GIXRF characterization of thin Ge1-xSnx films

The continuous demand for better performance in microelectronics raises the interest into the research of new materials beyond silicon to improve electrical properties or add functionalities to complementary metal oxide semiconductor (CMOS) technology. Ge1-xSnx alloy offers higher electron and hole mobility compared to silicon and germanium. Furthermore, Ge1-xSnx can be used to induce desired strain to channel regions of CMOS devices and boost carrier mobility, given the larger size of Sn atoms compared to Ge ones. Finally, it has been proved that Sn concentration between 3 and 7% can change the bandgap of Ge from indirect to direct. However, Sn solid solubility in Ge is rather low (<1%) and thus to realize Ge1-xSnx solution of technological relevance requires out-of-equilibrium processes. For all those applications, it is mandatory to define analytical approaches able to give accurate measurements of Sn content. X-ray diffraction (XRD) can provide clear evidences of the formation of Ge1-xSnx phase and of the expected Sn substitutional fraction, but not in depth distributions. Secondary ion mass spectrometry (SIMS) can provide depth distributions, but the high concentrations of Sn (>1%) requires quantification out of the dilute regime where SIMS is most accurate. Grazing incidence X-ray fluorescence (GIXRF) analysis showed high potential as a complementary technique to overcome SIMS limitation, particularly in the top nm’s of impurities distributions. In the presented study, reference samples were prepared by ion implantation of Sn+ in Ge in a way to control the total amount of tin introduced in Ge substrates and to have peak concentrations of Sn > 1% confined in the top 40 nm. Traditionally information about the retained dose are obtained through comparison with reference standard samples. In the presented study a standard-less approach is proposed: measured SIMS profiles are used as an input to an internal developed simulating program to recreate a model describing the sample under study. The total retained dose is then given as a free parameter to the fitting routine of the GIXRF scan. In previous works, doping profile were modeled as impurities inside the matrix failing in giving a precise description of the optical constant inside the doped material. A new modelling procedure consisting in splitting the modeled sample in several layers following the composition given by SIMS profile is currently in use.