Circular Photogalvanic Effect in SiGe Semiconductor Quantum Wells
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Circular Photogalvanic Effect in SiGe Semiconductor Quantum Wells. ossler1 , Wilhelm Prettl1 , Sergey D. Ganichev1,2 , Franz-Peter Kalz1 , Ulrich R¨ 2 2 Eugenius L. Ivchenko , Vasily V. Bel’kov , Robert Neumann3 , Karl Brunner3 , and Gerhard Abstreiter3 1 Fakult¨ at f¨ ur Physik, Universit¨ at Regensburg, 93040 Regensburg, Germany, 2 A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia 3 Walter Schottky Institute, TU Munich, D-85748 Garching, Germany ABSTRACT The photogalvanic effects, which require a system lacking inversion symmetry, become possible in SiGe based quantum well (QW) structures due to their built-in asymmetry. We report on observations of the circular and linear photogalvanic effects induced by infrared radiation in (001)and (113)-oriented p−Si/Si1−x Gex QW structures and analyse these observations in view of the possible symmetry of these structures. The circular photogalvanic effect arises due to optical spin orientation of free carriers in QWs with band splitting in k-space which results in a directed motion of free carriers in the plane of the QW. We discuss possible mechanisms that give rise to spin-splitting of the electronic subband states for different symmetries.
INTRODUCTION The spin-degree of freedom of charge carriers and its manipulation has become a hot topic in material science under the perspective of spin-based electronic devices. One particular aspect, which we want to address in this contribution, is the generation of spin-polarized carriers in semiconductor quantum structures. This aspect is specific for the semiconductor material and can be considered without technological problems, like spin injection, inherent with the paradigmatic spin transistor proposed by Datta and Das [1]. Recently it has been demonstrated that in quantum well structures based on III-V compounds, a directed current inseparably linked to spin-polarized carriers can be created by circularly polarized light employing nonlinear optical properties [2,3]. This effect belongs to the class of photogalvanic effects known for bulk semiconductors [4]. The nonlinear optical response of matter under excitation by light with frequencies ω1 and ω2 is described in lowest order by a third rank tensor. The response is at the sum or difference frequency Ω = ω1 ± ω2 . Special cases, with light from one intense light source (ω = ω1 = ω2 ), are second harmonic generation (Ω = 2ω) and the photogalvanic effect (Ω = 0). The latter is the generation of a direct current (double index summation understood) jλ = Dλµν Eµ (ω)Eν (−ω)
(1)
by applying light with the electric field amplitudes Eµ (t) = Eµ (ω)eiωt and Eµ∗ (ω) = Eµ (−ω). For jλ to be real one has Dλµν = Dλνµ . Thus by decomposition of Dλµν into real and imaginary parts (being symmetric and antisymmetric, respectively, under interchange of the indices µ and ν) jλ can be separated according to
jλ = χλµν Eµ (ω)Eν∗ (ω) + Eµ∗ (ω)Eν (ω) /2 + iγλκ (E × E∗ )κ ,
(2)
where χλµν is the real part of Dλµν and γλκ is a second-rank pseudo-tensor composed of the antisymmetric
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