Optical metrology of thickness and indium composition of epitaxial InxGa1−xAs layers on Si substrates
An optical metrology solution using a Spectroscopic Ellipsometer (SE) tool has been demonstrated to monitor the thickness and indium composition of InxGa1-xAs layers on Si substrates for III-V compound semiconductor channel devices. Fundamentally, the SE tool is based on light interference and phase shift effects corresponding to individual material properties. A wide range of wavelengths (190 ~ 840nm) of the SE spectra is used to collect elliptically polarized light and for dispersion model characterization to float n and k values at each wavelength. The SE data for InxGa1-xAs and indium composition is quantified using TEM (Transmission Electron Microscopy) and RBS (Rutherford Backscattering Spectrometry) showing excellent correlation. In the range of 190nm to 840nm wavelengths SE is able to provide InxGa1-xAs layer thickness through post data processing using InxGa1-xAs/GaAs layer thickness with surface roughness consideration. Reducing the surface roughness is expected to enable more accurate and higher intensity of elliptically polarized light from films so that the best film model can be applied to float n and k values. Indium composition of the InxGa1-xAs layer is quantified by dispersion (n and k) values extracted from a specific wavelength. In this work, the n value of InxGa1-xAs/GaAs film stack at 219nm shows excellent proportional relationship with RBS data on three different indium compositions, while k value at 248nm shows excellent inverse relationship. This is a promising solution to evaluate critical growth parameters such as indium composition and to offer detailed and immediate feedback on the deposition process. This solution can be extended to other compound materials - characterizing thickness and composition by correlating n and k values for each wavelength.
RELATIONSHIP BETWEEN SURFACE MORPHOLOGY AND STRAIN RELAXATION DURING GROWTH OF INGAAS STRAINED LAYERS
The relationship between the surface morphology and strain relaxation is explored in strained InxGa1−xAs layers grown on GaAs by molecular beam epitaxy. In situ light scattering, detected simultaneously along [110] and [110], reveals an asymmetric surface roughening which is consistent with ex situ scanning force microscopy. Transmission electron microscopy shows that strain relaxation by misfit dislocation formation occurs before the surface roughening is detected, for In0.18Ga0.82As films grown at 490 °C.
Impact of Nuclear Data Uncertainties on Transmutation of Actinides in Accelerator-Driven Assemblies
Abstract The potential impact of nuclear data uncertainties on a large number of performance parameters of reactor cores dedicated to the transmutation of radioactive wastes is discussed. An uncertainty analysis has been performed based on sensitivity theory, which underlines the cross sections, the energy range, and the isotopes that are responsible for the most significant uncertainties. To provide guidelines on priorities for new evaluations or validation experiments, required accuracies on specific nuclear data have been derived, accounting for target accuracies on major design parameters. The required accuracies (mostly in the energy region below 20 MeV), in particular for minor actinide data, are of the same order of magnitude of the achieved accuracies on major actinides. Specific requirements also concern the improvement of minor actinide data related to decay heat and effective delayed-neutron fraction assessment.