Bi alloying is predicted to transform GaAs from a semiconductor to a topological insulator or semimetal. To date, studies of the ${\mathrm{GaAs}}_{1-x}{\mathrm{Bi}}_x$ alloy band structure have been limited, and the origins of Bi-induced enhancement of the spin-orbit splitting energy, ${\mathrm{\Delta }}_{\mathrm{SO}}$, are unresolved. Here, we present high-resolution angle-resolved photoemission spectroscopy of droplet-free epitaxial ${\mathrm{GaAs}}_{1-x}{\mathrm{Bi}}_x$ films with $x_{\mathrm{Bi}}=0.06$. 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 ${\mathrm{\Delta }}_{\mathrm{SO}}$. 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 ${\mathrm{\Delta }}_{\mathrm{SO}}$.

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• Density of states
• Fermi surface
• Spin-orbit coupling
• Alloys
• Doped semiconductors
• Semiconductor compounds
• Single crystal materials
• Atomic force microscopy
• Density functional theory
• Epitaxy
• Ion beam analysis
• Mass spectrometry
• Photoemission spectroscopy
• k dot p method