基于三轴X射线衍射方法的n-GaN位错密度的测试条件分析

三轴X射线衍射技术广泛应用于半导体材料参数的精确测试,然而应用于纤锌矿n-GaN位错密度的测试却可能隐藏极大的误差.本文采用三轴X射线衍射技术测试了两个氢化物气相外延方法生长的n-GaN样品,发现两样品对应衍射面的半高全宽都基本一致,按照镶嵌结构模型,采用Srikant方法或Williamson-Hall方法,两样品的位错密度也应基本一致.但van der Pauw变温霍尔效应测试表明,其中的非故意掺杂样品是莫特相变材料,而掺Si样品则是非莫特相变材料,位错密度有数量级的差别.实验表明,位错沿晶界生长导致的晶粒尺寸效应,表现为三轴X射线衍射技术检测不到晶界晶格畸变区域的位错,给测试带来极大误差,这对正确使用Srikant方法和Williamson-Hall方法提出了测试要求.分析表明,当扭转角与倾转角之比β_(twist)/β_(tiit)≥2.0时,Srikant方法是准确的,否则需进一步由Williamson-Hall方法确定晶粒大小(面内共格长度L//),当L//≥1.5μm时,Srikant方法是准确的.

Analyses of determination conditions of n-GaN dislocation density by triple-axis X-ray diffraction

Dislocation densities of two hydride vapor phase epitaxy-grown hexagonal GaN samples, which are Si doped and unintentionally doped respectively, are determined by triple-axis X-ray diffractometry and van der Pauw variable temperature Hall-effect measurement. The dislocation densities of these two samples should be at the same level from the X-ray testing, the ω-FWHM (full width at half maximum) values of all corresponding reflections for these two samples are almost the same. But from the Hall-effect measurements, the dislocation density values should be different from each other remarkably, because the unintentionally doped sample belongs to Mott transition material, while the Si-doped one does not. This fact indicates that the X-ray testing is perhaps inaccurate under some conditions, although the triple-axis X-ray diffractometry is a highly suitable technique for discriminating different kinds of structural defects such as edge and screw dislocations that lead to characteristic broadening of symmetric and asymmetric Bragg reflection. The experimental result obtained so far (say, for hot-electron bolometer) shows that the dislocation density value from mobility fitting model is in good accordance with that from ω-FWHM fitting using Srikant method. The anomaly that the dislocation density from ω-FWHM fitting is much lower than that from mobility fitting for the same sample (sample 59^#), indicates that dislocations located in grain boundary may not be tested by triple-axis X-ray diffractometry. According to mosaic model, the layer is assumed to consist of single crystallites, called mosaic blocks, which are assumed to be slightly misoriented with respect to each other. The out-of-plane rotation of the block perpendicular to the surface normal is of the mosaic tilt, and the in-plane rotation around the surface normal is of the mosaic twist. The average absolute values of tilt and twist angles are directly related to the FWHM values of the corresponding distributions of crystallographic orientations. So, the X-ray testing can determine the average orientation of the grains with the same interplanar distance, excluding the information about the grain boundary at which X-ray cannot interfere because of disdortion of lattice. The experimental results and calculation analyses indicate that the dislocation density value from Srikant model is accurate when the ratio of twist angle to tilt angle exceeds 2.0, or the magnitude of the lateral coherence length is larger than 1.5 μm.