氮化镓单晶生长研究进展

氮化镓(GaN)作为第三代宽禁带半导体核心材料之一,具有高击穿场强、高饱和电子漂移速率、抗辐射能力强和良好的化学稳定性等优良特性,是制作宽波谱、高功率、高效率光电子、电力电子和微电子的理想材料.受制于氮化镓单晶衬底的尺寸、产能及成本的影响,当前的GaN基器件主要基于异质衬底(硅、碳化硅、蓝宝石等)制作而成,GaN单晶衬底的缺乏已成为制约GaN器件发展的瓶颈.近年来,国内外在GaN单晶衬底制备方面取得了较大的进展.本文综述了氮化镓单晶生长的最新进展,包括氢化物气相外延法、氨热法和钠助熔剂法的研究进展,分析了各生长方法面临的挑战与机遇,并对氮化镓单晶材料的发展趋势讲行了展望.     

GaN体单晶的氨热生长及应力调控

采用氨热法在碱性条件下生长了GaN体单晶,SEM照片显示晶体表面有大量裂纹.用拉曼光谱测量GaN晶体的E2 (high)声子拉曼峰,结果表明晶体内部应力波动较大.通过减小温度梯度、籽晶优选及降温程序优化后,得到了无裂纹的氮化镓晶体,(002)和(102)的X射线摇摆曲线FWHM分别为48 arcsec和54 arcsec,显微拉曼光谱表明经过工艺优化后的晶体应力显著降低,应力的来源主要为生长过程中杂质的引入.     

氮化镓籽晶的表面损伤处理及氨热生长研究

籽晶的表面损伤会导致后续生长的晶体位错增多.为了降低籽晶表面的损伤,通常采用粗磨-精磨-抛光的多步过程处理的晶片作为籽晶,工艺步骤多、复杂,成本高.本文采用磷酸去除表面损伤层的粗磨GaN与化学机械抛光的GaN分别作为籽晶,对比了两种籽晶氨热生长后晶体表面、生长速率、结晶质量、应力状况.光学显微镜表明两种籽晶生长后晶体的表面具有相似的丘状表面.氨热法生长速率较慢,化学机械抛光籽晶生长速率略高于粗磨籽晶.X射线单晶衍射(XRD) (002)和(102)的摇摆曲线半高宽显示抛光籽晶与粗磨籽晶生长得到GaN结晶质量基本一致.Raman E2(high)频移表明抛光籽晶生长的GaN晶体接近无应力状态,粗磨籽晶生长的晶体存在较小的压应力.     

Preparation of AlN film grown on sputter-deposited and annealed AlN buffer layer via HVPE

AlN films grown on sputter-deposited and annealed AlN buffer layer by high temperature hydride vapor phase epitaxy(HVPE)have been fabricated and structurally characterized.The crystalline quality and surface morphology of as-grown AlN films with various V/III ratios were studied and compared.The XRD results showed that the crystalline quality of the AlN film could be optimized when the growth V/III ratio was 150.At the same time,the full width at half-maximum(FWHM)values of(0002)-and(10ˉ12)-plane were 64 arcsec and 648 arcsec,respectively.As revealed by AFM,the AlN films grown with higher V/III ratios of 150 and 300 exhibited apparent hillock-like surface structure due to the low density of screw threading dislocation(TD).The defects microstructure and strain field around the HVPE-AlN/sputtered-AlN/sapphire interfaces have been investigated by transmission electron microscopy(TEM)technique combined with geometric phase analysis(GPA).It was found that the screw TDs within AlN films intend to turn into loops or half-loops after originating from the AlN/sapphire interface,while the edge ones would bend first and then reacted with others within a region of 400 nm above the interface.Consequently,part of the edge TDs propagated to the surface vertically.The GPA analysis indicated that the voids extending from sapphire to HVPE-AlN layer were beneficial to relax the interfacial strain of the best quality AlN film grown with a V/III ratio of 150.