横向过生长 (LEO)外延 GaN材料及其生长机理

由于没有合适的衬底材料与之匹配，使外延生长的GaN材料缺陷密度很大，从而限制了它的应用。采用LEO（横向过生长外延）技术能使缺陷密度降低3－4个数量级,可生长出高质量的GaN材料。本文简要介绍了应用LEO技术生长GaN材料的现状及对生长机理研究的进展。     

用MOCVD法在LiGaO2(001)上生长GaN的研究

LiGaO2单晶是目前所知的GaN最为理想的衬底材料，本研究用金属有机物气相沉积法（MOCVD）在LiGaO2(001)衬底上进行了外延生长GaN膜的试验，生长出了表面较为平整的GaN外延膜。应用原子力显微镜（AFM）、X射线粉末衍射（XRD）和高分辨X射线双晶衍射分别对衬底对外延膜和衬底材料进行了分析测试。结果表明，用MOCVD法可以在LiGaO2(001)衬底上生长出较高质量的无掺杂GaN(0001)外延膜。但由于MOCVD法是在高温还原气氛中生长GaN外延膜的，LiGaO2在这种气氛中不够稳定，实验发现衬底材料在生长过程中部分样品发生开裂，但没有发生相变。     

高质量GaN薄膜生长工艺的研究进展

概述了GaN异质外延生长中衬底的选择以及缺陷的形成机理,从缓冲层技术、横向外延技术、柔性衬底技术等生长工艺方面综述了国内外GaN基半导体薄膜生长的最新研究和进展,并对其优缺点进行了分析比较,认为发展同质外延将有希望解决现在异质外延生长中存在的问题,最后展望了GaN基薄膜同质外延生长的前景.     

N扩散对熔液法生长GaN晶体外延速率的影响

使用钠熔液液相外延在GaN/蓝宝石衬底上生长出GaN晶体;研究晶体生长速度与生长压力的关系,使用DCXRD对样品进行表征,发现在氮气压力为3.8MPa,温度为800℃的条件下,外延速度较快且结晶质量较高;研究还发现GaN晶体外延层的生长速率不但与溶液中N的输运有关,还与熔液中N的浓度有关。     

图形化衬底对高In组分InGaN材料分子束外延(MBE)生长的影响

研究了不同衬底上MBE技术生长的InGaN的光学和结构特征。结果表明,在相同生长条件下,用图形化衬底生长的InGaN材料比用普通衬底生长的材料有较低的表面粗糙度和背景载流子浓度及较强的发光强度。通过透射电子显微镜(TEM)观察,发现普通衬底生长的InGaN内部原子错排现象严重,而采用图形化衬底生长的InGaN原子排列清晰规则。这主要是因为图形化衬底材料外延初期为横向生长模式,这种生长模式可有效地抑制穿透位错在GaN材料体系中的纵向延伸,降低GaN缓冲层中的位错密度,进而抑制外延InGaN材料中穿透位错和V型缺陷的产生。     

“第5届欧洲SiC及相关材料会议”——重点讨论衬底和外延生长

在意大利召开的(2004年)第5届欧洲SiC及相关材料会议(ECSCRM)的重点是：消除SiC衬底中的微管道，实时改变生长条件以降低GaN／Si异质结中的应变，在Si衬底上生长立方SiC。     

立式高真空MOCVD装置CaN外延生长与器件制备

自行设计了一套具有创新性的研究型立式高真空MOCVD装置,能够较好的调节反应气体的流动状态,从而在衬底上生长大面积均匀的外延层.利用该装置在蓝宝石和硅单晶衬底上成功地生长出高质量的GaN晶体薄膜.在蓝宝石衬底上生长出n、p型GaN以及多量子阱多层结构材料,并成功制备了GaN基多层量子阱结构的蓝光发光二极管,性能良好,具有实用价值.     

GaN基材料及其在短波光电器件领域的应用

GaN具有禁带宽度大，热导纺高，电子饱和漂移速度大，临界击穿电压高和介电常数小等特点，在高亮度发光二极管，短波长激光二极管，高性能紫外探测器和高温、高频、大功率半导体器件等领域有着广泛的应用前景，本文介绍了GaN基半导体材料的各种特性，材料生长以及在光电器件领域的应用，并对存在的问题和今后的发展趋势提出了自己的看法。     

GaN材料的GSMBE生长

在国内首次用ＮＨ３作氮源的ＧＳＭＢＥ方法在α－Ａｌ２Ｏ３衬底上生长出了ＧａＮ单昌外延膜。ＧａＮ生长速率可达０．５μｍ／ｈ。ＧａＮ外延膜的（０００２）双晶Ｘ射线衍射峰回摆曲线的半高宽最窄为８ａｒｃｍｉｎ。霍尔迁移率为５０ｃｍ＾２／Ｖ．ｓ。对质量好的ＧａＮ膜，室温阴性发光谱上只有一个强而锐的近岸边发光峰，谱峰位于３７２ｎｍ处，谱峰半高宽为１４ｎｍ（１２５ｍｅＶ）。     

射频等离子体分子束外延GaN的极性研究

对采用射频等离子体分子束外延（RF-plasma MBE)生长得到的GaN进行极性研究。由于镓极性（Ga-polar)比氮极性（N－polar)有更好的化学稳定性，通过比较RF－plasma MBE生长得到的不同GaN样品对光辅助湿法刻蚀的稳定性，发现缓冲层生长条件对GaN外延层的极性有着重要影响：较高缓冲层生长温度得到的GaN外延层表现为N－polar，较低缓冲层生长温度得到的GaN外延层表现为Ga-polar。     

用MOCVD法在LiGaO2(001)上生长GaN的研究

LiGaO₂单晶是目前所知的GaN最为理想的衬底材料,本研究用金属有机物气相沉积法(MOCVD)在LiGaO₂(001)衬底上进行了外延生长GaN膜的试验,生长出了表面较为平整的GaN外延膜.应用原子力显微镜(AFM)、X射线粉末衍射(XRD)和高分辨X射线双晶衍射分别对衬底对外延膜和衬底材料进行了分析测试.结果表明,用MOCVD法可以在LiGaO₂(001)衬底上生长出较高质量的无掺杂GaN(0001)外延膜.但由于MOCVD法是在高温还原气氛中生长GaN外延膜的,LiGaO₂在这种气氛中不够稳定,实验发现衬底材料在生长过程中部分样品发生开裂,但没有发生相变.     

The influence of reactor height adjustment on properties in GaN films grown on 6H-SiC by metal organic chemical vapor deposition

The influence of reactor height adjustment on properties in GaN films grown on 6H-SiC by metal organic chemical vapor deposition (MOCVD) was investigated. The property of GaN epilayer was investigated by atomic force microscopy, X-ray diffraction, low-temperature (10 K) photoluminescence and the Raman scattering. It is found that, as the spacing between showerhead and susceptor decreased, the growth rate increased and the tensile stress decreased. This result may be useful to control the stress in GaN thin films grown on silicon carbide substrate by MOCVD.     

High‐Brightness Blue Light‐Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer

Single‐crystalline GaN‐based light‐emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid‐state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high‐quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high‐brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal–organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In_xGa_1?xN/GaN multiple quantum well structures. The as‐fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one‐step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high‐brightness LEDs.     

Aqueous lateral epitaxy overgrowth of ZnO on (0001) GaN at 90 °C: Part I. Increasing the critical thickness

Thick, epitaxial ZnO thin films have been grown on (0001) GaN buffered Al₂O₃ substrates using an aqueous solution at 90 °C. Films with improved structural, optical and electrical characteristics, were grown using a lateral epitaxial overgrowth (LEO) method. Different photoresist masks were used to enable LEO. The masks included linear windows and two different hexagonal arrays of circular windows. Films that exceeded a critical thickness mechanically failed through buckling, consistent with the large compressive stresses expected due to the mismatch of the ZnO lattice with the underlying GaN substrate. It was shown that improved mechanical stability could be achieved using the LEO method. Without LEO, a film thickness no greater than 4 µm could be grown without buckling. The critical thickness could be increased to 10 µm using linear windows, whereas a critical thickness of 50 μm was achieved with one array of circular windows, and 80 µm for a second array. The two different arrays of circular windows differed relative to their orientation on the substrate. It was also shown that the critical thickness increased with increasing distance between the growth windows. Optical transmission, micro-photoluminescence and Hall Effect measurements showed that the LEO method also results in improved optoelectronic properties.     

MOCVD growth of GaN layer on InN interlayer and relaxation of residual strain

100 nm InN layer was grown on sapphire c-plane using a metal-organic chemical vapor deposition (MOCVD) system. Low temperature (LT) GaN layer was grown on InN layer to protect InN layer from direct exposure to hydrogen flow during high temperature (HT) GaN growth and/or abrupt decomposition. Subsequently, thick HT GaN layer (2.5 μm thick) was grown at 1000 °C on LT GaN/InN/sapphire template. Microstructure of epilayer-substrate interface was investigated by transmission electron microscopy (TEM). From the high angle annular dark field TEM image, the growth of columnar structured LT GaN and HT GaN with good crystallinity was observed. Though thickness of InN interlayer is assumed to be about 100 nm based on growth rate, it was not clearly shown in TEM image due to the InN decomposition. The lattice parameters of GaN layers were measured by XRD measurement, which shows that InN interlayer reduces the compressive strain in GaN layer. The relaxation of compressive strain in GaN layer was also confirmed by photoluminescence (PL) measurement. As shown in the PL spectra, red shift of GaN band edge peak was observed, which indicates the reduction of compressive strain in GaN epilayer.     

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photoluminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system.     

On the interpretation of structural and light emitting properties of InGaN/GaN epitaxial layers grown above and below the critical layer thickness

In this contribution, the optical and structural properties of InGaN/GaN layers grown by metal-organic chemical vapour deposition (MOCVD) are studied. The main focus of this investigation is on the difference between microstructural and luminescence characteristics, for layers grown below and above the critical layer thickness (CLT) for elastic strain relaxation. By comparing the photoluminescence properties of samples grown under the same nominal conditions, except the deposition time, it is shown that in InGaN films grown above the CLT (x), an additional lower energy secondary luminescence component emerges. Specifically, for an InGaN layer with x∼0.1, the energy splitting between the two components is about 160 meV. The surface of samples with thicknesses larger than CLT(x), are found to be rough with pronounced islanding occurring, indicating that a Stranski-Krastanow 2D to 3D growth mode transition takes place after the CLT. A detailed structural characterization by high-resolution reciprocal space mapping reveals that the appearance of 3D islands is associated with elastic strain relaxation. Strong lateral and depth variations of the strain field associated to a peculiar 2D/3D growth mode can explain structural and optical properties, which are typically considered “anomalous” and frequently ascribed to phase segregation effects in InGaN. A simple calculation based on elastic strain relaxation, accounts for the observed energy splitting on the photoluminescence.     

AlGaN/GaN-heterostructures on (111) 3C-SiC/Si pseudo substrates for high frequency applications

We present the realization of high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures, which were grown on silicon substrates using an ultrathin SiC transition layer. The growth of AlGaN/GaN heterostructures on 3C-SiC(111)/Si(111) was performed using metalorganic chemical vapour deposition (MOCVD). The 3C-SiC(111) transition layer was realized by low pressure CVD and prevented Ga-induced meltback etching and Si-outdiffusion in the subsequent MOCVD growth. The two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface showed an electron sheet density of 1.5 × 10¹³ cm^− 3 and a mobility of 870 cm²/Vs. The HEMTs DC and RF characteristics were analysed and showed a peak cut-off frequency as high as 29 GHz for a 250 nm gate length.