Atomically resolved study of the unpinned GaN (10 1¯ 0) surface by cross-sectional scanning tunneling microscopy

Gallium nitride has been studied thoroughly in recent decades as the material is relevant for applications in blue-light-emitting devices and laser diodes, as well as high-power and high-frequency transistors. This material has rarely been explored by cross-sectional scanning tunneling microscopy (X-STM), in spite of the relevance of its m plane, which constitutes the output facet of commercial lasers and the sidewalls of GaN nanowires, and has been intensively explored as an alternative orientation for light emitters due to its nonpolar nature. Here we successfully investigate a clean m plane of wurtzite GaN and study the presence of gallium vacancies by X-STM at liquid nitrogen temperature. We observe Friedel oscillations around the defects and characterize the dependence of the oscillation radius on the applied bias. This physical phenomenon, combined with the tunneling conditions, clarifies the charge state of the vacancy and the origin of the free carriers in GaN that screen the Ga vacancies. We observe a predicted bi-stability of the N atom neighboring the Ga vacancy and perform density functional theory calculations to complement the experimental results. We obtain high-quality clean cleavages on the GaN m plane, which will allow for the study of other nanostructured GaN-based materials and devices.