The need for efficient, compact and robust solid-state UV optical sources and sensors had stimulated the development of optical devices based on III–nitride material system. Rapid progress in material growth, device fabrication and packaging enabled demonstration of high efficiency visible-blind and solar-blind photodetectors, deep-UV light-emitting diodes with emission from 400 to 250 nm, and UV laser diodes with operation wavelengths ranging from 340 to 350 nm. Applications of these UV optical devices include flame sensing; fluorescence-based biochemical sensing; covert communications; air, water and food purification and disinfection; and biomedical instrumentation. This paper provides a review of recent advances in the development of UV optical devices. Performance of state-of-the-art devices as well as future prospects and challenges are discussed.

We investigated the growth of GaN/Al0.20Ga0.80N multiple quantum wells (MQWs) on selective-area-grown a-plane GaN pillars over r-plane sapphire. In contrast to the MQWs grown on planar a-plane GaN templates, these GaN/Al0.20Ga0.8N MQWs on the pillars exhibited pit-free and atomically smooth surface morphology. Their structural quality and their UV emission (at 357 nm) increased with the underlying pillar height. The epitaxy of GaN/AlGaN MQWs on the selective-area-grown pillars is thus a promising and simple approach for fabricating stripe-geometry, high-efficiency, nonpolar UV emitters.

Self-heating imposes the major limitation on the output power of GaN-based HFETs on sapphire or SiC. SiC substrates allow for a simple device thermal management scheme; however, they are about a factor 20-100 higher in cost than sapphire. Sapphire substrates of diameters exceeding 4 in are easily available but the heat removal through the substrate is inefficient due to its low thermal conductivity. The authors demonstrate that the thermal impedance of GaN based HFETs over sapphire substrates can be significantly reduced by implementing flip-chip bonding with thermal conductive epoxy underfill. They also show that in sapphire-based flip-chip mounted devices the heat spread from the active region under the gate along the GaN buffer and the substrate is the key contributor to the overall thermal impedance.

In this paper, we report the pulsed atomic-layer epitaxy (PALE) of ultrahigh-quality AlN epilayers over basal-plane sapphire substrates and their use as templates to grow high-quality AlGaN layers with Al content ranging from 0.3 to 1. Symmetric/asymmetric x-ray diffraction (XRD) and room-temperature (RT) photoluminescence (PL) measurements were used to establish the high-structural and optical quality. The XRD (002) and (114) rocking-curve full-width at half-maximum (FWHM) values of the PALE-grown AlN epilayers were less than 60 arcsec and 250 arcsec, respectively. Using these ultrahigh-quality layers as templates, Si-doped AlGaN layers with a large Al content from 30% to 100% were grown and used for milliwatt power sub-280-nm, deepultraviolet (UV) light-emitting diodes (LEDs).

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.

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 Å for a 5 µm × 5 µm 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.

GaN and GaN/Al0.25Ga0.75N multiple quantum wells (MQWs) over c- and r-plane sapphire substrates have been grown by metal-organic chemical vapor deposition. A comparative study of photoluminescence (PL) in GaN epitaxial layers and AlGaN/GaN MQWs on these two types of substrates is reported. At low excitation levels, the measured room temperature PL signal in GaN layers grown over r-plane sapphire was more than order of magnitude lower than in GaN on c-plane substrates. In contrast, the emission intensity from AlGaN/GaN MQWs grown over r-plane substrates was almost 30 times stronger than in the structures grown over c-plane sapphire. Furthermore, with excitation power density up to 1 MW/cm(2), the PL peak position for the non-polar MQWs kept completely stable whereas the one for the c-plane structures exhibited a blue shift as large as 250 meV. We attribute this large difference in the ultraviolet emission intensity to the suppression of a strong quantum Stark effect in the AlGaN/GaN MQWs on the r-plane sapphire. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on the growth of quaternary AlInGaN layers and MQWs by two different metalorganic chemical vapor deposition (MOCVD) techniques such as pulsed atomic layer epitaxy (PALE) and pulsed MOCVD (PMOCVD). For both growth processes, emission wavelength of quaternary MQWs can be tuned from 350 nm to 300 nm by simply changing the unit growth cell configurations. The PALE grown AlInGaN MQWs have a very smooth surface, few band tail states and exhibit a band-to-band emission. The PMOCVD grown AlInGaN MQWs exhibit a high density of band tail states, which strongly enhance spontaneous emission. Based on the characterization by photoluminescence, X-ray diffraction and AFM, both MOCVD techniques grown quaternary samples are shown to be promising for fabricating the active region of deep UV LEDs.

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 report on an approach of using AlN/AlGaN superlattices (SLs) for threading-dislocation-density reduction to grow high quality thick AlGaN on sapphire. Using x-ray diffraction (XRD) measurements and etch pits counting by atomic force microscopy, we show that the insertion of AlN/AlGaN SLs suppresses the material mosaicity and decreases the threading dislocation density by two orders of magnitude, and then eliminates cracking. Dislocation densities deduced from the XRD results and those from chemical etching are in a good agreement.

We report on a study to compare the growth of thick AlGaN layers on GaN with different strain-relief interlayers. A set of ten period AlN/AlGaN superlattices was found to be the most efficient approach for the strain relief. The superlattice interlayer not only decreases the tensile strain but also improves the crystal structural quality. Thus, 2-μm-thick, high quality n+-Al0.2Ga0.8N layers can be grown on GaN epilayers without any cracks.

We report a pulsed atomic layer epitaxy (PALE) growth technique for quaternary AlInGaN films for ultraviolet optoelectronic applications. Using the PALE approach high quality quaternary AlInGaN/AlInGaN multiple quantum wells (MQWs) were successfully grown over sapphire substrates. From X-ray diffraction, atomic force microscopy, and photoluminescence study, a high structural and optical quality was established for the AlInGaN MQWs. Incorporating the PALE grown quaternary MQWs as the active layer of light emitting diode (LED) on sapphire or SiC substrates we also demonstrated room temperature deep ultraviolet electroluminescence under dc and pulsed electrical pumping. The peak emission wavelength can be tuned from 305 nm to 340 nm with spectrum FWHM of about 20 nm by varying the alloy compositions of the quaternary AlInGaN active layers using PALE. Comparative study of LEDs over sapphire and SiC substrates was also done in order to determine the influence of epilayer design on the performance parameters and the role of the substrate absorption.

In this letter, we report the pulsed atomic-layer epitaxy of ultrahigh-quality AlN epilayers and AlN/Al0.85Ga0.15N multiple quantum wells (MQWs) on basal plane sapphire substrates. Symmetric and asymmetric x-ray diffraction (XRD) measurements and room-temperature (RT) photoluminescence (PL) were used to establish the ultrahigh structural and optical quality. Strong band-edge RT PL at 208 and 228 nm was obtained from the AlN epilayers and the AlN/Al0.85Ga0.15N MQWs. These data clearly establish their suitability for sub-250-nm deep UV emitters.

We report homoepitaxial GaN growth on freestanding (11̄00) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition. Scanning electron microscopy, atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (11̄00) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (11̄00) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices.

We report homoepitaxial GaN growth on freestanding (1 (1) over bar 00) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition. Scanning electron microscopy, atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (1 (1) over bar 00) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (1 (1) over bar 00) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices. (C) 2002 American Institute of Physics.

We report on a novel pulsed atomic layer epitaxy (PALE) growth technique for quaternary AlInGaN films for ultraviolet optoelectronics applications. Using the PALE approach, quaternary AlInGaN/AlInGaN multiple quantum wells (MQWs) were successfully grown over sapphire substrates. These were characterized using X-ray diffraction, atomic force microscopy, and photoluminescence to establish structural and optical quality. Incorporating the PALE grown quaternary MQWs as the active layer we also demonstrated ultraviolet electroluminescence at 343 nm with an output power up to 0.12 mW at room temperature.

We report on a pulsed atomic layer epitaxy (PALE) technique for quaternary AlInGaN growth. PALE allows for the deposition of high-quality AlInGaN layers at much lower temperatures than those required for conventional low-pressure metalorganic chemical vapor deposition (MOCVD). The low growth temperature leads to an efficient incorporation of indium and, as a result, to a dramatic improvement in emission properties of the material. A deep ultraviolet optical gain of 300 cm—1 peaked at 330 nm is obtained in quaternary AlInGaN MQWs. Also, in PALE AlInGaN material, the optical emission properties do not degrade with increasing Al mole fraction. These results establish PALE as a promising technique for deep UV emitters.