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A low power ultra-fast current transient measuring device.

Description: We have studied the feasibility of an innovative device to sample 1ns low-power single current transients with a time resolution better than 10 ps. The new concept explored here is to close photoconductive semiconductor switches (PCSS) with a Laser for a period of 10 ps. The PCSSs are in a series along a Transmission Line (TL). The transient propagates along the TL allowing one to carry out a spatially resolved sampling of charge at a fixed time instead of the usual timesampling of the current. The fabrication of such a digitizer was proven to be feasible but very difficult.
Date: October 1, 2004
Creator: Doyle, Barney Lee; Rossi, Paolo; Armendariz, Marcelino G.; Sullivan, John Patrick; Foltynowicz, Robert J. & Zutavern, Fred J.
Partner: UNT Libraries Government Documents Department

Effects of chemical modifications on photophysics and exciton dynamics on {pi}-conjugation attenuated and metal-chelated photoconducting polymers

Description: Effects of two types of chemical modifications on photoconducting polymers consisting of polyphenylenevinylene (PPV) derivatives are studied by static and ultrafast transient optical spectroscopy as well as semi-empirical ZINDO calculations. The first type of modification inserts 2,2{prime}-bipyridyl-5-vinylene units (bpy V) in the PPV backbone, and the second type involves metal-chelation with the bpy sites. Photoluminescence and exciton dynamics of polymers 1 and 2 with PV:bpyV ratios of 1 and 3 were examined in solution, and compared to those of the homopolymer, poly(2,5-bis(2{prime}-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-PPV). Similar studies were carried out for several metal-chelated polymers. These results can be explained by changes in {pi}-conjugation throughout the polymer backbone. The attenuation in {pi}-conjugation by the chemical modifications transforms a conducting polymer from one-dimensional semiconductor to molecular aggregates.
Date: March 11, 2000
Creator: Chen, L. X.; Jager, W. J. H.; Gosztola, D. J.; Niemczyk, M. P. & Wasielewski, M. R.
Partner: UNT Libraries Government Documents Department

A Silicon Ingot Lifetime Tester for Large Crystals

Description: A lifetime-measurement instrument has been developed to characterize large silicon ingots prior to wafering and polishing. It uses the direct-current photoconductance decay method and localized probing and illumination to achieve the necessary sensitivity on low-resistivity, large samples. A 940-nm, 60-Wp, pulsed-laser diode beam (250-ms width, <100-ns cut-off) lights the as-cropped silicon surface between two ohmic-contact probes. A user-friendly graphical interface supports data acquisition, lifetime calculation, and data storage. Pneumatic systems position the ingot and probes. Three-dimensional, finite-element analysis indicates that the detection depth of this technique is much better than the microwave or radio-frequency techniques. It also shows that the as-cropped surface finish is adequate for measuring bulk lifetimes on the order of 50 ms or less-a typical range for Czochralski ingots used in photovoltaic module production. Measurement repeatability and clear distinction among different grades of feedstock materials have been demonstrated.
Date: May 2, 1999
Creator: Wang, T. H.; Ciszek, T. F.; Landry, M. (National Renewable Energy Laboratory); Matthaeus, A. (Siemens Solar Industries: Camarillo, California) & Mihalik, G. (Siemens Solar Industries: Vancouver, Washington)
Partner: UNT Libraries Government Documents Department

A DONOR COMPLEX WITH TUNNELING HYDROGEN IN PURE GERMANIUM

Description: A shallow donor complex observed by several authors in ultrapure germanium grown in a hydrogen atmosphere is attributed to an oxygen-hydrogen system. Photoconductivity data under stress are presented. An abrupt transition in the spectra at a well-defined stress (2.1 x 10{sup 8} dyn cm{sup -2}) is found. It is explained by a theory which involves dynamic tunneling of the hydrogen in the vicinity of an oxygen center. The comparison with other complex donors and acceptors supports the model.
Date: February 1, 1980
Creator: Joos, B.; Haller, E.E. & Falicov, L.M.
Partner: UNT Libraries Government Documents Department

Deposition of a-SiC:H, a-SiO{sub 2} and tetrahedral-C with programmable in-situ etching. Final performance report, March 1, 1988--November 30, 1991

Description: This research program was originally defined to investigate the deposition of semiconductor and dielectric thin films using a low pressure remote plasma chemical vapor deposition system incorporating a process for etching the films. This etching was to be performed in a periodic fashion during the deposition process to remove defect regions in the film being deposited. The goal was to remove voids and other defects which are characteristic of low temperature deposition processes. While the original research proposal suggested that the studies include the amorphous alloys (Si/C):H and (Si/Ge):H, subsequent funding reductions limited the work to the deposition of an amorphous silicon alloy material (a-Si:H). Intrinsic and doped forms of these materials have applications in the fabrication of single and multi-junction thin film solar cells.
Date: June 1, 1995
Creator: Collis, W.J.
Partner: UNT Libraries Government Documents Department

High gain GaAs photoconductive semiconductor switches: Switch longevity

Description: Optically activated, high gain GaAs switches are being tested for many different pulsed power applications that require long lifetime (longevity). The switches have p and n contact metallization (with intentional or unintentional dopants) configured in such a way as to produce p-i-n or n-i-n switches. The longevity of the switches is determined by circuit parameters and by the ability of the contacts to resist erosion. This paper will describe how the switches performed in test-beds designed to measure switch longevity. The best longevity was achieved with switches made with diffused contacts, achieving over 50 million pulses at 10 A and over 2 million pulses at 80 A.
Date: July 1, 1998
Creator: Loubriel, G.M.; Zutavern, F.J. & Mar, A.
Partner: UNT Libraries Government Documents Department

Photorefractivity in liquid crystalline composite materials

Description: We report recent improvements in the photorefractive of liquid crystalline thin film composites containing electron donor and acceptor molecules. The improvements primarily result from optimization of the exothermicity of the intermolecular charge transfer reaction and improvement of the diffusion characteristics of the photogenerated ions. Intramolecular charge transfer dopants produce greater photorefractivity and a 10-fold decrease in the concentration of absorbing chromophores. The mechanism for the generation of mobile ions is discussed.
Date: September 1, 1997
Creator: Wiederrecht, G.P. & Wasielewski, M.R.
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

A Collective Impact Ionization Theory of Lock-On

Description: PhotoConductive semiconductor switches (PCSS's), such as optically-triggered GaAs switches, have been developed for a variety of applications. Such switches exhibit unique properties associated with lock-on, a phenomenon associated with bistable switching. In this paper lock-on is explained in terms of collective impact ionization.
Date: September 2, 1999
Creator: HJALMARSON, HAROLD P.; KAMBOUR, CHARLES; KANG, SAMSOO; LOUBRIEL, GUILLERMO M.; MYLES, CHARLES W.; WAKE, DOUGLAS RAYMON 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

Influence of the Sb dopant distribution on far infrared photoconductivity in Ge:Sb blocked impurity band detectors

Description: Extended long wavelength response to {approx}200 {micro}m (50 cm{sup -1}) has been observed in Ge:Sb Blocked Impurity Band (BIB) detectors with N{sub D} {approx} 1 x 10{sup 16} cm{sup -3}. The cut-off wavelength increases from 150 {micro}m (65 cm{sup -1}) to 200 {micro}m (50 cm{sup -1}) with increasing bias. The responsivity at long wavelengths was lower than expected. This can be explained by considering the observed Sb diffusion profile in a transition region between the blocking layer and active layer. BIB modeling is presented which indicates that this Sb concentration profile increases the electric field in the transition region and reduces the field in the blocking layer. The depletion region consists partially of the transition region between the active and blocking layer, which could contribute to the reduced long wavelength response. The field spike at the interface is the likely cause of breakdown at a lower bias than expected.
Date: February 6, 2002
Creator: Bandaru, Jordana; Beeman, Jeffrey W.; Haller, Eugene E.; Samperi, Stacy & Haegel, Nancy M.
Partner: UNT Libraries Government Documents Department

Photoconductive Semiconductor Switch Technology for Short Pulse Electromagnetics and Lasers

Description: High gain photoconductive semiconductor switches (PCSS) are being used to produce high power electromagnetic pulses foc (1) compact, repetitive accelerators, (2) ultra-wide band impulse sources, (3) precision gas switch triggers, (4) optically-activated firesets, and (5) high power optical pulse generation and control. High power, sub-nanosecond optical pulses are used for active optical sensors such as compact optical radars and range-gated hallistic imaging systems. Following a brief introduction to high gain PCSS and its general applications, this paper will focus on PCSS for optical pulse generation and control. PCSS technology can be employed in three distinct approaches to optical pulse generation and control: (1) short pulse carrier injection to induce gain-switching in semiconductor lasers, (2) electro-optical Q-switching, and (3) optically activated Q-switching. The most significant PCSS issues for these applications are switch rise time, jitter, and longevity. This paper will describe both the requirements of these applications and the most recent results from PCSS technology. Experiments to understand and expand the limitations of high gain PCSS will also be described.
Date: August 5, 1999
Creator: Denison, Gary J.; Helgeson, Wesley D.; Hjalmarson, Harold P.; Loubriel, Guillermo M.; Mar, Alan; O'Malley, Martin W. et al.
Partner: UNT Libraries Government Documents Department

Recombination Parameters for Antimonide-Based Semiconductors using RF Photoreflection Techniques

Description: RF photoreflection measurements and PC-1D simulations have been used to evaluate bulk and surface recombination parameters in antimonide-based materials. PC-1D is used to simulate the photoconductivity response of antimonide-based substrates and doubly-capped epitaxial layers and also to determine how to extract the recombination parameters using experimental results. Excellent agreement has been obtained with a first-order model and test structure simulation when Shockley-Reed-Hall (SRH) recombination is the bulk recombination process. When radiative, Auger and surface recombination are included, the simulation results show good agreement with the model. RF photoreflection measurements and simulations using PC-1D are compatible with a radiative recombination coefficient (B) of approximately 5 x 10{sup -11} cm{sup 3}/s, Auger coefficient (C) {approx} 1.0 x 10{sup -28} cm{sup 6}/s and surface recombination velocity (SRV) {approx} 600 cm/s for 0.50-0.55 eV doubly-capped InGaAsSb material with GaSb capping layers using the experimentally determined active layer doping of 2 x 10{sup 17} cm{sup -3}. Photon recycling, neglected in the analysis and simulations presented, will affect the extracted recombination parameters to some extent.
Date: October 10, 2002
Creator: Kumar, R.J.; Borrego, J.M.; Dutta, P.S.; Gutmann, R.J.; Wang, C.A.; Martinelli, R.U. et al.
Partner: UNT Libraries Government Documents Department

ELECTRICAL PROPERTIES OF ORGANIC SOLIDS. I. KINETICS AND MECHANISMOF CONDUCTIVITY OF METAL-FREE PHTHALOCYANINE

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

ELECTRICAL PROPERTIES OF ORGANIC SOLIDS. II: EFFECTS OF ADDEDELECTRON ACCEPTOR ON METAL-FREE PHTHALOCYANINE

Description: The addition of ortho-chloranil to the surface of films of metal-free phthalocyanine has been found (a) to increase the dark conductivity of such films by as much as 10{sup 7}, (b) to increase the steady-state photoconductivity by as much as 10{sup 5}, and (c) to result in the formation of unpaired electrons whose concentration decreases reversibly as a result of illumination. These systems exhibit a light-induced polarization, the phthalocyanine layer becoming more positive with respect t o the ortho-chloranil layer. Kinetic studies demonstrate that, upon illumination, a single process (time constant = 40 seconds) results in the increase in conductivity, the decrease in unpaired spins, and the increase in polarization. The results are consistent with the following scheme. An electron transfer from phthalocyanine to ortho-chloranil occurs in the dark at room temperature, producing holes in the phthalocyanine layer and ortho-chloranil negative ion radicals (high conductivity, ESR signal). Illumination results in the transfer of an electron from an excited phthalocyanine molecule to the ortho-chloranil negative ion, producing further phthalocyanine holes and ortho-chloranil double-negative ion (increase in conductivity, increase in polarization, decrease in ESR signal). By equating spin concentration with charge - carrier concentration (phthalocyanine holes) it is possible to calculate a mobility of 10{sup -4} cm{sup 2}/volt/sec for holes in the phthalocyanine layer. By use of this value, a quantum yield of unity is calculated for the production of charge carriers in doped phthalocyanine. The experiments indicate a quantum yield of less than 10-1 for undoped phthalocyanine. The over-all results of adding a strong electron acceptor to a film of phthalocyanine are thus t o (a) produce charge carriers in the dark, (b) increase the quantum yield for production of charge carriers by light, and (c) increase charge-carrier lifetime.
Date: July 29, 1959
Creator: Kearns, David R.; Tollin, Gordon & Calvin, Melvin
Partner: UNT Libraries Government Documents Department

CORRELATIONS BETWEEN PHOTOINDUCED EPR AND PHOTOCONDUCTIVITY INTONE-THF SOLUTION CHARGE-TRANSFER COMPLEX

Description: Reversible photoinduced electron paramagnetic resonance (EPR) signals and photoconductivity were observed when a solution of tetracyancethylene (TCNE) in tetrahydrofuran (THF) was irradiated in the charge-transfer band of the complex formed between these two compounds. The eleven-line hyperfine structure of the EPR spectrum which was obtained demonstrated the presence of TCNE negative ion radical. The concentration of this radical was found to be directly proportional to the square root of the light intensity. Second order decay kinetics were followed when the light was shut off. Both the EPR signal and the photoconductivity rose initially as the square of the time. The latter portions of the growth curves could be fit to the latter portions of a hyperbolic tangential growth curve. From these data a reaction mechanism was proposed. The rate law dn/dt + kn{sup 2} = {alpha}L(1-e{sup -{beta}t}) = 0, where n = the concentration of radicals, t = the time, k, {alpha}, and {beta} are rate constants, and L = the light intensity, described both the photo-induced EPR and the photoconductivity within the limits of experimental accuracy.
Date: December 8, 1964
Creator: Ilten, David F. & Calvin, Melvin
Partner: UNT Libraries Government Documents Department

THE PHOTOVOLTAIC EFFECT AND PHOTOCONDUCTIVITY IN LAMINATED ORGANICSYSTEMS

Description: As a result of a wide variety of studies on photosynthesis in living plants and plant fragments, together with the development of photosensitive, photovoltaic junctions in inorganic crystals and the discovery and exploration of semiconduction in organic molecular substances, a suggestion has been made that the primary quantum conversion process in photosynthetic tissues involves the creation and separation of charge to opposite sides of an asymmetrically-constructed lamina, followed by the trapping of both the electrons and the holes which then lead to their respective chemical processes, namely reduction of carbon dioxide and oxidation of the water to oxygen. This has led us to study model systems as semiconductors with a view to creating an organic photovoltaic junction.
Date: September 1, 1958
Creator: Kearns, David & Calvin, Melvin
Partner: UNT Libraries Government Documents Department