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Nitride LEDs based on flat and wrinkled quantum wells

In this paper we propose a new type of nitride LED structure based on “wrinkled” rather than flat Multiple Quantum Wells (MQWs) grown along the polar [0001] direction. We show that this novel approach of employing “wrinkled” MQWs leads to the improvement of both the internal quantum efficiency as well the extraction efficiency. Identical GaN/AlGaN MQWs with well and barrier widths of 7 nm, were grown on both the textured and the smooth GaN templates by plasma-assisted MBE and their optical properties were evaluated by photoluminescence (PL) and cathodoluminescence (CL) measurements. The PL spectra of the smooth and “wrinkled” MQWs have significant differences. The PL from the smooth quantum wells has a single peak at 396 nm, consistent with the expected red-shift from the PL spectra of the bulk GaN films due to the quantum confined Stark effect (QCSE). The PL peak from the wrinkled QWs occurs at 358 nm, which is blue-shifted with respect to the PL spectra of the bulk GaN films, a result consistent with QWs having a square configuration. Furthermore, we found that the integrated photoluminescence intensity from the “wrinkled” multiple quantum wells (WMQWs) was about 700 times higher than that of the smooth MQWs. We attribute this significant enhancement of the photoluminescence from the WMQWs partly to enhancement in light extraction through the textured surface and partly to enhanced spontaneous emission rate. We believe that the increase in the internal quantum efficiency is due to the reduction of the QCSE, since the quantum wells are not perpendicular to polar [0001] direction. Further enhancement in internal quantum efficiency is expected due to quantum carrier confinement from "wedge" electronic eigen-modes. The latter has its origin to the transition in the carrier behavior from 2D to 1D due to the V-shaped intersecting planes of the quantum wells, and thus the "wedges" behave as quantum wires, which cause localization and trapping of excitons.