Spectrum simulation of rough and nanostructured targets from their 2D and 3D image by Monte Carlo methods

Université de Montréal

Corteo is a program that implements Monte Carlo (MC) method to simulate ion beam analysis (IBA) spectra of several techniques by following the ions trajectory until a sufficiently large fraction of them reach the detector to generate a spectrum. Hence, it fully accounts for effects such as multiple scattering (MS). Here, a version of Corteo is presented where the target can be a 2D or 3D image. This image can be derived from micrographs where the different compounds are identified, therefore brining extra information into the solution of an IBA spectrum, and potentially significantly constraining the solution. The image intrinsically includes many details such as the actual surface or interfacial roughness, or actual nanostructures shape and distribution. This can for example lead to the unambiguous identification of structures stoichiometry in a layer, or at least to better constraints on their composition. Because MC computes in details the trajectory of the ions, it simulates accurately many of its aspects such as ions coming back into the target after leaving it (re-entry), as well as going through a variety of nanostructures shapes and orientations. We show how, for example, as the ions angle of incidence becomes shallower than the inclination distribution of a rough surface, this process tends to make the effective roughness smaller in a comparable 1D simulation (i.e. narrower thickness distribution in a comparable slab simulation). Also, in ordered nanostructures, target re-entry can lead to replications of a peak in a spectrum.  Other improvements to Corteo include obtaining energy-depth maps that can be used to simulate the spectrum of IBA techniques based on photon emission such as PIXE and gamma-emitting NRA.   Finally, we show how, in thin layers made of heavy elements on light substrates, the broad background due to wide-angle MS in RBS is mainly due to ions flying parallel to the surface inside the heavy layer and scattering a second time. This could be the basis of an analytical model to account for this particular case of double wide-angle scattering.