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  • 1
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    MoS2 oxygen sensor with gate voltage stress induced performance enhancement (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-09-22
    Description: Two-dimensional (2D) materials have recently attracted wide attention and rapidly established themselves in various applications. In particular, 2D materials are regarded as promising building blocks for gas sensors due to their high surface-to-volume ratio, ease in miniaturization, and flexibility in enabling wearable electronics. Compared with other 2D materials, MoS 2 is particularly intriguing because it has been widely researched and exhibits semiconducting behavior. Here, we have fabricated MoS 2 resistor based O 2 sensors with a back gate configuration on a 285 nm SiO 2 /Si substrate. The effects of applying back gate voltage stress on O 2 sensing performance have been systematically investigated. With a positive gate voltage stress, the sensor response improves and the response is improved to 29.2% at O 2 partial pressure of 9.9 × 10 −5 millibars with a +40 V back-gate bias compared to 21.2% at O 2 partial pressure of 1.4 × 10 −4 millibars without back-gate bias; while under a negative gate voltage stress of −40 V, a fast and full recovery can be achieved at room temperature. In addition, a method in determining O 2 partial pressure with a detectability as low as 6.7 × 10 −7 millibars at a constant vacuum pressure is presented and its potential as a vacuum gauge is briefly discussed.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    In situ metalorganic chemical vapor deposition of Al2O3 on N-face GaN and evidence of polarity induced fixed charge (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-07-02
    Description: The in situ metalorganic chemical vapor deposition (MOCVD) of Al 2 O 3 dielectrics on N-face GaN is reported. The near-interface fixed charges are measured by using capacitance-voltage techniques on a metal-oxide-semiconductor (MOSCAP) structure, and the results are compared with those obtained on Ga-face MOSCAPs with the same in situ MOCVD Al 2 O 3 dielectrics. The influence of GaN polarity as well as other possible mechanisms on the formation of fixed charge are identified and discussed.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 3
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    On trapping mechanisms at oxide-traps in Al2O3/GaN metal-oxide-semiconductor capacitors (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-03-18
    Description: By means of combined current-voltage and capacitance-voltage sweep and transient measurements, we present the effects of forward-bias stress and charge trapping mechanisms at oxide traps in Al 2 O 3 /GaN metal-oxide-semiconductor capacitors grown in-situ by metalorganic chemical vapor deposition. Two main current-voltage regimes have been identified: a low-field regime characterized by low gate-current and low flat-band voltage instabilities, and a high-field regime triggered for oxide field greater than 3.3 MV/cm and characterized by the onset of parasitic leakage current and positive flat-band shift. In the low-voltage regime, gate current transients convey stress/relaxation kinetics based on a power-law, suggesting that tunneling trapping mechanisms occur at near-interface traps aligned with the GaN conduction-band minimum. In the high-voltage regime, devices experience parasitic conduction mechanisms and enhanced charge-trapping at oxide-traps revealed by very slow recovery transients.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 4
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    Field-angle and DC-bias dependence of spin-torque diode in giant magnetoresistive microstripe (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-11-08
    Description: The spin torque diode effect in all metal spintronic devices has been proposed as a microwave detector with a high power limit and resistivity to breakdown. The previous works have revealed the field-angle dependence of the rectified DC voltage ( V DC ) in the ferromagnetic stripe. The giant magnetoresistive (GMR) microstripe exhibits higher sensitivity compared with the ferromagnetic stripe. However, the influence of the magnetic field direction and bias current in the spin rectification of GMR microstripe is not yet reported. In this work, the angular dependence and bias dependence of resonant frequency ( f R ) and V DC are investigated. A macrospin model concerning the contribution of magnetic field, shape anisotropy, and unidirectional anisotropy is engaged to interpret the experimental data. f R exhibits a |sin  δ H | dependence on the in-plane field angle ( δ H ). V DC presents either |sin  δ H | or |sin2  δ H  cos  δ H | relation, depending on the magnitude of H ext . Optimized V DC of 24  μ V is achieved under 4 mT magnetic field applied at δ H  = 170°. Under out-of-plane magnetic field, f R shows a cos 2 θ H reliance on the polar angle ( θ H ), whereas V DC is sin  θ H dependent. The Oersted field of the DC bias current ( I DC ) modifies the effective field, resulting in shifted f R . Enhanced V DC with increasing I DC is attributed to the elevated contribution of spin-transfer torque. Maximum V DC of 35.2  μ V is achieved, corresponding to 47% increase compared with the optimized value under zero bias. Higher I DC also results in enlarged damping parameter in the free layer, resulting in increased linewidth in the spin torque diode spectra. This work experimentally and analytically reveals the angular dependence of f R and V DC in the GMR microstripe. The results further demonstrate a highly tunable f R and optimized V DC by bias current without the external magnetic field. GMR microstripe holds promise for application as a high-power, frequency-tunable microwave detector that works under small or zero magnetic field.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 5
    Electronic Resource
    Electronic Resource
    Surface-induced enhancement of internal structure in polymers and proteins (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8343-8351 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the conformations of chain molecules near interfaces by exhaustive simulation. We explore all the conformations accessible to a short isolated chain (16 monomers) on a 2-dimensional square lattice as a function of the distance of the center of mass of the chain from an interface. Our principal focus is on the "internal structure'' of the chain, certain simple patterns of intrachain contacts such as helices and sheets (planar zigzags). In the process of enumeration, we confirm the well-known result that chains near interfaces have fewer conformations than chains in the bulk. We also find that the persistence length increases, and the radius of gyration and end-to-end length become anisotropic as the chain approaches the interface. The main conclusion of this work is that chain molecules are predicted to often have enhanced amounts of internal structure when they are at or near interfaces. Steric constraints imposed by the interface are selective and exclusive, eliminating open conformations but not eliminating compact conformations such as helices and sheets. Therefore when a polymer, protein, or peptide chain is weakly attracted to an interface, internal structure should be induced or, if already present, it should be enhanced. In 2 dimensions, stronger attraction in some cases flattens the chain and obliterates this structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459317
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  • 6
    Electronic Resource
    Electronic Resource
    Piezoelectric coefficient of aluminum nitride and gallium nitride (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 5360-5363 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The piezoelectric coefficient d33 of aluminum nitride (AlN) and gallium nitride (GaN) thin films grown on silicon substrates by molecular beam epitaxy have been measured using a laser interferometer. X-ray diffraction reveals that the AlN and GaN films consist mainly of crystals with a hexagonal wurtzite structure. In order to grow epitaxial GaN films, an AlN film was first deposited on silicon as the buffer layer, so the d33 measurement for GaN was actually performed on GaN/AlN/Si multilayer systems. The relative permittivity and electrical resistivity of each constituent layer of the film and the potential drop across each layer were determined as a function of frequency. The potential drops were then used to calculate the piezoelectric coefficient d33 of GaN. After correcting for substrate clamping, d33 of AlN and GaN were found to be (5.1±0.1) and (3.1±0.1) pm V−1, respectively. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1317244
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  • 7
    Electronic Resource
    Electronic Resource
    Mode coupling in lead zirconate titanate/epoxy 1–3 piezocomposite rings (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 4122-4129 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Lead zirconate titanate (PZT)/epoxy 1–3 piezocomposite rings were fabricated with PZT volume fractions φ ranging from 0.82 to 0.94 and with a small epoxy width of 77 μm in order to investigate their resonance characteristics and to reveal the mode coupling. Four major resonance modes were observed, namely the coupled longitudinal-thickness fH and lateral fL1 and fL2 mode resonances of individual PZT elements inside the rings as well as the radial fR and wall-thickness fW mode resonances of the whole rings. No stopband resonances were observed in the frequency range of 1 to 10 MHz. fH was found to increase linearly with the decrease in element height while fL1 and fL2 remained constant. When the height and width of the elements became comparable, coupling of fH with fL1 and fL2 occurred. The observed fH, fL1, and fL2 for all samples agreed with those calculated by the mode-coupling theory. fR and fW were almost independent of the ring thickness but increased as φ increased. A guide of operating fH in the rings without causing mode coupling was presented to optimize the composite structure for transducer design. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1405139
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  • 8
    Electronic Resource
    Electronic Resource
    Taber vibration isolator for vacuum and cryogenic applications (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 2742-2750 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We present a procedure for the design and construction of a passive, multipole, mechanical high-stop vibration isolator. The isolator, consisting of a stack of metal disks connected by thin wires, attenuates frequencies in the kilohertz range, and is suited to both vacuum and cryogenic environments. We derive an approximate analytical model and compare its predictions for the frequencies of the normal modes to those of a finite element analysis. The analytical model is exact for the modes involving only motion along and rotation about the longitudinal axis, and it gives a good approximate description of the transverse modes. These results show that the high-frequency behavior of a multistage isolator is well characterized by the natural frequencies of a single stage. From the single-stage frequency formulas, we derive relationships among the various geometrical parameters of the isolator to guarantee equal attenuation in all degrees of freedom. We then derive expressions for the attenuation attainable with a given isolator length, and find that the most important limiting factor is the elastic limit of the spring wire material. For our application, which requires attenuations of 250 dB at 1 kHz, our model specifies a six-stage design using brass disks of approximately 2 cm in both radius and thickness, connected by 3 cm steel wires of diameters ranging from 25 to 75 μm. We describe the construction of this isolator in detail, and compare measurements of the natural frequencies of a single stage with calculations from the analytical model and the finite element package. For translations along and rotations about the longitudinal axes, all three results are in agreement to within 10% accuracy. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149839
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  • 9
    Electronic Resource
    Electronic Resource
    PVDF hydrophone and hydrophone array with side-looking elements (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 2376-2381 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Two single element hydrophones and an eight-element hydrophone array with side-looking polyvinylidene fluoride (PVDF) sensing elements are described. They were designed for measurements in which the ultrasonic beam insonifies the hydrophone from a direction perpendicular to the hydrophone axis. These hydrophones and the array have found various industrial and medical applications, especially for determining the ultrasound exposure of a patient in vivo. The construction and characterization of the hydrophones and the hydrophone array are reported and the directivities of the elements are compared with modeling results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1145209
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  • 10
    Electronic Resource
    Electronic Resource
    Electromechanical properties of 1-3 piezoelectric ceramic/piezoelectric polymer composites (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 484-489 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A study of the fabrication and electromechanical properties of piezoelectric 1-3 ceramic/polymer composites is reported in which the polymer phase, as well as the ceramic phase, is piezoelectric. Composites were prepared by both layering and dice-and-fill techniques. In the dice-and-fill technique, the major problem was preventing damage to the ceramic rods during the filling process. Both types of composite were found to present problems in the poling process. The electromechanical properties of the composites were measured and compared with modeling results. In general, it was found that the composite properties agreed with model predictions, with the exception of the piezoelectric coefficients, which were significantly lower than predicted.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357099
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