Jianping Zhang

A New Selective Area Lateral Epitaxy Approach for Depositing a-Plane GaN over r-Plane Sapphire

We report a new epitaxy procedure for growing extremely low defect density a-plane GaN films over r-plane sapphire. By combining selective area growth through a SiO2 mask opening to produce high height to width aspect ratio a-plane GaN pillars and lateral epitaxy from their c-plane facets, we obtained fully coalesced a-plane GaN films. The excellent structural, optical and electrical characteristics of these selective area lateral epitaxy (SALE) deposited films make them ideal for high efficiency III-N electronic and optoelectronic devices.

AlGaN Deep-Ultraviolet Light-Emitting Diodes

We report on the development of AlGaN based deep ultraviolet (DUV) light-emitting diodes (LEDs) grown by migration-enhanced metalorganic chemical vapor deposition (MEMOCVD). Improved quality of AlGaN has allowed us to achieve milliwatt-power at wavelengths ranging from 365 to 265 nm. For 295 and 280 nm LEDs, record CW powers with wall-plug-efficiency approaching 1.0% were realized. The CW power reached 1.2 and 1.0 mW at 20 mA for 280 and 295 nm LEDs, respectively. A multiple-chip package (UV lamp) emitted CW power of 11 mW at the wavelength of 280 nm. Under pulse operation, the 280 nm UV lamp produced power as high as 56 mW. The CW power levels at 20 mA were 0.5, 0.25 and 0.15 mW for a single-chip 275, 270 and 265 nm LEDs, respectively. A 265 nm UV lamp exhibited a record high CW power exceeding 1.5 mW. The applications of these DUV LEDs in bio-agents detection have been demonstrated and the preliminary results will be presented.

Sub-Milliwatt Power III-N Light Emitting Diodes at 285 nm

Using low defect density n+-Al0.4Ga0.6N buffer layers we fabricated AlGaN p-n junction light emitting diodes over sapphire substrates with peak emission at 285 nm. Powers as high as 0.15 mW were measured at 400 mA pulse pumping.

Lateral Epitaxial Overgrowth of Fully Coalesced A-Plane GaN on R-Plane Sapphire

Fully coalesced epitaxial laterally overgrown a-plane GaN films were characterized for their structural and optical quality. The films had a very smooth surface with a root mean square roughness as low as 4.6 ? for a 5 ?m ? 5 ?m atomic force microscope scan area. They exhibited a wing tilt of only 0.27? and optically pumped stimulated emission, which establish their high structural and optical quality. These non-polar films are ideal for fabricating high-efficiency optoelectronic and electronic devices.

10 Milliwatt Pulse Operation of 265 nm AlGaN Light Emitting Diodes

We report on the development of solid-state deep ultraviolet light sources optimized for the germicidal applications. Pulsed power levels in excess of 10 mW were achieved for AlGaN based 265 nm light emitting diodes by improving the material quality using Migration-Enhanced Metal Organic Chemical Vapor Deposition. Packaged devices reached the continuous-wave power of 237 µW at 30 mA and a pulse power exceeding 10 mW for 1.2 A driving current.

Influence of pulse width on electroluminescence and junction temperature of AlInGaN deep ultraviolet light-emitting diodes

The behavior of electroluminescence (EL) and junction temperature of AlInGaN deep ultraviolet light-emitting diodes under pulse-width modulation is investigated. The redshift of both emissions from quantum-well (P1) and localized (P2) states in the EL spectra and the increase of intensity ratio of P1 to P2 are observed with the increase of duty cycle. The photoluminescence of p-GaN contact layer is adopted to measure the junction temperature, which shows a linear relation with the duty cycle. Meanwhile, the duty cycle coefficient of junction temperature increases with the increase of injection current. The EL behaviors are explained by the thermal effect induced by pulse current at high duty cycle.

Submilliwatt Operation of AlInGaN Based Multifinger-Design 315 nm Light Emitting Diode (LED) over Sapphire Substrate

We report on sub-milliwatt operation of ultra-violet light emitting diodes on sapphire with a peak emission wavelength of 315 nm having quaternary AlInGaN multiple quantum well (MQW) active region. A significant increase in the device-emitted power was achieved using an interdigitated finger geometry design, which reduces the differential resistance and eliminates current crowding. The flip-chip mounting of the diced chips with sapphire substrates improves thermal management and increases the light extraction efficiency from the device active area. For 315 nm emission an optical power as high as 0.23 mW was obtained with 800 mA pulsed pumping current.

Pulsed metalorganic chemical vapor deposition of quaternary AlInGaN layers and multiple quantum wells for ultraviolet light emission

A new pulsed metalorganic chemical vapor deposition (MOCVD) technique has been used to grow quaternary AlxInyGa1-x-yN layers with a high density of localized states on sapphire substrates. These layers show very strong photoluminescence spanning from 320 to 350 nm due to the recombination of carriers (presumably as excitons) localized at the band tail states. We also used the pulsed MOCVD process to deposit quaternary multiple quantum wells with intense emission peak at 331 nm, thereby, making it very attractive for growth of active layers for deep ultraviolet light-emitting diodes.

247 nm Ultra-Violet Light Emitting Diodes

We report on AlGaN-based ultraviolet light emitting diodes with emission at the wavelengths as short as 247 nm. Migration-enhanced metal organic chemical vapor deposition technique allowed us to improve the material quality. The phonon energy gap engineering approach was used to optimize these UV sources and achieve continuous-wave regime at turn-on voltages lower than 8 V at 20 mA current. Power levels of 0.3 mW were achieved for the packaged devices driven at 90 mA DC current. In the pulse operation 9 mW output power was measured at 1.4 A current. The peak to noise emission ratio was close to 400 at cw current in excess of 20 mA.

Nanostructured stars of ZnO microcrystals with intense stimulated emission

We demonstrate the synthesis of single-crystalline star-shaped ZnO microcrystals with six arms by a simple solution method. Microscopic and diffraction data reveal that the as-grown products have a wurtzite structure. Each arm of the ZnO microstars has a sharp tip of several nanometers. Under moderate optical excitation, stimulated emission was observed at 388.4nm with a full width at half-maximum (FWHM) of less than 3nm and a threshold of 318kW∕cm2.