Correlation between quantum well morphology, carrier localization and the optoelectronic properties of GaInNAs/GaAs light emitting diodes

Abstract

The impact of carrier localization on the optoelectronic properties of GaInNAs/GaAs quantum well (QW) light emitting diodes (LED) grown by molecular beam epitaxy with different QW morphologies is studied. The QW morphology is determined by transmission electron microscopy, and is directly related to carrier localization, which is quantified with two different approaches: analysing the broadening of the low energy side of the photocurrent (PC) spectrum, and deriving a Stokes shift from electroluminescence (EL) and PC spectroscopic measurements. Carrier localization is found to be much stronger when the transition from a two- to a three-dimensional (2D, 3D, respectively) growth mode has started, indicating that once the 3D growth appears, carrier localization is dominated by the resulting upper interface modulation or island formation, regardless of the In and N contents. Nevertheless, within a given growth mode, 2D or 3D, carrier localization increases with the alloy effective band gap wavelength, i.e., with the In and N contents. This effect is also related to the QW morphology, because high In and N contents induce compositional fluctuations, which are responsible for generating localization states in 2D QWs, and accelerate the transition to the 3D growth mode. In 3D GaInNAs/GaAs QW LEDs, the EL emission is affected by carrier localization even at room temperature, while in 2D LEDs localization is only relevant at very low temperatures. Indeed, LEDs with 3D GaInNAs/GaAs QWs have external quantum efficiencies which are determined by the presence of localized carriers in states below the conduction band edge.