连续/脉冲激光再制造FeCrNiCu合金成形层温度场研究

采用非接触式红外高温测试仪对连续/脉冲激光成形两种模式下激光再制造FeCrNiCu合金成形层温度场进行分析,获取了熔池及热影响区温度场分布的一般规律,验证了脉冲激光工艺在控制热输入和成形形变以及降低熔池及热影响区温度方面的工艺优越性。结果表明:脉冲激光成形热影响区峰值温度为730.4~810.5℃,熔池峰值温度为998.7~1383.4℃,明显低于相同工艺下连续输出模式;脉冲激光成形层具有更快的升温及降温速率,利于形成细晶组织和获得良好的力学性能;实际成形实验也进一步验证脉冲激光工艺具有更小的热影响区范围。     

1.55 $mu$m InAs/InAlGaAs Quantum Dot DFB Lasers

For the fabrication of quantum dot (QD) distributed feedback (DFB) lasers, self-assembled InAs/InAlGaAs QDs were grown on the InP/InGaAs grating structures by a molecular beam epitaxy. Ridge-waveguide QD DFB lasers with a stripe width of 3 mum were fabricated. Single-mode lasing operation around the wavelength of 1.56 mum was successfully achieved under the pulsed and continuous-wave (CW) modes at room temperature. The lasing operation was also observed up to 70 degC under pulsed mode, which is the first observation on the single-mode lasing around 1.55 mum. The characteristic temperatures calculated from the temperature dependence of the threshold current density were 121.8 K from room temperature to 45 degC, which was reduced to 74.2 K from 45 degC to 70 degC.     

Large area double p–i–n heterostructure for signal multiplexing and demultiplexing in the visible range

Results on the use of a double a-SiC:H p–i–n heterostructure for signal multiplexing and demultiplexing applications in the visible range, are presented.Modulated monochromatic beams together (multiplexing mode), or a single polychromatic beam (demultiplexing mode) impinge in the device and are absorbed, accordingly to their wavelength, giving rise to a time and wavelength dependent electrical field modulation.Red, green and blue pulsed input channels are transmitted together, each one with a specific transmission rate. The combined optical signal is analyzed by reading out, under different applied voltages, the generated photocurrent. Results show that in the multiplexing mode the output signal is balanced by the wavelength and transmission rate of each input channel, keeping the memory of the incoming optical carriers. In the demultiplexing mode the photocurrent is controlled by the applied voltage allowing regaining the transmitted information. An electrical model gives insight into the device operation.     

Dynamics of the order parameter of superconducting aluminum films

The imaginary part of the pair-field susceptibility of dirty-limit, superconducting, aluminum films has been obtained both above and below the transition temperature by analyzing the I–V characteristic of asymmetric superconducting tunneling junctions. Results obtained above the transition temperature have been found to be consistent with a diffusive time-dependent Ginzburg-Landau equation. Below the transition temperature two modes have been found, one propagating and the other diffusing. The resonant frequency and damping constant of the propagating mode have been compared with the predictions of a phenomenological two-fluid hydrodynamic theory given by Bray and Schmidt. A somewhat more detailed comparison has been made with a microscopic theory given by Schön and Schmid in which the propagating and diffusing modes, respectively, are associated with transverse and longitudinal fluctuations of the order parameter. In both the microscopic and macroscopic theories the propagating mode is a high-frequency superconducting analog of second sound in that it consists of a counter motion of the normal and superfluid.Supported in part by the U.S. Energy Research and Development Administration under contract E(11-1) 1569 and by the Graduate School of the University of Minnesota.     

Thermoelectric charge imbalance in superconducting aluminum

The charge imbalance voltage produced in superconducting aluminum by the presence of a temperature gradient and an electric current has been studied over the temperature range 0.5–1.2 K. Measurements were obtained of the magnitude and temperature dependence of the charge imbalance voltage of seven samples, two of which contained magnetic impurities. The data are compared with recent theoretical models of the effect.Research sponsored by NSF grant DMR 78-09428.     

Sputtered YBCO bicrystal dc-SQUIDS

An analysis of YBa₂Cu₃O_{7 –} bicrystal dc-SQUIDs is reported. The devices have been fabricated by dc cylindrical magnetron sputtering from a single target. SQUIDs operating at the liquid nitrogen temperature have been obtained employing high-quality films fabricatedinsitu exhibiting a critical temperature as high as 89–90 K. Two different geometrical configurations for the loop inductance have been investigated. A good control of the transport parameters of the junctions, together with a reasonable correlation between the calculated inductance and the voltage response to an applied magnetic field, have been achieved. A study of the voltage response vs. the applied magnetic field as a function of both temperature and bias current has been carried out. SQUIDs working at temperatures as high as 87 K have been characterized.     

Polarization strategies to improve the emission of Si-based light sources emitting at 1.55 μm

We present a electroluminescence (EL) study of the Si-rich silicon oxide (SRSO) LEDs with and without Er³⁺ ions under different polarization schemes: direct current (DC) and pulsed voltage (PV). The power efficiency of the devices and their main optical limitations are presented. We show that under PV polarization scheme, the devices achieve one order of magnitude superior performance in comparison with DC. Time-resolved measurements have shown that this enhancement is met only for active layers in which annealing temperature is high enough (>1000 °C) for silicon nanocrystal (Si-nc) formation. Modeling of the system with rate equations has been done and excitation cross-sections for both Si-nc and Er³⁺ ions have been extracted.     

Nanofabrication of Self-assembled Molecular-scale Electronics

Single electron tunneling devices were made by combiningstandard electron beam lithography and the self-assembly ofchemically synthesized gold clusters. These clusters, withdiameters from 2 to 5 nm, were captured in a 5–10 nm gapbetween two gold electrodes. The gold particles as well as theelectrodes were covered with self-assembled monolayers (SAM)of organic molecules which served as tunnel barriers.Operating devices show a suppressed current due to the Coulombblockade of tunneling at room temperature. When cooled to4.2 K, a Coulomb staircase was observed. By applying a voltageto an oxidized aluminum gate beneath the electrodes and thetrapped gold cluster the current voltage characteristics weremodulated. Anomalous effects are observed such as constantcurrent plateaus whose positions are gate-voltage dependent.An electrodeposition method for gold has been used tofabricate gaps between electrodes smaller than 2 nm. A self-assembled monolayer was used successfully on the electrodes inorder to prevent the gold atoms from migrating on the surfacebetween the electrodes and thereby short-circuiting thejunction. The conductance of such a tunnel junction has beenmeasured and compared to the theory with good agreement. Fromthis comparison the capacitance of the junction was estimated,and we could use that value to calculate a rough estimation ofthe distance between the electrodes.     

Temperature and voltage sensings by mode‐mode interference in polarization‐maintaining optical fibers

Abstract

Two optic fiber sensing systems for temperature and voltage have been developed which utilize the mode‐mode interference of the two orthogonally polarized modes, HE^x _II and HE^y _II, in two commencal polarization‐maintaining fibers (bow‐tie and elliptical core fibers). A package of controlled programs in a Macintosh computer, which can record and process all related data automatically, is established for temperature sensing. The signal drifting problem in voltage sensings has been investigated, and the elimination of signal drifting is obtained by the phase tracking with direct current technology The agreement between the sensing results for temperature and dynamic voltage and those predicted by experimental principles is satisfactory, which confirms the validity of the developed sensing systems.     

Detection and high-precision positioning of liquid droplets using SAW systems

The capability to accurately handle liquids in small volumes is a key point for the development of lab-on-chip devices. In this paper, we investigate an application of surface acoustic waves (SAW) for positioning micro-droplets. A SAW device based on a 2 x 2 matrix of inter-digital transducers (IDTs) has been fabricated on a (YXI)/128 degrees LiNbO3 substrate, which implies displacement and detection in two dimensions of droplets atop a flat surface. Each IDT operates at a given frequency, allowing for an easy addressing of the active channel. Furthermore, very low cross-talk effects were observed as no frequency mixing arose in our device. Continuous as well as pulsed excitations of the IDTs have been studied, yielding, respectively, continuous and step-by-step droplet displacement modes. In addition, we also have used these two excitation types to control the velocity and the position of the droplets. We also have developed a theoretical analysis of the detection mode, which has been validated by experimental assessment.     

Pulse experiments on electroformed mim structures

Voltage pulses of amplitude from 0 to 10 V, width 1 μs to 10 ms and with a wide range of repetition frequencies have been applied to electroformed Al-SiO_x-Au devices and the pulsed current-voltage characteristics have been examined. The peak currents observed were up to 28 times larger than those obtained under d.c. bias conditions. It is argued that these results can be understood in terms of the filamentary model of conduction in electroformed devices but are in quantitative disagreement with the theory in its present form. Attempts to estimate the energy required for switching a device into a high impedance state were made using single 1 μs voltage pulses. These are compared with similar experiments reported in the literature. It is suggested that the present experiments yield the only unequivocal upper limit for the energy required to switch a device into a high impedance state.     

Efficiency and stability of polymer light-emitting diodes

The operation characteristics of polymer light-emitting diodes (PLEDs) are strongly dependent on materials, processing and the structure of the device. The device structure developed at Philips Research is presented together with some typical results for brightness, efficiency, response times and stability. The PLEDs typically operate at a voltage of 3–4 V for a brightness of 100 cd m^-2 and have an efficiency ranging from 2 cd A^-1 for orange emitting polymers (610 nm) up to 16 cd A^-1 for green emitting polymers (550 nm). The response time under conditions for display operation is determined by the charge carrier transport properties and amounts to 43 ns. Lifetimes of several thousand hours have been obtained for large orange emitting devices of 8 cm² for daylight visibility at room temperature.     

Semiconductor-superconductor hybrid electronics

This paper examines the feasibility and potential for applications of intimately combining semiconductor and superconductor devices at circuit and system levels. The focus is mainly on the temperature range 27–77 K. One of the main issues is the heat produced by the semiconductor devices, since the superconductor devices produce much less heat and are sensitive to temperature changes. It is shown that Josephson junctions made with high temperature superconductors can be placed very close to transistors on a properly heatsunk chip. A second important issue is interfacing the low voltages of superconducting devices to the much higher voltages needed for transistors; an existing technique is discussed in the context of high temperature superconductors. Only well developed semiconductor technologies have been considered; although there is some possibility of making low voltage transistors, this is not explored here. The paper concludes with an analysis of the various applications that can be realized, depending on the type of device available in high temperature superconductor technology: passive patterned films, nonhysteretic Josephson devices or Josephson tunnel juntions.     

Amorphous silicon twin photodiode structure for differential current measurements

In this paper we present a new amorphous silicon structure called “twin photodiode” constituted by two p–i–n series connected photodiodes biased at the same reverse voltage. The structure takes advantage of the differential current measurement to reveal very small variations of photocurrent in a large background current signal. The removal of the background allows us to increase the dynamic range of the input signal and the sensitivity of the detection system. In addition, the differential approach allows us to reduce the common mode signal due to the effect of temperature variations and instability of light source intensity.We have fabricated several twin structures for detection of ultraviolet radiation with different geometries utilizing a four mask-step process. Experimental results have demonstrated the ability of our structure to detect differential currents three orders of magnitude lower than the current of each sensor. The achieved common mode rejection ratio keeps constant with reverse bias voltage and increases with increasing wavelengths, varying from 30 dB nm at 254 nm to 42 dB at 365 nm.     

GaAs three-terminal double delta-doped triangular barrier switch prepared by molecular beam epitaxy

A novel S-shaped negative differential resistance (NDR) switching device, prepared by molecular beam epitaxy (MBE), has been successfully developed in a GaAs double delta-doped triangular barrier (TB) structure. All symmetrical S-shaped NDR characteristics, either unidirectional or bidirectional bistability phenomena, are observed experimentally. The bidirectional current-voltage (I–V) characteristics exhibit a new type of NDR caused by an avalanche multiplication process in the reverse-biased base-collector region and by barrier redistribution. Under a base current injection with respect to the cathode, the device behaves as a conventional transistor with a current gain of 1.2 at room temperature. Experimentally, electrical results can be easily understood by an equivalent circuit and a proposed model. In addition, a new optoelectronic switching device is also proposed which may have the potential for bidirectional wavelength emission. On the basis of the first utilization of a double delta-doped TB structure, a conceptual understanding of such a set of results enhances our understanding of the physics of double delta-doped TB devices in general.     

Dark and photoelectric conversion properties of p-MgPc/n-Si (Organic/Inorganic) heterojunction cells.

Heterojunction cells of p-MgPc/n-Si have been fabricated by thermal evaporation of MgPc thin films onto Si100 single crystal wafers. The devices exhibit strong photovoltaic characteristics with an open–circuit voltage of 0.35 V, a short–circuit current of 3.57 mA and a power conversion efficiency of 1.05%. These parameters have been estimated at room temperature and under a monochromatic illumination of 633 nm with an input power density of 50 mW/cm². The activation energy of the charge carriers of 0.32 eV and the cell series resistance of 2 kΩ have been evaluated from the measurements of the dark I–V characteristics. A free–carrier concentration of 2.2×10¹⁶ cm⁻³ and a barrier width of 75 nm have been estimated from C–V measurements. The temperature dependence of photocurrent, at constant illumination, has been also investigated.     

Mechanism generating current oscillations in BISPIN, a new semiconductor device

The operation of a new semiconductor device named BISPIN is analyzed. In certain modes the current oscillates with a frequency that depends on the incident light intensity, the supply voltage, the load impedance, and the temperature. Three different modes of current oscillation are distinguished by the state of the distributed p⁺-n junction and by whether the light does or does not have an effect. The mechanism by which those oscillations arise in the gate mode is considered in detail. Translated from Izmeritel'naya Tekhnika, No. 9, pp. 28–31, September, 1998.     

Electrical conduction through self-assembled monolayers in molecular junctions: Au/molecules/Au versus Au/molecule/PEDOT:PSS/Au

We fabricated and characterized a large number of octanedithiol (denoted as DC8) molecular devices as vertical metal–molecule–metal structure with or without using an intermediate conducting polymer layer of poly (3,4-ethylenedioxythiophene) stabilized with poly(4-styenesulfonic acid) (called as PEDOT:PSS). The electronic transport properties of DC8 molecular devices with and without PEDOT:PSS layer were statistically compared in terms of current density and device yield. The yields of the working molecular devices were found to be ~ 1.75% (84 out of 4800 devices) for Au/DC8/Au junctions and ~ 58% (74 out of 128 devices) for Au–DC8/PEDOT:PSS/Au junctions. The tunneling decay constants were obtained with the Simmons tunneling model and a multibarrier tunneling model for two kinds of molecular devices with and without PEDOT:PSS layer.     

Nonvolatile Memory Elements Based on Organic Materials

Many organic electronic devices exhibit switching behavior, and have therefore been proposed as the basis for a nonvolatile memory (NVM) technology. This Review summarizes the materials that have been used in switching devices, and describes the variety of device behavior observed in their charge–voltage (capacitive) or current–voltage (resistive) response. A critical summary of the proposed charge‐transport mechanisms for resistive switching is given, focusing particularly on the role of filamentary conduction and of deliberately introduced or accidental nanoparticles. The reported device parameters (on–off ratio, on‐state current, switching time, retention time, cycling endurance, and rectification) are compared with those that would be necessary for a viable memory technology.     

Diffusion Modes of an Equimolar Methane–Ethane Mixture from Dynamic Light Scattering

The hydrodynamic diffusion modes of an equimolar methane–ethane mixture have been investigated by dynamic light scattering. Measurements were performed over a wide temperature range between the plait critical point at 263.55 K and 310 K along the critical isochore. Two relaxation modes have been observed which are commonly associated with pure mass diffusion and pure thermal diffusion, but in near-critical binary fluid mixtures—according to recent theory—may alternatively be interpreted as two effective diffusivities resulting from a coupling between mass and thermal diffusion. Diffusivity values for the slow mode were obtained with typical standard deviations of 1% over the whole temperature range, whereas the low amplitude of the fast mode only allowed values of this component with a large measurement uncertainty. The results are discussed in connection with literature data available for the thermophysical properties of this binary fluid mixture and regarding the various possibilities of theoretical interpretation.