G. T. Andrews

Observation of hypersonic phononic crystal effects in porous silicon superlattices

Brillouin light scattering experiments were carried out on porous silicon superlattices with modulation wavelengths in the range 37–167 nm. Phonon frequencies deduced from the Brillouin spectra show good agreement with those obtained from an elastic continuum model for a system with one-dimensional periodicity. Evidence for the existence of a hypersonic phononic bandgap and zone-folded longitudinal acoustic phonons is reported.

Characterization of porous silicon by Raman scattering and photoluminescence

Abstract The structural and light-emitting properties of porous Si prepared from differently doped p-type Si (111) substrates have been studied by Raman scattering and photoluminescence (PL). A detailed analysis of the Raman line shapes was performed using a quantum phonon confinement model with realistic LO and TO phonon dispersion curves. Prevailing Si nanocrystallite types (spheres or wires) and characteristic sizes were determined for samples with porosities from 30 to 80%. The highly porous samples consist of fine Si spheres, while those of lower porosity are mostly wire-like. The PL spectra are less size-sensitive than the Raman ones and no clear correlation between the structural information from the Raman scattering and the PL has been observed.

Elastic characterization of a supported porous silicon layer by Brillouin scattering

Brillouin spectroscopy was used to study surface acoustic waves on a supported layer of (111)‐oriented porous silicon having a thickness of 2.7 μm and a porosity of 30%. The Rayleigh surface wave velocities were found to be significantly lower than corresponding velocities for crystalline silicon. A complete set of elastic constants for the porous layer was determined from the measured directional dependence of the surface wave velocity in the (111) plane. The best‐fit constants are C11=56.0±0.7 GPa, C12=6.7±0.3 GPa and C44=37.0±0.8 GPa. The anisotropy factor, η=1.50 indicates that the porous layer is elastically anisotropic.

Systematic Brillouin light scattering study of the elastic properties of porous silicon superlattices

Brillouin light scattering spectroscopy was used to determine elastic properties of porous silicon films and superlattices. The film elastic properties were compared with previously published results and also used as input for an effective elastic medium model (M Grimsditch and F Nizzoli 1986, Phys. Rev. B 33 5891) to predict the elastic constants of porous silicon superlattices with constituent layers comprised of thin copies of the films. Values of superlattice elastic constants c33 and c44 obtained from the model show reasonably good agreement with those determined directly from superlattice Brillouin data while constants c11 and c13 show crude agreement. This partial agreement suggests that the model holds promise as a means for accurately predicting the elastic properties of p-Si SLs. The results of this study show that it is possible to tailor porous silicon superlattices for custom applications as well as further fundamental studies.

Measuring soap bubble thickness with color matching

This paper describes a laboratory experiment designed to measure thickness variations across a soap bubble. The experiment uses the phenomenon of thin film interference and the principles of color perception to measure the thickness of the soap film at various points across the surface of the bubble. The students review the classical theory of interference and use a digital camera to make the measurements. The apparatus required for the experiment is inexpensive and easy to construct. This experiment is suitable for a senior undergraduate course and can potentially be used to study hydrodynamic effects in soap films.

Brillouin scattering from porous silicon-based optical Bragg mirrors

Brillouin light scattering experiments were done on porous silicon-based optical Bragg mirrors with modulation wavelengths of ∼100u2009nm. By using a combination of pseudo-reflection and backscattering geometries, phonon dispersion curves along the superlattice modulation axis were mapped. Excellent agreement is obtained with the bulk acoustic mode band structure calculated using a one-dimensional elastic continuum model. In addition to zone-folding of the bulk longitudinal mode dispersion curve, the samples are marked by a surface-localized acoustic mode at the superlattice-air interface. The frequency of this mode lies near the upper edge of a phononic band gap centered at ∼16u2009GHz. These results, along with optical reflectance data showing visible-range photonic band gaps, reveal that these samples are one-dimensional hypersonic phononic-photonic crystals.

Effect of anodization current density on pore geometry in macroporous silicon

Macroporous silicon films were fabricated by electrochemical etching of p?-type silicon with a resistivity range of 9.0?13.0 ? cm for 10?min in an electrolyte containing hydrofluoric acid, water, and acetonitrile. Samples were studied using scanning electron microscopy. The onset of macropore formation was observed to occur at a current density of ?3.6 mA?cm?2. At larger current densities, cross-sectional micrographs revealed macropores of approximately columnar shape. Average pore diameter was estimated from plan view micrographs using an image processing algorithm and found to be consistent with a square root dependence on the etching current density in the range investigated.

Off-axis phonon and photon propagation in porous silicon superlattices studied by Brillouin spectroscopy and optical reflectance

Brillouin light scattering experiments and optical reflectance measurements were performed on a pair of porous silicon-based optical Bragg mirrors which had constituent layer porosity ratios close to unity. For off-axis propagation, the phononic and photonic band structures of the samples were modeled as a series of intersecting linear dispersion curves. Zone-folding was observed for the longitudinal bulk acoustic phonon and the frequency of the probed zone-folded longitudinal phonon was shown to be dependent on the propagation direction as well as the folding order of the mode branch. There was no conclusive evidence of coupling between the transverse and the folded longitudinal modes. Two additional observed Brillouin peaks were attributed to the Rayleigh surface mode and a possible pseudo-surface mode. Both of these modes were dispersive, with the velocity increasing as the wavevector decreased.

Contact angle of ethanol-water solutions on crystalline and mesoporous silicon

Measurements of contact angle of ethanol?water solutions were performed on crystalline silicon and on mesoporous silicon films with porosities up to ?72%. Water contact angles of 44? and 76? were measured for untreated and HF-dipped crystalline silicon, respectively, consistent with previous studies. The contact angle for ethanol?water mixtures was found to decrease with increasing ethanol concentration for both untreated crystalline silicon and also for HF-dipped crystalline silicon up to an ethanol concentration of ?80%; at higher concentrations the contact angle approached zero. Similar behaviour was observed on mesoporous silicon surfaces for ethanol concentrations ? 40%, above which the contact angle dropped abruptly to an immeasurably small value. This behaviour is attributed to a decrease in surface tension with increasing ethanol concentration. For all ethanol?water solutions, a minimum value of contact angle was observed at a porosity of ?40%, above which it remained approximately constant. The behaviour of contact angle with porosity can be explained by a change in the Wenzel roughness parameter due to changes in the specific surface area of the film.

Brillouin light scattering study of surface acoustic phonons in p+ porous silicon layers

Brillouin light scattering experiments were carried out on porous silicon layers fabricated from [111]-oriented p-type crystalline silicon substrates with resistivity 0.01 Ω cm. The thickness and porosity of the layers ranged from 1.6 to 11.40 µm and 0.25 to 0.40, respectively. The only Brillouin peaks evident in the spectra were those due to the Rayleigh surface acoustic phonon. The velocity of this mode was found to be either weakly dispersive or nondispersive for all the samples studied. The porosity dependence of the surface acoustic phonon velocity for the layers of the present work and those of other studies fabricated from p-doped crystalline silicon with resistivity ≤0.01 Ω cm appears to be different from that of porous layers fabricated from p-doped crystalline silicon with resistivity >0.01 Ω cm. The origin of this difference is not completely clear, but it is likely that some of the most significant contributors include dopant-related morphological dissimilarities, rapid air-induced oxidation of porous silicon layers formed from lightly p-doped crystalline silicon compared to layers formed from heavily p-doped crystalline silicon, and/or porosity gradient influence.