Raman Scattering Studies of Aluminum-Gallium - Arsenide Based Semiconductor Superlattices (Phonons, Interfaces)

Abstract

Raman scattering measurements have been made on more than twenty semiconductor superlattices, all grown by molecular beam epitaxy and based on the Al(,x)Ga(,1-x)As system. The samples consist of undoped layers with alternating values of x along the {001} direction, and have periods ranging from 20(ANGSTROM) to 200(ANGSTROM). The data include a variety of scattering geometries, temperatures, and resonant and non-resonant laser excitation.

The most noticeable superlattice effect is the appearance in the A(,1) Raman spectra of folded LA phonon doublets near the frequencies (omega)(,m) = v(,SL)(2(pi)m/d (+OR-) q), where m is an integer, q the wavevector transfer, and v(,SL)('-1) (TURNEQ) (,rms) is an average of the bulk sound velocities in the layers. Their narrow linewidths indicate little inhomogeneous broadening. Peaks corresponding to m as large as five have been distinguished. Their intensities are well described out of resonance by a continuum model of photoelastic coupling between light and the phonon induced strain. Near resonance with the E(,0) and E(,0) + (DELTA)(,0) superlattice gaps, the peaks show behavior indicative of coupling to an electronic or two-phonon continuum. Disorder activated scattering from zone boundary phonons is identified in some samples, especially in those with high Al concentration.

The GaAs-like and AlAs-like optical phonons appear below the bulk frequencies, consistent with their confinement to alternate layers. Multiple peaks seen between the LO and TO frequencies are not fully understood. Their origins may lie in Brillouin zone folding or in disorder activated scattering from phonons propagating along an interface.