Hot Electrons In Semiconductors Research Articles

Real-Time Ab Initio Investigation on Hot Electron Relaxation Dynamics in Silicon.

The relaxation of hot electrons in semiconductors is pivotal for both energy harvesting processes and optoelectronics. Utilizing a self-developed non-adiabatic molecular dynamics simulation approach in the momentum space (NAMD_k), we have examined the dynamics of hot electrons in silicon. Whether excited from the Γ or L point, the relaxation dynamics exhibit two distinct stages. Initially, within 100 fs, electrons scatter with phonons throughout the Brillouin zone. Subsequently, over a few picoseconds, they further relax toward the conduction band minimum as a whole. This picture of hot electron relaxation is highly consistent with the quasi-equilibrium hot electron ensemble (HEE) concept. Throughout the hot electron relaxation process, energy transfer to phonons is efficient, leading to time-dependent phonon excitation, which is thoroughly analyzed. This investigation provides a novel perspective on hot electron relaxation in silicon, which holds substantive implications for the realm of photovoltaic and optoelectronic device applications.

Hot electrons generated from Mn‐doped quantum dots via upconversion for photocatalysis applications

AbstractThere is a growing interest in the application of photogenerated hot electrons in semiconductor or metallic nanostructures such as in photocatalysis and photovoltaics taking advantage of their large excess kinetic energy and long‐range transfer capability. Among various nanostructures that can generate hot electrons, Mn‐doped semiconductors quantum dots (QDs) have shown their unique ability to produce highly energetic hot electrons via upconversion process. Compared to the plasmonic hot electrons, upconverted hot electrons in Mn‐doped QDs possess several eV larger energy and can even produce photoelectron emission above the vacuum level from a subpopulation of hot electrons under weak visible excitation equivalent to the concentrated solar radiation. The present account reviews recent progresses made in the research on hot electron generation via upconversion and their application in photocatalytic reactions that highlights the benefits of long‐range transfer of energetic hot electrons.

Slow Electron Cooling in Colloidal Quantum Dots

Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.

Ambient temperature or moderately cooled semiconductor hot electron bolometer for mm and sub-mm regions

AbstractA model of semiconductor hot electron bolometer (SHEB), in which electromagnetic radiation heats only electrons in narrow-gap semiconductor without its lattice slow-response heating, is considered. Free carrier heating changes the generation-recombination processes that are the reason of semiconductor resistance rise. It is estimated, that Hg0.8Cd0.2Te detector noise equivalent power (NEP) for mm and sub-mm radiation wavelength range can reach NEP ∼10−11 W at Δf = 1 Hz signal gain frequency bandwidth. Measurements performed at electromagnetic wave frequencies v = 36, 39, 55, 75 GHz, and at 0.89 and 1.58 THz too, with non-optimized Hg0.8Cd0.2Te antenna-coupled bolometer prototype confirmed the basic concept of SHEB. The experimental sensitivity Sv ∼2 V/W at T = 300 K and the calculated both Johnson-Nyquist and generation-recombination noise values gave estimation of SHEB NEP ∼3.5 × 10−10 W at the band-width Δf = 1 Hz and v = 36 GHz.

Anisotropy of thermal conductivity and energy-flux relaxation time of hot electrons in semiconductors

In this article a theoretical and computational analysis of the hot-electron thermal conductivity and related quantities in semiconductors is given. Two types of anisotropy are analyzed: the first is related to the dependence of the thermal conductivity on the direction of an externally applied electric field; the second is associated to the difference between the longitudinal and transverse thermal conductivity (i.e., along the field direction and along a direction perpendicular to the field, respectively). Two theoretical approaches based on a set of generalized relaxation times or on a set of microscopic correlation functions are considered and compared. Numerical results are obtained using a Monte Carlo simulator for electrons in silicon at 77 and 300 K. This approach can be extended to other semiconductors of interest within a semiclassical approach where two-particle interactions are neglected.

Hot electron interference by 40 nm-pitch double slit buried in semiconductor

We report a double-slit experiment of hot electrons in semiconductor under a magnetic field. The pitch of the double slit buried in the semiconductor is 40 nm, and the electron energy is in the order of 100 meV. By applying a magnetic field, the change in current which passes through the slits is observed at the segmented collector. The measured current shows a minimum around B = 0 T. A simulation is presented which is based on the Fraunhofer diffraction and the path integral method. The simulation predicts the variation of current, and quantitative estimation is consistent with the order of measured current variation. We think that these results imply the hot electron interference by a double slit in a semiconductor.

Visualization of current filaments in n-GaAs by photoluminescence quenching

Hot electrons in semiconductors are known to quench radiative impurity and exciton recombination photoluminescence. This effect has been applied in a low invasive technique to determine the spatial form of current filaments generated by impurity breakdown in high purity n-GaAs epitaxial layers at low temperatures. Observations on samples with Corbino disc contacts clearly demonstrate symmetry breaking and self-organization by the current filamentation.

Harmonic generation of microwaves in semiconductors in a magnetic field

The characteristics of second harmonic generation due to hot electrons in semiconductors have been investigated in the presence of a magnetic field. The calculations are carried out for a parabolic law of dispersion and a scalar effective mass. Numerical results for n-Ge indicate the interesting effects the magnetic field may have on the harmonic generation.

The energy loss rate and dynamical resistivity of hot electrons in semiconductors

Among many proposed field theories for thermal phenomena, thermo-field dynamics (TFD) is very useful and simple tool to study non-equilibrium many-body systems, especially for a complicated system composed of several subsystems at different thermal equilibria. A complete perturbative calculation scheme in TFD for such systems is presented. In the TFD method, explicit investigation will be made of the energy loss rate (ELR) and resistivity of hot electrons in semiconductors. The ELR formula is shown to be different from that obtained by the Keldysh method. The possible ambiguity in the Keldysh method is discussed.

Theoretical study of time-resolved Raman scattering profiles of hot electrons in semiconductors

A comprehensive time-resolved electronic Raman scattering theory for nonequilibrium carrier excitations in semiconductors is presented. The following are simultaneously taken into account: (i) the effects of the ultrashort laser pulse for probing the excited carrier distribution function; (ii) the fact that the fluctuation-dissipation theorem is not valid under conditions of nonequilibrium carrier distributions; (iii) the effects of quasiparticle life time via a finite collision time in the Raman scattering cross section; and (iv) the effect of the time-dependent resonant enhancement factor due to the band structure. The single-particle scattering spectra for spin-density fluctuation contribution is found to be significantly broadened by an ultrashort laser pulse, but is substantially narrowed by the finite collision time. The effect of the time-dependent resonant enhancement factor has been demonstrated to broaden the line shape of single-particle scattering spectra for the spin-density fluctuation contribution as the probe photon energy increases.

An analytical model of the energy distribution of hot electrons

An analytical model of the energy distribution of hot electrons in semiconductors is presented. It is derived under the simplifying assumptions of a single nonparabolic conduction band, small space derivative of the external electric field, and emission of optical phonons as the dominant energy loss mechanism. The model, indicating that band nonparabolicity makes the electron energy distribution deviate markedly from the Maxwellian low-field case, is shown to be applicable to the case of silicon MOSFETs by means of comparisons with Monte Carlo simulations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hot electrons in semiconductors with long-lived resonance impurity states

The nonlinear carrier transport phenomena are considered in the semiconductor doped with the impurities which form the quasi-local electron states. The resonant scattering of carriers by these centres has two aspects, first, the scattering as such, i.e. the change of the carrier flight direction, and second, the long electron residence at the centre during the scattering which leads to the decrease of the free carrier concentration in the band. In this paper the influence of both these sides of the resonant scattering upon the electron transport is considered for the first time. The Boltzmann kinetic equation is not applicable to this case so the special approach has been developed. The possibility of carrier transfer to the resonant centres leads to appearance of the N-shaped parts of the current-voltage curves which is similar to the Gunn effect.

Correlation functions of hot electrons in semiconductors.

We present a general theory of the correlation functions for a steady state which is valid for arbitrary strengths of an applied electric field, as obtained in high-field transport in semiconductors. When limiting to the first two moments of the distribution function, we find a closed set of equations coupling energy and velocity which are amenable to an analytical solution. Thus, the theory provides an interpolation formula which gives smooth, analytical expressions for the correlation functions. These expressions can be made to fit the computer simulation by using these simulations to estimate the various parameters. The theory is found to be in excellent agreement with numerical calculations for a simple model semiconductor performed with an ensemble Monte Carlo technique.

Harmonic and intermodulation generation due to hot electrons in semiconductors

An exponential equation describing the nonlinear velocity-field relationship in a semiconductor is presented. Using this equation, closedform expressions are obtained for the harmonics and intermodulation products generate due to hot electrons in semiconductors.

Hot electrons in semiconductors

Hot electrons in semiconductors

Effect of recombination of the non-equilibrium carriers on the high frequency generation in semiconductors

The characteristics of second harmonic generation (SHG) due to hot electrons in semiconductors have been investigated with due regard to the recombination mechanisms. The results show that the recombination effect brings in significant changes in the characteristics.

Hot electrons in semiconductors

Electron heating by an electric field in basic semiconductors has been under study for several decades. Lasers are used for diagnostics of nonequilibrium distribution of field-heated electrons and, more recently, to heat an electron gas. The author discusses the heating of electrons by an electric field, its associated effects and its application to laser heating. He provides a description of the methods of optical heating and relaxation of hot electrons in a semiconductor.

Cathodoluminescence efficiency of binary compound phosphors

A theoretical treatment is given of binary compound cathodoluminescence efficiency on the basis of the statistical model treating the branching process of ionization scattering and phonon emission. Energy loss mechanisms of hot electrons in semiconductors are quantitatively examined using Penn’s model assuming an optical phonon deformation potential and polar-mode electron-phonon interactions. The quantum yield of e-h pairs is calculated with three parameters, the band-gap energy Eg, and two loss parameter constants Sd and Sp which are determined completely in terms of material parameters including Eg, the mass density, the lattice constant, the covalency, the optical phonon frequency, and dielectric constants. The material characterization of efficient phosphors is discussed in terms of empirical correlations between cathodoluminescence efficiency and the physical constants. Dependence of efficiency on the crystal structure of the phosphor host is particularly noticeable. Comparisons are made with previous work.

Effect of optical-phonon scattering on the energy distribution of hot electrons in semiconductors at low temperature

The variational method is used in finding the solution of the transport equation for a system of hot electrons inn-Ge atT=20 K in the presence of high electric field. The role of the emission of optical phonons by hot electrons together with the effect of electron capture by repulsive centres on the formulation of the distribution function are studied. It is shown that the emission of optical phonons plays a dominant role in the formulation of the distribution. The influence of electron capture is very small, it may become appreciable at higher trap concentrations. The obtained distribution function is then used in calculating the capture rate of electrons by negatively charged centres. It is shown that the capture rate increases with electric field.

On the photoconductivity of hot electrons in semiconductors at low temperatures

A theoretical study is made of the photoconductivity of hot photoelectrons interacting with acoustic phonons in n-type germanium at low temperature, T = 4 K, when the equipartition of the energy of phonons is not obeyed. Assuming that the energy relaxation of photoelectrons takes place by the spontaneous emission of acoustic phonons, the energy distribution of photoelectrons is obtained using the deformation potential scattering by acoustic phonons for the zero point lattice. The distribution function is then used in calculating the photoconductivity and average lifetime of photoelectrons. New temperature dependences of the photoconductivity and average lifetime arc obtained. It is shown that the new results differ from those reported when equipartition is obeyed.