Subband structure ofn-channel inversion layers on polar semiconductors

The subband structure of $n$-channel inversion layers on the surface of polar semiconductors has been investigated. The model used includes the effects of the Coulomb interaction and the polar, LO-phonon-mediated electron-electron interaction. The Coulomb interaction has been treated in the so-called diagonal random-phase approximation. This approximation ignores the effects of subband mixing, but is known to be valid for Coulomb forces. However, the phonon interaction is frequency dependent and may strongly mix states in different subbands, particularly when subband separation is near ${\omega }_{\mathrm{LO}}$, the LO-phonon frequency. Therefore, the calculation includes the effects of subband mixing due to an unscreened electron—LO-phonon interaction. A simple two-subband model has been used to calculate the quasiparticle energies and subband separation as a function of wave vector and density. In addition, the effect of the electron-phonon interaction on the depolarization shift has been investigated. It is found that the frequency dependence of this interaction splits each absorption peak into two branches which can be interpreted as mixed intersubband—LO-phonon excitations. The magnitude of the splitting and the relative strength of each absorption peak has been calculated.