| The frontiers of micro/nano photonic structures based light emitting device research spans a variety of III-nitride wide-bandgap semiconductors to achieve high efficiencies. The development of compact, robust, milliwatt-level LEDs that emit in ultraviolet (UV) and deep-ultraviolet (deep-UV) would greatly impact a number of applications from fluorescence-based biological agent detection to solid-state lighting. Continued research on the improvement of high efficiency short wavelength post-epi device fabrication procedures is one of high priorities.;This dissertation focuses on the understanding, design optimization, post-epi advance processing, flip chip packaging, and characterization of several key aspects of micro/nano photonic structures for UV/deep-UV light emitting devices. From the studies of AlInGaN-based UV LEDs epi-structures, we found that the high dislocation densities severely reduce emission efficiency. Therefore, reduced defect density substrates or nucleation process is key to realizing high brightness devices. For deep-UV LEDs (&lgr;∼300 nm), our focus has been to understand the physical phenomena and properties that dominate as the device size scales down. We concluded that the compact-size light sources offer benefits over lateral-geometry emitters. Device size dependence study showed that the circular disk and interdigitated LEDs design have been very effective to overcome current crowding and current spreading issues. Through a comparative study of various disk sizes, we found that the power density of the small sized ∼275 mum disk LEDs is optimal and can be considered ideal for high power compact UV/deep-UV LEDs. The dry plasma treatment damage in p-AlGaN/GaN was studied systematically using Schottky diode measurements. Various devices, including individual lateral geometry, hexagonal geometry, circular disk and interdigitated with varying mesa sizes (85 mum to 1 mm 2) were investigated. Monolithically integrated micro/nano structures, such as, microlens, photonic crystals and Fresnel lenses were also processed.;Based on the latest technical improvements, local thermal management (ILTM) of flip-chip packaged UV/deep-UV LEDs were explored for high efficiency. Contrary to conventional electroplating technique, void free bumps with better surface morphology were used with sputter-coating technology. The devices processed with ILTM techniques were thermally more stable, attributed to the unique metallurgical properties. With the L-I-V characterization of the processed devices, an enhanced optical power of 0.25 mW at 20 mA was obtained. Finally, the effect of multilayer's materials annealing, substrate surface nano-pattering/roughening, reflection multilayers and the packaging of the devices was also studied for the high efficiency of UV/deep-UV LEDs. After a careful optimization of post-epi processes individually, light emitting devices were fabricated in a single process flow. After performing a series of control experiments, we found that a modified metallization scheme such as Al/Ti/Au on bi-layer Ni/Au p-contacts could be a viable option for a stable reflection layer. An ILTM for the flip-chip devices (FCLEDs) were employed. With the comparison of conventional to FCLEDs, an output power of 0.45 mW with a forward voltage of 6.7 V at 20 mA was achieved. The thermal stability of these devices is far better compared to conventional packaged devices. Optoelectronic characterization of these devices suggested that UV/deep-UV light emitting device's life time has significantly improved with overall efficiency. |
