Nonlinear transport in far-from-equilibrium semiconductors

Summary Using a nonlinear transport theory, derived from the nonequilibrium statistical operator method, we obtain the equations that govern the evolution of the nonequilibrium state of a highly photoexcited direct-gap polar semiconductor, and its nonequilibrium mobility coefficient. It provides an analytic method that allows for a deep physical insight into the influence of the nonequilibrium irreversible evolution of the plasma in the semiconductor on its transport properties. We demonstrate that, under quite general conditions, the strong dependence of the momentum and energy relaxation times on the irreversible evolution of the macroscopic nonequilibrium state of the system results in the existence of a structured transient mobility,i.e. one with maxima and minima, with or without overshoot. A criterion for the occurrence of this structure is derived as well as several general properties of the ultrafast transient are discussed. The authors of this paper have agreed to not receive the proofs for correction.     

Bistable cyclotron resonance in semiconductors

We consider the cyclotron resonance in a semiconductor which has a band with a relativistic energy-momentum dispersion and show that, when subject to quasi-resonant radiation, the energy of a free carrier in this band and the relative potential drop between the two surfaces of the semiconductor facing the radiation exhibit bistability under classical single electron approximation.     

Nonlinear index of refraction in semiconductors

Summary Experimental work on light refraction in semiconductors suggests that there are two separate mechanisms by which the index of refraction becomes nonlinear: direct saturation, on the one hand, and the Burstein-Moss effect, on the other hand. We show that this picture is incorrect, since it was based on the usage of the Bloch equation. Electron systems are not Boltzmann systems and the appropriate Fermi-Dirac master equation leads to a saturation that differs from the saturation of the Bloch equation. We show, using a simple two-level system, that this result may look like the spin result in one extreme (phonon bottleneck saturation), while it shows the features of the Burnstein-Moss effect in the limit of small degeneracy of the upper level (Fermi saturation).

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Riassunto I lavori sperimentali sulla rifrazione della luce nei semiconduttori suggeriscono la presenza di due meccanismi separati grazie ai quali l'indice di rifrazione diventa non lineare: la saturazione diretta, da un lato, e l'effetto Burstein-Moss, dall'altro. Noi mostriamo che questo quadro non è corretto, in quanto basato sull'equazione di Bloch. I sistemi di elettroni non sono sistemi di Boltzmann e l'appropriata ?master equation? produce una saturazione differente da quella dell'equazione di Bloch. Si mostra, usando un semplice sistema a due livelli, che questo risultato assomiglia a quello di un sistema di ?spin? ad un limite estremo (saturazione fononica a collo di bottiglia), mentre mostra le caratteristiche dell'effetto Burstein-Moss nel limite di piccola degenerazione del livello superiore (saturazione di Fermi).

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Резюме Экспериментальная работа по преломлению света в полупроводниках предполагает, что существуют два различных механизма, благодаря которым преломление становится нелинейным: с одной стороны прямое насыщение, а с дуугой стороны эффект ъурштейна-Мосса. Мы показываем, что эта картина не является корректной, т.к.основана на использовании уравнения ълоха. Электронные системы не являются больцмановскими системами и соответствующее управляющее уравнение Ферми-Дирака приводит к насыщению, котрое отличается от насыщения уравнения Блоха. Мы показываем, используя простую двухуровневую систему, что этот результат похож на результат для спина в случае одного максимума, тогда как этот результат обнаруживает особенности эффекта Бурштейна-Мосса в пределе малой вырожденности верхнего уровня.

     

Electrophonon resonance in narrow band semiconductors

The theory of the electric current splashes on a current-voltage characteristic due to the phonon-induced-electron-transitions between Stark ladder levels is developed. Splitting and broadening of the current peaks because of an electric field vector deviation from the [ 111 ] direction is considered. The recent experimental results on current-voltage characteristic of the tellurium cluster superlattice in ceolite are accounted for by the electrophonon resonance studied in the present paper.     

Optically detected cyclotron resonance in semiconductors

In high quality semiconductor crystals, occurrence of cyclotron resonance is beautifully reflected on luminescence spectra. This feature is demonstrated in typical elemental semiconductor Ge, both doped and undoped. One obtains new information of kinetics in free carriers, free excitons, bound excitons and electron-hole drops.     

Nonlinear theory of recombination waves in semiconductors

A nonlinear problem in the theory of recombination waves is solved for infinite sample and a bounded sample. The nonlinearity is generated by allowing for the small terms that are responsible for the motion of the nonequilibrium electron-hole plasma in the field of the recombination wave. The aim of the paper is to establish the nature of the excitation of recombination waves when the external electric field exceeds the threshold by an infinitesimally small amount. It is shown that the growth of the instability occurs in a soft regime. The frequency of the waves increases at the same time.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 67–70, September, 1980.     

Nonlinear induced reflection of light waves in semiconductors

A scheme of nonlinear optical four-wave mixing of two counterpropagating laser beams on the surface of a semiconductor is proposed and analyzed. It is shown that the density modulation of the electron-hole plasma current carriers in the light-induced grating manifests itself in the probe beam depolarization signal after the reflection from the surface, which is sensitive to the state of the surface and presence of complex molecules on it.     

Nonlinear helicon-phonon resonance in bismuth

The effect of nonlinearity on the helicon-phonon resonance in bismuth is considered. It is shown that, in the nonlinear regime, the frequency range in which resonance is possible expands and the resonance curve changes in shape: the lower boundary of this range decreases, and the resonance maximum of the acoustic wave damping becomes higher and narrower.     

On the resonance two phonon absorption in zero-gap semiconductors

The resonance two phonon process is used to explain innormal value and temperature dependence of oscillator strength of additional phonon mode observed in zero-gap semiconductors.     

Electron-plasmon coupling in semiconductors: Harmonics of cyclotron resonance

Plasmon assisted absorption of infrared radiation leads to relatively intense absorption peaks close to harmonics of the electron cyclotron frequency for both the cyclotron resonance active and cyclotron resonance inactive circular polarizations.     

Level splitting in semimagnetic semiconductors under spin-magnetophonon resonance conditions

The splitting of electronic levels in quantum wells of semimagnetic semiconductors typically characterized by large effective g factors is analyzed theoretically. They are found to be capable of supporting resonance, provided the Zeeman spin-level splitting is equal to the energy of the longitudinal optical phonon ?ω_‖. The resonance condition can be written as ?ω_‖ = gμ_B B. This condition can be satisfied by choosing the magnetic field Bsuch that the sum of the energies of the lowest spin level and the optical phonon coincides with the energy of the highest level. It is shown that these two degenerate energy levels should experience mutual repulsion. The magnitude of the corresponding splitting depends on both the electron-phonon and spin-orbit interactions in semiconductors; moreover, it turns out substantially lower than the Zeeman energy gμ_B B. Resonant passage of light through and its reflection from a quantum well are considered as one of possible ways to observe this energy level splitting.     

On a possibility of negative cyclotron resonance in semiconductors

A possibility of negative conductance at the cyclotron resonance frequency of hot carriers in crossed electric and magnetic fields is investigated. It relies on sufficiently strong carrier-optical phonon interaction compared with other scattering processes. The condition is examined numerically but seems to hardly be satisfied in presently available crystals.     

Nonlinear response and chaos in semiconductors induced by impact ionization

A review is given of the nonlinear response and chaos induced by impact ionization of neutral shallow donors, observed in n-GaAs. Two kinds of the observation are described; (i) firing wave instability, and (ii) periodically driven current filament. For the firing wave instability, several important aspects are discussed including the selective excitation of the current filaments and the deterministic nature of the firing density wave. The nonlinear response of a periodically driven current filament has been investigated by applying a dc+ac bias of the form ofV _dc+V _ac sin(2f ₀ t), wheref ₀1 MHz. The carrier dynamics and the bifurcation routes to chaos are discussed in terms of the observed phase diagram and the bifurcation map. The deterministic nature of the strange attractors are described in detail in terms of the correlation dimension and the Kolmogorov entropy.