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Admittance-voltage profiling of AlxGa1-xN/GaN heterostructures was used to determine the frequency dependent capacitance and conductance of FET devices in the frequency range from 50 Hz to 1MHz. The nominally undoped low pressure metal-organic vapor-phase epitaxy structures were grown with an Al-content of 30%. An additional 1 nm thick AlN interlayer was placed in one structure before the Al0.3Ga0.7N layer growth. For frequencies below 108 Hz it is convenient to use equivalent circuits to represent electric or dielectric properties of a material, a method widely used, for example, in impedance spectroscopy. We want to emphasize the relation between frequency dependent admittance-voltage profiling and the corresponding equivalent circuits to the complex dielectric function. Debye and Drude models are used for the description of the frequency dependent admittance profiles in a range of depletion onset of the two-dimensional electron gas. Capacitance- and conductance-frequency profiles are fitted in the entire measured range by combining both models. Based on our results, we see contributions to the two-dimensional electron gas for our samples from surface states (80%) as well as from background doping in the Al0.3Ga0.7N barriers (20%). The specific resistance of the layers below the gate is above 105 Ω cm for both samples and increases with increasing negative bias, i.e., the layers below the gate are essentially depleted. We propose that the resistance due to free charge carriers, determined by the Drude model, is located between gate and drain and, because of the AlN interlayer, the resistance is lowered by a factor of about 30 if compared to the sample without an AlN layer. [ABSTRACT FROM AUTHOR]

Electrical properties of AlxGa1-xN/GaN heterostructures with an Al content below 15% and carrier concentrations as low as 1.0 × 1012 cm-2 were investigated by Hall effect measurements and capacitance-voltage profiling. The nominally undoped GaN capped structures were grown by low-pressure metal-organic vapor-phase epitaxy. The threshold voltage of transistor devices follows the trend already found for high Al-containing structures, which are described by a model indicating a surface potential independent of Al content. Photoreflectance spectroscopy confirms the results for as-grown heterostructures. The Hall effect measured on the as-grown samples, however, shows a stronger decrease in carrier concentration than expected from the effect of polarization and constant surface potential. In contrast, Hall effect data determined on samples with Ni Schottky contacts and capacitance-voltage profiling on as-grown samples yield the expected behavior, with surface potentials of 0.86 eV and 0.94 eV, respectively. The inconsistency is eliminated by describing the results of the Hall effect on as-grown samples by a two-carrier model. Self-consistent Schrödinger-Poisson calculations support these considerations if we take into account a transition range at the AlxGa1-xN/GaN interface. [ABSTRACT FROM AUTHOR]

The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped AlxGa1-xN/GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase (MOVPE) and plasma-assisted molecular beam epitaxial (PA-MBE) grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6×1012 to 1.2×1013 cm-2. A difference of the concentrations for a fixed Al-content was found between the different epitaxial techniques. This result indicates unambiguously different surface potentials determined quantitatively from the carrier concentration, and is verified in addition by the results of photoreflectance spectroscopy. The GaN surface potentials of MOVPE and PA-MBE grown samples amounts to (0.26±0.04) and (0.61±0.10) eV irrespective of the Al-content of the barrier layer. After device fabrication, we find that due to the identical surface potential defined by the Ni Schottky gate, the threshold voltage for a given Al-content is the same for samples grown with different techniques. Thus, the interplay between epitaxy and process technology defines the threshold voltage. [ABSTRACT FROM AUTHOR]

To extend the tuning capabilities of radio frequency devices, coupled microelectromechanical systems are often employed. In this letter, we demonstrate piezoelectrically actuated, electrically tuneable resonator systems based on coupled micromechanical oscillators operating in a flexural vibration mode. The substantial enhancement in electrostatic coupling was achieved due to the implementation of lateral nanogaps of 100-200 nm between single resonator bars. This allows for resonator synchronization and precise system frequency tuning by over a factor of two, relative to its initial value. Additionally, a simple electro-mechanical model has been developed to describe the dynamic behavior of the electrostatically coupled oscillators. [ABSTRACT FROM AUTHOR]

There is a high demand for compact low-cost ozone sensors. It has been shown recently that In2O3 nanolayers can act as ozone sensitive films activated at room temperature by ultraviolet light. In the present work, the authors integrate ultrathin layers of In2O3 nanoparticles and a GaInN/GaN based blue light emitting diode (LED) on a single sensor chip. The integrated sensor was found to be sensitive to O3 concentrations as low as ∼40 ppb. These results demonstrate that by integrating GaInN/GaN based blue LEDs and metal oxide sensing layers back to back on a single chip, compact and robust gas sensors can be realized. [ABSTRACT FROM AUTHOR]

Trap centers with an energy level positioned 1/3 of the band gap below the effective conduction band edge are observed in the electroluminescence spectra of InAs/(GaIn)Sb superlattice photodiodes with a cutoff wavelength of 11 µm. The trap centers are recognized by simulating the low-temperature current-voltage characteristics of the diodes. Excellent quantitative agreement on both, the I-V characteristic and the differential resistance between the experimental data and the theoretical prediction is achieved. The quantitative simulation of the I-V characteristics shows, that the 77 K performance of InAs/(GaIn)Sb photodiodes is dominated by generation-recombination processes even at long wavelengths. Above 50 K, tunneling currents are not of importance. [ABSTRACT FROM AUTHOR]

Strained triple-quantum-well, large-optical-cavity GaInSb/AlGaAsSb/GaSb diode lasers emitting at 1.98 µm at 300 K are investigated with regard to their high-power capability. As the heating of the active region is a limiting factor for these devices, a quantitative model is derived to simulate the performance of these lasers including thermal effects. The standard laser parameters, deduced from measurements on ridge waveguide lasers, and the measured thermal resistance of the mounted devices were then taken as input parameters. The output power and power efficiency of the lasers calculated using the presented model. Good agreement was found between calculated data and the measurements for different heatsink temperatures as well as for different laser geometries and mounting techniques. The maximum output power achieved for ρ-side down mounted 1000 x 150 µm² broad-area laser was 1.7 W at 300 K in cw operation. [ABSTRACT FROM AUTHOR]

Assesses the plasma etching damage in gallium arsenide. Use of resonant Raman scattering; Distinction between impurity-induced and electric-field-induced forbidden scattering; Changes in the space-charged electric field; Increase of defect concentration.

Magnetotransport and photoluminescence (PL) measurements on InAs/(GaIn)Sb superlattices (SLs) grown by molecular-beam epitaxy on GaSb substrates at different substrate temperatures are reported. With increasing growth temperature, a transition of the SLs from residual n type to residual p-type doping was observed. For n-type samples, a decrease in the electron concentration leads to a strong increase in the PL intensity. In contrast, the PL intensity of p-type samples is only weakly dependent on the hole concentration. This correlation can be used to control the residual doping of the SLs. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spectrally resolved measurements of the responsivity of infrared photodiodes based on InAs/(GaIn)Sb superlattices (SL) were performed in applied magnetic fields. For the field oriented parallel to the growth axis, interband Landau transitions related to both the center and the edge of the SL Brillouin zone in the growth direction were observed, in accordance with the parity selection rules that are expected for the type II system under consideration. For the field oriented perpendicular to the growth axis, the interband Landau resonances are broadened and the widening of the band gap is smaller. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]