Gallium nitride grown by molecular beam epitaxy at low temperatures

Growth of gallium nitride at low temperatures broadens the opportunity for its integration into optoelectronic devices that contain thermally sensitive substrates or active layers. As temperature is a very critical growth parameter, changes in crystallinity, defect density, optical, and structural properties are expected as temperatures fall below those typical of molecular beam epitaxy growth. In this contribution, energetic neutral atomic-beam lithography and epitaxy, a molecular beam epitaxy method that utilizes energetic neutral atomic nitrogen as the active nitrogen species, is used to grow gallium nitride directly on nitridized sapphire at temperatures between 800 and 200°C. Photospectroscopy, photoluminescence, Raman spectroscopy, scanning electron microscopy and X-ray diffractometry are applied to determine changes in optical, morphological and structural properties induced by the unconventional low-temperature growth process. As anticipated, we observe that defect density, disorder, and light absorptance increase as growth temperature decreases. Interestingly, X-ray diffraction and photoluminescence reveal the presence of the cubic phase of gallium nitride in films grown at low temperatures under a nitrogen-rich regime, which differs from growth conditions reported by plasma-assisted molecular beam epitaxy and metalorganic molecular beam epitaxy. These discrepancies are presented in a critical review of several studies reporting the stabilization of the cubic phase over the energetically-favored hexagonal phase, with emphasis on relation to growth temperature, Ga/N flux ratio and surface kinetics during growth.