Aluminium incorporation in polar, semi- and non-polar AlGaN layers: a comparative study of x-ray diffraction and optical properties

Core-shell GaN/AlGaN nanowires grown by selective area epitaxy

GaN/AlGaN core-shell nanowires with various Al compositions have been grown on GaN nanowire array using selective area metal organic chemical vapor deposition technique. Growth of the AlGaN shell using pure N₂ carrier gas resulted in a smooth surface for the nonpolar m-plane sidewalls with superior optical properties, whereas, growth using a mixed N₂/H₂ carrier gas resulted in a striated surface similar to the commonly observed morphology in the growth of nonpolar III-nitrides. The Al compositions in the AlGaN shells are found to be less than the gas phase input ratio. The systematic reduction in efficiency of Al incorporation in the AlGaN shells with increasing the Al molar flow in the gas phase is attributed to geometric loss, strain-limited Al incorporation, and increased gas phase parasitic reactions. Defect-related luminescence has been observed for AlGaN shells with Al content ≥ 30% and the origin of the defect luminescence has been determined as the (V_III-2O_N)^1- complex. Microstructural analysis of the AlGaN shells revealed that the dominant defects are partial dislocations. Growth of the nonpolar m-plane Al_xGa_1-xN/Al_yGa_1-yN quantum wells on the sidewalls of the GaN nanowires produced arrays with excellent morphology and optical emission, which demonstrated the viability of such a growth scheme for large area efficient ultraviolet LEDs as well as for next generation ultraviolet micro-LEDs.