AlN缓冲层厚度对脉冲激光沉积技术生长的GaN薄膜性能的影响

在蓝宝石(Al2O3)衬底上应用脉冲激光沉积技术(PLD)生长不同厚度的AlN缓冲层后进行GaN薄膜外延生长。采用高分辨X射线衍射仪(HRXRD)和扫描电子显微镜(SEM)对外延生长所得GaN薄膜的晶体质量和表面形貌进行了表征。测试结果表明: 相比直接在Al2O3衬底上生长的GaN薄膜, 通过生长AlN缓冲层的GaN薄膜虽然晶体质量较差, 但表面较平整; 而且随着AlN缓冲层厚度的增加, GaN薄膜的晶体质量和表面平整度均逐渐提高。可见, AlN缓冲层厚度对在Al2O3衬底上外延生长GaN薄膜的晶体质量和表面形貌有着重要的影响。     

AlN缓冲层厚度对脉冲激光沉积技术生长的GaN薄膜性能的影响

在蓝宝石(Al2O3)衬底上应用脉冲激光沉积技术(PLD)生长不同厚度的AlN缓冲层后进行GaN薄膜外延生长。采用高分辨X射线衍射仪(HRXRD)和扫描电子显微镜(SEM)对外延生长所得GaN薄膜的晶体质量和表面形貌进行了表征。测试结果表明:相比直接在Al2O3衬底上生长的GaN薄膜,通过生长AlN缓冲层的GaN薄膜虽然晶体质量较差,但表面较平整;而且随着AlN缓冲层厚度的增加,GaN薄膜的晶体质量和表面平整度均逐渐提高。可见,AlN缓冲层厚度对在Al2O3衬底上外延生长GaN薄膜的晶体质量和表面形貌有着重要的影响。     

多缓冲层对MOCVD生长的GaN性能的影响

采用多低温缓冲层法和高低温联合缓冲层法在 MOCVD系统上生长 Ga N外延膜 .对薄膜进行了 X射线衍射和光致发光谱 (PL)测试 ,(0 0 0 2 ) X射线摇摆曲线和 PL 谱的半高宽与常规的单低温缓冲层法制备的薄膜相比均有不同程度的改善 .实验结果表明改进的缓冲层法能提高 MOCVD生长的氮化镓外延膜晶体质量     

Growth of thick gan films on rf sputtered ain buffer layer by hydride vapor phase epitaxy

High crystalline quality thick GaN films were grown by vapor phase epitaxy using GaCl₃ and NH₃. The growth rate was in the range of 10~15 Μm/h. GaN films grown at higher temperatures (960~ 1020?C) were single crystalline with smooth surface morphologies. No chlorine impurity was incorporated in these films during growth. The best crystalline quality and surface morphology of grown films was achieved by sputtering a thin A1N buffer layer, prior to growth. According to reflection high energy electron diffraction and atomic force microscopy measurements, as-sputtered A1N buffer layer was amorphous with root means square roughness of 0.395 nm and then crystallized during the GaN growth. This improved the GaN growth due to more uniform distribution of GaN nucleation. Rutherford backscattering channeling experiments produced the lowest value from the GaN film grown on a-Al₂O₃ with a 500å A1N buffer layer at 1020?C.     

The effect of GaN and ain buffer layers on GaN film properties grown on both C-plane and A-plane sapphire

This paper presents a comparative study of the properties of GaN grown by organometallic vapor phase epitaxy, using both a GaN and A1N buffer layer, as a function of sapphire orientation (c-plane vs a-plane). Results are presented for varying the thickness of the buffer layer, varying the growth temperature of the GaN film, and also varying the ammonia/trimethylgallium mass flow ratio. The electron Hall mobilities of GaN films grown on an A1N buffer layer were, in general, higher compared to films grown using a GaN buffer layer. In addition, growth on a-plane sapphire resulted in higher quality films (over a wider range of buffer thicknesses) than growth on c-plane sapphire. The room temperature electron mobilities were also found to be dependent on, not only the growth temperature, but also the ammonia/trimethylgallium mass flow ratio.     

Structural and optical characterization of GaN grown on porous silicon substrate by MOVPE

GaN was grown on porous silicon (PS) substrates by Metalorganic Vapour Phase Epitaxy at temperature of 1050 °C. An additional AlN buffer layer is used between GaN and PS. The crystalline quality and surface morphology of GaN films were studied by X-ray diffraction and scanning electron microscope (SEM), respectively. Preferential growth of hexagonal GaN with 〈00.1〉 direction is observed and is clearly improved when the thickness of AlN buffer layer increases. Morphological changes in PS layer appearing after growth have been also discussed.GaN optical qualities were determined by photoluminescence at low and room temperature (RT).     

外延GaN基薄膜表面应变演变RHEED分析

介绍了反射式高能电子衍射仪(RHEED)衍射原理以及半导体薄膜表面原子间距与其衍射图像间距成反比例关系。分析了采用ECR-PEMOCVD生长技术,在α-Al2O3衬底上低温外延GaN基薄膜(氮化层、缓冲层、外延层)工艺过程。通过对RHEED图像分析软件获取不同工艺过程中的外延薄膜衍射条纹间距的数据分析、计算、比较,得到薄膜表面衍射图像间距的大小,依据RHEED衍射图像与原子面间距之间的对应关系,分析薄膜表面的应变状态演变情况。分析计算结果表明生长20min氮化层、20min缓冲层的表面原子层处于压应变状态,而生长180min的AlN外延层,表面则处于完全弛豫状态。     

Evolution of Threading Dislocation Density and Stress in GaN Films Grown on (111) Si Substrates by Metalorganic Chemical Vapor Deposition

We have studied the evolution of threading dislocations (TDs), stress, and cracking of GaN films grown on (111) Si substrates using a variety of buffer layers including thin AlN, compositionally graded Al _x Ga_1-x N (0 ≤ x ≤ 1), and AlN/Al _y Ga_1-y N/Al _x Ga_1-x N (0 ≤ x ≤ 1, y = 0 and 0.25) multilayer buffers. We find a reduction in TD density in GaN films grown on graded Al _x Ga_1-x N buffer layers, in comparison with those grown directly on a thin AlN buffer layer. Threading dislocation bending and annihilation occurs in the region in the graded Al _x Ga_1-x N grown under a compressive stress, which leads to a decrease of TD density in the overgrown GaN films. In addition, growing a thin AlN/Al _y Ga_1-y N bilayer prior to growing the compositionally graded Al _x Ga_1-x N buffer layer significantly reduces the initial TD density in the Al _x Ga_1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. In-situ stress measurements reveal a delayed compressive-to-tensile stress transition for GaN films grown on graded Al _x Ga_1-x N buffer layers or multilayer buffers, in comparison to the film grown on a thin AlN buffer layer, which subsequently reduces the crack densities in the films.     

GaN缓冲层对生长InN薄膜的影响

采用金属有机物化学气相沉积(MOCVD)方法生长六方相InN薄膜，利用氮化镓(GaN)缓冲层技术制备了高质量薄膜，得到了其能带带隙0.7eV附近对应的光致发光光谱(PL). 通过比较未采用缓冲层，同时采用低温和高温GaN缓冲层，以及低温GaN缓冲层结合高温退火三种生长过程，发现低温GaN缓冲层结合高温退火过程能够得到更优表面形貌和晶体质量的InN薄膜，同时表征了材料的电学性质和光学性质. 通过对InN薄膜生长模式的讨论，解释了薄膜表面形貌和晶体结构的差异.     

Effect of AlN buffer layer thickness on the properties of GaN films grown by pulsed laser deposition

The GaN films are grown by pulsed laser deposition (PLD) on sapphire, AlN(30 nm)/Al₂O₃ and AlN(150 nm)/Al₂O₃, respectively. The effect of AlN buffer layer thickness on the properties of GaN films grown by PLD is investigated systematically. The characterizations reveal that as AlN buffer layer thickness increases, the surface root-mean-square (RMS) roughness of GaN film decreases from 11.5 nm to 2.3 nm, while the FWHM value of GaN film rises up from 20.28 arcmin to 84.6 arcmin and then drops to 31.8 arcmin. These results are different from the GaN films deposited by metal organic chemical vapor deposition (MOCVD) with AlN buffer layers, which shows the improvement of crystalline qualities and surface morphologies with the thickening of AlN buffer layer. The mechanism of the effect of AlN buffer layer on the growth of GaN films by PLD is hence proposed.     

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.     

Investigations of GaN growth on the sapphire substrate by MOCVD method with different AlN buffer deposition temperatures

The effects of different AlN buffer deposition temperatures on the GaN material properties grown on sapphire substrate was investigated. At relatively higher AlN buffer growth temperature, the surface morphology of subsequent grown GaN layer was decorated with island-like structure and revealed the mixed-polarity characteristics. In addition, the density of screw TD and leakage current in the GaN film was also increased. The occurrence of mixed-polarity GaN material result could be from unintentional nitridation of the sapphire substrate by ammonia (NH₃) precursor at the beginning of the AlN buffer layer growth. By using two-step temperature growth process for the buffer layer, the unintentional nitridation could be effectively suppressed. The GaN film grown on this buffer layer exhibited a smooth surface, single polarity, high crystalline quality and high resistivity. AlGaN/GaN high electron-mobility transistor (HEMT) devices were also successfully fabricated by using the two-step AlN buffer layer.     

利用平片蓝宝石衬底制备高功率垂直结构LED

在平片蓝宝石衬底上,通过引入AlN缓冲层,优化成核层与粗糙层的生长条件,生长出了表面平整的GaN薄膜,晶体质量得到显著提升。通过引入AlN缓冲层,将X射线衍射(XRD)下样品(002)面的半高宽(FWHM)由232″降低至148″;通过减薄成核层厚度、提升粗糙层生长压力,将样品(102)面和(100)面的FWHM分别由243″和283″降低至169″和221″。研究了不同成核层和粗糙层的生长参数对GaN薄膜表面形貌的影响,随着(102)面和(100)面FWHM的降低,表面平整度亦得到改善,粗糙度由约3.8 nm下降到约1.6 nm。利用优化后的底层条件生长了高质量GaN薄膜,在3.5 A/mm^2电流密度下,与参考样品相比,制备出的LED样品的光输出功率由863 mW提升至942 mW,提升了约9%。     

GaN/AlGaN超晶格透射电镜分析

用MOCVD法生长了120周期的GaN/Al0.14Ga0.86N超晶格，激光剥离技术被有效地用于GaN/AlGaN超晶格截面透射电镜样品的制作. 用透射电子显微镜观察到了清晰的超晶格及晶胞周期结构，电子衍射也表明生长的超晶格样品质量较好. 在透射电镜图中看到了Al, Ga原子的聚居点，这些聚居点达到应力临界时将可能成为新缺陷的起点. 同时观察到GaN缓冲层中的线缺陷大多呈现弯曲的弧形，这是外延生长导致的.     

脉冲激光沉积GaN薄膜的研究进展

脉冲激光沉积(PLD)技术凭借其低温生长优势,逐步在GaN薄膜外延领域得到广泛应用。回顾了近年来PLD技术外延生长GaN薄膜的研究进展,包括新型衬底上的GaN薄膜外延研究进展,以及作为克服异质外延的重要手段——缓冲层技术的发展现状。从目前的研究进展可以看出,应用PLD技术制备GaN薄膜及其光电器件具有广阔的发展前景。     

Effects of Substrate Pretreatment Conditions on Quality of GaN Epilayer

The pure cubic GaN(c-GaN) has been grown on (001) GaAs substrates by ECR-PAMOCVD technique at low temperature using TMGa and high pure N2 as Ga and N sources,respectively.The effects of substrate pretreatment conditions on quality of cubic GaN epilayer are investigated by the measurements of TEM and XRD.It is found that hydrogen plasma cleaning,nitridation and buffer layer growth are very important for quality of cubic GaN epilayer.     

Initial stages of GaN/GaAs (100) growth by metalorganic chemical vapor deposition

We have investigated the growth of GaN buffers by metalorganic chemical vapor deposition (MOCVD) on GaAs (100) substrates. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to study the dependence of the nucleation on the growth temperature, growth rate, annealing effect, and growth time. A two-step growth sequence must be used to optimize and control the nucleation and the subsequent growth independently. The size and distribution of islands and the thickness of buffer layers have a crucial role on the quality of GaN layers. Based on the experimental results, a model was given to interpret the formation of hexagonal-phase GaN in the cubic-phase GaN layers. Using an optimum buffer layer, the strong near-band emission of cubic GaN with full-width at half maximum (FWHM) value as small as 5.6 nm was observed at room temperature. The background carrier concentration was estimated to be in the range of 10¹³ ∼ 10¹⁴ cm⁻³.     

蓝宝石图形衬底上GaN低温缓冲层的研究

低温下,在蓝宝石图形衬底上使用金属有机化学气相沉积(MOCVD)生长低温GaN(LT-GaN)缓冲层,并对其表面形貌进行了细致的观察,发现了不同于已报道的GaN选择性成核生长现象。基于不同厚度的低温GaN缓冲层生长了n型GaN(n-GaN),发现过厚或者过薄的缓冲层都会对n-GaN晶体质量产生负面影响,并结合初始成核阶段进行了原因分析。制备了基于不同厚度的n-GaN的发光二极管(LED)样品,分析了GaN晶体质量对LED输出功率的影响。同时发现,晶体质量较差的时候,光提取效率可能主导着对LED器件性能的影响。     

石墨烯上生长GaN纳米线

采用金属Ga升华法在石墨烯/蓝宝石衬底上生长了高质量GaN纳米线,研究了不同的生长条件,如NH3流量、反应时间、催化剂和缓冲层等对GaN纳米线形貌的影响,采用扫描电子显微镜(SEM)对GaN纳米线进行表征.研究发现,在适当的NH3流量且无催化剂时,衬底上可以生长出粗细均匀的GaN纳米线.反应时间为5 min时,纳米线密集分布在衬底上,表面光滑.在石墨烯/蓝宝石上预先低温生长GaN缓冲层,然后升温至1 100℃进行GaN纳米线生长,获得了具有择优取向的GaN纳米线结构.研究表明,石墨烯和缓冲层对获得GaN纳米线结构有序阵列具有重要的作用.     

Epitaxial growth of GaN layers on metallic TiN buffer layers

To improve GaN light-emitting diode light emission, we produced metal organic chemical vapor deposition (MOCVD)-grown, continuous, flat GaN layers on metallic TiN buffer layers deposited on sapphire substrates. Three important conditions were found: (a) the sapphire substrate surface plane should be (1120), (b) the TiN layer surface plane should be (111), and (c) the TiN buffer layer nitrogen content should be higher than that of stoichiometric TiN. Reduction of TiN layer thickness reduced TiN buffer layer surface roughness. Threading dislocation density in GaN layers grown on TiN buffer layers was much lower than that in GaN layers grown on AlN.