Both GaN and ZnO are widegap semiconductors and have similar properties in terms of crystal structure, optical and electrical properties. The conductivity control of GaN has been established, so that p-n junction devices such as light emitting devices (LEDs) and laser diodes (LDs) have been already commercially available. However, it is still difficult to grow ZnO with optical and electrical properties sufficient for light-emitting devices. One of the big emerging markets for GaN-based LEDs is general lighting, in which high-brightness white LEDs are strongly required. Although various high-brightness LED structures have been proposed, the best solution would be vertical LEDs. Vertical LEDs have been fabricated either by lift-off of substrates on which LED structures are fabricated or by growing LED structures on conductive substrates. Although the lift-off technique is considered to offer superior characteristics to the other methods in terms of process and cost, the inferior reliability of the laser-lift-off technique hampers form high-yield fabrication of the devices. We have most recently developed a chemical lift-off technique, in which chemicals are used to detach device layers from substrates. This process offers a reliable method for fabricating LED structures with high reproducibility. We would like to demonstrate the fabrication of vertical LEDs using this process in this presentation.

On the other hand, the most unique feature of ZnO lies in its high exciton binding energy (60 meV). Since excitons still survive even at room temperature, this material should show nonlinear optical phenomena even at low excitation level, if excitonic processes participate in the nonlinear optical phenomena. In order to exhibit nonlinear optical effects at low excitation level, phase matching conditions should be satisfied. For this purpose, quasi-phasing matching structures are very favorable, which can be realized by the fabrication of periodically polarity inverted (PPI) structures. We have developed the fabrication processes of ZnO-based PPI structures based on interface engineering. Second-harmonic generation of light from the fabricated PPIs will be demonstrated.

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1. Highly luminescent doped nanoparticles embedded in porous silicon oxide