Atomic-scale investigation and its device application for GaN crystal growth

在本项目中，我们主要研究了以下几个方面的内容：(1)建立了分子束外延扫描隧道显微镜(MBE-STM)实现GaN表面能量最小化和表面重构的一般原理；(2)了解了GaN外延层中掺杂和缺陷形成的原子过程。(3)阐明了掺杂对表面电学性质的影响。(4)阐明了干法刻蚀的原子化过程及其在器件制造中的应用。在此基础上，对器件制备的基本过程--原子相互作用、生长动力学、表面能级、应变和表面/薄膜电学性质等进行了讨论。我们利用MBE-STM对纤锌矿结构GaN(0001)的表面重构进行了系统研究。研究了GaN(0001)表面的各种重构，并在比较STM观测和第一性原理总能量计算的基础上，提出了一种适用于基本的2x2和4x4等结构的吸附原子方案。此外，为了了解纤锌矿GaN干法刻蚀过程中的原子过程，这对器件的应用具有重要意义，我们开展了Cl2分子对GaN表面的热激活干法刻蚀的STM研究。我们的广泛研究表明，在低温(~650℃)下，富Ga GaN(0001)的主要腐蚀是反台阶流腐蚀。另一方面，在较高的温度(~700℃)下，梯田上的三角形蚀坑开始形成。它的单步深度意味着逐层刻蚀。

In this project, we have studied on the following subjects as listed below; (1) establishing the general principles for the surface energy minimization and surface reconstruction of GaN surface by MBE-STM (2) understanding the atomistic process for the dopant incorporation and defect formation in GaN epitaxial layers. (3) clarifying the effect of dopant incorporation on the surface electronic properties (4) clarifying the atomistic process in dry etching and its application for the device fabrication. Based on those results, atomic interaction, growth dynamics, surface energetics, strain, and surface/film electronic properties, etc. which are the fundamental process for device fabrication, have been discussed.We have performed systematic investigations of surface reconstructions on Wurtzite GaN(0001) by MBE-STM. Various reconstructions on the GaN(0001) surface are studied and based on the comparison between the STM observations and first-principles total energy calculations, we propose an adatom scheme for the basic 2x2 and 4x4 structures, and others as well. Also, in order to understand atomistic process underlying dry etching processes of Wurtzite GaN, which is of great important for device application, we have carried out STM study of thermally activated dry etching of GaN surface by Cl_2 molecules. Our extensive studies show that at low temperature (〜650C) the dominated etching of Ga-rich GaN(0001) is anti-step-flow etching. On the other hand, at the higher temperature(〜700C), triangular etching pits on terraces start to form. Its single-step depth implies the bilayer-by-bilayer etching.