Carrier Kinetics in a Semiconductor with Light‐Induced Gaps

Abstract

Due to the influence of a strong electromagnetic field an induced gap is expected to orcur in the band structure of a semiconductor. I n this paper the influence of the damping by electron-phonon collisions and by recombination on the new band structure is investigated. Taking into account damping kinetic equations for the electrons are derived and discussed with the help of calculations.Unter dem EinfIuR eines starken elektromagnetischen Feldes kommt es in der Bandstruktur eines Halbleiters zur Ausbildung eines Zusatzgaps. In dieser Arbeit wird der EinfluB der Dilmpfung durch Elektron-PhononstBBe und Rekombination auf die neue Bandstruktur untersucht. Weiterhin werden kinetische Gleichungen unter Beriicksichtigung der Dilmpfung abgeleitet und an Hand von numerischen Berechnungen diskutiert. 1, IntroductionThe ground state of an intrinsic semiconductor is characterized by filled valence bands and empty conduction bands, the ground state and the excited states being separated by the energy gap E,. If light of the energy ho > E , is absorbed, electron-hole pairs are produced. Conventional optical experiments apply weak light intensities and, consequently, the electron-hole density remains small -there is practically no change in the state of the system, so it is the ground state which is studied in the linear response regime.With increasing, but not too high light intensities, however, band filling effects will be important. Under such conditions two times are of interest, the relaxation time trel and the recombination time trec ; zr,l characterizes the scattering process which tends t o establish equilibrium within the bands, t,,, measures the lifetime of the excited carriers. Because trel<< z , , , is fulfilled in most cases a quasi-equilibrium situation is established. On the other hand, on a very short time scale, the quasiequilibrium concept surely fails.A t higher light intensities of excitation no quasi-equilibrium concept is applicable independently of the pulse duration because the band structure is modified by the radiation field. Under such conditions highly excited semiconductors have to be treated as true nonequilibrium systems. Therefore, in the present contribution we shall use the nonequilibrium Green function formalism which has been introduced by Keldysh and others [l, 21. This formalism allows a consistent calculation of the single particle energies and of the carrier distribution function. An important advantage is that the Keldysh formalism provides us with strict diagrammatic rules for the calculation of higher-order many-body contributions under nonequilibrium conditions.

Carrier dynamics in silicon nanowires studied via femtosecond transient optical spectroscopy from 1.1 to 3.5 eV

“…In principle, by using the estimated photoexcited carrier density, the measured red shift of the bleaching signal due to this density, and the known hole effective mass, it is possible to obtain the effective electron mass of the Γ 15 conduction band minimum, which has been never estimated in experimental work nor explicitly published in theoretical papers. As described by Hartmann and coworkers [22], the bandgap narrowing is determined by carrier density, N, and carrier temperature, T c . They gave a simplified and manageable relationship:…”

Screening of the Coulomb Potential in the Athermal Stage of Highly Excited Direct‐Gap Semiconductors

“…Assuming that go moves mainly under the influence of 8, [7] the CTDMF-SSMA evolution equation is expressed as All well-known transient effects as laser-induced gap creation [13] or blue and red shift in resonant or nonresonant excitations [14] are involved in ( …”

Screening of the Coulomb Potential in the Athermal Stage in Highly Excited Semiconductors

“…the spectral hole burning [9], the dynamical Stark effect, e.g. [lo, 111, or the dynamical band mixing, coupled to a laser-induced additional gap creation [12].…”