Optical Properties of Low Dimensional Semiconductor Materials
Sammanfattning: This dissertation presents a serial study on optical properties of different semiconductor materials. Three main types of studies are addressed: The role of doping levels of N and Al atoms in the room-temperature photoluminescence (PL) of 4H-SiC films for optoelectronic applications; the use of a basic Monte Carlo method combined with probability calculations of the time-dependent Schroedinger equation to manifest multi-photon absorption and emission of II-VI compound quantum dots (QDs) for bioimaging; a theoretical quantum chemistry approach to study of structure and optical properties of InGaAsN and GaAs clusters for laser technology applications. 4H-SiC films were grown on AlN/SiC(100) substrates by a chemical vapour deposition (CVD) system. Three well-defined room-temperature PL peaks close to the band-gap energy were observed. By a detailed theoretical analysis of optical transitions in the samples, it was found that the PL peaks are most probably due to the optical transitions between impurity levels and band edges, and the transition between the second minimum of the conduction band and the top of the valance band. Special attention has been paid to effects of doping levels of N and Al impurities. Optical transitions in several II-VI semiconductor QDs have been studied by a quantum Monte Carlo method. We model the QD energy band structure by a spherical square quantum well and the electrons in the conduction band and holes in the valence band by the effective mass approximation. The optical probabilities of optical transitions induced by ultrafast and ultraintense laser pulses are calculated from the time-dependent Schroedinger equation. With the inclusion of the nonradiative electron-phonon processes, the calculated absorption and emission spectra are in agreement with experimental work. The dynamic processes and up-conversion luminescence of the QDs, required for many applications including bio-imaging, are demonstrated. Quantum chemistry is used to study InGaAsN and GaAs nano systems. The molecular structures of a series of dilute-nitride zinc blende InGaNAs clusters are examined from the energy point of view with a semi-empirical method. The optimum cluster configurations are identified by which we can identify the detailed bonding structures and the effects of In mole fraction. After proper geometry construction, an effective central insertion scheme has been implemented to study the electronic band structures of GaAs at the first-principles level. The formation of energy bands and quantum confinement effects have been revealed, thus providing theoretical support for laser design.
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