Optical properties of semiconductor microcrystallites from bulk like to quantum dot.

Abstract

A very small semiconductor crystal with a size on the order of the exciton Bohr radius exhibits discrete electronic states compared to the quasi-continuum of states in bulk semiconductor. These states have been studied using optical spectroscopy. In this dissertation, various optical spectroscopic methods have been used to investigate the energy levels of quantum dots. Transmission technique was used to obtain linear absorption spectra; photo-luminescence was measured; pump and probe spectroscopy was used to investigate nonlinear optical properties, excitation spectroscopy was employed to obtain two-photon absorption spectra, and a new spectroscopic method involving 3 beams was used to investigate energy levels in a system with large inhomogeneous broadening. Beginning with a very simple theoretical model, a single particle in an infinite confining potential using the effective mass approximation, the electronic states of quantum dots can be approximated. By including the Coulomb interaction, dielectric polarization, and eventually mixing of the bands, these simple states can be used as a basis for forming more realistic wavefunctions. The simple theoretical model, using a single conduction and a single valence band, can be used to describe the observed discrete resonances of quantum dots in the linear absorption spectra. To investigate relaxation processes, the photo-luminescent properties of quantum dots have been studied. For various reasons, donor-acceptor pairs appear to be responsible for the luminescence, however the interpretation still has some problems. In the investigation, non-radiative recombination is found to be very strong, but its origin can only be speculated upon. Employing pump and probe spectroscopy, nonlinear optical properties of CdS quantum dots have been investigated. From this investigation, bleaching due to phase space filling has been observed as well as transitions to biexciton states. However the transition to the ground state biexciton has only be seen in two samples. This is attributed to broad bleaching in the differential absorption spectra. In addition, the photo-darkening effect has been investigated with different matrix systems. The effect permanently changes the samples' optical properties. By controlling the matrix surrounding the dots we have significantly reduced the photo-darkening effect. From theoretical calculations, it is expected that confinement-induced mixing of the bands are to be observed by measuring one and two photon absorption spectra. We have observed this phenomenon and have explained the results by assuming mixing of the bands, especially the valence bands. With recently available quantum dot samples showing a narrow hole burning, a three beam experiment with pump and probe and saturator is used to obtain better resolution than can be obtained from conventional pump and probe spectroscopy. From these experimental results, biexciton binding energies, and discrete transitions from different valence bands can be observed. Also, enhancement of the biexciton binding energy with reduced dot size and strong mixing of the bands has been observed experimentally.