Terahertz excitation spectroscopy of Bi/GaAs heterostructures

Abstract

In this work, heterojunctions consisting of thin monoelemental bismuth (Bi) layers have been grown on crystalline GaAs substrates with various doping types and investigated using THz pulse generation excited via femtosecond optical pulses of different wavelengths. It was determined that the influence of the layers of Bi can be approached in two distinct ways: first, through the additional absorption of the optical pulse and second, through changes in the electron energy band line-ups at the heterojunction. The strongest influence of the latter effect was observed at the interface between Bi and p-GaAs when the internal electric field exceeded the saturation field of the drift velocity of the p-GaAs holes. Such high fields can lead to an overshooting effect of the holes when the holes move at speeds above 2 × 107 cm/s and emit a much stronger THz signal than in the case of p-GaAs. This finding is promising for the development of efficient surface THz pulse emitters based on p-GaAs and other p-type semiconductors.