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Resonant-cavity light-emitting diodes emitting at around 400 nm based on an undoped bottom AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric SiO(2)/ZrO(2) DBR with circular emitting apertures of diameters ranging from 5 to 200 µm are demonstrated. The current distribution is investigated by luminance distribution imaging and three-dimensional device simulations for different current densities. The current distribution exhibits a maximum in the aperture centre or is homogeneous up to an aperture diameter of 50 µm independent of the current density. A minimum occurs in the aperture centre for larger diameters increasing with increasing diameter and current density. The current distribution improves with larger n-GaN thickness and higher contact resistance between the transparent In(2)O(3):Sn (ITO) electrode and the p-GaN contact layer.

Currently available cochlear implants are based on electrical stimulation of the spiral ganglion neurons. Optical stimulation with arrays of micro-sized light-emitting diodes (µLEDs) promises to increase the number of distinguishable frequencies. Here, the development of a flexible GaN-based micro-LED array as an optical cochlear implant is reported for application in a mouse model. The fabrication of 15 µm thin and highly flexible devices is enabled by a laser-based layer transfer process of the GaN-LEDs from sapphire to a polyimide-on-silicon carrier wafer. The fabricated 50 × 50 µm(2) LEDs are contacted via conducting paths on both pand n-sides of the LEDs. Up to three separate channels could be addressed. The probes, composed of a linear array of the said µLEDs bonded to the flexible polyimide substrate, are peeled off the carrier wafer and attached to flexible printed circuit boards. Probes with four µLEDs and a width of 230 µm are successfully implanted in the mouse cochlea both in vitro and in vivo. The LEDs emit 60 µW at 1 mA after peel-off, corresponding to a radiant emittance of 6 mWmm(-2).

Eutectic aluminum-germanium wafer bonding was used to fabricate (AlGaIn)N thin-film light-emitting diodes (LEDs). Wafer bonding was carried out on 2`' wafer level at a bond temperature of 470 (0)C using patterned Al bond pads on the GaN-on-sapphire LED epiwafer and plain Ge substrates. The microstructure of the joint formation was characterized via cross-section analysis using scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy was used to investigate the bond interface. The shear strength was determined to be 1-2 kN/cm(2). The formation of a liquid Al-Ge phase is evident from crosssection analysis and optical microscopy. During solidification, Al and Ge are separated into distinct phases again, which is revealed by EDX. The obtained bond is not free of micro-voids, yet it is mechanically stable and suited for the fabrication of thin-film LEDs by removing the sapphire substrate via laser lift-off, which is also demonstrated.

So far, most of the studies on GaN doped with beryllium have mainly concentrated on possible ptype doping. Unfortunately, realization of p-type conductivity in such a way appeared to be very difficult. It seems, however, that bulk crystals doped with beryllium can be used as white light converters in the monolithic white light emitting diodes. To realize monolithic white light emitting diode, we used blue light emitting diodes and a single GaN:Be crystal as converter. High value of the Color Rendering Index gives hope for obtaining an effective light converter based on gallium nitride doped with beryllium.

Improving the internal quantum efficiency is still a major challenge for realizing efficient III-nitride-based lightemitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on the influence of a very low In amount of less than 1% on the luminescence properties of single Ga(In)N/AlGaN quantum well (QW) structures and the performance of AlGaN-based LEDs with an emission wavelength of about 350 nm. The samples were grown by metalorganic vapor phase epitaxy on low defect density AlGaN buffer layers on (0001) sapphire substrates. The temperature dependence of the QW emission energy reveals a clear "S-shape" behaviour indicating a significant carrier localization for In containing QWs. The electroluminescence efficiency of LEDs with a GaInN QW active region could be improved by a factor of 4 compared to LEDs with GaN QW, resulting in a continuous wave (cw) output power of 8.2 mW at 40 mA.

Excimer laser-based Laser Lift-Off the (LLO) is a key enabling technology in fabricating thin film LEDs. This article reviews the market drivers for high brightness LEDs, current LED designs and their limitations and the basics of thin film devices as well as the LLO process for manufacturing them.

The paper reports on the integration of resistive gas sensors from indium oxide nanoparticles with near-ultraviolet light emitting diodes (LEDs) for miniaturized and highly sensitive ozone sensing. For the integrated sensor an LED with an optical power as low as 250 microW is sufficient. An optimum operational window of the LED wavelength is determined to be between 380-420 nm. Furthermore, two operational modes of the used LED (pulse and continuous modes) have been compared and it is found that the pulse mode is more suitable for real applications.

A series of 400 nm emitting GaInN/GaN single-well light-emitting diodes, grown on ultra-low dislocation density GaN templates with well widths varying between 3 and 18 nm, were investigated by pulsed and time resolved electroluminescence measurements using small signal modulation technique for the latter. A reduction of the efficiency droop at high current densities with increasing well width was observed. The highest overall external quantum efficiency was obtained for LEDs with 11 nm thick double-heterostructure (or wide well) active region at current densities above 70 A/cm2. Furthermore, carrier lifetime and volume carrier density in the wells were determined. A model based on the interplay between defect assisted recombination, depending on dislocation density, and strongly carrier density dependent loss processes is presented explaining the well-thickness dependence of the efficiency droop.

We report on the reduction of the threading edge dislocation density of Al0.15Ga0.85N buffer layers for efficient 350 nm light-emitting diodes (LEDs). Structures were grown by metalorganic vapor phase epitaxy (MOVPE) on (0001) sapphire substrates using three-dimensional (3D) facetted GaN nucleation islands. The flattening of the overgrowing AlGaN buffer layers could be controlled by choosing appropriate growth conditions resulting in smooth surfaces. High-resolution X-ray diffraction (HRXRD) v-scans show that a prolonged 3D growth phase leads to a narrowing of the asymmetric diffraction peaks and hence to an effective reduction of the density of edge-type threading dislocations. Photoluminescence (PL) and electroluminescence (EL) measurements show directly the beneficial effect of the improved crystal quality on the optical emission properties. The output power of LED structures grown on an optimized buffer was increased by a factor of 6 compared to structures grown on a two-dimensional (2D) low Al content AlGaN nucleation layer.

We show that the conduction band structure of dilute AlxGa1-xAs1-yNy with x

There is a high demand for compact low-cost ozone sensors. It has been shown recently that In2O3 nanolayers can act as ozone sensitive films activated at room temperature by ultraviolet light. In the present work, the authors integrate ultrathin layers of In2O3 nanoparticles and a GaInN/GaN based blue light emitting diode (LED) on a single sensor chip. The integrated sensor was found to be sensitive to O-3 concentrations as low as similar to 40 ppb. These results demonstrate that by integrating GaInN/GaN based blue LEDs and metal oxide sensing layers back to back on a single chip, compact and robust gas sensors can be realized.

Two wavelength series (373-435 nm) of GaInN/GaN triple quantum well LED-structures, one on sapphire and one on GaN-templates, as well as a quantum well thickness series were grown by MOVPE. In order to study luminescence efficiency limiting effects the LED-structures were characterized by electroluminescence- and by temperature and excitation power dependent photoluminescence spectroscopy. The electroluminescence output and the internal quantum efficiency are peaked near 400 nm in both wavelength series, but output and efficiency are significantly enhanced for growth on templates. The photoluminescence data indicate that below 400 nm the efficiency, apart from insufficient vertical carrier confinement, is limited by poor localization within the quantum film plane while above 400 nm piezo-electric fields appear to be and important efficiency limiting effect.

We compare the electro-optical characteristics of violet-emitting ridge waveguide (AlGaIn)N quantum well diode lasers with etched laser facets, grown on GaN templates with a homogeneously reduced defect density, with those of reference devices grown on sapphire using conventional low-temperature GaN nucleation layer technology. Defect densities of the former and latter laser structures were around 1x10(exp 8) cm-2 and 2x10(exp 9) cm-2, respectively. In on-wafer pulsed-mode operation a 35% reduction in threshold current density and 40% reduction in injected electrical power at threshold was achieved upon reduction of defect density. Minimum values for threshold current density and injected electrical power at threshold are 7.5 kA/cm2 (500 mu m x 4 mu m) and 0.9 W (500 mu m x 2 mu m), respectively, for a ridge laser with uncoated facets. For improved heat sinking the sapphire substrate of selected laser diodes on GaN template was removed using laser lift-off, followed by n-side down soldering on copper heat sinks. This way, the maximum duty-cycle, limited by premature thermal rollover, could be increased by a factor of three for the substrate-less lasers.

The influence of nonradiative recombination processes on the output power P of InGaN light emitting devices (LEDs) with emission wavelength between 380 and 430 nm grown on sapphire substrates have been investigated. The electroluminescence intensity was studied as a function of the forward current I and the device temperature. The P-I curves of all LEDs show a super-linear behaviour at low currents (between 80 µA and 1 mA) and a nearly linear characteristic around the operating forward current of 20 mA (equivalent to a current density of 44 A/cm2). The super-linear behaviour is attributed to the saturation of nonradiative processes which also affect the output power at operating forward current. The reduction of the output power at 20 mA with increasing temperature is correlated with the super-linear behaviour at low currents indicating that nonradiative processes are thermally activated.

Resonant Raman scattering from Al(x)Ga(1-x)N(x

In(0.13)Ga(0.87)N/GaN double heterostructures and quantum wells (QWs) have been studied by room-temperature photoluminescence (PL) and spectroscopic ellipsometry (SE). PL revealed the presence of strong piezoelectric fields, which strongly influence the luminescence properties for InGaN QW widths in the 3-12 nm range. The pseudodielectric function spectrum derived from the SE measurements were analyzed using a multilayer approach, describing the dielectric function of the individual layers by a parametric oscillator model. The fundamental band-gap resonance in the InGaN dielectric-function spectrum was found to broaden for an InGaN layer width of 12 nm, as compared to bulk-like InGaN layers, due to piezoelectric field effects. For a much narrower QW width of 1.7 nm, however, quantum confinement was found to dominate over piezoelectric-field effects, resulting in a much sharper band-gap resonance shifted to higher energies and an increased oscillator strength.

The deep, yellow photoluminescence band well known in GaN has been studied in In(x)Ga(1-x)N (x

The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1*10/sup 19/ cm/sup 3/ and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor-acceptor (D-A) pair recombination character. It is suggested that the acceptor A is isolated Mg/sub Ga/ while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a Mg/sub Ga/ acceptor with a nitrogen vacancy, formed by self-compensation.

We report on the identification of molybdenum trace impurities in commercial 6H-SiC substrates by conventional and optically detected electron spin resonance (ESR, ODESR) and magnetic circular dichroism (MCD) absorption techniques. The two impurity charge states Mo(exp4+) (4d(exp 2)) and Mo(exp 3+) (4d( exp 3) are identified. Consequently, this impurity is electrically active as a deep level acceptor state. In addition we observe the 3.A2 -> 3.T2 crystal field absorptions of Mo(exp 4+) (4d(exp 2)) in the spectral range 1.1-1.25 eV. The consequences of the present findings, for the use of Mo as an electrical contact material, are pointed out.

A large number of epitaxial GaN samples as well as AlN ceramics have been studied by photoluminescence (PL) and PL excitation spectroscopy. In addition to the PL of residual iron, two new bands with zero-phonon-lines at 0.931 and 1.193 eV have been observed frequently in GaN. An analysis of the PL bands indicates that they arise from internal transitions within the 3d shell of residual vanadium and chromium impurities. The chromium PL has also been observed in polycrystalline AlN ceramics.

The 1.3 eV iron related emission bands in GaN and AlN were analysed by photoluminescence and photoluminescence excitation spectroscopy. The (-/0) acceptor levels of iron in GaN and AlN were determined. A value of 0.5 eV for the GaN/AlN valence band discontinuity is deduced. Values for the GaAs/GaN and the GaAs/AlN valence band discontinuities are discussed.

Nominally undoped zinc-blende polytype 3C-SiC single crystals have been studied by conventional electron-spin resonance (ESR), by the magnetic circular dichroism (MCD) absorption technique, and by optically detected ESR via the MCD (MCD-ESR). Apart from the dominant ESR signal ascribed to residual boron acceptors, an anisotropic spectrum with many fines is observed near liquid-He temperatures. It collapses into a hyperfine octet for H (111). This spectrum is identified as the anisotropic 2E ground-state resonance of V high4plus (3d high1) on the cubic Si site in 3C-SiC. Above 15 K this spectrum disappears and an isotropic hyperfine octet appears. It is interpreted as the motionally averaged spectrum of the anisotropic V high4plus spectrum. The data are analyzed in terms of Ham's theory of the dynamic Jahn-Teller effect for a high2 E state. A sharp optical line at 6681 cm high-1 has been found in MCD absorption. On this line the MCD-ESR of V high4plus is observed, which identifies the l ine as being related to V high4plus. It is interpreted as the zero-phonon line of the V high4plus high2 E about high2 T sub2 crystal-field transition.

Electron spin resonance (ESR) of silicon-substitutional vanadium in its neutral V (exp 4+)(ind Si) (3d (exp 1)) state has been observed in cubic bulk 3C-SiC single crystals. By photo-ESR the position of the (0/+) deep donor level of vanadium could be located at E (ind V)+1.7eV. Using this level as common reference in 3C-SiC and 6H-SiC, the valence-band discontinuity in the 3C-SiC/6H-SiC interface is predicted as Delta E (ind V)=0.l eV, with the valence-band of 3C-SiC lying lower in energy. We also offer an explanation for the absence of intra-3d-shell infrared luminescence of V (exp 4+) (3d (exp 1)) in 3C-SiC.

GaAS lightly doped with the heaviest group-V atom, bismuth (Bi), has been studied by conventional electron-spin resonance (ESR) and by ESR detected via the magnetic-circular-dichroism (MCD) absorption. A new Bi-related sharp-line MCD band has been observed on which two MCD-ESR lines have been discovered. They are shown to arise from the singly ionized BiGa double donor. Most remarkably, a substantial fraction, about 10 percent, of the total Bi content is found to occupy the Ga site. The BiGa MCD absorption band is tentatively assigned to an exciton deeply bound to the singly ionized double donor BiGa+.

Magnetic-circular-dichroism (MCD) absorption and optically detected electron-spin-resonance (ESR) data for the neutral vanadium impurity, Vhigh4plus(3dhigh1) on the alpha, beta and gamma sites in 6H-SiC are presented and supplemented by linearly polarized absorption and luminescence spectra. An analysis of the linearly polarized spectra yields crystal-field-level schemes in zero magnetic field and an intensity parameter u for Vhigh4plus on each of the three Si sites. This information is used to account for the Vhigh4plus MCD zero-phonon-line positions, signs, and relative intensities without an adjustable parameter. Evidence of Jahn-Teller effects is found in both the Vhigh4plus high2 E ground and the high2 T sub2 excited states. The MCD-ESR technique, in the present case, is site selective. The Vhigh4plus resonances associated with the beta and gamma site are virtually identical and cannot be distinguished by conventional ESR. The g factors for all three sites are satisfactorily expla ined within the framework of crystal-field theory.