Development of nano-topography during SIMS characterization of Ge1-xSnx alloy

Ge1-xSnx is a semiconductor alloy, compatible with silicon technology, with a bandgap tunable with Sn concentration (3%<x<7% can change the Ge bandgap from indirect to direct) [1], high electron and hole mobility [2,3]. For all those applications, it is mandatory to define analytical approaches able to provide accurate measurements of Sn content. SIMS can be a valuable choice but quantification and matrix issues due to the high Sn content need to be addressed. Therefore, we developed a SIMS protocol using Sn ion implants on Ge as reference samples. Ion implantation was carried out at liquid nitrogen temperature, in order to avoid the well-known phenomenon of Ge nanostructuration under heavy ion implantation at room temperature [4,5]. Implant fluences varied between 1x1014 at/cm2 and 5x1015 at/cm2 and implant energy was set at 45keV. SIMS characterization was performed in different configurations, i.e. using O2+ as primary beam and collecting positive secondary ions, Cs+ and negative secondary ions, Cs+ collecting MCs+ ions; the final results were compared with quantitative measurements obtained by RBS, revealing a good accuracy for the MCs+ protocol. However, it was observed that the applied sputtering conditions (Cs+ 1 keV, 55° incidence vs. normal) induced an early formation of surface topography resulting in a variation of sputtering yield. AFM images will be reported showing the peculiar topography developed on Ge and corrections to improve depth calibration accuracy will be discussed. The obtained protocol was then used to quantify also SIMS profiles of room temperature Sn implants, i.e. nanostructured Ge samples, with good accuracy. [1] S. Gupta et al., IEDM 2011. [2] G. He and H.A. Atwater, Phys. Rev. Lett., 79, (2007), 1937. [3] J.D. Sau and M.L. Cohen, Phys. Rev. B, 75, (2007), 045208. [4] I.H. Wilson, J. Appl. Phys. 53(3), (1982), 1698. [5] N.G. Rudawski and K.C. Jones, J. Mater. Res. 28(13), 1633, 2013