Orientation dependence of the elastic instability on strained SiGe films

At low strain, SiGe films on Si substrates undergo a continuous nucleationless morphological evolution known as the Asaro-Tiller-Grinfeld instability. We demonstrate experimentally that this instability develops on Si(001) but not on Si(111) even after long annealing. Using a continuum description of this instability, we determine the origin of this difference. When modeling surface diffusion in presence of wetting, elasticity and surface energy anisotropy, we find a retardation of the instability on Si(111) due to a strong dependence of the instability onset as function of the surface stiffness. This retardation is at the origin of the inhibition of the instability on experimental time scales even after long annealing.Comment: 3 pages, 4 figure

Population Dynamics of the Critically Endangered\ud Golden Lancehead Pitviper, Bothrops insularis: Stability\ud or Decline?

Little is known about vital rates of snakes generally because of the difficulty in collecting data. Here we used a robust design\ud mark-recapture model to estimate survival, behavioral effects on capture probability, temporary emigration, abundance and\ud test the hypothesis of population decline in the golden lancehead pitviper, Bothrops insularis, an endemic and critically\ud endangered species from southeastern Brazil. We collected data at irregular intervals over ten occasions from 2002 to 2010.\ud Survival was slightly higher in the wet season than in the dry season. Temporal emigration was high, indicating the\ud importance of accounting for this parameter both in the sampling design and modeling. No behavioral effects were\ud detected on capture probability. We detected an average annual population decrease (l= 0.93, CI = 0.47–1.38) during the\ud study period, but estimates included high uncertainty, and caution in interpretation is needed. We discuss the potential\ud effects of the illegal removal of individuals and the implications of the vital rates obtained for the future persistence and\ud conservation of this endemic, endangered species

Low-temperature solid phase epitaxy for integrating advanced source/drain metal-oxide-semiconductor structures

International audienceWe show that chemical vapor deposition using trisilane decomposition opens capabilities for the deposition of amorphous silicon on Si substrate at low temperature. Based on this behavior we developed a process including amorphous silicon deposition and crystallization. Transmission electron microscopy observations prove that solid phase epitaxy (SPE) occurs and produces monocrystalline layers, free of extended defects and compatible with complementary metal-oxide-semiconductor technology. We also show that during SPE films remain amorphous on oxidized areas while they transform into single crystal on Si. This process opens promising perspectives for the fabrication of advanced MOS structures

Insights into solid phase epitaxy of ultrahighly doped silicon

International audienceIn this study we investigate the mechanisms of growth and boron (B) incorporation into crystalline silicon (c-Si) during crystallization of amorphous doped silicon (a-Si:B) films. The process developed consists of two steps, first the chemical vapor codeposition at low temperature of Si and B atoms to form a-Si:B layer and second the crystallization of amorphous phase during in situ annealing to incorporate boron atoms on the substitutional sites of c-Si. We find that the crystallization rate linearly increases with the nominal boron concentration (CB) up to a critical CB∗ which corresponds to the maximum concentration of electrically active boron atoms in the crystalline phase. In these conditions, an increase in the crystallization rate by a factor 22 as compared to the intrinsic crystallization rate is obtained. We suggest that this remarkable behavior is attributed to D+ charged defects associated to the activated doping atoms in agreement with the generalized Fermi level shifting model. For larger CB, further boron atoms are incorporated in the amorphous phase in the form of ultrasmall clusters that do not contribute to shift the Fermi level of a-Si. As a consequence, for CB\textgreaterCB∗ the crystallization rate does not increase any more. We also show that crystallization provides a more complete incorporation of boron atoms already present in a-Si than the codeposition of Si and B atoms in the same experimental conditions (same growth rate and temperature). This result is attributed to the lower kinetic segregation at the amorphous-crystalline (a/c) interface than at the vacuum-crystalline interface. The lower kinetic segregation results from both a higher diffusion barrier of boron atoms at the a/c interface and a lower segregation energy (due to a low a/c interface energy)

On the influence of elastic strain on the accommodation of carbon atoms into substitutional sites in strained Si:C layers grown on Si substrates

International audienceMeasurements of strain and composition are reported in tensile strained 10- and 30-nm-thick Si:C layers grown by chemical vapor deposition on a Si (001) substrate. Total carbon concentration varies from 0.62% to 1.97%. Strain measurements were realized by high-resolution x-ray diffraction, convergent-beam electron diffraction, and geometric phase analysis of high-resolution transmission electron microscopy cross-sectional images. Raman spectroscopy was used for the deduction of the substitutional concentration. We demonstrate that in addition to the growth conditions, strain accumulating during deposition, thus depending on a layer thickness, has an influence on the final substitutional carbon composition within a strained Si:C laye