GaN-Based Photonic Crystal Microcavity Light Sources
III-Nitride optoelectronic devices have become important in a wide variety of applications ranging from semiconductor lighting to medical surgeries, where the demand for blue and ultraviolet light emitting diodes or laser diodes has dramatically increased in recent years. Highly directional and coherent light emission can be achieved by employing photonic crystal (PC) microcavities. In this proposal, the development and characterization of GaN-based PC microcavity light sources are proposed. In our research lab, we can design and optimize PC microcavities using three-dimensional finite difference time domain (3D-FDTD) and plane wave expansion (PWE) methods. By sharing processing and characterization equipments with other research groups, we can fabricate and characterize the PC microcavity devices efficiently. Currently we are capable of fabricating two-dimensional photonic crystals with the lattice constant of 200nm to 300 nm and the etch depth of 100nm. We will continually improve the process of PC fabrication for high aspect ratio and applications in the ultraviolet wavelength range. In the first year of this three-year research proposal, we plan to develop the methodology to design PC microcavities and integrate the fabrication process of photonic crystals and light emitting diodes. The research objectives include the demonstration of microcavity effects in optically-pumped GaN LEDs, and the optimization of the PC microcavity designs. In the second year, we will focus on the development and characterization of electrically-injected devices. Current confinement and microcavity design for polar and non-polar GaN materials will become crucial issues at this stage. Well-designed epitaxial structures, device topology, and PC microcavities are key elements to achieve our research objectives. In the third year, we plan to develop advanced PC microcavity emitters. Two device structures are of particular interest. One is the introduction of Bragg mirrors as the photon confinement in the third dimension. The other is to employ quantum wells (QWs) or quantum dots (QDs) as the active region. For the former structure, we have developed theoretical calculations on the microcavity design. If well-designed, such emitter can exhibit very low threshold current due to 3D photon confinement. However, the complex epitaxy and device fabrication will be the challenges. For the latter structure, the device could potentially have very high efficiency due to both the optical and electrical confinements. Moreover, recent developments in single photon sources have exploited the structure of semiconductor quantum dots placed in a high quality-factor (Q) microcavity. Thus, low-temperature characterization of fabricated devices will also be performed to investigate the potential of PC-QD emitters as the light sources for quantum communication and quantum encryption.