Surface-plasmon modes in Zn-doped InAs(001) and (111)

We have investigated the plasmon modes present at the (001) and (111) surfaces of heavily doped p -type InAs, using high-resolution electron-energy-loss spectroscopy (HREELS) together with dielectric theory simulations. Two-dimensional electron plasmon modes are supported in the inversion layer close to the polar surfaces, however, these modes appear in the experimental HREEL spectra only as a broadening of the elastic peak due to their low energy. Deeper into the bulk, the hole plasma is characterized by light- and heavy-hole components supporting both optical and acoustic plasmon modes. Observation of the optical hole plasmon in HREELS allows the hole density profile to be estimated using dielectric theory simulations, which employ two plasma oscillators incorporating spatial dispersion to model the two-component hole plasma. It is found that native spatial dispersion cannot account for the pronounced experimental plasmon dispersion observed following argon-ion bombardment and annealing of the samples. This procedure is shown to cause rapid diffusion of the zinc acceptors into the near-surface region, resulting in large, highly nonuniform hole concentrations over the length scales probed by HREELS. The acoustic hole plasmon mode, which cannot normally be observed in specular HREELS, is discussed in terms of a two-oscillator, two-layer model. It is shown that acoustic modes do not contribute to specular HREEL spectra even in the limit of highly nonuniform charge distributions.