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Enabling graphene nanoelectronics.

Description: Recent work has shown that graphene, a 2D electronic material amenable to the planar semiconductor fabrication processing, possesses tunable electronic material properties potentially far superior to metals and other standard semiconductors. Despite its phenomenal electronic properties, focused research is still required to develop techniques for depositing and synthesizing graphene over large areas, thereby enabling the reproducible mass-fabrication of graphene-based devices. To address these issues, we combined an array of growth approaches and characterization resources to investigate several innovative and synergistic approaches for the synthesis of high quality graphene films on technologically relevant substrate (SiC and metals). Our work focused on developing the fundamental scientific understanding necessary to generate large-area graphene films that exhibit highly uniform electronic properties and record carrier mobility, as well as developing techniques to transfer graphene onto other substrates.
Date: September 1, 2011
Creator: Pan, Wei; Ohta, Taisuke; Biedermann, Laura Butler; Gutierrez, Carlos; Nolen, C. M.; Howell, Stephen Wayne et al.
Partner: UNT Libraries Government Documents Department

Electronic transport in amorphous carbon

Description: Electronic transport in a-C films has been the subject of considerable debate. In this study, combined stress relaxation and electrical transport studies were used to identify the transport mechanism in a-C films prepared by pulsed-laser deposition. The stress relaxation was modeled by a first-order kinetic reaction involving transformation of 4-fold coordinated carbon atoms to 3-fold coordinated carbon atoms, and the distribution of activation energies for this process was determined. The activation energies were found to range from about 1 eV to over 2 eV, and using these activation energies, the increase in 3-fold carbon concentration with time-temperature annealing was obtained. Conductivity measurements were also performed as a function of time-temperature annealing. It was found that the conductivity of a-C films is exponentially proportional to increases in 3-fold carbon concentration. This result can be explained by thermally activated hopping along carbon 3-fold chains combined with chain-to-chain tunneling. From the data, a typical chain length was estimated to consist of 13 carbon atoms. The heterogeneous nature of the conductivity may explain the spatially localized electron emission which is observed in a-C assuming a tunnel barrier emission model.
Date: December 1, 1997
Creator: Sullivan, J.P. & Friedmann, T.A.
Partner: UNT Libraries Government Documents Department

The electronic transport mechanism in amorphous tetrahedrally-coordinated carbon films

Description: The electronic transport mechanism in tetrahedrally coordinated amorphous carbon was investigated using measurements of stress relaxation, thermal evolution of electrical conductivity, and temperature dependent conductivity measurements. Stress relaxation measurements were used to determine the change in 3-fold coordinated carbon concentration, and the electrical conductivity was correlated to this change. It was found that the conductivity was exponentially proportional to the change in 3-fold concentration, indicating a tunneling or hopping transport mechanism. It was also found that the activation energy for transport decreased with increasing anneal temperature. The decrease in activation energy was responsible for the observed increase in electrical conductivity. A model is described wherein the transport in this material is described by thermally activated conduction along 3-fold linkages or chains with variable range and variable orientation hopping. Thermal annealing leads to chain ripening and a reduction in the activation energy for transport.
Date: February 1, 1998
Creator: Sullivan, J.P.; Friedmann, T.A.; Dunn, R.G.; Stechel, E.B. & Schultz, P.A.
Partner: UNT Libraries Government Documents Department

The effect of aspect ratio and sp2/sp3 content on the field emission properties of carbon films grown by Ns-spiked PECVD

Description: The authors have deposited carbon films from mixtures of methane and N{sub 2} using Plasma Enhanced Chemical Vapor Deposition. By changing the percentage of N{sub 2} in the feed gas, they were able to produce films that have various aspect ratios and sp{sup 2}/sp{sup 3} contents. The film with the highest field emission contains spears of aspect ratio of 10:1. They also found that in their sp{sup 3}-rich films, higher sp{sup 2} content enhanced field emission. This is ascribed to improved charge transport to the field emission sites.
Date: April 1998
Creator: Tong, W.; Felter, T. E.; Pan, L. S.; Anders, S.; Cossy-Facre, A. & Stammler, T.
Partner: UNT Libraries Government Documents Department


Description: Techniques involving the use of high-intensity short-duration light pulses have been applied t o the study of the kinetics of photoconductivity in films of metal-free phthalocyanine. These experiments, in conjunction with measurements of steady-state photoconductivity, are consistent with the following scheme. The principal route for the formation of charge carriers is via the first excited singlet state, although the lowest triplet state can, t o some extent, contribute to charge - carrier production. The mobility of the carriers is low and is concentration-dependent, being lower at higher carrier concentration. The decay of the photocurrent is the result of a diffusion-limited bimolecular recombination, with a capture radius of approximately one molecular diameter. The experiments indicate that carriers produced thermally in the dark do not interact with light-produced carriers.
Date: July 29, 1959
Creator: Tollin, Gordon; Kearns, David R. & Calvin, Melvin.
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

High performance transparent conducting films of cadmium indate prepared by RF sputtering

Description: The authors are examining various spinel-structured thin films (e.g., Cd{sub 2}SnO{sub 4}, Zn{sub 2}SnO{sub 4}) to develop higher-quality transparent conducting oxides (TCO) than more conventional materials such as indium tin oxide. Here, the authors report on cadmium indate (CdIn{sub 2}O{sub 4}, CIO), which is another member of this family. Thin films of CIO were deposited by radio-frequency (RF) magnetron sputtering, from an oxide target, onto borosilicate glass substrates. The variables included the substrate temperature, sputtering gas composition, and pressure. Film properties were measured before and after heat treatment. Characterization involved Hall effect measurements, optical and infrared spectrophotometry, X-ray diffraction, and atomic-force microscopy. Film resistivities as low as 2.3 {times} 10{sup {minus}4} {Omega} cm were achieved for a film thickness of 0.55 {micro}m. The transmittance was 90% in the visible region of the spectrum, without correction for substrate losses and without an anti-reflection coating. The plasma resonance occurred at longer wavelengths than for other materials and this, with a bandgap of approximately 3.1 eV, presents a wide window for optical transmittance. The highest mobility was 54 cm{sup 2} V{sup {minus}1} s{sup {minus}1} and the highest carrier concentration was 7.5 {times} 10{sup 20} cm{sup {minus}3}.
Date: April 1, 1996
Creator: Coutts, T.J.; Wu, X. & Mulligan, W.P.
Partner: UNT Libraries Government Documents Department

Experimental evidence of improved thermoelectric properties at 300K in Si/Ge superlattice structures

Description: The authors have found that it may be possible to obtain significant enhancement in ZT at 300 K, over conventional bulk SiGe alloys, through the use of Si/Ge Superlattice (SL) structures. The Seebeck coefficient in Si/Ge SL structures was observed to increase rapidly with decreasing SL period with no loss of electrical conductivity. The carrier mobilities in Si/Ge SLs were higher than in a comparable thin-film Si/Ge alloy. The best power factor of the short-period Si/Ge SLs is 112.2 {micro}W/K{sup 2} cm, over five-fold better than state-of-the-art n-type, bulk SiGe alloys. Approximately a two to four-fold reduction in thermal conductivity in short-period SL structures, compared to bulk SiGe alloy, was observed. The authors estimate at least a factor of five improvement over current state-of-the-art SiGe alloys, in several Si/Ge SL samples with periodicity of {approximately}45 to 75 {angstrom}. The results of this study are promising, but tentative due to the possible effects of substrate and the developmental nature of the thermoelectric property measurements.
Date: April 1, 1997
Creator: Venkatasubramanian, R.; Colpitts, T.; Watko, E. & Malta, D.
Partner: UNT Libraries Government Documents Department

Characterization of SnO2 Films Prepared Using Tin Tetrachloride and Tetra Methyl Tin Precursors

Description: We have investigated the effect of deposition conditions of SnO2 films, deposited by chemical vapor deposition using tin tetrachloride and tetramethyltin precursors, on the film properties. The type of precursor and the deposition temperature affect the morphology of the films. The structure of the films is determined by the deposition temperature: films deposited at low temperatures show a mixed SnO and SnO2 phase. The processing temperature and type of substrate determine the impurity content in the films. Electrical properties (e.g. the carrier mobility) and optical properties of the films are affected by the structure and the impurity content in these layers.
Date: October 31, 1998
Creator: Dhere, R. G.; Moutinho, H. R.; Asher, S.; Li, X.; Ribelin, R.; Gessert, T. (National Renewable Energy Laboratory) et al.
Partner: UNT Libraries Government Documents Department

Effect of electron transport properties on unipolar CdZnTe radiation detectors: LUND, SpectrumPlus, and Coplanar Grid

Description: Device simulations of (1) the laterally-contacted-unipolar-nuclear detector (LUND), (2) the SpectrumPlus, (3) and the coplanar grid made of Cd{sub 0.9}Zn{sub 0.1}Te (CZT) were performed for {sup 137}Cs irradiation by 662.15 keV gamma-rays. Realistic and controlled simulations of the gamma-ray interactions with the CZT material were done using the MCNP4B2 Monte Carlo program, and the detector responses were simulated using the Sandia three-dimensional multielectrode simulation program (SandTMSP). The simulations were done for the best and the worst expected carrier nobilities and lifetimes of currently commercially available CZT materials for radiation detector applications. For the simulated unipolar devices, the active device volumes were relatively large and the energy resolutions were fairly good, but these performance characteristics were found to be very sensitive to the materials properties. The internal electric fields, the weighting potentials, and the charge induced efficiency maps were calculated to give insights into the operation of these devices.
Date: January 7, 2000
Creator: James, Ralph B.
Partner: UNT Libraries Government Documents Department

In-plane magneto-photoluminescence studies of modulation-doped GaAs/AlGaAs coupled double quantum wells

Description: In-plane magnetic field photoluminescence spectra from n series of n-type modulation doped GaAs/Al{sub 0.3}Ga{sub 0.7}As coupled double quantum wells show distinctive doublet structures related to the tunnel-split ground sub-level states. The magnetic field behavior of the upper transition from the antisymmetric state strongly depends on sample mobility. In a lower mobility sample, the transition energy displays an N-type kink with field (namely a maximum followed by a minimum), whereas higher mobility samples have a linear dependence. The former is attributed to a coupling mechanism due to homogeneous broadening of the electron and hole states. The results are in good agreement with recent theoretical calculations.
Date: May 11, 2000
Partner: UNT Libraries Government Documents Department

An accurate, efficient algorithm for calculation of quantum transport in extended structures

Description: In device structures with dimensions comparable to carrier inelastic scattering lengths, the quantum nature of carriers will cause interference effects that cannot be modeled by conventional techniques. The basic equations that govern these ``quantum`` circuit elements present significant numerical challenges. The authors describe the block recursion method, an accurate, efficient method for solving the quantum circuit problem. They demonstrate this method by modeling dirty inversion layers.
Date: May 1, 1994
Creator: Godin, T.J. & Haydock, R.
Partner: UNT Libraries Government Documents Department

Final report to the Department of Energy, Basic Energy Sciences, Grant No. DE-FG02-97ER45649 [Theoretical study of phonon modes and electron-phonon scattering]

Description: We have studied the effect of additional layering within a semiconductor quantum well on carrier mobility compared to a simple quantum well. The previous report indicated results concerning modification of the phonon dispersion of the quantum well with additional layering and enhanced scattering from interface phonons accompanied by a reduction in confined phonon scattering. New results for this report are calculated relaxation times for transitions into all subbands of the quantum well structures and subsequent mobilities. We have found that mobilities are enhanced in structurally modified quantum well systems compared to simple quantum wells. Structurally modified quantum wells have additional thin layers of barrier material inside the well regions. For the case considered here, we have studied GaAs/AlAs quantum wells with thin AlAs layers, 8 {angstrom} in width, at room temperature. We have found that structurally modified quantum wells in the approximate ranges of 50-90 {angstrom}, 200-250 {angstrom}, and 370-400 {angstrom} show higher mobilities when compared to simple quantum wells of comparable widths. Mobilities increase when the separation between subbands is unfavorable for single phonon emission or absorption. Upon reaching resonance there is a sharp decrease in the mobilities until conditions become favorable again. This is a final report on the theoretical study of phonon modes and electron-phonon scattering in structurally modified electronic materials.
Date: June 30, 2000
Creator: McIntyre, Dr. Cynthia R.
Partner: UNT Libraries Government Documents Department

Origin of Charge Density at LaAlO3 on SrTiO3 Heterointerfaces: Possibility of Intrinsic Doping

Description: As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO{sub 3} and LaAlO{sub 3}. Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.
Date: June 14, 2007
Creator: Siemons, W.; /Stanford U., Geballe Lab. /Twente U., Enschede; Koster, Gertjan; /Stanford U., Geballe Lab.; Yamamoto, Hideki; /Stanford U., Geballe Lab. /NTT Basic Res. Lab. et al.
Partner: UNT Libraries Government Documents Department

Comprehensive numerical model for cw vertical-cavity surface-emitting lasers

Description: The authors present a comprehensive numerical model for vertical-cavity surface-emitting lasers that includes all major processes effecting cw operation of axisymmetric devices. In particular, the model includes a description of the 2D transport of electrons and holes through the cladding layers to the quantum well(s), diffusion and recombination processes of these carriers within the wells, the 2D transport of heat throughout the device, and a multi-lateral-mode effective index optical model. The optical gain acquired by photons traversing the quantum wells is computed including the effects of strained band structure and quantum confinement. They employ the model to predict the behavior of higher-order lateral modes in proton-implanted devices, and to provide an understanding of index-guiding in devices fabricated using selective oxidation.
Date: March 1, 1995
Creator: Hadley, G.R.; Lear, K.L.; Warren, M.E.; Choquette, K.D.; Scott, J.W. & Corzine, S.W.
Partner: UNT Libraries Government Documents Department

Electrode design for coplanar-grid detectors

Description: The coplanar-grid charge sensing technique provides a method for improving the spectral response of gamma-ray detectors based on compound semiconductors, which typically have poor charge transport properties. The technique functions by effectively modifying the charge induction characteristics of the detector such that the dependence of detector signal on the depth of radiation interaction is minimized. The effectiveness of this technique however can be compromised by non-uniform charge induction characteristics across the detector. This paper examines such non-uniformity due to fringe effects near the detector edges. Alternate electrode configurations are studied that provide effective compensation for such effects. Results from experimental measurements and computer simulations are presented.
Date: November 1, 1996
Creator: Luke, P.N.; Amman, M.; Prettyman, T.H.; Russo, P.A. & Close, D.A.
Partner: UNT Libraries Government Documents Department

Performance status of 0.55 eV InGaAs thermophotovoltaic cells

Description: Data on {approximately} 0.55 eV In{sub 0.72}Ga{sub 0.28}As cells with an average open-circuit voltage (Voc) of 298 mV (standard deviation 7 mV) at an average short-circuit current density of 1.16 A/cm{sup 2} (sdev. 0.1 A/cm{sup 2}) and an average fill-factor of 61.6% (sdev. 2.8%) is reported. The absorption coefficient of In{sub 0.72}Ga{sub 0.28}As was measured by a differential transmission technique. The authors use a numerical integration of the absorption data to determine the radiative recombination coefficient for In{sub 0.72}Ga{sub 0.28}As. Using this absorption data and simple one-dimensional analytical formula the above cells are modeled. The models show that the cells may be limited more by Auger recombination rather than Shockley-Read-Hall (SRH) recombination at dislocation centers caused by the 1.3% lattice mismatch of the cell to the host InP wafer.
Date: October 1, 1998
Creator: Wojtczuk, S.; Colter, P.; Charache, G. & DePoy, D.
Partner: UNT Libraries Government Documents Department

A Simple Single Step diffusion and Emitter Etching Process for High Efficiency Gallium Antimonide Thermophotovoltaic Devices

Description: A single step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high efficiency GaSb thermophotovoltaic cells. The junction depth was controlled through monitoring of light current-voltage (I-V) curves (photovoltaic response) during the post diffusion emitter etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum efficiencies and the open circuit voltages in the fabricated devices. An optimum junction depth for obtaining highest quantum efficiency and open circuit voltage is presented based on diffusion lengths (or monitoring carrier lifetimes), carrier mobility and typical diffused impurity profile in GaSb.
Date: August 29, 2003
Creator: Rajagopalan, G.; Reddy, N.S.; Ehsani, E.; Bhat, I.B.; Dutta, P.S.; Gutmann, R.J. et al.
Partner: UNT Libraries Government Documents Department

Electrophoretic NMR measurements of lithium transference numbers in polymer gel electrolytes

Description: Polymer gel electrolytes are of increasing interest for plastic lithium batteries largely because of their high room temperature conductivity. Several studies have probed their conductivity and electrochemical stability but very little work has been done related to lithium transference numbers. Lithium ion transference numbers, the net number of Faradays carried by lithium upon the passage of 1 Faraday of charge across a cell, are key figures of merit for any potential lithium battery electrolytes. The authors describe here their application of electrophoretic NMR (ENMR) to the determination of transference numbers of lithium ions in polymer gel electrolytes. Two types of polymer gel electrolytes were selected for this study: PAN/PC/EC/LiX and Kynar/PC/LiX. Results obtained for the two types of gels are compared and the effects of anion, polymer-ion interactions and ion-ion interactions on lithium transference numbers are discussed. Significant differences in the behavior of transference numbers with salt concentration are observed for the two types of gels. This may be due to the extent of interaction between the polymer and the ions. Implications for solid polymer electrolytes are discussed.
Date: May 1997
Creator: Dai, H.; Sanderson, S.; Davey, J.; Uribe, F. & Zawodzinski, T. A., Jr.
Partner: UNT Libraries Government Documents Department

Growth and characterization of In{sub 0.2}Ga{sub 0.8}Sb device structures using metalorganic vapor phase epitaxy

Description: In{sub 0.2}Ga{sub 0.8}Sb epitaxial layers and thermophotovoltaic (TPV) device structures have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE). Control of the n-type doping up to 1 {times} 10{sup 18} cm{sup {minus}3} was achieved using diethyltellurium (DETE) as the dopant source. A Hall mobility of greater than 8,000 cm{sup 2}/Vs at 77 K was obtained for a 3 {times} 10{sup 17} cm{sup {minus}3} doped In{sub 0.2}Ga{sub 0.8}Sb layer grown on high-resistivity GaSb substrate. The In{sub 0.2}Ga{sub 0.8}Sb epilayers directly grown on GaSb substrates were tilted with respect to the substrates, with the amount of tilt increasing with the layer thickness. Transmission electron microscopy (TEM) studies of the layers showed the presence of dislocation networks across the epilayers parallel to the interface at different distances from the interface, but the layers above this dislocation network were virtually free of dislocations. A strong correlation between epilayer tilt and TPV device properties was found, with layers having more tilt providing better devices. The results suggest that the dislocations moving parallel to the interface cause lattice tilt, and control of this layer tilt may enable the fabrication of better quality device structures.
Date: May 1, 1997
Creator: Ehsani, H.; Bhat, I.; Hitchcock, C.; Gutmann, R.; Charache, G. & Freeman, M.
Partner: UNT Libraries Government Documents Department

Photocharge transport and recombination measurements in amorphous silicon films and solar cells by photoconductive frequency mixing. Annual subcontract report, 15 May 1995--15 May 1996

Description: Using the photomixing technique, the authors systematically studied the transport properties of intrinsic hydrogenated amorphous silicon (a-Si:H) samples that had hydrogen content ranging from over 10% to less than 1% and which were produced by the hot-wire technique at NREL. They investigated the continuous decay of electron drift mobility in intrinsic a-Si:H on light-soaking and determined the degradation of photoconductivity, lifetime, and drift mobility in these a-Si:H samples while light-soaking. In addition to the decay of the photoconductivity and electron lifetime, continuous decay of the electron drift mobility was found during the light-soaking process, which reveals a new phenomenon associated with the Staebler-Wronski effect. The drift mobility decreased by a factor of 2--4 for 5-hour light-soaking at 4-sun intensity. The authors investigated the effects of deposition conditions on transport properties of intrinsic a-Si:H films and, by using the photomixing technique, they determined the electron drift mobility, lifetime, and the conduction-band Urbach energy of a-Si:H films as a function of substrate temperature. 44 refs.
Date: October 1, 1996
Creator: Braunstein, R. & Dong, S.
Partner: UNT Libraries Government Documents Department

Minority-carrier transport in InGaAsSb thermophotovoltaic diodes

Description: Uncoated InGaAsSb/GaSb thermophotovoltaic (TPV) diodes with 0.56 eV (2.2 {micro}m) bandgaps exhibit external quantum efficiencies of 59% at 2 {micro}m. The devices have electron diffusion lengths as long as 29 {micro}m in 8-{micro}m-wide p-InGaAsSb layers and hole diffusion lengths of 3 {micro}m in 6-{micro}m-wide n-InGaAsSb layers. The electron and hole diffusion lengths appear to increase with increasing p- and n-layer widths. At 632.8 nm the internal quantum efficiencies of diodes with 1- to 8-{micro}m-wide p-layers are above 89% and are independent of the p-layer width, indicating long electron diffusion lengths. InGaAsSb has, therefore, excellent minority carrier transport properties that are well suited to efficient TPV diode operation. The structures were grown by molecular-beam epitaxy.
Date: May 1, 1997
Creator: Martinelli, R.U.; Garbuzov, D.Z.; Lee, H.; Morris, N.; Odubanjo, T. & Connolly, J.C.
Partner: UNT Libraries Government Documents Department