The evolution of surface morphology for single-crystal bulk Al-polar aluminum nitride substrates during ex situ cleaning, in situ cleaning, and subsequent homoepitaxy is investigated. Ex situ acid treatment is found to reveal atomic steps on the bulk AlN substrates. After in situ Al-assisted cleaning at high temperatures in a high vacuum environment monitored with reflection high-energy electron diffraction, cleaner atomic step edges are observed. Subsequent growth on the cleaned bulk AlN by molecular beam epitaxy is used to develop a phase-diagram for homoepitaxy on AlN single crystals. Secondary ion mass spectrometry profiles reveal high-purity epitaxial layers with undesired chemical impurity densities of Si, O, and C to be below detection limits. The grown homoepitaxial films are observed to oxidize in the ambient environment, but repeating the ex situ acid treatment again reveals atomic steps.

中文翻译：

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单晶 AlN 衬底上同质外延的表面控制和 MBE 生长图

研究了单晶块状铝极性氮化铝衬底在异位清洗、原位清洗和随后的同质外延过程中表面形貌的演变。发现异位酸处理揭示了块状 AlN 衬底上的原子台阶。在用反射高能电子衍射监测的高真空环境中高温原位铝辅助清洁后，观察到更清洁的原子台阶边缘。随后通过分子束外延在清洁的块状 AlN 上生长，用于开发 AlN 单晶同质外延的相图。二次离子质谱图显示高纯度外延层的 Si、O 和 C 化学杂质密度低于检测限。观察到生长的同质外延膜在周围环境中氧化，