Investigating coherent zone-folded acoustic phonons in Si/SiGe superlattice by transient thermoreflectance technique
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1221-CC08-03
Investigating coherent zone-folded acoustic phonons in Si/SiGe superlattice by transient thermoreflectance technique Hélène Michel1, Gilles Pernot2, Jean-Michel Rampnoux2, Stefan Dilhaire2, Stéphane Grauby2, Younès Ezzahri1, and Ali Shakouri1 1 Baskin School of Engineering, UC Santa Cruz, CA 95064, USA 2 CPMOH, Université de Bordeaux 1, Talence, 33405, France
ABSTRACT In this paper, we present a systematic study of coherent phonons in Si/SiGe superlattices with two different periods. The superlattice periodicity affects the acoustic properties of the structure. Transient thermoreflectance (TTR) technique is used to perform picosecond ultrasonics experiments and then, investigate coherent zone-folded acoustic phonons in different Si/SiGe superlattice structures. Several classes of coherent phonons are produced in the superlattice, whose generation mechanisms are different: Brillouin oscillations, coherent longitudinal-acoustic phonon Bragg reflection and impulsive stimulated Raman scattering (ISRS).
INTRODUCTION Folded acoustic phonons in semiconductor superlattices (SL) have been extensively studied during the three past decades. Most of the studies have been focused on GaAs/AlAs and GaAs/AlxGa1-xAs SLs [1-3]. More recently, Si/SixGe1-x SLs have received considerable attention due to high-quality growth, and the enhancement of their electrical and thermal properties, which become very promising for the design of CMOS compatible thermoelectric and phononic devices [4,5]. It is well-known that semiconductor SLs exhibit zone folding of the acoustic branches within the mini-Brillouin-zone (mini-BZ) due to the artificial periodicity of the elastic properties along the growth direction. Indeed, one of the most fundamental acoustic properties of a SL is the Bragg reflection of long-wavelength phonons. The Bragg condition requires:
2 D cos(θ ) = nΛ
(1)
Where θ is the angle of incidence of the phonon wave vector measured from the normal to the interfaces, Λ is the phonon wavelength, D is the period of the SL, and n is an integer. Each phonon that satisfies this condition is Bragg-reflected and cannot propagate though the SL. This Bragg reflection produces band gaps at the boundary and the center of the folded mini-BZ. Since more than two decades, lots of experiments focused on zone-folded acoustic modes of semiconductor SL, using cw Raman spectroscopy [6-8] or ultrafast laser pulses [9-11], have been reported. In the latter case, a femtosecond laser pulse, absorbed by the SL, excites coherent zone folded longitudinal-acoustic phonons. The excitation process has been attributed to impulsive stimulated Raman scattering (ISRS) [12,13], in which the formation of an electron-hole pair is accompanied by the creation of a longitudinal-acoustic phonon.
In the following we investigate coherent zone-folded acoustic phonons in several Si/SiGe SLs by using the transient thermoreflectance (TTR) technique to perform picosecond ultrasonics experiments [14].
EXPERIMENT Samples description
The structures studied are co
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