Fabrication of In_xGa_1-xN/GaN QDs with InAlGaN capping layer by coaxial growth on non-(semi-) polar n-GaN NWs using metal organic chemical vapor deposition for blue emission

This article demonstrates for the first time the merits of an immediate InAlGaN capping layer over self-assembled In_xGa_1-xN/GaN quantum dots (QDs) coaxially grown on the m-plane and r-plane of n-GaN nanowires on Si (111) substrate using metal organic chemical vapor deposition. For comparative analysis, we prepared InGaN/GaN QD samples both with and without quaternary capping. InAlGaN capping layer acted as a strain-driven phase separation alloy. Inhomogeneous surface strain over the dots helped this quaternary alloy in forming an indium concentration gradient over In_xGa_1-xN QDs and thus, indium out-diffusion from the dots was reduced. Quaternary alloy capped samples exhibited vertically stacked, highly dense, pyramidal In_xGa_1-xN/GaN QDs of improved carrier confinement grown as the active region on n-GaN NWs. In contrast, the nonexistence of InAlGaN capping over InGaN/GaN QDs caused deformation of the dots due to In-Ga inter-diffusion between the dots and the GaN barrier layer. Three kinds of In_xGa_1-xN/GaN QDs of different x with an InAlGaN capping layer were fabricated coaxially on n-GaN nanowire, whose emission wavelength were 380 nm, 450 nm and 510 nm respectively. These coaxially fabricated In_xGa_1-xN/GaN QDs on defect free n-GaN nanowires have various excellent characteristics and can be widely applicable to new optoelectronics semiconductor devices.