Spatio-time-resolved cathodoluminescence study on the midgap recombination lifetimes of nanowire-based InGaN/GaN multiple quantum shells

Spatial distributions of minority carrier lifetimes (τminority) in nanowire-based GaN and InGaN/GaN multiple quantum shells grown on a GaN template were investigated using macroarea time-resolved photoluminescence and spatio-time-resolved cathodoluminescence (STRCL) measurements. The τminority value of the GaN nanowires (approximately 101 ps) was uniformly distributed along the nanowire altitude and was longer than that of the GaN template (approximately 24 ps), reflecting the dislocation-free nature of nanowire structures. The τminority value of the InGaN wells grown under optimized conditions was also uniform along the height (approximately 86 ps). However, the value was more than one order of magnitude shorter than that of the planar c-plane single quantum well and several times shorter than that of the planar m-plane multiple quantum wells. Single τminority components observed for both GaN nanowires and InGaN wells, consistently confirmed by both macroarea time-resolved photoluminescence and STRCL measurements, indicated that spatial variations of NMGRC within individual nanowires or across the chip were likely negligible and that surface recombination effects were insignificant. Based on the quantified τminority, the concentrations of potentially dominant midgap recombination centers (MGRCs) in the GaN nanowires, which are likely divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN) [VGaVN] and/or carbon impurities resulting from extremely low V/III ratios, are estimated to be in the first half of 1016 cm−3. The MGRC concentration in the InGaN wells was estimated to be several times higher than that in the planar m-plane multiple quantum wells. Reducing the MGRC concentrations is essential for achieving highly luminescent InGaN/GaN multiple quantum shells.