Subband structure and plasmon-phonon coupled excitations in the accumulation layer of ZnO.

Abstract

Calculations of electronic subbands and collective excitations are presented for strong accumulation layers in ZnO. Our model for this system is that of a free-electron gas with an effective mass embedded in a dielectric medium that supports lattice vibrations. Allowance is made for the conduction electrons to tunnel into the surface barrier. The effects of a position-dependent effective mass and of exchange and correlation are also considered. Using a nonlocal dielectric response formalism based on the random-phase approximation, we obtain the dispersion and line shapes of both intrasubband and intersubband plasmons. Evidence is found for two-dimensional intrasubband plasmons, as well as intersubband and ``acoustic'' plasmons; the latter have a nearly linear dispersion relation. When lattice vibrations are included, coupled plasmon-phonon modes or ``plasmarons'' are obtained. Some of these modes have been observed in earlier high-resolution electron-energy-loss spectroscopy experiments.