In situ XRD measurements to explore phase formation in the near surface region

Abstract

Though X-ray diffraction (XRD) is a well-established technique—yet still continuously evolving—for characterization of thin films and diffusion layers, there are two inherent limitations due to the underlying physical principles leading to the diffraction: (i) all data are a weighted integral across the information depth and (ii) X-ray amorphous or nanocrystalline phases are very difficult to be detected. The purpose of this Perspective is to show how a straightforward combination of in situ XRD with depth profiling using a laboratory experiment in combination with a low-energy broad-beam ion source may allow us to overcome these limitations for functional surfaces. Whereas time-resolved investigations of diffusion and phase formation during ion implantation using reactive species are well established, using ion beam sputtering with nonreactive species for depth profiling of the phase composition during the measurements is a rather recent development. While employing analysis of differential spectra leads to a depth resolution below the information depth, ongoing in situ sputtering will lead to information obtained from layers deeper than the information depth. Keeping track of the total diffraction intensity permits the identification of phases without reflections in the XRD measurements, i.e., they appear to be amorphous, and their depth distribution. Selected examples drawn mainly from investigations of the nitriding kinetics of austenitic stainless steel will be used to demonstrate the potential and possible limitations of this method.