利用混合极性制备多孔缓冲层及其在GaN厚膜外延中的应用

使用分子束外延(MBE)技术在(0001)面蓝宝石衬底上生长混合极性的氮化镓(GaN)薄膜,利用不同极性面的GaN薄膜在强碱溶液中腐蚀特性的差异,混和极性样品经腐蚀处理后,得到了一层具有多孔结构的GaN层.以多孔结构的GaN作为缓冲层,用卤化物气相外延(HVPE)方法生长GaN厚膜.X射线双晶衍射和光致发光等测试结果表明,多孔结构的GaN缓冲层可以有效地释放GaN厚膜和衬底之间因热膨胀系数失配产生的应力,使GaN厚膜晶体的质量得到很大提高.     

利用混合极性制备多孔缓冲层及其在GaN厚膜外延中的应用

使用分子束外延(MBE)技术在(0001)面蓝宝石衬底上生长混合极性的氮化镓(GaN)薄膜,利用不同极性面的GaN薄膜在强碱溶液中腐蚀特性的差异,混和极性样品经腐蚀处理后,得到了一层具有多孔结构的GaN层.以多孔结构的GaN作为缓冲层,用卤化物气相外延(HVPE)方法生长GaN厚膜.X射线双晶衍射和光致发光等测试结果表明,多孔结构的GaN缓冲层可以有效地释放GaN厚膜和衬底之间因热膨胀系数失配产生的应力,使GaN厚膜晶体的质量得到很大提高.     

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.     

Si衬底GaN外延材料六角形缺陷分析

通过改变AlN形核层的生长温度分别在Si(111)衬底上生长了两个GaN样品,并对GaN外延材料表面的六角形缺陷进行了分析研究。通过显微镜和扫描电镜(SEM)观测发现,AlN形核层在高温下生长时,GaN材料表面会产生大量六角形缺陷。通过电子能谱(EDS)分析得出GaN六角形缺陷中含有大量的Si元素以及少量的Ga和Al元素,其中Si元素从Si衬底中高温扩散而来。在降低AlN形核层的生长温度后,GaN材料表面的六角形缺陷随之消失。表明AlN形核层在较低的温度下生长时可以有效地抑制Si衬底表面Si原子的扩散,减少外延层中由于衬底Si反扩散引起的缺陷。     

SiC衬底AlN缓冲层的应变对GaN外延层质量的影响

在3英寸(1英寸=2.54 cm)SiC衬底上采用金属有机物化学气相沉积(MOCVD)法生长GaN外延材料。研究了AlN缓冲层的应变状态对GaN外延层应变状态和质量的影响。使用原子力显微镜和高分辨率X射线双晶衍射仪观察样品表面形貌,表征外延材料质量的变化,使用高分辨喇曼光谱仪观察外延材料应力的变化,提出了基于外延生长的应变变化模型。实验表明,GaN外延层的张应变随着AlN缓冲层应变状态的由压变张逐渐减小,随着GaN张应力的逐渐减小,GaN位错密度也大大减少,表面形貌也逐渐变好。     

MBE growth and properties of GaN and AlxGa1−xN on GaN/SiC substrates

The growth of GaN and AlGaN by molecular beam epitaxy (MBE) has been studied using GaN/SiC substrates. The GaN/SiC substrates consisted of ∼3 μm thick GaN buffer layers grown on 6H-SiC wafers by metalorganic vapor phase epitaxy (MOVPE) at Crée Research, Inc. The MBE-grown GaN films exhibit excellent structural and optical properties—comparable to the best GaN grown by MOVPE. Al_xGa_1−xN films (x ∼ 0.06-0.08) and Al_xGa_1−xN/GaN multi-quantum-well structures which display good optical properties were also grown by MBE on GaN/SiC substrates.     

Cathodoluminescence and TEM studies of HVPE GaN layers grown on porous SiC substrates

Plain-view bright-field transmission electron microscopy and cathodoluminescence are used to study the defect structure of GaN films grown by hydride vapor-phase epitaxy on porous and nonporous SiC substrates. It is shown that the use of porous substrate reduces the mosaic structure of the films. This finding supports the compliance of porous SiC substrates, which was proposed by the authors earlier. The text was submitted by the authors in English.     

Si基GaN薄膜的制备方法及结构表征

分别采用射频磁控溅射、热壁化学气相沉积(CVD)、电泳沉积法制备GaN薄膜。利用扫描电镜(SEM)、荧光光谱仪对样品进行结构、形貌和发光特性的分析比较。射频磁控溅射方法中,把SiC中间层沉淀到Si衬底上,目的是为了缓冲由GaN外延层和Si衬底的晶格失配造成的应力。结果证实了SiC中间层提高了GaN薄膜的质量。热壁化学气相沉积法制备GaN晶体膜时,选择H2作反应气体兼载体,有利于GaN膜的形成。电泳沉积法显示所得样品为六方纤锌矿结构的GaN多晶薄膜。结果表明:溅射法制备的GaN薄膜结晶效果好;CVD法制备时GaN薄膜应用范围广;电泳沉积法操作方便、简单易行。     

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).     

High quality AIN and GaN grown on compliant Si/SiC substrates by gas source molecular beam epitaxy

Epitaxial layers of AlN and GaN were grown by gas source molecular-beam epitaxy on a composite substrate consisting of a thin (250 nm) layer of silicon (111) bonded to a polycrystalline SiC substrate. Two dimensional growth modes of AlN and GaN were observed. We show that the plastic deformation of the thin Si layer results in initial relaxation of the AlN buffer layer and thus eliminates cracking of the epitaxial layer of GaN. Raman, x-ray diffraction, and cathodoluminescence measurements confirm the wurtzite structure of the GaN epilayer and the c-axis crystal growth orientation. The average stress in the GaN layer is estimated at 320 MPa. This is a factor of two less than the stress reported for HVPE growth on 6H-SiC (0001).     

Effects of TiN Buffer Layer Thickness on GaN Growth

Smooth GaN layers were successfully grown on metallic TiN buffer layers by metalorganic chemical vapor deposition (MOCVD). One important factor in controlling GaN layer smoothness was the TiN layer thickness. We investigated systematically the effects of this thickness, and found an optimal thickness of 5 nm, at which the smallest average grain size (20 nm) and smoothest surface were obtained. The TiN layers increased surface coverage with GaN hexagons at an early stage of GaN growth, indicating that enhancing the GaN nucleation is essential for smooth GaN layer growth, and small grain size and smooth surface are needed to enhance GaN nucleation. Further reduction in TiN layer thickness to 2 nm decreased the surface coverage with GaN hexagons, and a high density of grooves and holes were observed in the surface of the 2-μm-thick GaN layers. Defect structures in the GaN layers grown on the TiN layers were remarkably changed on reduction of TiN layer thickness from 5 nm to 2 nm. GaN growth was found to be sensitive to the TiN layer thickness between 2 nm and 5 nm.     

GaN growth on porous silicon by MOVPE

GaN films have been grown at 1050 °C on porous silicon (PS) substrates by metalorganic vapour phase epitaxy. The annealing phase of PS has been studied in temperature range from 300 to 1000 °C during 10 min under a mixture of ammonia (NH₃) and hydrogen (H₂). The PS samples were characterized after annealing by scanning electronic microscope (SEM). We observed that the annealing under the GaN growth conditions does not affect the porous structure.For the growth of the active GaN layer we used a thin AlN layer in order to improve wetting between GaN and PS/Si substrate. The growth of AlN and GaN films was controlled by laser-reflectometry. We estimated the porosity of PS samples from the evolution of the reflectivity signal during the AlN growth. The crystalline quality and surface morphology of GaN films were determined by X-ray diffraction and SEM, respectively. Preferential growth of hexagonal GaN with (0002) direction is observed and is clearly improved when the thickness of AlN layer increases. Epitaxial GaN layers were characterized by photoluminescence.     

AlGaN缓冲层结构对Si基GaN材料性能的影响

Si衬底与GaN之间较大的晶格失配和热失配引起的张应力使GaN外延层极易产生裂纹,如何补偿GaN所受到的张应力是进行Si基GaN外延生长面临的首要问题.采用金属有机化合物化学气相沉积(MOCVD)技术在4英寸(1英寸=2.54 cm)Si (111)衬底上制备了GaN外延材料并研究了不同AlGaN缓冲层结构对Si基GaN外延材料性能的影响,并采用高分辨X射线衍射仪(HRXRD)、原子力显微镜(AFM)、喇曼光谱以及光学显微镜对制备的GaN材料的性能进行了表征.采用3层A1GaN缓冲层结构制备了表面光亮、无裂纹的GaN外延材料,其(002)晶面半高宽为428 arcsec,表面粗糙度为0.194 nm.结果表明,采用3层A1GaN缓冲层结构可以有效地降低GaN材料的张应力和位错密度,进而遏制表面裂纹的出现,提高晶体质量.     

Interfacial effects during GaN growth on 6H-SiC

GaN growth on 6H-SiC was investigated for heterojunction device applications. Dopant diffusion and surface reactions were discovered at the GaN/SiC heterojunction. A systematic study was therefore conducted focusing on: 1) SiC substrate preparation, 2) SiC nitridation; the effect of flowing ammonia (NH₃) at 1050°C on the SiC, and 3) the conductivity type and carrier concentration of the SiC substrate. Atomic force microscopy measurements revealed that the SiC substrates became smoother after the nitridation process possibly due to nitrogen chemisorption and etching. Current-voltage and capacitance-voltage measurements on Cr-Schottky diodes made on SiC revealed evidence for an increased potential barrier in the nitrided samples that can be explained by an interfacial monolayer ofSiN_x. Furthermore, we compared GaN/SiC heterojunction n-n and n-p diodes made from direct and selective GaN growth. Capacitancevoltage measurements on GaN/SiC n-p heterojunctions indicate that the effective doping in the junction increases as the growth temperature increases. Secondary ion mass spectrometry measurements exposed a tail of Al in the GaN due to acceptor out-diffusion from the p-SiC.     

锥形图形衬底上氮化镓薄膜生长和表征

GaN films are grown on cone-shaped patterned sapphire substrates(CPSSs)by metal-organic chemical vapor deposition,and the influence of the temperature during the middle stage of GaN growth on the threading dislocation(TD)density of GaN is investigated.High-resolution X-ray diffraction(XRD)and cathodeluminescence(CL)wereusedtocharacterizetheGaNfilms.TheXRDresultsshowedthattheedge-typedislocation density of GaN grown on CPSS is remarkably reduced compared to that of GaN grown on conventional sapphire substrates(CSSs).Furthermore,whenthegrowthtemperatureinthemiddlestageofGaNgrownonCPSSdecreases,the full width at half maximum of the asymmetry(102)plane of GaN is reduced.This reduction is attributed to the enhancement of vertical growth in the middle stage with a more triangular-like shape and the bending of TDs.The CL intensity spatial mapping results also showed the superior optical properties of GaN grown on CPSS to those of GaN on CSS,and that the density of dark spots of GaN grown on CPSS induced by nonradiative recombination is reduced when the growth temperature in the middle stage decreases.     

基于蓝宝石纳米图形衬底上氮化镓生长的研究

利用两步生长法在蓝宝石纳米图形衬底（NPSS）上生长得到高质量的氮化镓薄膜。通过XRD和SEM对薄膜质量的表征和研究发现,为得到高质量的氮化镓（GaN）薄膜,在NPSS上生长时得到的最优缓冲层厚度为15nm,而在微米级尺寸的图形衬底（MPSS）上得到的最优缓冲层厚度远大于15nm。同时,在NPSS上生长氮化镓薄膜的过程中观察到一个有趣的现象,即GaN在NPSS上生长的初始阶段,氮化镓晶粒主要在图形之间的平面区域生长,极少量的GaN在衬底图形的侧面上聚集生长。这一有趣的现象明显不同于GaN在MPSS上的生长过程。接着,又在NPSS上生长了GaN基LED结构,并对其光电性能进行了研究。     

利用低温插入层改善GaN外延层晶体质量研究

采用MOCVD系统在蓝宝石衬底上生长了GaN外延薄膜,在高温GaN生长中插入了低温GaN.通过改变低温GaN的生长温度和Ⅴ/Ⅲ比得到不同样品.对样品薄膜进行了高分辨X射线衍射(HRXRD)和光致发光谱(PL)测试,PL半峰宽变化不大,XRD半峰宽有明显变化.实验结果表明,低温GaN缓冲层可以使后续生长更好,达到二维生长...     

Si衬底上侧向外延生长GaN的研究

采用条形Al掩模在Si(111)衬底上进行了GaN薄膜侧向外延的研究.结果显示,当掩模条垂直于Si衬底[11-2]方向,也即GaN[10-10]方向时,GaN无法通过侧向生长合并得到表面平整的薄膜;当掩模条平行于Si衬底[11-2]方向,也即GaN[10-10]方向时,GaN侧向外延速度较快,有利于合并得到平整的薄膜.同时,研究表明,升高温度和降低生长气压都有利于侧向生长.通过优化生长工艺,在条形Al掩模Si(111)衬底上得到了连续完整的GaN薄膜.原子力显微镜测试显示,窗口区域生长的GaN薄膜位错密度约为1×109/cm2,而侧向生长的GaN薄膜位错密度降低到了5×107/cm2以下.     

6H-SiC衬底上采用不同厚度AlN缓冲层对GaN外延层的影响

采用不同厚度AlN作为缓冲层在6H-SiC衬底上生长了GaN外延层,并利用X射线衍射,拉曼散射和透射电子显微镜等对GaN性质进行了研究。AlN缓冲层的应变状态对GaN的晶体质量和表面形貌有很大影响。较厚的AlN缓冲层会导致GaN表面出现裂纹,而太薄的AlN缓冲层会导致GaN层较高的位错密度,从而恶化器件性能。分析了GaN产生裂纹和高位错密度的机制,并采用较优厚度（100nm）的AlN缓冲层生长出高质量的GaN外延层。     

Growth of GaN on Buffer Layers with Different Polarities by Hydride Vapor-Phase Epitaxy

This paper reports the properties of GaN grown by the hydride vapor-phase epitaxy (HVPE) technique on buffer layers with different polarities. The N-, mixed-, and Ga-polarity buffer layers were grown by molecular-beam epitaxy (MBE) on sapphire (0001) substrates; then, thicker GaN epilayers were grown on these by HVPE. The surface morphology, structural, and optical properties of these HVPE-GaN epilayers were characterized by atomic force microscopy (AFM), x-ray diffraction (XRD), scanning electron microscopy, and photoluminescence (PL) spectroscopy. The results indicate that the crystallinity of these HVPE-GaN epilayers depends on the polarity of the buffer layer.