Nanostructural characterization and two-dimensional electron-gas properties in high-mobility AlGaN∕AlN∕GaN heterostructures grown on epitaxial AlN/sapphire templates

${\mathrm{Al}}_{0.26}{\mathrm{Ga}}_{0.74}\mathrm{N}∕\mathrm{Al}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ heterostructures with a $1\mathrm{nm}$ -thick AlN interfacial layer were grown on epitaxial AlN/sapphire templates by metal-organic vapor phase epitaxy, and they exhibited excellent film qualities and very high electron mobilities, such as over $2100{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ at room temperature and over $\mathrm{25\hspace{0.17em}000}{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ at $15\mathrm{K}$ with a two-dimensional electron-gas (2DEG) density of approximately $1\times {10}^{13}∕{\mathrm{cm}}^2$ . Cross-sectional transmission electron microscopy images revealed that the thin AlN layer with a thickness of $1\mathrm{nm}$ is continuously grown between AlGaN and GaN layers with atomically abrupt and flat interfaces. The experimental and calculated results for 2DEG transport properties indicated that an AlN interfacial layer between AlGaN and GaN layers effectively suppresses alloy disorder scattering and that epitaxial AlN/sapphire templates largely contribute to the high electron mobility because they allow for the realization of a high-quality GaN channel with a low dislocation density and a smooth interface.