How does an In-containing underlayer prevent the propagation of defects in InGaN QW LEDs? identification of SRH centers and modeling of trap profile

Recent reports indicated that the use of an InAlN underlayer (UL) can significantly improve the efficiency of InGaN/GaN quantum well (QW) LEDs. Currently, this result is explained by considering that the UL reduces the density of nonradiative recombination centers in the QWs. However, an experimental proof of the reduction of defects in the QWs is not straightforward. In this paper, we use combined electrical (I-V), optical (L-I), capacitance (C-V), steady-state photocapacitance (SSPC) and light-assisted capacitance-voltage (LCV) measurements to explain why devices with UL have a much higher efficiency than identical LEDs without UL. Specifically, we demonstrated an improvement in both electrical and optical characteristic for the sample with the UL; SSPC measurements revealed a higher concentration of defects in the active region for the sample without the UL (9.2x1015 cm-3), compared to the sample with UL (0.8x1015 cm-3). In addition, we demonstrated that dominant defects are located near the midgap (EC 1.8 eV), thus acting as non-radiative recombination centers. Finally, we proposed a model to find the traps distribution in the active region. By comparing model and experimental results, we demonstrate the effectiveness of the UL in blocking the growth of defects in the bulk of the device, preventing their propagation towards the QW. The results presented in this paper give a proof of the effectiveness of the UL in limiting the propagation of defects towards the QWs and an experimental characterization of the related traps.