Ballistic Transport Of Hot Electrons Research Articles

Energy-Resolved Femtosecond Hot Electron Dynamics in Single Plasmonic Nanoparticles.

Efficient excitation and harvesting of hot carriers from nanoscale metals is central to many emerging photochemical, photovoltaic, and ultrafast optoelectronic applications. Nevertheless, direct experimental evidence of the energy-dependent femtosecond dynamics in ubiquitous tens-of-nanometer gold structures remains elusive, despite the potentially rich interplay between interfacial and internal plasmonic fields, excitation distributions, and scattering processes. To explore the effects of nanoscale structure on these dynamics, we employ simultaneous time-, angle-, and energy-resolved photoemission spectroscopy of single plasmonic nanoparticles. Photoelectron velocity and electric field distributions reveal bulk-like ballistic hot electron transport in different geometries, lacking any signatures of surface effects. Energy-resolved dynamics are measured in the 1-2 eV range and extrapolated to lower energies via Boltzmann theory, providing a detailed view of hot electron lifetimes within nanoscale gold. We find that particles with relevant dimensions as small as 10 nm serve as exemplary platforms for studying intrinsic metal dynamics.

Ballistic hot-electron transport in a quantum Hall edge channel defined by a double gate

Ballistic transport of hot electrons in a quantum Hall edge channel is attractive for studying the electronic analog of quantum optics, where the edge potential profile is an important parameter that governs the charge velocity and scattering by longitudinal-optical (LO) phonons. Here, we use a parallel double gate to control the electric field of the edge potential and investigate the ballistic length of the channel by using hot-electron spectroscopy. The ballistic length is significantly enhanced by reducing the LO phonon scattering rate in the tailored potential.

Effect of Co and Fe stoichiometry on terahertz emission from Ta/(CoxFe1−x)80B20/MgO thin films

Laser pulse induced terahertz (THz) emission from Ta/${({\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x})}_{80}{\mathrm{B}}_{20}$/MgO thin films, with varying compositions $x$ and annealing temperatures, is investigated. With increasing annealing temperature, the THz emission intensity exhibits significant dependence on $x$, with maxima at $x$ of 0.1--0.3. The $x$ dependence of the THz emission originated from the composition dependence of the spin currents induced in ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}$ formed by the crystallization of amorphous CoFeB after the annealing. The origin of the laser-induced spin current is qualitatively discussed in terms of a ballistic transport of hot electrons and a spin-dependent Seebeck effect with different compositions.

Spectroscopic study on hot-electron transport in a quantum Hall edge channel

Hot electron transport in a quantum Hall edge channel of an AlGaAs/GaAs heterostructure is studied by investigating the energy distribution function in the channel. Ballistic hot-electron transport, its optical-phonon replicas, weak electron-electron scattering, and electron-hole excitation in the Fermi sea are clearly identified in the energy spectra. The optical-phonon scattering is analyzed to evaluate the edge potential profile. We find that the electron-electron scattering is significantly suppressed with increasing the hot-electron's energy well above the Fermi energy. This can be understood with suppressed Coulomb potential with longer distance for higher energy. The results suggest that the relaxation can be suppressed further by softening the edge potential. This is essential for studying non-interacting chiral transport over a long distance.

Ultrafast Magnetization Manipulation Using Single Femtosecond Light and Hot-Electron Pulses.

Current-induced magnetization manipulation is a key issue for spintronic applications. This manipulation must be fast, deterministic, and nondestructive in order to function in device applications. Therefore, single- electronic-pulse-driven deterministic switching of the magnetization on the picosecond timescale represents a major step toward future developments of ultrafast spintronic systems. Here, the ultrafast magnetization dynamics in engineered Gdx [FeCo]1-x -based structures are studied to compare the effect of femtosecond laser and hot-electron pulses. It is demonstrated that a single femtosecond hot-electron pulse causes deterministic magnetization reversal in either Gd-rich and FeCo-rich alloys similarly to a femtosecond laser pulse. In addition, it is shown that the limiting factor of such manipulation for perpendicular magnetized films arises from the formation of a multidomain state due to dipolar interactions. By performing time-resolved measurements under various magnetic fields, it is demonstrated that the same magnetization dynamics are observed for both light and hot-electron excitation, and that the full magnetization reversal takes place within 40 ps. The efficiency of the ultrafast current-induced magnetization manipulation is enhanced due to the ballistic transport of hot electrons before reaching the GdFeCo magnetic layer.

Introduction to spin-polarized ballistic hot electron injection and detection in silicon

Ballistic hot electron transport overcomes the well-known problems of conductivity and spin lifetime mismatch that plague spin injection attempts in semiconductors using ferromagnetic ohmic contacts. Through the spin dependence of the mean free path in ferromagnetic thin films, it also provides a means for spin detection after transport. Experimental results using these techniques (consisting of spin precession and spin-valve measurements) with silicon-based devices reveals the exceptionally long spin lifetime and high spin coherence induced by drift-dominated transport in the semiconductor. An appropriate quantitative model that accurately simulates the device characteristics for both undoped and doped spin transport channels is described; it can be used to recover the transit-time distribution from precession measurements and determine the spin current velocity, diffusion constant and spin lifetime, constituting a spin 'Haynes-Shockley' experiment without time-of-flight techniques. A perspective on the future of these methods is offered as a summary.

Hot electron relaxation in the heavy-fermionYb1−xLuxAl3compound using femtosecond optical pump-probe spectroscopy

Femtosecond time-resolved optical spectroscopy was used to systematically study photoexcited carrier re­ laxation dynamics in the intermediate-valence heavy-fermion system Yb 1_xLuxAl 3 (0 oSx oS I). Given the dem­ onstrated sensitivity of this experimental technique to the presence of the low-energy gaps in the charge excitation spectrum, the aim of this work was to study the effect of dilution of the Kondo lattice on its low-energy electronic structure. The results imply that in Yb 1_xLuxAl3 the hybridization gap, resulting from hybridization of local moments and conduction electrons, persists up to 30% doping. Interestingly, below some characteristic, doping dependent temperature T(x) the relaxation-time divergence, governed by the relaxation bottleneck due to the presence of the indirect hybridization gap, is truncated. This observation is attributed to the competing ballistic transport of hot electrons out of the probed volume at low temperatures. The derived theoretical model accounts for both the functional form of relaxation dynamics below T'(x), as well as the doping dependence of the low-temperature relaxation rate in Yb J._xLuxAI3.

Hot electron transport and a quantitative study of ballistic electron magnetic imaging on Co/Cu multilayers

We present a study of ballistic hot electron transport in Co/Cu/Co spin valves, using the ballistic electron emission microscope. By comparing samples with various Co thicknesses, we determine the hot electron attenuation length for both the minority- and majority-spin populations in the 1--2 eV energy range. Results are compared to recent calculations in order to qualitatively understand the hot electron attenuation lengths. Using the scanning ability of the microscope, we also present the imaging of different magnetic structures in the Co layers. In particular, we present a detailed study of the variation in $360\ifmmode^\circ\else\textdegree\fi{}$ domain-wall morphology versus applied field. The images are compared with micromagnetic simulations. Good agreement is found that ultimately allows for an estimation of the resolution limits of this microscope.

Ballistic hot electron transport in graphene

We theoretically study the inelastic scattering rate and the carrier mean free path for energetic hot electrons in graphene, including both electron-electron and electron-phonon interactions. Taking account of optical phonon emission and electron-electron scattering, we find that the inelastic scattering time τ∼10−2–10−1ps and the mean free path l∼10–102nm for electron densities n=1012–1013cm−2. In particular, we find that the mean free path exhibits a finite jump at the phonon energy 200meV due to electron-phonon interaction. Our results are directly applicable to device structures where ballistic transport is relevant with inelastic scattering dominating over elastic scattering.

The genesis and importance of oxide–metal interface controlled heterogeneous catalysis; the catalytic nanodiode

Much of the research by Konrad Hayek’s group is focused on the influence of oxide–metal interfaces in heterogeneous catalysis. In this paper, we show that electronic excitations at the metal surface induced by exothermic catalytic reactions lead to the generation of energetic (hot) electron flows. We detected the flow of hot electrons during oxidation of carbon monoxide using Pt/TiO2 Schottky diodes. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. We found that heat generated by the reaction caused a negligible increase of temperature in our experimental range, suggesting that this thermal effect is not responsible for the generation of hot electron flow. We tested the stability and reversibility of chemicurrent generated during CO oxidation at 413~573 K. The activation energy calculated from the measurement of chemicurrent is quite close to that of the turnover rate of chemical reaction, which indicates that the generation mechanism of hot electrons is closely correlated with the chemical reaction. This correlation suggests that hot electron flow could be a new tool to probe the role of oxide–metal interfaces in heterogeneous catalysis.

Continuity-Equation Analysis of Hot Electron Base Transport in InP/InGaAs Heterojunction Bipolar Transistors

The static and small-signal properties of the hot electron base transport in InP/InGaAs heterojunction bipolar transistors are analyzed theoretically within the framework of the continuity equation. By introducing a probability distribution function for a hot electron in the base, which is a continuous function of position, the nonequilibrium electron transport is integrated into the conventional drift-diffusion theory. The dc analysis of the continuity equation shows that the proposed theory is feasible for the seamless description of ballistic electron transport in thin bases and diffusion-like electron transport in thick bases. In the small-signal ac analysis, an analytic expression for the base transport factor is derived taking into account the hot electron transport. Moreover, a simple expression for the base transit time in the presence of hot electrons is obtained through the analysis of the derived base transport factor. The expression is given as a linear combination of the transit time for ballistic transport of hot electrons and the transit time for diffusive transport of energy-relaxed electrons.

Terahertz negative conductivity of heterostructure barriers during ballistic transport of hot electrons

A mechanism for achieving the terahertz negative conductivity on transmit time effects during ballistic transport of hot carriers in nanoscale semiconductor heterostructures was theoretically considered. It was shown that selection of the heterostructure potential profile can significantly increase high-frequency negative conductivity.

Dynamics of Surface Catalyzed Reactions; the Roles of Surface Defects, Surface Diffusion, and Hot Electrons

The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the high-pressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H(2) and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), pi-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, pi-allyl c-C6H9, 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography.

Energy conversion from catalytic reaction to hot electron current with metal-semiconductor Schottky nanodiodes

Exothermic catalytic reactions induce electronic excitation at the metal surface, leading to the production of energetic hot electrons. We monitored the flow of hot electrons for over several hours using two types of metal-semiconductor Schottky diodes, Pt∕TiO2 or Pt∕GaN, during the platinum catalyzed oxidation of carbon monoxide. The thickness of Pt film used as the catalyst was 5nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. The chemicurrent was well correlated with the turnover rate of CO oxidation separately measured by gas chromatography, suggesting the possibility of application as chemical sensors with high sensitivity.

Ballistic hot-electron transport in nanoscale semiconductor heterostructures: Exact self-energy of a three-dimensional periodic tight-binding Hamiltonian

As the length scale for semiconductor heterostructures approaches the regime of the lattice constant, our current theory for calculating ballistic hot-electron transport becomes inapplicable. In this case, a method such as the Green's function formalism should be used to calculate ballistic electron transmission functions from the exact, periodic lattice potential. We present a method for directly calculating the exact surface Green's function for three-dimensional periodic leads which is necessary for such a scheme. Except in cases of high crystal symmetry, the method is limited by the difficulty to solve a nonsymmetric matrix Riccati equation.

Infrared multiple-quantum-well phototransistor

The multiple-quantum-well (MQW) phototransistor is considered theoretically. The MQW phototransistor utilizes intersub-band optical absorption and exhibits giant photocurrent gain which can lead to very high responsivity and detectivity. This effect is due to the thermionic injection of hot electrons across the emitter barrier and fast electron transit through the MQW base. An analytic theory of the MQW phototransistor utilizing parameters evaluated by Monte Carlo simulation is proposed. Transition from near ballistic hot electron transport to diffusive transport decreases the responsivity but its value can be significant in this case as well.

Hot Electron Reduction at n‐Si/Au Thin Film Electrodes

Schottky barrier electrodes were prepared by deposition of thin films of Au on device quality n‐Si. Barrier heights for fourteen day old films, determined by high frequency capacitance measurements at open circuit in the presence of , were found to be 0.60 eV in the thickness range of 200 to 1000 Å. The exchange current densities were determined as a function of film thickness and aging time up to about one month after deposition. Ideal solid‐state‐controlled Tafel slopes of 60 mV/decade were observed for overpotentials up to 120 mV. Mott‐Schottky plots confirmed the symmetry factor was unity. The ratio of the exchange current was about 100 for measurements made within 1 h after deposition. A similar increase in with decreasing thickness was observed at five and fourteen days, but the magnitude of the currents were lower by one to two orders of magnitude, with larger changes for thinner films. The lower currents after aging, were attributed to larger barrier heights, perhaps caused by changes in the interfacial layer such as oxidation. The observed dependence of exchange current on thickness after each aging period, was successfully modeled by assuming ballistic transport of hot electrons across the metal film. The ballistic mean‐free path of electrons in our Au films, determined by application of the theory developed in Paper I , was .

Hot electron transport in two dimensional electron gas

We demonstrate ballistic transport of hot electrons in a high mobility two dimensional electron gas (2DEG) over distances as large as two micrometers and tunneling through a barrier induced in the 2DEG. Various inelastic scattering mechanisms are studied experimentally. For electrons with excess energy larger than the longitudinal optical (LO) phonon energy quanta we show sequential emission of up to three phonons. Below that energy we find a surprisingly long mean free path, approximately an order of magnitude longer than expected theoretically for electron-electron scattering. The combination of a very short mean free path for LO phonon emission and the long mean free path below the phonon energy leads to a new type of periodic oscillations in the maximal energy of the collected electrons. Finally we report on a study of the angular distribution of hot electrons injected from a point contact and demonstrate electrostatic steering of the electronic beam.

Ballistic hot-electron transistors

We present an overview of work at the IBM Thomas J. Watson Research Center on the tunneling hot-electron transfer amplifier (THETA) device—including its use as an amplifier and as a tool for investigating ballistic hot-electron transport. In the initial, vertically configured version of the device, a quasi-monoenergetic, variable-energy, hot-electron beam is generated (via tunneling) which traverses a thin GaAs region and is then collected and energy-analyzed. As the hot electrons traverse the device, they are used to probe scattering events, band nonparabolicity, size-quantization effects, and intervalley transfer. A recent, lateral version of the device has been used to demonstrate the existence of ballistic hot-electron transport in the plane of a two-dimensional electron gas, and the associated possibility of achieving high gain.

Inelastic scattering effects on carrier relaxation in quantum well-based hot electron structures

We calculate the inelastic scattering rate of “hot” electrons injected into doped quantum well structures by including on an equal footing, both the electron-electron (Coulomb) and the electron-polar optical phonon (Fröhlich) interaction effects in the theory within the dynamical random-phase-approximation. Our theory includes in a consistent diagrammatic approximation effects of quantum statistics, dynamical screening and phonon renormalization (i.e. the so-called “plasmon-phonon coupling” effect). Our results, obtained for the two-dimensional, the quasi-two-dimensional and the three-dimensional models, are appropriate for a number of transport and optical experiments including ballistic hot-electron transport, resonant tunneling, femtosecond carrier relaxation, and, planar parallel transport.