Intervalley Transitions Research Articles

Effect of electron-electron scattering on intervalley transition rates of photoexcited carriers in GaAs.

Etude par methode de Monte Carlo couplee avec une approche de dynamique moleculaire. Mise en evidence de l'interaction de divers mecanismes de diffusion et de l'ecrantage dynamique des porteurs de charge chauds dans des semiconducteurs. A une densite pour laquelle GaAs est degenere, la diffusion du volume d'excitation est domine, dans les 10 femtasecondes initiales, par une diffusion electron-electron. Cette diffusion modifie les vitesses de transition et la densite des porteurs dans les vallees satellites

Calculation of intravalley and intervalley transition probabilities in the course of electron scattering by a gallium arsenide surface

We employed the surface Green's function method in the discontinuous surface potential model to calculate the coefficients of reflection from (111) and (110) planes for electrons propagating in gallium arsenide. The complex band structure required for the calculation was obtained by the empirical pseudopotential method. The results make it possible to determine the intravalley and intervalley scattering probabilities as the electrons are reflected by the crystal surface. In particular, it is demonstrated that in the case of the (111) gallium arsenide surface γL and Lγ intervalley transitions are more probable than γγ and LL intravelley transitions.

Hot electron magnetophonon resonance in n-type Si at high fields up to 40 T

Hot electron magnetophonon resonances (MPR) in n-type Si were investigated at a temperature T=80 K in pulsed high magnetic fields up to 40 T. Although the oscillation profile is very complicated due to the overlap of several different intervalley MPR series, the assignment of each peak has become possible owing to the high resolution measurements in high magnetic fields. In the longitudinal magnetoresistance for B//(100) and B//(110) configuration, the dominant series of the observed MPR oscillation peaks M=2-19 (B//(100)) or N=3-19 (B//(110)) were assigned as the intervalley transitions from the excited states to the ground state of different valleys with the lighter electron mass by emitting a f-110 Sigma 3 phonon with a momentum of (2 pi h(cross)/a) (0.85, 0.85, 0). The electronic field dependence of the oscillation amplitude of each series and the damping factor were investigated in electric fields up to 300 V cm-1. The damping factor as a function of electric field was found to have a minimum in the warm electron range around 200 V cm-1.

Electron-electron scattering modifications of intervalley transition rates and ultrafast relaxation of hot photoexcited carriers in GaAs

The femtosecond relaxation of photoexcited carriers in GaAs is investigated by the use of ensemble Monte Carlo calculations coupled with a molecular dynamics approach for the carrier-carrier interaction, to probe various scattering mechanisms and the dynamic screening of hot carriers in semiconductors. At a density for which the GaAs is degenerate (in equilibrium), scattering out of the excitation volume is dominated in the initial tens of femtoseconds by electron-electron scattering, and the lifetime decreases with increasing density. This decrease agrees both in lifetime magnitude and in density dependence with recent experimental measurements. The presence of electron-electron scattering modifies both the population transition rates and the carrier densities in the satellite valleys, primarily by reshaping the energy distribution of carriers in the central valley. Intervalley scattering rates slow by two orders of magnitude as the electronic system cools in the first picosecond. Thus, while intervalley processes do play a role in the initial decay, the same processes play a role in the picosecond-scale luminescence decay. We find that rates, for particular mechanisms of carrier transfer to the satellite valleys, must be estimated carefully since the Γ-L population shift contains a significant fraction of electrons that reach the L valleys by way of the X valleys.

Intervalley scattering in GaAs.

We report on a measurement of the rise of luminescence intensity in GaAs excited by sub- picosecond pulses at 1.66 eV so that the photoexcited electrons are not energetically allowed to transfer to the L valley. The rise in luminescence is faster in this case compared to that obtained with photoexcitation at 2.04 eV when intervalley transitions are allowed. These results confirm the importance of the intervalley transitions for high photoexcitation energies.

Diffusion and inter-valley noise in (100) n-channel Si-MOSFETs from T = 4.2 to 295 K

Noise in the drain current of (100) n-channel Si-MOSFETs was investigated as a function of both source-drain voltage and gate voltage, from T = 4.2 to 295 K. The frequency range covered was from 1 to 250 kHz. Three contributions to the noise could be distinguished: 1/ f-noise, generation-recombination noise and white noise. This paper deals mainly with the white noise component. The current-voltage characteristics at T = 295 K can be described by simple MOSFET-theory, but at T = 4.2 K hot carrier effects and field-induced transitions occur, and the theory has to be amended accordingly. The white noise that was measured at room temperature is diffusion noise. At T = 4.2 K however we observed extra white noise over and above diffusion noise. We also observed extra white noise at intermediate temperatures but it was less pronounced. We have developed a model which interprets this extra noise as the frequency-independent part of generation-recombination noise caused by inter-valley transitions between three groups of valleys. There is good agreement between the model and our experimental results.

Hidden valley structure of Ising spin glasses

The hidden valley structure of the phase space of short-range spin glasses is investigated. In the spirit of a similar investigation for liquids by Stillinger and Weber, valleys are defined as the set of states which by steepest descent lead to the same metastable state. The lifetime of valleys and the size of spin clusters involved in intervalley transitions on thermal Monte Carlo trajectories are calculated as a function of temperature.

Monte Carlo simulation of AlGaAs/GaAs hot-electron transistors

Monte Carlo simulation of AlGaAs/GaAs hot-electron transistors (HET) was carried out on the following model: electrons are injected by tunneling through the emitter barrier and are scattered by polar optical phonons, acoustic phonons, ionized impurities, intervalley transitions, and plasmons in the base, and by quantum mechanical reflection at the boundary between the base and the collector barrier. Intervalley scattering and plasmon scattering are shown to play essential roles in limiting the characteristics of the HET. The simulation predicts that the transfer ratio can be as high as 0.9 with a base width of 100 A at 77 K, and that the transit time is about 1 ps with a total device length of 2200 A at a base-emitter voltage and a collector-base voltage of 1.0 V. The simulated result of the transfer ratio is in good agreement with the experimental result, if plasmon scattering is taken into account.

Electron transport in the conduction channel of the HEMT

The electron transport in the conduction channel of the HEMT is investigated using an Ensemble Monte Carlo method, which includes the first two subbands in the quantum well. The interaction between the channel and the rest of the device is accounted for, including intravalley and intervalley transitions between confined and 3-D states. The model allows an evaluation of the real-space transfer effects.

Selection rules for matrix elements of electron–phonon scattering processes in crystals with TlSe structure

AbstractSelection rules for intervalley transitions in TlSe structural crystals are obtained. The results are presented both, without and with regard to the electron spin.

Intraband Absorption Due to Alloy Scattering in Lead Chalcogenide Semiconductors

AbstractFree carrier absorption allowed by the chemical disorder is calculated for lead chalcogenide mixed semiconductors. It is found that the absorption due to the intervalley transitions may be greater than that due to the intravalley transitions.

Observation of intervalley optical transitions in PbTe by magneto-photo response

Far infrared photo-response of epitaxial PbTe films at 4.2 K is found to be much more sensitive to optical transitions than reflection or transmission. Extra peaks are found which are identified as impurity induced spin-flip transitions and intervalley magnetic subband transitions.

Spectrum and polarization of hot-electron photoluminescence in semiconductors

Experimental and theoretical research on the recombination photoluminescence of free hot electrons in semiconductors (primarily GaAs) is reviewed. The polarization characteristics are discussed. These characteristics reflect, in particular, a momentum alignment of the electrons by linearly polarized light and an effect of a ripple in the constant-energy surfaces in the valence band. The dependence of the linear polarization on the spectrum is discussed in connection with various mechanisms for the energy relaxation of the hot electrons. The depolarization of the hot-electron photoluminescence in a magnetic field is discussed. A procedure is discussed for determining the energy relaxation times and the scale times for intervalley transitions through an analysis of depolarization curves. The energy distribution of the hot electrons is found from the hot-electron photoluminescence spectrum. The recombination luminescence of hot holes is discussed. These holes appear when the semiconductor is illuminated in the spin-split-off subband.

Hot electron luminescence

The linear polarization of the hot luminescence in GaAs crystals and GaAs-based solid solutions is created by linearly polarized light aligning the hot electrons in momentum. A study of the structure of the spectrum, of its polarization and of the magnetic field effect on polarization permits one to establish the distribution function of the hot electrons, to investigate their relaxation and to determine the scattering times τ involving optical phonon emission and for intervalley transitions.

Violation of selection rules for phonon-induced intervalley transitions in silicon

It is shown that the usually accepted selection rules governing intervalley scattering in silicon with emission or absorption of phonons may be strongly broken by perturbations of the wavefunction such that the envelope wavefunction varies by as little as several percent over a lattice cell. Such may be the case in the presence of a magnetic field (magnetophonon effect, impurity centres or electron-hole interactions (excitons). This is due to the fact that the symmetry inside the unit cell is changed so as to introduce integrals in the transition matrix element much larger than those which enter in the group-theoretically allowed transitions.

Hot electrons in Si inversion layer

The energy losses per electron to phonons (both acoustic and optical) are calculated for Si(100) inversion layer taking the subband structure of electron energy spectrum fully into account. The electron mobility variations with applied field are studied by energy balance method. We examine two extreme cases: (1) the intervalley transitions connect any pair of subbands in respective valleys with the same transition matrix; (2) the intervalley transitions connect only the lowest subbands in each valley. It turns out that we can choose the magnitude of intervalley transition amplitude such that experimental data lie in between the theoretical results in two cases mentioned above at both lattice temperatures T = 77 and 300 K.

Intervalley transitions in inversion layers

Transitions between sub‐band systems deriving from valleys with different effective mass in the direction of surface quantization involve large momentum transfer. As a consequence the probabilities for impurity‐induced transitions between the respective sub‐band systems are low. The corresponding transitions by phonon emission are energetically forbidden if the sub‐band splitting does not exceed the energy of acoustic phonons near the zone boundary. The transfer of carriers into a sub‐band system with lower mobility provides an explanation of the experimentally observed negative photoconductivity and of the extremely long photoresponse times in n‐type Si<100≳ inversion layers. The dependence of the photoconductive response on the band splitting provides information about the relative position of the electric sub‐bands in different sub‐band systems. Relaxation losses from transitions between different sub‐band systems also imply a possibility for measuring the sub‐band energies if the system is no longer in the electric quantum limit.

Edge luminescence and light absorption in GaSxSe1−x solid solutions at low temperatures

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Selection Rules for the Spinel Structure

AbstractSelection rules are presented for indirect optical, intervalley transitions, and two‐phonon and three‐phonon processes for the points \documentclass{article}\pagestyle{empty}\begin{document}$ \Gamma (000),\,{\rm X}\frac{{2\pi }}{a}(100),\,{\rm L}\frac{\pi }{a}(111),\,{\rm W}\frac{\pi }{a}(120) $\end{document} of the Brillouin zone. The results obtained may be used for any compound of the symmetry group O.

Dielectric function for a model, two-band semimetal

An approximate expression for the frequency- and wave-vector-dependent random-phase-approximation longitudinal dielectric function at 0 \ifmmode^\circ\else\textdegree\fi{}K, for a model, two-band semimetal, is derived. In the vicinity of electron and hole pockets, energy bands are assumed to be parabolic and ellipsoidal; the small-pocket approximation as well as the tight-binding approximation have been used for derivations. Contributions to the total polarizability due to interband (insulatorlike) transitions, intravalley (metal-like) transitions, and intervalley transitions, which are specific for a semimetal, have been given in a closed analytic form. The ellipsoidal symmetry of the carrier pockets does not destroy the isotropy of the Thomas-Fermi screening, whereas an anisotropy of interband polarizability does. Carrier transitions between different electron and hole valleys are shown to lead to small contributions to the total dielectric function, which are logarithmically sloped and rather localized in the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}\ensuremath{\rightarrow}0$ space. The multipocket plasma frequency in the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ limit is anisotropic. A semiheuristic formula for the static dielectric matrix has also been derived with the aid of the generalized sum rule for oscillator strengths. Problems of inverting the dielectric matrix are reviewed.