On the accuracy of strain measurements by different TEM techniques

Despite the lengthy, generally difficult and destructive sample preparation, transmission electron microscopy (TEM) is also considered an attractive analytical tool, in addition to its imaging capabilities, when high spatial resolution is required. Several TEM methods to evaluates strain in crystalline structures were developed in the last decades and applied to different materials. In this paper we report on the application of three TEM techniques to strain measurements in in three different SiGe films, epitaxially grown on perfect Si substrates, with nominal Ge concentration and thicknesses of 10%/45 nm, 30%/29 nm and 40%/22 nm. The data were collected in the framework of the CHALLENGES project, financed by the EC (Grant Agreement n. 861857) and act as reference measurements, together with other standard analytical techniques, such as Raman and x-ray diffraction (XRD), for the purposes of the project. Convergent Beam Electron Diffraction (CBED), High Resolution Geometrical Phase Analysis (GPA) and selected area micro-diffraction (SAD) were used to measure the SiGe film strain with respect to the Si substrate. All the three samples were thinned with focused ion beam (FIB) at different thicknesses for TEM analysis to both evaluate the influence of strain relaxation at the free surfaces on the final measurement and check the lamellae thickness range applicability of the three techniques. The results were also compared with those obtained by optical (Raman and TERS) and structural XRD analyses in bulk samples. The nominal Ge composition in the three samples was also checked by EDX analysis, that confirmed the expected values. While good CBED patterns were obtained in the Si substrate, it was not possible to evaluate strain in the SiGe films by means of this technique. As expected, the sensitive high order Laue zones (HOLZ) lines appeared blurred in the patterns. This result confirmed that CBED can generally be applied when strain values are below 10-3: larger values in relatively thin films may cause a pattern blurring even at relatively large thicknesses. It was possible to measure strain in the three samples by means of GPA and SAD techniques. While GPA relies on high resolution (HREM) imaging by detection of lattice fringes, and consequently can be used only at relatively low sample thicknesses (about 100 nm) in conventional TEMS, SAD can be applied even at crystal thickness up to 500 nm in Si. It was found that SAD measurements were in good accord with the expected strain values, as determined by assuming that Ge atoms occupy substitutional sites in the lattice. On the other hand, considerably smaller strain values were obtained with GPA, thus demonstrating that strain relaxation in these samples at 100 nm thickness values cannot be neglected. These results in standard samples with a simple geometry confirmed that reliable strain measurements in TEM prepared lamellae can only be obtained at relatively large sample thicknesses.