Influence of Bi Alloying on GaAs Valence Band Structure

TL;DR

This work addresses how Bi alloying alters the valence-band structure of GaAs to promote topological phases. Using droplet-free GaAs$_{1-x}$Bi$_x$ films with $x_{Bi}=0.06$, the authors combine high-resolution ARPES, $k\cdot p$ modeling, and all-electron x2C-DFT to quantify Bi-induced shifts in the LH, HH, and SO bands and to identify Bi $p$-orbitals as the main driver of the enhanced $ΔSO$. They find a 0.23 eV upward shift of the LH/HH bands and a 0.03 eV downward shift of the SO band relative to GaAs:Si, yielding a total $ΔSO$ increase to ~0.59 eV, primarily due to the VBM upward shift from Bi states. Si doping shifts band energies rigidly without significantly affecting $ΔSO$, establishing Bi's primary role in tuning GaAs valence-band structure toward III–V topological materials.

Abstract

Bi alloying is predicted to transform GaAs from a semiconductor to a topological insulator or semi-metal. To date, studies of the GaAs$_{1-x}$Bi$_x$ alloy band structure have been limited, and the origins of Bi-induced enhancement of the spin-orbit splitting energy, $Δ_\mathrm{SO}$, are unresolved. Here, we present high-resolution angle-resolved photoemission spectroscopy (ARPES) of droplet-free epitaxial GaAs$_{1-x}$Bi$_x$ films with $x_{\mathrm{Bi}}$ = 0.06. In addition to quantifying the Bi-induced shifts of the light-hole and heavy-hole valence bands, we probe the origins of the Bi-enhanced $Δ_\mathrm{SO}$. Using exact-two-component density functional theory calculations, we identify the key role of Bi p-orbitals in the upward shift of the light-hole and heavy-hole bands that results in the Bi-enhanced $Δ_\mathrm{SO}$.

Figures (7)

• Figure 1: GaAs$_{1-x}$Bi$_x$ atomic structure: (a) illustration of the GaAs unit cell containing BiAs (red) and SiGa (gray). (b) GaAs Brillouin zone, with high-symmetry points labeled. Local $x_{\mathrm{Bi}}$ from local-electrode atom-probe tomography, shown as (c) a 2D contour plot created from a 1414 nm$^{3}$ cubic region-of-interest (bin size = 1.0 nm) and (e) a 3D rendering of the xz-cross section. (d) Pair correlation functions, C(r), vs pair separation for Bi–Bi (red), Bi–Ga (green), and Ga–Ga (black) pairs. Error bars are within the size of the data points for most values of C(r) mitchell_influence_2024.
• Figure 2: GaAs$_{1-x}$Bi$_x$ film composition and morphology: (a) Rutherford backscattering spectrometry (RBS) yield versus backscattered particle energy (and depth) for GaAs1-xBix:Si (red), GaAs:Si (gray), and GaAs (black) films. SIMNRA fitting of the GaAs$_{1-x}$Bi$_x$:Si spectrum yields an average Bi composition of $x_{\mathrm{Bi}}$ = 0.060 and layer thickness of 210 nm for the GaAs$_{1-x}$Bi$_x$:Si film. (b) Normalized XPS core level spectra for GaAs (black), GaAs:Si (gray), and GaAs1-xBix:Si (red) centered around the energies corresponding to the Bi 4f and Ga 3s core levels, with Voigt fits as dashed lines. (c-d) high-resolution x-ray rocking curves (XRC), consisting of diffraction intensity vs. $\Delta\omega$ about the (c) (004) and (d) (224) GaAs for GaAs (black), GaAs:Si (gray), and GaAs$_{1-x}$Bi$_x$:Si (red) films. Analysis of the $\Delta\omega_{(004)}$ and $\Delta\omega_{(224)}$ data reveals a residual in-plane compressive strain of 0.62 $\%$ for the GaAs$_{1-x}$Bi$_x$:Si film. (e-g) atomic force microscopy (AFM) images for (e) GaAs$_{1-x}$Bi$_x$:Si, (f) GaAs:Si, and (g) GaAs. The color-scale ranges displayed are (e) 2.0 nm, (f) 1.5 nm, and (g) 23 nm.
• Figure 3: Comparison of electronic structures for GaAs, GaAs:Si, and GaAs1-xBix:Si. (a) Out-of-plane and (b) in-plane constant energy contours at a binding energy of 3.5 eV for GaAs taken with p-polarized light. The pinked dashed lines depict the first Brillouin zone with high symmetry points labeled. (c)-(e) Band dispersion of GaAs along the high symmetry lines (c) $\Gamma$ - X with p-polarized light, (d) $\Gamma$ - K with p-polarized light, and (e) $\Gamma$ - K with s-polarized light. (f) - (j) and (k) - (o) are the same as (a) - (e) except for GaAs:Si and GaAs1-xBix:Si.
• Figure 4: Effects of Si Doping on the Electronic Structure of GaAs. (a) Band dispersion taken with p-polarized light of GaAs along the $\Gamma$ - K high symmetry line. (b) Band positions found through the energy-dispersion curves (EDCs) (markers) and the corresponding $k \cdot p$ fits (solid lines). The dashed lines denote $k*$, the limit at which 1st order $k \cdot p$ theory no longer is accurate. (c), (d) Same as (a),(b) but for GaAs:Si. The enhanced n doping of GaAs:Si is evident through the downward shift in energy of the bands relative to GaAs. The red arrow in (c) highlights a kink in the LH band dispersion of GaAs:Si that is absent in GaAs.
• Figure 5: Enhanced Spin-Orbit Splitting in GaAs1-xBix:Si. (a) Energy dispersion and (b) the corresponding second derivative taken along the momentum axis of GaAs:Si along the $\Gamma$ - K high symmetry line. (c) - (d) Same as (a) - (b), respectively, for GaAs1-xBix:Si. The pink and orange dashed lines show the LH/HH and SO band maxima, labeled as ELH and ESO, respectively. The difference between these band positions is the spin-orbit splitting, labeled as $\Delta$SO, which is enhanced in GaAs1-xBix:Si. All plots are a summation of the dispersion measured with p- and s-polarized light.