Surface Stress, Morphological Development, and Dislocation Nucleation During SixGe1-x Epitaxy

Utilizing Ge marker layer experiments combined with atomic number contrast (Z-contrast) imaging, we have studied the evolving surface morphology of SixGe1-x alloys during growth by molecular beam epitaxy. The marker layers map out the instability transition between planar two-dimensional (2D) growth and three-dimensional (3D) growth. The transition occurs via the gradual formation of a surface ripple as anticipated from instability theory. However, these undulations rapidly develop into crack-like surface instabilities which we simulate and explain by the mechanism of stress-driven surface diffusion. Finally, we model the large stresses associated with these features within a fracture mechanics formalism. This analysis demonstrates that crack-like instabilities provide ideal candidate sites for the nucleation of misfit dislocations

Determination of reference values for optical properties of liquid phantoms based on Intralipid and India ink

A multi-center study has been set up to accurately characterize the optical properties of diffusive liquid phantoms based on Intralipid and India ink at near-infrared (NIR) wavelengths. Nine research laboratories from six countries adopting different measurement techniques, instrumental set-ups, and data analysis methods determined at their best the optical properties and relative uncertainties of diffusive dilutions prepared with common samples of the two compounds. By exploiting a suitable statistical model, comprehensive reference values at three NIR wavelengths for the intrinsic absorption coefficient of India ink and the intrinsic reduced scattering coefficient of Intralipid-20% were determined with an uncertainty of about 2% or better, depending on the wavelength considered, and 1%, respectively. Even if in this study we focused on particular batches of India ink and Intralipid, the reference values determined here represent a solid and useful starting point for preparing diffusive liquid phantoms with accurately defined optical properties. Furthermore, due to the ready availability, low cost, long-term stability and batch-to-batch reproducibility of these compounds, they provide a unique fundamental tool for the calibration and performance assessment of diffuse optical spectroscopy instrumentation intended to be used in laboratory or clinical environment. Finally, the collaborative work presented here demonstrates that the accuracy level attained in this work for optical properties of diffusive phantoms is reliable

Density of amorphous SixGe1-x alloys prepared by high-energy ion implantation

The atomic density of amorphous SixGe1 12x alloys (0 x 1), has been measured. The amorphous alloys were made by high-ion-energy implantation into monocrystalline SixGe1 12x layers, deposited epitaxially on silicon substrates. During the bombardments, a steel contact mask was used to create alternating lines of amorphous and crystalline material. The ratio between the densities of crystalline and amorphous alloys was measured with 0.1\u20130.2% accuracy using surface profilometry and Rutherford backscattering spectrometry in conjunction with channelling. Amorphous pure elements and alloys are less dense by 1.5\u20132.1% than the crystalline pure elements and alloys. By comparing both the amorphous and crystalline densities with Vegard's law, it is found that this law underestimates the a-SixGe1 12x densities by the same amount as those of c-SixGe1 12x.NRC publication: Ye