Optical Characterization of Defect States within the Metamorphic GaAs_yP_1-y Alloy System

The use of lattice-mismatched (metamorphic) heteroepitaxy is an accepted approach for expanding the III-V compound semiconductor materials palette for a range of optoelectronic applications. However, this flexibility comes at the cost of unavoidable crystal defects, namely dislocations, which can be detrimental to device performance and reliability. Although the net impact of dislocations is well recognized, the details of their physical and electronic structure are generally not well understood for most III-V alloys. To this end, we are investigating dislocation-related defect states within direct-bandgap GaAs_yP_1-y — of interest for use in III-V/Si multijunction solar cells and other forms of integrated photonics — using a wide range of multi-scale, multi-modal structural (XRD, electron microscopy) and spectroscopic (PL, CL, Raman) characterization techniques, seeking to determine the impact of As/P composition, doping polarity, and threading dislocation density (TDD). To date, we have identified a number of interesting phenomena, including a previously unreported deep-level state in n-type GaAs_yP_1-y alloys that exhibits strong lattice coupling and a clear dependance of optical bandgap on both dopant polarity and TDD. This highlights a larger discrepancy between experimental and theoretical bandgap, and thus a more complex structure-property relationship in these metamorphic compounds than previously reported.