Lifetimes Of Quasibound States Research Articles

Analytical estimation of lifetime of quasi-bound states in iii-v semiconductors quantum well

The lifetime of electrons in the Quasi Bound States in a quantum well formed by III-V semiconductors has been calculated numerically by several researchers in the past. In this work, we obtain an analytical expression for the lifetime by finding the poles of the scattering matrix in small-width approximation. Airy functions which are solutions of the Schrödinger equation for triangular potentials are expanded asymptotically for large arguments and in power series for small arguments. A scattering problem for the triangular potential well is solved with the help of mixed boundary conditions to derive the expression for the decay width that further gives the net tunneling current from the quantum well. Heterostructures of III-V materials are modeled with triangular wells whose experimental measurements are in close approximation with the theoretical calculations presented in this paper. The analytically calculated decay width in this work is also compared with the reported values by different numerical methods and found in close agreement with them.

Lifetime enhancement of quasibound states in graphene quantum dots via circularly polarized light

Permanent localization of electrons inside a graphene quantum dot (GQD) is known to be forbidden, as a manifestation of Klein tunneling. However, an electron which scatters on a GQD may be transiently trapped inside and one known practice is the usage of magnetic field. These electronic states discussed here, called quasibound states, are scattering resonances typically characterized by a finite lifetime (trapping time). In this paper, we present a theoretical perspective concerning the opportunity to enhance the lifetime of quasibound states excited in a GQD placed in a uniform magnetic field, using circularly polarized light. Generally speaking, electron trapping inside GQDs is achievable for certain well-defined conditions, for instance, magnetic field intensity. We report here that the trapping time of an electron inside a GQD may be successfully enhanced by adjusting the light intensity while keeping the magnetic field constant.

Finite-Element Modeling of Retention in Nanocrystal Flash Memories With High- $k$ Interpoly Dielectric Stack

The analysis of retention characteristics is essential for reliability assessment of any type of flash memory. A high- $k$ (HK) material in the Interpoly dielectric (IPD) stack is known to improve the program–erase characteristics and makes the control oxide thicker for better retention time; however, it also brings-in defects along with it, creating undesirable charge loss path that plays a significant role in retention. Accurate modeling of this leakage process is important for optimizing the architecture that is less prone to oxide defects and exhibiting improved charge retention. In this paper, we present a physical model considering all possible charge leakage mechanisms that include inelastic tunneling through multiple traps in addition to direct tunneling (DT). The trap capture process is modeled as an inelastic process from nanocrystal (NC) to trap state, while the trap-to-trap tunneling is calculated by Bardeen’s Transfer Hamiltonian approach. The trap emission process and DT from the spherical NC are modeled by computing the lifetime of quasi-bound states with very small decay widths in the finite-element setup. We show that a single trap model in HK cannot explain the retention characteristics. We propose a new asymmetric IPD structure that show significant improvement in retention time.

Experimental determination of quantum-well lifetime effect on large-signal resonant tunneling diode switching time

An experimental determination is presented of the effect the quantum-well lifetime has on a large-signal resonant tunneling diode (RTD) switching time. Traditional vertical In0.53Ga0.47As/AlAs RTDs were grown, fabricated, and characterized. The switching time was measured with a high-speed oscilloscope and found to be close to the sum of the calculated RC-limited 10%–90% switching time and the quantum-well quasibound-state lifetime. This method displays experimental evidence that the two intrinsic resonant-tunneling characteristic times act independently, and that the quasibound-state lifetime then serves as a quantum-limit on the large-signal speed of RTDs.

Tuning resonant transmission through geometrical configurations of impurity clusters

Resonant tunneling features through impurity clusters embedded in an insulating matrix have been examined through the inter-play between the size of the clusters and the inter-cluster distance. Constructive interference phenomena were tuned through a systematic study of different geometrical configurations, thereby controlling confinement in quasi-bound states. Gaussian trap potentials have been used to simulate the imperfect barrier-well interface associated with disordered materials. Strongly localized states can be formed successfully despite weak disorder as illustrated by breaking the symmetry in the horizontal configuration. To this end, triangular cluster configurations were investigated under a variety of conditions including various shapes and orientations. The effects of disorder created effectively by the arbitary configurations destroy the Fano resonance, which is previlent in conductance spectra and consequently reduce the peak to valley ratio of the resonant peak in current vs. voltage curves. However the formation of two quasi-bound states is demonstrated, suggesting possible applications for disordered naturally grown systems of impurity clusters. This work addresses the controlled lifetime of quasi-bound states and can inform the design of fast switching devices based on high band gap materials by the astute incorporation of impurity clusters with specific geometrical configurations.

The Green's function and quasibound states for N delta potentials in a Stark field

The energy Green's function for a system with N delta-function potentials (delta potentials) in a constant force field is derived and the condition for finding the poles of the energy Green's function is found from the recurrence relation between the Nth and (N − 1)st energy Green's functions. The energy Green's function is decomposed in terms of the residues and complex energies of the poles (the quasibound states of the system), and this complex spectral decomposition is used to derive the survival probability of an initially bound state. We find that the lifetime of quasibound states can oscillate and poles can appear to merge as the spatial position of the delta potentials is changed. A particle that is initially trapped in the delta-potential system can be significantly stabilized by proper choice of positions of the delta potentials.

Structure dependence of tunneling lifetimes in double-barrier structures

It is known that the structure dependence of the lifetimes of quasi-bound states in rectangular multi-barrier structures is significant but still ambiguous. In this paper, we report the elaborate dependence of the lifetimes of higher quasi-bound states in symmetrical double-barrier structures on structure parameters, such as well width, barrier width and composition mole fraction. Based on the energy uncertainty condition at quasi-bound states method, numerical calculations are carried out to investigate the structure dependence of the lifetimes of higher quasi-bound states. The calculation results show that the lifetimes of higher quasi-bound states vary with structure parameters as a simple and exact function, and that the lifetimes of higher quasi-bound states increase exponentially with structure parameters.

Resonance-enhanced group delay times in an asymmetric single quantum barrier

It is shown that transmission and reflection group delay times in an asymmetric single quantum barrier are greatly enhanced by the transmission resonance when the energy of incident particles is larger than the height of the barrier. The resonant transmission group delay is of the order of the quasibound state lifetime in the barrier region. The reflection group delay can be either positive or negative, depending on the relative height of the potential energies on the two sides of the barrier. Its magnitude is much larger than the quasibound state lifetime. These predictions have been observed in microwave experiments.

Lifetime of quasi‐bound states in open semiconductor quantum structures

In this work, we compare two different methods to calculate the lifetime of quasi-bound states of one-dimensional semiconductor open quantum structures as a function of the size of confinement layers and barrier thicknesses that separate them from the particle reservoirs. Besides the excellent agreement between both methods, we found the lifetime depends inversely on the well width and decreases exponentially with the barrier thickness. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mean life times of quasi-bound states in δ-doped GaAs quantum wells

We present a theoretical study of the mean life times of the electron and hole quasi-bound states in δ-doped GaAs quantum wells. The full width at half-maxima and the mean life time are found for a series of donor and acceptor concentrations. The spatial localizations of these states are calculated and discussed in relation with theirs line shapes. The numerical calculations are performed within the framework of the semi-empirical sp3s* tight-binding model taking into account the spin.

Quasi-bound state theory of field-dependent photogeneration from polymer-embedded nanoparticles

A quantum mechanical model of field-dependent photogeneration from polymer-embedded nanoparticles is presented. In this formalism, the rate of escape of a photocreated charge from the nanoparticle into the surrounding polymer matrix is found by calculating the lifetime of its quasi-bound state (QBS) wavefunction. The theory is used to model some typical nanoparticles: CdSe and CdS quantum dots and a CdSe-CdS core/shell nanostructure. It is found that the QBS lifetime undergoes a rapid collapse at critical values of applied field, resulting in a sudden increase in photogeneration efficiency.

Displacement currents and the real part of high-frequency conductance of the resonant-tunneling diode

I have shown that weak variation of the tunnel transparency of the collector barrier with bias has substantial (and frequently crucial) effect on the high-frequency properties of the resonant-tunneling diodes (RTDs). Also it has been shown that the real part of the RTD conductance can be negative and large at the frequencies much higher than the reciprocal quasibound-state lifetime in the quantum well between the barriers of RTD, if (as opposed to common practice) the RTD collector is heavily doped and does not have thick spacer layers. The displacement currents are responsible for the effects. A simple equivalent circuit of RTD is proposed, and it fairly well describes the published experimental data.

Semiclassical and wave mechanical modeling of charge control and direct tunneling leakage in MOS and H-MOS devices with ultra-thin oxides

Charge control and gate leakage in metal-oxide-semiconductor (MOS) structures and heterojunction-MOS structures with ultrathin oxide (1 nm) are investigated using both classical and wave-mechanical calculations. In the classical approach, direct tunneling gate current is determined using the formalism of transmission probability whereas the notion of quasibound state lifetime is applied in the wave mechanical model. For conventional MOS structure, the threshold voltage V/sub T/ significantly depends on the applied model but an excellent agreement between both approaches is found about gate leakage provided that the correction of V/sub T/ is taken into account. For buried-channel H-MOS structures the quantum-induced V/sub T/-shift is smaller but the degradation of gate control efficiency dn/sub s//dV/sub g/ is increased, due to a large charge displacement from the oxide interface resulting from 2-D confinement in the buried strained layer. Using semiclassical approach the error of inversion charge distribution yields an overestimation of gate leakage compared with the more rigorous wave-mechanical calculation. It is finally shown by properly solving self-consistently Poisson and Schrodinger equations that a heterojunction-channel architecture may reduce the gate leakage by at least two orders of magnitude compared with conventional MOS design. This improvement would be in addition to the expected increase of device performance due to the strain-induced enhancement of electron transport properties.

Does the quasibound-state lifetime restrict the high-frequencyoperation of resonant-tunnelling diodes?

We have shown, firstly, that the response time(τresp) of the double-barrier resonant-tunnelling diode (RTD) can be muchsmaller as well as much larger than the quasibound-state lifetime inthe quantum well (τdwell). Secondly, the real part of the RTDconductance can be negative and large at the frequencies higher thanthe reciprocal τdwell in the RTDs with a heavily dopedcollector without spacer layers. The Coulomb interaction of the electrons in the quantumwell with emitter and collector is responsible for the effects. Asimple analytical expression for the impedance of the RTD has beenderived and an equivalent circuit has been proposed.

Effect of the Coulomb interaction on the response time and impedance of the resonant-tunneling diodes

We demonstrated that the response time of the resonant-tunneling structures (τresp) can be much smaller as well as much larger than the quasibound-state lifetime. A simple analytical expression for the impedance of the resonant-tunneling diode has been derived, it takes into account the Coulomb interaction and the quasibound-state lifetime. A simple equation relating τresp to the static differential conductance has also been obtained; it allows one to get τresp in the static measurements of the current–voltage curve.

Bound and quasibound states of the Na⋯FH van der Waals molecule

A recently developed diabatic representation for the two lowest A'2 electronic states of the NaFH system [M.S. Topaler, D.G. Truhlar, X.Y. Chang, P. Piecuch, and J.C. Polanyi, J. Chem. Phys. 108, 5349 (1998)], augmented with highly accurate multireference configuration interaction data, is used to construct a new potential energy function for the ground electronic state of NaFH. The refined potential is used to calculate the bound and quasibound rovibrational states of the Na⋯FH molecule. The focus is on the quasibound states corresponding to excitation of the H–F mode in the complex, although states in which the H–F mode is not excited are also studied. All rovibrational calculations are performed within the framework of the Sutcliffe–Tennyson Hamiltonian for triatomic molecules. The energy positions and lifetimes of quasibound states are obtained using the stabilization method. Three methods are employed to solve the rovibrational problem: (i) the variational approach, in which the Hamiltonian matrix, as defined by a discrete basis set, is diagonalized, (ii) the coupled-channel method, in which the van der Waals stretching coordinate is handled by direct numerical propagation on a grid, and (iii) two perturbative approaches based on the adiabatic separation of vibrational motions. The effect of rotational excitation on the lifetimes of calculated resonances is studied. The main results of this study are the strong evidence for the existence of many long-lived rovibrational resonances corresponding to excitation of the H–F mode in the complex and the rationalization of this finding in terms of effective potentials defining adiabatic separations. Possible impact of the results obtained in this study on new experimental ways of probing potential energy surfaces of the NaFH system, with emphasis on the dynamics of photo-induced charge transfer in Na⋯FH, is discussed.

Quantum-mechanical modeling of electron tunneling current from the inversion layer of ultra-thin-oxide nMOSFET's

Quantum-mechanical modeling of electron tunneling current from the quantized inversion layer of ultra-thin-oxide (<40 /spl Aring/) nMOSFET's is presented, together with experimental verification. An accurate determination of the physical oxide thickness is achieved by fitting experimentally measured capacitance-versus-voltage curves to quantum-mechanically simulated capacitance-versus-voltage results. The lifetimes of quasibound states and the direct tunneling current are calculated using a transverse-resonant method. These results are used to project an oxide scaling limit of 20 /spl Aring/ before the chip standby power becomes excessive due to tunneling currents,.

Accurate equivalent-network modelling of GaAs/AlAs based resonant tunnelling diodes with thin barrier layers

The small-signal intrinsic impedance of GaAs/AlAs based resonant tunnelling diodes with thin barriers has been measured at room temperature over the full 0-2 V bias-voltage and 0.05-40.05 GHz frequency ranges, on stable, non-oscillating devices. The classical Esaki and the quantum-inductance equivalent circuits were used to model the impedance for CAD purposes. Information about the quasibound-state lifetime against bias-voltage was extracted.

Resonant level lifetime in GaAs-AlAs double-barrier structures including Γ-X mixing

A quantitative model for calculation of the lifetime of quasibound states, including the Γ-X transfer, in a AlAs-GaAs-AlAs double-barrier structure is presented. When device is designed that a Γ-like energy level approaches to an X-like energy level, anticrossing of the Γ-X transition occurs and the lifetime of the state can be several orders larger than that of a pure Γ system.

Oscillations up to 420 GHz in GaAs/AlAs resonant tunneling diodes

We report room-temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1-nm-thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound-state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak-to-valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak-to-valley ratio, should allow oscillations up to about 1 THz.