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The optical properties of GaN films have been found to be sensitive to SiNx and SiO2 surface passivation. The main effect of such passivation on photoluminescence (PL) data is an increase of the PL intensity for near-band-edge emission. This effect is attributed to the removal of oxygen from the surface of GaN and the subsequent formation of a protective layer during passivation. The increase in PL intensity is more pronounced for samples passivated with SiO2, which demonstrate initially lower PL intensity and a lower equilibrium concentration of free electrons. A nearly constant band bending of approximately 1.0 eV at the surface has been observed for as-grown and passivated samples by scanning Kelvin probe microscopy (SKPM). This constant value is explained by pinning of the Fermi level at the surface. In addition, we have demonstrated that passivation of the GaN surface between the contacts of a Schottky diode leads to a reduction of the leakage current observed at reverse bias. It was found that the surface potential measured by SKPM increases as a function of distance from the Schottky contact much faster after SiNx passivation. We suggest that the passivation reduces the total density of surface states and therefore reduces surface recombination. [ABSTRACT FROM AUTHOR]

The current-voltage characteristics of n-GaN/u-AlGaN/n-GaN heterostructure devices are investigated for potential pressure sensor applications. Model calculations suggest that the current decreases with pressure as a result of the piezoelectric effect, and this effect becomes more significant with thicker AlGaN layers and increasing AlN composition. The change in current with pressure is shown to be highly sensitive to the change in interfacial polarization charge densities. The concept is verified by measuring the current versus voltage characteristics of an n-GaN/u-Al0.2Ga0.8N/n-GaN device under hydrostatic pressure over the range of 0–5 kbars. The measured current is found to decrease approximately linearly with applied pressure in agreement with the model results. A gauge factor, which is defined as the relative change in current divided by the in-plane strain, approaching 500 is extracted from the data, demonstrating the considerable potential of these devices for pressure sensing applications. [ABSTRACT FROM AUTHOR]

We report on the effectiveness of porous TiN nanonetworks on the reduction of threading dislocations (TDs) in GaN grown by metal-organic vapor-phase epitaxy (MOVPE). The porous TiN networks were formed by in situ annealing of thin-deposited Ti films deposited ex situ on GaN templates within the MOVPE growth chamber. Different annealing parameters in relation to surface porosity of TiN networks were investigated. Transmission electron micrographs indicated dislocation reduction by factors of up to 10 in GaN layers grown on the TiN nanonetwork, compared with a control sample. TiN prevented many dislocations present in the GaN templates from penetrating into the upper layer. Microscale epitaxial lateral overgrowth of GaN above TiN also contributed to TD reduction. The surface porosity of the TiN network had a strong impact on the efficiency of TD reduction. X-ray-diffraction and time-resolved photoluminescence measurements further confirmed the improved GaN quality. [ABSTRACT FROM AUTHOR]

We report on the growth of and evolution of defects in GaN epilayers having single- and double-layer SiNx nanoporous insertion layers. The SiNx was formed in situ in the growth chamber of an organometallic vapor-phase epitaxy system by simultaneous flow of diluted silane and ammonia. The GaN epilayers and SiNx interlayers were grown on 6H-SiC substrates using three different nucleation layers, namely, low-temperature GaN, high-temperature GaN, and high-temperature AlN nucleation layers. X-ray-diffraction rocking curves and cross-sectional and plan-view transmission electron microscope analyses indicated that a nanoporous SiNx layer can reduce the dislocations density in the GaN overgrown layer to ∼3×108 cm-2 range; below this level the defect blocking effect of SiNx would saturate. Therefore the insertion of a second SiNx layer becomes much less effective in reducing dislocations, although it continues to reduce the point defects, as suggested by time-resolved photoluminescence measurements. The insertion of SiNx interlayers was found to improve significantly the mechanical strength of the GaN epilayers resulting in a much lower crack line density. [ABSTRACT FROM AUTHOR]

Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on SiN and TiN porous network templates. The room-temperature decay times obtained from biexponential fits to time-resolved photoluminescence data are increased with the inclusion of SiN and TiN layers. The carrier lifetime of 1.86 ns measured for a TiN network sample is slightly longer than that for a 200 mm-thick high-quality freestanding GaN. The linewidth of the asymmetric x-ray diffraction (XRD) (1012) peak decreases considerably with the use of SiN and TiN layers, indicating the reduction in threading dislocation density. However, no direct correlation is yet found between the decay times and the XRD linewidths, suggesting that point defect and impurity-related nonradiative centers are the main parameters affecting the lifetime. [ABSTRACT FROM AUTHOR]

We report on the structural analysis of ferromagnetic Mn-doped ZnO thin films deposited by radio frequency magnetron sputtering, using transmission electron microscopy (TEM), high-resolution x-ray diffraction, and Rutherford backscattering spectroscopy (RBS) measurements. The ferromagnetic Mn-doped ZnO film showed magnetization hysteresis at 5 and 300 K. A TEM analysis revealed that the Mn-doped ZnO included a high density of round-shaped cubic and elongated hexagonal MnZn oxide precipitates. The incorporation of Mn caused a large amount of structural disorder in the crystalline columnar ZnO lattice, although the wurtzite crystal structure was maintained. The observed ferromagnetism is discussed based on the structural characteristics indicated by TEM and the behavior of Mn when it is substituted into a ZnO lattice derived from RBS measurements. [ABSTRACT FROM AUTHOR]

Carrier transport properties of AlGaN/GaN heterostructures have been analyzed with the quantitative mobility spectrum analysis (QMSA) technique. The nominally undoped Al0.08Ga0.92N/GaN sample was grown by plasma-assisted molecular beam epitaxy on a GaN/sapphire template prepared with hydride vapor phase epitaxy. Variable-magnetic-field Hall measurements were carried out in the temperature range of 5–300 K and magnetic field range of 0.01–7 T. QMSA was applied to the experimental variable-field data to extract the concentrations and mobilities associated with the high-mobility two-dimensional electron gas and the relatively low-mobility bulk electrons for the temperature range investigated. The mobilities at T=80 K are found to be 7100 and 880 cm2/V s, respectively, while the corresponding carrier densities are 7.0×1011 and 8×1014 cm-3. Any conclusions drawn from conventional Hall measurements at a single magnetic field would have been highly misleading. [ABSTRACT FROM AUTHOR]

High-power photoconductive semiconductor switching devices were fabricated on 4H–SiC. In order to prevent current crowding, reduce the contact resistance, and avoid contact degradation, a highly n-doped GaN subcontact layer was inserted between the contact metal and the high resistivity SiC bulk. This method led to a two orders of magnitude reduction in the on-state resistance and, similarly, the photocurrent efficiency was increased by two orders of magnitude with the GaN subcontact layer following the initial high current operation. Both dry etching and wet etching were used to remove the GaN subcontact layer in the channel area. Wet etching was found to be more suitable than dry etching. [ABSTRACT FROM AUTHOR]

We have determined the absolute indium content incorporated in the crystalline lattice of InGaN films and InGaN/GaN multiple quantum wells using anomalous x-ray scattering (AXS). AXS spectra were obtained near the In K absorption edge at the InGaN (0006) Bragg peak where the InGaN Bragg reflection is well-resolved from the GaN reflections. By comparing the indium composition obtained by AXS to regular x-ray scattering results, which are also sensitive to the lattice strain, we determine the Poisson ratio of InGaN to be ν≈0.23. The AXS method can be effective in determining absolute chemical composition of InGaN independent of the lattice strain, which is especially valuable for InGaN multiple quantum wells. [ABSTRACT FROM AUTHOR]

A correction to the research "Efficacy of Single and Double SiNx Interlayers on Defect Reduction in GaN Overlayers Grown by Organometallic Vapor-Phase Epitaxy," that was published in the 2005 issue of "Journal Applied Physics" is presented.

A new piezoelectric lithotripter, the EDAP LT02, differs from the earlier LT01 in two important respects: a stone-localization system consisting of ultrasound and fluoroscopy and more energy per shockwave. The results of SWL monotherapy with the LT02 in 1363 patients and with the LT01 in 1586 patients who completed SWL treatment were compared. All of the stones could be localized by ultrasound (62.7%), fluoroscopy (0.3%), or both (37%) on the LT02, but the renal stones of three patients who had a history of open renal surgery and 29 upper ureteral stones could not be localized by real-time ultrasound on the LT01. The success rate (stone free or residual fragments < 4 mm in diameter) with the LT02 and LT01 according to stone location was not significantly different: 86.4% and 88.5%, respectively, for renal stones, 89.1% and 88.9% for ureteral stones, and 87.0% and 89.0% overall. The overall stone-free rate of LT02 and LT01 SWL was 78.8% and 87.9%, respectively. For mid-ureteral stones, the success rate with LT02 SWL was 76.8%. (Such stones were not an indication for LT01 SWL.) The mean number of treatment sessions with the LT02 (1.9) was much less (P < 0.001) than with the LT01 (3.3). The retreatment rates of LT02 and LT01 SWL were 68.2% and 198.2%, and the auxiliary treatment rates were 3.4% and 6.3%, respectively. Thus, the efficiency quotient was 45.9% in LT02 SWL, which was much higher than the 28.9% of the LT01. There was no significant difference in the occurrence of complications between LT02 and LT01 SWL. Therefore, LT02 piezoelectric lithotripsy is more efficient than the LT01 in localizing stones and can lessen the number of treatment sessions.