Influence of Non-stoichiometry and Local Atomic Environments on Carrier Transport in GaAs_1-x-yN_xBi_y Alloys

Due to the significant bandgap narrowing induced by the incorporation of dilute fractions of N and Bi into GaAs, dilute nitride-bismide alloys are of interest for optoelectronic devices operating in the near- to mid-infrared range. However, the low substrate temperatures (≤ 400°C) required to incorporate both N and Bi into GaAs during molecular-beam epitaxy (MBE) lead to non-stoichiometry, primarily due to excess arsenic incorporation. Thus, non-stoichiometry should be considered to understand the electronic properties of GaAsNBi. Here, we have investigated the influence of non-stoichiometry and local atomic environments on carrier transport in GaAs(N)Bi alloy films using local-electrode atom probe tomography (LEAP) and high-resolution x-ray diffraction (HRXRD) in conjunction with time-resolved THz photoconductivity measurements. Through direct measurement of the excess As concentration ([As]_excess) using LEAP, quantified by calibration using HRXRD, we demonstrate that [As]_excess increases with decreasing substrate temperature and observe that it is suppressed in layers with y_Bi > 0.035. The local concentrations of N and Bi, as well as Bi pair-correlations, are also quantified using LEAP. Using time-resolved THz photoconductivity measurements, we show that carrier transport is primarily limited by excess As, with the highest carrier mobilities for layers with y_Bi > 0.035.