58 Matching Results

Search Results

Advanced search parameters have been applied.

Anomalously high photocurrents in nanostructured electrodes : a new local microchip power source.

Description: An increase in photocurrent has been observed at silicon electrodes coated with nanostructured porous silica films as compared to bare, unmodified silicon. Ultimately, to utilize this effect in devices such as sensors or microchip power supplies, the physical phenomena behind this observation need to be well characterized. To this end, Electrochemical Impedance Spectroscopy (EIS) was used to characterize the effect of surfactant-templated mesoporous silica films deposited onto silicon electrodes on the electrical properties of the electrode space-charge region in an aqueous electrolyte solution, as the electrical properties of this space-charge region are responsible for the photobehavior of semiconductor devices. A significant shift in apparent flat-band potential was observed for electrodes modified with the silica film when compared to bare electrodes; the reliability of this data is suspect, however, due to contributions from surface states to the overall capacitance of the system. To assist in the interpretation of this EIS data, a series of measurements at Pt electrodes was performed with the hope of decoupling electrode and film contributions from the EIS spectra. Surprisingly, the frequency-dependent impedance data for Pt electrodes coated with a surfactant-templated film was nearly identical to that observed for bare Pt electrodes, indicating that the mesoporous film had little effect on the transport of small electrolyte ions to the electrode surface. Pore-blocking agents (tetraalkylammonium salts) were not observed to inhibit this transport process. However, untemplated (non-porous) silica films dramatically increased film resistance, indicating that our EIS data for the Pt electrodes is reliable. Overall, our preliminary conclusion is that a shift in electrical properties in the space-charge region induced by the presence of a porous silica film is responsible for the increase in observed photocurrent.
Date: February 1, 2004
Creator: Hughes, Robert Clark; Dunphy, Darren Robert; Brinker, C. Jeffrey & Brozik, Susan Marie
Partner: UNT Libraries Government Documents Department

Device Physics of Thin-Film Polycrystalline Cells and Modules; Final Subcontract Report; 6 December 1993-15 March 1998

Description: This report describes work performed under this subcontract by Colorado State University (CSU). The results of the subcontract effort included progress in understanding CdTe and Cu(In1-xGax)Se2-based solar cells, in developing additional measurement and analysis techniques at the module level, and in strengthening collaboration within the thin-film polycrystalline solar-cell community. A major part of the CdTe work consisted of elevated-temperature stress tests to determine fabrication and operation conditions that minimize the possibility of long-term performance changes. Other CdTe studies included analysis of the back-contact junction, complete photon accounting, and the tradeoff with thin CdS between photocurrent gain and voltage loss. The Cu(In1-xGax)Se2 studies included work on the role of sodium in enhancing performance, the conditions under which conduction-band offsets affect cell performance, the transient effects of cycling between light and dark conditions, and detailed analysis of several individual series of cells. One aspect of thin-film module analysis has been addressing the differences in approach needed for relatively large individual cells made without grids. Most work, however, focused on analysis of laser-scanning data, including defect signatures, photocurrent/shunting separation, and the effects of forward bias or high-intensity light. Collaborations with other laboratories continued on an individual basis, and starting in 1994, collaboration was through the national R&D photovoltaic teams. CSU has been heavily involved in the structure and logistics of both the CdTe and CIS teams, as well as making frequent technical contributions in both areas.
Date: May 3, 1999
Creator: Sites, J. R. (Department of Physics, Colorado State University, Ft. Collins, Colorado)
Partner: UNT Libraries Government Documents Department

Investigation of Deep Levels in GaInNas

Description: This paper presents and discusses the first Deep-Level transient spectroscopy (DLTS) data obtained from measurements carried out on both Schottky barriers and homojunction devices of GaInNAs. The effect of N and In doping on the electrical properties of the GaNInAs devices, which results in structural defects and interface states, has been investigated. Moreover, the location and densities of deep levels related to the presence of N, In, and N+In are identified and correlated with the device performance. The data confirmed that the presence of N alone creates a high density of shallow hole traps related to the N atom and structural defects in the device. Doping by In, if present alone, also creates low-density deep traps (related to the In atom and structural defects) and extremely deep interface states. On the other hand, the co-presence of In and N eliminates both the interface states and levels related to structural defects. However, the device still has a high density of the shallow and deep traps that are responsible for the photocurrent loss in the GaNInAs device, together with the possible short diffusion length.
Date: November 12, 1998
Creator: Abulfotuh, F.; Balcioglu, A.; Friedman, D.; Geisz, J. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

An image sensor capable of detecting nano-ampere transient signals with strong background illumination

Description: A readout detector integrated circuit (IC) has been developed which is capable of detecting nano-ampere photo-current signals of interest in a high (micro-ampere) background illumination or DC noise level (SNR=92dB). The readout detector sensor IC processes transient signals of interest from a separate photodiode array chip. Low noise signal conditioning, filtering, and signal thresholding implement smart sensor detection of only ``active pixels.`` This detector circuit can also be used to perform signal conditioning for other sensor applications that require detection of very small signals in a high background noise environment.
Date: October 1, 1995
Creator: Chu, D.D.; Dixon, D.A. & Thelen, D.C. Jr.
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

A new time-dependent analytic model for radiation-induced photocurrent in finite 1D epitaxial diodes.

Description: Photocurrent generated by ionizing radiation represents a threat to microelectronics in radiation environments. Circuit simulation tools such as SPICE [1] can be used to analyze these threats, and typically rely on compact models for individual electrical components such as transistors and diodes. Compact models consist of a handful of differential and/or algebraic equations, and are derived by making simplifying assumptions to any of the many semiconductor transport equations. Historically, many photocurrent compact models have suffered from accuracy issues due to the use of qualitative approximation, rather than mathematically correct solutions to the ambipolar diffusion equation. A practical consequence of this inaccuracy is that a given model calibration is trustworthy over only a narrow range of operating conditions. This report describes work to produce improved compact models for photocurrent. Specifically, an analytic model is developed for epitaxial diode structures that have a highly doped subcollector. The analytic model is compared with both numerical TCAD calculations, as well as the compact model described in reference [2]. The new analytic model compares well against TCAD over a wide range of operating conditions, and is shown to be superior to the compact model from reference [2].
Date: April 1, 2012
Creator: Verley, Jason C.; Axness, Carl L.; Hembree, Charles Edward; Keiter, Eric Richard & Kerr, Bert (New Mexico Institute of Mining and Technology, Socorro, NM)
Partner: UNT Libraries Government Documents Department

Measurements of High-Field THz Induced Photocurrents in Semiconductors

Description: THz pulses have provided a useful tool for probing, with time resolution, the free carriers in a system. The development of methods to produce intense THz radiation has been slow since spectroscopists and condensed matter physicists first began probing materials with THz pulses. We have developed a method for producing intense ultra-short THz pulses, which have full width half maximum of 300 fs - approximately a half cycle of THz radiation. These intense half cycle pulses (HCPs) allow us to use THz radiation not only as a probe of the free carriers in a system but also as a source of excitation to alter a system in some way. In particular, HPCs perturb free carriers considerably in short time scales but show minimal effect to individual free carriers over long time. By exposing the semiconductor indium antimonide (InSb) to our intense THz HCP radiation, we have observed non-linear optical effects which suggest the generation of new free carriers by below band-gap THz photons. This generation of free carriers appears to be caused by an avalanche multiplication process, which should amplify the number of free carriers already in the system and then induce a current in the timescale of our THz pulse. This amplification on such a short timescale suggests the possibility of an ultra-fast detector of weak above band-gap radiation. We constructed a device which detects these currents by painting an electrode structure on the surface of the semiconductor. The currents induced across the electrodes by this avalanche multiplication process were measured and compared with other measurements of this non-linear optical process. We successfully measured THz induced currents in InSb, which indicate promise towards the development of an ultra-fast detector, and we gain insight into a possible physical explanation of the THz induced free carriers we observe in InSb.
Date: November 7, 2007
Creator: Wiczer, Michael & /Illinois U., Urbana
Partner: UNT Libraries Government Documents Department

PHOTOVOLTAIC PROPERTIES OF AU-MEROCYANINE-TiO2 SANDWICH CELLS. I. DARK ELECTRICAL PROPERTIES AND TRANSIENT EFFECT

Description: The electrical properties of thin films (200-3000 {angstrom}) of merocyanine photosensitizing dye sandwiched between a TiO{sub 2} single crystal doped n type and a thin (200 {angstrom}) Au metal layer has been studied. Dark current voltage measurements revealed that the current is space-charge limited at high current densities with an electron trapping density of -10{sup 17} cm{sup -3}. This was determined by using TiO{sub 2} as an electron injecting contact. Interpretation of the kinetics of rise and decay of the photocurrent suggests that the mobility of holes, the majority carriers in merocyanine, is dependent on traps, the dominant trapping level having a depth of 0.11 eV. The decay of the photocurrent is monomolecular at short times and dominated by bimolecular recombination kinetics for long times of the order of seconds. The high series resistance in the merocyanine prevents any band bending in the TiO{sub 2}, as the entire built-in voltage in the junction falls across the merocyanine film. This is supported by capacitance voltage data showing a complete absence of mobile charge carriers in the junction region.
Date: July 1, 1980
Creator: Skotheim, T.; Yang, J.-M.; Otvos, J. & Klein, M.P.
Partner: UNT Libraries Government Documents Department

Electrolytic production of hydrogen utilizing photovoltaic cells

Description: Hydrogen has the potential to serve as both an energy storage means and an energy carrier in renewable energy systems. When renewable energy sources such as solar or wind power are used to produce electrical power, the output can vary depending on weather conditions. By using renewable sources to produce hydrogen, a fuel which can be stored and transported, a reliable and continuously available energy supply with a predictable long-term average output is created. Electrolysis is one method of converting renewable energy into hydrogen fuel. In this experiment we examine the use of an electrolyzer based on polymer-electrolyte membrane technology to separate water into hydrogen and oxygen. The oxygen is vented to the atmosphere and the hydrogen is stored in a small pressure vessel.
Date: October 1, 1996
Creator: Daugherty, M.A.
Partner: UNT Libraries Government Documents Department

Measurement techniques for single junction thermophotovoltaic cells

Description: Several measurement systems and techniques for the electrical and thermal characterization of thermophotovoltaic (TPV) cells are discussed. One computer controlled system measures the quantum efficiency of cells from 0.8 to 2.6 microns. A probe resistor is used to account for cells with low shunt resistances. In the second system, a production-style robot provides automated measurements of I-V characteristics under dark, blackbody, and flashed illumination conditions. The system measures the length and width of each cell, and calculates the open circuit voltage, short circuit current, fill factor, and maximum power for each cell. The mean and standard deviation of the measured parameters are also computed. The third system measures the overall cell efficiency by a calorimetric technique. Heat losses due to radiation, conduction, and convection are factored into the analysis method.
Date: October 1, 1998
Creator: Danielson, L.R.; Parrington, J.R.; Charache, G.W.; Nichols, G.J. & Depoy, D.M.
Partner: UNT Libraries Government Documents Department

Experimental study of the factors governing the Staebler-Wronski photodegradation effect in a-Si:H solar cells. Annual subcontract report 1 April 1995--30 June 1996

Description: This report describes continuing experiments on electroluminescence (EL), field profile, and H-microstructure studies of a-Si:H-based solar cells and materials. By using EL spectroscopy, we observed that both the band-tail width and the defect energy distribution are narrowed by H-dilution. We demonstrated the existence of the `fast` and `slow` defects in the cell performance and identified their energy positions as a {approx}0.9 eV and a {approx}0.75 eV defect EL band. Our results also reinforced the notion that H-dilution eliminates the microstructure that causes the creation of `slow` defects and hence stabilizes rapidly under light illumination. We demonstrated that the internal-electric field profile of a-Si:H p-i-n structures can be measured by the transient-null-current method. For the first time, hot-wire-deposited a-Si:H films were characterized by {sup 1}H nuclear magnetic resonance. Surprisingly, about 90 percent of the H atoms give rise to the 50-kHz line, and only a very small percentage of the H atoms give rise to the 3-kHz-narrower resonance line, which suggests that H-bonding in hot-wire films is very different from that in a-Si:H produced by plasma-enhanced chemical vapor deposition.
Date: October 1, 1996
Creator: Han, D.
Partner: UNT Libraries Government Documents Department

Optical and electrical properties of III-V nitride wide bandgap semiconductors. Annual report, April 1, 1997--May 31, 1998

Description: The objectives of this project were to investigate the optical and electrical properties of III-nitride wide bandgap semiconductors (GaN, InGaN, AlGaN) and quantum wells, to understand the fundamental optical transitions and impurity properties of these materials, to study the physics components of GaN-based devices, and to provide input for new approaches toward the improvement of materials quality and the optimization of device design. We were the first group to employ transport measurement techniques on the persistent photoconductivity (PPC) state to study the impurity properties of III-nitrides. We were also one of the few research groups m in the world to employ picosecond time-resolved photoluminescence (PL) measurement technique to study mechanisms of optical transitions, LED emission, and lasing m in GaN materials. During this funding period, we have investigated a variety of GaN samples and structures grown by MBE as well as by MOCVD. We have also made a significant progress in MOCVD GaN materials growth. This report briefly discusses the following accomplishments: effects of deep level impurities in the AlGaN/GaN heterostructures; materials characterization of III-nitrides alloys; optical studies of III-nitride epilayers and quantum wells; fabrication and optical studies of III-nitride microdisk arrays; and materials growth by MOCVD.
Date: June 1, 1998
Partner: UNT Libraries Government Documents Department

Experimental investigation of dynamic pressure in a cryosorbing beam tube exposed to synchrotron radiation

Description: Results of photodesorption experiments on a 4.2-K beam tube irradiated with synchrotron radiation from the VEPP-2M storage ring are being reported. The experiments have been performed on SSC1 and SSC2 beamlines. Synchrotron radiation parameters of the SSC1 beamline are the same as the SSCL 20 TeV proton collider; critical energy = 284 eV, photon intensity 1{center_dot}10{sup 16} photons/m/s. Photon intensity of the SSC2 beamline is eight times higher than intensity of the SSC1 beamline. We have used two experimental configurations to observe the density increase due to: (1) photodesorption of tightly bound molecules not previously desorbed and (2) photodesorption of weakly bound cryosorbed molecules. The two configurations used were a simple 4.2-K beam tube and a 4.2-K tube with a coaxial perforated liner. The photo-desorption coefficient of tightly bound H{sup 2} measured on the SSC1 beamline was observed to decrease monotonically with photon exposure, reaching {eta}4{center_dot}10{sup {minus}4} molecules per photon at the end of exposure ({approximately}1{center_dot}10{sup 22} photons/m). The same experiment on the SSC2 beamline gave a similar result at photon dose 3.5{center_dot}10{sup 22} photons/m. The photodesorption coefficient of cryosorbed H{sub 2} increased with increasing H{sup 2} surface density, reaching {eta}`{sigma}{sub w}{approximately}7 molecules/photon at one monolayer surface density (s{sub m}{approximately}3{center_dot}10{sup 15} H{sub 2}/cm{sup 2}), where {sigma}{sub w} is the sticking coefficient. The liner was shown to effectively shield cryosorbed molecules from synchrotron radiation.
Date: June 1, 1994
Creator: Anashin, V.V.; Malyshev, O.B.; Osipov, V.N.; Maslennikov, I.L. & Turner, W.C.
Partner: UNT Libraries Government Documents Department

Photoconductivity and Non-Exponential Relaxation at Insulating LaAlO3/SrTiO3 Interfaces

Description: LaAlO{sub 3} is grown on SrO terminated (100) SrTiO{sub 3}, and (110) SrTiO{sub 3}, producing insulating heterointerfaces without light. Photocurrent spectroscopy at low temperatures reveals a broad distribution of interface states between 2 eV and 2.7 eV at both interfaces, with a higher density in the (110) case concomitant with relatively shallow traps. The photocurrent relaxation can be well fitted by a stretched exponential form, confirming energetically distributed electron traps. Photo-carrier lifetimes are larger than a few hundred seconds for optical excitation approaching the SrTiO{sub 3} band-gap energy, providing the opportunity to study transient light-induced properties at low temperatures.
Date: August 17, 2012
Creator: Kim, Minu; Bell, C.; Hikita, Y.; Kozuka, Y.; Kim, B.G. & Hwang, H.Y.
Partner: UNT Libraries Government Documents Department

Analytic 1D pn junction diode photocurrent solutions following ionizing radiation and including time-dependent changes in the carrier lifetime.

Description: Circuit simulation tools (e.g., SPICE) have become invaluable in the development and design of electronic circuits in radiation environments. These codes are often employed to study the effect of many thousands of devices under transient current conditions. Device-scale simulation tools (e.g., MEDICI) are commonly used in the design of individual semiconductor components, but require computing resources that make their incorporation into a circuit code impossible for large-scale circuits. Analytic solutions to the ambipolar diffusion equation, an approximation to the carrier transport equations, may be used to characterize the transient currents at nodes within a circuit simulator. We present new transient 1D excess carrier density and photocurrent density solutions to the ambipolar diffusion equation for low-level radiation pulses that take into account a finite device geometry, ohmic fields outside the depleted region, and an arbitrary change in the carrier lifetime due to neutron irradiation or other effects. The solutions are specifically evaluated for the case of an abrupt change in the carrier lifetime during or after, a step, square, or piecewise linear radiation pulse. Noting slow convergence of the raw Fourier series for certain parameter sets, we use closed-form formulas for some of the infinite sums to produce 'partial closed-form' solutions for the above three cases. These 'partial closed-form' solutions converge with only a few tens of terms, which enables efficient large-scale circuit simulations.
Date: April 1, 2011
Creator: Axness, Carl L.; Keiter, Eric Richard & Kerr, Bert (New Mexico Tech, Socorro, NM)
Partner: UNT Libraries Government Documents Department

Development of Novel Nanocrystal-based Solar Cell to Exploit Multiple Exciton Generation: Cooperative Research and Development Final Report, CRADA Number CRD-07-00227

Description: The purpose of the project was to develop new design and fabrication techniques for NC solar cells with the goal of demonstrating enhanced photocurrent and efficiency by exploiting multiple exciton generation and to investigate multiple exciton generation and charge carrier dynamics in semiconductor NC films used in NC-based solar cells.
Date: August 1, 2010
Creator: Ellingson, R.
Partner: UNT Libraries Government Documents Department

Ultrahigh Efficiency Multiband Solar Cells Final Report forDirector's Innovation Initiative Project DII-2005-1221

Description: The unique properties of the semiconductor ZnTeO were explored and developed to make multiband solar cells. Like a multijunction cell, multiband solar cells use different energy gaps to convert the majority of the solar spectrum to electrical current while minimizing losses due to heating. Unlike a multijunction cell, this is accomplished within a single material in a multiband cell. ZnTe{sub 1-x}O{sub x} films with x up to 2% were synthesized and shown to have the requisite unique band structure (2 conduction bands) for multiband function. Prototype solar cells based on an n-type ZnTe{sub 1-x}O{sub x} multiband top layer and a p-type ZnTe substrate were fabricated. Contacts to the cell and the series resistance of the substrate were identified as challenges for good electrical performance. Both photovoltage and small photocurrents were demonstrated under AMO illumination. A second semiconductor system, GaN{sub x}As{sub 1-y-x}P{sub y}, was shown to have multiband function. This alloy system may have the greatest potential to realize the promise of high efficiency multiband solar cells because of the relatively advanced technology base that exists for the manufacturing of III-V-alloy-based IC and opto-electronic devices (including multijunction solar cells).
Date: March 29, 2006
Creator: Ager, Joel W., III; Walukiewicz, W. & Yu, Kin Man
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

Device physics of thin-film polycrystalline cells and modules. Annual subcontract report, December 6, 1993--December 5, 1994

Description: Progress has been made in several applications of device physics to thin-film polycrystalline cells and modules. At the cell level, results include a more quantitative separation of photon losses, the impact of second barriers on cell operation, and preliminary studies of how current-voltage curves are affected by band offsets. Module analysis includes the effects of the typical monolithic, series-connected cell geometry, analytical techniques when only the two module leads are accessible, and the impact of chopping frequency, local defects, and high-intensity beams on laser-scanning measurements.
Date: May 1, 1995
Creator: Sites, J.R.
Partner: UNT Libraries Government Documents Department

Interfacial recombination in In(Al)GaAsSb/GaSb thermophotovoltaic cells

Description: The authors have studied efficient p-on-n homo- (InGaAsSb/GaSb) and heterojunction (InGaAsSb/AlGaAsSb/GaSb) thermophotovoltaic (TPV) cells with respect to the recombination velocity at the cap-layer/emitter interface, S. In both cell types the open-circuit voltage, V{sub oc}, and the short-circuit current, J{sub ac}, have about the same sensitivity to S. The dark current, J{sub 0}, is the most sensitive of all. An examination of the essential factors in the one-dimensional minority-carrier diffusion model shows that under short-circuit conditions, photogenerated electrons diffuse rapidly away from the interface to the junction, whereas under open-circuit conditions, they remain in the emitter for a much longer bulk-recombination time, and therefore, they are more likely to recombine at the interface. A factor of 2.2 increase in S from 5 to 11 {times} 10{sup 4} cm/s produces a 25-mV decrease in V{sub oc}, a 12-% decrease in J{sub ac} or quantum efficiency, and a factor of two increase in J{sub 0}. This work points out the critical importance of interfacial recombination even in efficient TPV cells.
Date: October 1, 1998
Creator: Khalfin, V.B.; Garbuzov, D.Z.; Lee, H.; Taylor, G.C.; Morris, N.; Martinelli, R.U. et al.
Partner: UNT Libraries Government Documents Department

Fabrication and characterization of GaSb based thermophotovoltaic cells using Zn diffusion from a doped spin-on glass source

Description: The GaInSb material system is attractive for application in thermophotovoltaic (TPV) cells since its band gap can be tuned to match the radiation of the emitter. At present, most of the TPV cells are fabricated using epitaxial layers and hence are expensive. To reduce the cost, Zn diffusion using elemental vapors in a semi-closed diffusion system is being pursued by several laboratories. In this paper, the authors present studies carried out on Zn diffusion into n-type (Te-doped) GaSb substrates in an open tube diffusion furnace. The dopant precursor was a 2,000 {angstrom} thick, zinc doped spin-on glass. The diffusion was carried out at temperatures ranging from 550 to 600 C, for times from 1 to 10 hours. The diffused layers were characterized by Hall measurements using step-and-repeat etching by anodic oxidation, secondary ion mass spectrometry (SIMS) measurements and TPV device fabrication. For diffusion carried out at 600 C, the junction depth was 0.3 {micro}m, and the hole concentration near the surface was 5 {times} 10{sup 19}/cm{sup 3}. The external quantum efficiency, measured without any anti-reflection coating, of the TPV cells fabricated using mesa-etching had a maximum value of 38%. Masked diffusion was also carried out by opening windows in a Si{sub 3}N{sub 4} coated, GaSb wafer. TPV cells fabricated on these structures had similar quantum efficiency, but lower dark current.
Date: June 1, 1998
Creator: Dakshinamurthy, S.; Shetty, S.; Bhat, I.; Hitchcock, C.; Gutmann, R.; Charache, G. et al.
Partner: UNT Libraries Government Documents Department

Optical interconnections to focal plane arrays

Description: The authors have successfully demonstrated an optical data interconnection from the output of a focal plane array to the downstream data acquisition electronics. The demonstrated approach included a continuous wave laser beam directed at a multiple quantum well reflectance modulator connected to the focal plane array analog output. The output waveform from the optical interconnect was observed on an oscilloscope to be a replica of the input signal. They fed the output of the optical data link to the same data acquisition system used to characterize focal plane array performance. Measurements of the signal to noise ratio at the input and output of the optical interconnection showed that the signal to noise ratio was reduced by a factor of 10 or more. Analysis of the noise and link gain showed that the primary contributors to the additional noise were laser intensity noise and photodetector receiver noise. Subsequent efforts should be able to reduce these noise sources considerably and should result in substantially improved signal to noise performance. They also observed significant photocurrent generation in the reflectance modulator that imposes a current load on the focal plane array output amplifier. This current loading is an issue with the demonstrated approach because it tends to negate the power saving feature of the reflectance modulator interconnection concept.
Date: November 1, 2000
Creator: Rienstra, J.L. & Hinckley, M.K.
Partner: UNT Libraries Government Documents Department

Application of a-Si:H radiation detectors in medical imaging

Description: Monte Carlo simulations of a proposed a-Si:H-based current-integrating gamma camera were performed. The analysis showed that the intrinsic resolution of such a camera was 1 {approximately} 2.5 mm, which is somewhat better than that of a conventional gamma camera, and that the greater blurring, due to the detection of scattered {gamma}-rays, could be reduced considerably by image restoration techniques. This proposed gamma camera would be useful for imaging shallow organs such as the thyroid. Prototype charge-storage a-Si:H pixel detectors for such a camera were designed, constructed and tested. The detectors could store signal charge as long as 5 min at {minus}26C. The thermal generation current in reverse biased a-Si:H p-i-n photodetectors was investigated, and the Poole-Frenkel effect was found to be the most significant source of the thermal generation current. Based on the Poole-Frenkel effect, voltage- and time-dependent thermal generation current was modeled. Using the model, the operating conditions of the proposed a-Si:H gamma camera, such as the operating temperature, the operating bias and the {gamma}-scan period, could be predicted. The transient photoconductive gain mechanism in various a-Si:H devices was investigated for applications in digital radiography. Using the a-Si:H photoconductors in n-i-n configuration in pixel arrays, enhancement in signal collection (more than 200 times higher signal level) can be achieved in digital radiography, compared to the ordinary p-i-n type a-Si:H x-ray imaging arrays.
Date: June 1, 1995
Creator: Lee, Hyoung-Koo
Partner: UNT Libraries Government Documents Department