Growth and optoelectronic properties of III-nitride quaternary alloys

In_xAl_yGa_1-xN quaternary alloys with different In and Al compositions were grown on sapphire substrates with GaN buffer by metal-organic chemical vapor deposition (MOCVD). Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. Our studies have revealed that In_xAl_yGa_1-xN quaternary alloys with lattice matched with GaN (y∼4.7x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensities (or quantum efficiencies) in In_xAl_yGa_1-x-yN quaternary alloys than that of GaN. The quantum efficiency of In_xAl_yGa_1-xN quaternary alloys was also enhanced significantly over AlGaN alloys with a comparable Al content. We have also fabricated ultraviolet (UV) photoconductive detectors based on In_xAl_yGa_1-x-yN/GaN quaternary alloy heterostructures. We found that with varying In and Al compositions, the cut-off wavelength of the In_xAl_yGa_1-x-yN detectors could be varied to the deep UV range and that the responsivity of the In_xAl_yGa_1-x-yN quaternary alloys exceeded that of AlGaN alloys with comparable cut-off wavelengths by a factor of five. This showed that In_xAl_yGa_1-x-yN quaternary alloys is a very important material for solar-blind UV detector applications particularly in the deep UV range where Al rich AlGaN alloys have problems with low quantum efficiency and cracks due in part to lattice mismatch with GaN. Our results strongly suggested that In_xAl_yGa_1-x-yN quaternary alloys open a new avenue for the fabrication of many novel optoelectronic devices such as high efficient light emitters and detectors, particularly in the UV region.