Near valence-band electronic properties of semiconducting $\beta {-\mathrm{Ga}}_2{\mathrm{O}}_3$ (100) single crystals

$\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ is a transparent wide-band-gap semiconductor that has attracted considerable interest in recent years due to its suitable electrical conductivity and transparency in the ultraviolet spectral region. In this work we investigate the electronic properties of the near valence-band-edge region for semiconducting $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ (100) bulk single crystals using core-level photoelectron spectroscopy and ab initio theory within the framework of density functional theory and the $\mathit{GW}$ approach. We find good agreement between the experimental results and the theoretical calculations. This is explained by the hybridization of the Ga $3d$ and O $2s$ states, similar as for ${\mathrm{In}}_2{\mathrm{O}}_3$.

Figure 1

(a) XPS wide scan of the $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ (100) air-cleaved crystal prior (black, bottom) and after (blue, top) annealing showing the element specific peaks. The spectra are shifted vertically for clarity. The inset shows the LEED ($2\times 1$) pattern from the bulk taken at 85 eV after annealing. (b) and (c) show the photoemission peaks for O $1s$ and Ga $3d$, respectively.

Figure 2

(a) VB photoemission spectrum of the $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ (100) cleaved surface showing the extrapolation to the VBM at 4.56 eV. ($E_B=0$ refers to the experimentally determined value of the Fermi level of the sample holder in electrical contact with the sample.) (b) A schematic representation of the near-surface electronic bands.

Figure 3

Band structure and corresponding DOS obtained from a $\mathit{GW}$ quasiparticle band-structure calculation. The energy zero has been set equal to the energy of the VBM. The solid (black) line denotes the total DOS, the dashed (blue) line the $s$-projected DOS, the dotted (green) line the $p$-projected DOS, and the dotted-dashed (red) line the $d$-projected DOS.

Figure 4

(a) Total DOS, and $s$ and $p$ contributions to the near VB region as calculated from the $\mathit{GW}$ quasiparticle approach compared to the experimental photoemission spectra (line with circles). The calculated data have been shifted by 4.56 eV to fit the experimental $E_B=0$. ($E_B=0$ refers to the experimentally determined value of the Fermi level of the sample holder in electrical contact with the sample.) (b) Detail of the VB edge region, showing good agreement between the theory and the experiment for the O $2p$ VB levels.