Study of carrier dynamics in LT GaAs by means of time-resolved emission terahertz spectroscopy

Abstract

Biased large aperture semiconductor emitters are widely used to generate ultrashort broadband terahertz (THz) pulses. The temporal shape of such pulses can be determined through a phase sensitive electro-optic or photoconduction sampling technique. The measured profile of the THz pulse then depends on several factors: (i) incoming optical pulse shape, (ii) dynamics of photo-excited carriers in the THz emitter, (iii) far-infrared optics transforming the THz beam, and (iv) receiving antenna or sensor spectral detectivity. We used 80 fs laser pulses at 800 nm generated by a self-mode-locked Ti:sapphire laser to excite biased GaAs samples. They consisted of a 1.3 /spl mu/m layer of MBE-GaAs grown at low temperatures (LT-GaAs) on a semi-insulating GaAs substrate with a 72.5 nm GaAs buffer layer. We studied several samples characterised by different deposition temperatures ranging between 175 and 250/spl deg/C. The irradiated THz waveform was measured using a ZnTe electro-optic sensor (without any focusing optics to avoid the waveform reshaping). The THz signal was studied as a function of several parameters (applied voltage, pump laser intensity, distance between biased electrodes). Lateral scans of the emitter position (with regard to the pump beam) were also performed.