Improvements of the internal quantum efficiency by reduction of the threading dislocation density and of the light extraction by using UV transparent p-type cladding and contact layers, UV reflecting ohmic contact, and chip encapsulation with optimized shape and refractive index allowed us to obtain the external quantum efficiency of 10.4% at 20 mA CW current with the output power up to 9.3 mW at 278 nm for AlGaN-based deep-ultraviolet light-emitting diodes grown on sapphire substrates.

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7 ϫ10 17 cm Ϫ3) annealed in forming gas at 600°C reached a minimum contact resistivity of 8 ϫ10 Ϫ6 ⍀ cm 2 and had much better thermal stability than reported by previous researchers. Ti/Al ͑35/115 nm͒ contacts on n-GaN (5ϫ10 17 cm Ϫ3) had resistivities of 7ϫ10 Ϫ6 and 5 ϫ10 Ϫ6 ⍀ cm 2 after annealing in Ar at 400°C for 5 min and 600°C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400°C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400-600°C range includes Ti reducing the GaN native oxide and an Al-Ti intermetallic coming into intimate contact with the GaN.

We report on a metal–insulator–semiconductor heterostructure field-effect transistor (MISHFET) using Si3N4 film simultaneously for channel passivation and as a gate insulator. This design results in increased radio-frequency (rf) powers by reduction of the current collapse and it reduces the gate leakage currents by four orders of magnitude. A MISHFET room temperature gate current of about 90 pA/mm increases to only 1000 pA/mm at ambient temperature as high as 300 °C. Pulsed measurements show that unlike metal–oxide–semiconductor HFETs and regular HFETs, in a Si3N4 MISHFET, the gate voltage amplitude required for current collapse is much higher than the threshold voltage. Therefore, it exhibits significantly reduced rf current collapse.

We report blue-purple pn-junction light-emitting diodes (LEDs) with a-plane GaN–InGaN multiple quantum well active region. The LEDs were grown over r-plane sapphire substrates. Our study has shown the low pump intensity photoluminencence and electroluminescence to be dominated by emission from the band-tail states which then saturates rapidly giving rise to band-edge emission.

a-plane GaN templates and coalesced epitaxial lateral overgrown (ELOG) films on r-plane sapphire substrates were investigated by x-ray diffraction (XRD). The a-plane GaN templates were found to have [0001]-oriented stripe-features, which is related to anisotropic mosaicity. For the mosaic blocks, the mosaicity reached the largest and the smallest values along the [11̄00] and the [0001] directions. The ELOG procedure with the SiO2 mask stripes perpendicular to the [0001] direction limits the preferable growth along this direction, and thereby enhances the [11̄00] growth. This leads to large-area, featureless, a-plane GaN films for which the wing tilt and not the fine mosaic block size becomes the major XRD line-broadening mechanism.

We present the results of a comparative photoluminescence (PL) study of GaN and InGaN-based epilayers, and InGaN/GaN multiple quantum wells (MQWs). Room-temperature PL spectra were measured for a very broad range of optical excitation from 10 mW/cm2 up to 1 MW/cm2. In contrast to GaN epilayers, all In-containing samples exhibited an excitation-induced blueshift of the peak emission. In addition, the blueshift of the emission in the InGaN epilayers with the same composition as the quantum well was significantly smaller. The comparison of the blueshift in the “bulk” InGaN and in the MQWs allowed us to separate two different mechanisms responsible for this effect: (i) filling of the localized states in In-rich areas and (ii) screening of the polarization electric field in strained MQW structures.

We report on an AlN/AlGaN superlattice approach to grow high-Al-content thick n+-AlGaN layers over c-plane sapphire substrates. Insertion of a set of AlN/AlGaN superlattices is shown to significantly reduce the biaxial tensile strain, thereby resulting in 3-μm-thick, crack-free Al0.2Ga0.8N layers. These high-quality, low-sheet-resistive layers are of key importance to avoid current crowding in quaternary AlInGaN multiple-quantum-well deep-ultraviolet light-emitting diodes over sapphire substrates.

We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.