Local Density of States at Metal-Semiconductor Interfaces: An Atomic Scale Study

We investigate low temperature grown, abrupt, epitaxial, nonintermixed, defect-free $n$-type and $p$-type $\mathrm{Fe}/\mathrm{GaAs}(110)$ interfaces by cross-sectional scanning tunneling microscopy and spectroscopy with atomic resolution. The probed local density of states shows that a model of the ideal metal-semiconductor interface requires a combination of metal-induced gap states and bond polarization at the interface which is nicely corroborated by density functional calculations. A three-dimensional finite element model of the space charge region yields a precise value for the Schottky barrier height.

Figure 1

$10\times 5{\mathrm{nm}}^2$ constant current topography of an ideal $\mathrm{Fe}/n\text{−}\mathrm{GaAs}(110)$ interface ($V_{\mathrm{s}}=2\mathrm{V}$, $I_{\mathrm{T}}=100\mathrm{pA}$) and cross-sectional geometry. The (green) solid line at $x=0$ indicates the interface plane. The inset shows the interface slab for DFT calculations in top view. White, black, and gray filled circles represent As, Ga, and Fe atoms, respectively.

Figure 2

250 topography-normalized (see text) $I\text{−}V$ spectra taken along the space charge region of (left) an $n$-type and (right) a $p$-type $\mathrm{Fe}/\mathrm{GaAs}(110)$ interface. The tunnel current is plotted color coded as $\log (|I_{\mathrm{T}}|)$ in dependence of bias voltage between tip and sample and $x$ position (interface at $x=0\mathrm{nm}$). Each $I\text{−}V$ curve is averaged over three spectra in the $y$ direction. Data ($p$ type) taken at $T=6\mathrm{K}$ and ($n$ type) at room temperature (RT). The horizontal green dashed lines serve as a guide to the eye for the flat band condition.

Figure 3

(a) Scheme of the vacuum tunneling (VT) energy range ${\mathrm{\Phi }}_{\mathrm{VT}}$ in the rigid band model. For more details see text. (b) and (c) The tunnel current is depicted color coded as $\log (|I_{\mathrm{T}}|)$ for 250 topography-normalized $I\text{−}V$ spectra along the $x$ axis across (b) an $n$-type and (c) a $p$-type interface (interface at $x=0\mathrm{nm}$). Data taken at (b) RT and (c) $T=6\mathrm{K}$. The black dots represent isolines of constant current. The light blue solid lines represent isolines from 3D finite element calculations (see text). Yellow isolines represent the band edges in the rigid band model.

Figure 4

Color-coded $dI/dV$ spectra calculated from (a) 230 respective $I\text{−}V$ spectra taken across a $p$-type interface along the atomic line in the $x$ direction depicted above ($V_s=+1.75\mathrm{V}$, $I_{\mathrm{T}}=75\mathrm{pA}$) and (b) from 100 respective $I\text{−}V$ spectra taken across a $p$-type $\mathrm{GaAs}(110)\text{−}(1\overline{1}0)$ edge. The data are averaged over 10 spectra in the $y$ direction. The interface and the edge are both located at $x=0\mathrm{nm}$.

Figure 5

(a) Variation of the LDOS inside the valence band for a $p$-type junction extracted from the STS data set in Fig. 3 (interface at $x=0\mathrm{nm}$). The white dashed line represents the valence band edge $E_V$. (b) Density functional calculations for the ideal $\mathrm{Fe}/\mathrm{GaAs}(110)$ interface: (upper panels) The total (sum of spin-up and -down) density of states of the $n$th GaAs layer off the interface. The Fermi energy is located at the valence band edge. (lower panels) Variation of the LDOS in the $n$th GaAs layer with respect to the 6th layer. At 0.35 eV below the valence band edge (horizontal dashed line) we observe a sharp drop in $\mathrm{\Delta }\mathrm{LDOS}$ (transparent red area) due to Fe-As hybridization (see text).