Time-resolved second harmonic generation studies of carrier and carrier-phonon dynamics at gallium arsenide surfaces (Gallium arsenide)

Carrier dynamics in the depletion region of GaAs(110) were studied using femtosecond time-resolved second-harmonic generation. Due to its short probing depth, SHG is ideally suited for Angstrom-scale sensitivity to carrier density near the surface. A modified one-dimensional drift and diffusion model that includes separate surface trap capture dynamics for electrons and holes was used to fit the data. Contrary to analysis using the ambipolar diffusion model, no significant surface, trap recombination was observed.; Surface physical changes (disorder/damage) due to femtosecond pulsed laser irradiation were observed in a new regime—extremely low laser fluence (<10−3 of the single shot bulk damage fluence) and large number of laser shots (>109 pulses). Disorder of the (1 x 1)-relaxed GaAs(110) surface was monitored using SHG intensity and time-resolved SHG-based surface phonon spectroscopy. No bulk damage was detected during the laser irradiation. Disorder is reversible by thermal annealing at 580°C. A new mechanism for femtosecond laser-induced surface physical reactions is proposed. Disorder is caused by a surface dangling bond hole-induced lattice instability. On rare high density hole fluctuations the reaction barrier is lowered to ∼k_BT. This is remarkably different from traditional thermal chemical reactions where the rate limiting process is rare vibrational fluctuations that overcome eV-scale reaction barriers.; The interband transition of the dangling bonds on the GaAs(110)-(1 x 1) relaxed surface was studied with a new time-domain technique based on SHG. Two TRSHG field intensities and their interference are used to deconvolve the laser pulse parameters from the coherent media response. In typical coherent electronic transition studies, strict “energy-conservation” within the electronic system requires that only transition frequencies that fall within the laser bandwidth may be driven coherently. Surprisingly, media oscillation frequencies several 100 meV outside the laser bandwidth were observed. This implies that the full quantum coherence of the electronic and other material systems must be considered. The large energy shift implies that the electronic system is coupled to electronic rather than phonon excitation. Possibilities include plasmon-polaritons, exchange interaction, or band gap renormalization. The large density of “incoherent” surface dangling bond holes needed for these effects would be driven to the surface by the depletion field.