Elastic Tunnelling Research Articles

Exciton-assisted electron tunnelling in van der Waals heterostructures.

The control of elastic and inelastic electron tunnelling relies on materials with well-defined interfaces. Two-dimensional van der Waals materials are an excellent platform for such studies. Signatures of acoustic phonons and defect states have been observed in current-to-voltage measurements. These features can be explained by direct electron-phonon or electron-defect interactions. Here we use a tunnelling process that involves excitons in transition metal dichalcogenides (TMDs). We study tunnel junctions consisting of graphene and gold electrodes separated by hexagonal boron nitride with an adjacent TMD monolayer and observe prominent resonant features in current-to-voltage measurements appearing at bias voltages that correspond to TMD exciton energies. By placing the TMD outside of the tunnelling pathway, we demonstrate that this tunnelling process does not require any charge injection into the TMD. The appearance of such optical modes in electrical transport introduces additional functionality towards van der Waals material-based optoelectronic devices.

Imaging work and dissipation in the quantum Hall state in graphene.

Topology is a powerful recent concept asserting that quantum states could be globally protected against local perturbations1,2. Dissipationless topologically protected states are therefore of major fundamental interest as well as of practical importance in metrology and quantum information technology. Although topological protection can be robust theoretically, in realistic devices it is often susceptible to various dissipative mechanisms, which are difficult to study directly because of their microscopic origins. Here we use scanning nanothermometry3 to visualize and investigate the microscopic mechanisms that undermine dissipationless transport in the quantum Hall state in graphene. Simultaneous nanoscale thermal and scanning gate microscopy shows that the dissipation is governed by crosstalk between counterpropagating pairs of downstream and upstream channels that appear at graphene boundaries as a result of edge reconstruction. Instead of local Joule heating, however, the dissipation mechanism comprises two distinct and spatially separated processes. The work-generating process that we image directly, which involves elastic tunnelling of charge carriers between the quantum channels, determines the transport properties but does not generate local heat. By contrast, the heat and entropy generation process-which we visualize independently-occurs nonlocally upon resonant inelastic scattering from single atomic defects at graphene edges, and does not affect transport. Our findings provide an insight into the mechanisms that conceal the true topological protection, and suggest routes towards engineering more robust quantum states for device applications.

(Invited) Investigating the Electronic Properties of Narrow Band Gap III-V Mos Systems

The electronic properties of the III-V/oxide MOS system play a central role in applications of compound semiconductors in RF, sensor and photonics based applications. The technique commonly used to probe the electronic properties of oxide/semiconductor interfaces in MOS structures is impedance spectroscopy, where the capacitance and conductance of the MOS system is measured as a function of bias, frequency and temperature [1, 2]. There is a growing interest in the integration of RF/mm wave [3] and photonic [4] components onto a large area silicon platform, to save on the overall use of III-V materials and also to allow more compact system integration. Considering the case of RF and mm wave devices in a MOSFET implementation, there is a need to understand and control electrically active defects at the interface between the III-V channel and the insulating gate oxide. In recent years there has been considerable progress in reducing the interface state density (Dit) [5-8], in narrow gap InGaAs and InGaSb [9] MOS structures to the point where genuine surface inversion can be observed for both n- and p-type InGaAs [8,9]. The research focus for III-V MOS systems is now turning to the issue of defects states in the oxide (border traps) which can communicate with the conduction and valence bands by elastic or inelastic tunnelling [10]. Analysis of these border traps to date has been based on a distributed capacitor/resistor model [10, 11]. In this presentation, we extend these studies by simulating the AC response of border traps (BTs) and interface states by employing a simulator [12, 13] that solves the Poisson, drift-diffusion and charge conservation equations, taking into account the elastic band-to-trap tunneling model [14] for BTs, yielding estimates for the trap volume as well as variation in border trap density with energy and depth into the oxide. The presentation will also provide an overview of the relationship between specific functions of the capacitance (C) conductance (G) for the case of narrow band gap III-V MOS structures which exhibit genuine surface inversion, where the capacitance and conductance of the MOS system as a function of ac angular frequency (ω) are related, and in particular the peak values of G/ω and −dC/dloge(ω) (≡− ωdC/dω) are equal, and that these equal peak magnitudes occur at the same value of ω [15]. The relationship is also confirmed by physics based ac simulations of MOS structures and through analysis of the equivalent circuit model in inversion. Results will be presented for InGaAs and InGaSb MOS structures (Al2O3 and Al2O3/HfO2 ALD oxides) where genuine inversion of the III-V/oxide surface is confirmed by the G/ω and −dC/dloge(ω) functions, which have peak values of Cox 2/2(Cox+Cd) when the surface is inverted (where Cox is the gate oxide capacitance and Cd is the maximum capacitance of the semiconductor surface in inversion). Extending on previous studies [15], this presentation will presents experimental and physics based simulations which indicate the relationship between G/ω and −dC/dloge(ω) holds true in all bias regions of the MOS response. Finally, it is noted that the relation between G/ω and −dC/dloge(ω) is not specific to III-V MOS systems, and is generally true for all MOS systems, and examples will be provides for MOS structures based on a 2 D dimensional semiconductor. All authors acknowledge the financial support of the EU through the INSIGHT project (688784). The authors from Tyndall acknowledge Science Foundation Ireland (15/IA/3131). [1] E. H. Nicollian and J. R. Brews, MOS Physics and Technology. Wiley, 1982 [2] E. H. Nicollian and A. Goetzberger, Bell Syst. Tech. J., 46, 6, pp. 1055–1133, 1967. [3] C. Zota, et. al., IEEE Electron Dev. Lett. vol 37, p1264 (2016) [4] S. Chen, et al., Optics Express Vol. 25, Issue 5, pp. 4632-4639 (2017) [5] É. O’Connor et al., J. Appl. Phys., vol. 109, no. 2, p. 024101, 2011. [6] H.-D. Trinh et al., Appl. Phys. Lett., vol. 97, no. 4, pp. 042903-1–042903-3, 2010. [7] T. D. Lin et al., Appl. Phys. Lett., vol. 100, no. 17, p. 172110, 2012. [8] E. O'Connor, et al. Appl. Phys. Lett 110, 032902 (2017) [9] D. Millar et al., Applied Materials & Interfaces (submitted) [10] Y. Yuan et al., IEEE EDL, 32, 485 (2011) [11] G. Sereni et al., IEEE TED 62, 705, (2015) [12] Synposys Inc., Sentaurus Device TM, v. L-2016.03-SP2 [13] S.E. Laux, IEEE Trans. on Computer-Aided Design of Integrated Circuits and Systems, 4, 478 (1985) [14] F. Jiménez-Molinos et al., JAP, 91 (2002), 5116 [15] Scott Monaghan et al., IEEE Transaction on Electron Devices, 61, 4176 (2014)

(Invited) Anodic Oxide Formation and Oxygen Evolution on Metals Such As Al and Ta - Experiment and Simulation

This presentation recalls the peculiarities of anodic polarization valve metals namely aluminium and Tantalum. Both metals are very unnoble and react immediately with oxygen or water. They form an anodic oxide that hinders further transport of anions and cations through this film. Upon higher polarization the electrical field in the oxide exceeds the critical value of oxide formation field strength enabling metal cations and oxygen anions to migrate within the high field gradient in a thermally activated and field supported hopping process. This mechanism that is known as the high field mechanism of oxide formation nicely explains the most important aspects of oxide formation but some of the kinetic observations require a more sophisticated description that is called the extended high field model. As a result of the ions injected into the adjacent phases through the interface, a space charge is formed that drastically changes the local field strength. Since the potential drop over the entire film thickness remains constant, other regions of the film must compensate the reduced interface potential drop. This leads to a temporal imbalance of the driving forces within the oxide. For a limited time the charge carrier concentration can become extremely high, resulting in an experimentally observed maximum in current density named overshoot. This overshoot is not only observed in potentiostatic experiments but also in potentiodynamic scans. Both types of experiments can be satisfyingly simulated on the basis of this model. Under specific conditions however, oxide formation is not the only occurring process, meaning that the current efficiency in terms of oxide formation is below 100%. There are at least two different charge transport mechanism leading to oxygen evolution. The first one is the more obvious one in which the oxide breakdown field strength is exceeded resulting in a, at least temporal and/or local, destruction of the oxide film, that allows charge flow into this side reaction. The second one is less obvious since direct elastic tunnelling of electrons is not possible over a range of a few nm. A large number of defects in the insulating oxide film which can also be described as states in the band gap may allow charge transport in an unexpected way. The importance of these findings, the model behind, and the industrial applications will be discussed.

Tunable photonic cavity coupled to a voltage-biased double quantum dot system: Diagrammatic nonequilibrium Green's function approach

We investigate gain in microwave photonic cavities coupled to voltage-biased double quantum dot systems with an arbitrary strong dot-lead coupling and with a Holstein-like light-matter interaction, by adapting the diagrammatic Keldysh nonequilibrium Green's function approach. We compute out-of-equilibrium properties of the cavity : its transmission, phase response, mean photon number, power spectrum, and spectral function. We show that by the careful engineering of these hybrid light-matter systems, one can achieve a significant amplification of the optical signal with the voltage-biased electronic system serving as a gain medium. We also study the steady state current across the device, identifying elastic and inelastic tunnelling processes which involve the cavity mode. Our results show how recent advances in quantum electronics can be exploited to build hybrid light-matter systems that behave as single-atom amplifiers and photon source devices. The diagrammatic Keldysh approach is primarily discussed for a cavity-coupled double quantum dot architecture, but it is generalizable to other hybrid light-matter systems.

Modelling of conductive atomic force microscope probes for scanning tunnelling microscope operation

A comprehensive model is proposed that can be used to select a proper conductive atomic force microscopy (CAFM) probe for use in stable scanning tunnelling microscopy (STM) operation. This type of operation mode could be useful for scanning and patterning heterogeneous surfaces with both conductive and insulating parts using electrical principles in a non-contact fashion. The model includes elastic contact deformation, intermolecular forces, electrostatic attraction and tunnelling current generation between the tip and the sample and the snap-into contact criterion of the probe. Using the model, snap-into contact distances of the probes with varying stiffness values under different bias voltages are found, and is verified with experiments. It is shown that, for a given sample and tip materials, an optimal bias voltage for STM operation with CAFM cantilevers exists. The results also show that, to successfully utilise CAFM probes as STM end-effectors, there is a minimum normal stiffness limit for a given bias voltage. For operation on metal surfaces using metal-coated probes with tip radius values smaller than 50 nm, the model predicts that probes with high stiffness values (>24 N/m) enable both STM and AFM operations reliably with potential resolution reduction in AFM force sensing. The model also implies that probes with longer tips are better for minimising the electrostatic attraction between the cantilever and the substrate. The model would help researchers to select proper CAFM probes, which could enable simultaneous AFM and STM imaging and manipulation capabilities for tip-based nanofabrication applications.

Local enhancement of inelastic tunnelling in epitaxial graphene on SiC(0001)

AbstractWe have measured the elastic and inelastic tunnelling properties of epitaxial graphene on SiC(0001) using cryogenic scanning tunnelling spectroscopy. We find that the dominant inelastic channel of the out‐of‐plane acoustic graphene phonon at 70 mV is spatially localized to particular regions of the graphene–SiC system that contain localized states. At these locations the maximum inelastic tunnelling channel reaches up to half of the total tunnelling current. The local enhancement of the inelastic tunnelling is found at the localized electron states of the graphene/SiC interface layer. Nonequilibrium Green's function formalism theory calculations indicate that this intense inelastic channel arises from graphene phonon modes strongly coupled to narrow electron states.

Theory of elastic and inelastic tunnelling microscopy and spectroscopy: CO on Cu revisited.

Based on density functional calculations of an ordered CO structure and a CO dimer on Cu(111), we show that the Tersoff-Hamann approach to tunnelling in a scanning tunnelling microscope (STM) junction and its generalization to vibrational inelastic tunnelling give a useful description of observed STM images and vibrational spectra and microscopy. In particular, this approach provides new physical understanding of vibrational inelastic tunnelling that should not only be useful for the development of single molecule vibrational spectroscopy and microscopy but also of single-bond making and breaking by inelastic electron tunnelling in an STM junction.

Multiple-trapping model with field-dependent effects on carrier time of flight in a-Si:H

We have performed Monte Carlo (MC) calculations of the temperature ( T) and field ( F) dependence of the behavior of the multiple-trapping model assuming exponential band tails of localized states with two different models for field-dependent effects for high field carrier transport. The properties calculated are the effective temperature, T eff, the drift mobility, μ d( T, F), and the dispersion parameters α 1( T, F) and α 2( T, F), before and after the transit time, t T. The calculation with elastic tunnelling clearly shows a separation of the field and temperature effects in the dispersion parameters α( T, F). In this case, T eff is not clearly defined from the electron distribution function, f i ( E), in localized states. The calculation with field-assisted detrapping (diagonal jump from localized states to the band) shows a smooth transition in the α( T, F) and a well-defined effective temperature. We suggest that this model is more appropriate for charge transport in the presence of high field. Our MC results for α and drift mobility are compared with high field time-of-flight (TOF) experimental data for temperatures ranging from 100 to 400 K, with very acceptable agreement with the diagonal jump model (DJM).

Electron spectroscopy at Diamond (100)2 × 1: H with a scanning tunnelling microscope

Topographic scanning tunnelling microscopy on an undoped 2 × 1-reconstructed diamond surface of a polycrystalline film was possible in air, but not in ultra-high vacuum. Elastic electron tunnelling spectroscopy with the scanning tunnelling microscope was when applied to study electronic states at the surface. A surface bandgap was not found for the surface in air, whereas a gap approximately 3.5 eV wide was found in ultrahigh vacuum. This gives strong evidence for the monohydrogenated form of the 2 × 1 reconstruction, (100)2 × 1: H. The Fermi level appeared within the gap about 0.5 eV below the upper band edge which thus explained the impossibility of topographic scanning tunnelling microscopy in ultra-high vacuum. Comparison with data for ultraviolet photoelectron spectroscopy taken from the literature indicates that the surface states form one single, approximately 2-eV-wide band to filled states located just above the valence band edge. The Fermi level is accordingly located approximately 0.5 eV below the conduction band edge and not pinned by surface states.

On inversion of normal metal tunnelling data

A short summary of the theoretical and experimental status of elastic tunnelling between normal metals is given. Methods for the determination of the electron-phonon interaction function, alpha 2F( omega ), from the conductance. sigma ( omega ), of a normal metal tunnel junction are presented and their performance is discussed. A complete list of sum rules applicable to the inversion procedures is given for the first time. Elimination of the thermal smearing of the conductance data with the help of the fast Fourier transform algorithm and the regularization method is described. The possibility of the existence of 'interference' between the elastic and inelastic parts of the tunnelling current is discussed for conductivity data of Pb and Sn.

Elastic and inelastic electron tunnelling

This work includes a brief review of the technique of electron tunnelling in relation to other vibrational spectroscopies. A strong emphasis is given to preparation techniques. Some mathematical theory and numerical techniques used in relation to the study are also covered. The approach is an introductory one aimed at postgraduate or postdoctoral fellows entering this field of research. It is a complementary text to the Compilation of some 179 figures, displaying spectra contributed from over 20 different research groups, published by Progress in Surface Science in 1985.

Mechanism of electron transfer through monomolecular films of neutral organic species adsorbed at an electrode surface

The kinetics of reduction of Co(NH 3) 3+ 6, CrO 2- 4, S 2O 2- 8 and O 2 at a Hg electrode coated with monolayers of quinoline, iso-quinoline and 3-methyl-iso-quinoline have been investigated. The mechanism of electron transfer at the film-covered electrode depends on the structure of the superficial film. At a liquid-like film, the reacting ions are discharged both from inside and outside the film. Within the potential range where sold-like films are observed, it has been found that: (1) the rate of ETR reactions is slowed down by 6–7 orders of magnitude; (2) the decelerating effect of the film is distributed equally between changes in the probability of the electron transition and the energy of activation of ETR; (3) the logarithm of the reaction rate decreases linearly with the film thickness; (4) Tafel plots are linear and their slopes are slightly smaller than those observed for the bare electrode surface; and (5) the electronic transitions are probably elastic in character although distinction between elastic and resonance tunnelling is difficult for thin films.

An elastic tunnelling study of ring substituent effects in tunnel barriers doped with aromatic aldehydes

The barrier shapes of a series of doped tunnel junctions of the general form A1-I(X)-M have been determined using a three parameter (φ 1, φ 2 and d) trapezoidal analysis of their observed I– V characteristics. The parameter φ 2 represents the step between doped oxide layer and the metal M (= Pb or Ag), and d the effective barrier thickness. The adsorbates are a set of 4-substituted benzaldehydes chemisorbed, following surface reaction on the insulating oxide layer, as the corresponding carboxylate anions. Correlations are found between the fitted values of φ 2 and the commonly recognised Hammett substituent parameters σ and σ R and the pK a values of the related substituted benzoic acids as well as the calculated molecular dipole moments of the derived carboxylate anions. These imply prescriptive possibilities in the choice of adsorbates which may show enhanced Raman responses.

An elastic tunnelling study of ring substituent effects in tunnel barriers doped with aromatic aldehydes

Abstract The barrier shapes of a series of doped tunnel junctions of the general form A1-I(X)-M have been determined using a three parameter (φ1, φ2 and d) trapezoidal analysis of their observed I–V characteristics. The parameter φ2 represents the step between doped oxide layer and the metal M (= Pb or Ag), and d the effective barrier thickness. The adsorbates are a set of 4-substituted benzaldehydes chemisorbed, following surface reaction on the insulating oxide layer, as the corresponding carboxylate anions. Correlations are found between the fitted values of φ2 and the commonly recognised Hammett substituent parameters σ and σR and the pKa values of the related substituted benzoic acids as well as the calculated molecular dipole moments of the derived carboxylate anions. These imply prescriptive possibilities in the choice of adsorbates which may show enhanced Raman responses.

Test of short range effects in surface enhanced Raman spectroscopy: An elastic tunnelling study of pyridine — metal interfaces

The elastic tunnel current versus bias over the range ± 1 V in junctions of the type A1-I(X)-M where M = Pb, Ag, Cu or Au and X is 2- 3- or 4-pyridinecarboxaldehyde has been used to determine parameters which describe the barrier in each. Little variation attributable to the different dopants is observed but there are substantial effects attributable to the top metal M in both clean and doped junctions. It is suggested that electron densities of states are responsible. The potential step between dopant and metal M decreases in the sequence Au:Pb:Ag. Evidence is collated from a number of sources to suggest that hydroxyl dipole layers influence the metal-organic interface potential step both in aqueous-solution-doped tunnel junctions and in SERS systems.

Test of short range effects in surface enhanced Raman spectroscopy: An elastic tunnelling study of pyridine - metal interfaces

Test of short range effects in surface enhanced Raman spectroscopy: An elastic tunnelling study of pyridine - metal interfaces

A comparison of the charge transfer kinetics between platinum and ferrocene in solution and in the surface attached state

Linear and cyclic potential sweep techniques were used to measure the kinetics of electron transfer between a platinum electrode and ferrocene, both in solution and chemically attached to its surface. The temperature dependencies of the rate constants conformed with Arrhenius behaviour in both cases although the observed activation energies were not equal. The kinetics of the reaction involving dissolved ferrocene were satisfactorily accounted for in terms of Marcus theory. For the surface reaction, interpretation in terms of absolute reaction rate theory was abandoned in favour of an approach based on elastic electron tunnelling through a surface layer where quantitative agreement between calculated and observed preexponential factors in the rate equation was obtained.

Elastic and inelastic tunnelling in single-layer Langmuir films

The electron tunnel effect in single-layer Langmuir films sandwiched between metal electrodes (MIM devices) has been investigated in the present work by a study of both the elastic and inelastic tunnelling components. Results of inelastic tunnelling experiments in single-layer barium stearate Langmuir films are presented and it is shown that this offers convincing evidence that tunnelling is the predominant electron transport mechanism in these films over a more extensive voltage regime than previously studied. Data obtained from d.c. investigations of such MIM devices is shown to fit Stratton's model for tunnelling in symmetrical barriers for applied voltage V ⩽ 0.6 V. A value of 1.8 eV is obtained for the aluminium-barium stearate barrier height. It is also shown that the same d.c. data can alternatively be fitted to a log I versus V 1 4 relation. The voltage-current characteristics of such MIM devices were studied at low temperature and low voltage. Structure related to the superconducting energy gap is clearly visible when one electrode is made superconducting. The modified fabrication technique employed is described in detail and the close agreement with results of other workers gives confidence that the method is adequate.

A comparison of the charge transfer kinetics between platinum and ferrocene in solution and in the surface attached state

Linear and cyclic potential sweep techniques were used to measure the kinetics of electron transfer between a platinum electrode and ferrocene, both in solution and chemically attached to its surface. The temperature dependencies of the rate constants conformed with Arrhenius behaviour in both cases although the observed activation energies were not equal. The kinetics of the reaction involving dissolved ferrocene were satisfactorily accounted for in terms of Marcus theory. For the surface reaction, interpretation in terms of absolute reaction rate theory was abandoned in favour of an approach based on elastic electron tunnelling through a surface layer where quantitative agreement between calculated and observed preexponential factors in the rate equation was obtained.