Valence band-anticrossing in GaP${}_{1-x}$Bi${}_x$ dilute bismide alloys: giant bowing of the band gap and spin-orbit splitting energy

Abstract

Using spectroscopic ellipsometry measurements on GaP${}_{1-x}$Bi${}_x$/GaP epitaxial layers up to $x=3.7$% we observe a giant bowing of the direct band gap ($E_g^{\Gamma }$) and valence band spin-orbit splitting energy (${\Delta }_{\text{SO}}$). $E_g^{\Gamma }$ (${\Delta }_{\text{SO}}$) 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 ${\Delta }_{\text{SO}}$ in going from GaP to GaP${}_{0.99}$Bi${}_{0.01}$. The evolution of $E_g^{\Gamma }$ and ${\Delta }_{\text{SO}}$ 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 $E_g^{\Gamma }$ and ${\Delta }_{\text{SO}}$ with $x$. In contrast to the well-studied GaAs${}_{1-x}$Bi${}_x$ alloy, in GaP${}_{1-x}$Bi${}_x$ 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 $E_g^{\Gamma }$ and ${\Delta }_{\text{SO}}$ and in particular for the giant bowing observed for $x\lesssim 1$%. Our analysis provides insight into the action of Bi as an isovalent impurity, and constitutes the first detailed experimental and theoretical analysis of the GaP${}_{1-x}$Bi${}_x$ alloy band structure.

7 pages, 2 figures