The hot carrier dynamics of Zn0.91Cd0.09Se/ZnSe multi-quantum wells were studied using the femtosecond time-resolved photoluminescence upconversion technique. The carrier cooling behavior was investigated for different compositions at various lattice temperatures. The hot carriers generated photoexcitation by 405nm Ti:sapphire laser pulses release their excess energy primarily through carrier-LO-phonon interaction. As the excess energy reduce to the amount that lower than the energy of LO phonon, the excess energy was released by carrier-TA-phonon scattering before radiative recombination occurs. We have determined the scattering times of carrier-LO-phonon scattering at different lattice temperatures. No hot phonon effects was found at low photoexcited carrier density. The dependence of photoluminescence lifetime on wavelength was also discussed.

Abstract We have extended the spectral range of the current PL-upconversion apparatus to be operated in infrared. Using the IRPL-upconversion¡Awe study the behavior of carrier cooling of InN film and the relationship between the spacer and lifetime in InAs/GaAs stacked QDs . We excited InN film of the band gap of 0.74eV with ultrafast Ti:sapphire laser of the wavelength 404nm. We found the phonon emission time by hot carriers of InN is 14fs. The hot carriers release their excess energy to the lattice of 35K with a timescale of 100ps. We observed in InAs/GaAs QDs that the shorter life time for samples with thin spacer is due to tunneling effect.

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This theses investigates the carrier dynamics in Indium Nitride thin films grown on Si(111) substrates by means of ultrafast time-resolved photoluminescence (TRPL) apparatus. The study of energy relaxation shows hot phonon effective is prominent at photogenerated carrier concentration above 4¡Ñ10^18cm^-3 and become insignificant at carrier concentration below 7¡Ñ10^17cm^-3. Effective phonon emission times in the range of 116 to 23 femtoseoncds are obtained from the time evolution of carrier temperature assuming that the carrier-LO-phonon interaction is the dominant energy relaxation process. In the study of carrier recombination, the TRPL¡¦s are studied at the peak energies of the time-integrated PL at various lattice temperatures and are converted to decay rates with a rate equation, which includes the nonradiative and radiative coefficients, and a nonlinear dependence of PL intensity on the photogenerated carrier concentration. The increase with temperatures of the Shockley-Read-Hall rates implies that, in addition to the mid-gap defect states, a thermally activated trapping may become prominent at high lattice temperatures due to the increased kinetic energy gained by the carriers. The radiative recombination is the dominated recombination mechanism at low temperature but become trivial at high temperature. The fitted radiative coefficient at a temperature of 35K is consistent to the theoretical prediction. The Auger recombination exhibits a quadratic dependence on carrier concentration and becomes effective at high carrier concentration and at high temperature. The fitted Auger recombination coefficients are comparable to those of InGaAs and InGaAsP materials with band gap energies in the range of 0.6-0.8eV.

This thesis is concerned with the influence of nanoscale boundaries and interfaces upon the electronic processes that occur within the inorganic semiconductors. Inorganic semiconductor nanowires and their blends with semiconducting polymers have been investigated using state-of-the-art ultrafast optical techniques to provide information on the sub-picosecond to nanosecond photoexcitation dynamics in these systems. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising nanowires in hybrid organic photovoltaic devices, revealing the performances to date. The experimental methods used during the thesis are detailed in Chapter 3. Chapter 4 describes the crucial roles of surface passivation on the ultrafast dynamics of exciton formation in gallium arsenide (GaAs) nanowires. By passivating the surface states of nanowires, exciton formation via the bimolecular conversion of electron-hole plasma can observed over few hundred picoseconds, in-contrast to the fast carrier trapping in 10 ps observed in the uncoated nanowires. Chapter 5 presents a novel method to passivate the surface-states of GaAs nanowires using semiconducting polymer. The carrier lifetime in the nanowires can be strongly enhanced when the ionization potential of the overcoated semiconducting polymer is smaller than the work function of the nanowires and the surface native oxide layers of nanowires are removed. Finally, Chapter 6 shows that the carrier cooling in the type-II wurtzite-zincblend InP nanowires is reduced by order-of magnitude during the spatial charge-transfer across the type-II heterojunction. The works decribed in this thesis reveals the crucial role of surface-states and bulk defects on the carrier dynamics of semiconductor nanowires. In-addition, a novel approach to passivate the surface defect states of nanowires using semiconducting polymers was developed.