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We report our current development progress of ultraviolet vertical-cavity surface-emitting lasers, which included the development of an electrically conducting n-DBR consisting of 40-pairs of Si-doped quarter-wavelength layers of Al0.12Ga0.88N and GaN. A peak reflectivity of 91.6% at 368 nm was measured and a series resistance of 17.7Ω was extracted near the maximum measured current of 100 mA. Furthermore, a micro-cavity light emitting diode was demonstrated by utilizing the established n-DBR. A 2λ cavity was subsequently grown on the 40-pair Al0.12Ga0.88N/GaN n-DBR and a peak wavelength of 371.4 nm was observed with spectral linewidth of 5.8 nm.

High-resolution imaging in ultraviolet (UV) bands has many a pplications in defense and commercial systems. The shortest wavelength is desired for increased spatial resolution, which allows for small pixels and large formats. The next frontier is to develop UV avalanche photodiode (UV-APD) arrays with high gain to demonstrate high-resolution imaging. We compare performance characteristics of front-illuminated Al 0.05 Ga 0.95 N UV-APDs grown on a free-standing (FS) GaN substrate and a GaN/sapphire template. UV-APDs grown on a FS-GaN substrate show lower dark current densities for all fabricated mesa sizes than similar UV-APDs grown on a GaN/sapph ire template. In addition, stable avalanche gain higher than 5×10 5 and a significant increase in the responsivity of UV-APDs grown on a FS-GaN substrate are observed as a result of avalanche multiplication at high reverse bias. We believe that the high crystalline quality of Al 0.05 Ga 0.95 N UV-APDs grown on a FS-GaN substrate with low dislocation density is responsible for the observed improvement of low leakage currents, high performance photodetector characteristics, and reliability of the devices. Keywords: GaN, AlGaN, avalanche photodiodes, ultraviolet, high gain

A 245.3 nm deep ultraviolet optically pumped AlGaN based multiple-quantum-well laser operating at room temperature is described. Epitaxial growth was performed by metalorganic chemical vapor deposition on a c-plane bulk AlN substrate at a growth temperature of ~ 1130 °C. The wafer was fabricated into cleaved bars with a cavity length of ~1.45 mm and the lasing threshold was determined to be 297 kW/cm 2 under pulsed 193 nm ArF excimer laser excitation. A further ~20% reduction in threshold pumping power density was observed with six pairs of SiO 2 /HfO 2 distributed Bragg reflector deposited at the rear side of facets.

Blue LED progress has laid the ground works of nitride technology to tackle the higher challenge of longer wavelength direct emitters of green, yellow, and orange. Use of bulk GaN substrate allows leapfrogging epitaxy development and offers crystallographic planes that allow higher crystal perfection and a control over piezoelectric polarization. Their combination allows stabilization of emission wavelength with current. Further improvement is found in substrate patterning on the micro and nano-meter length scale where we find roughly equal performance enhancement due to both, enhancement in light extraction and enhanced crystalline perfection.

Energy efficiency has been the primary driving force for solid state lighting to replace wasteful incandescent lamps by light emitting diodes (LEDs). Recently, rising cost for rare earth metals has redoubled the push to also replace fluorescent and compact fluorescent lighting. Phosphors in fluorescent lamps heavily rely on rare earth metals and even first generation LEDs use such phosphors, albeit at much lower quantities. The role of phosphors to expand a narrow wavelength source into a wider spectrum is a very lossy process in itself and can be circumvented altogether by second generation LEDs, where the full visible solar spectrum is directly replicated by direct emitting LEDs. We here report progress of our work towards this goal, in particular by the development of high brightness direct emitting green group- III nitride LEDs.

The green spectral region provides a formidable challenge for energy efficient light emitting diodes. In metal organic vapor phase epitaxy we developed GaInN/GaN quantum well material suitable for 500 - 580 nm LEDs by rigorous defect reduction and thrive for alloy uniformity. We achieve best results in homoepitaxy on polar c-plane, and non-polar a -plane and m -plane bulk GaN. By the choice of crystal orientation, the dipole of piezoelectric polarization in the quantum wells can be optimized for highest diode efficiency. We report progress towards the goal of reduced efficiency droop at longer wavelengths.

InGaN/GaN multiple quantum well (MQW) light emitting diode (LED) structures with blue and green light emissions have been grown on sapphire substrates by metalorganic chemical vapor deposition. They are investigated by high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL) and photoluminescence excitation (PLE). HR-XRD showed multiple satellite peaks up to 10th order due to the quantum well superlattice confinement effects. HR-TEM determined the MQW structures and parameters, indicating the high quality of layer interfaces of these LED samples. Excitation power-dependent PL predicates that both piezoelectric field-induced quantum-confined Stark effect and band filling effect influence the luminescent properties. Temperature-dependent PL shows that the QW PL emission peak exhibits a monotonic red-shift and that the full width at half maximum of the PL band shows a W-shaped temperature-dependent behavior with increasing temperature. From the PLE results, a large energy difference, so-called quantum confined Stokes shift, between the band-edge absorption and emission was observed. Penetrating TEM revealed the V-shape defects, and quantum dot-like structures within the InGaN well region, which leads to intense light emissions from these MQW LEDs.

Increasing emission power and efficiency in green light emitting diodes is one of the big challenges towards all-solidstate lighting. The prime challenge lies in the combination of extension of wavelength from 470 nm blue to 525 nm green while maintaining the emission power level. Commonly a steep decrease in power is observed. In a broad development effort we have been able to ameliorate that decrease significantly and obtain bare die performance at 525 nm of 1.6 mW at 20 mA for 350x350 µm 2 dies. Here we discuss critical die performance and wafer yield aspects of our optimization approach to the active layer of the GaInN/GaN quantum wells.

Transverse mode in GaN-based violet laser diodes for both vertical and horizontal directions was investigated. In order to achieve stable fundamental mode operation in vertical direction, thick AlGaN contact layer is found to be effective. For the stabilization of a transverse-mode in horizontal direction of the conventional ridge-waveguide structure, it is necessary to precisely control the remaining thickness of p-AlGaN cladding layer. In comparison, inner stripe structure using AlGaN current blocking layer has wide feasibility of the device parameter, excellent stability of large optical confinement, and small aspect ratio of beam divergence, under the condition of the precise control of the AlN molar fraction in AlGaN current blocking layer.

Among the current flame detectors, UV selective photosensor has the highest performance thanks to its high ability to follow the turn down of burners. However, its utilization is limited to industrial-use because of its lifetime of 1-2 years, operation temperature below 120 degrees C, and its high system cost including power supply of 300V and cooling. Accordingly, solid sate flame detector is strongly desired. The required elements for the UV flame detection is as follows: a) photo current given by low intensity light should be greater than dark current; b) high rejection to the background is indispensable. AlGaN was chosen through material selection and feasibility study had been conducted. For a), using pn-GaN grown on single buffer layer, the leakage current comes from both mesa sidewall leakage and tunneling current through PN junction. Both leakages were approximately 2 orders larger to detect 1nW/cm2, but thought to be within outreach using multi-buffer crystal growth technique and sensor design. For b), AlGaN photocurrent fabricated on the multibuffer layer has shown responsivity difference of 3 orders at the bandage of 4.4 eV, which satisfied the high rejection in UV region of 300- 400nm.