Effect of heteroepitaxial growth on LT-GaAs: ultrafast optical properties

TL;DR

This study compares the optical and electronic properties of heteroepitaxial LT-GaAs grown on silicon with those grown on semi-insulating GaAs, revealing differences in carrier dynamics due to bonding variations.

Contribution

It provides new insights into how heteroepitaxial growth on silicon affects the ultrafast optical properties and trap states of LT-GaAs, which is relevant for terahertz applications.

Findings

LT-GaAs/Si exhibits lower As-related electron trap density.

Carrier decay dynamics differ significantly under high excitation fluence.

Heteroepitaxial growth induces bonding changes affecting electronic properties.

Abstract

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.