5,369 research outputs found
Crystal growth of device quality GaAs in space
It was established that the findings on elemental semiconductors Ge and Si regarding crystal growth, segregation, chemical composition, defect interactions, and materials properties-electronic properties relationships are not necessarily applicable to GaAs (and to other semiconductor compounds). In many instances totally unexpected relationships were found to prevail. It was further established that in compound semiconductors with a volatile constituent, control of stoichiometry is far more critical than any other crystal growth parameter. It was also shown that, due to suppression of nonstoichiometric fluctuations, the advantages of space for growth of semiconductor compounds extend far beyond those observed in elemental semiconductors. A novel configuration was discovered for partial confinement of GaAs melt in space which overcomes the two major problems associated with growth of semiconductors in total confinement. They are volume expansion during solidification and control of pressure of the volatile constituent. These problems are discussed in detail
Semiconductor technology program: Progress briefs
Measurement technology for semiconductor materials, process control, and devices, is discussed. Silicon and silicon based devices are emphasized. Highlighted activities include semiinsulating GaAs characterization, an automatic scanning spectroscopic ellipsometer, linewidth measurement and coherence, bandgap narrowing effects in silicon, the evaluation of electrical linewidth uniformity, and arsenicomplanted profiles in silicon
Electronic transport in two dimensional graphene
We provide a broad review of fundamental electronic properties of
two-dimensional graphene with the emphasis on density and temperature dependent
carrier transport in doped or gated graphene structures. A salient feature of
our review is a critical comparison between carrier transport in graphene and
in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells,
inversion layers) so that the unique features of graphene electronic properties
arising from its gap- less, massless, chiral Dirac spectrum are highlighted.
Experiment and theory as well as quantum and semi-classical transport are
discussed in a synergistic manner in order to provide a unified and
comprehensive perspective. Although the emphasis of the review is on those
aspects of graphene transport where reasonable consensus exists in the
literature, open questions are discussed as well. Various physical mechanisms
controlling transport are described in depth including long- range charged
impurity scattering, screening, short-range defect scattering, phonon
scattering, many-body effects, Klein tunneling, minimum conductivity at the
Dirac point, electron-hole puddle formation, p-n junctions, localization,
percolation, quantum-classical crossover, midgap states, quantum Hall effects,
and other phenomena.Comment: Final version as accepted for publication in Reviews of Modern
Physics (in press), 69 pages with 38 figure
Structural and Magnetic Characteristics of MnAs Nanoclusters Embedded in Be-doped GaAs
We describe a systematic study of the synthesis, microstructure and
magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of
MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created
during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by
molecular beam epitaxy. Transmission electron microscopy and magnetometry
studies reveal two distinct classes of nanoclustered samples whose structural
and magnetic properties depend on the Mn content of the initial (Ga,Mn)As
layer. For Mn content in the range 5% - 7.5%, annealing creates a
superparamagnetic material with a uniform distribution of small clusters
(diameter around 6 nm) and with a low blocking temperature (T_B approximately
10 K). While transmission electron microscopy cannot definitively identify the
composition and crystalline phase of these small clusters, our experimental
data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich
regions of (Ga,Mn)As. At higher Mn content (> 8 %), we find that annealing
results in an inhomogeneous distribution of both small clusters as well as much
larger NiAs-phase MnAs clusters (diameter around 25 nm). These samples also
exhibit supermagnetism, albeit with substantially larger magnetic moments and
coercive fields, and blocking temperatures well above room temperature
Investigation of new semiinsulating behavior of III-V compounds
The investigation of defect interactions and properties related to semiinsulating behavior of III-V semiconductors resulted in about twenty original publications, six doctoral thesis, one masters thesis and numerous conference presentations. The studies of new compensation mechanisms involving transition metal impurities have defined direct effects associated with deep donor/acceptor levels acting as compensating centers. Electrical and optical properties of vanadium and titanium levels were determined in GaAs, InP and also in ternary compounds InGaAs. The experimental data provided basis for the verification of chemical trends and the VRBE method. They also defined compositional range for III-V mixed crystals whereby semiinsulating behavior can be achieved using transition elements deep levels and a suitable codoping with shallow donor/acceptor impurities
Scanning probe force microscopy of III-V semiconductor structures
In this dissertation, cross-sectional potential imaging of GaAs-based homoepitaxial, heteroepitaxial and quantum well structures, all grown by atmospheric pressure Metal-organic Vapor Phase Epitaxy (MOVPE) is investigated. Kelvin probe force microscopy (KPFM), using amplitude modulation (AM) and frequency modulation (FM) modes in air and at room temperature, is used for the potential imaging. Studies performed on n-type GaAs homoepitaxial structures have shown two different potential profiles, related to the difference in electron density between the semi-insulating (SI) substrate and the epilayers. It is shown that the contact potential difference (CPD) between the tip and sample is higher on the semi-insulating substrate side than on the n-type epilayer side. This change in CPD across the interface has been explained by means of energy band diagrams indicating the relative Fermi level positions. In addition, it has also been found that the CPD across the interface increases with electron density. This result is in qualitative agreement with theory. In addition, as known from literature, even under ambient conditions FM mode KPFM provides better lateral resolution and more realistic CPD values than AM mode KPFM. Compared to the case of AM mode analysis, where the experimental CPD values were on average of the theoretical values, the CPD values from FM mode analysis are on average of the theoretical ones. Furthermore, by using FM mode, the transition across the interface is sharper and the surface potential flattens/saturates as expected when scanning sufficiently far away from the junction. The non-neutral space charge region of the sample with an electron density of for example, is as measured by FM-KPFM, whereas for AM-KPFM, the width is even more than and the potential profiles do not saturate. For the p-type GaAs homoepitaxial structures, FM mode measurements from a sample with a dopant density of are presented. As in the case of n-type GaAs,a similar potential profile showing two main domains has been obtained. However, unlike the case of type GaAs where the potential measured on the epilayer side is higher than that on the substrate side, the potential on the epilayer side of the junction is lower in this case due to the fact that the Fermi level of p-type GaAs is below that of the substrate
Ku-band field-effect power transistors
A single stage amplifier was developed using an 8 gate, 1200 micrometer width device to give a gain of 3.3 + or - 0.1 dB over the 14.4 to 15.4 GHz band with an output power of 0.48 W and 15% minimum efficiency with 0.255 W of input power. With two 8 gate devices combined and matched on the device carrier, using a lumped element format, a gain of 3 dB was attained over the 14.5 to 15.5 GHz band with a maximum efficiency of 9.9% for an output power of 0.8 W
ECFA Detector R&D Panel, Review Report
Two special calorimeters are foreseen for the instrumentation of the very
forward region of an ILC or CLIC detector; a luminometer (LumiCal) designed to
measure the rate of low angle Bhabha scattering events with a precision better
than 10 at the ILC and 10 at CLIC, and a low polar-angle
calorimeter (BeamCal). The latter will be hit by a large amount of
beamstrahlung remnants. The intensity and the spatial shape of these
depositions will provide a fast luminosity estimate, as well as determination
of beam parameters. The sensors of this calorimeter must be radiation-hard.
Both devices will improve the e.m. hermeticity of the detector in the search
for new particles. Finely segmented and very compact electromagnetic
calorimeters will match these requirements. Due to the high occupancy, fast
front-end electronics will be needed. Monte Carlo studies were performed to
investigate the impact of beam-beam interactions and physics background
processes on the luminosity measurement, and of beamstrahlung on the
performance of BeamCal, as well as to optimise the design of both calorimeters.
Dedicated sensors, front-end and ADC ASICs have been designed for the ILC and
prototypes are available. Prototypes of sensor planes fully assembled with
readout electronics have been studied in electron beams.Comment: 61 pages, 51 figure
- …