Overcoming bismuth saturation in GaAsBi by manipulating film strain

Epitaxially grown III-V-Bismides offer new band gap and lattice constant combinations for near- and mid-IR device applications on GaAs and InP substrates. High bismuth content GaAsBi is typically grown compressively strained on GaAs, though droplet-free thick films have been difficult to realize in that system. Surface Ga-Bi droplets inhibit bismuth incorporation and cause vertical phase separation, yielding films unusable for optoelectronic applications. To produce bismide films that are suitable for IR optoelectronics, we explored the growth of increasingly Bi-rich GaAsBi compounds under a variety of strain conditions ranging from highly compressively strained on GaAs to tensile strained on InGaAs buffer layers. High resolution x-ray diffraction and Rutherford backscatter spectrometry showed higher Bi content can be achieved without droplet formation by reducing film compressive strain. Atomic force microscopy and phase contrast microscopy showed reduced surface droplet density, indicating bismuth is being incorporated in the film rather than surface segregating. Overall, we show that reducing the compressive strain in GaAsBi films is a successful method for reducing droplet formation and enabling increased Bi incorporation.