Optical Phonon Hardening and Anisotropy at a Strained and Chemically Diffuse AlN/GaN Interface

A comprehensive understanding of the phonon properties at semiconductor heterointerfaces is crucial for tailoring heat control in electronic devices. We study the phonon behavior of an ultrathin AlxGa1-xN layer (∼ 4 nm in width) with variable stoichiometry and strain gradient formed at an epitaxial AlN/GaN interface. Using momentum-resolved vibrational electron energy loss spectroscopy, we measured the phonon dispersion of the AlxGa1-xN layer with nanoscale sensitivity. We find an anisotropic phonon response within the layer, with modes distinct from the surrounding materials. Insights into the partial phonon density of states of the AlxGa1-xN layer are provided. We discover that the ultrathin layer behaves as an Al0.5Ga0.5N alloy in the low energy range (<65 meV), while it exhibits significant phonon hardening in the high energy range. The roles of stoichiometry, strain, and interfacial effect are assessed in the phonon hardening. They represent key tuning parameters of phonon behavior at interfaces, with implications for electronic device performance.