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Advanced fabrication technologies for nano-electronics

Description: Three novel fabrication technologies are presented which greatly increase the tools available for the realization of nano-electronic devices. First, a sub-micron area post structure descending from a metallic airbridge allows gating of regions as small as 0.1 {mu}m in diameter. This has enabled the study of such quantum phenomena as coupling of parallel quantum point contacts, and electron focusing around a tunable quantum antidot. We also describe two new techniques for backgating multiquantum well structures with submicron lateral resolution. These techniques enable separate ohmic contacts to individual quantum wells spaced as closely as 100 {Angstrom}, and thus allow the fabrication of novel quantum tunneling devices. The first technique uses regrowth over a patterned ion-implanted substrate. The second involves a novel epoxy-bond-and-stop-etch (EBASE) processing scheme, whereby the original substrate is etched away and the backside then patterned using conventional methods.
Date: March 1, 1996
Creator: Simmons, J.A.; Weckwerth, M.V. & Baca, W.E.
Partner: UNT Libraries Government Documents Department

Nuclear Spin Orientation Dependence of Magnetoconductance: A New Method for Measuring the Spin of Charged Excitations in the QHE

Description: A new method for measuring the spin of the electrically charged ground state excitations m the Q$j~j quantum Hall effect ia proposed and demonstmted for the tirst time in GaAs/AIGaAs nndtiquantum wells. The method is &sed on the nuclear spin orientation dependence of" the 2D dc conductivity y in the quantum Hall regime due to the nuclear hyperfine interaction. As a demonstration of this method the spin of the electrically charged excitations of the ground state is determined at filling factor v = 1.
Date: December 1, 1998
Creator: Bowers, C.R.; Reno, J.L.; Simmons, J.A. & Vitkalov, S.A.
Partner: UNT Libraries Government Documents Department

Final report on LDRD Project: The double electron layer tunneling transistor (DELTT)

Description: This report describes the research accomplishments achieved under the LDRD Project ``Double Electron Layer Tunneling Transistor.`` The main goal of this project was to investigate whether the recently discovered phenomenon of 2D-2D tunneling in GaAs/AlGaAs double quantum wells (DQWs), investigated in a previous LDRD, could be harnessed and implemented as the operating principle for a new type of tunneling device the authors proposed, the double electron layer tunneling transistor (DELTT). In parallel with this main thrust of the project, they also continued a modest basic research effort on DQW physics issues, with significant theoretical support. The project was a considerable success, with the main goal of demonstrating a working prototype of the DELTT having been achieved. Additional DELTT advances included demonstrating good electrical characteristics at 77 K, demonstrating both NMOS and CMOS-like bi-stable memories at 77 K using the DELTT, demonstrating digital logic gates at 77 K, and demonstrating voltage-controlled oscillators at 77 K. In order to successfully fabricate the DELTT, the authors had to develop a novel flip-chip processing scheme, the epoxy-bond-and-stop-etch (EBASE) technique. This technique was latter improved so as to be amenable to electron-beam lithography, allowing the fabrication of DELTTs with sub-micron features, which are expected to be extremely high speed. In the basic physics area they also made several advances, including a measurement of the effective mass of electrons in the hour-glass orbit of a DQW subject to in-plane magnetic fields, and both measurements and theoretical calculations of the full Landau level spectra of DQWs in both perpendicular and in-plane magnetic fields. This last result included the unambiguous demonstration of magnetic breakdown of the Fermi surface. Finally, they also investigated the concept of a far-infrared photodetector based on photon assisted tunneling in a DQW. Absorption calculations showed a narrowband absorption which persisted to temperatures much higher ...
Date: June 1, 1998
Creator: Simmons, J.A.; Moon, J.S. & Blount, M.A.
Partner: UNT Libraries Government Documents Department

Novel magnetic-field-induced minigap and transport in coupled double quantum wells

Description: A review is given of recent theoretical and experimental work on in-plane electron transport in strongly coupled double quantum wells (QWs) in the presence of an in-plane magnetic field B{sub {parallel}} {parallel} x. This system displays unusual electronic and transport properties arising from a partial minigap ({approximately} a few meV) formed in the transverse in-plane direction k{sub y} {perpendicular} B{sub {parallel}} in k-space due to the anticrossing of the two QW dispersion curves displaced relative to each other by {Delta}k{sub y} {proportional_to} B{sub {parallel}}. Sweeping B{sub {parallel}} moves the minigap through the Fermi level ({mu}), deforming the Fermi surface from a two-component surface (with one orbit inside the other) to a single-orbit surface, and then back to a two-separated-orbit structure, accordingly as {mu} lies above, inside, and below the gap, respectively. The authors show that the density of states develops a sharp van Hove singularity at the lower gap edge, while transport properties such as the in-plane conductance and the cyclotron mass show sharp B{sub {parallel}}-dependent structures as {mu} passes through the gap edges.
Date: August 1, 1995
Creator: Lyo, S.K.; Simmons, J.A.; Harff, N.E. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

Enhancement of the Two-Dimensional Conduction Electron Zeeman Energy Near v=1 by Optical Dynamic Nuclear Polarization

Description: Enhancement of the Zeeman energy of 2D conduction electrons near v = 1 by optical dynamic nuclear polarization (lINP), as observed by the Overhauser shift of the transport detected electron spin resonance, is measured quantitatively for the first time in GaAs/AIGaAs mukiquantum wells. The NMR signal enhancement is obtained under similar conditions in the same sample, allowing the hyperke coupling constant of 3.7T between between the nuclei and 2D conduction electrons to be measured for the first time. The potential to suppress the Zeeman energy by optical DNP is discussed in the context of its potential influence on Skyrmion formation.
Date: November 6, 1998
Creator: Bowers, C.R.; Reno, J.L.; Simmons, J.A. & Vitkalov, S.A.
Partner: UNT Libraries Government Documents Department

Technical-economic assessment of the production of methanol from biomass. Assessment of biomass resource and methanol market. Final research report

Description: Detailed information is presented on the following: feasibility of biomass feedstocks for methanol production, biomass availability and costs, potential demand for methanol from biomass, comparison of potential methanol demand and supply, and market penetration assessment. (MHR)
Date: July 12, 1979
Creator: Wan, E.I.; Simmons, J.A.; Price, J.D. & Nguyen, T.D.
Partner: UNT Libraries Government Documents Department

Technical-economic assessment of the production of methanol from biomass. Executive summary. Final research report

Description: The results are presented of a comprehensive systems study which assessed the engineering and economic feasibilities of the production of methanol from biomass utilizing existing technology. The three major components of the biomass to methanol system assessed are the availability of biomass feedstocks, the thermochemical conversion of biomass to methanol fuels, and the distribution and markets for methanol fuels. The results of this study show that methanol fuel can be produced from biomass using commercially available technology in the near term, and could be produced economically in significant quantities in the mid-to-late 1980's when advanced technology is available.
Date: July 12, 1979
Creator: Wan, E.I.; Simmons, J.A.; Price, J.D. & Nguyen, T.D.
Partner: UNT Libraries Government Documents Department

Technical-economic assessment of the production of methanol from biomass. Conversion process analysis. Final research report

Description: A comprehensive engineering system study was conducted to assess various thermochemical processes suitable for converting biomass to methanol. A summary of the conversion process study results is presented here, delineating the technical and economic feasibilities of producing methanol fuel from biomass utilizing the currently available technologies. (MHR)
Date: July 12, 1979
Creator: Wan, E.I.; Simmons, J.A.; Price, J.D. & Nguyen, T.D.
Partner: UNT Libraries Government Documents Department

Relative consequences of transporting hazardous materials

Description: The objective of this paper is to discuss methods under study at Transportation Technology Center to develop a perspective on how technical measures of hazard and risk relate to perception of hazards, harm, and risks associated with transporting hazardous materials. This paper is concerned with two major aspects of the relative hazards problem. The first aspect is the analyses of the possible effects associated with exposure to hazardous materials as contained in the following two parts: outlines of possible problems and controversies that could be encountered in the evaluation and comparisons of hazards and risks; and description of the various measures of harm (hazards or dangers) and subsequent comparisons thereof. The second aspect of this paper leads into a presentation of the results of a study which had the following purposes: to develop analytical techniques for a consistent treatment of the phenomenology of the consequences of a release of hazardous materials; to reduce the number of variables in the consequence analyses by development of transportation accident scenarios which have the same meteorological conditions, demography, traffic and population densities, geographical features and other appropriate conditions and to develop consistent methods for presenting the results of studies and analyses that describe the phenomenology and compare hazards. The results of the study are intended to provide a bridge between analytical certainty and perception of the hazards involved. Understanding the differences in perception of hazards resulting from transport of various hazardous materials is fraught with difficulties in isolating the qualitative and quantitative features of the problem. By relating the quantitative impacts of material hazards under identical conditions, it is hoped that the perceived differences in material hazards can be delineated and evaluated.
Date: January 1, 1980
Creator: Fullwood, R.R.; Rhyne, W.R.; Simmons, J.A. & Reese, R.T.
Partner: UNT Libraries Government Documents Department

Magnetic field induced minigap in double quantum wells

Description: We report discovery of a partial energy gap, or minigap, in strongly coupled double quantum wells (QWs), due to an anticrossing of the two QW dispersion curves. The anticrossing and minigap are induced by an in-plane magnetic field B{sub {parallel}}, and give rise to large distortions in the Fermi surface and density of states, including a Van Hove singularity. Sweeping B{sub {parallel}} moves the minigap through the Fermi level, with the upper and lower gap edges producing a sharp maximum and minimum in the low-temperature in-plane conductance, in agreement with theoretical calculations. The gap energy may be directly determined from the data.
Date: July 1, 1994
Creator: Simmons, J. A.; Lyo, S. K.; Klem, J. F. & Harff, N. E.
Partner: UNT Libraries Government Documents Department

Electron transport in coupled double quantum wells and wires

Description: Due to inter-quantum well tunneling, coupled double quantum wells (DQWs) contain an extra degree of electronic freedom in the growth direction, giving rise to new transport phenomena not found in single electron layers. This report describes work done on coupled DQWs subject to inplane magnetic fields B{sub {parallel}}, and is based on the lead author`s doctoral thesis, successfully defended at Oregon State University on March 4, 1997. First, the conductance of closely coupled DQWs in B{sub {parallel}} is studied. B{sub {parallel}}-induced distortions in the dispersion, the density of states, and the Fermi surface are described both theoretically and experimentally, with particular attention paid to the dispersion anticrossing and resulting partial energy gap. Measurements of giant distortions in the effective mass are found to agree with theoretical calculations. Second, the Landau level spectra of coupled DQWs in tilted magnetic fields is studied. The magnetoresistance oscillations show complex beating as Landau levels from the two Fermi surface components cross the Fermi level. A third set of oscillations resulting from magnetic breakdown is observed. A semiclassical calculation of the Landau level spectra is then performed, and shown to agree exceptionally well with the data. Finally, quantum wires and quantum point contacts formed in DQW structures are investigated. Anticrossings of the one-dimensional DQW dispersion curves are predicted to have interesting transport effects in these devices. Difficulties in sample fabrication have to date prevented experimental verification. However, recently developed techniques to overcome these difficulties are described.
Date: April 1997
Creator: Harff, N. E.; Simmons, J. A. & Lyo, S. K.
Partner: UNT Libraries Government Documents Department

Bandgap renormalization studies of n-type GaAs/AlGaAs single quantum wells

Description: Bandgap energy renormalization due to many body effects has been studied in a series of n-type 8-nm-wide GaAs/AlGaAs single quantum wells using magnetoluminescence spectroscopy at 1.4K. The 2D-carrier densities varied between 1 and 12 {times} 10{sup 11}/sq cm. At the maximum 2D-carrier density, the bandgap energy reduction compared to an undoped specimen was found to be about 34 meV.
Date: September 1, 1997
Creator: Jones, E.D.; Blount, M.; Chow, W.; Hou, H. & Simmons, J.A.
Partner: UNT Libraries Government Documents Department

Magnetoresistance and cyclotron mass in extremely-coupled double quantum wells under in-plane magnetic fields

Description: The authors experimentally investigate the transport properties of an extremely-coupled AlGaAs/GaAs double quantum well, subject to in-plane magnetic fields (B{sub {parallel}}). The coupling of the double quantum well is sufficiently strong that the symmetric-antisymmetric energy gap ({Delta}{sub SAS}) is larger than the Fermi energy (E{sub F}). Thus for all B{sub {parallel}} only the lower energy branch of the dispersion curve is occupied. In contrast to systems with weaker coupling such that {Delta}{sub SAS} < E{sub F} the authors find: (1) only a single feature, a maximum, in the in-plane magnetoresistance, (2) a monotonic increase with B{sub {parallel}} in the cyclotron mass up to 2.2 times the bulk GaAs mass, and (3) an increasing Fermi surface orbit area with B{sub {parallel}}, in good agreement with theoretical predictions.
Date: December 1, 1997
Creator: Blount, M.A.; Simmons, J.A.; Lyo, S.K.; Harff, N.E. & Weckwerth, M.V.
Partner: UNT Libraries Government Documents Department

Tuning a double quantum well Fermi surface with in-plane magnetic fields

Description: A double quantum well (QW) subject to in-plane magnetic fields B{sub {parallel}} has the dispersion curves of its two QWs shifted in k-space. When the QWs are strongly coupled, an anticrossing and partial energy gap occur, yielding a tunable multi-component Fermi surface. We report measurements of the resultant features in the conductance, capacitive density of states, and giant deviations in cyclotron effective masses.
Date: July 1, 1995
Creator: Simmons, J.A.; Harff, N.E.; Eiles, T.M.; Lyo, S.K. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

Magnetic modulations of optical and transport properties of N-doped coupled double quantum wells

Description: Magnetoquantum resistance (MR) in a perpendicular magnetic field (B{sub {perpendicular}}) and photoluminescence (PL) spectra are shown to be sensitively modulated by an in-plane field (B{sub {parallel}}) due to the B{sub {parallel}}-induced anticrossing of the energy-dispersion curves of the two quantum wells (QWs). Using a self-consistent density functional theory, they find very different B{sub {parallel}}-evolutions of the PL spectra for symmetric and asymmetric double QWs consistent with recent data. The MR is calculated using a linear response theory. The results consist of a superposition of two series of MR oscillations represented by ridges running nearly perpendicular to each other in the B = (B{sub {parallel}}, B{sub {perpendicular}}) plane. The data from GaAs/AlGaAs double QWs agree with this behavior.
Date: August 1, 1998
Creator: Lyo, S.K.; Simmons, J.A.; Huang, D. & Harff, N.E.
Partner: UNT Libraries Government Documents Department

Unipolar Complementary Circuits Using Double Electron Layer Tunneling Tansistors

Description: We demonstrate unipolar complementary circuits consisting of a pair of resonant tunneling transistors based on the gate control of 2D-2D interlayer tunneling, where a single transistor - in addition to exhibiting a welldefined negative-differential-resistance can be operated with either positive or negative transconductance. Details of the device operation are analyzed in terms of the quantum capacitance effect and band-bending in a double quantum well structure, and show good agreement with experiment. Application of resonant tunneling complementary logic is discussed by demonstrating complementary static random access memory using two devices connected in series.
Date: October 19, 1998
Creator: Blount, M.A.; Hafich, M.J.; Moon, J.S.; Reno, J.L. & Simmons, J.A.
Partner: UNT Libraries Government Documents Department

Magnetic-field-induced tunneling and minigap transport in double quantum wells

Description: We review recent theoretical and experimental results on low- temperature tunneling and in-plane transport properties in double quantum wells (DQWs) in an in-plane magnetic field B{parallel}. These properties arise from combined effect of B{parallel}-induced relative displacement of the wave vectors in the two QWs and the interwell tunneling. In weakly coupled DQWs, the tunneling conductance has two sharp maxima as a function of B{parallel}. In strongly coupled DQWs, a partial minigap is formed due to anticrossing of the two QW dispersion curves, yielding sharp B{parallel}-dependent structures in the density of states and in- plane transport properties. Excellent agreement is obtained between theory and data from GaAs/AlGaAs DQWs.
Date: August 1, 1995
Creator: Lyo, S.K.; Simmons, J.A.; Harff, N.E.; Eiles, T.M. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

Magnetic breakdown in double quantum wells

Description: The authors find that a sufficiently large perpendicular magnetic field (B{sub {perpendicular}}) causes magnetic breakdown (MB) in coupled double quantum wells (QWs) that are subject to an in-plane magnetic field (B{sub {parallel}}). B{sub {parallel}} shifts one QW dispersion curve with respect to that of the other QW, resulting in an anticrossing and an energy gap. When the gap is below the Fermi level the resulting Fermi surface (FS) consists of two components, a lens-shaped inner orbit and an hour-glass shaped outer orbit. B{sub {perpendicular}} causes Landau level formation and Shubnikov-de Haas (SdH) oscillations for each component of the FS. MB occurs when the magnetic forces from B{sub {perpendicular}} become dominant and the electrons move on free-electron circular orbits rather than on the lens and hour-glass orbits. MB is observed by identifying the peaks present in the Fourier power spectrum of the longitudinal resistance vs. 1/B{sub {perpendicular}} at constant B{sub {parallel}}, an arrangement achieved with an in-situ tilting sample holder. Results are presented for two strongly coupled GaAs/AlGaAs DQW samples.
Date: August 1, 1996
Creator: Harff, N.E.; Simmons, J.A.; Klem, J.F.; Boebinger, G.S.; Pfeiffer, L.N. & West, K.W.
Partner: UNT Libraries Government Documents Department

Magnetic breakdown and Landau level spectra of a tunable double-quantum-well Fermi surface

Description: By measuring longitudinal resistance, the authors map the Landau level spectra of double quantum wells as a function of both parallel (B{sub {parallel}}) and perpendicular (B{sub {perpendicular}}) magnetic fields. In this continuously tunable highly non-parabolic system, the cyclotron masses of the two Fermi surface orbits change in opposite directions with B{sub {parallel}}. This causes the two corresponding ladders of Landau levels formed at finite B{sub {perpendicular}} to exhibit multiple crossings. They also observe a third set of landau levels, independent of B{sub {parallel}}, which arise from magnetic breakdown of the Fermi surface. Both semiclassical and full quantum mechanical calculations show good agreement with the data.
Date: December 31, 1997
Creator: Simmons, J.A.; Harff, N.E.; Lyo, S.K.; Klem, J.F.; Boebinger, G.S.; Pfeiffer, L.N. et al.
Partner: UNT Libraries Government Documents Department

A Planar Quantum Transistor Based on 2D-2D Tunneling in Double Quantum Well Heterostructures

Description: We report on our work on the double electron layer tunneling transistor (DELTT), based on the gate-control of two-dimensional -- two-dimensional (2D-2D) tunneling in a double quantum well heterostructure. While previous quantum transistors have typically required tiny laterally-defined features, by contrast the DELTT is entirely planar and can be reliably fabricated in large numbers. We use a novel epoxy-bond-and-stop-etch (EBASE) flip-chip process, whereby submicron gating on opposite sides of semiconductor epitaxial layers as thin as 0.24 microns can be achieved. Because both electron layers in the DELTT are 2D, the resonant tunneling features are unusually sharp, and can be easily modulated with one or more surface gates. We demonstrate DELTTs with peak-to-valley ratios in the source-drain I-V curve of order 20:1 below 1 K. Both the height and position of the resonant current peak can be controlled by gate voltage over a wide range. DELTTs with larger subband energy offsets ({approximately} 21 meV) exhibit characteristics that are nearly as good at 77 K, in good agreement with our theoretical calculations. Using these devices, we also demonstrate bistable memories operating at 77 K. Finally, we briefly discuss the prospects for room temperature operation, increases in gain, and high-speed.
Date: December 14, 1998
Creator: Baca, W.E.; Blount, M.A.; Hafich, M.J.; Lyo, S.K.; Moon, J.S.; Reno, J.L. et al.
Partner: UNT Libraries Government Documents Department

Composite fermions in 2 x 10{sup 6} cm{sup 2}/Vs mobility A1GaAs/GaAs heterostructures grown by MOCVD

Description: Recent growth by MOCVD (metalorganic chemical vapor deposition) of 2.0x10{sup 6} cm{sup 2}/Vs mobility heterostructures are reported. These mobilities, the highest reported to date, are attributed to use of tertiarybutylarsine as the arsenic precursor. Measurements in tilted magnetic fields of the fractional quantum Hall effect states near filling factor 3/2 are consistent with a spin-split composite fermion (CF) model proposed earlier. Extracted values of the product of the CF g-factor and CF effective mass agree with values previously obtained for MBE samples.
Date: December 31, 1996
Creator: Simmons, J.A., Chui, H.C., Harff, N.E., Hammons, B.E. & Du, R.R., Zudov, M.A.
Partner: UNT Libraries Government Documents Department

Composite fermions in 2 {times} 10{sup 6} cm{sup 2}/Vs mobility AlGaAs/GaAs heterostructures grown by MOCVD

Description: The authors report on the recent growth by MOCVD of 2.0 {times} 106 cm2/Vs mobility heterostructures. These mobilities, the highest reported to date, are attributed to the use of tertiarybutylarsine as the arsenic precursor. Measurements in tilted magnetic fields of the fractional quantum Hall effect (FQHE) states near filling factor 3/2 are consistent with a spin-split composite fermion (CF) model proposed earlier. The extracted values of the product of the CF g-factor and CF effective mass agree with values previously obtained for MBE samples.
Date: August 1, 1996
Creator: Simmons, J.A.; Chui, H.C.; Harff, N.E.; Hammons, B.E.; Du, R.R. & Zudov, M.A.
Partner: UNT Libraries Government Documents Department

Photoluminescence studies of modulation doped coupled double quantum wells in magnetic fields

Description: We have studied the photoluminescence spectra of a series of mudulation doped couple double quantum well structures in parallel and perpendicular magnetic fields to 62 tesla at 4K and 77K, for B{parallel}a, the spectra display distinct Landau level transitions which show anti-crossing with the e1-hh1 exciton. At high fields, the lowest conduction band-valence exciton approaches the extrapolated 0- 0 Landau level. About 25 Tesla, there is valence band mixing of the e1-lh1, e1-hh2, e1-hh1 transitions. The spectral peaks display a diamagnetic shift in low in-plane magnetic fields which become linear in high fields. At magnetic fields beyond 40T, spin splitting is observed for both B{parallel}z and B{perpendicular} geometries. The partial energy gap discovered in conductance measurements in in-plane fields was not conclusively observed using photoluminescence spectroscopy, although anomalies in the energy dependence of the lowest level with magnetic field were evident at similar field values.
Date: September 1, 1996
Creator: Kim, Y.; Perry, C.H.; Simmons, J.A.; Klem, J.F.; Jones, E.D. & Rickel, D.G.
Partner: UNT Libraries Government Documents Department

Bandgap renormalization: GaAs/AlGaAs quantum wells

Description: Bandgap energy renormalization by many-body interactions has been studied in a series of n-type 8-nm-wide GaAs/AlGaAs single quantum wells using magnetoluminescence spectroscopy at 1.4K and for magnetic fields up to 30T. The 2D- carrier densities varied between 1 and 12 x 10(11) cm(-2). At the maximum 2D-carrier density, the bandgap energy difference between the doped and undoped samples was about 34 meV.
Date: January 1, 1998
Creator: Jones, E.D.; Blount, M.; Chow, W.; Hou, H.; Simmons, J.A.; Kim, Yongmin et al.
Partner: UNT Libraries Government Documents Department