Abstract
Using spectroscopic ellipsometry measurements on GaP1−χBiχ/GaP epitaxial layers up to χ = 3.7% we observe a giant bowing of the direct band gap (EΓg) and valence band spin-orbit splitting energy (ΔSO). EΓg (ΔSO) is measured to decrease (increase) by approximately 200 meV (240 meV) with the incorporation of 1% Bi, corresponding to a greater than fourfold increase in ΔSO in going from GaP to GaP0.99Bi0.01. The evolution of EΓg and ΔSO with χ is characterised by strong, composition-dependent bowing. We demonstrate that a simple valence band-anticrossing model, parametrised directly from atomistic supercell calculations, quantitatively describes the measured evolution of EΓg and ΔSO with χ. In contrast to the well-studied GaAs1−χBiχ alloy, in GaP1−χBiχ substitutional Bi creates localised impurity states lying energetically within the GaP host matrix band gap. This leads to the emergence of an optically active band of Bi-hybridised states, accounting for the overall large bowing of EΓg and ΔSO and in particular for the giant bowing observed for χ ≲ 1%. Our analysis provides insight into the action of Bi as an isovalent impurity, and constitutes the first detailed experimental and theoretical analysis of the GaP1−χBiχ alloy band structure.