Raman-scattering study of GaP/InP strained-layer superlattices.

Strained-layer superlattices (SL's) made up of alternate GaP and InP layers constitute a system where the strain is symmetrized when grown on GaAs substrates. While both components present a large lattice mismatch to GaAs ( —3.6% GaP, +3.8% InP) giving rise to large biaxial deformations in the individual layers, the net value of strain in the whole SL is very small, thus favoring their stability. ' An incentive for studying GaP/InP SL's consists in their potential applications in optoelectronic devices. They are an advantageous alternative to GaInP alloys. ' Moreover, strain adds a powerful degree of freedom which introduces additional useful features and can be used as a design parameter. In the present work, we use Raman scattering to investigate the phonon spectra of GaP/InP SL's. The in-plane extensive (compressive) strain present in the GaP (InP) layers shifts the phonon modes to lower (higher) frequencies with respect to their bulk values. These shifts are of the order of 25 cm ' for both materials, thus causing the flatter LO branch of bulk InP to fall within that of GaP. The lattice dynamics of these SL's reflects then mainly the strain present in the layers. We compare our experiments to simulations of the Raman spectra made using the bond-polarizability model ' with the eigenvectors obtained from a linear-chain calculation. '