Impact Ionization Induced by Terahertz Radiation in HgTe Quantum Wells of Critical Thickness
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Impact Ionization Induced by Terahertz Radiation in HgTe Quantum Wells of Critical Thickness S. Hubmann1 · G.V. Budkin2 · M. Urban1 · V.V. Bel’kov2 · A.P. Dmitriev2 · J. Ziegler1 · D.A. Kozlov3 · N.N. Mikhailov3 · S.A. Dvoretsky3 · Z.D. Kvon3 · D. Weiss1 · S.D. Ganichev1 Received: 28 January 2020 / Accepted: 20 March 2020 / © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency ω and momentum relaxation time τl larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light-induced impact ionization is proportional to exp(−E02 /E 2 ), with the radiation electric field amplitude E and the characteristic field parameter E0 . As observed in experiment, it exhibits a strong frequency dependence for ωτ 1 characterized by the characteristic field E0 linearly increasing with the radiation frequency ω. Keywords Terahertz · Nonlinearities · Impact ionization · HgTe quantum wells
1 Introduction Impact ionization across the band edges and its inverted process - Auger recombination - and impact ionization of impurities are the most important autocatalytic
S.D. Ganichev
[email protected] 1
Terahertz Center, University of Regensburg, 93040, Regensburg, Germany
2
Ioffe Institute, 194021, St. Petersburg, Russia
3
Rzhanov Institute of Semiconductor Physics, 630090, Novosibirsk, Russia
International Journal of Infrared and Millimeter Waves
processes in semiconductors. They have been studied extensively not only because of fundamental interest in these nonlinear phenomena but also due to their great practical importance for IMPATT diodes (impact ionization avalanche transit time) [1], high-efficiency solar cells [2], and photodetectors with internal amplification like avalanche photodiodes, particularly useful in the case of fiber-optic communication systems [3]. Aside from being excited by a dc electric field like the aforementioned processes, impact ionization can also be excited by the ac electric field of THz radiation. Such a process has been observed first in bulk InSb crystals and was called light impact ionization [4, 5]. With the development of high-power THz laser systems like molecular lasers, free-electron lasers, and Ti:Sapphire-based THz systems, there has been a steady increase on experimental and theoretical research interest in the field of THz radiation-induced impact ionization, carrier multiplication, and nonperturbative nonlinearities in three- and two-dim
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