Different experimental and simulation techniques aiming at a better understanding of lateral mode absorption in light-emitting diodes (LEDs) are presented in this paper. A measurement of transmitted power versus propagation distance allows us to derive the absorption losses of LED layer structures at their emission wavelength. Two models for the observed intensity distribution are presented: one is based on scattering, whereas the other relies on selective absorption. Both models were applied to InGaN-on-sapphire-based LED structures. Material absorption losses of 7 cm/sup -1/ for the scattering model and 4 cm/sup -1/ for the absorbing-layer model were obtained. Furthermore, these values are independent of the emission wavelength of the layer structure in the 403-433-nm range. The losses are most likely caused by a thin highly absorbing layer at the interface to the substrate. In a second step, interference of the modal field profile with the absorbing layer can be used to determine its thickness (d=75 nm) and its absorption coefficient (/spl alpha/ /spl ap/ 3900 cm/sup -1/). This method has also been tested and applied on AlGaInP-based layer structures emitting at 650 nm. In this case, the intensity decay of /spl alpha/=30 cm/sup -1/ includes a contribution from the absorbing substrate.

GaN is the first highly polar semiconductor used in field effect transistors. Polarization charge dipoles are an essential part contributing to the device performance. Especially at the surface such charges may influence stability and large signal characteristics. Switching transients in output current of AlGaN/GaN-FETs related to charge storage effects are discussed and related to the polar nature of this material system. Alternative structures not suffering of the surface charge problem are introduced, namely an InGaN-channel FET and a AlGaN/GaN-double barrier structure. For realization doping screening of the polarization field is applied as main tool. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

DC current-switching and power-switching transients of various GaN-based FET structures are investigated. Two different characteristics are compared, namely, thermal and electronic transients. While the thermal transients are mainly reflected in changes in channel carrier mobility, the electronic transients are dominated by charge instabilities caused by the polar nature of the material. The discussion of the electronic transients focuses, therefore, on instabilities caused by polarization-induced image charges. Three structures are discussed, which are: 1) a conventional AlGaN/GaN heterostructure FET, 2) an InGaN-channel FET, and 3) an AlGaN/GaN double-barrier structure. In structures 2) and 3), field-induced image charges are substituted by doping impurities, eliminating this source of related instability. This is indeed observed.

Dislocation analysis of epitaxially grown GaN was performed using an HCl vapor phase etching process. The effects of the major process parameters temperature, pressure and gas composition have been studied in detail. For reliable results a large size of the etch pits is required, but a merging of the pits has to be avoided. A temperature of 600 degreesC, a reactor pressure of 940 mbar and an HCl concentration of 10% lead to best results and highest etch rates, respectively (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Wemheim

The absorption of lateral guided modes in light emitting diodes is determined by the photocurrent measurement method. A theory for waveguide dispersion is presented and extended by ray-tracing simulations. Absorption coefficients of InGaN-on-sapphire and AlGaInP-based structures is evaluated by comparison with simulation curves. For nitride-based samples with emission wavelengths of 415 nm and 441 nm an absorption of 7 cm(-1) is obtained. It is found that scattering is present in the buffer layer and influences the lateral intensity distribution. The investigated AlGaInP-based sample exhibits an absorption of alpha = 30 cm(-1) at 650 nm emission wavelength.

Under pulsed operation, time dependent spectral and electro-optical measurements on GaN-based laser diodes show a considerable red shift in the emission wavelength and a decreasing voltage drop across the device. These changes appear on a rather short time scale in the microsecond range. During a 3.7 microsecond long pulse, a temperature increase of approximately 50 K is obtained using different experimental methods. This value agrees well with numerical simulations based on the thermal properties of the material. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The dipole nature of the polarization image charges are a new feature of polar wide bandgap semiconductor heterostructures and need thus to be taken into account in the design of FET structures. The ambi-polar nature of the image charges, which are mobile in most cases, needs to be considered carefully. If the image charge dipole creates an ambi-polar channel, the hole charge needs to be compensated by shallow donor doping, preferably by modulation doping. If the countercharge to the channel charge is located on the surface, this may result in drift and current dispersion, which indeed plagues many AlGaN/GaN power devices. The countercharge can also be removed from the surface by a double barrier arrangement. The most attractive structure to. obtain such a configuration is the InGaN channel FET.

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.

We have carried out spatially resolved micro-Raman spectroscopy, cathodoluminescence microscopy and scanning capacitance microscopy in order to obtain a comprehensive understanding about the properties of different domains formed in epitaxial laterally-overgrown GaN. For this purpose a spherical pit was fabricated into the sample by mechanical grinding and polishing, penetrating through to the buffer layer at its center. We found areas showing sharp excitonic luminescence corresponding to local free-carrier concentrations n below 1017 cm—3 as well as domains exhibiting broad luminescence originating from recombination of a doping plasma with n reaching 1019 cm—3. Simultaneously, we observed in the scanning microscopy investigations a substructure which could be explained by the existence of internal space charge regions.

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.

Using hydride vapor phase epitaxy the influence of growth parameters on the crack density is studied for thick epitaxially lateral overgrown (ELOG) GaN layers. Reactor pressure, growth rate, and substrate temperature are key factors to obtain crack-free thick GaN layers. The cracking mechanism is discussed and void formation on top of the SiO2 stripes is proposed to play a key role in stress relaxation and crack suppression.

Making use of the polar nature of III-nitride heterostructures, a new FET device concept is proposed. The structure contains an InGaN QW channel sandwiched in between two GaN barrier layers. The charge inthis structure is mainly generated by the strain field in the InGaN layer and is an electron/hole dipole sheet charge located at the opposite InGaN/GaN interfaces. To obtain nchannel characteristics the hole charge at the rear interface (for Ga-face oriented material) is compensated by donor doping of the channel or by modulation doping from the real GaN barrier layer. Growth, processing technology and characteristics of first fabricated devices is discussed.

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 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.

Columnar ELOG growth domains formed in a 65 μm thick HVPE GaN layer during overgrowth of hexagonal SiO2 masks are three-dimensionally characterized using spatially and spectrally resolved scanning cathodoluminescence (CL) microscopy. In conjunction with cross-sectional imaging perpendicular to the c-plane, a direct visualization of the 3D domain formation is achieved by consecutive vertical series of CL mappings parallel to the c-plane. A perfect agreement between the local luminescence properties and the evolution of the local free carrier concentration during the different stages of overgrowth is confirmed by micro-Raman measurements. While mask-periodic domains with specific optical and electronic properties are observed for initial growth, homogeneously high crystal quality at the sample surface is evidenced.

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.

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.