Electric field effects on resonance structures in negative ion photodetachment

Abstract

The photodetachment of negative ions in a static electric field exhibits some new characteristic features and has beer considered in various theortical approaches.¹ Most of them, however, neglect the short-range interaction between the escaping electron and the atomic core, and must be modified to describe various resonant effects. Experiments² have shown very rich resonant structure in a dc-field, which can be attributed to the mixing of different excited states in the negative ion, to competition between elastic and inelastic decay channels, and to tunneling effects induced by the field. It is known that various resonant structures in Photoprocesses can be successfully described within standard multichannel quantum defect theory (MQDT). We present a modified MQDT frame transformation approach to extend the standard method to long-range potentials with nonspherical symmetry. In our treatment both the electron-field and electron-atom interactions are treated nonperturbatively and on an equal footing. The resulting theoretical calculations are compared with experimental data on field-modified H^? photodetachment in the vicinity of the n = 2 resonances.     

Resonant Josephson tunneling through S-I-S junctions of arbitrary size

The Josephson tunneling current in S-I-S structures where the main current transport channel is resonant tunneling through an isolated localized state is calculated using the Bogolyubov-de Gennes equations. It is shown that the efficiency of equilibrium Josephson resonant tunneling is determined only by the ratio of the width of the resonance level to the absolute value of the order parameter for the superconducting electrodes with arbitrary relationships among the system parameters. Zh. éksp. Teor. Fiz. 112, 342–352 (July 1997)     

Suppression of the equilibrium tunneling current between slightly disordered two-dimensional electron systems with different electron concentrations in a high magnetic field

Tunneling between parallel two-dimensional electron gases (2DEG) in accumulation layers formed on both sides of the single doped AlGaAs barrier are examined in both zero and high magnetic field. Accumulation layers are separated from highly n-doped contact regions which freely supply electrons to the 2DEGs via 80 nm thick lightly n-doped spacer layers. Strongly oscillating current with magnetic field along the 2DEGs is absent in this arrangement. Without magnetic field resonant tunneling between 2DEGs with different as grown electron concentration could be settle by application of external voltage bias. High magnetic fields (ν<1) shift resonant tunneling to zero external bias and suppresses tunneling current, creating wide gap in the tunneling density of states at the Fermi level arisen from the in-plane Coulomb interaction in the 2DEGs. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 236–241 (10 February 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Discrete tunneling of holes in porous silicon

It is pointed out that in the partial oxidation of porous silicon (PS) formed on heavily doped crystals, the topology of the pores can result in the formation of an anisotropic material with strings of nanometersized silicon granules embedded in insulating silicon dioxide SiO₂. In this range of granule sizes the correlation effects in the tunneling of electrons (holes) are strong on account of their Coulomb interaction. This should be manifested as discrete electron and hole tunneling at temperatures comparable to room temperature. The room-temperature current-voltage characteristics of n ⁺-PSp ⁺-p ⁺ diode structures with a PS interlayer on p ⁺-Si, which exhibit current steps on the forward and reverse branches, are presented. The current steps are attributed to discrete hole tunneling along the silicon strings in SiO₂. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 794–797 (25 May 1998)     

Magneto-tunneling spectroscopy of GaAs---AlGaAs double barrier tunneling devices in pulsed high magnetic fields up to 40T

The magneto-tunneling effect was investigated in GaAs---AlGaAs double barrier resonant tunneling devices in pulsed high magnetic fiels up to 40T applied parallel(B) and perpendicular (B) to the barrier layers. In a sample with , oscillatory structures due to the 2D electrons in the emitter and the LO phonon assisted resonant tunneling were observed when the magnetic field (B) was swept at constant bias voltages. A large drop of the current was found in the quantum limit at applied voltages below the negative differential conductivity region. A striking hysteresis was observed in the voltage-current (V - I) curves. In a wide well sample with , rich structures were observed in the V - I curve for B, corresponding to the tunneling to different cyclotron orbits from the emitter.     

Self-excitation of 2D plasmons in resonant tunneling diodes

Resonant tunneling is accompanied by the accumulation of 2D electrons in the quantum well between the barriers of resonant tunneling diodes. In high-quality structures this gives a Z-shaped current-voltage characteristic, and it is shown that self-excitation of 2D plasmons occurs in this quantum well for any external circuit at completely realistic parameters of the structures. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 628–633 (25 October 1998)     

Overcoming of the gamow tunneling insufficiencies by maximizing the damp-matching resonant tunneling

The resonant quantum tunneling current through the barrier between two wells may be maximized when the damp (absorption) in one well matches the barrier parameters. The maximum resonant tunneling current is much greater than the conventional expectation by a factor ofθ (1/θ ² is the Gamow tunneling factor). It is shown that with all the established quantum mechanics, very much higher reaction probabilities between nuclei in contrary to the Gamow theory can be explained in agreement with experiments. Particularly, the resonance will select the sub-barrier fusion with a suitable fusion rate which matches the barrier parameters. This selective resonant tunneling model is able to explain both the hot fusion data (e.g. the width of resonance in¹¹B(p,α)2α reaction) and the cold fusion data (e.g. “excess heat” without any commensurate neutron andγ radiation). This work is supported by the State Commission of Science and Technology, the Natural Science Foundation of China (Contract #19645005), and the Fundamental Research Fund of Tsinghua University.     

Deep impurity-center ionization by far-infrared radiation

An analysis is made of the ionization of deep impurity centers by high-intensity far-infrared and submillimeter-wavelength radiation, with photon energies tens of times lower than the impurity ionization energy. Within a broad range of intensities and wavelengths, terahertz electric fields of the exciting radiation act as a dc field. Under these conditions, deep-center ionization can be described as multiphonon-assisted tunneling, in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field. The field dependence of the ionization probability permits one to determine the defect tunneling times and the character of the defect adiabatic potentials. The ionization probability deviates from the field dependence e(E) ∝ exp(E ²/E _c ² ) (where E is the wave field, and E _c is a characteristic field) corresponding to multiphonon-assisted tunneling ionization in relatively low fields, where the defects are ionized through the Poole-Frenkel effect, and in very strong fields, where the ionization is produced by direct tunneling without thermal activation. The effects resulting from the high radiation frequency are considered and it is shown that, at low temperatures, they become dominant. Fiz. Tverd. Tela (St. Petersburg) 39, 1905–1932 (November 1997)     

Surface-field induced interband tunneling in InAs

Metal/insulator/semiconductor junctions are prepared on degeneratep-type InAs substrates with hole concentrations ranging from 2.3×10¹⁷ cm^–3 to 2.7×10¹⁸ cm^–3. The low work function of the top metal Yb, Al, or Au and charged interface states influence a two-dimensional (2D) electron inversion layer at the InAs surface. The insulator barrier that is formed by thermal oxidation is designed sufficiently thin, so that the bias voltage applied at the metal electrode mainly drops across the depletion layer separating the electron channel from the bulk. The current-voltage (I–V) characteristics exhibit strong negative differential conductance due to interband, tunneling from the 2D subband into the 3D valence band with peak-to-valley current ratios up to 3.1, 18, and 32 at 300 K, 77 K, and 4.2 K, respectively. In agreement with a theoretical model based on coherentelastic tunneling, the form of the I–V curves resembles those of double-barrier resonant tunnel devices rather than those of 3D Esaki diodes. The series resistance is obtained from the saturation of the differential conductance dI/dV at high forward bias and from the shift of structures in d² I/dV ² arising from phonon assisted tunneling.Dedicated to G. Lautz on the occasion of his 65th birthday     

Plasmons in asymmetric double quantum well structures

We investigate the effects of spatial asymmetry, tunneling coupling, and exchange-correlation correction on the plasmon modes in asymmetric double quantum well (DQW) structures in a time-dependent local-density approximation. Special attention is paid to the properties of the ω _- mode which is always damped in symmetric DQW systems. In addition, the results on the spectral weight of the excitations are also presented. In general, all the modes carry finite spectral weights and should be observable in resonant inelastic light scattering experiments for the specified values of the parameters. Received 2 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: c412-1@aphy.iphy.ac.cn     

Room-temperature observation of current bistability and fine structures in germanium quantum dots/SiO2 resonant tunneling diodes

The steady-state and time-dependent current–voltage (I–V) characteristics are experimentally investigated in Ge quantum dot (QD)/SiO₂ resonant tunneling diodes (RTDs). Ge QDs embedded in a SiO₂ matrix are naturally formed by thermal oxidation of Si_0.9Ge_0.1 nanowires (30 nm×50 nm) on silicon-on-insulator substrates. The average dot size and spacing between dots are 9±1 and 25 nm, respectively, from TEM observations, which indicate that one or two QDs are embedded between SiO₂ tunneling barriers within the nanowires. Room-temperature resonant oscillation, negative differential conductance, bistability, and fine structures are observed in the steady-state tunneling current of Ge-QD/SiO₂ RTDs under light illumination. Time-dependent tunneling current characteristics display periodic seesaw features as the Ge-QDs RTD is biased within the voltage regime of the first resonance peak while they exhibit harmonic swing behaviors as the RTD is biased at the current valleys or higher-order current peaks. This possibly originates from the interplay of the random telegraph signals from traps at the QD/SiO₂ interface as well as the electron wave interference within a small QD due to substantial quantum mechanics effects.     

Theory of inelastic tunneling spectroscopy of a single molecule - Competition between elastic and inelastic current

A theory of inelastic electron tunneling spectroscopy of a single molecule with scanning tunneling microscope is presented using the Keldysh Green’s function method for an adsorbate-induced resonance coupled to the molecular vibration. It is found that the correction to the tunneling current is expressed in terms of the transmission probability; the correction is negative for the elastic part of the current and positive for the inelastic one. The differential conductance (dI/dV) exhibits an increase or decrease at the threshold corresponding to the opening of inelastic channel depending on the sign of the correction, and the size of this conductance jump is scaled with the vibrational damping due to electron-hole pair excitation. The lineshape of d²I/dV²-spectra calculated using a renormalized adsorbate Green’s function evolves from an antisymmetric dip to a peak through the derivative-like one as the position of the adsorbate resonance recedes from the Fermi level of the substrate.     

Electron transport in granular amorphous silicon dioxide films with ferromagnetic nanoparticles placed in a magnetic field

Electron transport in amorphous silicon dioxide films with embedded nanoparticles (Co, Nb, Ta) was studied. The mean number of localized states in the interparticle tunneling channel was derived from the temperature dependence of conductivity for various grain concentrations under the assumption of the electron transport being governed by resonance tunneling in a chain of localized states between grains. To confirm the assumption of the inelastic character of tunneling, the dependences of the magnetoresistance on grain concentration, temperature, and magnetic field were studied. Accepting the single-orbital model, where the intergrain tunneling magnetoresistance is determined by s-s tunneling, it was found that the existence of weakly split localized states in the tunneling channel results in a lack of magnetoresistance saturation in strong magnetic fields. The combined effect of a decrease in the s-s tunneling coefficient and of growth in the probability of inelastic electron spin scattering with increasing length of the chain of localized states between particles in which the electron is tunneling accounts for the characteristic temperature-concentration dependences of the magnetoresistance. The experimental observation of these features provides an argument for the electron transport in a-SiO₂(Co,Nb,Ta) structures being governed by inelastic resonance tunneling through intergrain localized states.     

Impact of proton irradiation with different fluences on the characteristics of InP/InGaAs heterostructure

ABSTRACT

The effect of traps to C–V and I–V plots of InP/InGaAs heterostructure with 3?MeV proton irradiation at different fluences has been discussed. After proton irradiation, the total reverse capacitance increases, which does not only include the variation of the depletion region width, but also the charging and discharging effect of traps. The total actual traps density N_SS of InP/InGaAs heterostructure could reach 13 orders of trap density, which is from the peak under reverse bias. The forward current is dominated by recombination current at low voltage and by the tunneling current at high voltage. The tunneling current and trap-assisted tunneling current are dominant in the reverse current.     

On resonance states in “split” germanium

Maxima have been observed experimentally in the dependences of the current on the uniaxial pressure in p-type germanium for crossed directions of the uniaxial strain and electric field. The effect, which is observed at T=77 K and is absent at T=4.2 K, can be explained by tunneling transitions of holes, with the participation of acoustic phonons, from a resonant impurity state to unoccupied states in the valence subbands of germanium. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 638–642 (10 May 1999)     

Time-dependent analysis of tunneling effect in the formation of ultracold molecules via photoassociation of laser-cooled atoms

The paper contains a time-dependent investigation of the tunneling effect observed in the photoassociation spectrum of Cs₂ and attributed to the 0_g ^-(6s, 6p _3/2) double well. When by photoassociation of two cold cesium atoms a vibrational level of the outer well is populated, tunneling is an efficient mechanism for transferring the population to the inner well (R < 15a ₀), where spontaneous emission may lead to formation of cold molecules in low vibrational levels of the a ³Σ⁺ _u(6s, 6s) electronic state. This tunneling effect is analyzed by wavepackets propagation, first considering the double well potential alone, and following a packet made by a superposition of states initially located at large distances. Characteristic times for the vibration dynamics, corresponding to a beating phenomenon between the two wells, to partial “revival” at large distances, and to maxima in the population localized in the inner well are reported and discussed. Second, we simulate the two-channels a ³Σ⁺ _u(6s, 6s)↦0_g ^-(6s, 6p _3/2) photoassociation at detunings around 2.9 cm^-1: the inner well can be populated either by the excitation of a vibrational level of the external well (resonant excitation), or by tuning the photoassociation laser at the energy of the inner well level which displays tunneling (“off-resonance excitation”). In the first case the photoassociation is efficient, while the tunneling probability is small; in the second, the tunneling probability is large, so that despite the poor efficiency of the photoassociation process, more population can be transferred to the inner well. This second choice is shown to be very sensitive to the laser intensity, which could be used to control the population of the inner well and hence the formation of ultracold molecules in low vibrational levels. Received 19 April 2002 Published online 1st October 2002 RID="a" ID="a"e-mail: francoise.masnou@lac.u-psud.fr     

Ultra-high detectivity room temperature THZ-IR photodetector based on resonant tunneling spherical centered defect quantum dot (RT-SCDQD)

In this paper, a novel structure for THZ-IR photodetector based on resonant tunneling spherical centered defect quantum dot (RT-SCDQD) operating at room temperature is proposed. The proposed structure includes a quantum dot with centered defect following a resonant tunneling double barrier. It is shown that inserting a centered defect leads to considerable enhancement in absorption coefficient at long wavelength in small dot size (1.05 × 10⁶-7.33 × 10⁶ m⁻¹ at 83 μm). This effect guarantees large responsivity of the proposed system for THZ-IR photodetector. In this proposal, intersublevel transitions in related states positioned at mid energies of large conduction-band-offset materials (GaN/AlGaN) are used to depress the thermal effect in dark current. Adding the resonant tunneling double barrier to the quantum dot resolves the basic problem of collecting electrons from deep excited state without applying large bias voltage. Also, employing the RT double barrier reduces the ground state dark current term. Reduction of the dark current and increasing the responsivity yields ultra-high detectivity, 5 × 10¹⁶ and 2.25 × 10⁹ cm Hz^1/2/W at 83 μm, at 83 and 300 K, respectively. Analysis of the proposed structure is done analytically.     

Resonant tunneling and a nonlinear response in RF fields

The problem of resonant tunneling through a double-barrier nanostructure in a strong alternating electric field is solved completely. To this end, a perturbation method is proposed. Electron wavefunctions and a nonlinear response are obtained in analytical form over wide ranges of field frequencies and amplitudes, using the perturbation method and the semiclassical approximation. The semiclassical expression for the current allows for contributions of all orders with respect to the field, i.e., electron transitions with the emission and absorption of any number of photons. This enables one to find the limits of resonant current and output power. The case of ?ω?Γ is considered, where ? is the rationalized Planck constant, ω is the field frequency, and Γ is the resonance level width. It is established that the maximum resonant current is approximately as high as half the resonant constant current. For the quantum regime of oscillation, the output power can be 10⁶–10⁷ W/cm² at ω=10¹³ s^?1 and the output power rises with ω, in contrast to the well-known classical regime, where the power decreases rapidly.     

Adsorbate motions induced by vibrational mode coupling

Adsorbate motions are discussed with a primary attention focused on the coupling between a vibrational mode excited by ultrafast laser heated hot-electrons or by inelastic tunneling electrons with scanning tunneling microscope and the reaction coordinate (RC) mode. Recent experimental results have demonstrated an efficient reaction pathways involving an indirect excitation of a frustrated translational mode, rather than its direct excitation for adsorbate hopping on surfaces. Elementary processes are briefly described for hopping of CO molecules on a laser heated stepped Pt surface, where excitation of the frustrated rotation mode has been found to plays an indispensable. Calculation of the inelastic tunneling current (ITC) for excitation of the C-O stretch mode of a CO molecule is combined with a theory of anharmonic mode coupling to activate the frustrated translation mode above the barrier. The hopping rate as a function of the bias voltage agrees with the experimental result. An unified theory of single-, and two-electron processes for ITC-induced motions induced by an indirect excitation of the RC-mode via mode coupling is also applied to reproduce a crossover from hopping to desorption of a single NH₃ molecule on Cu(1 0 0) with an increase in the tunneling current.     

A comprehensive model for the I –V characteristics of metal-Ta 2 O 5 /SiO 2 -Si structures

The I–V characteristics of metal-Ta₂O₅/SiO₂-Si structures are precisely described with a comprehensive model, for both polarities, in the whole measurement range where there is no noticeable degradation and over seven orders of magnitude of the current. Hopping conduction and tunneling were elucidated to be the dominant conduction mechanisms in the SiO₂ layer and Poole–Frenkel internal field-assisted emission in the Ta₂O₅ layer. Other possible relevant mechanisms were discussed and subsequently discarded, based on their minor contribution. Theoretical calculations are made with fitted values of the defect related constants for hopping conduction of SiO₂ and Poole–Frenkel emission of Ta₂O₅ and the thickness of the SiO₂ layer. For gates positively biased, tunneling of electrons from the silicon conduction band through the SiO₂ is considered, while for gates negatively biased, tunneling of holes from the silicon valence band. Approximations for practical use are proposed and thus introduced limitations of the model discussed. The model is demonstrated on Al-insulator-Si structures containing thermally grown Ta₂O₅, previously studied in terms of microstructural, dielectric and electrical properties. The experimental results suggest that at higher current densities (>10 nA/cm²) an effect of compensation of the existing oxide charges by accumulated charges occurs. PACS 73.40.Qv; 73.50.-h; 73.61.-r; 77.55.+f