Ultrafast Dynamics In Semiconductor Research

In this work, the ultrafast dynamics in semiconductor was studied using femtosecond laser technique. Different experimental methods were used to investigate different ultrafast processes. By means of ultrafast photocurrent spectroscopy, the dephasing (namely momentum relaxation) in Ge quantum dots was investigated and theoretically simulated. Using pump-probe reflectivity spectroscopy, the reflectivity dynamics of photoexcited carriers in Fe implanted InP was studied, then the processes of its energy relaxation and diffusion were investigated. Using ultrafast pump-probe absorption spectroscopy, the relaxing process of nonequilibrium carriers in ZnSe nanocrystal was investigated.1. The femtosecond ultrafast measurement system controlled by computer was set up, and it can measure ultrafast photocurrent spectroscopy, ultrafast pump-probe absorption spectroscopy, ultrafast pump-probe reflectivity spectroscopy and transient photoluminescence.2. As we know, the study of the dephasing in Ge quantum dots was firstly reported using ultrafast photocurrent spectroscopy. The optical Bloch equation about two-level system eradiated by the optical pulse pair was deduced, it can be used to simulate the dephasing in Ge quantum dots. The dephasing time of two sublevels in Ge quantum dots is 130 fs, however, the dephasing time of transition from band to band in bulk Si is 70 fs. The dephasing in Ge quantum dots possibly results from carrier-carrier scattering and carrier-photon scattering. Due to the probability of carrier in zero dimension being smaller than that in three dimension, the dephasing time in Ge quantum dots is lightly longer than that in bulk Si.3. Using ultrafast pump-probe reflectivity spectroscopy, the reflectivity dynamics of photoexcited carriers in Fe implanted InP was studied. The transient decaying curves are characterized by initial rapid decay in several ps and by a following slower decay whose time is more than several tens ps. The first process corresponds to thermalization process, and the latter is diffusion process. Considering surface recombination, the theoretical model was proposed to simulate diffusion process. Thefitting results indicate that the surface recombination of Fe implanted InP is much larger than that of undoped InP, and the diffusion coefficient of Fe implanted InP is smaller than that of undoped InP. Using the excitation of high-energy-photon, the thermalization process of photoexcited carriers and the relationship between the relaxing time and the concentration of photoexcited carrier were investigated. The experimental results indicate that the higher the concentration of photoexcited carrier is, the shorter the time of thermalization is. The proportion between the thermalization process and diffusion process possess in all the decay process is determined by photon energy of excitation. The higher is the photon energy of excitation, the larger is the proportion of thermalization process. However, the lower is photon energy of excitation, the larger is the proportion of diffusion.4. The ultrafast absorption spectra of ZnSe nanocrystal indicate that the time of electron-electron scattering is 6.6 ps (for the average size of nanocrystal ~ 75 nm) and 2.5 ps (45 nm). With the decrease of nanocrystal, the probability of inelastic collision among carriers and nanocrystal surface increases, which results in the intension of carrier-photon coupling enhances and the time of carrier-photon scattering shortens. According to the transient photoluminescence spectra, in the lower temperature range (13 K~ 100 K), the transition probability between singlet state and triple state rapidly increases with the increasing of tenperature, which results in the radiation decay time of triple state rapidly increases. In higher temperature range (100 K~ 300 K), with the increasing of temperature, radiation recombination from singlet state to ground state dominates and the nonradiation recombination between the competitive singlet state and triple state is restrained, and therefore the radiation decay ti...