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(a) Cross-sectional scanning electron micrograph of a SL. The bright (dark) regions correspond to layers with a porosity of 0.56 (0.46). (b) Scattering geometry. The wavevectors of the probed phonon and the incident (scattered) light are and , respectively.
Spectra collected from (a) a 0.59/0.52 superlattice, (b) a 0.59/0.52 superlattice, (c) a 0.59/0.52 superlattice, and (d) a 0.56/0.46 superlattice (smoothed using three-point averaging). The arrow indicates a weak feature that was difficult to fit to a Lorentzian and, hence, has a large uncertainty in frequency shift. This and similar features in other spectra appear in Fig. 3 and are indicated by large frequency-axis error bars.
Phonon frequency vs modulation wavelength for superlattices with constituent layer porosities of (a) 0.59 and 0.52 and (b) 0.56 and 0.46. Thick (thin) broken lines: calculated curves for the longitudinal (transverse) mode; shaded region: values of for which the probing light is in a photonic bandgap. Arrows mark the approximate values of for which coincides with the edges of the first, second, and third phononic Brillouin zones, respectively. Open symbols: experimental data; diamonds: duplicate sample. The frequency scale on both plots is the same.
Porosity, refractive index, and bulk acoustic phonon velocities for single layer porous silicon films.
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