High Electron Mobility Al$_{x}$Ga$_{1-x}$N/GaN Heterostructures Grown by PAMBE on GaN Templates Prepared by MOCVD

A series high mobility Al$_{x}$Ga$_{1-x}$N/GaN heterostructures samples were grown on MOCVD-grown GaN templates by molecular beam epitaxy with different Al fractions(x = 0.017$\sim $0.355). The highest mobility in this series samples at liquid nitrogen temperature is 14110 cm$^{2}$/Vs with carrier concentration 2.87 x 10$^{12}$ cm$^{-2}$ and Al fraction x = 0.022. In our experiments, the carrier density decreases as Al content reduces. While the carrier density decreases from 1.54 x 10$^{13}$ cm$^{-2}$ to 2.87 x 10$^{12}$ cm$^{-2}$, the mobility increases. But as the carrier density decreases from 2.87 x 10$^{12}$ cm$^{-2}$, the mobility decreases.