Free Path Of Hot Electrons Research Articles

Revealing the Loss Mechanism of Chemically-Induced Hot Electron Transport.

Hot electrons are crucial for unraveling the intrinsic relationship between chemical reactions and charge transfer in heterogeneous catalysis. Significant research focused on real-time detection of reaction-driven hot electron flow (chemicurrent) to elucidate the energy conversion mechanisms, but it remains elusive because carrier generation contributes to only part of the entire process. Here, a theoretical model for quantifying the chemicurrent yield is presented by clarifying the contributions of hot carrier losses from the internal emission and multiple reflections. The experimental chemicurrent yield verifies our model with a reliable mean free path of hot electrons, emphasizing the importance of comprehensive consideration of the transport process besides hot electron generation. Moreover, Pt nanoparticles (NPs)-decorated Au/TiO2 is examined, showing the role of NPs-induced carrier losses in the performance of catalytic nanodiodes. These findings are expected to contribute to understanding the hot electron detection efficiency and designing nanodiodes with enhanced hot carrier flow and catalytic activity.

Probing Hot Electron Flow Generated on Pt Nanoparticles with Au/TiO2Schottky Diodes during Catalytic CO Oxidation

Hot electron flow generated on colloid platinum nanoparticles during exothermic catalytic carbon monoxide oxidation was directly detected with Au/TiO2 diodes. Although Au/TiO2 diodes are not catalytically active, platinum nanoparticles on Au/TiO2 exhibit both chemicurrent and catalytic turnover rate. Hot electrons are generated on the surface of the metal nanoparticles and go over the Schottky energy barrier between Au and TiO2. The continuous Au layer ensures that the metal nanoparticles are electrically connected to the device. The overall thickness of the metal assembly (nanoparticles and Au thin film) is comparable to the mean free path of hot electrons, resulting in ballistic transport through the metal. The chemicurrent and chemical reactivity of nanoparticles with citrate, hexadecylamine, hexadecylthiol, and TTAB (tetradecyltrimethylammonium bromide) capping agents were measured during catalytic CO oxidation at pressures of 100 Torr O2 and 40 Torr CO at 373-513 K. We found that chemicurrent yield varies with each capping agent but always decreases with increasing temperature. We suggest that this inverse temperature dependence is associated with the influence of charging effects due to the organic capping layer during hot electron transport through the metal-oxide interface.

Impact ionization of excitons in an electric field in GaN

The impact ionization of excitonic states is studied in epitaxial GaN filmsand GaN/AlGaN quantum well structures. The investigation is carried out byoptical methods involving the observation of quenching of the excitonphotoluminescence under an applied electric field. It is found that impurityscattering rules the momentum and energy relaxation, rather than the acousticphonon scattering. The effective mean free path of hot electrons is estimated.In GaN/AlGaN quantum wells the mean free paths of hot electrons appear to bean order of magnitude larger than those of GaN films due to the decrease inscattering probability of the electron in the two-dimensional case.

Impact Ionization of Excitons in an Electric Field in GaN

Quenching of the exciton luminescence of GaN epitaxial films and GaN/AlGaN quantum wells in an electric field is observed and attributed to impact ionization of excitonic states by hot electrons. It is found that impurity scattering rules the momentum relaxation of electrons rather than the acoustic phonon scattering. The effective mean free path of hot electrons l(eff) is estimated. The value of l(eff) for quantum wells appears to be one order of magnitude larger than that for epitaxial films.

Enhancement of Electrochemical Hot Electron Injection into Electrolyte Solutions at Oxide-Covered Tantalum Electrodes by Thin Platinum Films

The previously reported experimental evidence for hot solution-phase electrons generated at Ta2O5-covered Ta electrodes in both acetonitrile and aqueous solutions using electrogenerated chemiluminescence (ECL) and electrochemical measurements was extended by observing the effect of thin Pt films. Hot electron injection was monitored by noting the photoemission following reduction of the thianthrene radical cation (TH•+), signaling the direct formation of the excited state (TH•+ + ehs → TH*), a process that does not occur at a bulk Pt electrode. We report the enhancement of hot electron injection efficiency by a factor of ∼5 by deposition of a thin (<40 nm) Pt film on the Ta/Ta2O5 electrode. This enhancement of ECL efficiency at the metal/oxide/Pt/liquid interface is ascribed to the suppression of the Ta2O5 surface states by the Pt film. The effect of Pt film thickness was investigated and showed a decrease in emission with increased film thickness, in accord with the expected mean free path of hot electrons in the Pt (ehPt). In examining the Ta/Ta2O5/Pt/solution system, one can contrast the behavior observed when an electrical connection is made directly to the Pt with that observed when a connection to the Pt is made via the Ta, where ehPt species are generated.

Investigation of the mean free path of hot electrons in GaAs-AlGaAs heterostructures

The mean free path of hot electrons in GaAs/AlGaAs heterostructures is investigated. For the first time grating-induced Smith-Purcell emission is studied as a function of the grating period length. The mean free path is obtained from the changes in intensity and width of the experimental spectra by performing a Fourier analysis of the effective interaction potential. In the direction of the applied electric field a mean free path of lambda approximately 300 nm is obtained, whereas perpendicular to the electric field the mean free path is smaller ( lambda approximately=150 nm).

Ballistic electron emission microscopy study of PtSi–n-Si(100) Schottky diodes

PtSi–n-Si(100) Schottky contacts have been studied by ballistic electron emission microscopy (BEEM) for PtSi layers from 30 to 195 Å thickness. Locally measured barrier heights were typically close to or above those determined from the diode I–V characteristics, with a tendency to higher barriers for the thinner silicide films. The ballistic transmission probability also showed local variations with a clear tendency to decrease for increasing PtSi layer thicknesses, which can be understood in terms of the mean free path of hot electrons in PtSi. From simultaneous measurements at several voltages over the same area, maps of the apparent barrier height and transmission rate have been obtained showing sometimes strong local variations of these quantities. Very low and very high apparent barriers that were occasionally observed seemed to show that the simple BEEM picture is not always applicable, possibly due to interface roughness.

Long mean free path of hot electrons selectively injected to higher subbands.

The long hot-electron mean free path observed by Sivan, Heiblum, and Umbach in high-mobility 2D electron-gas heterostructures is explained by electron transport in the second subband. Our estimates show that the mean free path in the second subband is longer by 2 orders of magnitude than that in the first subband. New magnetic-focusing measurements reveal hot-electron velocities lower than the Fermi velocity, as expected for a higher-subband transport. The electrostatic potential near a biased constriction used as the hot-electron injector is shown to induce nonadiabatic intersubband transfer.

Trapped-electron and generated interface-trap effects in hot-electron-induced MOSFET degradation

A new experimental method is proposed to distinguish the electron-trapping effect in the gate oxide from the interface-trap generation effect in hot-electron-induced nMOSFET degradation. In this method, by selecting the appropriate bias conditions, hot electrons and/ or hot holes are intentionally injected into the oxide region above the channel outside the drain layer, which affects MOSFET characteristics such as threshold voltage and transconductance. The negative charges of electrons trapped in the oxide during hot-electron injection are completely compensated for by the positive charges of subsequently injected and trapped holes, and the trapped electron effect in the degradation is eliminated. Using this method, the causes for hot-electron-induced transconductance degradation (Δg <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> /g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> ) are analyzed. As the degradation increases, the trapped-electron effect decreases, and the generated interface-trap effect increases. The relationship of (Δg <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> /g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> )_{it}, = <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</tex> (Δg <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> /g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> ) -- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</tex> is obtained, where (Δg <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> /g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> )_{it} is g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> degradation due to generated interface-traps, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</tex> are fixed numbers. Furthermore φ_{it}/λ (the ratio of the critical value in hot-electron energy for interface-trap generation to the mean free path of hot electrons in Si) is experimentally obtained to be 5.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> eV/cm. Using λ = 9.2 nm [1], a value of φ_{it} = 5.2 eV is derived.

Optical Determination of Mean Free Path of Hot Electrons in Metals

AbstractLight emission via surface plasmon excitation can be used as a sensitive detector for hot electrons in metal‐insulator‐metal structures. Ballistic and electron‐electron scattering mean free pathvalues obtained for silver with this method in the 2.19 to 2.44 eV energy range are in good agreement with theoretical predictions.

Mean free path of hot electrons at the surface of boron-doped silicon

Electrons can be injected from silicon into silicon dioxide in a metal-oxide-semiconductor-transistor (MOST) structure in which electrons are accelerated in the depletion layer of the channel region in a direction perpendicular to the Si-SiO2 interface. An analysis of the resulting oxide current as a function of the reverse voltage applied to the source and drain regions is carried out using an extension of a model proposed by Shockley. This leads to an estimate of the mean free path λ of the hot electrons in the silicon. It appears that a heavily doped source and drain, formed by a phosphorus diffusion, or the dislocations generated by this diffusion, have the effect of increasing λ by, presumably, the removal of scattering centers. The largest value of λ observed, λ=135 Å, is possibly equal to the value of the mean free path for the generation of high-energy phonons.

Hot electron scattering and optical density of states of yttrium

Abstract The mean free path of hot electrons in Y has been determined as ∼ 10A at 7 eV above ther Fermi level EF using a new photoemission method. The importance of this strong electron≅electron scattering in photoemission spectroscopy work is shown via an analysis of the optical density of states of Y. We find an occupied d-band with a peak at EF and a width of 1.6 ± 0.2 eV (at 1 2 maximum).

Mean Free Path of Hot Electrons and Holes in Metals

Mean Free Path of Hot Electrons and Holes in Metals

Mean Free Path of Hot Electrons and Holes in Metals

The mean free paths and attenuation lengths of hot electrons and holes in metals are calculated by Morte Cario methods. The results are compared with experimental results for electrons in Au,-Ag, Cu, and Pd and holes in Au. (T.F.H.)