Growth of Semi-Insulating GaN Using N2 as Nucleation Layer Carrier Gas Combining with an Optimized Annealing Time

Semi-insulating （SI） GaN is grown using N2 as the nucleation layer （NL） carrier gas combined with an optimized annealing time by metalorganic chemical vapour deposition. Influence of using 1-12 and N2 as the NL carrier gas is investigated in our experiment. It is found that the sheet resistance of unintentionally doped GaN can be increased from 10^4 Ω/sq to 10^10 Ω/sq by changing the NL carrier gas from 1-12 to N2 while keeping the other growth parameters to be constant, however crystal quality and roughness of the tilm are degraded unambiguously. This situation can be improved by optimizing the NL annealing time. The high resistance of GaN grown on NL using N2 as the carrier gas is due to higher density of threading dislocations caused by the higher density of nucleation islands and small statistic diameter grain compared to the one using 1-12 as carrier gas. Annealing the NL for an optimized annealing time can decrease the density of threading dislocation and improve the tilm roughness and interface of AlGaN/GaN without degrading the sheet resistance of as-grown GaN signiticantly. High-quality SI GaN is grown after optimizing the annealing time, and AlGaN/GaN high electron mobility transistors are also prepared.     

使用AlN/GaN超晶格势垒层生长高Al组分AlGaN/GaN HEMT结构

在蓝宝石衬底上生长了以AlN/GaN超晶格准AlGaN合金作为势垒的HEMT结构材料,并与传统AlGaN合金势垒样品进行了对比.在高Al组分(≥40%)情况下,超晶格势垒样品的表面形貌明显改进,电学性能特别是2DEG面电子浓度也有所改进.对超晶格势垒生长参数进行了初步优化,使得HEMT结构薄层电阻进一步降低,最后获得了251 Ω/□的薄层电阻. 关键词： AlGaN/GaN 结构 AlN/GaN超晶格 二维电子气 高电子迁移率晶体管     

Raman Spectrum of Epitaxial Graphene on SiC （0001） by Pulsed Electron Irradiation

We report new Raman features of epitaxial graphene （EG） on Si-face 4H-SiC prepared by pulsed electron irradiation （PEI）. With increasing graphene layers, frequencies of G and 2D peaks show blue-shifts and approach those of bulk highly-oriented pyrolytic graphite. It is indicated that the EG is slightly tension strained and tends to be strain-free. Meanwhile, single Lorentzian line shapes are well fitted to the 2D peaks of EG on SiC（O001） and their full widths at half maximum decrease with the increasing graphene layers, which indicates that the multilayer EG on Si-face can also contain turbostratic stacking by our PEI route instead of only AB Bernal stacking by a traditional thermal annealing method. It is worth noting that the stacking style plays an important role on the charge carrier mobility. Therefore our findings will be a candidate for growing quality graphene with high carrier mobility both on the Si- and C-terminated SiC substrate. Mechanisms behind the features are studied and discussed.     

Enhanced Photoluminescence of InGaN/GaN Green Light-Emitting Diodes Grown on Patterned Sapphire Substrate

Green InGaN/GaN based light-emitting diodes （LEDs） are fabricated both on planar and wet-etched patterned sapphire substrates by metalorganic vapour phase epitaxy （MOVPE）. Their photoluminescence （PL） properties of the two samples are studied. The results indicate that the PL integral intensity of the green LED on the patterned substrate is nearly two times of that on the planar one within the whole measured temperature range. The enhanced PL intensity in the green LED on the patterned substrate is shown completely contributed from the extraction efficiency, but not from the internal quantum efficiency. The conclusion is supported by temperature-dependent PL analysis on the two samples, and the mechanisms axe discussed.     

Asymmetrical plasmon reflections in tapered graphene ribbons with wrinkle edges

Asymmetrical graphene plasmon reflection patterns are found in infrared near-field images of tapered graphene ribbons epitaxially grown on silicon carbon substrates. Comparing experimental data with numerical simulations, the asymmetry of these patterns is attributed to reflection of plasmons by wrinkled edges naturally grown in the graphene. These graphene wrinkles are additional plasmon reflectors with varying optical conductivity, which act as nanometer scale plasmonic modulators and thus have potential applications in photoelectric information detectors, transmitters, and modulators.     

宽禁带半导体碳化硅单晶生长和物性研究进展

本文介绍了最近几年在SiC单晶生长和晶片加工技术产业化进程中的系列进展。研究出SiC单晶生长的扩径技术,4英寸SiC晶体单晶直径达105 mm。晶体质量逐步提高,至2011年,大部分晶片微管密度小于1个/cm2,反映晶体结晶质量的X射线摇摆曲线半高宽小于20″;生长的导电型SiC晶体电阻率小于0.02Ω.cm,半绝缘型SiC晶体电阻率大于108Ω.cm,电阻率分布均匀性良好。研究出即开即用SiC晶片批量加工技术,晶片表面粗糙度低于0.2 nm,翘曲度和总厚度变化满足工业化批量生产要求。与此同时,在基础物性研究方面也取得了系列研究成果。首次在实验上给出了直接证据证明SiC晶体中的双空位能够诱导出磁性,并从理论上予以证明。利用多种方法在SiC衬底上成功制备出大面积、高质量、性能优异的石墨烯。     

Interface Properties of InAs／AlSb Superlattices Characterized by Grazing Incidence X-Ray Reflectivity

Two kinds of superlattice interfaces of InAs/AlSb superlattices are realized in an optimized interface growth process, where one is AlAs-like and the other is InSb-like grown on a relaxed AlSb buffer layer. The superlattice properties such as interface roughness and layer thickness are studied by grazing incidence x-ray reflectivity. The reflectivity curves are simulated by standard software till the simulation curves match well with the experimental curves. The simulation indicates that AlAs-like interfaces are much rougher than InSb-like interfaces. Grazing incidence x-ray reflectivity is also discussed as a powerful tool to assessing the structure properties of superlattices.     

Characteristics of High In-Content InGaN Alloys Grown by MOCVD

InN and In0.46 Ca0.54N films are grown on sapphire with a CaN buffer by metalorganic chemical vapour deposition （MOCVD）. Both high resolution x-ray diffraction and high resolution transmission electron microscopy results reveal that these films have a hexagonal structure of single crystal. The thin InN film has a high mobility of 4 75 cm^2V^-1s^-1 and that oflno.46 Gao.54N is 163 cm^2 V^-1s^-1. Room-temperat ure photoluminescence measurement of the InN film shows a peak at 0.72eV, confirming that a high quality InN film is fabricated for applications to full spectrum solar cells.     

AlGaN/AlN/GaN结构中二维电子气的输运特性

对使用金属有机物汽相沉积法生长的AlGaN/AlN/GaN结构进行的变温霍尔测量，测量结果指出在AlN/GaN界面处有二维电子气存在且迁移率和浓度在2K时分别达到了1.4×10⁴cm²·V^-1·s^-1和9.3×10¹²cm^-2，且在200K到2K范围内二维电子气的浓度基本不变，变磁场霍尔测量发现只有一种载流子(电子)参与导电.在2K温度下，观察到量子霍尔效应，Shubnikov-de Haas (SdH) 振荡在磁场约为3T时出现，证明了此结构呈现了典型的二维电子气行为.通过实验数据对二维电子气散射过程的半定量分析，推出量子散射时间为0.23ps，比以往报道的AlGaN/GaN结构中的散射时间长，说明引入AlN层可以有效减小合金散射，进一步的推断分析发现低温下以小角度散射占主导地位.     

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.