Nonpolar a-plane GaN grown on r-plane sapphire using multilayer AlN buffer by metalorganic chemical vapor deposition

Mirror-like and pit-free non-polar a-plane (1 1 −2 0) GaN films are grown on r-plane (1 −1 0 2) sapphire substrates using metalorganic chemical vapor deposition (MOCVD) with multilayer high-low-high temperature AlN buffer layers. The buffer layer structure and film quality are essential to the growth of a flat, crack-free and pit-free a-plane GaN film. The multilayer AlN buffer structure includes a thin low-temperature-deposited AlN (LT-AlN) layer inserted into the high-temperature-deposited AlN (HT-AlN) layer. The results demonstrate that the multilayer AlN buffer structure can improve the surface morphology of the upper a-plane GaN film. The grown multilayer AlN buffer structure reduced the tensile stress on the AlN buffer layers and increased the compressive stress on the a-plane GaN film. The multilayer AlN buffer structure markedly improves the surface morphology of the a-plane GaN film, as revealed by scanning electron microscopy. The effects of various growth V/III ratios was investigated to obtain a-plane GaN films with better surface morphology. The mean roughness of the surface was 1.02 nm, as revealed by atomic force microscopy. Accordingly, the multilayer AlN buffer structure improves the surface morphology and facilitates the complete coalescence of the a-plane GaN layer.     

图形硅衬底GaN基发光二极管薄膜去除衬底及AlN缓冲层后单个图形内微区发光及 应力变化的研究

研究了图形硅衬底上外延生长的氮化镓(GaN)基发光二极管(LED)薄膜、去除硅衬底后的无损自由状态LED薄膜以及去除氮化铝(AlN)缓冲层后的自由状态LED薄膜单个图形内的微区光致发光(PL)性能, 用荧光显微镜与扫描电镜观测了去除AlN缓冲层前后LED薄膜断面弯曲状况的变化. 研究结果表明: 1)去除硅衬底后, 自由支撑的LED薄膜朝衬底方向呈柱面弯曲状态, 且相邻图形的柱面弯曲方向不一致, 当进一步去除AlN缓冲层后薄膜会由弯曲变为平整; 2)LED薄膜在去除硅衬底前后同一图形内不同位置的PL谱具有显著差异, 而当去除AlN缓冲层后不同位置的PL谱会基本趋于一致; LED薄膜每一位置的PL 谱在去除硅衬底后均出现明显红移, 进一步去除AlN缓冲层后PL谱出现程度不一的微小蓝移; 3)自由支撑的LED薄膜去除AlN缓冲层后, PL光强随激光激发密度变化的线性关系增强, 光衰减得到改善.     

Effect of double AlN buffer layer on the qualities of GaN films grown by radio-frequency molecular beam epitaxy

This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire （0001） substrate with a double A1N buffer layer. The buffer layer consists of a high-temperature （HT） A1N layer and a low-temperature （LT） A1N layer grown at 800℃ and 600℃, respectively. It is demonstrated that the HT-A1N layer can result in the growth of GaN epilayer in Ga-polarity and the LT-A1N layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm^2/V.s at room temperature when the thickness of LT-A1N layer varies from 0 to 20nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilavers by the utilization of LT-A1N layer.     

Influence of AlN buffer layer thickness on structural properties of GaN epilayer grown on Si (111) substrate with AlGaN interlayer

We present the growth of GaN epilayer on Si (111) substrate with a single AlGaN interlayer sandwiched between the GaN epilayer and AlN buffer layer by using the metalorganic chemical vapour deposition. The influence of the AlN buffer layer thickness on structural properties of the GaN epilayer has been investigated by scanning electron microscopy, atomic force microscopy, optical microscopy and high-resolution x-ray diffraction. It is found that an AlN buffer layer with the appropriate thickness plays an important role in increasing compressive strain and improving crystal quality during the growth of AlGaN interlayer, which can introduce a more compressive strain into the subsequent grown GaN layer, and reduce the crack density and threading dislocation density in GaN film.     

AlN缓冲层对Si基GaN外延薄膜性质的影响

采用金属有机化合物化学气相沉积（MOCVD）方法制备了不同AlN缓冲层厚度的GaN样品,研究了AlN缓冲层厚度对GaN外延层的应力、表面形貌和晶体质量的影响。研究结果表明：厚度为15 nm的AlN缓冲层不仅可以有效抑制Si扩散,而且还给GaN外延层提供了一个较大的压应力,避免GaN薄膜出现裂纹。在该厚度AlN缓冲层上制备的GaN薄膜表面光亮、无裂纹,受到的张应力为0.3 GPa,（0002）和（1012）面的高分辨X射线衍射摇摆曲线峰值半高宽分别为536 arcsec和594 arcsec,原子力显微镜测试得到表面粗糙度为0.2 nm。     

GROWTH MODE AND SURFACE RECONSTRUCTION OF GaN(0001) THIN FILMS ON 6H-SiC(0001)

Two-dimensional growth of GaN thin films on an atomically flat C-face 6H-SiC(0001) surface prepared by ultra-high vacuum Si-etching is observed when using an AlN buffer layer in N plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy and reflection high energy electron diffraction observations reveal a series of Ga-stabilized reconstructions which are consistent with those reported for an N-polar GaN(0001) film. The result, including the effect observed previously for GaN thin film on Si-terminated 6H-SiC(0001), agrees with the polarity assignment of heteroepitaxial wurtzite GaN films on polar 6H-SiC substrates, i.e., GaN film grown on SiC(0001) is <0001> oriented (N-face) while that on SiC(0001) is <0001> oriented (Ga-face).     

The influence of AlN/GaN superlattice intermediate layer on the properties of GaN grown on Si（111） substrates

AlN/GaN superlattice buffer is inserted between GaN epitaxial layer and Si substrate before epitaxial growth of GaN layer. High-quality and crack-free GaN epitaxial layers can be obtained by inserting AlN/GaN superlattice buffer layer. The influence of AlN/GaN superlattice buffer layer on the properties of GaN films are investigated in this paper. One of the important roles of the superlattice is to release tensile strain between Si substrate and epilayer. Raman spectra show a substantial decrease of in-plane tensile strain in GaN layers by using AlN/GaN superlattice buffer layer. Moreover, TEM cross-sectional images show that the densities of both screw and edge dislocations are significantly reduced. The GaN films grown on Si with the superlattice buffer also have better surface morphology and optical properties.     

高阻GaN的MOCVD外延生长

利用金属有机化合物气相沉积(MOCVD)在蓝宝石衬底上生长了高阻GaN薄膜。对GaN成核层生长的反应室压力、生长时间和载气类型对GaN缓冲层电学特性的影响进行了分析。实验结果表明,延长GaN成核层的生长时间,降低成核层生长时的反应室压力,载气由H₂换为N₂都会得到高阻的GaN缓冲层。样品的方块电阻R_s最高为2.49×10¹¹ Ω/□。以高阻GaN样品为衬底制备了AlGaN/AlN/GaN结构HEMT器件,迁移率最高达1 230 cm²/(V·s)。     

国产SiC衬底上利用AIN缓冲层生长高质量GaN外延薄膜

采用高温AlN作为缓冲层在国产SiC衬底上利用金属有机物化学气相外延技术生长GaN外延薄膜.通过优化AlN缓冲层的生长参数得到了高质量的GaN外延薄膜,其对称(0002)面和非对称(1012)面X射线衍射摇摆曲线的半峰宽分别达到130 arcsec和252 arcsec,这是目前报道的在国产SiC衬底上生长GaN最好的...     

脉冲激光沉积GaN薄膜的结构和光学特性研究

采用准分子脉冲激光,在Si(111)衬底上生长了带有AlN缓冲层的GaN薄膜, 利用X射线衍射(XRD)、原子力显微镜(AFM)和光致发光光谱(PL)等测试手段研究了不同沉积温度所生长的GaN薄膜结构特征和光学性能.研究表明:沉积温度影响GaN薄膜结构和光学性能,黄带发射峰主要与晶体缺陷有关.在400～700℃沉积范围内随着温度升高,GaN薄膜结构和光学性能提高.     

利用MOCVD在r面蓝宝石上生长的α面GaN中两步AlN缓冲层的优化

采用两步AlN缓冲层(一层低温AlN和一层高温AlN)在r面蓝宝石衬底上生长了非极性的α面GaN,并利用高分辨X射线衍射和光致荧光谱对所生长的材料进行了研究.两步AIN缓冲层在我们之前的工作中已被证明比单步高温AlN或低温GaN缓冲层更有利于减小材料各向异性和提高晶体质量,本文进一步优化了两步AlN缓冲层的结构,并得到了各向异性更小,晶体质量更好的α面GaN薄膜.分析表明,两步AlN缓冲层中的低温AlN层在减小各向异性中起着关键作用.低温AlN层能抑制了优势方向(c轴)的原子迁移,有利于劣势方向(m轴)的原子迁移,从而减小了Al原子在不同方向迁移能力的差异,并为其后的高温AlN缓冲层和GaN层提供"生长模板",以得到各向异性更小、晶体质量更好的α面GaN材料.     

Influence of AlN Buffer Thickness on GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

Hexagonal GaN is grown on a Si(111) substrate with AlN as a buffer layer by gas source molecular beam epitaxy (GSMBE) with ammonia. The thickness of AlN buffer is changed from 9 to 72nm. When the thickness of AlN buffer is 36nm, the surface morphology and crystal quality of GaN is optimal. The in-situ reflection high energy electron diffraction (RHEED) reveals that the transition to a two-dimensional growth mode of AlN is the key to the quality of GaN. However, the thickness of AlN buffer is not so critical to the residual in-plane tensile stress in GaN grown on Si(111) by GSMBE for AlN thickness between 9 to 72nm.     

氮化铝单晶薄膜的ECR PEMOCVD低温生长研究

采用电子回旋共振等离子体增强金属有机物化学气相沉积（ECR－PEMOCVD）技术，在c轴取向的蓝宝石即α Al2O3（0001）衬底上，以氮化镓（GaN）缓冲层和外延层作为初始层，分别以高纯氮气（N2）和三甲基铝(TMAl)为氮源和铝源低温生长氮化铝（AlN）薄膜.并利用反射高能电子衍射（RHEED）、原子力显微镜（AFM）和x射线衍射（XRD）等测量结果，研究了氢等离子体清洗、氮化和GaN初始层对六方AlN外延层质量的影响，从而获得解理性与α Al2O3衬底一致的六方相AlN单晶薄膜，其XRD半高宽为1 关键词： AlN 氢等离子体清洗 氮化 GaN     

刻蚀AlN缓冲层对硅衬底N极性n-GaN表面粗化的影响

研究了等离子体刻蚀AlN缓冲层对硅衬底N极性n-GaN表面粗化行为的影响. 实验结果表明, 表面AlN缓冲层的状态对N极性n-GaN的粗化行为影响很大, 采用等离子体刻蚀去除一部分表面AlN缓冲层即可以有效提高N极性n-GaN在KOH溶液中的粗化效果, AlN缓冲层未经任何刻蚀处理的样品粗化速度过慢, 被刻蚀完全去除AlN缓冲层的样品容易出现粗化过头的现象. 经X射线光电子能谱分析可知, 等离子体刻蚀能够提高样品表面AlN缓冲层Al 2p的电子结合能, 使得样品表面费米能级向导带底靠近, 原子含量测试表明样品表面产生了大量的N空位, N空位提供电子, 使得材料表面费米能级升高, 这降低了KOH溶液和样品表面之间的肖特基势垒, 从而有利于表面粗化的进行. 通过等离子体刻蚀掉表面部分AlN缓冲层, 改善了N极性n-GaN在KOH溶液中的粗化效果, 明显提升了对应发光二级管器件的出光功率.     

Characteristics of GaN epilayer grown on Al2O3 with AlN buffer layer by molecular beam epitaxy

Double crystal rocking curve (DCRC), atomic force microscopy (AFM), scanning electron microscopy (SEM), and photoluminescence (PL) measurements on GaN epilayers grown with various thicknesses (3.4, 8.5, 17, 34, and 51 nm) of the AlN buffer layer were carried out to investigate the surface, the structural, and the optical properties of the GaN epilayers. The results of the DCRC, AFM, SEM, and PL measurements showed that the GaN epilayer grown on a 3.4 nm AlN buffer layer had the best quality. These results indicate that GaN active layers grown on 3.4 nm AlN buffer layers hold promise for potential applications in optoelectronic devices.     

MOCVD生长Si衬底上HT-AlN缓冲层低生长压力对GaN薄膜的影响

采用金属有机化学气相沉积（MOCVD）技术在Si（111）衬底上外延GaN薄膜,对高温AlN（HT-AlN）缓冲层在小范围内低生长压力（6.7~16.6 kPa）条件下对GaN薄膜特性的影响进行了研究。研究结果表明GaN外延层的表面形貌、结构和光学性质对HT-AlN缓冲层的生长压力有很强的的依赖关系。增加HT-AlN缓冲层的生长压力,GaN薄膜的光学和形貌特性均有明显改善,当HT-AlN缓冲层的生长压力为13.3 kPa时,得到无裂纹的GaN薄膜,其（002）和（102）面的X射线衍射峰值半高宽分别为735 arcsec和778 arcsec,由拉曼光谱计算得到的张应力为0.437 GPa,原子力显微镜（AFM）观测到表面粗糙度为1.57 nm。     

在6H-SiC衬底上用低压MOVPE法生长AlN和GaN的过程中用(AlN/GaN)多层缓冲层来控制残余应力

人们已提出用BAlGaN四元系材料制备紫外光谱区的光发射器件.GaN和AlN二元系是这种四元材料在器件应用中的基础材料.6H-SiC衬底在氮化物生长中因其晶格失配小是一大优势,而且SiC衬底的热膨胀系数也和AlN的很接近.然而,对于AlN外延层来说,需要控制其中的残余应力,因为在SiC衬底上直接生长的AlN外延层中存在着因晶格失配所产生的压缩应力.另一方面,在SiC衬底上直接生长的GaN外延层中存在着拉伸应力.这种拉伸应力起源于GaN比衬底有着更大的热膨胀系数.本文讨论了在6H-SiC衬底上生长的氮化物外延层中残余应力的类型、数量及控制.为此目的,提出了在6H-SiC衬底上,无论是生长AlN,还是生长GaN,都可以采用(GaN/AlN)多层缓冲层的办法,作为控制残余应力的有效方法.我们还讨论了AlN和GaN外延层的结晶质量和残余应力间的关系.     

SiC/AlN上外延GaN薄膜的黄带发光与晶体缺陷的关系

利用室温光致发光(PL)技术研究了在6HSiC(0001)上用金属有机物化学汽相沉积(MOCVD)外延生长的GaN薄膜“黄带”发光(YL)特点,与扫描电子显微镜(SEM)、X射线衍射(XRD)技术得到的GaN薄膜的表面形貌质量和内部结晶质量的结果相对照,表明“YL”发光强度与GaN薄膜的扩展缺陷多少直接相对应通过二次离子质谱(SIMS)技术获取的GaN薄膜中Ga元素深度分布揭示出镓空位(V_Ga)最可能是“YL”发光的微观来源分析认为,虽然宏观扩展缺陷(丝状缺陷、螺形位错等)和微观点缺陷V_Ga及其与杂质的络合物(complexes)都表现出与“YL”发射密切相关,但V_Ga及其与杂质的络合物更可能是“YL”发射的根本微观来源室温下获得的样品“YL”发射强度和光谱精细结构可用于分析GaN薄膜缺陷和晶体质量.     

PEALD-deposited crystalline GaN films on Si(100) substrates with sharp interfaces

Polycrystalline gallium nitride(GaN) thin films were deposited on Si(100) substrates via plasma-enhanced atomic layer deposition(PEALD) under optimal deposition parameters. In this work, we focus on the research of the GaN/Si(100)interfacial properties. The x-ray reflectivity measurements show the clearly-resolved fringes for all the as-grown GaN films, which reveals a perfectly smooth interface between the GaN film and Si(100), and this feature of sharp interface is further confirmed by high resolution transmission electron microscopy(HRTEM). However, an amorphous interfacial layer(~ 2 nm) can be observed from the HRTEM images, and is determined to be mixture of Ga_xO_y and GaN by xray photoelectron spectroscopy. To investigate the effect of this interlayer on the GaN growth, an AlN buffer layer was employed for GaN deposition. No interlayer is observed between GaN and AlN, and GaN shows better crystallization and lower oxygen impurity during the initial growth stage than the GaN with an interlayer.     

Effect of Ⅲ/Ⅴ Ratio of HT-AIN Buffer Layer on Polarity Selection and Electrical Quality of GaN Films Grown by Radio Frequency Molecular Beam Epitaxy

We investigate the effect of A/N ratio of the high temperature （HT） AIN buffer layer on polarity selection and electrical quality of GaN films grown by radio frequency molecular beam epitaxy. The results show that low Al/N ratio results in N-polarity GaN films and intermediate Al/N ratio leads to mixed-polarity GaN films with poor electrical quality. GaN films tend to grow with Ga polarity on Al-rich AIN buffer layers. GaN films with different polarities are confirmed by in-situ reflection high-energy electron diffraction during the growth process. Wet chemical etching, together with atomic force microscopy, also proves the polarity assignments. The optimum value for room-temperature Hall mobility of the Ga-polarity GaN film is 703cm^2/V.s, which is superior to the N-polarity and mixed-polarity GaN films.