Intrinsic Electron Accumulation at Clean InN Surfaces

The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low $\Gamma$-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the $\Gamma$ point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of $2.5(\pm 0.2)\times {10}^{13}{\mathrm{c}\mathrm{m}}^{-2}$ and a surface Fermi level of $1.64\pm 0.10\mathrm{e}\mathrm{V}$ above the valence band maximum.

Figure 1

Specular HREEL spectra recorded at 300 K from an atomic hydrogen cleaned InN(0001)-$(1\times 1)$ surface with incident electron energies of 10, 15, and 30 eV (points) and the corresponding semiclassical dielectric theory simulations (solid lines).

Figure 2

The plasma frequency (solid line) and the electron effective mass at the Fermi level (dashed line) calculated as a function of the carrier concentration for InN. These carrier statistics were calculated from the conduction band dispersion relation modeled by a two-band $\mathbf{k}·\mathbf{p}$ band structure that incorporates electron-electron interactions and electron-ionized impurity interactions in the high wave vector regime.

Figure 3

The layered charge profile used in the HREELS simulations (solid line) and the corresponding smooth charge profile $n(z)$ calculated by solving the Poisson equation within the MTFA (dotted line). Also shown is the potential profile $V(z)$ (dashed line). The bulk and surface Fermi level values are referenced to the valence band maximum.

Figure 4

The conduction and valence band edges ($E_C$ and $E_V$, solid lines) and the branch-point energy ($E_B$, dotted line) with respect to the Fermi level ($E_F$, dashed line) in the near-surface region of InN(0001). The donor-type surface states ($D_{\mathrm{s}\mathrm{s}}$) are also shown, where the unoccupied states above the Fermi level are shown to be positively charged.