TUNNELING ESCAPE TIME OF ELECTRONS THROUGH DOUBLE-BARRIER RESONANT-TUNNELING STRUCTURES

Tunneling escape of electrons from quantum wells (QWs) has systematically been studied in an arbitrarily multilayered heterostructures, both theoretically and experimentally. A wave packet method is developed to calculate the bias dependence of tunneling escape time (TET) in a three-barrier, two-well structure. Moreover, by considering the time variation of the band-edge profile in the escape transient, arising from the decay of injected electrons in QWs, we demonstrate that the actual escape time of certain amount of charge from QWs, instead of single electron, could be much longer than that for a single electron, say, by two orders of magnitude at resonance. The broadening of resonance may also be expected from the same mechanism before invoking various inhomogeneous and homogeneous broadening. To perform a close comparison between theory and experiment, we have developed a new method to measure TET by monitoring transient current response (TCR), stemming from tunneling escape of electrons out of QWs in a similar heterostructure. The time resolution achieved by this new method reaches to several tens ns, nearly three orders of magnitude faster than that by previous transient-capacitance spectroscopy (TCS). The measured TET shows an U-shaped, nonmonotonic dependence on bias, unambiguously indicating resonant tunneling escape of electrons from an emitter well through the DBRTS in the down-stream direction. The minimum value of TET obtained at resonance is accordance with charging effect and its time variation of injected electrons. A close comparison with the theory has been made to imply that the dynamic build-up of electrons in DBRTS might play an important role for a greatly suppressed tunneling escape rate in the vicinity of resonance.

STUDIES ON TUNNELING ESCAPE TIME OF ELECTRONS FROM A QUANTUM-WELL IN MULTILAYERED HETEROSTRUCTURES

By considering the time variation of band-edge profile arising from the decay of injected charge in quantum wells(QWs), we employ a wave packet method to verify that the actual escape time of certain amount of electrons from QWs could be much larger than that for a single electron. The theoretical result is also in agreement with our measurement of escape time, performed by using a newly developed method--transient current response.

InP-base resonant tunneling diodes

We have fabricated In_0.53Ga_0.47As/AlAs/InP resonant tunneling diodes (RTDs) based on the air-bridge technology by using electron beam lithography processing.The epitaxial layers of the RTD were grown on semi-insulating (100) InP substrates by molecular beam epitaxy.RTDs with a peak current density of 24.6 kA/cm~2 and a peak-to-valley current ratio of 8.6 at room temperature have been demonstrated.

A Monolithic Integrated Logic Circuit of Resonant Tunneling Diodes and a HEMT

A technology for the monolithic integration of resonant tunneling diodes (RTDs) and high electron mobility transistors (HEMTs) is developed. Molecular beam epitaxy is used to grow an RTD on a HEMT structure on GaAs substrate. The RTD has a room temperature peak-to-valley ratio of 5.2:1 with a peak current density of 22.5kA/cm~2. The HEMT has a 1μm gate length with a-1V threshold voltage. A logic circuit called a monostableto-bistable transition logic element (MOBILE) circuit is developed. The experimental result confirms that the fabricated logic circuit operates successfully with frequency operations of up to 2GHz.

Fabrication of an AlAs/In0.53Ga0.47As/InAs Resonant Tunneling Diode on InP Substrate for High-Speed Circuit Applications

A high performance AlAs/In0.53 Ga0.47 As/InAs resonant tunneling diode （RTD） on InP substrate is fabricated by inductively coupled plasma etching. This RTD has a peak-to-valley current ratio （PVCR） of 7. 57 and a peak current density Jp = 39.08kA/cm^2 under forward bias at room temperature. Under reverse bias, the corresponding values are 7.93 and 34.56kA/cm^2 . A resistive cutoff frequency of 18.75GHz is obtained with the effect of a parasitic probe pad and wire. The slightly asymmetrical current-voltage characteristics with a nominally symmetrical structure are also discussed.

Design and Realization of Resonant Tunneling Diodes with New Material Structure

A new material structure with Al0.22Ga(>. 78 As/Ino.i5 Gao.ss As/GaAs emitter spacer layer and GaAs/Ino.15-Gao.8ii As/GaAs well for resonant tunneling diodes is designed and the corresponding device is fabricated. RTDs DC characteristics are measured at room temperature. Peak-to-valley current ratio and the available current density for RTDs at room temperature are computed. Analysis on these results suggests that adjusting material structure and optimizing fabrication processes will be an effective means to improve the quality of RTDs.

Resonant tunneling of holes in GaMnAs-related double- barrier structures

Using the multiband quantum transmitting boundary method (MQTBM), hole resonant tunneling through AlGaAs/GaMnAs junctions is investigated theoretically. Because of band-edge splitting in the DMS layer, the current for holes with different spins are tuned in resonance at different biases. The bound levels of the "light" hole in the quantum well region turned out to be dominant in the tunneling channel for both "heavy" and "light" holes. The resonant tunneling structure can be used as a spin filter for holes for adjusting the Fermi energy and the thickness of the junctions.

Sequential coupling transport for the dark current of quantum dots-in-well infrared photodetectors

The dark current characteristics and temperature dependence for quantum dot infrared photodetectors have been investigated by comparing the dark current activation energies between two samples with identical structure of the dots-in-well in nanoscale but different microscale n-i-n environments. A sequential coupling transport mechanism for the dark current between the nanoscale and the microscale processes is proposed. The dark current is determined by the additive mode of two activation energies: E-a,E-micro from the built-in potential in the microscale and E-a,E-nano related to the thermally assisted tunneling in nanoscale. The activation energies E-a,E-micro and E-a,E-nano decrease exponentially and linearly with increasing applied electric field, respectively.

From Tunneling to Hopping: A Comprehensive Investigation of Charge Transport Mechanism in Molecular Junctions Based on Oligo(p-phenylene ethynylene)s

The charge transport mechanism of oligo(p-phenylene ethynylene)s with lengths ranging from 0.98 to 5.11 nm was investigated using modified scanning tunneling microscopy break junction and conducting probe atomic force microscopy methods. The methods were based on observing the length dependence of molecular resistance at single molecule level and the current-voltage characteristics in a wide length distribution. An intrinsic transition from tunneling to hopping charge transport mechanism was observed near 2.75 nm. A new transitional zone was observed in the long length molecular wires compared to short ones. This was not a simple transition between direct tunneling and field emission, which may provide new insights into transport mechanism investigations. Theoretical calculations provided an essential explanation for these phenomena in terms of molecular electronic structures.

Study on current-random wave forces acting on a framework

The forces of random wave plus current acting on a simplified offshore platform (jacket) model have been studied numerically and experimentally. The numerical results are in good agreement with experiments. The mean force can be approximated as a function of equivalent velocity parameter and the root-mean-square force as a function of equivalent significant wave height parameter.

Steady Current Induced Seabed Scour around a Vibrating Pipeline

Most of the existing researches either focus on vortex-induced vibrations (VIVs) of a pipeline near a rigid boundary, or on seabed scour around a fixed pipeline. In this study, the coupling effects between pipeline vibration and sand scour are investigated experimentally. Experimental results indicate that there often exist two phases in the process of sand scouring around the pipeline with an initial embedment, i.e. Phase I: scour beneath pipe without VIV, and Phase II: scour with VIV of pipe. During Phase II, the amplitude of pipe vibration gets larger and its frequency gets smaller while the sand beneath the pipe is being scoured, and finally the pipe vibration and sand scour get into an equilibrium state. This indicates that sand scouring has an influence upon not only the amplitude of pipe vibration but also its frequency. Moreover, the equilibrium scour depth decreases with increasing initial gap-to-diameter ratio for both the fixed pipes and vibrating pipes. For a given value of initial gapto- diameter ratio (e0/D), the vibrating pipe may induce a deeper scour hole than the fixed pipe in the examined range of initial gap-to-diameter ratios (−0.25 < e0/D < 0.75).

Experiments on interaction between current-induced vibration and scour of submarine pipelines on sandy bottom

In order to understand the dynamic behavior of submarine pipelines exposed to current and the mechanism of the interaction between current-induced vibration and scour of pipelines on a sandy bottom, an experimental investigation is conducted with a small scale model A test model which can be tested in the flume is set up by taking into account the typical working conditions of the pipelines and by applying the similarity theory. The interactions between the shape of the scour hole and the behavior of the pipeline as well as the flow patterns of the current are detailed, and the interaction mechanism outlined. The effect of vibration of the pipeline on the development of dynamic scour at different stages is found out. The proposed experimental method and test results provide an effective means for design of marine pipelines against scouring.

海流-管道-海床之间动力相互作用的量纲分析及实验模拟装置研制=Dimensional Analysis and Experimental Apparatus on Interaction between Ocean Current-Pipeline and Seabed

海底管道涡激振动和管道周围海床冲刷是海流--管道--海床之间复杂的动力耦合问题.文章应用量纲分析方法对海流、管道与海床之间的动力耦合作用进行了分析,确定了在实验模拟中应遵循的相似准则.在此基础上,研制了一套能模拟海流、海床与海底管道之间相互作用的实验模拟装置.初步实验结果表明文中研制的实验模拟装置能够模拟典型海洋环境下海底管道的涡激振动和管道周围海床冲刷等问题.

Arc Root Motions In An Argon-Hydrogen Direct-Current Plasma Torch At Reduced Pressure

Arc root motions in generating dc argon-hydrogen plasma at reduced pressure are optically observed using a high-speed video camera. The time resolved angular position of the arc root attachment point is measured and analysed. The arc root movement is characterized as a chaotic and jumping motion along the circular direction on the anode surface.