Giant bowing of the band gap and spin-orbit splitting energy in GaPBi dilute bismide alloys.

Using spectroscopic ellipsometry measurements on GaPBi/GaP epitaxial layers up to x = 3.7% we observe a giant bowing of the direct band gap ([Formula: see text]) and valence band spin-orbit splitting energy (Δ). [Formula: see text] (Δ) 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 Δ in going from GaP to GaPBi. The evolution of [Formula: see text] and Δ with x 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 [Formula: see text] and Δ with x. In contrast to the well-studied GaAsBi alloy in GaPBi 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 [Formula: see text] and Δ and in particular for the giant bowing observed for x ≲ 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 GaPBi alloy band structure.