Surface control and MBE growth diagram for homoepitaxy on single-crystal AlN substrates

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.