Peiji Zhao

Simulation of resonant tunneling structures: Origin of the I–V hysteresis and plateau-like structure

Hysteresis and plateau-like behavior of the I–V curves of a double-barrier resonant tunneling structure are simulated in the negative differential resistance region. Our simulation results show that the creation of an emitter quantum well after the current passes its maximum value is the key point in understanding the origin of the I–V plateau-like structure. It is demonstrated that the plateau-like behavior of the I–V curves is produced by the coupling between the energy level in the emitter quantum well and that in the main quantum well. The hysteresis is a manifestation of the above-mentioned energy level coupling, the accumulation and distribution of electrons in the emitter, and the coupling between the energy level in the quantum well and the conduction band edge or the three-dimensional continuum states in the emitter. The effects of the structural parameters on the bistability of the I–V curves of resonant tunneling devices are discussed. The creation and disappearance mechanism of the emitter qua...

Equivalent circuit parameters of resonant tunneling diodes extracted from self-consistent Wigner-Poisson simulation

The equivalent circuit parameters of resonant tunneling diodes (RTD) are extracted from numerical simulation results for RTDs. The RTD models used in this paper are double barrier structures. The influence of the resonant tunneling structure (RTS) parameters, such as the height of barriers, the width of the quantum well, the width of the spacers, and the width of the barriers, on the device parameters are systematically discussed. The effects of device temperature on device parameters are also discussed. Scattering between electrons and phonons greatly affects device parameters and thereby the function of the RTDs. Physical explanations about how the structure parameters and device temperature influence the device parameters are provided. Based on the analysis results, a general way to get an RTD oscillator with a higher maximum frequency is suggested.

THz-frequency intrinsic oscillations in double-barrier quantum well systems

Abstract Traditional implementations of double-barrier quantum well structures (DBQWSs) have not been successful as oscillator sources at THz frequencies because they are utilized in an extrinsic (i.e., external charge exchange) manner. Indeed, the true failing of a “traditional” DBQWS-based oscillator is tied directly to the physical principles associated with its implementation. In this paper, greater insight into the physics of instabilities within nanoscale tunneling structures is revealed. Here, self-consistent, time-dependent Wigner–Poisson simulations demonstrate sustained THz-frequency current-oscillations in a DBQWS that arise without the benefit of external charging processes. More importantly, dependencies between the emitter-boundary structure and the DBQWS are identified that strongly influence the internal instability phenomenon. These new results offer potential methodologies for inducing and controlling intrinsic oscillations in DBQWSs.

MIXED-VALENCE TRANSITION METAL COMPLEX BASED INTEGRAL ARCHITECTURE FOR MOLECULAR COMPUTING (I): ATTACHMENT OF LINKER MOLECULE TO SILICON (100) - 2×1 SURFACE

Based on density functional theory, we have calculated and analyzed the electrical characteristics of silicon (100) - 2×1 surfaces. The calculation results show that the sulfur atom establishes a Schottky-like contact when it is used to tethering the surface and the molecule. In contrast to the Si – S linkage, a direct Si – C linkage does not build up a barrier between the molecule and the surface of silicon. This type of contact is a Ohmic-like contact. The origin of the characteristics of the contacts is analyzed. A basic rule for designing the electrical characteristics of the molecule - surface of silicon is presented.

Creation and quenching of interference-induced emitter-quantum wells within double-barrier tunneling structures

The initial creation and subsequent quenching of the emitter quantum well within double-barrier resonant tunneling structures (RTSs) is the key process that explains the origin of the hysteresis and plateau-like structure of the I–V characteristics. This fundamental process, which evolves out of quantum-mechanical interference, defines the basic mechanism that can lead to intrinsic high-frequency oscillations. This article presents numerical results, derived from a coupled Wigner–Poisson model, that illustrate the underlying mechanisms responsible for the creation and disappearance of the emitter-quantum well. Additional theoretical results are also given that demonstrates how subband state coupling, between the emitter-quantum well (EQW) and the main-quantum well (MQW) defined by the double-barrier heterostructure, leads to the hysteresis and instability behavior. This article will reveal how the quantum interference that develops between the incident and reflected electron wave function (i.e., from the ...

Suppression of electron-phonon scattering in double-quantum-dot based-quantum gates

The authors propose a nanostructure design which can significantly suppress longitudinal-acoustic-phonon–lectron scattering in double-quantum-dot based quantum gates for quantum computing. The calculated relaxation rates versus voltage exhibit a double-peak feature with a minimum approaching 105s−1. In this matter, the energy conservation law prohibits scattering contributions from phonons with large momenta; furthermore, increasing the barrier height between the double quantum dots reduces coupling strength between the dots. Hence, the joint action of the energy conservation law and the decoupling greatly reduces the scattering rates.

Electrostatic characteristics of tether atoms in connecting organic molecules to the surface of silicon

In this letter, the authors analyze the electrostatic characteristics of the tether atoms connecting organic molecules onto silicon (100)-2×1 surfaces, which is a key factor in the design of molecular devices for information processing and biomolecular sensing. Design principles for silicon surfaces with required electrostatic functionality are presented.

Origin of intrinsic oscillations in double-barrier quantum-well systems

Abstract We present a theory accounting for the origin of high-frequency current oscillations in a double barrier quantum-well system. The origin of such current oscillations is traced to the development of a dynamic emitter quantum-well and the concomitant coupling of the energy levels in the double barrier quantum-well system. The relationship between the oscillation frequency and the energy level structure of the system is expressed as ν=ΔE0/h: A self-consistent, time-dependent Wigner–Poisson numerical computer experiment is used to exhibit remarkable intrinsic, sustained current oscillations in the double-barrier quantum well at terahertz frequencies; and a procedure for calculating ΔE0, the energy difference at time t0 (defined such that the contribution to the energy difference from the potential oscillation is zero) is also presented. The simulated oscillation frequency determined using the Wigner–Poisson analysis is in very good agreement with that calculated using a Schrodinger equation with a self-consistent potential determined from the Poisson equation.

Parallel parameter study of the Wigner-Poisson equations for RTDs

We will discuss a parametric study of the solution of the Wigner-Poisson equations for resonant tunneling diodes. These structures exhibit self-sustaining oscillations in certain operating regimes. We will describe the engineering consequences of our study and how it is a significant advance from some previous work, which used much coarser grids. We use LOCA and other packages in the Trilinos framework from Sandia National Laboratory to enable efficient parallelization of the solution methods and to perform bifurcation analysis of this model. We report on the parallel efficiency and scalability of our implementation.

Parallel-platform based numerical simulation of instabilities in nanoscale tunneling devices

We present theoretical results on instability processes in nanoscale tunneling structures that were obtained from a computationally improved physics-based simulator. The results were obtained from a numerical implementation of the Wigner-Poisson electron transport model upon a parallel-computing platform. These investigations considered various forms of multi-barrier resonant tunneling structures (RTSs) and they were used to test the robustness of the new modeling code. This improved modeling tool is shown to be fast and efficient with the potential to facilitate complete and rigorous studies of this time-dependent phenomenon. This is important because it will allow for the study of RTSs embedded in realistic circuit configurations. Hence, this advanced simulation tool will allow for the detailed study of RTS devices coupled to circuits where numerical simulations in time and iterative numerical optimization over the circuit parameters are required. Therefore, this work will enable the future study of RTS-based circuits operating at very high frequencies.