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(0001)-oriented epitaxial wurtzite III-nitride layers grown on mismatched substrates have no resolved shear stress on the natural basal and prismatic slip planes; however, strained Ill-nitride layers may gradually relax. We report on the stress relaxation of Al0.49Ga0.51N layers grown on nominally relaxed Al0.62Ga0.38N buffer layers on sapphire. The reduction in elastic strain of the Al0.49Ga0.51N was enhanced by Si doping which caused an increased surface roughness. Despite the Si doping, the films always sustained step-flow growth. The extent of relaxation of the Al0.49Ga0.51N layer was determined by on-axis omega-2 theta scans of (0001) peaks and reciprocal space maps of inclined (off-axis) peaks. Cross-section and plan-view transmission electron microscopy studies showed that the threading dislocations in the Al(0.49)Gao(0.5)N layer inclined from the [0001] direction towards (1 (1) over bar 00) directions by similar to 15-25 degrees, perpendicular to their Burgers vector (1/3 ) These inclined threading dislocations have a misfit dislocation component and thus provide stress relief. The contribution of the dislocation inclination to the degree of relaxation has been formulated and the energy release has been determined for dislocation inclination in mismatched stressed layers. (c) 2005 American Institute of Physics.

An analysis is presented on the effect of the strain field originating from a subsurface stressor (point source of dilatation or a dilatating ellipsoidal inclusion) on the electronic properties of nitride semiconductors. With good accuracy, real quantum dots can be modeled as such stressors. We consider the following material structure design: a uniform semi-infinite GaN matrix with a buried stressor or a GaN matrix with a single (In,Ga)N quantum well, which is grown pseuodomorphically between the stressor and the free surface. We utilize isotropic elasticity to determine the strain field in the structures under investigation. We then apply a k.p perturbation theory approach to examine the shifts of the conduction and valence band edges caused by the stressor. We find lateral confinement for electrons and holes, which can be proposed for the realization of strain-induced quantum dots in the quantum well. (C) 2005 American Institute of Physics.

We report AlGaN-GaN high electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) on SiC substrates with excellent microwave power and efficiency performance. The GaN buffers in these samples were doped with carbon to make them insulating. To reduce gate leakage, a thin silicon nitride film was deposited on the AlGaN surface by chemical vapor deposition. At 4 GHz, an output power density of 6.6 W/mm was obtained with 57% power-added efficiency (PAE) and a gain of 10, dB at a drain bias of 35 V. This is the highest PAE reported until now at 4 GHz in AlGaN-GaN HEMTs grown by MBE. At 10 GHz, we measured an output power density of 7.3 W/mm with a PAE of 36% and gain of 7.6 dB at 40-V drain bias.

Dislocation-related conduction paths in n-type GaN grown by molecular-beam epitaxy and a mechanism for local suppression of current flow along these paths are analyzed using conductive atomic force microscopy, scanning Auger spectroscopy, and macroscopic current-voltage measurements. Application of an electric field at the GaN surface in an ambient atmospheric environment is shown to lead to local formation of gallium oxide in the immediate vicinity of the conduction paths, resulting in the strong suppression of subsequent current flow. Current-voltage measurements for Schottky diodes in which local conduction paths have been suppressed in this manner exhibit reverse-bias leakage currents reduced by two to four orders of magnitude compared to those in Schottky diodes not subjected to any surface modification process. These results demonstrate that the dislocation-related current leakage paths are the dominant source of leakage current in Schottky contacts to n-type GaN grown by molecular-beam epitaxy, and elucidate the nature of a microscopic process for their suppression. (C) 2003 American Institute of Physics.

Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) have been used to image surfaces of GaN grown by molecular beam epitaxy. Detailed analysis of the same area using both techniques allowed imaging and comparison of both surface potential variations arising from the presence of negatively charged threading dislocations and localized current leakage paths associated with dislocations. Correlations between the charge state of dislocations, conductivity of current leakage paths, and dislocation type were thereby established. Analysis of correlated SKPM and C-AFM images revealed a density of negatively charged features of similar to3 x 10(8) cm(-2) and a localized current leakage path density of similar to3 x 10(7) cm(-2), With approximately 25% of the leakage paths spatially correlated with negatively charged dislocation features. Based on correlated topography and previous studies quantifying the densities of edge, screw, and mixed dislocations, our results suggested that dislocations having an edge component behave as though negatively charged while pure screw dislocations are solely responsible for the observed leakage paths and are uncharged. (C) 2003 American Institute of Physics.

Evaluation of the structural properties of 200-nm-thick Si-doped Al0.49Ga0.51N films, grown on nominally relaxed 1-mum-thick Al0.62Ga0.38N buffer layers on sapphire, revealed that increased Si doping promoted the relaxation of the compressively strained layers. The degree of strain relaxation R of the Al0.49Ga0.51N films, as determined by x-ray diffraction (XRD), increased from R=0.55 to R=0.94 with an increase in disilane injection from 1.25 nmol/min to 8.57 nmol/min. Transmission electron microscopy analysis showed that the edge threading dislocations (TDs) in the Al0.49Ga0.51N layers were inclined, such that the redirected TD lines had a misfit dislocation component. The calculated strain relaxation due to the inclined TDs was in close agreement with the values determined from XRD. We propose that the TD line redirection was promoted by the Si-induced surface roughness. (C) 2003 American Institute of Physics.

We present a simple strategy that minimizes the impact of surface segregation of In during the growth of (In,Ga) N/GaN multiple quantum wells by plasma-assisted molecular-beam epitaxy and simultaneously results in abrupt interfaces. The two ingredients of this strategy are (i) the use of a higher substrate temperature than commonly employed, that is, well above the In desorption point and (ii) the use of a modulated stoichiometry, that is, N-rich during growth of the well and Ga-stable during growth of the barrier. (C) 2003 American Institute of Physics.

An electrochemical surface treatment has been developed that decreases the reverse-bias leakage current in Schottky diodes fabricated on GaN grown by molecular-beam epitaxy (MBE). This treatment suppresses current flow through localized leakage paths present in MBE-grown GaN, while leaving other diode characteristics, such as the Schottky barrier height, largely unaffected. A reduction in leakage current of three orders of magnitude was observed for Schottky diodes fabricated on the modified surface compared to diodes fabricated on the unmodified surface for reverse-bias voltages as large as -20 V. In addition to suppressing reverse-bias leakage, the surface treatment was found to improve substantially the ideality factor of the modified surface diodes compared to that of unmodified surface diodes, suggesting that such a surface modification process could be useful for a variety of GaN-based electronic devices. (C) 2003 American Institute of Physics.

M-plane In0.1Ga0.9N(1 (1) over bar 00) multiple quantum wells were grown on gamma-LiAlO2(100) by plasma-assisted molecular-beam epitaxy. The high brightness of the photoluminescence of these multiple quantum wells and the observation of band-edge emission from the barriers are consistent with the expected absence of spontaneous and piezoelectric polarization fields along the growth direction. Despite this fact, the recombination dynamics observed is rather complex and is shown to originate from the superposition of exciton relaxation towards a band of localized states and simultaneous recombination.

Memory extinction, defined as a decrease of a conditioned response as a function of a non-reinforced conditioned stimulus presentation, has high biological and clinical relevance. Extinction is not a passive reversing or erasing of the plasticity associated with acquisition, but a novel, active learning process. Nifedipine blocks L-type voltage gated calcium channels (LVGCC) and has been shown previously to selectively interfere with the extinction, but not the acquisition, of fear memory. We studied here the effect of retrograde and anterograde shifts of nifedipine application, with respect to an extinction training, on the extinction of fear conditioning. Subcutaneous injection of 30 mg/kg nifedipine, at least up to 4 h before the extinction session, significantly impaired extinction, as did intraperitoneal injection of 15 mg/kg nifedipine, at least up to 2 h before extinction training. However, the injection of nifedipine also induced a strong and protracted stress response. The pharmacokinetics of nifedipine suggest that it was mainly this stress response that triggered the specific inhibition of extinction, not the blockade of LVGCC in the brain. Our results support recent findings that stress selectively interferes with the extinction, but not the acquisition, of fear memory. They also indicate that a pharmacological approach is not sufficient to study the role of brain LVGCC in learning and memory. Further research using specific genetically modified animals is necessary to delineate the role of LVGCC in fear memory extinction.

The effect of 6H-SiC or A12O3 substrates on the E3(high)-phonon Raman line was studied.

Author(s): P. Waltereit (corresponding author); O. Brandt; A. Trampert; H. T. Grahn; J. Menniger; M. Ramsteiner; M. Reiche; K. H. Ploog Compact solid-state lamps based on light-emitting diodes (LEDs) [1, [...]