Time domain terahertz spectroscopy of semiconductor bulk and multiple quantum wells structures.

摘要

A time-domain terahertz spectroscopic system with high source power (average power > 10 nW) and high signal-to-noise ratio (>104) was developed and used to study ultrafast electronic processes in semiconductor structures. The physics of the spectroscopy, the theoretical basis of the interferometry, the model of the electron-electromagnetic field interaction, and the principle of experimental data processing are presented.; The first direct measurement of the intervalley scattering time in In 0.53Ga0.47As was performed. The intervalley scattering time constants obtained were tauLGamma = 35 fs and tauLGamma = 450 fs. The spectroscopic data showed that at low carrier density the carrier-carrier scattering is unimportant. The intervalley deformation potential was obtained from the measured intervalley scattering time constant tau LGamma. The transient conductivity was obtained using time-domain terahertz spectroscopy. The frequency dependent terahertz spectroscopy enabled us to uniquely determine the transient mobility and density. The transient electron mobility is ∼5200 cm2/Vs, which is less than the Hall mobility. For large photocarrier densities, this discrepancy is attributed to the additional momentum relaxation associated with electron-hole scattering.; Using pump pulses with wavelength of 810 run, the electron trapping time in low-temperature-grown GaAs was accurately determined. The measured trapping time is slightly larger than that observed from a band-edge pump-probe measurements. We argue that the terahertz technique provides the most reliable measure of carrier lifetime due to the unique interaction.; The carrier dynamics of low-temperature-grown InGaAs bulk and InGaAs/InAlAs multiple quantum wells were investigated. We were able to differentiate the two dominant mechanisms in the electron decay process, trapping and recombination. A trapping time as fast as 1.3--2.6 ps was observed for photo-excited electrons. The effects of Be-doping and growth temperature on the trapping dynamics in both bulk and multiquantum wells were determined. The role of Be-As complexes and simple compensation were discussed. The photo-electron trapping time of isolated As defects was found to be 16--24 ps. Annealing of low-temperature-grown materials was found to dramatically decrease the recombination time of isolated As defects but hardly changed the effective trapping rate. We propose that the isolated As defects still play a major role in carrier trapping and recombination in the annealed low-temperature-grown InGaAs multi-quantum wells. However, the formation of As precipitates after annealing may dramatically decrease the recombination time of isolated As defects. The initial transient conductivity and the residual conductivity were determined using numerical methods in a quasi-static fashion. The mobility mu and the static carrier density No are uniquely determined. This allows us to calculate the initial change of the conductivity Deltasigma i due to photoexcitation. A significant residual (t > 10ns) photoconductivity has been observed in low-temperature-grown InGaAs. This residual conductivity has significant consequences for device performance. Be-doping was shown to systematically reduce the residual conductivity and simultaneously reduce the decay time of the initial conductivity transient. The Be related defect may act in the form of a Be-As complex which may be a superior recombination center. However, it is not possible to disregard the compensation effects of the beryllium.; The carrier density and mobility of a GaN sample were determined using static terahertz spectroscopy. Assuming the carrier density and mobility are independent parameters, the terahertz transmission through a multi-layer structure was calculated and shown to fit the experimental data. The values of carrier density and mobility obtained in the fitting are consistent with the Hall measurement results.; A wide well-width GaAs-AlGaAs multi-quantum well