Quantum dots-templated growth of strain-relaxed GaN on a c-plane sapphire by radio-frequency molecular beam epitaxy

We investigated the quantum dots-templated growth of a (0001) GaN film on a c-plane sapphire substrate. The growth was carried out in a radio-frequency molecular beam epitaxy system. The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images, as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer. The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra. The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves. Meanwhile, the threading dislocation density in the quantum dots-templated film was estimated to be 7.1×10⁷ cm^-2, which was significantly suppressed compared with that of the AlN-buffered GaN film. The room-temperature Hall measurement showed an electron mobility of up to 1860 cm²/V·s in the two-dimensional electron gas at the interface of the Al_0.25Ga_0.75N/GaN heterojunction.