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.     

蓝宝石上横向外延GaN薄膜

在蓝宝石衬底上利用金属有机物气相外延(MOCVD)方法对横向外延(ELO)GaN薄膜的生长条件进行了研究.在蓝宝石衬底上利用化学腐蚀的方法刻饰出图案,再沉积低温GaN缓冲层作为外延层的子晶层,以降低外延层与衬底的晶格失配与热失配,制备出低位错密度的GaN外延层.分别利用X射线衍射、原子力显微镜及湿法腐蚀对外延层进行检测.     

蓝宝石上横向外延GaN薄膜

在蓝宝石衬底上利用金属有机物气相外延(MOCVD)方法对横向外延(ELO)GaN薄膜的生长条件进行了研究.在蓝宝石衬底上利用化学腐蚀的方法刻饰出图案,再沉积低温GaN缓冲层作为外延层的子晶层,以降低外延层与衬底的晶格失配与热失配,制备出低位错密度的GaN外延层.分别利用X射线衍射、原子力显微镜及湿法腐蚀对外延层进行检测.     

ZnO缓冲层的厚度对HVPE-GaN外延层的影响

为了得到高质量的GaN材料,首先在c面蓝宝石(Al2O3)衬底上射频磁控溅射不同厚度的ZnO缓冲层,然后采用氢化物气相外延(HVPE)法在ZnO缓冲层上生长约5.2μm厚的GaN外延层,研究ZnO缓冲层的厚度对GaN外延层质量的影响。用微分干涉显微镜(DIC)、扫描电子显微镜(SEM)、X射线衍射(XRD)和光致发光(PL)技术研究分析了GaN外延层的表面形貌、结晶质量和光学特性。结果表明,ZnO缓冲层的厚度对GaN外延层的特性有着重要的影响,200 nm厚的ZnO缓冲层最有利于高质量GaN外延层的生长。     

MOVPE生长的GaN中有关Zn的电致发光特性

GaN是制造发蓝光器件的理想材料之一。在蓝宝石(0001)(C-面)衬底上用HVPE法(氢气气相外延法)己生长出GaN单晶。但是,要获得具有无裂痕的表面平滑的高质量GaN薄膜是相当困难的,这是因为在外延膜和衬底之间的热膨胀系数不同以及存在着较刀的晶格失配。在用MOVPE生长GaN之前预先沉积的AlN薄膜缓冲层对改进表面形态及未掺杂GaN膜的结晶质量有明     

射频溅射ZnO作缓冲层的HVPE法生长GaN厚膜

在低温HVPE-GaN/c-Al2O3模板上射频溅射ZnO作为缓冲层,采用氢化物气相外延(HVPE,hydridevapoarphaseepitaxy)法外延生长了高质量的GaN320μm厚膜。用高分辨率双晶X射线衍射仪(DCXRD)、原子力显微镜(AFM)和扫描电子显微镜(SEM)分析了制备的GaN厚膜特性。结果表明,GaN(0002)面的X射线摇摆曲线衍射峰半高宽(FWHM)为336.15arcsec,穿透位错密度(TDD)为107cm-2,外延生长的GaN厚膜晶体质量较好,可以作为自支撑GaN衬底。     

正晶向SiC衬底上表面光滑的N极性GaN薄膜材料的生长

采用MOCVD技术在101.6 mm(4英寸)正晶向半绝缘C面SiC衬底上生长了厚度为1μm的GaN缓冲层。通过深入研究不同AlN成核层上生长的GaN缓冲层的性质,揭示了正晶向SiC衬底上氮极性GaN的生长机制,发现成核层表面AlN岛的密度是决定缓冲层表面形貌的重要参数,对后续GaN的成核以及GaN岛间的合并具有显著影响。通过在AlN成核层外延期间引入N2作为载气并且以脉冲方式供应铝源,研制的成核层表面成核岛尺寸小且致密性高,有利于促进GaN成核岛之间的横向合并,并抑制GaN缓冲层的岛状生长模式,从而获得了表面光滑的N极性GaN薄膜材料,原子力显微镜20μm×20μm扫描范围下的均方根(rms)粗糙度值为1.5 nm。此外,在表面平滑的GaN薄膜上生长的GaN/AlGaN异质结的电子输运特性也同步得到提升,这有利于N极性GaN材料的微波应用。     

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。     

蓝宝石图形衬底上利用AlN缓冲层生长高质量GaN

在图形化衬底上以AlN作为缓冲层生长高质量的GaN薄膜,国内相关的报道较少。通过引入两步缓冲层生长方法,在蓝宝石图形衬底上生长基于AlN缓冲层的高质量GaN薄膜,利用低温AlN层生长时内部的缺陷,选择性进行腐蚀,形成衬底与外延层界面间的侧向倒斜角,提高光萃取效率;同时在其上继续生长高温AlN,为后续GaN薄膜提供高质量模板。从外延角度出发,以表面形貌及其上生长的GaN薄膜的晶体质量为衡量依据,优化了低温AlN缓冲层以及高温AlN缓冲层的生长参数,优化后LED样品在20 mA测试电流下的光输出功率较参考样品提升了4%。     

Gradual tilting of crystallographic orientation and configuration of dislocations in GaN selectively grown by vapour phase epitaxy methods

Cross-sectional transmission electron microscope (TEM) observation was performed for selectively grown gallium nitride (GaN) in order to examine the dependence of GaN microstructure on the growth conditions. The GaN films were grown by hydride vapour phase epitaxy (HVPE) or metalorganic vapour phase epitaxy (MOVPE) on GaN covered with a patterned mask. Thin foil specimens for TEM observation were prepared with focused ion beam (FIB) machining apparatus. It was demonstrated that the c-axis of GaN grown over the terrace of the mask tilts towards the centre of the terrace when the GaN is grown in a carrier gas of N2. The wider terrace results in a larger tilting angle if other growth conditions are identical. The tilting is attributed to 'horizontal dislocations' (HDs) generated during the overgrowth of GaN on the mask terrace. The HDs in HVPE-GaN have a semi-loop shape and are tangled with one another, while those in MOVPE-GaN are straight and lined up to form low-angle grain boundaries.     

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.     

Ultraviolet luminescence of thin GaN films grown by radical-beam gettering epitaxy on porous GaAs(111) substrates

GaN films with a thickness of 0.1 μm were grown by radical-beam gettering epitaxy on porous GaAs(111) substrates. Excitonic luminescence bands are dominant in the photoluminescence spectra measured at 4.2 K. The energy positions of excitonic-band peaks are analyzed; as a result, it is concluded that there are stresses in the grown GaN films. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 11, 2003, pp. 1303–1304. Original Russian Text Copyright ? 2003 by Kidalov, Sukach, Revenko, Potapenko.     

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)村底蒸发了Au/Cr和Au/Ti/AI/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.两种缓冲层的表面部比较平整和均匀,都是具有Au(111)面掸优取向的立方相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AIN缓冲层直接生长GaN,得到的是多品GaN;先在800℃生长一层AIN缓冲层,然后在710℃生长GaN,得到的足沿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.     

Giant traps on the surface of hydride vapor phase epitaxy-grown free-standing GaN

Extended defects on the top surface of a 250-μm-thick free-standing GaN sample, grown by hydride vapor phase epitaxy (HVPE), were studied by deep level transient spectroscopy (DLTS) and scanning surface potential microscopy (SSPM). For comparison, similar studies were carried out on as-grown HVPE-GaN samples. In addition to the commonly observed traps in as-grown HVPE-GaN, the DLTS measurements on free-standing GaN reveal a very high concentration of deep traps (∼1.0 eV) within about 300 nm of the surface. These traps show nonexponential capture kinetics, reminiscent of those associated with large defects, that can accumulate multiple charges. The SSPM measurements clearly reveal the presence of charged microcracks on the top surface of the sample. It appears that the “giant traps” may be associated with these microcracks, but we cannot rule out the involvement of other extended defects associated with the near-surface damage caused by the polishing/etching procedure.     

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.     

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.     

Growth of GaN on porous SiC and GaN substrates

We have studied the growth of GaN on porous SiC and GaN substrates, employing both plasma-assisted molecular-beam epitaxy (PAMBE) and metal-organic chemical-vapor deposition (MOCVD). For growth on porous SiC, transmission electron microscopy (TEM) observations indicate that the epitaxial-GaN growth initiates primarily from surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. The dislocation density in the GaN layers is similar to, or slightly less than, that observed in layers grown on nonporous substrates. For the case of GaN growth on porous GaN, the overgrown layer replicates the underlying dislocation structure (although considerable dislocation reduction can occur as this overgrowth proceeds, independent of the presence of the porous layer). The GaN layers grown on a porous SiC substrate were found to be mechanically more relaxed than those grown on nonporous substrates; electron-diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature.     

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.