Applications of Optical Spectroscopy to Studies of Electronic and Vibrational States in Semiconductor Nanostructures

This dissertation, "Applications of Optical Spectroscopy to Studies of Electronic and Vibrational States in Semiconductor Nanostructures" by Jiqiang, Ning, 宁吉強, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled APPLICATIONS OF OPTICAL SPECTROSCOPY TO STUDIES OF ELECTRONIC AND VIBRATIONAL STATES IN SEMICONDUCTOR NANOSTRUCTURES Submitted by Ning Jiqiang for the degree of Doctor of Philosophy at The University of Hong Kong in December 2007 Time-resolved Kerr rotation (TRKR) spectroscopy, reflectance spectroscopy, and Raman spectroscopy were employed to investigate the electronic and vibrational properties of semiconductor nanostructures, namely InAs/GaAs ultrafine multiple quantum wells (MQW's), the nanosized space-charge double layer formed on the surface of a ZnO bulk crystal, and GaN nanowires, respectively. From the measured TRKR data of the InAs/GaAs MQW's at 5 K, the transition energies from the heavy- (hh) and light-hole (lh) to the conduction band were determined to be 1.4346 and 1.4475 eV, respectively, resulting in an energy separation of 12.9 meV. Such a well-separated valence band structure allows selective excitation of electrons with opposite spin orientations. By selective excitation of spin-up and down electrons, it was found that the spin polarized electrons with opposite spin orientations show distinctive dynamical behaviors. For the electrons from the hh subband, their spin polarizations experience a fast relaxation process with a time constant of tens of picoseconds. In strong contrast to the hh case, the spin polarizations of electrons excited from the lh subband exhibit a slow relaxation process within hundreds of picoseconds since the light holes lose their spin orientations almost immediately after the excitation. Therefore, the long relaxation time basically reflects the spin relaxation process of electrons resonantly excited from the lh subband. Due to the long-lived heavy-hole spins, bi-excitons with defined spin orientation can form, leading to the observation of quantum beats between the bi-excitons and original excitons in the TRKR spectra. The bi-exciton binding energy of 2.72 0.02 meV was determined from the oscillation period. The beating amplitudes exhibit a clear resonance effect with respect to the wavelength of the excitation light, which gives a confirmative evidence of the exciton effect. Regular slow oscillations were observed in the low-temperature reflectance spectra measured from a bulk crystalline ZnO rod at almost normal incident. The oscillations can be interpreted as Fabry-Perot interfering due to the formation of a nanosized space-charge double layer on the ZnO surface. The oscillation periods possess a linear dependence on the wavelengths of the incident light, which suggests a constant extending rate of the surface space-charge double layer under illumination. From the experimental data, the extending rate of about 1.3 nm/min was deduced. Confocal micro-Raman scattering measurements were carried out to identify a -1 vibrational mode at 418 cm which is commonly observed in GaN nanomaterials. The spectra measured from a series of GaN nanowires systematically prepared by nitriding β-Ga O nanowires under different conditions show a clear evolution of the 2 3 mode, which reveals that the mode most likely originates from the local octahedral Ga-N bonding structures which can abundantly form in GaN nanostructures. This assignment is additionally supported by the high-resolution electron transmission microscopy observation. DOI: 10.5353/th_b3963422 Subjects