In GaN layers grown by molecular beam epitaxy as well as metal organic vapor phase epitaxy significant differences were found in the appearance of deep defects de-tected by thermal admittance spectroscopy as compared for deep level transient spectros-copy measurements. While, thermal admittance spectroscopy measurements which were made under zero bias conditions only show thermal emissions at activation energies between 130 and 170 meV, further deep levels existing in these GaN layers were evidenced by transient spectrocopy. This discrepancy is explained by a pinning effect of the Fermi level at the metal / GaN interface induced by high a concentration of the deep levels showing up in thermal admittance spectroscopy. We compare our results with a GaAs:Te Schottky- diode as a refernec sample. Here, both spectroscopic methods give exactly the same deep level emissions.

AlInN/GaN heterostructures with indium contents between 20% and 35% were grown by metal organic vapour phase epitaxy on high purity silicon (111) substrates. The samples were investigated by photovoltage (PV) spectroscopy whereby the individual layers were distinguished by their different absorption edges.The near band-edge transitions of GaN and of Si demonstrate the existence of space charge regions within the GaN layers and the Si substrate. In sandwich geometry the Si substrate significantly influences the PV spectra which are strongly quenched by additional 690 nm laser light illumination. The intensity dependence and the saturation behaviour of quenching suggest a recharging of Si-and GaN-related interface defects causing a collapse of the corresponding PV signals in the space charge region.From additional scanning surface potential microscopy measurements in bevel configuration further evidence of the existence of different space charge regions at the GaN/AlN/Si and AlInN/GaN interfaces is obtained.The properties of the Si/seed layer/GaN heterostructure are discussed in terms of a p-type Si/n-type GaN layer interface generated by diffusion of Si atoms into GaN and of Ga or Al atoms into the Si substrate.

Little is known about the effects of various types, modes, and routes of hormone replacement therapy (HRT) on the risk of colorectal cancer (CRC) among postmenopausal women. We conducted a population-based case-control study with validation of self-reported hormone use and no upper age limit. In 1,456 postmenopausal women aged 45-94 years (546 cases, 910 controls), the use of HRT was associated with reduction in CRC risk among ever users (adjusted odds ratio (OR) 0.65, 95% confidence interval 0.50-0.84), current users, and recent users. There was no evidence that risk reduction among current users varies by age. Risk reduction was seen both in estrogen-only therapy (0.42, 0.23-0.78) and in combination therapy (0.60, 0.41-0.87), the latter regardless of the mode of therapy, whether with hormone patches (0.40, 0.17-0.90) or with oral tablets (0.59, 0.39-0.90). In combination with estrogen, progestagens of the norethisterone and levonorgestrel families were associated with strong reduction in CRC risk.

This report concerns investigations of the activation process of nerve cells cultured on large planar interdigitated electrodes. Cultured neurons from embryonic rat cerebral cortex form electrophysiologically active networks after one week in vitro. The treatment with antagonists to ionotropic glutamate and GABA(A) receptors blocks synaptic neurotransmission without affecting intrinsic excitability of neurons. Standard stimulation parameters, which were previously determined to trigger a generalized network response in control networks, were ineffective for activation of networks under a synaptic activity blockade. Here we show that in these conditions a pulse sequence of ten biphasic pulses with amplitude of +/- 2.2 V is adequate for global network stimulation, without cell damage. In specially designed compartment cultures we analyzed further the stimulation process in control and synaptically blocked networks with cell bodies spatially separated from the electrodes. Our results suggest that the direct stimulation of few neurons trigger network-wide responses by the activation of synaptic connections. Further, the stimulation of axonal neuritic extensions is sufficient to active single neurons.

We report on the implementation of GaN-based, blue light emitting diodes on 150 mm Si(001) substrates grown by m, etalorganic vapor phase evitaxy, The 2,8 pro thick samples, are completely crack-free and the optically active layers consist of fivefold In.Ga1-xN/GaN multiple quantum wells. The homogeneity of the fight, emission across the whole sample is determined by photoluminescence wafer mapping exhibiting an averaged peak wavelength of 456 nm at room temperature and a standard deviation of less than 1%. A bright blue light emission is obtained by an electrical excitation with a turn-on, voltage of below 2.8 V. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present AlGaN/GaN-based FETs on Si(0 0 1) grown by metalorganic vapor phase epitaxy (MOVPE). The influence of the substrate off-orientation on the crystallographic quality is investigated by spatially resolved electron backscatter diffraction. A stringent correlation of the surface morphology of GaN layers grown on differently misoriented Si(0 0 1) with two different in-plane alignments of adjacent GaN crystallites is observed. On a 2.5 μm thick single-crystalline and crack-free GaN-based buffer layer structure, an AlGaN/GaN FET heterostructure was realized. A drain–source current of 245 mA/mm with a transconductance of 90 mS/mm was achieved.

We describe the successful homoepitaxial growth of ZnO layers on oxygen-face ZnO substrates by metalorganic vapor-phase epitaxy in two-dimensional growth mode. In detail, we discuss the impact of the oxygen/zinc precursor ratio using N2O and O2 as oxygen precursors, growth temperature, and reactor pressure on structural properties and surface morphology as obtained by X-ray diffraction, field emission scanning electron microscopy, and atomic force microscopy measurements. Optimizing the growth parameters leads to smooth layers, ending up in a mirror-like surface grown in two-dimensional growth mode. The structural layer properties are found to be significantly governed by the substrate properties.

AlN layers were grown on Si and sapphire substrates in a horizontal and a vertical metalorganic chemical vapour phase epitaxy system. AlN grown in the horizontal reactor is smooth and shows intense band-edge cathodoluminescence but Ga can be found in the layers even after several microns of AlN growth. After cleaning the reactor and using a new Ga-free quartz and graphite system the layer quality is drastically reduced and we observe a rough morphology. In the vertical system pure AlN layers with smooth surfaces can be grown even on a Ga-contaminated susceptor and no Ga is found in the layers. In cathodoluminescence (CL) measurements an enhancement of oxygen or silicon-related luminescence peaks is found for the growth on sapphire or silicon substrates, respectively. A comparison of CL and Raman measurements reveals a strong tensile stress for AlN on Si that corresponds to ∼1 GPa and relaxes at cracks where a wavelength shift of 4 nm is observed. AlN grown on sapphire is found to be under compressive stress at room temperature.

We report on scanning capacitance microscopy (SCM) investigations of Fe-doped GaN and nitrogen-doped ZnO layers. Macroscopically, these samples electrically behave in conventional I–V and C–V measurements like semi-insulating or n-type material, respectively. However, in SCM we found local p-type regions surrounded by an n-type matrix instead of homogeneous and uniform layer conductivity. A comparison with topography reveales that these p-type islands with extensions in the micrometer scale exclusively appear in the vicinity of structural defects and grain boundaries. This doping related effect is discussed in terms of selective dopant incorporation at these defects.

Extended defects on the top surface of a 250-µm-thick free-standing GaN sample, grown by hydride vapor phase epitaxy (HVPE), were studied by deep level transient spectroscopy (DLTS) and scanning surface potential microscopy (SSPM). For comparison, similar studies were carried out on as-grown HVPE-GaN samples. In addition to the commonly observed traps in as-grown HVPE-GaN, the DLTS measurements on free-standing GaN reveal a very high concentration of deep traps (∼1.0 eV) within about 300 nm of the surface. These traps show nonexponential capture kinetics, reminiscent of those associated with large defects, that can accumulate multiple charges. The SSPM measurements clearly reveal the presence of charged microcracks on the top surface of the sample. It appears that the “giant traps” may be associated with these microcracks, but we cannot rule out the involvement of other extended defects associated with the near-surface damage caused by the polishing/etching procedure.

A dipole like strain state is induced by threading edge dislocations emerging at the surface of gallium nitride (GaN) bulk substrates. This local strain is calculated by means of a three-dimensional elastic deformation potential model, taking into account the free surface of the sample. The calculations are in excellent quantitative agreement with the strain state derived from line shifts of the near band edge excitonic spectrum, measured by micro-photoluminescence (μPL). Scanning surface potential microscope (SSPM) measurements show that the dipole structure is not reflected in the local electrical potential distortions around the dislocations and the potential profile decreases laterally faster than the strain distortion, which is detectable even in several micrometer distance from the dislocation core.

We report on the influence of dielectric pulse injection on the energy metabolism of yeast cells with a planar interdigitated electrode interface. The energy metabolism was measured via NADH fluorescence. The application of dielectric pulses results in a distinct decrease of the fluorescence, indicating a response of the energy metabolism of the yeast cells. The reduction of the NADH signal significantly depends on the pulse parameters, i.e., amplitude and width. Furthermore, the interface is used to detect electrical changes in the cell-electrolyte system, arising from glucose-induced oscillations in yeast cells and yeast extract, by dielectric spectroscopy at 10 kHz. These dielectric investigations revealed a β1-dispersion for the system electrolyte/yeast cells as well as for the system electrolyte/yeast extract. In agreement with control measurements we obtained a glycolytic period of 45s for yeast cells and of 11min for yeast extract.

GaN on Si offers a promising technology for the low-cost production of wide-bandgap devices. Here, we present approaches towards the growth of GaN on technologically most relevant Si(001) substrates and methods to grow single-crystalline c-axis-oriented GaN on Si(001) with omega-scan FWHMs of 986 arc sec for the (0002) Bragg reflection. Strain is still the major issue for the established growth on Si(111). A study on the generation of strong tensile stress by Si-doping is presented. We find that tensile stress generation is dominantly dependent on the Si doping concentration and the edge dislocation density. (c) 2006 Elsevier B.V. All rights reserved.

InAlN has been investigated as barrier layer material for GaN -HEMT structures, potentially offering higher sheet charge densities [1] and higher breakdown fields [2]. Lattice matched growth of the barrier layer can be achieved with 17 % In content, avoiding piezo polarization. In this configuration the sheet charge density is only induced by spontaneous polarization. First experimental results of unpassivated undoped samples realized on 111- Si substrate exceed a DC output current density of 1.8 A/mm for a gate length of 0.5 μm. Small signal measurements yield a f t = 26 GHz and f max = 14 GHz , still limited by the residual conductivity of the Si -substrate. A saturated output power at 2 GHz in class A bias point yielded a density of 4.1 W/mm at V DS = 24 V .

In GaN layers grown by metal-organic vapor phase epitaxy on sapphire substrates the temperature-dependent Hall (TDH) and photo-Hall-effect (PHE) measurements show essential differences between undoped and Si-doped GaN. In undoped GaN the maximum of the Hall mobility occurs at temperatures near 300K with a low value. In PHE, an illumination introduces an enhancement of the mobility and a decrease of the electron density. In contrast, in Si-doped GaN the maximum Hall mobility is higher by a factor of 10 and is observed at temperatures between 100 and 180K. The photoinduced changes in the mobility and electron density are only marginal. Intensity dependent PHE measurements suggest the existence of internal potential barriers caused by inhomogeneities in the undoped samples. These results are combined with the surface-potential roughness on a microscale, as determined by scanning surface-potential microscopy (SSPM). In SSPM the undoped layers show strong potential fluctuations while they are lower for the Si-doped GaN samples. A correlation among the rms roughness of the surface potential, the maximum Hall mobility in TDH, and the maximum changes of the photo-Hall mobility is observed. In undoped GaN the mobility seems to be determined by the scattering at inner potential barriers stemming from structural inhomogeneities.

We report on the realization of a planar large area electrode interface which reproducibly allows the global excitation of neurons and the generation of stimulated network activity. The interface is formed by two double finger-shaped Ti–Au-electrodes without any isolating coating deposited by electron beam evaporation on microscope cover slips. Dissociated nerve cells from embryonic rat cerebral cortex were cultured on these electrodes forming electrophysiologically active networks within seven days of culture. These networks were electrically excited by application of voltage pulses, resulting either in an activity of single neurons or in a stimulated synchronous network activity in dependence on the pulse parameters. The impact of these parameters, such as the number of pulses, the pulse amplitude and the delay between distinct pulse events, on the stimulation success was systematically investigated. We found threshold values for the voltage pulse amplitude of 1.8–2.2 V and for the voltage pulse duration of 1 ms to reproducibly obtain stimulation success with our system. These results are repeated for differently aged cell cultures and at different sections of the whole network. The stimulation procedure does not significantly damage the nerve cells.

We present a study on AlInN/GaN field effect transistors (FETs) grown by metalorganic chemical vapor phase epitaxy. AlInN can be grown lattice-matched to GaN with an In concentration of 18%. In this study samples with In concentrations ranging from 9.5 to 24%, covering a range from tensely to compressively strained AlInN layers, were grown on GaN layers on Si(l I I). From Hall effect and capacitance-voltage measurements we find high sheet carrier densities for most of the samples indicating a high electron density at the AlInN/GaN heterointerface. This is also reflected in the behavior of processed FETs. Nearly lattice-matched structures show sheet carrier densities of 3.2 x 10(13) cm(-2) and mobilities up to similar to 406 cm(2)/Vs. Such Al0.84In0.16N FETs have maximum DC currents of 1.33 A/mm for devices with 1 mu m gate length and 100 Vim gate width and an output power of 2.5 W/mm at 2 GHz. The best devices with In concentrations of 19% show maximum output powers of 4.1 W/mm at 2 GHz. In contrast to that a compressively strained AlInN layer with an In concentration of 24% leads to a decreased polarization charge at the heterointerface and a low DC current of similar to 70 mA/mm. (c) 2005 WILEY-VCH Verlag GmbH & Co.

Extended defects in semiconductors can trap charge and lead to changes in carrier concentration and mobility. Here we consider the trapping effects of pores, microcracks, and dislocations in GaN and SiC, as analyzed by deep level transient spectroscopy (DLTS), transmission electron microscopy (TEM), and scanning surface potential microscopy (SSPM). The defect structures are modeled as spheres, plates, and cylinders for pores, cracks, and dislocations, respectively, and their potentials are directly compared with those measured by holographic TEM and SSPV. The dynamics of the capture and emission processes are investigated by DLTS, although the standard DLTS analysis framework is not applicable here and must be replaced by a more general formalism. As an example, 40-nm-dia. nanopores in SiC can each hold more than 100 electrons, and they exhibit anomalous capture and emission properties. (c) 2005 WILEY-VCH Verlag GmbH E Co. KGaA, Weinheim.

A doping approach for p-type ZnO is reported which is reproducible and long-time stable. For p-type doping the zinc oxide layers were doped simultaneously with nitrogen and arsenic in metal organic vapor phase epitaxy. The conductivity type of the layers was investigated by scanning capacitance microscopy, a technique based on local capacitance-voltage analysis (C-V) with submicron spatial resolution. Depending on the growth parameters, largely extended p-type domains were observed, surrounded by n-type regions. The differences in local conductivity type are directly correlated to the topography as measured with atomic force microscopy revealing p-type for smooth, two-dimensional surfaces and n-type signals in the case of three-dimensional island growth or structural defects, i.e., microcracks or surface pits.

The presence of a threading edge dislocation terminated at the surface of GaN bulk substrates causes a dipole-like strain state ranging over a several micrometer square area. The local strain state is derived from microphotoluminescence mappings of the near-band-edge spectrum and is quantitatively reproduced by a three-dimensional elastic deformation model approach. These results are compared with the local electrical potential distortion due to the core charge and attracted defects as analyzed by scanning surface-potential microscopy. In contrast to the local strain, the potential profile does not show a dipole-like behavior and decreases laterally faster.