3,412 research outputs found
Aging behavior of Au-based ohmic contacts to GaAs
Gold based alloys, commonly used as ohmic contacts for solar cells, are known to react readily with GaAs. It is shown that the contact interaction with the underlying GaAs can continue even at room temperature upon aging, altering both the electrical characteristics of the contacts and the nearby pn junction. Au-Ge-Ni as-deposited (no heat treatment) contacts made to thin emitter (0.15 micrometer) GaAs diodes have shown severe shunting of the pn junction upon aging for several months at room temperature. The heat-treated contacts, despite showing degradation in contact resistance did not affect the underlying pn junction. Au-Zn-Au contacts to p-GaAs emitter (0.2 micrometer) diodes, however, showed slight improvement in contact resistance upon 200 C isothermal annealing for several months, without degrading the pn junction. The effect of aging on electrical characteristics of the as-deposited and heat-treated contacts and the nearby pn junction, as well as on the surface morphology of the contacts are presented
The interaction between the Moon and the solar wind
We study the interaction between the Moon and the solar wind using a
three-dimensional hybrid plasma solver. The proton fluxes and electromagnetical
fields are presented for typical solar wind conditions with different magnetic
field directions. We find two different wake structures for an interplanetary
magnetic field that is perpendicular to the solar wind flow, and for one that
is parallell to the flow. The wake for intermediate magnetic field directions
will be a mix of these two extreme conditions. Several features are consistent
with a fluid interaction, e.g., the presence of a rarefaction cone, and an
increased magnetic field in the wake. There are however several kinetic
features of the interaction. We find kinks in the magnetic field at the wake
boundary. There are also density and magnetic field variations in the far wake,
maybe from an ion beam instability related to the wake refill. The results are
compared to observations by the WIND spacecraft during a wake crossing. The
model magnetic field and ion velocities are in agreement with the measurements.
The density and the electron temperature in the central wake are not as well
captured by the model, probably from the lack of electron physics in the hybrid
model.Comment: Accepted for publication in Earth, Planets and Spac
The achievement of low contact resistance to indium phosphide: The roles of Ni, Au, Ge, and combinations thereof
We have investigated the electrical and metallurgical behavior of Ni, Au-Ni, and Au-Ge-Ni contacts on n-InP. We have found that very low values of contact resistivity rho(sub c) in the E-7 omega-sq cm range are obtained with Ni-only contacts. We show that the addition of Au to Ni contact metallization effects an additional order of magnitude reduction in rho(sub c). Ultra-low contact resistivities in the E-8 omega-sq cm range are obtained with both the Au-Ni and the Au-Ge-Ni systems, effectively eliminating the need for the presence of Ge in the Au-Ge-Ni system. The formation of various nickel phosphides at the metal-InP interface is shown to be responsible for the observed rho(sub c) values in the Ni and Au-Ni systems. We show, finally, that the order in which the constituents of Au-Ni and Au-Ge-Ni contacts are deposited has a significant bearing on the composition of the reaction products formed at the metal-InP interface and therefore on the contact resistivity at that interface
A very low resistance, non-sintered contact system for use on indium phosphide concentrator/shallow junction solar cells
An investigation is made into the possibility of providing low resistance contacts to shallow junction InP solar cells which do not require sintering and which do not cause device degradation even when subjected to extended annealing at elevated temperatures. We show that the addition of In to Au contacts in amounts that exceed the solid solubility limit lowers the as-fabricated (unsintered) contact resistivity (R sub c) to the 10(exp -5) ohm cm(exp 2) range. We next consider the contact system Au/Au2P3, which has been shown to exhibit as-fabricated R sub c values in the 10(exp -6) ohm cm(exp 2) range, but which fails quickly when heated. We show that the substitution of a refractory metal (W, Ta) for Au preserves the low R sub c values while preventing the destructive reactions that would normally take place in this system at high temperatures. We show, finally, that R sub c values in the 10(exp -7) ohm cm(exp 2) range can be achieved without sintering by combining the effects of In or Ga additions to Au contacts with the effects of introducing a thin Au2P3 layer at the metal-InP interface
Lateral spreading of Au contacts on InP
The contact spreading phenomenon observed when small area Au contacts on InP are annealed at temperatures above about 400 C was investigated. It was found that the rapid lateral expansion of the contact metallization which consumes large quantities of InP during growth is closely related to the third stage in the series of solid state reactions that occur between InP and Au, i.e., to the Au3In-to-Au9In4 transition. Detailed descriptions are presented of both the spreading process and the Au3In-to-Au9In4 transition along with arguments that the two processes are manifestations of the same basic phenomenon
Semiconductor structural damage attendant to contact formation in III-V solar cells
In order to keep the resistive losses in solar cells to a minimum, it is often necessary for the ohmic contacts to be heat treated to lower the metal-semiconductor contact resistivity to acceptable values. Sintering of the contacts, however can result in extensive mechanical damage of the semiconductor surface under the metallization. An investigation of the detailed mechanisms involved in the process of contact formation during heat treatment may control the structural damage incurred by the semiconductor surface to acceptable levels, while achieving the desired values of contact resistivity for the ohmic contacts. The reaction kinetics of sintered gold contacts to InP were determined. It was found that the Au-InP interaction involves three consecutive stages marked by distinct color changes observed on the surface of the Au, and that each stage is governed by a different mechanism. A detailed description of these mechanisms and options to control them are presented
Multiphase flow and hysteresis phenomena in oil recovery by water alternating gas (WAG) injection
Water flooding and gas injection are two widely used improved oil recovery techniques
that can be applied individually or combined as water alternating gas (WAG) or
simultaneous gas and water (SWAG) injection. Laboratory data on WAG and SWAG
injections for non-water-wet systems are very limited especially for near-miscible (very
low IFT) gas-oil systems. Near-miscible gas injection represents a number of processes
of great importance to reservoir engineers including high pressure hydrocarbon gas
injection and CO2 flooding.
Simulation of these processes (WAG and SWAG injections) requires three-phase
relative permeability (kr) data. Most of the existing three-phase relative permeability
correlations (such as Stone-I, Stone-II or Baker) have been developed for water-wet
conditions and are unable to adequately account for all the complex multi-phase and
multi-physics processes involved in these oil recovery techniques. Another major
problem in the prediction of the performance of Water Alternating Gas (WAG) process
is the uncertainty associated with the changes in three-phase relative permeability (kr)
values of oil, gas and water in different cycles, which is known as cyclic hysteresis.
The current approach in the industry (except hysteresis model proposed by Larsen and
Skauge) is to use two-phase bounding imbibition and drainage relative permeabilities
along with a two-phase hysteresis model (such as Land, Carlson or Killough to generate
two-phase scanning curves) and input the result into a three-phase correlation (Stone-I,
Stone-II, Baker etc) to simulate hysteresis in WAG injection. The other approach in the
industry to account for hysteresis in WAG injection is the WAG-hysteresis model
(proposed by Larsen and Skauge) coupled with Stone-I correlation. None of these
models and approaches is developed and assessed based for low oil/gas IFT and/or nonwater-
wet system. Nevertheless, the majority of oil reservoirs are believed to be mixedwet
and hence, prediction of the performance of WAG injection in these reservoirs is
associated with significant uncertainties.
Accurate determination of relative permeability values and their hysteresis behaviour is
crucial for obtaining a reliable prediction of the performance of water-alternating-gas
(WAG) injection in oil reservoirs. Performing reliable laboratory experiments is the key
to evaluating the performance of these oil recovery techniques under different reservoir
and operational conditions. The experimental data can be also used for assessment of
different relative permeability and hysteresis models, and developing new
methodologies for reliable simulation of WAG and SWAG injections (if required).
The content of the thesis can be divided into two sections: a) two-phase flow and b)
three-phase flow.
I present the results of comprehensive series of two-phase and three-phase (WAG
injections) coreflood experiments for a gas/oil system at near-miscible (IFT= 0.04
mN.m-1) conditions. Two different cores; a high-permeability (1000 mD) and a lower
permeability (65 mD) core were used in the experiments and both water-wet and mixedwet
conditions were examined. Experimental data have been used to obtain reliable
relative permeabilities and investigate their cyclic hysteresis behavior.
In the first section of the thesis (two-phase flow), effects of different parameters such as
permeability, wettability (water-wet and mixed-wet), immobile water and saturation
history on two-phase flow of oil and gas at near-miscible condition have been investigated. Contrary to the open literature reports which are based on high IFT oil/gas, the results (for very-low oil/gas IFT) showed the importance of the wettability and immobile water saturation on the recovery profiles and estimated relative permeabilities. In addition contrary to the near-miscible liquid-liquid systems, it was observed significant hysteresis effect in the gas-liquid system.
I have also investigated different two-phase systems (gas-oil, gas-water and oil-water) in mixed-wet systems. This is crucial, considering the importance of the two-phase systems as a backbone to better understand three-phase flow as well as their importance as an input to two-phase hysteresis models (for simulation of WAG including hysteresis). The investigation in this study shows that currently available two-phase hysteresis models in simulators (Carlson and Killough) are not able to capture the observed cyclic hysteresis behavior in these systems. The results suggest that for mixed-wet systems, it is necessary to consider irreversible hysteresis loops for both the wetting and non-wetting phases. Such capability currently does not exist in reservoir simulators due to lack of appropriate predictive tools. Results highlight the differences between cyclic hysteresis behaviors of the relative permeabilities in these three systems.
In the second section of the thesis, I first evaluated the performance of different injection scenarios in the mixed-wet system. These processes include primary waterflooding (WF), primary gasflooding (GF), WAG injection (either starting with water injection or gas injection), and SWAG injection (with different gas/water ratios). For some of these processes (WF, GF and WAG injection started with primary WF) the effect of wettability was also investigated. The results show that in both the water-wet and mixed-wet cores, the performance of WAG injection is better than water injection and gas injection alone. The results show that in mixed-wet core, oil recovery by the WAG test which had started with water injection was higher than the WAG test started with gas injection. WAG injections had superior performance over SWAG injections. SWAG performed better compared to primary gas injection. However, surprisingly, SWAG resulted in lower oil recovery compared to primary waterflood in the mixed-wet system. Compared to the other injection strategies, a very high pressure drop across the core was observed during SWAG injection indicating injectivity problems with the application of the process in mixed-wet rocks.
Using results of the WAG injection experiments, I also investigated the cyclic hysteresis effect on three-phase relative permeabilities of each phase (gas, oil and water). The results show the importance of properly accounting for irreversible kr hysteresis loops in the processes involving cyclic injection under three-phase flow conditions. Gas relative permeability (krg) dropped in successive cycles under both water-wet and mixed-wet conditions. krg hysteresis was larger in the water-wet system compared to the mixed-wet case. The results also reveal saturation history dependency for oil relative permeability (kro), which tends to increase in successive gas injection periods. The improvement in kro was larger in the water-wet system. In both water-wet and mixed-wet systems, the largest krw hysteresis happens for the transition from two-phase (oil/water system) to three-phase system (from 1st water injection into 1st gas injection) and the subsequent WAG cycles does not show much hysteresis for krw in the experiments. I addressed some serious shortcomings of the existing reservoir simulators for reliable simulation of oil recovery processes involving three-phase flow and flow reversal
An investigation of mass composition of ultra-high energy cosmic rays with energies above 1019 eV via the study of extensive air showers
The electron and muon components of extensive air shower (EAS) with energies
above 1019 eV are analyzed via various giant EAS arrays. A varying property
of showers is observed for two energy ranges; higher and lower than (3 − 4) x
1019 eV. The age parameter, zenith angle, shower size dependence on muon size
and shower size dependence on primary energy show an increment of mass
composition (MC) above (3−4)x 1019eV. Comparison of the observed EAS results
with the simulations of Capdevielle et al. (2000) and Shinozaki et al. (2005)
gives at most 20% photon fraction for primary energies above 1019 eV. The
arrival directions of showers above 4x1019 eV indicate an increasing
concentration towards the super galactic plane
Texturing of InP surfaces for device applications
A unique process for texturing InP (100) wafers by anisotropic etching was developed. The process produces irregular V-grooves on the surface, which reduce the surface reflectivity. The process does not require photolithography or masking. The etching characteristics depend on doping, with etching tending to proceed more rapidly on the more heavily doped samples. Reduced reflectivity surfaces formed using this process can be applied to solar cells, photodetectors, and other optoelectronic devices
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