Picosecond acoustics in superlattices
Semiconductor superlattices (SLs) are attracting considerable interest as their low dimensionality may increase the thermoelectric figure-of-merit. Indeed, the vibrational properties of periodic multilayers and SLs have been investigated as a possible mechanism that could reduce thermal conductivity of such structures. Understanding coherent phonons propagation in SL is of significant in optimizing SL based thermoelectric materials. For monolithic integration, conventional semiconductor materials with good thermoelectric properties are required. SiGe alloy is one of the best thermoelectric materials for this use. Si/SixGe1-x SLs have therefore 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.
We used the transient thermoreflectance (TTR) technique to perform picosecond ultrasonics experiments to investigate coherent zone-folded acoustic phonons in different Si/SiGe superlattice structures. A schematic of a typical structure studied is represented in Figure 1.
Figure 1. Schematic diagram of the Si/SiGe SL structure.
By using an appropriate aluminum transducer on a top of the SL structure, three classes of coherent phonon could be generated and detected, each having a different generation mechanism: Brillouin oscillations, coherent longitudinal-acoustic phonon Bragg reflection and impulsive stimulated Raman scattering (ISRS).
When the pump pulse energy is absorbed by the aluminum thin film, coherent longitudinal acoustic phonons are generated. However, if the transducer is thin enough to let the light pass through, an opto-acoustic interaction occurs, and then Brillouin oscillations are detected. Due to the acoustical impedance mismatch between the different layers of the structure and the periodicity of the SL, one part of the longitudinal acoustic wave, which propagates from the transducer to the substrate, is Bragg-reflected to the free surface. Then, acoustical echoes with a burst shape are detected. The frequencies of the burst correspond exactly to the gap frequencies of dispersion relation of the SL. Finally, because the light reaches the SL, there is a direct excitation of folded phonons in the SL by ISRS process. Light couples to zone folded acoustic modes of the SL, and both forward and back scattering modes could be observed.
Figure 2a illustrates the TTR signal obtained on a sample composed with the same structure as shown in Figure 1, with a SL period of 30nm and a 15nm of transducer. This TTR signal represents the relative reflectivity variations of the surface of the sample as function of the pump-probe time delay. The inset shows the acoustic contribution to the signal after subtraction of the thermal background. The signal exhibits several oscillations associated to the 3 classes of coherent phonons mentioned above. In order to get good frequency information, a numerical Fast Fourier transform is applied to the time resolved acoustic signal. The “acoustic spectrum” obtained is superposed to the longitudinal acoustics branches of the dispersion curves of the SL (Figure 2b). This superposition facilitates the distinction between each type of oscillation involved as each oscillation corresponds to one class of coherent phonons.
Figure 2. (a) TTR signal of sample A; the inset shows the acoustic contribution to the TTR signal after subtraction of the thermal background, (b) FFT of the acoustic contribution to the TTR signal
By performing a systematic study of coherent phonons in Si/SiGe superlattices having different SL periods and transducer thicknesses, we were able to experimentally distinguish each class of phonons.