范围中的4μm2面积上不存在明显的阶状形貌.透射电子显微镜的观测表明在ELO范围内位错密度很低.在接合的界面上没观察到有空隙存在.但观测到晶格的弯曲高达3.3°,这被归因为由GaN层下的"籽层”和接合界面处的水平倾料和猝灭所产生的螺旋位错的积聚.     

使用SiNx原位淀积方法生长的GaN外延膜中的应力研究

采用低压MOCVD系统,在生长过程中使用SiNx原位淀积的方法产生纳米掩模,并在纳米掩模上进行选区生长和侧向外延制备了GaN外延薄膜.使用拉曼光谱和光荧光的手段对GaN外延膜中的残余应力进行了研究.研究发现,用SiNx原位淀积出纳米掩模后,GaN生长将由二维向三维转变,直到完全合并为止.利用拉曼光谱和光荧光谱分别研究了薄膜中的残余应力,两者符合得很好;这种方法生长出的GaN薄膜的应力分布较传统的侧向外延更加均匀;并且从中发现随着生长过程中SiNx原位淀积时间的增加,生长在其上的GaN外延膜中的残余应力减小.这是因为,随着SiNx原位淀积时间的增加,SiNx纳米掩模的覆盖度也增大.因此侧向外延区的比例增大,残余应力随之减小.     

分子束外延生长SiGe合金中位错密度的研究

Schimmel腐蚀液腐蚀结合高倍光学显微镜观察发现，不同尺寸的掩膜窗内生长的SiGe外延层中的位错密度在整个外延层中从SiGe/Si界面到SiGe外延层表面由少到多，再由多到少明显地分成3个区，无掩膜窗限制的大面积区内的SiGe层则只呈现2个区，掩膜材料与掩膜窗尺寸不同，这3个区的位错密度也不同，掩膜形成过程中产生的应力对衬底晶格的影响，以及掩膜边界对衬底与外延层的影响是造成这种不同的根本原因。     

使用SiNx原位淀积方法生长的GaN外延膜中的应力研究

采用低压MOCVD系统，在生长过程中使用SiN_x原位淀积的方法产生纳米掩模，并 在纳米掩模上进行选区生长和侧向外延制备了GaN外延薄膜.使用拉曼光谱和光荧光的手段对 GaN外延膜中的残余应力进行了研究.研究发现，用SiN_x原位淀积出纳米掩模后 ，GaN生长将由二维向三维转变，直到完全合并为止.利用拉曼光谱和光荧光谱分别研究了薄 膜中的残余应力，两者符合得很好；这种方法生长出的GaN薄膜的应力分布较传统的侧向外 延更加均匀；并且从中发现随着生长过程中SiN_x原位淀积时间的增加，生长在 其上的GaN外延膜中的残余应力减小.这是因为，随着SiN_x原位淀积时间的增加 ，SiN_x纳米掩模的覆盖度也增大.因此侧向外延区的比例增大，残余应力随之减 小. 关键词： GaN x原位淀积')" href="#">SiN_x原位淀积 拉曼 光荧光 残余应力     

腐蚀时间对蓝宝石衬底上外延生长GaN质量的影响

使用熔融的KOH在高温下对c面蓝宝石衬底进行不同时间的腐蚀,借助扫描电镜、原子力显微镜对衬底表面进行了表征,然后利用金属有机物化学气相沉积设备在不同腐蚀时间的衬底样品上进行了GaN材料的外延生长。通过X射线衍射结果比较了两组衬底上外延材料的质量,利用原子力显微镜结果对外延层表面形貌进行了分析,最后论述了腐蚀时间的调整对蓝宝石衬底上外延生长氮化镓质量的影响机理。     

GaN氢化物气相外延生长系统的设计与制作

根据GaN氢化物气相外延生长(HVPE)的原理,设计制作了双温区卧式HVPE系统．根据实际生长中出现的问题和CaN样品的测试情况,对系统进行了逐步的调试和改进．     

SiNx插入层的生长位置对GaN外延薄膜性质的影响

系统研究了纳米量级的多孔 SiNx插入层生长位置对高质量GaN外延薄膜性质的影响.高分辨X射线衍射测量结果表明:SiNx插入层生长在CaN粗糙层上能够得到最好的晶体质量.利用测量结果分别计算出了螺位错和刃位错的密度.此外,GaN薄膜的光学、电学性质分别用Raman散射能谱、低温光致发光能谱和霍尔测量的方法进行了表征.实...     

SiO2对多层GaN外延片热应力分布影响的仿真分析

为了研究SiO2对多层结构GaN外延片的热应力的影响,以直径d为40 mm的GaN外延片为研究对象,利用有限元分析法分别对蓝宝石/AlN/GaN和蓝宝石/SiO2/AlN/GaN这两种光阴极组件外延片表面热应力进行理论计算和仿真。在其他结构参数相同的情况下,分别分析了两种光阴极组件外延片径向和厚度方向的应力分布,分析了外延片热应力分布及影响因素。分析结果显示:在1 200 ℃的生长温度下,径向区域内的热应力分布比较均匀,厚度方向的热应力均在衬底和外延层的界面上发生突变。最后分析了外延片生长温度、蓝宝石衬底和GaN、AlN过渡层厚度对表面热应力的影响。     

Self-organized domain formation in low-dislocation-density GaN

The growth of high-quality GaN layers on a wafer size appropriate for device applications is based on heteroepitaxy on foreign substrates. Heteroepitaxial GaN layers with low densities (below 10⁶ cm⁻²) of extended structural defects can be achieved by lateral overgrowth of mask-patterned templates or by the growth of extremely thick GaN layers as a route towards free-standing GaN-pseudosubstrates. We present the microscopic analysis of such low-dislocation-density GaN layers by means of scanning cathodoluminescence microscopy (CL). Several state-of-the-art concepts of lateral overgrowth are compared, including two-step epitaxial lateral overgrowth of stripe masks (ELO), multi-stack ELO comprising several mask layers as well as an alternative approach involving in situ SiN nano-masks. The self-organized formation of typical microscopic growth domains with characteristic optical properties is evidenced by CL for all lateral overgrowth techniques. This behavior directly fingerprints the different growth rates and the specific impurity incorporation on non-equivalent GaN facets, e.g. (0001), or , always present during lateral overgrowth and closely related to the mask geometry. Accordingly, characteristic CL line shapes found in ELO on periodic, micrometer scale mask patterns are also detected for GaN on in situ SiN nano-masks and clearly reveal the individual facet structure during overgrowth. For thick GaN layers, CL is used to detect the spontaneous appearance of inclined facets inside inverted pyramidal defects. Optimized, thick GaN layers exclusively formed by (0001)-growth are proven to be laterally homogeneous despite periodically varying residual stress and dislocation density of the underlying ELO template.     

Comparative study of different properties of GaN films grown on(0001) sapphire using high and low temperature AlN interlayers

Comparative study of high and low temperature AlN interlayers and their roles in the properties of GaN epilayers prepared by means of metal organic chemical vapour deposition on (0001) plane sapphire substrates is carried out by high resolution x-ray diffraction, photoluminescence and Raman spectroscopy. It is found that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN interlayers, which prevent the threading dislocations from extending, especially for the edge type dislocation. The analysis results based on photoluminescence and Raman measurements demonstrate that there exist more compressive stress in GaN epilayers with high temperature AlN interlayers. The band edge emission energy increases from 3.423~eV to 3.438~eV and the frequency of Raman shift of $E_{2 }$(TO) moves from 571.3~cm$^{ - 1}$ to 572.9~cm$^{ - 1}$ when the temperature of AlN interlayers increases from 700~$^{\circ}$C to 1050~$^{\circ}$C. It is believed that the temperature of AlN interlayers effectively determines the size, the density and the coalescence rate of the islands, and the high temperature AlN interlayers provide large size and low density islands for GaN epilayer growth and the threading dislocations are bent and interactive easily. Due to the threading dislocation reduction in GaN epilayers with high temperature AlN interlayers, the approaches of strain relaxation reduce drastically, and thus the compressive stress in GaN epilayers with high temperature AlN interlayers is high compared with that in GaN epilayers with low temperature AlN interlayers.     

Luminescence of a GaN grain with a nonpolar and semipolar plane in relation to microstructural characterization

We report on the growth of the high-quality GaN grain on a r-plane sapphire substrate by using a self-organized SiN interlayer as a selective growth mask.Transmission electron microscopy,scanning electron microscopy,and Raman spectroscopy are used to reveal the effect of SiN on the overgrown a-plane GaN growth.The SiN layer effectively terminates the propagation of the threading dislocation and basal plane stacking faults during a-plane GaN regrowth through the interlayer,resulting in the window region shrinking from a rectangle to a "black hole".Furthermore,strong yellow luminescence(YL) in the nonpolar plane and very weak YL in the semipolar plane on the GaN grain is revealed by cathodoluminescence,suggesting that C-involved defects are responsible for the YL.     

Anisotropic defect reduction in non-polar a-plane GaN grown by hydride vapor phase epitaxy on maskless patterned templates

Several non-polar a-plane GaN films had been grown by hydride vapor phase epitaxy (HVPE) on different designed metal organic chemical deposition (MOCVD) GaN templates, which exhibited various ridge-like sidewall facets surface morphologies. The templates induced a lateral growth at the early stage of the HVPE growth, and resulted in a kind of maskless epitaxy lateral overgrown (ELO) process. It is found that the dislocation reduced differently along [1 0 0 0] and [] directions in these HVPE a-plane GaN layers. In [0 0 0 1] direction, the dislocation reduction resulted from the optimal surface roughness value of the template. In [] direction, the inclined facet might be a main factor for the dislocation reduction in HVPE-GaN films. The maskless ELO process had a significant influence on decreasing the dislocation density.     

蓝宝石衬底上RF-MBE生长的GaN中的极性控制和螺旋位错的降低

近年来人们报道了用MBE方法生长GaN的飞速进展,利用RF-MBE方法可以获得高的GAN生长速率和高的电子迁移率.本文讨论了用RF-MBE方法在蓝宝石衬底上生长GaN过程中的极性控制和螺旋位错的降低.在充分氮化的蓝宝石衬底上直接生长GaN,使GaN的极性控制为N-极性,并用高温生长的AlN核化层实现GaN的Ga-极性.对于N-和Ga-极性的GaN这两种情况,高温生长的AlN中间迭层的引入,可以有效地抑制螺旋位错的扩散.位错的降低使GaN的室温电子迁移率得到提高,对于Ga-极性的GaN,其值为332cm²/V·s;而对于N-极性的GaN,其值为688cm²/V·s.     

高温预生长对图形化蓝宝石衬底GaN薄膜质量的提高

在图形化蓝宝石衬底生长低温缓冲层之前,通入少量三甲基镓（TMGa）和大量氨气进行短时间的高温预生长,通过改变TMGa流量制备了4个蓝光LED样品。MOCVD外延生长时使用激光干涉仪实时监测薄膜反射率,外延片使用高分辨率X射线衍射（002）面和（102）面摇摆曲线估算位错密度,并使用光致发光谱表征发光性能,制备成芯片后测试了正向电压和输出光功率。结果表明,高温预生长可促进薄膜的横向外延,使得三维岛状GaN晶粒在较小的薄膜厚度内实现岛间合并,有利于降低位错密度,提高外延薄膜质量,LED芯片的输出光功率的增强幅度达29.1%,而电学性能无恶化迹象;但高温预生长工艺中TMGa的流量应适当控制,过量的TMGa导致GaN晶粒过大,将延长岛间合并时间,降低晶体质量。     

Dynamics of threading dislocations in porous heteroepitaxial GaN films

Behavior of threading dislocations in porous heteroepitaxial gallium nitride (GaN) films has been studied using computer simulation by the two-dimensional discrete dislocation dynamics approach. A computational scheme, where pores are modeled as cross sections of cylindrical cavities, elastically interacting with unidirectional parallel edge dislocations, which imitate threading dislocations, is used. Time dependences of coordinates and velocities of each dislocation from dislocation ensembles under investigation are obtained. Visualization of current structure of dislocation ensemble is performed in the form of a location map of dislocations at any time. It has been shown that the density of appearing dislocation structures significantly depends on the ratio of area of a pore cross section to area of the simulation region. In particular, increasing the portion of pores surface on the layer surface up to 2% should lead to about a 1.5-times decrease of the final density of threading dislocations, and increase of this portion up to 15% should lead to approximately a 4.5-times decrease of it.     

Elimination of Crystallographic Wing Tilt of Canti-Bridged Epitaxial Laterally Overgrown GaN Films by Optimizing Growth Procedure

Canti-bridged epitaxial lateral overgrowth （CBELO） of GaN is performed by metalorganic chemical vapour deposition （MOCVD） on maskless V-grooved sapphire substrates prepared by wet chemical etching with different mesa widths. The wing tilt usually observed in ELO is not found in the CBELO GaN with wide mesa widths, while it can be detected obviously in the GaN with narrow mesa widths. The wing tilt of CBELO GaN grown on a grooved sapphire substrate with narrow mesa can be controlled by adjusting the thickness of the nucleation layer. The dependence of the wing tilt on the nucleation layer thickness is studied. Cross-sectional scanning electron microscopy is used to characterize the geometry of the wing regions, and double crystal x-ray diffraction is used to analyse the structural characteristics and to measure the magnitude of the crystalline wing tilt. It is found that the crystalline wing tilt can be eliminated completely by first growth of a thin nucleation GaN layer then the CBELO GaN. Possible reason and the origin of the wing tilt in CBELO GaN films are also discussed.     

Stress evolution influenced by oxide charges on GaN metal–organic chemical vapor deposition on silicon-on-insulator substrate

GaN epilayers have been deposited on silicon-on-insulator (SOI) and bulk silicon substrates. The stress transition thickness and the initial compressive stress of a GaN epilayer on the SOI substrate are larger than those on the bulk silicon substrate, as shown in in situ stress measurement results. It is mainly due to the difference of the three-dimensional island density and the threading dislocation density in the GaN layer. It can increase the compressive stress in the initial stage of growth of the GaN layer, and helps to offset the tensile stress generated by the lattice mismatch. PACS 81.15.Gh     

磁控反应溅射AlN缓冲层对GaN基LED器件性能的影响

以直流磁控反应溅射法(RMS)在图形化蓝宝石衬底上制备的AlN薄膜作为缓冲层,采用金属有机化学气相沉积法(MOCVD)外延生长了GaN基LED。与MOCVD生长的低温GaN缓冲层相比,RMS制备的AlN缓冲层具有表面更平整、颗粒更小的形核岛,有利于促进GaN外延的横向生长,减少了形核岛合并时的界面数量和高度差异,降低了缺陷和位错产生的几率。研究结果表明,溅射AlN缓冲层取代传统低温GaN缓冲层后,外延生长的GaN材料具有更高的晶体质量,LED器件在亮度、漏电和抗静电能力等光电特性上均有明显提升。