Interface recombination in Ga- and N-polar GaN/(Al,Ga)N quantum wells grown by molecular beam epitaxy

TL;DR

This study compares the morphological, structural, and optical properties of GaN/(Al,Ga)N quantum wells grown on different polar substrates, revealing polarity-dependent differences in exciton decay and photoluminescence efficiency at room temperature.

Contribution

It provides a systematic comparison of Ga- and N-polar GaN/(Al,Ga)N quantum wells grown by molecular beam epitaxy, highlighting polarity effects on optical properties and recombination mechanisms.

Findings

N-polar MQWs show higher PL intensity at room temperature.

Exciton decay is dominated by nonradiative processes at defects.

N-polar orientation reduces carrier capture by interface traps.

Abstract

We explore and systematically compare the morphological, structural and optical properties of GaN/(Al,Ga)N multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PA-MBE) on freestanding GaN$(0001)$ and GaN$(000\bar{1})$ substrates. Samples of different polarity are found to be comparable in terms of their morphological and structural perfection and exhibit essentially identical quantum well widths and Al content. Regardless of the crystal orientation, the exciton decay in the MQWs at 10 K is dominantly radiative and the photoluminescence (PL) energy follows the quantum confined Stark effect (QCSE) for different quantum well widths. A prominent free-to-bound transition involving interface shallow donors is, however, visible for the N-polar MQWs. At room-temperature, in contrast, the exciton decay in all samples is dominated by nonradiative recombination taking place at point defects, presumably Ca or V N located at the bottom QW interface. Remarkably, the N-polar MQWs exhibit a higher PL intensity and longer decay times than the Ga-polar MQWs at room-temperature. This improved internal quantum efficiency is attributed to the beneficial orientation of the internal electric field that effectively reduces the capture rate of minority carriers by interface trap states.