Optical properties of polar and nonpolar GaN/AlN multiquantum well systems—DFT study

Abstract

Ab initio simulations were applied for nonpolar GaN/AlN superlattices of various widths for the first time giving band diagrams and optical transition oscillator strengths. The results confirmed the absence of electric fields in m-plane GaN/AlN multiquantum well (MQW) systems leading to drastically different properties of polar and nonpolar systems, narrow polar and nonpolar GaN/AlN well properties, such as bandgap or oscillator strength. Wider wells show drastic differences related to the presence of electric fields in polar systems. Wide nonpolar wells have bandgaps following parabolic dependence resulting from localization energy. Polar wide well gaps are determined by spontaneous and piezoelectric fields that reduce bandgap for wider wells and barriers. Oscillator strength shows a more dramatic difference between polar and nonpolar wells. Drastic reduction in the oscillator strength is observed for wide polar MQWs. The decrease in the oscillator strength by several orders of magnitude is observed for the increase in the width of wells and barriers. The wide polar wells should have extremely ineffective radiative emission from both direct and across the barrier overlap of electron–hole wavefunctions.