A method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix. A synthetic sandstone was manufactured from Sand cemented with an epoxy glue. The cracks of known geometry were introduced into the material in the manufacturing stage, by emplacing thin metallic discs in the Sand-epoxy matrix. These discs were chemically leached out of the consolidated porous sandstone. Acoustic anisotropy. and shear-wave splitting were observed in the synthetic sandstones. For the dry sample the observed angular dependence of the P-and S-wave velocities (at 100 kHz) compares well, qualitatively, with the theoretical models of Hudson and of Thomsen. Quantitatively, however, the experimental data fits Hudson's model better. For the case of a saturated sample the experimental results are in excellent agreement with Thomsen's model. Hudson's model, on the other hand, predicts a different angular dependence for P-waves. This demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process. The results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry .
We systematically investigate the compositional uniformity, degree of strain relaxation (DSR), defect structure and surface morphology of GeSn epitaxial layers with 16% Sn, grown by low temperature molecular beam epitaxy (MBE) on Ge-buffered Si(001) substrates. Combining atom probe tomography, reciprocal space mapping, cross-sectional transmission electron microscopy, and atomic force microscopy analyses, we demonstrate that for a layer thickness of
t
GeSn
=
250
nm
, a high DSR (∼70%) can be achieved, while maintaining compositional uniformity at the atomic scale. We find no evidence of Sn clustering in the bulk, or Sn segregation to the surface, for at least this value of
t
GeSn
. The observed compositional uniformity contrasts the well-established phenomenon of strain-relaxation enhancement of Sn content in chemical vapour deposition (CVD) growth of GeSn. The defect structure leading to strain relaxation in these MBE-grown GeSn epitaxial layers is also distinctly different from that observed in CVD growth of the alloy. We observe the co-existence of highly strain-relaxed and pseudomorphically strained regions in the grown epilayers, tentatively explained by bunching of threading dislocations. Considering that MBE growth of GeSn epitaxial layers, with such high-Sn content and layer thickness, has not been reported before, our results are encouraging for future improvements in design and fabrication of group-IV-based mid-infrared photonic devices.
Molecular beam epitaxy of Ge (111) thin films on epitaxial-Gd 2 O 3 /Si(111) substrates is reported, along with a systematic investigation of the evolution of Ge growth, and structural defects in the grown epilayer. While Ge growth begins in the Volmer-Weber growth mode, the resultant islands coalesce within the first ~ 10 nm of growth, beyond which a smooth two-dimensional surface evolves.Coalescence of the initially formed islands results in formation of rotation and reflection microtwins, which constitute a volume fraction of less than 1 %. It is also observed that while the stacking sequence of the (111) planes in the Ge epilayer is similar to that of the Si substrate, the (111) planes of the Gd 2 O 3 epilayer are rotated by 180° about the [111] direction. In metal-semiconductor-metal schottky photodiodes fabricated with these all-epitaxial Ge-on-insulator (GeOI) samples, significant suppression of dark current is observed due to the presence of the Gd 2 O 3 epilayer. These results are promising for application of these GeOI structures as virtual substrates, or for realization of high-speed group-IV photonic components.
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