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Magnetoconductance of Independently Tunable Tunnel-Coupled Double Quantum Wires

Description: The authors report on their recent experimental studies of vertically-coupled quantum point contacts subject to in-plane magnetic fields. Using a novel flip-chip technique, mutually aligned split gates on both sides of a sub micron thick double quantum well heterostructure define a closely-coupled pair of ballistic one-dimensional (1D) constrictions. They observe quantized conductance steps due to each quantum well and demonstrate independent control of each ID constriction width. In addition, a novel magnetoconductance feature at {approximately}6 T is observed when a magnetic field is applied perpendicular to both the current and growth directions. This conductance dip is observed only when 1D subbands are populated in both the top and bottom constrictions. This data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.
Date: July 13, 2000
Creator: BLOUNT,MARK A.; MOON,J.S.; SIMMONS,JERRY A.; LYO,SUNGKWUN K.; WENDT,JOEL R. & RENO,JOHN L.
Partner: UNT Libraries Government Documents Department

Determination of the Built-in Electric Field near Contacts to Polycrystalline CuInSe{sub 2} - Probing Local Charge Transport Properties by Photomixing

Description: The built-in electric field in polycrystalline CuInSe{sub 2} (CIS) near gold co-planar contacts was quantitatively revealed for the first time by the photomixing technique. A He-Ne laser beam was focused locally on the CIS sample near one of its contact. While both dc dark and photo-currents showed ohmic behavior, the high frequency ac current was non-zero for zero applied dc bias, which reveals a built-in electric field of {approx}1000V/cm. The capability of the photomixing technique to probe local charge transport properties is expected to be very useful for, e.g., the quantitative evaluation of the quality of ohmic contacts and the investigation of electric field induced p-n junction formation in CIS and related materials.
Date: November 19, 1998
Creator: Tang, Y.; Dong, S.; Sun, G. S.; Braunstein, R. & von Roedern, B.
Partner: UNT Libraries Government Documents Department

Micromachined VLSI 3D electronics. Final report for period September 1, 2000 - March 31, 2001

Description: The phase I program investigated the construction of electronic interconnections through the thickness of a silicon wafer. The novel aspects of the technology are that the length-to-width ratio of the channels is as high as 100:1, so that the minimum amount of real estate is used for contact area. Constructing a large array of these through-wafer interconnections will enable two circuit die to be coupled on opposite sides of a silicon circuit board providing high speed connection between the two.
Date: March 31, 2001
Creator: Beetz, C.P.; Steinbeck, J. & Hsueh, K.L.
Partner: UNT Libraries Government Documents Department

Low resistivity ohmic contacts to moderately doped n-GaAs with low temperature processing

Description: A low-temperature process for forming ohmic contacts to moderately doped GaAs has been optimized using a PdGe metallization scheme. Minimum specific contact resistivity of 1.5 {times} 10{sup {minus}6} {minus}cm{sup 2} has been obtained with a low anneal temperature of 250 C. Results for optimizing both time and temperature are reported and compared to GeAu n-GaAs contacts. Material compositions was analyzed by x-ray photoelectron spectroscopy and circuit metal interconnect contact resisitivity to the low-temperature processed PdGe contacts is reported. For the lowest temperature anneals considered, excess Ge on the ohmic contact layer is suspected of degrading interconnect metal contacts, while higher temperature anneals permitted interconnect metal formation with negligible contact resistivity. Atomic force microscopy measurements showed that the PdGe surface morphology is much more uniform than standard GeAu contacts.
Date: December 31, 1994
Creator: Lovejoy, M.L.; Howard, A.J.; Zavadil, K.R.; Rieger, D.J.; Shul, R.J. & Barnes, P.A.
Partner: UNT Libraries Government Documents Department

Technology support for initiation of high-throughput processing of thin-film CdTe PV modules. Phase II technical report, March 14, 1996--March 13, 1997

Description: Research at Solar Cells Inc. is focused on developing processes which will lead to high volume and low cost manufacturing of solar cells and to increase the performance of their present technology. The process research has focused on developing vapor transport deposition of the semiconductors, eliminating wet chemistry steps while minimizing the chloride treatment time, forming a low-loss back contact using only dry processing, and an improved interconnection technique. The performance improvement work has focused on the increase of the photocurrent by a combination of more transparent glass substrates and a thinner CdS window layer deposited on an i-SnO{sub 2} buffer layer. SCI record 13.0% 1 cm{sup 2} devices have been fabricated using these techniques. Stability monitoring continues and shows minimal degradation for over 20,000 hours of continuous light soak at 0.8 sun illumination.
Date: September 1, 1997
Creator: Sasala, R.; Powell, R. & Dorer, G.
Partner: UNT Libraries Government Documents Department

A hardware review of electrical contact aging and performance in electromechanical stronglinks

Description: Contacts from the functional switch assembly have been examined for a series of MC2969 stronglinks varying from 9 to 14 years of age. Wear tracks are apparent on the contacts as a result of oxide removal by wiping action as the switch is exercised. Typical contaminants observed on the contacts include C, O, S, Cl, F and Si, all of which vary with position on the contacts. All of the contacts show segregation of Ag into the near-surface region. Measurement of the local contact resistance on the ends of the contacts provide resistance values that are reasonable for this material, but with variation among contacts as a result of changes in the local surface chemistry.
Date: September 1, 1997
Creator: Pebbles, D.E.; Ohlhausen, J.A.; Varga, K.S. & Bryan, R.M.
Partner: UNT Libraries Government Documents Department

The Onset of Pileup in Nanometer-Scale Contacts

Description: The interfacial force microscope (IFM) was used to indent and image defect free Au(111) surfaces, providing atomic-scale observations of the onset of pileup and the excursion of material above the initial surface plane. Images and load-displacement measurements demonstrate that elastic accommodation of an indenter is followed by two stages of plasticity. The initial stage is identified by slight deviations of the load-displacement relationship from the predicted elastic response. Images acquired after indentations showing only this first stage indicate that these slight load relaxation events result in residual indentations 0.5 to 4 nm deep with no evidence of pileup or surface orientation dependence. The second stage of plasticity is marked by a series of dramatic load relaxation events and residual indentations tens of nanometers deep. Images acquired following this second stage document 0.25 nm pileup terraces which reflect the crystallography of the surface as well as the indenter geometry. Attempts to plastically displace the indenter 4-10 nanometers deep into the Au(111) surface were unsuccessful, demonstrating that the transition from stage I to stage H plasticity is associated with overcoming some sort of barrier. Stage I is consistent with previously reported models of dislocation nucleation. The dramatic load relaxations of stage II plasticity, and the pileup of material above the surface, require cross-slip and appear to reflect a dynamic process leading to dislocation intersection with the surface. The IFM measurements reported here offer new insights into the mechanisms underlying the very early stages of plasticity and the formation of pileup.
Date: January 18, 2000
Creator: JARAUSCH,K.F.; KIELY,J.D.; HOUSTON,JACK E. & RUSSELL,P.E.
Partner: UNT Libraries Government Documents Department

Highlights of the DARPA eletrically conductive adhesive FCA project. Quarterly report, April 1, 1996--June 30, 1996

Description: The accomplishments are broken down into three different categories: materials development (polymer metal composite with increased bond strength), process development (reliability testing, bonding optimization evaluation, testing of bulk conductive adhesives, paste deposition process development), and equipment development (laboratory bonding equipment, cost estimation for flip chip attach methods).
Date: June 30, 1996
Creator: Saraf, R.; Roldan, J.; Sambucetti, C.; Gaynes, M.; Woychik, C. & Snyder, M.
Partner: UNT Libraries Government Documents Department

Doped Contacts for High-Longevity Optically Activated, High Gain GaAs Photoconductive Semiconductor Switches

Description: The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses. This was achieved by improving the ohmic contacts through the incorporation of a doped layer that is very effective in the suppression of filament formation, alleviating current crowding. Damage-free operation is now possible with virtually infinite expected lifetime at much higher current levels than before. The inherent damage-free current capacity of the bulk GaAs itself depends on the thickness of the doped layers and is at least 100A for a dopant diffusion depth of 4pm. The contact metal has a different damage mechanism and the threshold for damage ({approx}40A) is not further improved beyond a dopant diffusion depth of about 2{micro}m. In a diffusion-doped contact switch, the switching performance is not degraded when contact metal erosion occurs, unlike a switch with conventional contacts. This paper will compare thermal diffusion and epitaxial growth as approaches to doping the contacts. These techniques will be contrasted in terms of the fabrication issues and device characteristics.
Date: December 17, 1999
Creator: MAR,ALAN; LOUBRIEL,GUILLERMO M.; ZUTAVERN,FRED J.; O'MALLEY,MARTIN W.; HELGESON,WESLEY D.; BROWN,DARWIN JAMES et al.
Partner: UNT Libraries Government Documents Department

Longevity improvement of optically activated, high gain GaAs photoconductive semiconductor switches

Description: The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses at 23A, and over 100 pulses at 1kA. This is achieved by improving the ohmic contacts by doping the semi-insulating GaAs underneath the metal, and by achieving a more uniform distribution of contact wear across the entire switch by distributing the trigger light to form multiple filaments. This paper will compare various approaches to doping the contacts, including ion implantation, thermal diffusion, and epitaxial growth. The device characterization also includes examination of the filament behavior using open-shutter, infra-red imaging during high gain switching. These techniques provide information on the filament carrier densities as well as the influence that the different contact structures and trigger light distributions have on the distribution of the current in the devices. This information is guiding the continuing refinement of contact structures and geometries for further improvements in switch longevity.
Date: March 2, 2000
Creator: MAR,ALAN; LOUBRIEL,GUILLERMO M.; ZUTAVERN,FRED J.; O'MALLEY,MARTIN W.; HELGESON,WESLEY D.; BROWN,DARWIN JAMES et al.
Partner: UNT Libraries Government Documents Department

Doped Contacts for High-Longevity Optically Activated, High Gain GaAs Photoconductive Semiconductor Switches

Description: The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 50 million pulses. This was achieved by improving the ohmic contacts through the incorporation of a doped layer beneath the PCSS contacts which is very effective in the suppression of filament formation and alleviating current crowding to improve the longevity of PCSS. Virtually indefinite, damage-free operation is now possible at much higher current levels than before. The inherent damage-free current capacity of the switch depends on the thickness of the doped layers and is at least 100A for a dopant diffusion depth of 4pm. The contact metal has a different damage mechanism and the threshold for damage ({approximately}40A) is not further improved beyond a dopant diffusion depth of about 2{micro}m. In a diffusion-doped contact switch, the switching performance is not degraded when contact metal erosion occurs. This paper will compare thermal diffusion and epitaxial growth as approaches to doping the contacts. These techniques will be contrasted in terms of the fabrication issues and device characteristics.
Date: August 5, 1999
Creator: Baca, A.G.; Brown, D.J.; Donaldson, R.D.; Helgeson, W.D.; Hjalmarson, H.P.; Loubriel, G.M. et al.
Partner: UNT Libraries Government Documents Department

Copper electroplating process for sub-half-micron ULSI structures

Description: We have utilized electroplating technology in a damascene process to produce low resistance copper interconnects in sub-half-micron ULSI patterns having aspect ratios of 2.4:1. The use of a pulsed-voltage plating technique allows trench filling capability without voids. Samples of 150 mm diameter were patterned and sputtered with a barrier layer, followed by a copper seed layer. Pulsed-voltage electroplating, deposited about 2 microns of copper uniformly (1 sigma < 5%) over the surface. The electroplated copper has low levels of impurities, excellent adhesion, excellent step coverage, and rates comparable to other deposition methods. We present details of the electroplating equipment, and data on the filling characteristics of the copper metallization which prevent void formation and reduce contact resistance.
Date: May 15, 1995
Creator: Contolini, R.J.; Tarte, L.; Graff, R.T.; Evans, L.B.; Cox, J.N.; Puich, M.R. et al.
Partner: UNT Libraries Government Documents Department

Effects of pre-stressing and flux on the flow of solder on PWB copper surfaces

Description: A variety of test methods are available to evaluate the solderability of printed wiring board [PWB] surface finishes. A new test has been developed which better simulates the capillary flow physics of typical solder assembly processing, especially surface mount soldering. The work was conducted under a cooperative research and development agreement between Sandia National Laboratories, the National Center for Manufacturing Sciences, and several PWB fabricators (AT&T, IBM, Texas Instruments, and United Technologies Corporation/Hamilton Standard) to advance PWB interconnect systems technology. Particular attention has been given at Sandia to characterizing the effects of accelerated aging in a simulated indoor industrial environment on subsequent PWB solderability. The program`s baseline surface finish was copper. Solderability testing on ``as-fabricated`` and ``pre-stressed copper`` pad-strip geometries was performed with Sn-Pb eutectic solder and three different fluxes at four different reflow temperatures.
Date: December 31, 1994
Creator: Hernandez, C.L. & Hosking, F.M.
Partner: UNT Libraries Government Documents Department

The influence of grain structure on the reliability of narrow Al- based interconnects

Description: The work reported here concerns the effect of grain structure on electromigration failure in pure A1 and A1-2wt.% Cu-1 wt.% Si lines. The grain structure of fine lines were controlled by annealing after pattering to promote the formation of ``bamboo`` structures. Significant improvements in the median time to failure (MTF) and the deviation of the time to failure (DTF) were observed with the development of near-bamboo structures with polygranular-segment lengths shorter than {approximately} 5 {mu}m. The most common failure sites are voids or slits across bamboo grains at the upstream ends of polygranular segments. The time-to-failure decreases with the polygranular segment length, and can be significantly enhanced by controlling the grain structure.
Date: May 1, 1995
Creator: Kang, S.H.; Kim, C.l Morris, J.W. Jr. & Genin, F.Y.
Partner: UNT Libraries Government Documents Department

Mechanism of electromigration failure in Al thin film interconnects containing Sc

Description: In order to understand the role of Sc on electromigration (EM) failure, Al interconnects with 0.1 and 0.3 wt.% Sc sere tested as a function of post-pattern annealing time. In response to the evolution of the line structure, the statistics of lifetime evolved. While the addition of Sc greatly reduces the rate of evolution of the failure statistics because the grain growth rate decreases, the MTF variation was found to be very similar to that of pure Al. These observations seem to show that Sc has little influence on the kinetics of Al EM; however, it has some influence on the EM resistance of the line since it is an efficient grain refiner. Unlike Cu in Al, Sc does not seem to migrate, which may explain its lack of influence on the kinetics of Al EM.
Date: May 1, 1995
Creator: Kim, Choong-un; Kang, S.H.; Morris, J.W. Jr. & Genin, F.Y.
Partner: UNT Libraries Government Documents Department

Ohmic contacts to Si-implanted and un-implanted n-type GaN

Description: We report on ohmic contacts to Si-implanted and un-implanted n-type GaN on sapphire. A ring shaped contact design avoids the need to isolate the contact structures by additional implantation or etching. Metal layers of Al and Ti/Al were investigated. On un-implanted GaN, post metalization annealing was performed in an RTA for 30 seconds in N{sub 2} at 700, 800, and 900 C. A minimum specific contact resistance (r{sub c}) of 1.4{times}10{sup -5} {Omega}{minus}cm{sup 2} was measured for Ti/Al at an annealing temperature of 800 C. Although these values are reasonably low, variations of 95% in specific contact resistance were measured within a 500 {mu}m distance on the wafer. These results are most likely caused by the presence of compensating hydrogen. Specific contact resistance variation was reduced from 95 to 10% by annealing at 900 C prior to metalization. On Si-implanted GaN, un-annealed ohmic contacts were formed with Ti/Al metalization. The implant activation anneal of 1120 C generates nitrogen vacancies that leave the surface heavily n-type, which makes un-annealed ohmic contacts with low contact resistivity possible.
Date: February 1996
Creator: Brown, J.; Ramer, J.; Zheng, L. F.; Hersee, S. D. & Zolper, J.
Partner: UNT Libraries Government Documents Department

Behavior of W and WSi(x) Contact Metallization on n- and p- Type GaN

Description: Sputter-deposited W-based contacts on p-GaN (N{sub A} {approximately} 10{sup 18} cm{sup {minus}3}) display non-ohmic behavior independent of annealing temperature when measured at 25 C. The transition to ohmic behavior occurs above {approximately} 250 C as more of the acceptors become ionized. The optimum annealing temperature is {approximately} 700 C under these conditions. These contacts are much more thermally stable than the conventional Ni/Au metallization, which shows a severely degraded morphology even at 700 C. W-based contacts may be ohmic as-deposited on very heavily doped n-GaN, and the specific contact resistance improves with annealing up to {approximately} 900 C.
Date: January 5, 1999
Creator: Abernathy, C.R.; Cao, X.A.; Cole, M.W.; Eizenberg, M.; Lothian, J.R.; Pearton, S.J. et al.
Partner: UNT Libraries Government Documents Department

Electrical Contact Performance Degradation in Electromechanical Components

Description: Detailed materials evaluations have been performed for MC2969 Intent Stronglink switch monitor circuit parts returned from the field out of retired weapon systems. Evaluations of local contact resistance, surface chemical composition and surface roughness and wear have been determined as a function of component level contact loop resistance testing position. Several degradation mechanisms have been identified and correlated with the component level measurements. Operational degradation produces surface smoothing and wear with each actuation of the monitor circuit, while aging degradation is observed in the segregation of contaminant species and alloy constituent elements to the surface in the stressed wear regions.
Date: March 23, 1999
Creator: Peebles, D.E.; Dugger, M.T.; Neff, S.G.; Sorroche, E.H.; Robinson, J.A.; Fanska, J. et al.
Partner: UNT Libraries Government Documents Department

Amorphous silicon/crystalline silicon heterojunctions for nuclear radiation detector applications

Description: Results on characterization of electrical properties of amorphous Si films for the 3 different growth methods (RF sputtering, PECVD [plasma enhanced], LPCVD [low pressure]) are reported. Performance of these a-Si films as heterojunctions on high resistivity p-type and n- type crystalline Si is examined by measuring the noise, leakage current, and the alpha particle response of 5mm dia detector structures. It is demonstrated that heterojunction detectors formed by RF sputtered films and PECVD films are comparable in performance with conventional surface barrier detectors. Results indicate that the a-Si/c-Si heterojunctions have the potential to greatly simplify detector fabrication. Directions for future avenues of nuclear particle detector development are indicated.
Date: October 1, 1996
Creator: Walton, J.T.; Hong, W.S.; Luke, P.N.; Wang, N.W. & Ziemba, F.P.
Partner: UNT Libraries Government Documents Department

Multichip module technology development

Description: This is the final report of a one-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). A Multichip Module (MCM) was designed and submitted for fabrication to the Lockheed Martin foundry using a licensed process called High Density Interconnect (HDI). The HDI process uses thin film techniques to create circuit interconnect patterns on multiple layers of dielectric film which are deposited directly on top of unpackaged electronic die. This results in an optimally small package that approaches the area of the bare die themselves. This project tested the capability of the Lockheed Martin foundry to produce, in an HDI process, a complex mixed-mode (analog and digital) circuit on a single MCM substrate.
Date: October 1, 1997
Creator: Kapustinsky, J.S.; Boissevain, J.G.; Muck, R.C.; Smith, G.D.; Wong-Swanson, B.G. & Ziock, H.J.
Partner: UNT Libraries Government Documents Department

Material development of polymer/metal paste for flip-chip attach interconnection technology. Quarterly report

Description: In the last leg of the project the major thrust has been on the assembly process using the conductive adhesive, viz., the optimization of the process conditions and the bonding equipment. The past at this point is deemed optimum in terms of the three basic properties: adhesion, screenability and conductivity. The reliability and wafer level screening is proven reproducibly over several experiment constituting assembly of more than one part. Using the optimum paste the authors have provided an uninterrupted supply of reproducible (optimum) paste. By tweaking the compounding conditions a first-level scale-up was successfully achieved. The initial 30g batch to Endicott is increased to as high as 300 g batches with similar properties. The large batch material is shown to behave similar to the small batch materials. Also, it has been essential to do large wafer level studies: Endicott has scaled up their screening from 5 inch wafer to 8 inch wafer.
Date: November 4, 1997
Creator: Saraf, R.F.; Roldan, J.M. & Sambucetti, C.J.
Partner: UNT Libraries Government Documents Department

Effectiveness of nickel plating in inhibiting atmospheric corrosion of copper alloy contacts

Description: A series of tests was run to determine the effect of Ni plating thickness on connector contact resistance. Copper coupons were plated with an electrolytic nickel strike followed by electroless nickel to produce Ni layers of 10, 20, 55 and 100 {micro}in. The coupons were then exposed to a simulated industrial environment. Pore corrosion was observed after the exposure, which correlated with Ni thickness. In a second series of tests, beryllium-copper four-tine contacts with 50 {micro}in of gold plate over electrolytic nickel strike/electroless-nickel plates of varying thickness were exposed the same corrosive environment. Contact resistance of mated pairs was monitored over a two-month period. The degradation in contact resistance correlated with the Ni thickness used in the connectors.
Date: December 31, 1997
Creator: Ernest, T.; Sorensen, R. & Guilinger, T.
Partner: UNT Libraries Government Documents Department

A.c. transport and collective excitation in a quantum point contact

Description: The authors calculate the a.c.-admittance of a two dimensional quantum point contact (QPC) using a Boltzmann-like kinetic equation derived for the partial Wigner distribution function. An integral equation for a potential inside a QPC is solved numerically. The dependence of the admittance on the frequency of the a.c. field is found in a wide frequency range {omega} {approx} 0--50 GHz. The contribution to the imaginary part of the admittance due to the quantum capacitance and inductance is numerically calculated. It is shown that the crossover from localized parameters--quantum capacitance and inductance--to distributed behavior takes place at {omega} {approximately} 10 GHz. A transition from 2D plasmons to quasi-1D plasmons is analyzed as a function of two dimensionless parameters: k{sub x}d{sub 0} (where k{sub x} is the longitudinal wave vector, and d{sub 0} is the width of the QPC), and the number of open electron channels, N.
Date: February 1, 1998
Creator: Berman, G.P.; Doolen, G.D.; Mainieri, R.; Aronov, I.E.; Campbell, D.K.; Beletskii, N.N. et al.
Partner: UNT Libraries Government Documents Department