New method for producing ideal metal-semiconductor ohmic contacts

A new understanding of ohmic-contact alloying in terms of liquid-epitaxy processes has led to improved contact fabrication by using a slow alloying cycle with an As overpressure chamber. A conveniently wide range of As pressures is possible when Ga is deposited together with the other metals.     

White-light emission from GaAs m.o.s. structures

Emission of light from GaAs m.o.s. structures with anodic native oxides is reported. The spectrum is continuous, covers the visible range and has substantial parts that have higher photon energies than the GaAs energy gap. A part of the emission therefore seems to originate from the amorphous native GaAs oxide with its wide energy gap of about 4.5 eV. The light appears white to the eye and its intensity, but not its spectral distribution, can be controlled by the bias applied to the m.o.s. structure.     

X band Gunn oscillators triggered by baseband Gunn diodes

A 155 ?m-long Gunn diode produces output pulses of 1.5 V amplitude in resistive loading. These pulses are used to trigger a transistor multivibrator and an X band Gunn oscillator.     

Improvements of the SiC homoepitaxy process in a horizontal cold-wall CVD reactor

In this research effort, we investigate the influence of the cold-wall reactor geometry on the chemical vapor deposition (CVD) growth process of 4H-SiC and the quality of lightly doped epitaxial layers. Stable growth conditions with respect to growth rate and C/Si ratio of the gas-phase can be achieved by the appropriate choice of the distance between susceptor and walls of the inner quartz tube. A background doping concentration in the range of 10¹⁴ cm⁻³ is realized by employing a high temperature stable and hydrogen etch resistant coating of the graphite susceptor. Doping and thickness homogeneity of epitaxial layers on 35 mm diam. 4H-SiC substrates, expressed by σ/mean, are as low as 6.9 and 7.7%, respectively. From deep level transient spectroscopy measurements, the concentration of the frequently reported intrinsic Z₁-center in 4H-SiC is determined to be below the detection limit of 10¹² cm⁻³.     

FDFD full-wave analysis and modeling of dielectric and metalliclosses of CPW short circuits

A finite-difference method in the frequency domain (FDFD) is used to analyze the influence of lossy materials on the scattering behavior of CPW short ends. Not only dielectric losses but also realistic metallic losses are taken into account for the first time in an FDFD method. Both, the numerical results for the three-dimensional structure and the complex propagation constants of the homogeneous waveguide are presented. These are compared with those yielded by an analytical method and shown to be of good agreement. Finally, a simple model is presented, which describes the CPW short end with good accuracy     

Free-standing AlGaAs thermopiles for improved infrared sensordesign

Our group introduced the GaAs/AlGaAs material system for various integrated and micromachined thermoelectric sensors. Investigating the material parameters of Al_xGa_1-xAs in detail indicates that this material system can be optimized with respect to thermoelectric properties. We demonstrate that figures of merit Z as high as 1.4×10^-4 K^-1 are predicted. Simultaneously, this material is compatible for micro-machining purposes. The presented infrared sensor is optimized with respect to the material parameter and design. The sensors do not need a supporting membrane and hence undesirable parallel thermal conductance is reduced. Sensors of different geometrical dimensions have been fabricated and compared. Black body measurements result in responsivities up to R=365 V/W and maximum relative detectivities of D^*=6.9×10⁸ cm√(Hz)/W which compare to the predicted performance     

Continuous-wave single-sampling-point characterisation of optoelectronic on-chip terahertz transceiver

Fast electro-optic modulation of the terahertz phase, single-samplingpoint lock-in detection and the use of interdigitated-finger photomixers improve the dynamic range of the transceiver by several orders of magnitude compared with standard chopping techniques. This allowed the frequency range of the continuous-wave on-chip terahertz transceiver to be extended beyond 1THz, which is roughly a factor of five improvement compared to previous reports.     

Terahertz imaging system based on LT-GaAsSb antenna driven by all-semiconductor femtosecond source

A source of terahertz radiation based on a LT-GaAsSb antenna excited by a 1.04 mum 480 fs pulse train from a vertical external cavity surface emitting laser is described. Images of samples were acquired using bolometer detection     

Model for the decrease in HBT collector current under DC stress based on recombination enhanced defect reactions

Based on simplifying assumptions a model is developed to describe decreasing HBT collector current under DC bias stress. The underlying processes are recombination enhanced defect reactions (REDR), namely defect generation and defect annealing. The simplification of the derived equation under particular boundary conditions leads to a form of the Eyring relationship generally employed to describe the decrease in collector current under DC bias stress. Additionally, pulsed tests with an emitter current density of J_E=200 kA/cm² have been performed. The comparison between measured data from these tests and their fit proofs the applicability of the derived equation. Of course this can not be an evidence for the real running physical processes.