Heavy-hole intersubband scattering by confined optical phonons in a Si/ZnS superlattice

摘要

The confinement of optical modes of vibration in a superlattice consisting of polar and nonpolar materials is described by a continuum model, Specifically, the structure under investigation is the Si/ZnS superlattice. Optical phonon modes in Si and ZnS layers are totally confined within their respective layers since both layers can be treated as infinitely rigid with respect to the other layer. Since there are no associated electric fields with nonpolar optical phonons in Si layers, only a mechanical boundary condition needs to be satisfied for these nonpolar optical modes at the Si-ZnS interface. The optical phonons in Si layers can be described by guided modes consisting of an uncoupled s-TO mode and a hybrid of LO and p-TO modes with no interface modes. In ZnS layers, a continuum model hybridizing the LO, TO, and IP modes is necessary to satisfy both the mechanical and electrostatic boundary conditions at the heterointerface. A numerical procedure is provided to determine the common frequency between LO, TO, and TP modes. This is a procedure for obtaining the eigenmodes of a mixed polar-nonpolar heterosystem. Analytical expressions are obtained for the ionic displacement and associated electric field as well as scalar and vector potentials. The established model for the confined optical phonons is used in calculating the intersubband heavy-hole scattering rate by optical phonons in the Si/ZnS superlattice. Our results indicate that contributions to the intersubband scattering rate from Si or ZnS confined optical phonons depend strongly on the distribution of envelope wave functions over the respective layers within which different types of optical phonons are confined. [References: 30]