Inelastic electron tunneling

Inelastic Electron Tunneling Spectroscopy (IETS) is a new technique for measuring the vibrational spectrum of minute quantities of organic compounds. Sensitivity is its key advantage over the conventional techniques of infrared and Raman spectroscopy. This article will first discuss the technique itself: its theoretical basis, selection rules, sensitivity, vibrational mode shifts due to surface interactions and superconductivity, and sample preparation. Then it will discuss applications of the technique to problems in biology, radiation physics, surface physics, and catalysis.     

On resonant tunneling

Localized electron states in oxides adjacent to metals hybridize with conduction electron states forming interface states, which at the localized site have an amplitude resonantly enhanced over the amplitude of the conduction electron states. The interface states mediate a continuous transition between the metal and the semiconducting or insulating oxide. Resonant tunneling via these interface states to an opposing metal surface can dominate over direct and intermediate-state tunneling. Resonant tunneling is obstructed by the correlation (Coulomb) energy which causes voltage, temperature and time dependencies. The obstruction increases with distance of the localized state from the metal and this increased obstruction causes the transition from resonant to intermediate-state tunneling. This corresponds to a space-wise metal-insulator transition. In oxides, like Nb₂O₅, the correlation energy is small and the hybridization is strong and thus resonant tunneling through localized states at the Fermi energy can account for various tunnel anomalies observed in the normal or superconducting state.     

Inelastic tunneling in Al-AlN-Pb junctions

The technique of the prepagation of the tunnel junctions with a barrier made of AlN has been presented. To produce a barrier layer a freshly evaporated Al film was exposed to a glow discharge in atmosphere of pure ammonia. The features shown in the tunnel spectra of these junctions have been interpreted as AlN optic phonons and a vibration spectrum of NH₃ molecules adsorbed in the junctions.     

Inelastic tunneling assisted by molecular impurities

A recently published many-body theory of tunneling is used to calculate the effect of a molecular impurity localized in the barrier. The variation of the conductance with the position of the impurity is worked out for finite temperatures in a renormalized perturbation treatment.     

Superconductivity-induced macroscopic resonant tunneling

We show analytically and by numerical simulations that the conductance through pi-biased chaotic Josephson junctions is enhanced by several orders of magnitude in the short-wavelength regime. We identify the mechanism behind this effect as macroscopic resonant tunneling through a macroscopic number of low-energy quasidegenerate Andreev levels.     

Experimental analysis of resonant tunneling transit time using high-frequency characteristics of resonant tunneling transistors

We systematically estimate the resonant tunneling transit time from the high-frequency characteristics of resonant tunneling transistors. It is found that the transit time across an InGaAs/InAlAs resonant tunneling structure is more than one order of magnitude shorter than that for GaAs/AlAs with the same barrier layer thickness. In addition, the obtained times agree reasonably well with calculated phase time. It is probably the elastic resonance width and not the inelastic scattering effect that mainly determines the resonant tunneling transit time.     

Inelastic resonance tunneling in S-Sm-S tunnel structures

Inelastic resonance tunneling through junctions with an amorphous interlayer and superconducting electrodes is studied. The form of the current-voltage characteristic I(V) at low temperature and the temperature dependence of the conductance G(0) at low bias are calculated and are found to be much different from the analogous dependences of structures with normal electrodes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 159–163 (25 January 1997)     

A lateral resonant tunneling FET

A lateral resonant tunneling FET (RTFET) is proposed. The RTFET has three closely spaced gates. The outer gates control the barrier heights, and the inner gate controls the potential of the quantum well. These gates are capacitively coupled to the barriers and the well, therefore, the gate currents are very small. Modeling and computer simulation show that the RTFET should have an improved peak-to-valley ratio, narrower current peak widths, and more uniform distribution of peak currents than that of a conventional resonant tunneling diode with the same structure. Furthermore, a unique feature of this device is that the barrier height can be adjusted, which allows the current peak, the peak-to-valley ratio, and the peak positions to be tuned.     

Subband current in resonant tunneling diode

An accumulation layer is formed on the emitter side of a biased resonant tunneling diode (RTD) leading to a similar subband structure as in the ordinary MOS-system. Electrons occupying the subbands can tunnel through the RTD-structure and give rise to a significant contribution to the diode current. We calculate the subband current from our semiclassical transport model developed earlier for the ordinary tunneling current. The model includes quantum interference and bulk scattering by utilizing an optical approximation for the coherent part of the wave function. The subband current turns out to be of the same order of magnitude as the ordinary tunneling current component. It is shifted to higher voltages and therefore it increases the valley current. In order to reduce the subband current and improve the peak-to-valley current ratio (PVCR), we propose a novel RTD-structure with a grading in front of the emitter barrier. The purpose of the grading is to suppress the formation of the accumulation layer and thereby decrease the valley current. Calculations show that PVCR increases by a factor of two using a proper design of the grading.     

PT-symmetry breaking in resonant tunneling heterostructures

We present fermionic model based on symmetric resonant tunneling heterostructure, which demonstrates spontaneous symmetry breaking in respect to combined operations of space inversion (P) and time reversal (T). PT-symmetry breaking manifests itself in resonance coalescence (collapse of resonances). We show that resonant energies are determined by eigenvalues of auxiliary pseudo-Hermitian PT-invariant Hamiltonian.     

Inelastic electron tunneling spectroscopy of an alkanethiol self-assembled monolayer using scanning tunneling microscopy

We report inelastic electron tunneling spectroscopy (IETS) of a C8 alkanethiol self-assembled monolayer using a scanning tunneling microscope (STM). High-resolution STM IETS spectra show clear features of the C-H bending and C-C stretching modes in addition to the C-H stretching mode, which enables a precise comparison with previously reported vibrational spectroscopy, especially electron energy loss spectroscopy data. Intensity variation of vibrational peaks with tip position is discussed with the STM IETS detection mechanism.     

Inelastic electron tunneling spectroscopy of a single nuclear spin

Detection of a single nuclear spin constitutes an outstanding problem in different fields of physics such as quantum computing or magnetic imaging. Here we show that the energy levels of a single nuclear spin can be measured by means of inelastic electron tunneling spectroscopy (IETS). We consider two different systems, a magnetic adatom probed with scanning tunneling microscopy and a single Bi dopant in a silicon nanotransistor. We find that the hyperfine coupling opens new transport channels which can be resolved at experimentally accessible temperatures. Our simulations evince that IETS yields information about the occupations of the nuclear spin states, paving the way towards transport-detected single nuclear spin resonance.