摘要:

We have investigated the response of the flux-locked output of several high-TcSQUID magnetometers to small changes in temperature and for magnetic fields 0–30 &mgr;T. The temperature responseDT≡d&Fgr;S/dTis observed to be linear in the applied magnetic fieldBaand can be as large as500 m&Fgr;0/Kwhere&Fgr;Sis the flux through the SQUID loop and&Fgr;0is the flux quantum. Our measurements can be explained using a simple model that takes into account the geometry of a given device and is based on the idea thatDTis due to the temperature dependence of the superconducting penetration depth. Our results can be used to optimize device performance in applications where the noise of a device is dominated by ambient temperature fluctuations. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366519

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A device topology is proposed and analyzed which is based upon quantum wire components. The device controls the conductance between an input line and a pair of output lines by varying the phase shift around a quantum wire ring connecting the external nodes. The phase shift is controlled using a simple electron stub tuner. Switching characteristics with efficiencies greater than 95&percent; for certain geometries have been calculated. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366520

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The transport properties of electrons in a heterostructure high electron mobility transistor are analyzed by considering the capture and escape phenomena as scattering events in Monte Carlo simulation. Through the variation of the equivalent charge in time, the drain current of an AlInAs/InGaAs/InP quantum-well device with planar doping is deduced for given drain and gate voltages. We studied the energy distribution of carriers in the device and we compared the results to those obtained by other quantum mechanical treatment and the classical treatment of the confinement. We demonstrated that by using our capture procedure for the quantum mechanical treatment, carriers have higher energy values than in the case of classical treatment and therefore the current values are different by an amount of 30&percent;–40&percent;. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366521

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present a unified device model for single layer organic light emitting diodes (LEDs) which includes charge injection, transport, and space charge effects in the organic material. The model can describe both injection limited and space charge limited current flow and the transition between them. We specifically considered cases in which the energy barrier to injection of electrons is much larger than that for holes so that holes dominate the current flow in the device. Charge injection into the organic material occurs by thermionic emission and by tunneling. For Schottky energy barriers less than about 0.3–0.4 eV, for typical organic LED device parameters, the current flow is space charge limited and the electric field in the structure is highly nonuniform. For larger energy barriers the current flow is injection limited. In the injection limited regime, the net injected charge is relatively small, the electric field is nearly uniform, and space charge effects are not important. At smaller bias in the injection limited regime, thermionic emission is the dominant injection mechanism. For this case the thermionic emission injection current and a backward flowing interface recombination current, which is the time reversed process of thermionic emission, combine to establish a quasi-equilibrium carrier density. The quasi-equilibrium density is bias dependent because of image force lowering of the injection barrier. The net device current is determined by the drift of these carriers in the nearly constant electric field. The net device current is much smaller than either the thermionic emission or interface recombination current which nearly cancel. At higher bias, injection is dominated by tunneling. The bias at which tunneling exceeds thermionic emission depends on the size of the Schottky energy barrier. When tunneling is the dominant injection mechanism, a combination of tunneling injection current and the backflowing interface recombination current combine to establish the carrier density. We compare the model results with experimental measurements on devices fabricated using the electroluminescent conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] which by changing the contacts can show either injection limited behavior or space charge limited behavior. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366522

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Current–voltage, impedance, and transient conductance measurements have been carried out on indium-tin-oxide/poly(phenylene vinylene)/Al light emitting diodes. In these devices injection and transport is expected to be dominated by positive carriers. Fowler–Nordheim tunneling theory cannot account for the temperature dependence, the thickness dependence, or the current magnitude of the current–voltage characteristics. Space-charge limited current theory with an exponential distribution of traps is however in extremely good agreement with all of the recorded current–voltage results in the higher applied bias regime (approximately0.7⩽V/d⩽1.6×106 V cm−1). This gives a trap densityHtof5(±2)×1017 cm−3and the product of&mgr;NHOMOof between1014and5×1012 cm−1 V−1 s−1.AssumingNHOMOis1020 cm−3gives an effective positive carrier mobility between10−6and5×10−8 cm2 V−1 s−1.The characteristic energyEtof the exponential trap distribution is 0.15 eV at higher temperatures(190⩽T⩽290 K),but this decreases as the devices are cooled, indicating that the distribution is in fact a much steeper function of energy closer to the highest occupied molecular orbital (HOMO) levels. The current–voltage characteristics in the lower applied bias regime (approximatelyV/d⩽0.7×106 V cm−1) can be fitted to pure space-charge limited current flow with a temperature and field dependent mobility of Arrhnenius form with a mobility at 290 K close to the above values. IfNHOMOlies between1021and1019 cm−3,then the trap filled limit bias gives a mobility independent value ofHtof3(±1)×1017 cm−3.Capacitance–voltage measurements show that at zero bias the devices are fully depleted, and that the acceptor dopant densityNAmust be less than about1016 cm−3.The impedance results show that the devices can be modeled on a single, frequency independent, parallel resistor-capacitor circuit with a small series resistor. The variation of the resistor and capacitor in the parallel circuit with applied bias and temperature are consistent with the space-charge limited current theory with the same exponential trap distribution used to model the current–voltage characteristics. Initial results for transient conductance measurements are reported. The transients have decay times greater than 300 s and exhibit a power-law dependence with time. This is shown to be exactly the behavior expected for the decay of an exponential trap distribution. Measurements at higher temperatures(290⩾T⩾150 K)give anEtof 0.15 eV, in excellent agreement with that found from the current–voltage measurements. This value ofEtis exactly that found by similar analysis of the current–voltage characteristics in negative carrier dominated dialkoxy poly(phenylene vinylene) andMq3devices. It is proposed that this bulk transport dominated behavior is purely a consequence of hopping conduction through an approximately Gaussian density of states in which the deep sites act as traps. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366523

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The shape and stability of a floating liquid zone between two solids are analytically investigated using the principle of variation. The formulas for its shape and stability are obtained. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366524

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

N-doped 6H–SiC wafers have been annealed in hot hydrogen for two doping levels, corresponding ton-(n∼1017 cm−3)andn+-type(n∼1019 cm−3)material. Electron spin resonance shows little passivation of N by hydrogen inn-type material, where secondary-ion-mass spectroscopy shows better penetration of deuterium. Both observations are accounted for in terms of Fermi-level control of the hydrogen charge state. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366525

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Degradation ofHgBa2CaCu2Oxsuperconducting thin films witha- andc-axis oriented normal to the substrates in water has been studied with electrical measurement, x-ray diffractometry, scanning electron microscopy, and energy dispersive x-ray spectrometry. We observed the different degradation behavior ofa- andc-axis orientedHgBa2CaCu2Oxthin films normal to substrates in water immersion experiments. Thea-axis oriented films are much more stable than thec-axis oriented films in water. After a 60 min immersion, the zero-resistance temperature of thea-axis oriented film is still above 110 K, but the zero resistance is not observed in thec-axis oriented film in water even for 5 min. The intensity of(00l)peaks is suppressed at one order of magnitude faster than that of(l00)peaks as the total immersion time increase. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366526

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effect of Zr additions on the microstructure and magnetic properties of melt-spunSm2Fe(15.5−x)ZrxAl1.5C1.5alloys has been systematically studied. The additions of Zr toSm2(Fe,Al,Zr)17C1.5alloys result in the precipitation of a ZrC compound within intergranular regions. It is found that the Zr addition is helpful in eliminating free iron dendrites, and effective on inhibiting grain growth during annealing. The x-ray diffraction analysis and transmission electron microscopy observation show that annealed ribbons with the Zr addition consist of a multiphase of the 2:17-type carbide, &agr;-Fe and ZrC compound with an average grain size of about 25–35 nm. A remanence of up to 55 emu/g along with a coercivity of up to 11.6 kOe is obtained forSm2Fe15.3Zr0.2Al1.5C1.5ribbons annealed at 750 °C for 30 min. It is suggested that the high coercivity and remanence may be attributed to the Zr addition. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366527

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have investigated the visible-region transmission spectra of light propagating in directions which lie outside the array plane of a two-dimensional photonic crystal. These photonic crystals consist of arrays of glass rods inside a glass matrix, with periodicities comparable to the wavelengths of interest. When the propagation direction is within the array plane, attenuations corresponding to the Brillouin-zone boundaries are observed, while for out-of-plane propagation all attenuation features shift to shorter wavelengths. Interesting polarization-dependent effects appear for out-of-plane propagation.

ISSN:0021-8979    

DOI:10.1063/1.366534

出版商:AIP    

年代:1997

数据来源: AIP