Riccardo Gatti

Misfit dislocation gettering by substrate pit-patterning in SiGe films on Si(001)

We show that suitable pit-patterning of a Si(001) substrate can strongly influence the nucleation and the propagation of dislocations during epitaxial deposition of Si-rich Si1-xGex alloys, preferentially gettering misfit segments along pit rows. In particular, for a 250 nm layer deposited by molecular beam epitaxy at xGe = 15%, extended film regions appear free of dislocations, by atomic force microscopy, as confirmed by transmission electron microscopy sampling. This result is quite general, as explained by dislocation dynamics simulations, which reveal the key role of the inhomogeneous distribution in stress produced by the pit-patterning.

Dislocation distribution across ultrathin silicon-on-insulator with epitaxial SiGe stressor

We studied the plastic deformation of an ultrathin silicon-on-insulator with epitaxial Si1−xGex by transmission electron microscopy, Raman spectroscopy, and finite-element method. We analyzed a top Si layer of 10 nm (testing also a 2 nm layer) with epitaxial Si0.64Ge0.36 stressors of 50 and 100 nm. SiGe plastically deforms the top Si layer, and this strain remains even when Si1−xGex is removed. For low dislocation densities, dislocations are gettered close to the Si/SiO2 interface, while the SiGe/Si interface is coherent. Beyond a threshold dislocation density, interactions between dislocations force additional dislocations to position at the Si1−xGex/Si interface.

Physically Justified Models for Crystal Plasticity Developed with Dislocation Dynamics Simulations

This article highlights how dislocation dynamics (DD) simulations provide a unique opportunity for establishing scale transitions in crystal plasticity. Recent progress in this numerical method is briefly reviewed. Based on the standard problem of plasticity in fcc crystals, we show that DD simulation insight provides guidelines for modeling material mechanical properties controlled by the collective behavior of dislocation microstructures. Hence, DD simulation allows more physical input to be incorporated into continuum models for strain hardening, thereby improving their predictive ability.

Dislocation driven nanosample plasticity: new insights from quantitative in-situ TEM tensile testing

Intrinsic dislocation mechanisms in the vicinity of free surfaces of an almost FIB damage-free single crystal Ni sample have been quantitatively investigated owing to a novel sample preparation method combining twin-jet electro-polishing, in-situ TEM heating and FIB. The results reveal that the small-scale plasticity is mainly controlled by the conversion of few tangled dislocations, still present after heating, into stable single arm sources (SASs) as well as by the successive operation of these sources. Strain hardening resulting from the operation of an individual SAS is reported and attributed to the decrease of the length of the source. Moreover, the impact of the shortening of the dislocation source on the intermittent plastic flow, characteristic of SASs, is discussed. These findings provide essential information for the understanding of the regime of ‘dislocation source’ controlled plasticity and the related mechanical size effect.

Strain and dislocation gradients from diffraction. Edited by Rozaliya I. Barabash and Gene E. Ice. Imperial College Press, 2014. Price (hardcover) GBP 104. ISBN-978-1-908979-62-9.

In recent years the development of X-ray diffraction (XRD) has opened the door to the possibility of accurately measuring strain gradients in crystalline materials. At variance with other experimental techniques, XRD allows one to investigate structural properties without destroying the samples and to perform in situ measurements. In this context, the book Strain and Dislocation Gradients from Diffraction highlights how XRD can be exploited to study mesoscale strain gradients and dislocation distributions in deformed crystals. This book is arranged in 13 chapters; different research groups contribute to each chapter. Chapter 1, written by the two editors of the book, gives a theoretical description of how distributions of defects in a crystal produce strain gradients that can be probed by diffraction. Furthermore, the reader is guided to the conclusion that different dislocation arrangements generate characteristic features in the diffracted pattern. While the first chapter shows that, from a theoretical point of view, dislocation arrangements can be detected by XRD, the following chapters describe the developments achieved in the past few years in the micro-diffraction techniques to measure the strain gradient and dislocation distribution in crystalline materials. Complementary micro-diffraction techniques, developed by different research groups around the world, are compared, identifying advantages, capabilities and limitations. Chapter 2 and chapter 10 are devoted to the description of polychromatic X-ray micro-diffraction (PXM), developed mainly at the Oak Ridge National Laboratory and at the Advanced Photon Source, and to the computer routines used to analyze experimental data. The PXM methodology is useful to study strain and dislocation gradients in polycrystalline materials. Chapter 3 describes recent developments in the high-energy transmission Laue (HETL) micro-beam diffraction technique. The HETL method is suitable to study material volumes embedded deep in experimental samples. Chapter 4 illustrates the capabilities of the XMAS software, a tool to analyze micro-diffraction data coming from synchrotron facilities. Chapter 5 presents a technique similar to PXM and developed at the European Synchrotron Radiation Facility (ESRF). In this chapter, the Laue micro-diffraction station of the ESRF is described and perspectives about the planned upgrade are given. Chapters 6–9 provide an overview of three-dimensional X-ray diffraction microscopy. This approach, useful to study polycrystalline materials, is based on the employment of highly penetrating X-rays and the application of a tomographic algorithm to reconstruct the three-dimensional shape of each grain in the sample. Chapter 11 shows the capabilities of energy variable X-ray diffraction (EVD). By varying the X-ray energy, the EVD technique allows one to study residual stresses and strains in a given sample with high depth resolution. Chapter 12 presents a description of the electron backscatter diffraction (EBSD) methods, which are complementary to X-ray micro-diffraction. This chapter highlights that, while EBSD is fundamentally a surface technique, it can achieve a spatial resolution greater than X-ray diffraction methods. Finally Chapter 13 depicts how X-ray micro-diffraction can be applied to the high-pressure study of materials. The scientific level of the book is excellent. I recommend this book to scientists interested in understanding the state of the art in characterizing local crystal structures and defects using X-ray micro-diffraction.