Characterization of proton-irradiated InGaAs/GaAs multiple quantum well structures by nonresonant transient four-wave mixing technique

Effects of proton irradiation and thermal annealing on free carrier lifetime and transport in a InGaAs/GaAs multiple quantum well (MQW) structure and its substrate have been studied by using the nonresonant transient four-wave mixing technique. Under the excitation of the surface region of the structure by 0.53 mum wavelength, we determined the carrier lifetime tau = 150-600 ps, bipolar diffusion coefficient D = 24-26 cm(2) s(-1), and mobility-lifetime product mu tau x (2-6) x 10(-7) cm(2) V-1 in 3 MeV proton irradiated and annealed MQW samples. At excitation by 1.06 mum wavelength, we studied the full proton penetration range. including the substrate. We found an unexpected twofold increase in carrier bulk lifetime with increasing irradiation dose. Features of this effect at low photoexcitation and its absence at high photoexcitation are attributed to carrier separation by potential fluctuations in the vicinity of irradiation-created deep defects. These features indicate that proton-irradiated semi-insulating GaAs exhibits properties of mesoscopic pointlike defects, similar to those previously reported in nonstoichiometric crystals of GaAs. This result is important to understand the mechanism of carrier compensation in proton-irradiated semiconductors.