Assessing the optical and optoelectronic characteristics of bulk and multiple quantum well GaAs(1-x)Bi(x)/GaAs devices for photovoltaic applications

Incorporating dilute bismuth (Bi) into GaAs is a potential route to the 1 eV sub-cell of multijunction photovoltaics. Bi alloying reduces the GaAs bandgap by creating localised states within the bandgap above the valence band edge. A 1 eV bandgap can be achieved with 6 % Bi, which induces 0.7 % strain. Replacing the Ge in GaInP/GaAs/Ge three-junction photovoltaic devices with 1 eV GaAsBi can enhance the efficiency. Furthermore, it can be utilised in the GaInP/GaAs/GaAsBi/Ge quad junction architecture, which has the potential to achieve an efficiency exceeding 44 %. Growing GaAsBi is known to be challenging. This work assesses the optical and optoelectronic characteristics of GaAsBi, with the aim of identifying areas for material development.
A series of GaAsBi samples (100 nm) grown systematically at different temperatures and Bi fluxes are characterised using electroluminescence, photoluminescence, and photocurrent techniques. The samples grown at high temperature exhibit segregation of Bi, which reduces the growth-related defects in the material. Although the bandgap decreases for the high Bi flux samples, their quality also deteriorates due to Bi-induced defects in the material.
Systematic optical and optoelectronic characterisations are conducted on GaAsBi/GaAs multiple quantum well (MQW) strained and relaxed samples to evaluate their optical quality. The MQW structures exhibit higher luminescence intensity, indicating better optical quality compared to GaAsBi bulk samples. The power-dependent photoluminescence of InGaAs/GaAs MQW structures is compared with GaAsBi/GaAs MQW structures at both room and low temperatures. This work shows that GaAsBi exhibits superior behaviour compared to the well-established InGaAs/GaAs MQW structures in terms of bandgap reduction and optical quality. In other words, GaAsBi MQW devices have the potential to achieve a 1 eV bandgap at lower compressive strain than InGaAs MQWs.