Valence band-anticrossing in GaP1−xBix dilute bismide alloys: giant bowing of the band gap and spin-orbit splitting energy

Using spectroscopic ellipsometry measurements on GaP1−xBix/GaP epitaxial layers up to x = 3.7% we observe a giant bowing of the direct band gap (EgΓ) and valence band spin-orbit splitting energy (∆SO). EgΓ (∆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 ∆SO in going from GaP to GaP0.99Bi0.01. The evolution of EgΓ and ∆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 EgΓ and ∆SO with x. In contrast to the well-studied GaAs1−xBix alloy, in GaP1−xBix 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 EgΓ and ∆SO 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 GaP1−xBix alloy band structure.通过对x最高达3.7%的GaP₁₋ₓBiₓ/GaP外延层开展光谱椭偏术（spectroscopic ellipsometry）测量，我们观测到直接带隙（EgΓ）与价带自旋轨道分裂能（ΔSO）存在巨幅弯曲效应。当掺入1%的铋时，EgΓ（ΔSO）分别降低（升高）约200 meV（240 meV）；从GaP转变为GaP₀.₉₉Bi₀.₀₁时，ΔSO的增幅超过四倍。EgΓ与ΔSO随组分x的变化规律呈现出强烈的组分依赖性弯曲特征。我们证明，直接由原子级超胞计算（atomistic supercell calculations）得到参数的简单价带反交叉模型，可定量描述EgΓ与ΔSO随x变化的实测演化规律。与研究较为充分的GaAs₁₋ₓBiₓ合金不同，在GaP₁₋ₓBiₓ中，替位铋会在GaP基体的带隙内形成局域杂质态，由此催生一类由铋杂化形成的光学活性能带，可解释EgΓ与ΔSO整体的大幅弯曲效应，尤其是x≤1%时观测到的巨幅弯曲现象。本研究分析揭示了铋作为等价杂质（isovalent impurity）的作用机制，同时也是首次对GaP₁₋ₓBiₓ合金的能带结构开展详尽的实验与理论分析。