Epitaxial growth on GaN single bulk crystals sets new standards in GaN material quality. The outstanding properties provide new insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) being not accessible by heteroepitaxial growth on sapphire or SiC. With MOVPE and MBE we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. Those layers revealed an exceptional optical quality as determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec.Only recently, progress in surface preparation allowed morphologies of the layers suitable for device applications. We report on InGaN/GaN MQW structures as well as the first GaN pn- and InGaN/GaN double heterostructure LEDs on GaN single bulk crystals. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping. Time resolved electroluminescence measurements proof that band/band recombination is the dominant emission mechanism for the InGaN/GaN LEDs.

Different substrates for gallium nitride growth are discussed. The commercially relevant substrates, silicon carbide and sapphire, and the two most promising alternatives, silicon and gallium nitride, are compared in terms of suitability for epitaxial processes and in their effects on devices. An estimation on future market success is given.

We present the first GaN based MOSFET with wet chemical processed gate oxide. The oxide was grown photoelectrochemically (PEC) in KOH based aqueous solutions and was determined to be AlxGa2−xO3. This process offers low surface damage. The gate contact for our created PEC-MOSHFET (metal oxide semiconductor heterostructure field effect transistor) was fabricated by e-beam evaporation of tungsten on the AlxGa2−xO3 layer, followed by a lithographic step and wet etch by H2O2. Source and drain contacts were placed by the liftoff technique using Ti/Al. Peak values for the mutual conductance (gm) are 64 mSmm−1 for MOVPE (metalorganic vapour pressure epitaxy) structures with 2DEG mobility of 190 cm2V−1s−1. We achieve a maximum drain current IDmax of 540 mAmm−1 for the PEC-MOSHFET. The results obtained for transistor operation are compared to other gate dielectrics such as SiO2 with different pre-treatments and to a conventional HFET with a Ni/Au Schottky gate. Depletion starts at threshold voltages Vth of −4 V in the case of the PEC-MOSHFET, for the conventional HFET structure Vth is about −9 V and for the SiO2-MOSHFETs it varies between −11.5 and −14 V depending on the wet chemical pre-treatment. Leakage currents depend on device isolation and on gate currents, which are lowest for the SiO2-MOSHFETs (∼2 pA) and several orders of magnitude bigger for the HFET (∼4 μA). Gate currents for the PEC-MOSHFET depend on the oxide growth and vary between microamperes and a few picoamperes.

In this article, multiple-step rapid thermal annealing (RTA) processes for the activation of Mg doped GaN are compared with conventional single-step RTA processes. The investigated multiple-step processes consist of a low temperature annealing step at temperatures between 350°C and 700°C with dwell times up to 5min and a short time high temperature step. With optimized process parameters, and multiple-step processes, we achieved p-type free carrier concentrations up to 1–2×1018cm−3. The best achieved conductivity, so far, lies at 1.2Ω−1cm−1. This is a 50% improvement compared to conventional single-step process at 800°C, 10min.

Two-step thermal annealing processes were investigated for electrical activation of magnesium- doped galliumnitride layers. The samples were studied by room-temperature Hall measurements and photoluminescence spectroscopy at 16 K. After an annealing process consisting of a short-term step at 960 °C followed by a 600 °C dwell step for 5 min a resistivity as low as 0.84 Ω cm is achieved for the activated sample, which improves the results achieved by standard annealing (800 °C for 10 min) by 25% in resistivity and 100% in free hole concentration. Photoluminescence shows a peak centered at 3.0 eV, which is typical for Mg-doped samples with high free hole concentrations.

The influence of the mirror reflectivity on the L–I characteristics of GaN-based lasers has been studied. A cleaved, Al-coated fiber is used as an external micro-mirror to control the reflectance of the end facets allowing for a continuous adjustment of mirror losses of a particular laser. In contrast to other methods, this eliminates all ambiguities usually arising from the comparison of different or differently coated devices. An increase in the single facet external quantum efficiency by 45% is observed for uncoated lasers and simultaneously, the threshold current is reduced by 12%. Internal losses of approximately 20–30cm−1 are derived from the differential quantum efficiency variation depending on the particular device under investigation.

The DC characteristics of an AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor are presented. The unique feature of this device is its oxide, which is formed photoelectrochemically at room temperature. For a device with a gate length of 2 /spl mu/m state-of-the-art values of 540 mA/mm and 62 mS/mm were obtained for the drain current and transconductance, respectively.

In this work, we investigate the extraction efficiency for UV emitting rectangular 300 x 300 mum(2) gallium nitride (GaN) based light emitting diodes (LEDs) by simulation with a raytracer tool. It is shown that the extraction efficiency depends strongly on slight variations of the absorption in the GaN layers. Furthermore, the influence of the substrate shape is studied. For standard rectangular sapphire substrate based LEDs the calculated extraction efficiency is 12.4%, whereas for silicon carbide substrate based devices the higher refractive index causes a lower efficiency (4.5%). Using a shaped SiC substrate the extraction efficiency can be improved to 17.2% and 19.9% for a sapphire substrate. The influence of geometric design parameters like sidewall angle are analyzed as well.

Dry-etching of laser facets is commonly used for (InAl)GaN/sapphire-based structures since the epitaxial planes of the nitride layers are rotated with respect to the substrate planes making cleaving impractical. To achieve steep and smooth facets by chemically assisted ion beam etching, a 3-layer resist system is developed for patterning. Characterization by scanning electron microscopy and atomic force microscopy shows facets with root-mean-square roughnesses of 7 nm and inclination angles of 2–4°. Optically pumped lasers yield low threshold excitation densities for fully doped separate confinement heterostructure lasers.

We have measured temperature-dependent reflectance (RF) and photoluminescence (PL) spectra on a high-quality unstrained homoepitaxial GaN layer grown by MOVPE on a pre-treated GaN single crystal In the reflectance spectra, intrinsic exciton-polariton features show up related to all three valence bands. For the A and B valence band, we find exceptional low damping. A full fit to the experimental curves including spatial dispersion allows to deduce exact exciton energies for a broad range of sample temperatures. Besides the dominant donor- and acceptor-bound excitons the photoluminescence spectra show relatively strong, non-thermalized emission from exciton-polaritons in so-called bottleneck states. The analysis of these lines confirms the RF data.

We report on photoassisted wet chemical formation of thin oxide films on n-GaN layers in potassium hydroxide based electrolytes at room temperature. The kinetics of the oxide formation and dissolution were examined via photocurrent transients. The tendency of the photocurrent to level out during photoelectrochemical etching experiments is associated with a quasiequilibrium state at the semiconductor/electrolyte interface. Homogeneous oxide films were grown in weak alkaline solutions (11 < pH < 13) under potentionstatic control with oxidation rates of up to 250 nm/h and characterized by Auger electron spectroscopy. Consequences on wet photochemical etch strategies are discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)00826-3].

Electroluminescence (EL) is the most significant measure for light-emitting diodes since it probes the most relevant properties of the fully processed device during operation. In addition to the information gained by conventional spectrally resolved EL, scanning micro-EL provides spatially resolved information. The devices under investigation are InGaN/GaN-LEDs with single peak band-band emission at about 400 nm grown by MOVPE on sapphire substrates. The {mu}-EL-characterization is performed as a function of injection current densities and the emission is investigated from the epitaxial layer as well as from substrate side. Spatially resolved wavelength images reveal emission peaks between 406 nm and 417 nm, corresponding either to In fluctuations of 1--1.5% or local fluctuations of piezo electric fields. Beside the information on the emission wavelength fluctuations {mu}-EL is used to determine the temperature distribution in the LEDs and to investigate transparent contacts.

Epitaxial growth on GaN bulk single crystal substrates sets new standards in GaN material quality. The outstanding properties provide insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) with a precision not obtainable from heteroepitaxial growth on sapphire or SiC. With metalorganic vapor phase epitaxy (MOVPE) we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. By the use of dry etching techniques for surface preparation, an important improvement of crystal quality is achieved. Those layers reveal an exceptional optical quality as determined by a reduction of the low-temperature photoluminescence (PL) linewidth from 5 meV to 0.1 meV and a reduced X-ray diffraction CI(RD) rocking curve width from 400 to 20 arcsec. As a consequence of the narrow PL linewidths, new features as, e. g. a fivefold fine structure of the donor-bound exciton line at 3.471 eV was detected. Additionally, all three free excitons as well as their excited states are visible in PL at 2K.Dry etching techniques for surface preparation allow morphologies of the layers suitable for device applications. We report on InGaN/GaN multi-quantum-well (MQW) structures as well as GaN pn- and InGaN/GaN double heterostructure light emitting diodes (LEDs) on GaN bulk single crystal substrates. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping.

In this paper, Hydride Vapour Phase Epitaxy (HVPE) of GaN layers under reduced pressures is reported. First results show that the HVPE grown GaN layers exhibit excellent electrical, crystallographic and optical quality. By reducing the reactor pressure from 950 to 250 mbar, improvements in background doping (down to 2 x 10(16) cm(-3)) and Hall mobility (up to 300 cm(2)/Vs) are observed. Experiments on MOVPE overgrowth on HVPE GaN layers show excellent results. Low temperature PL spectra of the overgrowth MOVPE layer reveal all three free exciton levels (FE A, FE B, FE C) without any visible bound excitons.

Homoepitaxial GaN layers grown by MOVPE on a pre-treated GaN single crystal are almost strain-free, have very low defect densities and low impurity levels. These layers show strong, extraordinarily sharp photoluminescence spectra. Especially, the donor-(D(0), X) and acceptor-(A(0), X) bound excitons at approximate to 3.471 eV and 3.465 eV, respectively, exhibit line widths of approximate to 100 mu eV, and ample fine structures can be resolved. For the (DO, X), complicated spectra are observed in the spectral region of two-electron-transitions and in the excited bound exciton state. For both (DO, X) and (AO, X), the respective bound exciton states belonging to all three valence bands are found. By variation of the sample temperature and excitation density, we separate the spectral features contributed by different impurities and reassign several lines in the band gap region. (C) 1999 Elsevier Science B.V. All rights reserved.

Heat:generated by ohmic losses is a critical parameter for performance and lifetime of light-emitting diodes (LEDs). The temperature of InGaN MQW LEDs during operation has been investigated depending on the forward current using three independent methods. First, the temperature of the active region was derived from the electroluminescence spectra of the devices. Second, temperature dependent micro-Raman scattering by phonons was used to determine,the local temperature. Finally a finite element simulation was performed to get a full temperature profile of the device. While the first method yields the temperature of the active region, the latter two can map the thermal distribution. All three independent methods reveal maximum operation temperatures about 120 to 130 degrees C at a forward current of 30 mA, corresponding to a power density of 705 W cm(-2).

We report on highly resolved photoluminescence (PL) and reflectance (RF) spectra of a homoepitaxial GaN layer grown by metal-organic vapor phase epitaxy. This sample exhibits narrow linewidths of several PL emission peaks, down to approximate to 100 mu eV full width at half maximum for some bound exciton transitions. As a consequence, we have detected new PL features as, e.g., a fivefold fine structure of the donor-bound exciton line at approximate to 3.471 eV, and other known PL transitions could be determined with high precision. In RF measurements, the extraordinary quality of the epitaxial layer allowed observation of weakly damped excitonic groundstate transitions and of narrow excited exciton transitions with high signal-to-noise ratio. [S0163-1829(99)14027-X].

A novel approach of spectrally resolved scanning electroluminescence microscopy is introduced as a powerful characterization tool directly imaging the spectral emission characteristics of luminescence:devices. This fast non-destructive technique allows the direct correlation of morphological and optical properties of final devices at the microscopically identical sample position. it directly visualizes the inhomogeneities in epitaxial growth as well as in processing. Furthermore, it shows where and how much light and with which spectral characteristic is emitted by the device which is most important for device design, The characterization of an InGaN/GaN Light Emitting Diode is given as an example of the power of scanning electroluminescence microscopy. The luminescence intensity maps and the emission peak wavelength images are taken under-operation of the device. They directly image the optical quality of the device and yield direct images of the In fluctuations with a spatial resolution of 1 mu m The indium concentration is found to fluctuate from 5% to 8%. The InGaN layer shows a strong spatially localized emission characteristic.

The frequencies and dampings of the zone-center optical phonons E2 and A1(LO) in wurtzite-type GaN and AlN layers have been measured by Raman spectroscopy in the temperature range from 85 to 760 K. The GaN layer was grown by metalorganic vapor phase epitaxy and the AlN layer by molecular beam epitaxy both on sapphire substrate. The experimentally obtained frequencies and dampings are modeled by a theory taking into account the thermal expansion of the lattice, a symmetric decay of the optical phonons into two and three phonons of lower energy, and the strain in the layers induced by the different thermal expansion coefficients of layer and substrate. The results were used to determine the local temperature of a GaN pn diode in dependence on the applied voltage.