Nonlocal Theory of Surface Plasmons in N-Type Gallium-Arsenide Films.

This dissertation presents a theoretical study of collective electronic excitations in semiconducting films, particularly n-type GaAs. We explore the effect of surface charge, or an applied electric field, on the free carrier density profile near the surface; and subsequently explore the effect of the induced depletion or accumulation regions on the surface plasmon spectrum. The free carrier density profiles and corresponding potential profiles are calculated self-consistently. The associated wavefunctions and energy states are then used to explore the dynamics of the plasmons in the inhomogeneous near-surface region. This investigation may have some general interest since the dynamics of the inhomogeneous electron gas have not been well-studied as yet. The random phase approximation (RPA) is used to describe the collective excitations. This dissertation presents the results of the first full calculations of the surface plasmon contribution to the Electron Energy Loss Spectrum (EELS) of a doped semiconductor (here, n -type GaAs). The results presented show significant deviations from the predictions of classical dielectric theory, pointing up the usefulness of the nonlocal dielectric theory for the case of GaAs. We find and discuss the properties of surface plasmons, standing wave bulk plasmons, and in the case of a strong accumulation layer, we also find collective modes localized in the accumulation region that are only weakly Landau damped even at large wavevectors. The results of these theoretical calculations are compared with recent experiments, and are shown to account nicely for some very interesting trends. Finally, the numerical methods used in the calculations are discussed in considerable detail.