Single-crystalline, colorless, GaN has been prepared by a vapor-phase growth technique previously used to prepare GaAs, GaP, and GaSb. These crystals are the first reported specimens of GaN suitable for good electrical and optical evaluation of this compound. It has been determined that GaN has a direct energy bandgap of 3.39 eV, and that undoped crystals prepared by this method have a very high, inherent electron co:, entration, typically above 10 19/cm3 , which is probably related r i a high density of nitrogen vacancies. Conducting p-type specimens have been prepared using Ge as the dopant; but this result has been difficult to reproduce, and the samples have been electrically inhomogeneous.

This letter reports on a low thermal impedance flip-chip packaged deep ultraviolet light-emitting diodes emitting at 280 nm featuring a finned heat sink. For a single 100μm×100μm device flip-chip packaged onto an AlN submount and mounted on a TO-66 header with attached finned heat sink, a total junction to ambient thermal impedance as low as 33°C∕W was obtained. Numerical simulations of the heat transfer show that the excessive value of effective thermal impedance is mostly limited by insufficient junction area.

We report the detailed structure analysis of our AlN∕AlGaN superlattice (SL) grown by pulsed atomic-layer epitaxy (PALE) for dislocation filtering. Due to the nature of PALE, the AlGaN well material itself in the SL was found to be composed actually of an AlxGa1−xN∕AlyGa1−yN short-period superlattice (SPSL), with the periodicity of 15.5Å (≈6 monolayer), determined consistently from high-resolution x-ray diffraction and high-resolution transmission electron microscopy measurements. The SPSL nature of the AlGaN layers is believed to benefit from the AlN∕AlGaN SL’s coherent growth, which is important in exerting compressive strain for the thick upper n-AlGaN film, which serves to eliminate cracks. Direct evidence is presented which indicates that this SL can dramatically reduce the screw-type threading dislocation density.

We report on the reduction of defect densities in non-polar a-plane GaN films over r-plane sapphire achieved by epitaxial laterally overgrowth (ELOG) approach. A mask pattern was used to produce ELOG GaN with wing region width of about 30 mu m. Based on transmission electron microscopy (TEM) results, the window regions have stacking faults density of similar to 10(6)cm(-1) and threading dislocation density of similar to 10(10) cm(-2). Both ELOG Ga-face and N-face wing regions have stacking fault density of similar to 10(5) cm(-1), and dislocation density less than 10(8) cm(-2). Cathodoluminescence studies reveal the difference in defect densities between N-faced and Ga-faced wings. GaN-based UV light-emitting diode formed on Ga-faced wing shows stronger quantum well emission and weaker parasitic emission than that formed on N-faced wing. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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 report on the growth of superior quality highly resistive thick GaN films by hydride vapor phase epitaxy (HVPE). HVPE growth conditions resulting in high quality crack-free films with sheet resistances in the range of 3.5 × 105 Ohm/sq are described. Thick films are grown using a vertical HVPE reactor with a unique gas flow delivery system. High quality 1 micron thick GaN layers grown by MOCVD on c-plane sapphire substrates are used as templates. The films are characterized by Hall effect measurements, photoluminescence, the light-induced transient grating technique, X-ray diffractometry, and scanning electron microscopy. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

(10{und 1}0) GaN wafers grown on (100) face of {gamma}-LiAlO{sub 2} were studied using transmission electron microscopy. Despite good lattice matching in this heteroepitaxial system, high densities of planar structural defects in the form of stacking faults on the basal plane and networks of boundaries located on prism planes inclined to the layer/substrate interface were present in these GaN layers. In addition, significant numbers of threading dislocations were observed. High-resolution electron microscopy indicates that stacking faults present on the basal plane in these layers are of low-energy intrinsic I1type. This is consistent with diffraction contrast experiments.

Using a micro-pixel design, we report the demonstration of high-power deep UV AlGaN-based light-emitting diodes (LEDs) with peak emission wavelength at 280nm. The design comes in response to lateral current crowding problems, which severely limit the maximum possible active area and the overall performance of ordinary square geometry III-nitride LEDs fabricated on insulating substrates. It is shown that the interconnected micro-pixel geometry significantly reduces both the device series resistance and the thermal impedance, thereby improving heat dissipation and increasing the maximum optical power. The design imparts ever-increasing advantages as the operating wavelength decreases (and the aluminum content increases). The optical power of the 10×10pixel array with an effective area of 222×222μm2 only saturates at dc currents higher than 200mA, which is nearly 50% greater than found for a square geometry LED with identical junction area, fabricated from the same wafer. These 280nm LEDs demonstrated a high on-wafer cw power of 145μW with 200mA of pumping current.

Freestanding GaN templates with (1010) orientation (M-plane) were obtained by halide vapor phase epitaxy (HVPE) on nearly lattice-matched LiAlO2 and subsequent removal of the substrate by wet chemical etching. Photoluminescence (PL) spectrum from both sides of the GaN template investigated is dominated by peaks at 3.47, 3.42 and 3.36 eV, tentatively attributed to an exciton bound to the neutral shallow donor and two unidentified structural defects, respectively. The quantum efficiency of the exciton-related emission exceeds 10%, whereas that of the combined emission from the defect-related bands (red, yellow and blue) is below 0.1%. The evolution of the PL spectrum with temperature and excitation intensity is analyzed in detail. Effects of polishing and etching on the PL properties are also discussed.

We report AlGaN deep ultraviolet light-emitting diodes (LEDs) at 250 and 255nm that have short emission wavelengths. For an unpackaged 200×200μm square geometry LED emitting at 255nm, we measured a peak power of 0.57mW at 1000mA of pulsed pump current. For a similar device emitting at 250nm the peak output power of 0.16mW was measured at 300mA of pulsed pump current. Progress is based on the development of high quality AlGaN cladding layers with an Al content up to 72%, which were grown over AlGaN∕AlN superlattice buffer layers on sapphire substrates. These n-Al0.72Ga0.28N layers were doped with Si up to about 1×1018cm−3 and electron mobilities up to 50cm2∕V∙s were estimated. High resolution x-ray diffraction studies gave a narrow (002) rocking curve with full width at half maximum of only 133arcsec.

A review is given of efforts to prepare thick gallium nitride films on lattice-matched gamma-LiAlO2 substrates. Much progress in the design of new high performance nitride device structures is presently impeded by the lack of GaN substrates, leading to large defect concentrations in layers grown on foreign materials. These problems could be alleviated if a true GaN substrate were to become available, allowing homoepitaxial growth. The preparation of 50 mm diameter boules of gamma-LiAlO2 from the melt will be discussed, including wafer preparation. Growth of thick (300-400 mum) GaN layers on the gamma-LiAlO2 wafers will be presented. The GaN is deposited by the halide vapour phase epitaxy (HVPE) method. Characteristics of these 50 mm diameter wafers are explained in detail. Much progress has been made, but several problems remain to be overcome.

Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched γ-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [101_0]. Stacking faults in the basal plane are seen when viewing the specimen in the [12_10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3×108 cm−2 when viewing the [12_10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as “seeds” for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations.

Structural and optical properties of GaN/AlGaN multiple quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy on (11̄00) plane free-standing GaN substrates and (0 0 0 1) GaN quasi-substrates have been compared. Atomic force microscopy studies indicate that the films and MQW structures grown on both substrates replicate the surface morphology of the substrates. MQWs with AlGaN barriers grown in the presence of In flux have stronger photoluminescence (PL) intensity than those with AlGaN barriers without In. X-ray diffraction spectra of MQWs grown on the (0 0 0 1) GaN substrates show larger number of superlattices peaks than those grown on (11̄00) substrates suggesting that the former have smoother interfaces. The PL spectra of MQWs deposited on (0 0 0 1) GaN substrates, where the growth is in a polar direction, exhibit a red-shift as well as a decrease in peak intensity with increase in well widths. Similar MQW structures on the (11̄00) GaN, on which the growth is in a non-polar direction, do not exhibit this phenomenon, which we attribute to the absence of internal electric fields in these structures. PL intensity of MQWs with a well width of 75 Å is 20 times stronger for those grown on the (11̄00) plane than on the (0 0 0 1) plane GaN substrate.

(1010) GaN wafers grown on (100) face of γ-LiAlO2 were studied using transmission electron microscopy. Despite good lattice matching in this heteroepitaxial system, high densities of planar structural defects in the form of stacking faults on the basal plane and networks of boundaries located on prism planes inclined to the layer/substrate interface were present in these GaN layers. In addition, significant numbers of threading dislocations were observed. High-resolution electron microscopy indicates that stacking faults present on the basal plane in these layers are of low-energy intrinsic I1 type. This is consistent with diffraction contrast experiments.

We report the development of large 50 mm diameter free standing wafers of GaN. GaN layers up to 350 mum thick have been grown by HVPE on lattice-matched LiAlO2 substrates. The original oxide substrates were removed by wet chemical etching, and the defect structure of the GaN wafers has been investigated. The issue of stacking faults vs. polar inversion domains has been resolved in favor of ABABACAC stacking sequences. A UV-emitting LED (368 nm) has been developed on these substrates, using molecular beam epitaxy.

We report on the homoepitaxial growth of GaN on freestanding [1 (1) over bar 00] oriented GaN substrates using metalorganic chemical vapor deposition. A proper pretreatment of the substrates was found to be essential for the GaN homoepitaxy. The influence of growth conditions such as V/III molar-ratio and temperature on the surface morphology and optical properties of epilayers was investigated. Optimized pretreatment and 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. Based on these GaN epilayer, AlGaN/GaN multiple quantum wells have been grown on the freestanding M-plane GaN. Photoluminescence data confirm that built-in electric field for M-plane structures is very weak, and this situation results in a stronger PL intensity in comparison with C-plane multiple quantum wells in tests at low excitation level.

Structural and optical properties of GaN/AlGaN multiple quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy on (1 (1) over bar 0 0) plane free-standing GaN substrates and (0 0 0 1) GaN quasi-substrates have been compared. Atomic force microscopy studies indicate that the films and MQW structures grown on both substrates replicate the surface morphology of the substrates. MQWs with AlGaN barriers grown in the presence of In flux have stronger photoluminescence (PL) intensity than those with AlGaN barriers without In. X-ray diffraction spectra of MQWs grown on the (0 0 0 1) GaN substrates show larger number of superlattices peaks than those grown on (1 (1) over bar 0 0) substrates suggesting that the former have smoother interfaces. The PL spectra of MQWs deposited on (0 0 0 1) GaN substrates, where the growth is in a polar direction, exhibit a red-shift as well as a decrease in peak intensity with increase in well widths. Similar MQW structures on the (1 (1) over bar 0 0) GaN, on which the growth is in a non-polar direction, do not exhibit this phenomenon, which we attribute to the absence of internal electric fields in these structures. PL intensity of MQWs with a well width of 75 A is 20 times stronger for those grown on the (1 (1) over bar 0 0) plane than on the (0 0 0 1) plane GaN substrate. (C) 2002 Elsevier Science B.V. All rights reserved.

Freestanding GaN templates with (10 (1) over bar0) orientation (M-plane) were obtained by halide vapor phase epitaxy (HVPE) on nearly lattice-matched LiAlO2 and subsequent removal of the substrate by wet chemical etching. Photoluminescence (PL) spectrum from both sides of the GaN template investigated is dominated by peaks at 3.47, 3.42 and 3.36 eV, tentatively attributed to an exciton bound to the neutral shallow donor and two unidentified structural defects, respectively. The quantum efficiency of the exciton-related emission exceeds 10%, whereas that of the combined emission from the defect-related bands (red, yellow and blue) is below 0.1%. The evolution of the PL spectrum with temperature and excitation intensity is analyzed in detail. Effects of polishing and etching on the PL properties are also discussed.