Temperature dependence of the Ge(111) surface electronic structure probed by inelastic H atom scattering

Experimental methods capable of determining the electronic properties of the surfaces of materials suffer from severe limitations, including interference from bulk electronic states, insensitivity to unoccupied states, and the requirement that the material be conducting. In this work, we introduce inelastic H atom scattering as a tool to probe the electronic structures of surfaces, which can be applied to both conducting and nonconducting samples while exhibiting exceptional surface sensitivity. We illustrate the method for the example of Ge(111). The measurements show a semiconducting surface at low temperature that continuously becomes increasingly metallic at high temperature.

Figure 1

Temperature-dependent energy-loss distributions for H atoms scattered from a Ge(111) surface. The polar incidence ${\vartheta }_{\mathrm{i}}$ and scattering ${\vartheta }_{\mathrm{f}}$ angles are both ${45}^{\circ }$. The energy-loss distributions are normalized to the integrated signal. The incident H atoms have a translational energy of $E_{\mathrm{i}}=0.99\mathrm{eV}$ and travel along the $\left[\overline{1}10\right]$ surface direction.

Figure 2

Metallized surface of Ge(111) at elevated surface temperature. Inelastic H atom scattering data for Ge(111) at $T_{\mathrm{S}}=1082\mathrm{K}$ (○) is compared to the results of MDEF simulations for Cu(111) at $T_{\mathrm{S}}=950\mathrm{K}$ (black solid line). The H atoms travel along the $\left[10\overline{1}\right]$ direction of the Cu(111) surface. The energy-loss distributions are normalized to the integrated signal. Conditions are otherwise the same as in Fig. 1.

Figure 3

Fit of the energy-loss distributions for H atoms scattered from Ge(111) at different surface temperatures with three components. Panels (a) to (h) show a fit of the energy-loss distributions (○) of H atoms scattered from Ge(111) at different surface temperatures, $T_{\mathrm{S}}$. The data were fitted to a sum of three components (blue solid line): S1 (red dashed line), S2 (green dotted line), and M (orange dashed-dotted line). Details about the fit can be found in the text and the Supplemental Material [37]. The incidence translational energy is $E_{\mathrm{i}}=0.99\mathrm{eV}$, the polar incidence and scattering angles are ${\vartheta }_{\mathrm{i}}={\vartheta }_{\mathrm{f}}={45}^{\circ },$ and the incident H atoms travel along the $\left[\overline{1}10\right]$ surface direction.

Figure 4

Electronic character of the Ge(111) surface as a function of surface temperature. Panel (a) shows the relative intensities determined from the fits in Fig. 3 for the sum of the semiconducting components, S1+S2 (■) and the metallic component, M (•). Error bars reflect the uncertainty of the fit due to the overlap of the S2 and M components. Shaded areas correspond to the fractions of S1 (dark gray) and S2 (light gray) of the overall semiconductor component, respectively. Panel (b) shows the surface band gap as a function of surface temperature, determined from the onset of the S2 component in Fig. 3; for details see Supplemental Material [37]. In both panels, lines between data points are given as a guide to the eye.