ZnO Fin Optical Cavities

Nanostructured semiconductors have shown great promise toward miniaturization of electrically driven semiconductor lasers. Previously, we reported on an electrically driven sub-micron size ZnO-GaN nanofin (fin) light-emitting diode (LED) architecture that at high current densities showed a droop-free behavior and achieved lasing. In this work, we provide a deeper analysis of lateral ZnO fins as an optical gain medium and an optical resonator to better understand their unexpected performance. As a gain medium, its ultraviolet (UV) excitonic emission is explored and contrasted with upright ZnO fins and bulk ZnO single crystal. A study of excitonic transitions in the range of 5-383 K shows that band gap of the lateral fin is nearly independent of temperature, while upright fins and bulk ZnO single crystal demonstrate significant band gap decrease due to lattice dilation. Results show the efficiency of electron-hole (e-h) pair collection in lateral fins is directly dependent on the temperature, that is, as the heterojunction temperature increases, more carrier confinement occurs within the fin. These observations are explained by the planar geometry of the lateral fin and its large interface with the GaN substrate that becomes advantageous in heat dissipation. Formation of resonance modes in lateral fins was analyzed using wavelength- and angle-resolved cathodoluminescence spectroscopy. Unique coherency and anisotropy in emission are observed due to light reflection from the internal sidewalls that appears as interference fringes. These fringes depend on the fin cavity height and, interestingly, are only observed for UV wavelengths of light. Results show ZnO due to its large exciton binding energy, and the fin-shape could be a great candidate both for enhancing the internal quantum efficiency and for realizing bright, small footprint cavities for LED and surface-emitting laser applications.