Giant Bowing of the Bandgap and Spin-Orbit Splitting in GaP1-xBix Dilute Bismide Alloys

Highly-mismatched III-V semiconductor alloys containing dilute concentrations of bismuth (Bi) have attracted significant attention in recent years since their unique electronic properties open up a range of possibilities for practical applications in semiconductor lasers, photovoltaics, spintronics, photodiodes, and thermoelectrics. Research on dilute bismide alloys has primarily focused to date on $\text{GaAs}_{1-x}\text{Bi}_{x}$ , where incorporation of Bi brings about a strong reduction of the direct $\Gamma$ -point band gap ( $E_{g}{}^{\Gamma}$ ) –by up to 90 meV per % Bi at low Bi compositions $x$ –characterised by strong, composition-dependent bowing. This unusual behaviour derives from the large differences in size (covalent radius) and chemical properties (electronegativity) between As and Bi.Bi, being significantly larger and more electropositive than As, acts as an isovalent impurity which primarily impacts and strongly perturbs the valence band (VB) structure. This is in contrast to dilute nitride alloys, in which small electronegative nitrogen (N) atoms strongly perturb the conduction band (CB) structure in $\text{GaN}_{x}\text{As}_{1-x}$ and related alloys. Additionally, Bi, being the largest stable group-V element, has strong relativistic (spin-orbit coupling) effects. As such, the reduction of $E_{g}{}^{\Gamma}$ in $(\text{In})\text{GaAs}_{1-x}\text{Bi}_{x}$ is accompanied by a strong increase in the VB spin-orbit splitting energy ( $\Delta_{\text{SO}}$ ).