Nanoscopic spectroscopy of potential barriers formed around V-pits in InGaN/GaN multiple quantum wells on moderate temperature GaN pit expansion layers

We used photoluminescence (PL) spectroscopy combined with scanning near-field optical microscopy to directly observe the potential barriers in InGaN/GaN multiple quantum wells (MQWs) on a GaN layer grown under moderate temperatures (MT-GaN) as the V-pit expansion layer. Area-averaged PL spectra revealed shoulders on the higher-energy side of the InGaN/GaN MQW emission (HE emission) even at room temperature (RT), as well as at low temperature. These HE emission regions, which possessed island-like shapes in the PL intensity maps at RT, corresponded to the dark regions in the PL intensity maps of GaN emission. Low-temperature local PL spectra acquired at the measurement points where HE emissions were observed revealed multiple HE emission peaks at two distinct energies. Most of the regions where the HE emission occurred corresponded to the dark regions of GaN emission and their outer circumference. These experimental observations demonstrate the formation of potential barriers around threading dislocations in the InGaN/GaN MQWs on an MT-GaN layer as the pit expansion layer. The energy difference between the HE emissions and the MQW emissions on the MT-GaN layer tended to increase with increasing V-pit diameter and was much larger than the energy difference for an InGaN/GaN superlattice structure. This was rationalized by the greater change in transition energy for thinner MQWs on V-pits than for planar MQWs, when the thickness in the MQWs on MT-GaN decreases owing to strain effects.