Development of blue-ultraviolet wavelength semiconductor lasers is of considerable current interest. Essentially all current effort has been focused on ZnSe- and GaN-based heterostructures, however ZnO has received little attention for such applications. This thesis reports the studies of UV lasing in ZnO thin films....[ Read more ]

Development of blue-ultraviolet wavelength semiconductor lasers is of considerable current interest. Essentially all current effort has been focused on ZnSe- and GaN-based heterostructures, however ZnO has received little attention for such applications. This thesis reports the studies of UV lasing in ZnO thin films.

The first observation of room temperature stimulated and lasing emission in ZnO thin films are presented. The optical gain, threshold, and polarization properties of the laser emission are investigated in order to understand the mechanisms responsible for the observed room-temperature UV lasing.

The ZnO films studied in this thesis are grown by the laser-molecular-beam-epitaxy method and consist of self-assembled, ordered arrays of hexagonal microcrystallites. Structural data shows that the hexagonal facets of these rnicrocrystallites are parallel to those of the others and form natural Fabry-Perot lasing cavities.

Under moderate optical excitation, a stimulated emission band, labeled as P band, is observed at room temperature. The P stimulated emission band is identified to be due to an exciton-exciton collision (ex-ex) process. At higher pumping intensity, when the excitation density exceeds the Mott density, a new stimulated emission band, N band, is observed. The N band occurs at a lower photon energy and exhibits red-shift with increasing excitation intensities. N band is identified to be due to radiative recombination of electrons and holes in an electron-hole plasma (EHP). Under optical pumping using a stripe geometry, UV lasing (388 nm) is observed in these thin films. At low pumping intensity, lasing is due to the ex-ex collision process. At higher pumping intensity, the lasing is due to recombination of electrons and holes in a EHP. The lasing cavity is formed by the hexagonal microcrystallites. Lasing threshold is observed to be the lowest for ZnO films with a thickness near 55 nm as a result of the enhanced spontaneous emission into the TE₀ mode in a micro-photonic waveguide structure.

The optical gain spectra are measured using the variable stripe length method. The peak optical gain is determined to be 280 cm^-1 for a 55 nm thick film at a pumping intensity of 40 kw/cm², which is about an order of magnitude larger than the largest value reported for bulk ZnO crystals measured using much higher fluence. The gain spectra exhibit a transition from excitonic gain to electron-hole plasma gain as increasing pumping intensity.

Stimulated emission as well as lasing due to the ex-ex process is only observed in films containing microcrystallites with diameters in the range 50-60 nm. We believe that the ex-ex collision is enchanced by weak quantum confinement in this size regime.