Dynamical characteristics of a coherent longitudinal optical phonon in a GaAs buffer layer optically covered with a GaSb top epitaxial layer investigated with use of terahertz spectroscopy

Abstract

We demonstrate that, in a GaSb/GaAs epitaxial structure, a coherent longitudinal optical (LO) phonon in a GaAs buffer layer optically covered with a GaSb top layer is observed utilizing terahertz spectroscopy. It is confirmed from Raman scattering measurements that only the optical phonons in the GaSb layer is optically observable. In the terahertz-wave measurement, the Fourier power spectrum of a terahertz waveform exhibits both the coherent GaAs and GaSb LO phonon bands; namely, the coherent LO phonon in the optically covered GaAs buffer layer is observed in the terahertz-wave measurement. This fact demonstrates that the instantaneous surface potential modulation, which originates from the impulsive carrier excitation by the pump beam, reaches the GaAs buffer layer. Consequently, the above-mentioned surface potential modulation generates the coherent GaAs LO phonon that cannot be optically excited. In addition, we perform a time-partitioning Fourier transform analysis in order to investigate the decay dynamics of the coherent GaAs and GaSb LO phonons. The decay times of the coherent GaAs and GaSb LO phonons are estimated to be 2.0 and 3.3 ps, respectively. The longer decay time of the coherent GaSb LO phonon is attributed to the fact that the phonon density of state in a final state of the decay process of GaSb is relatively small in comparison with that of GaAs.