Effect of Ga-rich growth conditions on the optical properties of GaN films grown by plasma-assisted molecular beam epitaxy

Abstract

While it is well known that high quality III–nitride material is deposited by plasma-assisted molecular beam epitaxy (PA-MBE) only under III-rich conditions, the extent of III-flux required for optimal film growth is unclear. In this article, we focus on the dependence of the optical properties of the material on the III/V flux ratio. Specifically, we report on the photoluminescence (PL) intensity and the lifetime of photogenerated carriers in homoepitaxially grown GaN films, deposited under Ga-rich conditions, as a function of the Ga flux employed during growth. The samples investigated consist of ∼1.2 μm thick GaN films deposited homoepitaxially on ∼85 μm thick GaN templates grown by the hydride vapor phase epitaxy method. The films were deposited under Ga-rich conditions but without the formation of metal droplets. The Ga flux was varied from 0.89 to 1.0×10−6 Torr while the N-plasma conditions (300 W, 0.6 sccm) were held constant. The intensity of the room temperature PL measured for films grown under intermediate flux (9.2×10−7 Torr) conditions is observed to be 3.5–4 times brighter than what is seen for films grown under the lower (8.9×10−7 Torr) and higher (1×10−6 Torr) flux conditions. Similarly, the lifetime of photogenerated carriers, as measured by pump–probe time-resolved reflectivity, in films grown at the higher Ga flux was found to be ∼2 times smaller than that observed for films grown at the intermediate flux. The dependence of the carrier lifetime on the density of photogenerated carriers in these films suggests that films grown under high flux conditions have a larger density of deep trap states than films grown under intermediate conditions. These results can be explained by the formation of defects under Ga-rich growth conditions that suppress band-to-band recombination in these materials, and are important for the development of PA-MBE-grown optoelectronic devices.