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  • 1
    Electronic Resource
    Electronic Resource
    Effect of the carbon acceptor concentration on the photoquenching and following enhancement of the photoacoustic signals of semi-insulating GaAs (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 7567-7574 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spectral and the time dependent piezoelectric photoacoustic (PPA) measurements under the continuous light illumination were carried out at 85 K to investigate nonradiative recombination processes involving EL2 defect levels in carbon concentration controlled and not intentionally doped semi-insulating (SI) GaAs. The decrease of the PPA signal due to the photoquenching effect of EL2 is observed for a short period of illumination in the photon energy region from 1.0 to 1.3 eV. Since almost all of the carbon acceptors are compensated by deep donor EL2 in SI GaAs, electron occupancy of EL2 level can be controlled by changing the carbon acceptor concentration. It is found that the photoquenching becomes drastic with increasing the carbon concentration. After fully photoquenching, the PPA signal increases again through a local minimum by the continuous light illumination and finally exceeds the initial value before illumination until the saturation level is reached. The deep donor level EL6 and its metastable state are proposed. EL6 level donates electrons to compensate a part of carbon acceptors after EL20 to EL2* transition is accomplished. The nonradiative recombination through this level generates the PPA signal. Since the PPA measurement can detect lower concentration of EL6 than that of EL2, the higher sensitivity of the PPA measurements than the optical absorption measurements is pointed out. The usefulness of the PPA technique for studying the nonradiative transition through deep levels in semiconductors is also suggested. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.365300
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  • 2
    Electronic Resource
    Electronic Resource
    Current–voltage characteristics of GaAs/AlAs double-barrier resonant tunneling diodes with a Si-planar-doped barrier (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 1801-1806 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Current–Voltage (I–V) characteristics of GaAs/AlAs double-barrier resonant tunneling diodes (RTDs) with a Si-planar-doped barrier are studied. At 4.2 K the peak-to-valley current ratio decreased when Si was introduced to the barrier next to the electrode from which electrons are emitted (emitter barrier). On the other hand, the ratio did not decrease considerably when Si was introduced to the other barrier, i.e., next to the electrode into which electrons flow (collector barrier). To investigate the origin of the decrease in the peak-to-valley ratio, we evaluated the transmission coefficient experimentally, and found that the resonance transmission widths for the double-barrier RTDs are almost the same. This means that the reduction in the peak-to-valley current ratio is not caused by the broadening of resonance transmission width. To explain why the peak-to-valley current ratio is degraded, we then introduced a new model in which an excess current flows through donor levels in the emitter barrier. To confirm this model, we investigated the I–V characteristics of GaAs/AlAs single-barrier diodes. We found an excess current, which is probably due to a tunneling current through the donor level created in the barrier, when the barrier is planar-doped. It is, therefore, concluded that the decrease in the peak-to-valley current ratio observed for the RTD with the doped emitter barrier is due to this excess current flowing through the doped emitter barrier. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360972
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  • 3
    Electronic Resource
    Electronic Resource
    Electroluminescent features of oligothiophenes dispersed as a dopant in host matrices (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5706-5711 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electroluminescent (EL) features of oligothiophenes dispersed as a dopant in the host matrices comprising tris(8-hydroxyquinoline)aluminum have been investigated. We chose the oligothiophenes that are substituted with phenyl or methyl groups at both the molecular terminals and possess various degrees of polymerization. Regarding both the phenyl- and methyl-substituted materials, the EL spectra are progressively red-shifted with the increasing number of thiophenes. Comparing these spectra with the photoluminescent spectra, we have found out that the EL arises mostly from the dopant molecules of the oligothiophenes. The emission is dominated by energy transfer from host matrices to the dopant molecules, leading to the enhanced device efficiencies. The specific effects of the phenyl- or methyl-substitution and the extension of the π-delocalization in the molecules are also discussed. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1465510
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  • 4
    Electronic Resource
    Electronic Resource
    Surface structure of niobium-dioxide overlayer on niobium(100) identified by scanning tunneling microscopy (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 4772-4776 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The surface structures of an oxide overlayer on Nb(100) single crystal formed by numerous cycles of Ar-ion sputtering and flash annealing at 1973 K in an ultra-high-vacuum chamber have been investigated by scanning tunneling microscopy. Flat terraces with a monatomic step height of Nb(100) are observed; where two orthogonal domains are alternately evident. A ladder-like (3×10) structure of Nb(100) with no long-range order is identified at atomic resolution in the domain on the terrace. The ladder-like (3×10) structure is interpreted to be a modulated NbO2(010) overlayer on the Nb(100) surface and an atomic structural model of the ladder-like (3×10) structure is proposed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1364649
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  • 5
    Electronic Resource
    Electronic Resource
    Investigation of deep levels in semi-insulating GaAs by means of the temperature change piezoelectric photo-thermal measurements (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 1751-1754 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The temperature variation of the piezoelectric photo-thermal (PPT) signal intensity of semi-insulating (SI) GaAs from 20 to 150 K was measured. Four peaks at 50, 70, 110, and 125 K were observed in the PPT signal. From the theoretical analysis based on the rate equations of electrons in the conduction band and deep levels, we concluded that the observed four peaks were due to the nonradiative electron transitions through EL6, EL7, EL15, and an unspecified deep level, respectively. Deep levels with extremely low concentration (1012–1015 cm−3) were clearly identified in SI GaAs by using the PPT method. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1336560
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  • 6
    Electronic Resource
    Electronic Resource
    Apparatus for materials testing in high-pressure hydrogen at low temperatures (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 4232-4235 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new type of apparatus for materials testing in high-pressure hydrogen up to 10 MPa at low temperatures down to 20 K was developed. The apparatus consists of the pressure vessel, cooled by a two-step external cooling method, and the hydraulic loading system. In the first cooling step the specimen in the pressure vessel is cooled to 84 K using a cooling tube between the coolant shroud filled with liquid nitrogen and the pressure vessel. In the second cooling step the specimen is cooled to 20 K using a helium refrigerator. The apparatus is designed to measure the actual load on the specimen with an external load cell irrespective of the axial load caused by high pressure and friction at the sliding seals. As an example of the application of the apparatus, the tensile properties of type 304L stainless steel in 1 MPa hydrogen and helium at low temperature are measured and hydrogen environment embrittlement of the steel is briefly discussed. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148337
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  • 7
    Electronic Resource
    Electronic Resource
    Piezoelectric photothermal study of AlxGa1−xAs epitaxial layer (x=0.22, 0.28, and 0.5) grown on semi-insulating GaAs substrate (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 4385-4391 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Piezoelectric photothermal measurements of an AlxGa1−xAs (x=0.22, 0.28, and 0.5) epitaxial layer grown on a GaAs substrate were carried out in the temperature range of 297 to 80 K. In addition to the band gap signal of the GaAs substrate, the direct transition gaps of AlGaAs were clearly observed in the higher photon energy region. It was experimentally confirmed that the temperature coefficient of the direct transition gap of AlxGa1−xAs alloy decreases with increasing Al mole fraction. By conducting the quenching light illumination measurements at 80 K we concluded that the photoexcited electrons in the AlGaAs epitaxial layer drifted under the influence of an electric field present at the AlGaAs/GaAs interface. The drifted electrons eventually recombined with the ionized EL2 centers in the SI GaAs substrate. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1407309
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  • 8
    Electronic Resource
    Electronic Resource
    Surface superstructure on fullerenes annealed at elevated temperatures (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 3763-3767 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Scanning tunneling microscopy images of fullerenes annealed at elevated temperatures are investigated. Three types of ringlike superstructures of ((square root of 3)×(square root of 3))R30° of graphite, the circular type, the U type, and the semicircular type, are observed on the curved surface of the fullerenes annealed in the temperature range from 2073 to 2673 K. A new modified interference model, in which both the α and β site atoms contribute to the tunneling current caused by the curved surface, is proposed. The model well simulates the three types of the ringlike superstructure of graphite and the continuous change from the triangular structure of graphite to the ringlike superstructure of graphite. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.372411
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  • 9
    Electronic Resource
    Electronic Resource
    The effect of interposing thin oxide layers on the photovoltaic properties of a-Si:H solar cells II between the middle n and p layers of a tandem-type cell (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 394-398 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The properties of the p/n junction of hydrogenated amorphous silicon (a-Si) were studied by use of some kinds of a-Si cells. It was concluded that the photoelectromotive force generated at the p/n junction reduces the photovoltage of the tandum-type solar cells, and the rate of the electron-hole recombination lowers their fill factor. However, by interposing a 2-nm-thick TiO2 layer between p and n layers of the tandem-type solar cells, the open-circuit photovoltage was raised from 1.50 to 1.64 V, the fill factor from 0.705 to 0.775, and the energy conversion efficiency was improved ∼10%. Among the metal oxides examined i.e., V2O5, TiO, TiOx, TiO2, NiO, WO3, ITO, Fe2O3, and SiO2, the TiOx (x=1.7) layer showed the best result. In addition, it was observed that the thickness of the p and n layers, which was necessary to form the p/n junction, could be reduced by interposing a metal-oxide layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341440
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  • 10
    Electronic Resource
    Electronic Resource
    Piezoelectric detection of the photoacoustic signals of n-type GaAs single crystals (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 2408-2413 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Piezoelectric photoacoustic (PA) measurements on liquid-encapsulated-Czochralski-grown n-GaAs were carried out at room temperature. A continuous broad band below 1.35 eV and a peak at 1.383 eV were observed in the PA amplitude spectra. By comparing with the optical-absorption spectra, it is concluded that the broad band is due to the electron transition involving the EL2 deep-lying defect levels. For the observed peak at 1.383 eV, the origin is considered to be dislocation related. The possibility that this peak is an apparent one expected from the proposed models for the PA signal generation is denied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351098
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