  Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has
the potential for terahertz (THz) photoconductive antenna applications.
However, crystalline, optical and electrical properties of heteroepitaxial
grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs
substrates (reference). In this study, we investigate optical properties of an
epitaxial grown LT-GaAs/Si sample, compared to a reference grown under the same
substrate temperature, and with the same layer thickness. Anti-phase domains
and some crystal misorientation are present in the LT-GaAs/Si. From coherent
phonon spectroscopy, the intrinsic carrier densities are estimated to be
~$10^{15}$ cm$^{-3}$ for either sample. Strong plasmon damping is also
observed. Carrier dynamics, measured by time-resolved THz spectroscopy at high
excitation fluence, reveals markedly different responses between samples. Below
saturation, both samples exhibit the desired fast response. Under optical
fluences $\geq$ 54 $\mu$ J/cm$^2$, the reference LT-GaAs layer shows saturation
of electron trapping states leading to non-exponential behavior, but the
LT-GaAs/Si maintains a double exponential decay. The difference is attributed
to the formation of As-As and Ga-Ga bonds during the heteroepitaxial growth of
LT-GaAs/Si, effectively leading to a much lower density of As-related electron
traps.
