Excitonic gain and room-temperature UV lasing in ZnO microcrystallite thin films

Development of blue-ultraviolet wavelength semiconductor lasers is of considerable current interest. Essentially all current effort is focused on ZnSe- and GaN-based heterostructures. As an oxide, ZnO is superior to nitrides and selenides in thermal stability and in resistance to chemical attack and oxidation. ZnO has received little attention on short wavelength optoelectronic applications for the last years due to relative low optical gain and difficulty of making high quality thin film of ZnO.; This thesis reports the observation of room temperature stimulated and lasing emission of ZnO hexagonal microcrystallite film, and investigation of optical gain, threshold, and polarization properties in order to understand the mechanism of lasing.; The films are grown by laser-molecular-beam-epitaxy method and consist of self-assembled, ordered arrays of hexagonal microcrystallites. Structural data show that the hexagonal facets of these microcrystallites are parallel to those of the others and form natural Fabry-Perot lasing cavities.; Two stimulated emission bands which labeled as P band and N band are observed. Under moderate optical excitation, P band stimulated emission is resulted by an exciton-exciton collision (ex-ex) process. At high pumping intensity, the stimulated emission mechanism is switched to that of electron-hole recombination in an electron-hole plasma after the Mott transition. The stimulated emission of the N band occurs at a lower photon energy and exhibits the characteristic red-shift with increasing excitation intensities. The RT UV laser of ZnO microcrystallite thin film is also investigated. The lasing mechanism can be excitonic recombination at low threshold, or electron-hole plasma at high threshold. The laser cavity is formed by the hexagonal microcrystallites. TE and TM modes are observed in both the spontaneous emission spectrum and the stimulated emission spectrum. At the threshold pumping intensity, lasing is observed to occur in TE{dollar}{bsol}sb0{dollar} mode because of the enhanced spontaneous emission into the lower order modes in a waveguide structure.; The gain spectra are measured using the variable stripe method. The peak optical gain is determined to be 280 cm{dollar}{bsol}sp{lcub}-1{rcub}{dollar} for a 55 nm thick film under the pumping intensity 40 kW/cm{dollar}{bsol}sp2,{dollar} which is about an order of magnitude larger than the largest value reported for a bulk ZnO crystal. Gain spectra show the transition from excitonic gain to electron-hole plasma gain with increasing of pump rate. This phenomenon disappears when the diameters of hexagonal grain increase. We believe that it comes from quantum confinement of exciton at this region. The mechanisms responsible optical gain is investigated and discussed in detail.; The observations reported here should encourage further investigations of ZnO as another possible material for short-wavelength optoelectronics applications.