Effect of electron-optical phonon interaction on resonant tunneling in coupled quantum wells

The spatial distribution of the wave functions for electrons in a coupled-quantum-well system of GaAs/Al _x Ga_1?x As with triple barriers is discussed. Within the framework of the dielectric continuum model, the dispersion relations of interface optical phonon modes are given. Furthermore, the interaction between an electron and optical phonons and the ternary mixed crystal effect in these structures are investigated in detail. The optical phonon-assisted tunneling (PAT) is studied using the Fermi golden rule to obtain numerically the PAT currents. The results reveal that the interface optical phonons are more important than the confined longitudinal optical phonons. Only one PAT peak does appear when the middle barrier is wide enough or its Al component is high enough, and the peak moves to the higher applied voltage direction, whereas two PAT peaks do appear when the middle barrier is narrow enough or its Al component is low enough.     

Investigation of the performance and band-to-band tunneling effect of a new double-halo-doping carbon nanotube field-effect transistor

Carbon nanotube field-effect transistors (CNTFETs) can be fabricated with Ohmic- or Schottky-type contacts. We focus here on Ohmic CNTFETs. The CNTFETs suffer from band-to-band tunneling which in turn causes the ambipolar conduction. In this paper, to suppress the ambipolar behavior of CNTFETs and improve the performance of these devices, we have proposed application of symmetric double-halo (DH)-doping in CNTFETs. In this new structure, the source-side halo doping reduces the drain-induced barrier lowering (DIBL) and the drain-side halo reduces the band-to-band tunneling effect. Simulation results show in the DH-CNTFET, subthreshold swing below the 60 mV/decade conventional limit can be achieved. Also it decreases significantly the leakage current and drain conductance and increases on–off current ratio and voltage gain as compared to conventional CNTFET.     

Electron-phonon scattering in an asymmetric double barrier resonant tunneling structure

We calculate the electron-phonon scattering rate for an asymmetric double barrier resonant tunneling structure based on dielectric continuum theory, including all phonon modes, and show that interface phonons contribute much more to the scattering rate than do bulk-like LO phonons for incident energies which are approximately within an order of magnitude of the Fermi energy. The maximum scattering rate occurs for incident electron energies near the quantum well resonance. Subband nonparabolicity has a significant influence on electron-phonon scattering in these structures. We show that the relaxation time is comparable to the dwell time of electrons in the quantum well for a typical resonant tunneling structure. Received: 23 December 1997 / Revised: 24 March 1998 / Accepted: 9 March 1998     

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

S-shaped current bistability in a bipolar resonant tunneling diode

The bipolar tunneling transport through p–i–n double barrier structures has been studied by means of simultaneous electrical transport measurements and electroluminescence spectroscopy. An “inverted” hysteresis loop is observed at the onset of the first electronic resonance in the current–voltage characteristics with an electrical ON/OFF ratio of more than two orders of magnitude. Relating the different branches of the current–voltage characteristic to the space charges accumulated throughout the structure the inverted hysteresis loop is interpreted in terms of an S-shaped current bistability. The S-shaped current bistability is similar to the current driven negative differential resistivity as known for instance from thyristor action. This analogy between the bipolar double barrier structure with alloyed n-type emitter and the thyristor will be briefly discussed.     

Energy and effective mass of a polaron in asymmetric semiconductor quantum well structures

In this paper, the correct electron extended states wave functions and the density of states in asymmetric single quantum wells (QWs) are given for the first time, we put right mistakes from some previous papers of some other authors. Within the framework of the secondorder perturbation theory, the ground-state polaron binding energy and effective mass correction in asymmetric single QWs are studied including the full energy specturm, i.e., the discrete energy levels in the well and the continuum energy spectrum above the barrier, and all possible optical-phonon modes. The effects of the finite electronic confinement potential and the subband nonparabolicity are considered. The relative importance of the different phonon modes is investigated. Our results show that the polaron energy and effective mass are sensitive to the asymmetry of the structure and have a close relation to the interface phonon dispersion. When well width and one side barrier height of asymmetric QWs are fixed and identical with those of symmetric QW, the polaron binding energy and effective mass in asymmetric QWs are always less than those in symmetric QW. It is necessary to include the continuum energy spectrum as intermediate states in the study of polaron effects in QWs in order to obtain the correct results. The subband non-parabolicity has little influence on the polaron effects. The polaron energies given in this paper are excellent agreement with our variational results.     

Level width of a quasibound state in a double-barrier parabolic-well resonant tunneling structure

Taking exact Airy functions and Hermitian functions as envelope functions, we investigate in detail the level width of a quasibound state for electrons coherent resonant tunneling through symmetric and asymmetric double-barrier parabolic-well resonant tunneling structures (DBRT) with the transfer-matrix formalism. It is found that for the symmetric structure and the asymmetric structure with left barrier thicker than the right one, both the level width and the peak value vary monotonously with increasing applied bias, but for the asymmetric DBRT structure with left barrier thinner than the right one, they change nonmonotonously. The nonmonotonous variations of the level width and the peak value reflect the transition of tunneling type (i.e. first from incompletely resonant tunneling to completely resonant tunneling, and then from completely resonant tunneling back to incompletely resonant tunneling). The effects of well width, barrier thickness and barrier height on the level width and the peak value are also inspected.     

The hole transport characteristics in SiGe/Si double and triple barrier resonant tunneling structures

Experimental measurements and theoretical calculations have been used to study the hole transport characteristics in SiGe/Si double and triple barrier resonant tunneling structures. The main emphasis is put on discussing the symmetry of I–V characteristics with forward and reverse bias, their temperature dependences and relations to quantum well designs. The calculations show that at current resonance, the sub-level can be much lower (e.g, for heavy hole resonance) or much higher (e.g, for light hole resonance) than the quasi-Fermi-level in the spacer. The distinctly different features of the measured first and second resonances for SiGe/Si double and triple barrier resonant tunneling, can be understood, by considering the different population of the heavy hole and light hole bands in the spacer region and the temperature dependences of Fermi-level, carrier mobility and effective masses. The analysis of dependences of the transmission and I–V curve with quantum well designs presents the possibility of using an asymmetric triple barrier structure to improve the resonant tunneling performance.     

Phonon-assisted tunneling process in amorphous silicon nanostructures and GaAs nanowires

Experimental results on the current–voltage (I–V) characteristics of amorphous Si nanostructures reported by Irrera et al. [A. Irrera, F. Iacona, I. Crupil, C.D. Presti, G. Franzo, C. Bongiorno, D. Sanfilippo, G. Di Stefano, A. Piana, P.G. Fallica, A. Canino, F. Priolo, Nanotechnology 17 (2006) 1428] are reinterpreted in terms of a phonon-assisted tunneling model. It is shown that temperature dependence of current can be caused by the temperature dependence of electron tunneling rate from traps in the metal–semiconductor interface to the conduction band of the semiconductor. A good fit of experimental data with the theory is achieved in all measured temperature range from 30 to 290 K using for calculation the effective mass of 0.5m_e, and for the phonon energy the value of 12 meV. An advantage of this model over that of Irrera et al. used model is the possibility of describing the behavior of I–V data measured at both high and low temperatures with the same set of parameters characterizing this material. The temperature-dependent I–V data by Schricker et al. [A.D. Schricker, F.M. Davidson III, R.J. Wiacek, B.A. Korgel, Nanotechn. 17 (2006) 2681.] of GaAs nanowires, are also explained on the basis of this model.     

Electron spin filtering in all-semiconductor tunneling structures

In this work we briefly review the present day perspectives for exploiting conventional non-magnetic semiconductor nano-technology to design high speed spin-filter devices. In recent theoretical investigations a high spin polarization has been predicted for the ballistic tunneling current in semiconductor single- and double-barrier asymmetric tunnel structures of III–V semiconductors with strong Rashba spin–orbit coupling. We show in this paper that the polarization in the tunneling can probability be sufficiently increased for producing realistic single-barrier structures by including of the Dresselhaus term into consideration.     

Exciton-assisted tunneling transport in heterojunction microstructures

Longitudinal tunneling transport in the low-dimensional heterojunction structures induced by the excitonic Coulomb interaction has been formulated and discussed in the framework of Fermi's golden rule. We have investigated the tunneling transition of free carriers to quantum-well Wannier–Mott excitons incorporated in the sequential tunneling Hamiltonian. The modeling is evaluated by a set of coupled rate equations involving subband states of electron, hole and exciton. The exciton-assisted tunneling (EAT) phenomenon has its characteristic fingerprint causing tunneling current prior to the resonance electric fields, and a significant modulation of the I–V characteristics. It is also found that the bias offset and the FWHM of the EAT current spectrum can be comparable to that of resonant tunneling (RT) current, depending both on the 2D hole density of the confined subband and the excitonic properties in the active region. The EAT effect has a different I–V spectral line shape, compared to that of the RT or its replica, tailing off in the resonance regime induced by the exciton binding energy.     

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     

Interaction between an electron and optical phonons in polar semiconductor heterostructures

Interface phonons and bulk-like longitudinaloptical (LO) phonons and their interaction with an electron are studied for a finite four-layer heterostructure (FFLHS). An analysis of the field eigenvectors shows that, in the vicinity of the Brillouin-zone center, an interface transverse-optical (TO) mode oscillates at the bulk LO frequency, and an interface LO mode oscillates at the bulk TO frequency. Analytic expressions and numerical illustrations for dispersion relations of interface modes and for electron-phonon coupling functions and scattering rates are obtained for finite, semi-infinite and infinite quantum well (QW) structures which are important special cases of an FFLHS. It is shown that the scattering rates depend strongly on the well width of a QW structure, and that interface modes are much more important than bulk LO modes when the well width is small. The calculated results also show that the usual selection rules for intersubband and intrasubband transitions break down in asymmetric heterostructures. Moreover, we have found an interesting result. That is, in comparison with the negligibly small interaction between an electron and the lowest-frequency interface-mode in symmetric single QWs and commonly used step QWs, this interaction may be very large in asymmetric single QWs and general step QWs.     

k//=0 filtering effects in ballistic electron transport through sub-surface GaAs–AlGaAs double barrier resonant tunneling structures

In ballistic electron emission microscopy on Au–GaAs double barrier resonant tunneling diodes, electrons are transferred across an interface between an area of high and low effective mass and subsequently through a low-dimensional state. Experimentally, the resonant level in the double barrier structure becomes evident as clear step in the ballistic current measured as a function of sample bias. To analyze the spectrum, an extended transfer matrix method, together with the commonly accepted Bell Kaiser model is used. In terms of this model we show that only electrons with zero wave vector parallel to the barriers can be transmitted resonantly.     

Time Correlation in Tunneling of Photons

I propose to consider photon tunneling as a space-time correlation phenomenon between the emission and absorption of a photon on the two sides of a barrier. Standard technics based on an appropriate counting rate formula may then be applied to derive the tunneling time distribution without any ad hoc definition of this quantity. General formulae are worked out for a potential model using Wigner–Weisskopf method. For a homogeneous square barrier in the limit of zero tunneling probability a vanishing tunneling time is obtained.     

Oscillation modes,transient chaos and its control in a modulation-doped semiconductor double-heterostructure

The nonlinear dynamics of real space transfer for a 2D electron gas in a system of two adjacent Al _x Ga_1–x As/GaAs-heterolayers under parallel current conduction has been investigated numerically. The mechanisms which have been taken into account are transfer of electrons by thermionic emission and nonresonant tunneling and the delayed dielectric relaxation of the interface potential barrier. We predict bistability of an asymmetric and a symmetric self-generated oscillation mode, a quasiperiodic route to chaos and transient chaos with mean transient times obeying a universal critical scaling law. Unstable periodic orbits of the chaotic repeller can be stabilized by a simple delayed feedback control, thus providing a widely tunable semiconductor oscillator.     

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.     

Vibration Spectra of Quasi-confined Optical Phonon Modes in an Asymmetric Wurtzite AlxGa1-xN/GaN/AlyGa1-yN Quantum Well

Based on the dielectric continuum model and Loudon's uniaxial crystal model,the properties of the quasiconfined (QC) optical phonon dispersions and the electron-QC phonons coupling functions in an asymmetric wurtzite quantum well (QW) are deduced via the method of electrostatic potential expanding.The present theoretical scheme can naturally reduce to the results in symmetric wurtzite QW once a set of symmetric structural parameters are chosen.Numerical calculations on an asymmetric A1N/GaN/Al0.15Ga0.85N wurtzite QW are performed.A detailed comparison with the symmetric wurtzite QW was also performed.The results show that the structural asymmetry of wurtzite QW changes greatly the dispersion frequencies and the electrostatic potential distributions of the QC optical phonon modes.     

Influence of interface within the composite barrier on the tunneling electroresistance of ferroelectric tunnel junctions with symmetric electrodes

The interface with a pinned dipole within the composite barrier in a ferroelectric tunnel junction（FTJ） with symmetric electrodes is investigated.Different from the detrimental effect of the interface between the electrode and barrier in previous studies,the existence of an interface between the dielectric SrTiO₃ slab and ferroelectric BaTiO₃ slab in FTJs will enhance the tunneling electroresistance（TER） effect.Specifically,the interface with a lower dielectric constant and larger polarization pointing to the ferroelectric slab favors the increase of TER ratio.Therefore,interface control of high performance FTJ can be achieved.     

Experimental evidence of resonant tunneling via localized DQW states in an asymmetric triple barrier structure

In this work we report on field-induced features appearing in the tunneling current traces of a biased asymmetric triple barrier resonant tunneling device in the presence of an in-plane magnetic field. A theoretical model that satisfactorily explains the origin of these features is discussed. The reported data evidences the localized nature of the quantum states in thin layer asymmetric double-quantum-well structures.