Mean Free Path Of Hot Electrons Research Articles

Annealing effect on terahertz emission enhancement from ferromagnetic heterostructures

We systematically investigate the influence of annealing effect on terahertz (THz) generation from CoFeB/heavy metal heterostructures driven by femtosecond laser pulses. The THz yield is achieved to increase triply in W/CoFeB through annealing effect, and doubly in Pt/CoFeB. The annealing effect originates from both the decrease of synthetic effect of THz absorption and the increase of hot electron mean free path induced by crystallization, with the latter being dominant, which is experimentally corroborated by THz transmission measurement of time-domain spectrum and four-probe resistivity <i>t</i>. Our observations not only deepen understand the spintronic THz radiation mechanism but also provide a novel platform for high speed spintronic opto-electronic devices.

Heteroepitaxial vertical perovskite hot-electron transistors down to the monolayer limit

Two-dimensional heterostructures combined with vertical geometries are candidates to probe and utilize the physical properties of atomically-thin materials. The vertical configuration enables a unique form of hot-carrier spectroscopy as well as atomic-scale devices. Here, we present the room-temperature evolution of heteroepitaxial perovskite hot-electron transistors using a SrRuO3 base down to the monolayer limit (∼4 Å). As a fundamental electronic probe, we observe an abrupt transition in the hot-electron mean free path as a function of base thickness, coinciding with the thickness-dependent resistive transition. As a path towards devices, we demonstrate the integrated synthesis of perovskite one-dimensional electrical edge contacts using water-soluble and growth-compatible Sr3Al2O6 hard masks. Edge-contacted monolayer-base transistors exhibit on/off ratios reaching ∼108, complete electrostatic screening by the base manifesting pure hot-electron injection, and excellent scaling of the output current density with device dimensions. These results open new avenues for incorporating emergent phenomena at oxide interfaces and in heterostructures.

An Efficient Nanophotonic Hot Electron Solar-Blind UV Detector

A solar-blind UV photodetector is designed and fabricated based on a nanophotonic metal–oxide–semiconductor structure. A large potential barrier of ∼3.8 eV at the metal/oxide interface enables solar-blind UV detection by blocking the electrons excited by visible photons while allowing UV-excited hot electrons to pass through. By selecting metal absorbers with high density of states near the Fermi level and employing photon management in self-assembled pseudoperiodic metal nanostructures, we managed to achieve ∼74% spectrally averaged UV absorption at λ = 200–300 nm. Furthermore, such a high UV absorption is achieved within the hot electron mean free path of ∼50 nm from the metal/oxide interface, effectively facilitating the ballistic transport of the UV-excited hot electrons across the interfacial barrier and improving the overall photocurrent. The device has demonstrated a responsivity of 29 mA/W and an internal quantum efficiency of ∼18% at λ = 269 nm, a significant advance compared to ∼1% internal quan...

Measurement of the hot electron mean free path and the momentum relaxation rate in GaN

We present a method for measuring the mean free path and extracting the momentum relaxation time of hot electrons in GaN using the hot electron transistor (HET). In this device, electrons are injected over a high energy emitter barrier into the base where they experience quasi-ballistic transport well above the conduction band edge. After traversing the base, high energy electrons either surmount the base-collector barrier and become collector current or reflect off the barrier and become base current. We fabricate HETs with various base thicknesses and measure the common emitter transfer ratio (α) for each device. The mean free path is extracted by fitting α to a decaying exponential as a function of base width and the relaxation time is computed using a suitable injection velocity. For devices with an injection energy of ∼1 eV, we measure a hot electron mean free path of 14 nm and calculate a momentum relaxation time of 16 fs. These values are in agreement with theoretical calculations where longitudinal optical phonon scattering is the dominant momentum relaxation mechanism.

Hot-electron mean free path of ErAs thin films grown on GaAs determined by metal-base transistor ballistic electron emission spectroscopy

We present an experimental investigation of the hot-electron mean free path in ErAs thin films grown on GaAs. Using an $\mathrm{Al}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}∕\mathrm{Al}$ tunnel junction as a hot-electron source for ballistic electron emission spectroscopy, we investigate ErAs films of thicknesses $\ensuremath{\sim}100--\ensuremath{\sim}300\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Our results indicate a mean free path of order $100\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for electrons $1--2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the Fermi level at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Electric-field-induced impact ionization of excitons in GaN and GaN/AlGaN quantum wells

Impact ionization of exciton states in epitaxial GaN films and GaN/AlGaN quantum-well structures was studied. The study was done using an optical method based on the observation of exciton photoluminescence quenching under application of an electric field. It was established that electron scattering on impurities dominates over that from acoustic phonons in electron relaxation in energy and momentum. The mean free path of the hot electrons was estimated. The hot-electron mean free path in GaN/AlGaN quantum wells was found to be an order of magnitude larger than that in epitaxial GaN films, which is due to the electron scattering probability being lower in the two-dimensional case.

Measurement of Electron Ballistic Length in p-Doped InGaAs

The electroluminescence of InP/AlAs/InGaAs n–p heterodiodes is carefully analyzed as a function of temperature, p-doping level and InGaAs layer thickness. It provides an original method to measure hot electron mean free path in p-type InGaAs. Furthermore, this measurement can be performed for various p-doping levels, ballistic electron energies or temperatures.

Development of the spin-valve transistor

As the easiest experimental approach, GMR (giant magnetoresistance) is usually measured using the current in plane (CIP)-GMR. The spin-valve transistor has previously been presented as a spectroscopic tool to measure current perpendicular to the planes (CPP)-GMR. Hot electrons cross the magnetic multilayer base quasi-ballistically and the number reaching the collector depends exponentially on the perpendicular hot electron mean free path. Collector current changes of 390% at 77 K have already been measured. Apart from the substantial fundamental value, such properties may be useful for sensor applications. The electron energy range fills the gap between the Fermi surface transport in resistance measurements and other hot electron techniques such as spin polarised electron energy loss spectroscopy (SPEELS). The preparation problem of the spin-valve transistor and metal base transistor structures in general, the deposition of a device quality semiconductor on top of a metal, has now been tackled by bonding of two semiconductor substrates during vacuum deposition of a metal: an excellent bond is achieved at room temperature. TEM photos show a continuous buried metal film. Apart from preparation of various metal base transistor like structures, many other fields may benefit form this new technique.

Experiments of highly emissive metal–oxide–semiconductor electron tunneling cathode

An effective mean-free path of hot electrons in the conduction band of SiO2 in a Si-gate metal–oxide–semiconductor (MOS) electron tunneling cathode was measured and found to be about 0.7 nm. Following these observations, we proposed and fabricated a depletion gate MOS electron tunneling cathode. The highest transfer ratio of 13.3% was achieved in the cathode at the low emission current level, which was considerably higher than that of tunneling cathodes studied in the past. However, the ratio decreased drastically at high current due to the hole injection into the depletion region from the gate.

Smith Purcell effect in GaAs/AlGaAs heterostructures

We report a new experimental technique to study the form of the hot electron distribution function in GaAs/AlGaAs heterostructures. A weak periodic surface potential induces Smith-Purcell-FIR-radiation of the electric field driven hot electrons in the 2-dimensional electron gas directly reflecting their velocity distribution. The FIR- radiation is detected by a magnetic field tuned InSb-detector. In samples with very low electron concentration and high mobility the emission spectra show a significant shift to higher energies and develop a steep high energy slope with increasing electric field when we use the geometry with grating vector q directed parallel to the electric field (q ∥ E). In the geometry q ⊥ E smooth decays are observed at lower energies. Comparison of the results with theory gives experimental evidence of a non-equilibrium shape of the distribution caused by the onset of LO-Phonon emission. In addition, the hot electron mean free path of the heated distribution is derived by investigating the experimental emission spectra as a function of the grating period length. The influence of a limited hot electron mean free path on the spectral width is described in terms of a Fourier-analysis of the interaction potential. In drift direction a mean free path of λ = 200 nm is obtained, whereas the mean free path is smaller in the direction perpendicular to the drift direction.

Direct measurement of hot electron energy distributions in Al0.35Ga0.65As field-assisted photocathodes

A spectroscopic investigation of the electron energy distributions and spectral photosensitivities of the photoemission from field-assisted caesiated metal-semiconductor structures has been performed. Strong field heating of the electron distribution in the semiconductor crystal due to the applied electric field was directly observed and an increase in the photoemission sensitivity by a factor of approximately=30 was seen upon the application of an applied electric field of approximately=2.7*107 V m-1. From an analysis of the variation of the photoemission current with reverse bias an estimate of 5.2+or-1.3 nm was obtained for the hot electron mean free path in Al0.35Ga0.65As. The energy distribution curves of the emitted photoelectrons were measured and were modelled to extract electron temperatures as a function of field in the high-field regime. Effective temperatures in excess of 5000 K were observed for fields of approximately=2*107 V m-1.

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.

Coherent enhancement of the hot-electron mean free path by superlattice transmission resonances.

We report a strong enhancement of the optical responsivity in GaAs/AlGaAs multi-quantum-well infrared detectors. This is explained by a resonant increase in the hot-electron mean free path as a result of the coherent transmission resonances in the continuum states above the energy barriers of the superlattice.

Energy-resolved study of the spin precession in photoemission from activated (110) GaAs.

The measurement of the spin vector S of electrons photoemitted from a III-V compound semiconductor activated to negative electron affinity is shown to be a powerful means to explore band hybridization properties, surface effects, and to estimate characteristic times of electron relaxation. Because of the lack of inversion symmetry (i.e., to the conduction-band spin splitting), an internal spin precession occurs in the bulk semiconductor and also in the band-bending region (BBR) near the surface. From the modulus of S we determine the hot-electron mean free path. From the S direction we deduce the angle of spin precession ${\ensuremath{\theta}}_{p}$ due to the internal precession vector \ensuremath{\omega}. The experimental originality of the present work stands in the combination of the S measurement with the high-resolution energy analysis of the photoemitted electrons. The dependence of ${\ensuremath{\theta}}_{p}$ on the electron kinetic energy reflects the memory of the initial anisotropic distribution of electron momenta, and its modifications during thermalization and transit through the BBR. Our analysis relates the D'ymnikov-D'yakonov-Perel' formalism for the calculation of spin-polarization properties under circularly polarized light excitation with the widely used Kane k\ensuremath{\cdot}p? band description.

High field hot electron transport through Al xGa 1-xAs multiquantum well superlattices

We report the first measurement of the high electric field hot electron mean free path L for transport through Al xGa 1-xAs multiquantum well superlattices by fabricating a novel detector whose γ = 10 μm infrared sensitivity critically depends on hot electron transport. We find L to be extremely large namely L = 4500 A ̊ for a 50 period superlattice composed of 72Å GaAs quantum wells separated by 133Å Al 0.38Ga 0.62As tunneling barriers (i.e., L = 22 periods). In addition, we have achieved, for the first time, an avalanche gain of 300% at 10μm in these quantum well superlattices. The electrons are generated by resonant intersubband tunneling and then avalanche by hot electron impact ionization of carriers out of the quantum wells. By analyzing these measurements we find a highly nonthermal peaked electron distribution with a relatively narrow energy width of 60 meV.

New quantum well long-wavelength ( lambda =10 mu m) detectors and novel superlattice transport physics

The authors have demonstrated the first 10 mu m detector based on bound-to-continuum absorption in doped GaAs/AlGaAs quantum wells which have been designed to contain only one bound state. The linear voltage dependence of the responsivity is strikingly different from these previous intersubband tunneling detectors, containing two bound states in the quantum well, which show an exponential tunneling voltage dependence. The hot electron mean free path (1700 AA) for transport above the superlattice as well as the mobility and carrier lifetime (2 ps) have been determined. The authors have measured the continuum infrared ( lambda approximately 10 mu m) photoconductivity spectrum for a dual-bound-state intersubband absorption photoexcited tunneling quantum-well detector. The line shape is broadened and asymmetrical with respect to the zero-bias Lorentzian absorption spectrum. They report intersubband absorption experiments in doped InGaAs/InAlAs multiquantum well superlattices and observe a resonance peak at a wavelength of lambda =4.4 mu m, which is in good agreement with theory. The authors have measured a novel negative differential photoconductance (at 10 mu m) in an alternately doped multi-quantum-well structure. From this measurement, the density of the electrons dynamically stored in the undoped wells can be deduced at different external biases.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Superconductor-Base Hot-Electron Transistor I. Theory and Design

This paper proposes a theoretical design for a superconductor-base hot-electron transistor (Super-HET), a new device which can be regarded as one of the ultimate hot-electron transistors. Our study reveals that a Super-HET with a GaAs/Nb-based compounds/InSb/metal structure has excellent characteristics, although it works only in a limited temperature range. The total current transmission coefficient and the maximum oscillation frequency are predicted to be more than 96% and in excess of 1000 GHz, respectively. The estimation and design are performed taking into account the quantum mechanical reflection effect, the hot-electron mean free path in superconductor and semiconductor, Monte Carlo simulation, and unilateral power gain.

Injected-hot-electron transport in GaAs.

Using the new experimental technique of hot-electron spectroscopy we have, for the first time, observed electrons in extreme nonequilibrium. When the transit-region width is comparable to the calculated hot-electron mean free path, two peaks are observed in the measured spectrum. The high-energy peak corresponds to quasiballistic electrons suffering few, if any, collisions in the transit region. The low-energy peak is due to excitation of the Fermi sea by the injected hot electrons.

Photoemission from activated gallium arsenide. II. Spin polarization versus kinetic energy analysis.

The spin polarization of the electrons emitted by a GaAs photocathode under circularly polarized light excitation is investigated as a function of the electron kinetic energy. The photocathode is activated by cesium and oxygen coadsorption under ultrahigh-vacuum conditions to achieve a negative electron affinity. The spin polarization is measured by Mott scattering. The study is performed with a very-high-energy resolution (20 meV), at 300 and 120 K, under well-focused ${\mathrm{Kr}}^{+}$-laser light excitation (photon energy ranging from 1.55 to 2.60 eV). The polarization-versus-energy distribution curves show typical features related to those observed in the energy distribution curves, which are analyzed in detail in the preceding paper [H.-J. Drouhin, C. Hermann, and G. Lampel, Phys. Rev. B 31, 3859 (1985)]. A model is developed to account for the largest measured polarization, which arises from electrons excited from the heavy-hole band and emitted without suffering any collision: A (2/3) maximum value is expected, which is reduced by spin precession in the internal D'yakonov and Perel' (DP) field, due to the absence of space-inversion symmetry in GaAs. An estimation of the hot-electron mean free path (\ensuremath{\sim}0.1 \ensuremath{\mu}m for photon energy above 1.96 eV) is deduced. The photoemission polarizations of the electrons excited from each of the two other valence bands are also calculated using a nonparabolic Kane band model. The L and X subsidiary minima give rise to polarization plateaus originating from energy relaxation in the band-bending region. The main contribution to the photocurrent is due to electrons which were thermalized in the central minimum of the bulk crystal and have relaxed their energy in the band-bending region prior to emission into vacuum. Their polarization is studied in relation with the luminescence polarization, measured on the same samples, in the framework of a one-dimensional diffusion model. An additional depolarization, occurring during the escape process, is evidenced and attributed to the DP relaxation mechanism in the band-bending region. Finally, the performances of GaAs photocathodes as monochromatic and polarized electron sources are analyzed with use of the physical concepts developed in the present paper and in the preceding one.

Internal photoemission mechanisms at interfaces between germanium and thin metal films

The quantum efficiency associated with the internal photoemission of electrons over the Schottky barrier (of height φ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> ) at the metal-Ge interface has been studied experimentally for several metals (Au, Cu, Ag, Pb, and Ni). A theoretical description of this mechanism has been developed in which we take into account the front and back optical absorptance, hot electron scattering, and multiple reflections of excited electrons from the surfaces of the thin electrode film. We have found it necessary to impose a modification of the Fowler theory of photoemission when applied to internal photoemission from thin metal films over a Schottky barrier. This modification relates to an enhanced photoexcitation within the metal films which is attributed in the present theory to a density of states which exhibits a peaked distribution in energy rather than the simple parabolic bands assumed by Fowler. It is clear from the present study that the majority of photoelectron excitation occurs from a small region of energy of the order of a fraction of an electron volt near the Fermi energy. The theoretical model presented here defines two important parameters: a hot-electron mean free path (L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> ) and an energy (E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ef</inf> ) given by the difference between the Fermi level and the effective conduction hand minimum associated with the region of energy in the metal near the Fermi level where the electron distribution is strongly peaked. Values of L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> for Au is 550 Å, Ag is 570 Å, Cu is 450 Å, and Pb is 55 Å. E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ef</inf> for Au is 0.1 eV, Ag is 0.152 eV, Cu is 0.11 eV, Pb is 0.1 eV. The validity of this model is confirmed by the experimental finding that the parameters L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> and E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ef</inf> are independent of metal thickness.