Irreversible nucleation in molecular beam epitaxy: From theory to experiments

Recently, the nucleation rate on top of a terrace during the irreversible growth of a crystal surface by MBE has been determined exactly. In this paper we go beyond the standard model usually employed to study the nucleation process, and we analyze the qualitative and quantitative consequences of two important additional physical ingredients: the nonuniformity of the Ehrlich-Schwoebel barrier at the step-edge, because of the existence of kinks, and the steering effects, due to the interaction between the atoms of the flux and the substrate. We apply our results to typical experiments of second layer nucleation.Comment: 11 pages. Table I corrected and one appendix added. To be published in Phys. Rev. B (scheduled issue: 15 February 2003

Near-infrared light trapping and avalanche multiplication in silicon epitaxial microcrystals

The chemical vapor deposition of silicon on a patterned silicon substrate leads to the formation of 3D microcrystals, which, due to their inclined top facets and high aspect ratio, produce a light-trapping effect enhancing the optical absorption in the near-infrared (NIR). In this work, it is demonstrated that Si microcrystals can form the building blocks of a new class of NIR sensitive photodetectors operating in a linear or avalanche regime. Microcrystal-based devices are designed by coupling a 2D kinetic-growth model with a Poisson drift-diffusion solver and fabricated by combining electron beam lithography and low-energy plasma-enhanced chemical vapor deposition (LEPECVD). The optoelectronic properties of microcrystal-based p–i–n photodiodes are investigated both theoretically and experimentally by means of finite-difference time-domain (FDTD) simulations and responsivity measurements. At 1000 nm wavelength, the responsivity of microcrystal-based devices is six times higher than that of an equivalent mesa diode. Moreover, the photocurrent gains of Si microcrystals operating as an avalanche photodiode (APD), at the same wavelength, reaches 2 × 104 demonstrating the potentialities of substrate patterning, combined with epitaxial growth, for amplified photodetection applications

Cell killing and resistance in pre-operative breast cancer chemotherapy

<p>Abstract</p> <p>Background</p> <p>Despite the recent development of technologies giving detailed images of tumours <it>in vivo</it>, direct or indirect ways to measure how many cells are actually killed by a treatment or are resistant to it are still beyond our reach.</p> <p>Methods</p> <p>We designed a simple model of tumour progression during treatment, based on descriptions of the key phenomena of proliferation, quiescence, cell killing and resistance, and giving as output the macroscopically measurable tumour volume and growth fraction. The model was applied to a database of the time course of volumes of breast cancer in patients undergoing pre-operative chemotherapy, for which the initial estimate of proliferating cells by the measure of the percentage of Ki67-positive cells was available.</p> <p>Results</p> <p>The analysis recognises different patterns of response to treatment. In one subgroup of patients the fitting implied drug resistance. In another subgroup there was a shift to higher sensitivity during the therapy. In the subgroup of patients where killing of cycling cells had the highest score, the drugs showed variable efficacy against quiescent cells.</p> <p>Conclusion</p> <p>The approach was feasible, providing items of information not otherwise available. Additional data, particularly sequential Ki67 measures, could be added to the system, potentially reducing uncertainty in estimates of parameter values.</p