93 research outputs found
Electron capture in GaAs quantum wells via electron-electron and optic phonon scattering
Electron capture times in a separate confinement quantum well (QW) structure
with finite electron density are calculated for electron-electron (e-e) and
electron-polar optic phonon (e-pop) scattering. We find that the capture time
oscillates as function of the QW width for both processes with the same period,
but with very different amplitudes. For an electron density of 10^11 cm^-2 the
e-e capture time is 10-1000 times larger than the e-pop capture time except for
QW widths near the resonance minima, where it is only 2-3 times larger. With
increasing electron density the e-e capture time decreases and near the
resonance becomes smaller than the e-pop capture time. Our e-e capture time
values are two-to-three orders of magnitude larger than previous results of
Blom et al. [Appl. Phys. Lett. 62, 1490 (1993)]. The role of the e-e capture in
QW lasers is therefore readdressed.Comment: 5 pages, standard LaTeX file + 5 PostScript figures (tarred,
compressed and uuencoded) or by request from [email protected],
accepted to Appl. Phys. Let
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Compatibility of lead-free solders with lead containing surface finishes as a reliability issue in electronic assemblies
Enhanced performance goals and environmental restrictions have heightened the consideration for use of alternative solders as replacements for the traditional tin-lead (Sn-Pb) eutectic and near-eutectic alloys. However, the implementation of non-Pb bearing surface finishes may lag behind solder alloy development. A study was performed which examined the effect(s) of Pb contamination on the performance of Sn-Ag-Bi and Sn-Ag-Cu-Sb lead-free solders by the controlled addition of 63Sn-37Pb solder at levels of 0.5 {minus} 8.0 wt.%. Thermal analysis and ring-in-plug shear strength studies were conducted on bulk solder properties. Circuit board prototype studies centered on the performance of 20I/O SOIC gull wing joints. Both alloys exhibited declines in their melting temperatures with greater Sn-Pb additions. The ring-in-plug shear strength of the Sn-Ag-Cu-Sb solder increased slightly with Sn-Pb levels while the Sn-Ag-Bi alloy experienced a strength loss. The mechanical behavior of the SOIC (Small Outline Integrated Circuit) Sn-Ag-Bi solder joints reproduced the strength levels were insensitive to 10,106 thermal cycles. The Sn-Ag-Cu-Sb solder showed a slight decrease in the gull wing joint strengths that was sensitive to the Pb content of the surface finish
Spatio-temporal dynamics of quantum-well excitons
We investigate the lateral transport of excitons in ZnSe quantum wells by
using time-resolved micro-photoluminescence enhanced by the introduction of a
solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps,
respectively. Strong deviation from classical diffusion is observed up to 400
ps. This feature is attributed to the hot-exciton effects, consistent with
previous experiments under cw excitation. The coupled transport-relaxation
process of hot excitons is modelled by Monte Carlo simulation. We prove that
two basic assumptions typically accepted in photoluminescence investigations on
excitonic transport, namely (i) the classical diffusion model as well as (ii)
the equivalence between the temporal and spatial evolution of the exciton
population and of the measured photoluminescence, are not valid for
low-temperature experiments.Comment: 8 pages, 6 figure
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Assessment of circuit board surface finishes for electronic assembly with lead-free solders
The suitability of various metallic printed wiring board surface finishes was assessed for new technology applications that incorporate assembly with Lead-free solders. The manufacture of a lead-free product necessitates elimination of lead (Pb) from the solder, the circuit board as well as the component lead termination. It is critical however for the selected interconnect Pb-free solder and the corresponding printed wiring board (PWB) and component lead finishes to be mutually compatible. Baseline compatibility of select Pb-free solders with Pb containing PWB surface finish and components was assessed. This was followed by examining the compatibility of the commercially available CASTIN{trademark} (SnAgCuSb) Pb-free solder with a series of PWB metallic finishes: Ni/Au, Ni/Pd, and Pd/Cu. The compatibility was assessed with respect to assembly performance, solder joint integrity and long term attachment reliability. Solder joint integrity and mechanical behavior of representative 50 mil pitch 20I/O SOICs was determined before and after thermal stress. Mechanical pull test studies demonstrated that the strength of SnAgCuSb solder interconnections is notably greater than that of SnPb interconnections
Semiconductor Superlattices: A model system for nonlinear transport
Electric transport in semiconductor superlattices is dominated by pronounced
negative differential conductivity. In this report the standard transport
theories for superlattices, i.e. miniband conduction, Wannier-Stark-hopping,
and sequential tunneling, are reviewed in detail. Their relation to each other
is clarified by a comparison with a quantum transport model based on
nonequilibrium Green functions. It is demonstrated how the occurrence of
negative differential conductivity causes inhomogeneous electric field
distributions, yielding either a characteristic sawtooth shape of the
current-voltage characteristic or self-sustained current oscillations. An
additional ac-voltage in the THz range is included in the theory as well. The
results display absolute negative conductance, photon-assisted tunneling, the
possibility of gain, and a negative tunneling capacitance.Comment: 121 pages, figures included, to appear in Physics Reports (2001
n.m.r. Studies of Supercooled Viscous Fluids and Silica Gels (Nmr)
130 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.Pulsed N.M.R. techniques are used to study the dynamic behavior of viscous fluids ((eta) > 1 Poise) in the supercooled state. The applicability of the hydrodynamic equations to high viscosity systems is investigated at a molecular level in a series of selectively deuterated compounds of varying molecular symmetry and structure. Pressure is used as an experimental variable to extend the range of observable viscosities, and in addition to separate the effects of kinetic energy and density on the molecular interactions in the supercooled state.It is found that the rotational-translational coupling parameter exhibits a strong density dependence at high viscosities. The nature and extent of the dependence is primarily determined by molecular structure. The results are interpreted in terms of simple molecular models based on free volume concepts. The coupling parameter is found insensitive to kinetic energy changes. It is concluded that volume rather than kinetic energy plays the decisive role in determining the rotational-translational coupling in supercooled viscous fluids.The origin of the anomalous discontinuity in the temperature dependence of viscosity is investigated using isopropyl benzene for a model compound. The rotational and translational contributions to the rotating frame relaxation time are separated through an isotopic dilution study. It is shown that the viscosity anomaly of isopropyl benzene is associated with a change in the type of translational rather than reorientational dynamics.The effect of pressure on the polymerization kinetics of sol-gel processes is investigated at a molecular level using high resolution ('29)Si spectroscopy. The condensation subsequent to hydrolysis of the initiating silicon-alkoxide reagent, Si(OCH(,3))(,4), is monitored as a function of elapsed time at different pressures. It is shown that high pressures have a dramatic accelerating effect on the condensation rate, but do not alter the mechanism via which the polycondensation rate, but do not alter the mechanism via which the polycondensation reaction proceeds. The extent of the condensation rate enhancement is quantitatively evaluated using kinetic principles. Transition state theory is employed to provide a detailed mechanism of the pressure induced acceleration of the gelation process.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD
Monte Carlo calculations parallel and perpendicular electron transport in gallium arsenide heterolayers
U of I OnlyThesi
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An Evaluation of Prototype Circuit Boards Assembled with a Sn-Ag Bi Solder
An evaluation was performed which examined the aging of surface mount solder joints assembled with 91.84Sn-3.33Ag-4.83Bi solder. Defect analysis of the as-fabricated test vehicles revealed excellent solderability, good package alignment, and a minimum number of voids. Continuous DC electrical monitoring of the solder joints did not reveal opens during as many as 10,000 thermal cycles (0 C, 100 C). The solder joints exhibited no significant degradation through 2500 cycles, based upon an absence of microstructural damage and sustained shear and pull strengths of chip capacitors and J-leaded solder joints, respectively. Thermal cycles of 5000 and 10,000 resulted in some surface cracking of the solder fillets and coatings. In a few cases, deeper cracks were observed in the thinner reaches of several solder fillets. There was no deformation or cracking in the solder located in the gap between the package I/O and the circuit board pad nor in the interior of the fillets, both locations that would raise concerns of joint mechanical integrity. A drop in the chip capacitor shear strength was attributed to crack growth near the top of the fillet
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