1,344 research outputs found
Effect of impurity trapping on the capacitanceâvoltage characteristics of nâGaAs/NâAlGaAs heterojunctions
We have studied the capacitance-voltage (C- V) characteristics of Schottky barriers on inverted nGaAs/ N-AIGaAs and normal N-AIGaAs/n-GaAs heterojunctions. Impurities introduced during film growth produced a negative sheet charge of 6.0 X 10 II cm -2 at the interface of the inverted n-GaAs/N-AIGaAs heterojunction. The effectiveness of GaAs quantum wells in trapping these impurities was investigated. GaAs quantum wells 20 A wide were placed in intervals of 2500 A for the first 0.75 pm of the AIGaAs layer; in the last 0.25 pm, the periodicity of the quantum wells was progressively decreased by half with the last quantum well placed at about 160 A from the GaAs/ AIGaAs interface. The resulting measured interface charge concentration of 4.4 X 1010 cm -2 is more than a magnitude lower than measured before the use of the quantum wells and is essentially at the limit of the accuracy of the C-V technique for this structure
Minority Hole Mobility in n+ GaAs
The minority hole diffusivity, or equivalently the hole mobility, was measured in n+GaAs with the zeroâfield timeâofâflight technique. The minority hole mobility was measured for the donor doping range of 1.3Ă1017 cmâ3 to 1.8Ă1018 cmâ3 and was found to vary from 235 to 295 cm2/Vâs. At the lower doping level, the minority hole mobility is comparable to the corresponding majority hole mobility, but at 1.8Ă1018 cmâ3 the minority hole mobility was 30% higher than the majority carrier hole mobility. These results have important implications for the design of devices such as solar cells and pnpâheterojunction bipolar transistors
The Smell of Age: Perception and Discrimination of Body Odors of Different Ages
Our natural body odor goes through several stages of age-dependent changes in chemical composition as we grow older. Similar changes have been reported for several animal species and are thought to facilitate age discrimination of an individual based on body odors, alone. We sought to determine whether humans are able to discriminate between body odor of humans of different ages. Body odors were sampled from three distinct age groups: Young (20â30 years old), Middle-age (45â55), and Old-age (75â95) individuals. Perceptual ratings and age discrimination performance were assessed in 41 young participants. There were significant differences in ratings of both intensity and pleasantness, where body odors from the Old-age group were rated as less intense and less unpleasant than body odors originating from Young and Middle-age donors. Participants were able to discriminate between age categories, with body odor from Old-age donors mediating the effect also after removing variance explained by intensity differences. Similarly, participants were able to correctly assign age labels to body odors originating from Old-age donors but not to body odors originating from other age groups. This experiment suggests that, akin to other animals, humans are able to discriminate age based on body odor alone and that this effect is mediated mainly by body odors emitted by individuals of old age
Decoherence due to contacts in ballistic nanostructures
The active region of a ballistic nanostructure is an open quantum-mechanical
system, whose nonunitary evolution (decoherence) towards a nonequilibrium
steady state is determined by carrier injection from the contacts. The purpose
of this paper is to provide a simple theoretical description of the
contact-induced decoherence in ballistic nanostructures, which is established
within the framework of the open systems theory. The active region's evolution
in the presence of contacts is generally non-Markovian. However, if the
contacts' energy relaxation due to electron-electron scattering is sufficiently
fast, then the contacts can be considered memoryless on timescales coarsened
over their energy relaxation time, and the evolution of the current-limiting
active region can be considered Markovian. Therefore, we first derive a general
Markovian map in the presence of a memoryless environment, by coarse-graining
the exact short-time non-Markovian dynamics of an abstract open system over the
environment memory-loss time, and we give the requirements for the validity of
this map. We then introduce a model contact-active region interaction that
describes carrier injection from the contacts for a generic two-terminal
ballistic nanostructure. Starting from this model interaction and using the
Markovian dynamics derived by coarse-graining over the effective memory-loss
time of the contacts, we derive the formulas for the nonequilibrium
steady-state distribution functions of the forward and backward propagating
states in the nanostructure's active region. On the example of a double-barrier
tunneling structure, the present approach yields an I-V curve with all the
prominent resonant features. The relationship to the Landauer-B\"{u}ttiker
formalism is also discussed, as well as the inclusion of scattering.Comment: Published versio
Microsecond Lifetimes and Low Interface Recombination Velocities in Moderately Doped n-GaAs Thin Films
We have observed lifetimes greater than 1 ps in moderately doped, thin film, n-GaAs/A1a,Gae,As double heterostructure membranes formed by etching away the substrate. We attribute these ultralong lifetimes to enhanced photon recycling caused by the removal of the substrate. Nonradiative recombination in the bulk and at the interfaces is very low; the upper limit of the interface recombination velocity is 25 cm/S.-Such long lifetimes in GaAs doped at N,= 1.3 X 10â cme3 suggest that thin-film solar cells offer a potential option for achieving very high efficiencies
Zero-Field Time-of-Flight Measurements of Electron Diffusion in P+-GaAs
Minority electron diffusivities in p+-GaAs-doped NA =~1.4Ă1018 and ~1019 cm-3 have been measured in zero-field conditions with an extension of the zero-field time-of-flight technique. Extension of the technique to make it applicable to heavily doped p+-GaAs is described and zero-field data are discussed. Unexpectedly, majority carrier drag effects are not evident in a comparison of this data with recently reported high-field data. Low zero-field mobility of electrons in p+-GaAs has important implications for high-speed devices such as heterojunction bipolar transistors
A study of minority carrier lifetime versus doping concentration in nâtype GaAs grown by metalorganic chemical vapor deposition
Timeâresolved photoluminescence decay measurements are used to explore minority carrier recombination in nâtype GaAs grown by metalorganic chemical vapor deposition, and doped with selenium to produce electron concentrations from 1.3Ă1017 cmâ3 to 3.8Ă1018 cmâ3. For electron densities n0\u3c1018 cmâ3, the lifetime is found to be controlled by radiative recombination and photon recycling with no evidence of ShockleyâReadâHall recombination. For higher electron densities, samples show evidence of ShockleyâReadâHall recombination as reflected in the intensity dependence of the photoluminescence decay. Still, we find that radiative recombination and photon recycling are important for all electron concentrations studied, and no evidence for Auger recombination was observed
Ultrathin compound semiconductor on insulator layers for high performance nanoscale transistors
Over the past several years, the inherent scaling limitations of electron
devices have fueled the exploration of high carrier mobility semiconductors as
a Si replacement to further enhance the device performance. In particular,
compound semiconductors heterogeneously integrated on Si substrates have been
actively studied, combining the high mobility of III-V semiconductors and the
well-established, low cost processing of Si technology. This integration,
however, presents significant challenges. Conventionally, heteroepitaxial
growth of complex multilayers on Si has been explored. Besides complexity, high
defect densities and junction leakage currents present limitations in the
approach. Motivated by this challenge, here we utilize an epitaxial transfer
method for the integration of ultrathin layers of single-crystalline InAs on
Si/SiO2 substrates. As a parallel to silicon-on-insulator (SOI) technology14,we
use the abbreviation "XOI" to represent our compound semiconductor-on-insulator
platform. Through experiments and simulation, the electrical properties of InAs
XOI transistors are explored, elucidating the critical role of quantum
confinement in the transport properties of ultrathin XOI layers. Importantly, a
high quality InAs/dielectric interface is obtained by the use of a novel
thermally grown interfacial InAsOx layer (~1 nm thick). The fabricated FETs
exhibit an impressive peak transconductance of ~1.6 mS/{\mu}m at VDS=0.5V with
ON/OFF current ratio of greater than 10,000 and a subthreshold swing of 107-150
mV/decade for a channel length of ~0.5 {\mu}m
Highâefficiency Al0.22Ga0.78As solar cells grown by molecular beam epitaxy
The quality of pn junction photodetectors made of Al0.2Ga0.8As has been investigated as a first step in the optimization of tandem solar cells. We have obtained 1 sun AM1.5 efficiencies of 16.1% for 0.25 cm2 Al0.22Ga0.78As solar cellsfabricated from molecular beam epitaxy (MBE) material. This efficiency is 3.2 percentage points higher than the previously best reported efficiency of 12.9% for an Al0.2Ga0.8As solar cell fabricated from MBE material
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