Hot-electron Transport Properties Research Articles

Simulation of Space-Charge-Limited Current for Hot Electrons With Initial Velocity in a Vacuum Diode

In this article, a sheet model is utilized to model and simulate hot electron transport properties in vacuum gap. Based on this model, emitted electrons with constant initial velocities and Maxwell–Boltzmann distribution are considered. Numerical analyses of the potential, electric field, current density, velocity, and phase-space distribution are also performed. The accuracy of this model is verified by comparing with analytical results. Furthermore, we studied the velocity distribution of the emitted electrons at any position in the interelectrode gap and observed a significant change in the distribution.

Hot-electron transport studies of the Ag/Si(001) interface using ballistic electron emission microscopy

Ballistic electron emission microscopy has been utilized to investigate the hot-electron transport properties of the Ag/Si(001) Schottky diode utilizing metal films deposited both in situ and ex situ. The Schottky barrier heights are measured to be 0.57±0.02 and 0.59±0.02 eV for the ex situ and in situ depositions, respectively. The metal overlayers demonstrate typical Volmer–Weber growth when deposited on the Si(001) semiconducting substrate, as seen in the scanning tunneling microscopy images. An enhancement in hot-electron transmission is measured for the in situ deposited metal films when compared to the ex situ films.

Temperature and doping dependencies of hot electron transport properties in bulk GaP, InP and Ga0.5In0.5P

An ensemble Monte Carlo simulation has been carriedout to study electron transport properties in GaP, InP and Ga 0.5 In 0.5 P materials. The simulation results show that intervalley electron transfer plays a dominant role in higher electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. In addition, the electron velocity in GaP is less sensitive to temperature than other group III-V semiconductors like InP and Ga 0.5 In 0.5 P. So GaP devices are expected to be more tolerant to self-heating and high ambient temperature device modeling.

Hot electron transport studies of the Cu/Si(001) interface using ballistic electron emission microscopy

The hot electron transport properties of the Cu/Si(001) interface have been studied using ballistic electron emission microscopy (BEEM). The Schottky barrier height was measured to be 0.64±0.02 eV. The scanning tunneling microscopy images provide evidence of Volmer–Weber growth of the metal, while Rutherford backscattering spectrometry data corroborated with Auger depth profiling indicate distinct Cu and Si regions with little intermixing. Comparison with Au/Si(001) BEEM data provides some insight into the hot electron transport and scattering properties of the Cu/Si(001) interface.

Hot-electron transport properties of CoFe/n-Si and CoFe/Cu/n-Si junctions

The hot-electron transport properties of different thickness of CoFe films deposited on n-Si substrate with and without Cu layer were investigated. Diode characteristics were tested to obtain the heights of Schottky barrier for different samples. The dependences of Schottky heights on CoFe thickness were studied. The research shows that the height of the Schottky barrier can be adjusted and a good Schottky diode can be obtained by controlling the thickness of CoFe film accurately. The results are very important for the application of spintronic devices, such as spin valve transistor (SVT) and magnetic tunnel transistor (MTT).

Hot electron transport across manganese silicide layers on the Si(001) surface

Ballistic electron emission microscopy (BEEM) has been performed on MnSi∕Si(001) Schottky diodes at 80K to study the hot electron transport properties. The BEEM spectra best fit the thermally broadening 5∕2 power law model with two threshold heights at 0.71 and 0.86eV, indicating a complex interface band structure. In addition, the normalized BEEM current in the MnSi overlayer was found to be approximately seven times less than is observed in Au∕Si(001) samples of similar thicknesses, indicating a larger amount of hot electron scattering in the MnSi∕Si(001) samples.

The Transport Properties of Hot Electrons in a Xenon/Methane Mixture

The results are given of numerical solution of the Boltzmann equation in a binomial approximation in view of elastic and inelastic electron collisions in Xe+CH4 mixtures and in pure methane. The electron energy distribution functions obtained are used to calculate the electron transport coefficients for E/N values of up to several Townsends, i.e., the drift velocity, mobility, average and characteristic energies, diffusion coefficient. The results of calculation for pure methane fit the available experimental data. A similarity rule is found for the electron transport coefficients in a Xe+CH4 mixture with different concentrations of methane molecules, which enables one to determine the values of transport coefficients in a mixture with a minor (less than 30 percents) methane content.

Nonparabolic multivalley balance-equation approach to high-field electron transport and impact ionization in ZnS: Comparison with full-band Monte Carlo simulations

Extended nonparabolic multivalley balance equations including impact ionization (II) process are presented and are applied to study electron transport and impact ionization in ZnS with a $\ensuremath{\Gamma},$ X, and L conduction-band structure at high electric field up to 2000 kV/cm. Hot-electron transport properties, such as electron drift velocity, electron energy, intervalley electron transfer, and electron-hole pair generation, are calculated by taking account of the scattering from polar optical, deformation potential, and intervalley interactions. Quantitative agreement is obtained among the results for drift velocity and II coefficient derived from the present calculations, from the recent full-band Monte Carlo simulations, and from experiments, thus confirming the validity of the present multivalley nonparabolic balance-equation approach.

Optical characterization of individual semiconductor nanostructures using a scanning tunneling microscope.

By injecting low-energy minority carriers from the tip of a scanning tunneling microscope (STM) and analyzing the light emitted from the tip-sample gap of the STM, it is possible to study the optical and electronic properties of individual semiconductor nanostructures with an extremely high spatial resolution close to the atomic scale. This technique has been applied to investigate the transport properties of hot electrons injected into AlGaAs/GaAs quantum well structures and the optical properties of single self-assembled InAs/AlGaAs quantum dots. The physical principles, usefulness and future expectations of this novel technique are discussed.

Transport in a Polarization-Induced 2D Electron Gas

AlGaN/GaN structures constitute a new class of 2D systems in that a large population of electrons can be produced without doping as a result of spontaneous and strain-induced polarization. Electron transport can, in principle, be mediated solely by phonon scattering and, for the first time, it is possible to realistically envisage the formation of a drifted Maxwellian or Fermi-Dirac distribution in hot-electron transport. We first describe a simple model that relates electron density in a heterostructure to barrier width and then explore electron-electron (e-e) energy and momentum exchange in some depth. We then illustrate the novel hot-electron transport properties that can arise when only phonon and e-e scattering are present. These include S-type NDR, electron cooling and squeezed electrons.

TRANSPORT IN A POLARIZATION-INDUCED 2D ELECTRON GAS

AlGaN/GaN structures constitute a new class of 2D systems in that a large population of electrons can be produced without doping as a result of spontaneous and strain-induced polarization. Electron transport can, in principle, be mediated solely by phonon scattering and, for the first time, it is possible to realistically envisage the formation of a drifted Maxwellian or Fermi-Dirac distribution in hot-electron transport. We first describe a simple model that relates electron density in a heterostructure to barrier width and then explore electron-electron (e-e) energy and momentum exchange in some depth. We then illustrate the novel hot-electron transport properties that can arise when only phonon and e-e scattering are present. These include S-type NDR, electron cooling and squeezed electrons.

Ballistic current transport studies of ferromagnetic multilayer films and tunnel junctions (invited)

Three applications of ballistic electron microscopy are used to study, with nanometer-scale resolution, the magnetic and electronic properties of magnetic multilayer thin films and tunnel junctions. First, the capabilities of ballistic electron magnetic microscopy are demonstrated through an investigation of the switching behavior of continuous Ni80Fe20/Cu/Co trilayer films in the presence of an applied magnetic field. Next, the ballistic, hot-electron transport properties of Co films and multilayers formed by thermal evaporation and magnetron sputtering are compared, a comparison which reveals significant differences in the ballistic transmissivity of thin film multilayers formed by the two techniques. Finally, the electronic properties of thin aluminum oxide tunnel junctions formed by thermal evaporation and sputter deposition are investigated. Here the ballistic electron microscopy studies yield a direct measurement of the barrier height of the aluminum oxide barriers, a result that is invariant over a wide range of oxidation conditions.

Nonparabolic multivalley balance-equation approach to impact ionization: Application to wurtzite GaN

Extended nonparabolic multivalley balance equations including impact ionization (II) process are presented and are applied to study electron transport and impact ionization in wurtzite-phase GaN with a , L-M, and conduction band structure at high electric field up to 1000kV/cm. Hot-electron transport properties and impact ionization coefficient are calculated taking account of the scatterings from ionized impurity, polar optical, deformation potential, and intervalley interactions. It is shown that, for wurtzite GaN when the electric field approximately equals 530kV/cm, the II process begins to contribute to electron transport and results in an increase of the electron velocity and a decrease of the electron temperature, in comparison with the case without the II process. Similar calculations for GaAs are also carried out and quantitative agreement is obtained between the calculated II coefficients by this present approach and the experimental data. Relative to GaAs, GaN has a higher threshold electric field for II and a smaller II coefficient.

Transport of electrons in atomic liquids in high electric fields

To describe the transport properties of hot electrons in high electric fields, a theory for the scattering of excess electrons in atomic liquids with high atomic polarizability is proposed. The theory is based on the variable phase method and it does not require information about the short-range part of the electron-atom potential. The scattering of electrons by density fluctuations in a liquid of a given macroscopic density (the liquid density) is taken into account in the framework of the theory. Mobility, mean and characteristic energies of electrons as functions of the electric field strength are calculated for different liquid densities. The results are presented for liquid Ar at the triple point, and at the density at which the scattering of electrons by density fluctuations is predominant.

Measurements of epitaxially grown Pt/CaF2/Si(111) structures by ballistic electron emission microscopy and scanning tunneling microscopy

The hot electron transport properties and growth morphology of ultrathin Pt/CaF2/Si(111) metal-insulator-semiconductor structures have been characterized in situ by ballistic electron emission microscopy (BEEM) and scanning tunneling microscopy (STM). Platinum thicknesses from submonolayer to 50 Å and CaF2 thicknesses from 2 to 10 ML have been characterized. The STM images of the Pt/CaF2/Si(111) structures show the atomic steps of the underlying CaF2 morphology, as well as the formation of Pt nodules, which nucleate at step edges and defect sites. Some BEEM spectra show an anomalous peak near 2 eV, which has not been observed in previous studies of other metal/CaF2/Si(111) structures. The localized nature of this peak indicates that it results from an interaction between the ballistic electrons and fluorine vacancies at the Pt–CaF2 interface.

Effect of Intersubband Coulomb Interaction on Hot-Electron Transport in Two- and One-Dimensional Electron Systems

We study the hot-electron transport properties of model GaAs-based quasi-two-dimensional (quantum-well) and quasi-one-dimensional (quantum wire) systems having two occupied subbands by using the Lei-Ting balance-equations for two types of carriers. Both the intersubband electron–phonon interaction and intersubband Coulomb interaction are taken into account. Our numerical results show that when the electron density is high enough, the intersubband Coulomb interaction is substantially strong in thermalizing the electrons between the different subbands. As a consequence, the one-type-of-carriers model (OTCM) is a good approximation for electron transport. However, in the cases of the lower electron densities, the intersubband Coulomb interaction is not strong enough to fire the electrons in differents subbands to share a common electron temperature and a more accurate two-types-of-carriers model (TTCM) must be used for analysis.

Monte Carlo study of electron-plasmon scattering effect on hot electron transport in GaAs

It is shown using Monte Carlo simulation that electron-plasmon scattering affects substantially the hot-electron energy distribution function and transport properties in bulk GaAs. However, this effect is found to be much less than that predicted in earlier papers of other authors.

Electrical transport properties of hot electrons at metal, insulator, and semiconductor interfaces

The application of ballistic electron emission microscopy (BEEM) to study hot electron transport phenomena over a kinetic energy range of 0–8 eV is discussed. A basic discussion of hot electron scattering in metals, semiconductors, and insulators precedes the presentation of three case studies, representing examples of transport parameters extracted from BEEM experiments. They include the determination of the energy dependence of (a) the quasielastic and inelastic electron mean free paths in Pd films, (b) the quantum yield of electron–hole pair generation through impact ionization in Si, and (c) the transmission probability of electrons through thin SiO2 layers in metal–oxide–semiconductor structures.

Intersubband coulomb scattering effecton high-field hot-electron transport in a quantum wire

Hot-electron transport properties of AlxGa1−xAs/GaAs quantum wires under high electric fields are studied by means of the balance-equation approach using a model with multiple species of carriers. Each transverse subband is assumed to have its own electron temperature, Fermi level, and mean drift velocity differing from other nondegenerate subbands. The intersubband Coulomb interactions are taken into account perturbatively for the first time together with acoustic and polar optic phonon scatterings in the numerical calculation. Our calculation shows that although the electron temperatures, Fermi levels, and drift velocities of different subbands differ markedly from each other, the overall average drift velocity and the resultant nonlinear mobility are very close to those predicted by the conventional model assuming single species of carriers in the multisubband quantum wire system.

Observation of quantum mechanical reflections of electrons at an in situ grown GaAs/aluminum Schottky barrier

The room temperature observation of quantum mechanical reflections of electrons at an aluminum/gallium arsenide Schottky barrier is reported here. Molecular-beam epitaxy was used to grow an AlAs/GaAs/AlAs double barrier resonant tunneling diode (RTD) followed by an epitaxial in situ grown aluminum/gallium arsenide Schottky barrier. This RTD was used to inject a nearly monoenergetic beam of electrons towards the Schottky barrier. The measured I–V curves show resonances associated with the reflections of electrons at the Schottky interface. Understanding the transport properties of hot electrons at a Schottky barrier may prove important for understanding the physics of metal base transistors and other device structures that employ the use of epitaxial metals.