一种测量纤锌矿n-GaN位错密度的新方法

A new method to determine the dislocation density in wurtzite n-GaN

采用点缺陷线性分布模型,利用能量弛豫方法得到了基于van der Pauw变温霍尔效应测量来确定纤锌矿n-GaN位错密度的新方法.用高分辨率X射线衍射仪测试了两个分别用MOCVD方法和用HVPE方法生长的n-GaN样品,用Srikant方法拟合得到了位错密度.结果表明两种方法高度一致.进一步的研究表明,新方法和化学腐蚀方法的测试结果基本一致,相关拟合参数与采用Rode迭代法精确求解Boltzmann输运方程的理论结果也基本一致.研究还表明,新方法能有效消除施主杂质带和界面简并层对测试结果的影响,测试剔除界面层影响后的整个外延层的刃、螺位错密度,而不是穿透位错密度.该方法适合霍尔迁移率曲线峰位在200 K左右及以下并且峰位明确的各种生长工艺、各种厚度、各种质量层次的薄膜和体材料,具有对迁移率曲线高度拟合,材料参数精确,计算简便、收敛速度快等优点.

We develop a new method to determine the edge and screw dislocation density in wurtzite n-GaN film. The method is to fit the van der Pauw variable temperature Hall-effect measurements with a analytic expression of low-field electron mobility in n-GaN. Our calculations take the comprehensive effect between the dislocation line and the shallow-donor defects as the main cause to depress the carrier mobility. Because of the crystal distortion near the dislocation line, the energy is so high that shallow-donor defects in the GaN crystal can be captured near the dislocation line. In other words, the shallow-donor defects distribute in lines along the dislocation line, but the shallow-donor defects along the screw and edge dislocation line have different energy levels. The shallow-donor defects take energy from lattice and the carrier, which is in relaxation process, then deliver the energy through ionizing. So, it is found that the following assumptions need to be made in order to obtain the model function for the mobility over a wide temperature range： i） there are 6 shallow-donor defect lines around one dislocation line;ii） two donor energy levels belonging to the screw and edge dislocation respectively must be taken into account; iii） the exchange energy between the carrier and the shallow-donor defect is nωLO, the energy value of polar optical phonon. Under these assumptions, experiments indicate that our calculation function can fit the experimental curve best. The values of dislocation density from our model and others determined by x-ray diffraction or by chemical etching method are in good agreement, and the values of donor energy levels from our model and Rode iterative method to solve the Boltzmann equation are also in good accordance with each other. This method is applicable for the wurtzite n-GaN films grown by various preparation technologies under any condition, which is for the sample with the peak-mobility temperature about or under 200 K, not for the sample with the peak-mobility temperature about or above 300 K, which room-temperature mobility usually is about or less than 100 cm2/（V·s）.