Morphological and Functional Modifications of Optical Thin Films for Space Applications Irradiated with Low-Energy Helium Ions.

Future space missions will operate in increasingly hostile environments, such as those in low-perihelion solar orbits and Jovian magnetosphere. This exploration involves the selection of optical materials and components resistant to the environmental agents. The conditions in space are reproduced on ground through the use of ion accelerators. The effects of He particles coming from the solar wind impinging on a gold thin film have been systematically investigated, considering absorbed doses compatible with the duration of the European Space Agency Solar Orbiter mission. Structural and morphological changes have been proved to be dependent not only on the dose but also on the irradiation flux. A predictive model of the variation of thin film reflectance has been developed for the case of lower flux irradiation. The results are discussed regarding reliability and limitations of laboratory testing. The outcomes are important to address the procedures for the space qualification tests of optical coatings.

Monolayer doping of germanium by phosphorus-containing molecules.

The DPP (diethyl 1-propylphosphonate) and ODPA (octadecylphosphonic acid) molecules are studied as precursors for the monolayer doping (MLD) of germanium. Their adsorption behaviour is investigated, revealing different physicochemical interactions between the phosphorus-containing molecules and the Ge surfaces. It is discovered that DPP adsorption occurs after the oxidation of Ge surface, while the ODPA undergoes chemisorption on -H terminated surfaces. Quantitative phosphorus analysis demonstrates that in the first case more than one monolayer is formed (from 2 to 4), while in the second a single monolayer is formed. Moreover, the analysis of phosphorus diffusion from the surface layers into the Ge matrix reveals that conventional thermal annealing processes are not suitable for Ge injection due to a higher activation energy of the process in comparison with silicon. On the contrary, pulsed laser melting is effective in forming a doped layer, owing to the precursor's decomposition under UV light.

Extended point defects in crystalline materials: Ge and Si.

B diffusion measurements are used to probe the basic nature of self-interstitial point defects in Ge. We find two distinct self-interstitial forms--a simple one with low entropy and a complex one with entropy ∼30 k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket--we name it a morph. Computational modeling suggests that morphs exist in both self-interstitial and vacancylike forms, and are crucial for diffusion and defect dynamics in Ge, Si, and probably many other crystalline solids.

First direct measurement of the 17O(p,γ)18F reaction cross section at Gamow energies for classical novae.

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive (18)F, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The (17)O(p,γ)(18)F reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes, but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range E(c.m.)~/= 200-370 keV appropriate to hydrogen burning in classical novae. In addition, the E(c.m.)=183 keV resonance strength, ωγ=1.67±0.12 µeV, has been measured with the highest precision to date. The uncertainty on the (17)O(p,γ)(18)F reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis.

Room temperature migration of boron in crystalline silicon.

We demonstrate that substitutional B in silicon can migrate even at room temperature and below, stimulated by a high interstitial flux. Once mobile B is formed, it migrates for long distances with a diffusivity >5 x 10(-13) cm(2)/s, until it assumes an immobile configuration with a migration length independent of the temperature. This phenomenon is present during secondary ion mass spectrometry (SIMS) analyses of B profiles, altering the profile during the analysis itself. These results shed new light on all the data based on SIMS analyses and reported in literature in the last decades.

Computation of the strain field generated by dislocations with a position-dependent Burgers' vector distribution

A new phenomenon of strain relaxation will be presented. In a series of InxGa1-xAs graded composition buffer layers grown on well cut (001) GaAs substrates, a curvature of the epilayer lattice has been found, i.e. a tilt of the epilayer lattice orientation with respect to the substrate which varies coherently along the sample surface on the scale of several mm. The most recent data analysis performed on a buffer layer compositionally graded with a six-step profile shows also a thickness functional dependence of the curvature. The epilayer lattice curvature has been attributed to a coherent lateral distribution of the Burgers' vectors. An analytical model has been developed in the framework of the continuum elasticity theory to compute the related strain field. The results show small but unexpected contributions to the parallel strain.