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  • Electronic Resource  (20)
  • 1995-1999  (20)
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  • Electronic Resource  (20)
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  • 1
    Electronic Resource
    Electronic Resource
    A direct current superconducting quantum interference device gradiometer with a digital signal processor controlled flux-locked loop and comparison with a conventional analog feedback scheme (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 222-229 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A double-washer dc superconducting quantum interference device (SQUID) gradiometer with a flux-locked loop (FLL) based on a digital signal processor (DSP) has been developed for biomagnetic applications. All of the analog electronics in the conventional FLL are replaced and implemented by the DSP except for the low-noise field-effect transistor preamplifier at the front end of the signal recovery components. The DSP performs the signal demodulation by synchronously sampling the recovered signals and applying the appropriate full wave rectification. The signals are then integrated, filtered, and applied to the output. At 4.2 K, the white flux noise of the gradiometer measured in a DSP FLL mode is about 4μΦ0/(square root of)Hz and the noise at 1 Hz is 13 μΦ0/(square root of)Hz. The corresponding noise levels in the gradiometer operated by the conventional FLL are 1.8 and 3μΦ0/(square root of)Hz. The poorer system performance in the DSP FLL compared to the analog FLL is mainly caused by the ambient field noise and interference signals picked up through the connecting cables. Additional noise is also added to the overall noise floor by the instruments employed in the DSP system in the present prototype setup. Further improvement in the noise characteristics and the dynamic behavior of the DSP SQUID gradiometer is expected when a better configuration of DSP with the associated I/O devices is implemented. Additional improvements of the DSP programs are expected by incorporating higher-order integration, adaptive control, and noise reduction schemes. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146575
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  • 2
    Electronic Resource
    Electronic Resource
    Observation of room temperature surface-emitting stimulated emission from GaN:Ge by optical pumping (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 6544-6546 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optically pumped surface-emitting stimulated emission at room temperature was observed from GaN:Ge grown by metalorganic chemical vapor deposition. The sample was optically pumped perpendicularly on the top surface while the stimulated emission was collected from the back colinearly with the pump beam. The cavity was formed by the GaN/air and GaN/sapphire interfaces without any other structure. The stimulated emission was gain guided by the pump beam. The threshold optical pump density for stimulated emission was approximately 2.8 MW/cm2 and the linewidth was 2.5 nm. The emission from GaN:Ge showed a redshift as the pump density increased. The comparison between theoretical calculations and experimental results suggested that many-body interactions can account well for the redshift. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363674
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  • 3
    Electronic Resource
    Electronic Resource
    Kinetics of photoconductivity in n-type GaN photodetector (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 3792-3794 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High-quality ultraviolet photoconductive detectors have been fabricated using GaN layers grown by low-pressure metalorganic chemical vapor deposition on (11⋅0) sapphire substrates. The spectral responsivity remained nearly constant for wavelengths from 200 to 365 nm and dropped sharply by almost three orders of magnitude for wavelengths longer than 365 nm. The kinetics of the photoconductivity have been studied by the measurements of the frequency-dependent photoresponse and photoconductivity decay. Strongly sublinear response and excitation-dependent response time have been observed even at relatively low excitation levels. This can be attributed to redistribution of the charge carriers with increased excitation level. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.115385
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  • 4
    Electronic Resource
    Electronic Resource
    Growth of AlxGa1−xN:Ge on sapphire and silicon substrates (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 1745-1747 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: AlxGa1−xN were grown on (00.1) sapphire and (111) silicon substrates in the whole composition range (0≤x≤1). The high optical quality of the epilayers was assessed by room-temperature optical absorption and photoluminescence measurements. Layers with higher Al composition are more resistive. Resistive AlxGa1−xN epilayers were successfully doped with Ge and free-electron concentration as high as 3×1019 cm−3 was achieved. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.115036
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  • 5
    Electronic Resource
    Electronic Resource
    Photovoltaic effects in GaN structures with p-n junctions (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 2028-2030 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Large-area GaN photovoltaic structures with p-n junctions have been fabricated using atmospheric pressure metalorganic chemical vapor deposition. The photovoltaic devices typically exhibit selective spectral characteristics with two narrow peaks of opposite polarity. This can be related to p-n junction connected back-to-back with a Schottky barrier. The shape of the spectral characteristic is dependent on the thickness of the n- and p-type regions. The diffusion length of holes in the n-type GaN region, estimated by theoretical modeling of the spectral response shape, was about 0.1 μm. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.114776
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  • 6
    Electronic Resource
    Electronic Resource
    High quality AIN and GaN epilayers grown on (00⋅1) sapphire, (100), and (111) silicon substrates (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 2958-2960 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The growth of high quality AlN and GaN thin films on basal plane sapphire, (100), and (111) silicon substrates is reported using low pressure metalorganic chemical vapor deposition. X-ray rocking curve linewidths of about 100 and 30 arcsec were obtained for AlN and GaN on sapphire, respectively. Room-temperature optical transmission and photoluminescence (of GaN) measurements confirmed the high quality of the films. The luminescence at 300 and 77 K of the GaN films grown on basal plane sapphire, (100), and (111) silicon was compared. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.114242
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  • 7
    Electronic Resource
    Electronic Resource
    High-speed, low-noise metal–semiconductor–metal ultraviolet photodetectors based on GaN (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 762-764 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present the fabrication and characterization of nonintentionally doped GaN and GaN:Mg Schottky metal–semiconductor–metal (MSM) photodetectors, grown on sapphire by metalorganic chemical vapor deposition. Low-leakage, Schottky contacts were made with Pt/Au. The devices are visible blind, with an ultraviolet/green contrast of about five orders of magnitude. The response times of the MSM devices were 〈10 ns and about 200 ns for GaN and GaN:Mg, respectively. The noise power spectral density remains below the background level of the system (10−24 A2/Hz) up to 5 V, for the undoped GaN MSM detector. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.123303
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  • 8
    Electronic Resource
    Electronic Resource
    Band-gap narrowing and potential fluctuation in Si-doped GaN (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 102-104 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We investigate the optical properties of two sets of Si-doped GaN epitaxial layers with different degree of compensation. The electron concentration dependence of the band-gap energy measured by photoluminescence is interpreted as band-gap narrowing effect and evaluated by a simple relation. The photoluminescence peak positions of heavily compensated samples are shifted downward with respect to those of moderately compensated samples, and the down shift becomes larger at higher electron density. Based on analysis of photoluminescence spectra, these prominent behaviors are accounted for by band-edge potential fluctuation associated with inhomogeneous residual impurities. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.122964
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  • 9
    Electronic Resource
    Electronic Resource
    Metalorganic chemical vapor deposition of monocrystalline GaN thin films on β-LiGaO2 substrates (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 2116-2118 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the metalorganic chemical vapor deposition growth and characterization of monocrystalline GaN thin films on β-LiGaO2 substrates. The influence of the growth temperature on the crystal quality was studied. The structural, electrical, and optical properties of the films were assessed through scanning electron microscopy, x-ray diffraction, Hall measurements, optical transmission, photoluminescence. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.116898
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  • 10
    Electronic Resource
    Electronic Resource
    High-quality visible-blind AlGaN p-i-n photodiodes (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 1171-1173 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the fabrication and characterization of AlxGa1−xN p-i-n photodiodes (0≤x≤0.15) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible rejection of six orders of magnitude with a cutoff wavelength that shifts from 365 to 338 nm. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances, the transient response becomes non-exponential, with a decay time longer than the RC constant. This behavior is justified by the strong frequency dependence of the device capacitance. By an admittance analysis, we conclude that speed is not limited by deep levels, but by substitutional Mg capture and emission time. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.123960
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