Influence of Bi Alloying on GaAs Valence Band Structure

Bi alloying is predicted to transform GaAs from a semiconductor to a topological insulator or semi-metal. To date, studies of the GaAs${}_{1-x}$Bi${}_x$ alloy band structure have been limited, and the origins of Bi-induced enhancement of the spin-orbit splitting energy, ${\mathrm{\Delta }}_{\mathrm{S}\mathrm{O}}$, are unresolved. Here, we present high-resolution angle-resolved photoemission spectroscopy (ARPES) of droplet-free epitaxial GaAs${}_{1-x}$Bi${}_x$ films with $x_{\mathrm{B}\mathrm{i}}$ = 0.06. In addition to quantifying the Bi-induced shifts of the light-hole and heavy-hole valence bands, we probe the origins of the Bi-enhanced ${\mathrm{\Delta }}_{\mathrm{S}\mathrm{O}}$. Using exact-two-component density functional theory calculations, we identify the key role of Bi p-orbitals in the upward shift of the light-hole and heavy-hole bands that results in the Bi-enhanced ${\mathrm{\Delta }}_{\mathrm{S}\mathrm{O}}$.