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.

• Received 18 September 1989

DOI:https://doi.org/10.1103/PhysRevB.41.5991