Spin Dynamics In Semiconductors And Semiconductor Nanostructures

Processing speed and integration of traditional electronic devices based on the degree of freedom of a charge still follow the prediction of Moore's law. However, quantum effect of the device becomes more and more predominant with the decrease of the size, which restricts the development of the traditional electronic devices. Semiconductor spintronics aims at utilizing or incorporating the spin degree of freedom in electronics for a new generation of spintronic devices. Spintronic devices are more stable, smaller size, faster speed, and lower power consumption than traditional electronic devices.Spin relaxation/dephasing time is an important parameter in the fabrication of spintronic devices. Experimental results about the spin relaxation/dephasing time are extreme complex because it is affected by many factors, such as temperature, density, bandgap structure, and external field etc. In this dissertation, carrier density and photon energy dependence of spin dynamics in bulk semiconductor and semiconductor nanostructrues are systematically investigated by time resolved pump-probe technique. The main results in this dissertation are concluded as follows:1. The transient carrier dynamics of intrinsic cadmium telluride (CdTe) was investigated by femtosecond time-resolved pump-probe reflectivity (fs-TRPPR) method at different photon energy and carrier density. Two relaxation processes are observed. The fast one related to carrier cooling process with typical decay time of several picoseconds (ps) increases with the carrier density, while the slow one owing to the electron hole recombination almost keeps a constant.2. Electron spin dynamics in intrinsic bulk CdTe and Indium Phosphide (InP) semiconductor crystal was studied by fs-TRPPR technique using the co-circularly and counter-circularly polarized femtosecond pulses at room temperature and 70 K. The results show that spin relaxation time decreases monotonously with increasing photon energy.3. With increasing carrier density, the electron spin relaxation time in bulk semiconductor increases initially and then decreases after reaching a maximum value. Our experimental results agree well with the recent theoretical prediction based on a fully microscopic kinetic spin Bloch equation (KSBE) approach and D'yakonov-Perel'mechanism is considered as a dominate contribution to the electron spin relaxation in intrinsic bulk semiconductor.4. The reflectivity change from bleaching to absorption in InP crystal is observed with increasing pump photon energy, which has been explained successfully in terms of the spin sensitive band filling and band gap renormalization effects.5. Exciton spin dynamics of CdSe as well as CdSe/ZnS quantum dots (QDs) was investigated with circularly polarized pump-probe transmission spectroscopy at room temperature. The excitation photon energy is tuned to be on-resonant with the 1S(h)-1S(e) exicton of the CdSe QDs. The spin dynamics of core CdSe QD shows single exponential decay with typical time constant of about several ps, while the spin dynamics in the CdSe/ZnS core/shell structure shows biexponential relaxation: a several-ps-fast-component and a slow-component with time constant of hundreds of ps. The time constant of slow process decreases with increasing the excited power. The fast spin relaxation component in both CdSe and CdSe/ZnS arises from the surface-state-trapping effect. ZnS-capped-CdSe QD can greatly reduce the surface states at the CdSe surface, and the slow spin decay comes from the long exicton lifetime in core/shell structure. Spin relaxation mechanisms based on the spin-orbit interaction are strongly inhibited in spatial confined systems, and the absence of translational motion in QDs prolongs the carriers spin lifetimes as compared to the bulk counterpart.6. Magneto-optical dynamics of water dispersed CdTe quantum dots is investigated by time-resolved Faraday rotation technique at room temperature. Wavelength dependence of Faraday rotation and ellipticity are studied across the lowest exciton absorption in CdTe QDs. It is found that a sign reversal of Faraday rotation occurs around the exciton abosprtion peak, where the Faraday ellipticity reaches the maximum. This phenomenon is caused by the level splitting owing to the quantum confinement effect.