Columnar nano-void formation on Germanium under Sn+ ion implantation: Ge1-xSnx walls

As it is well known, Ge undergoes a peculiar surface nanostructuration under heavy ion implantation at room temperature [1,2]. In fact, once a threshold dose (~5-10x1014 at/cm2) is reached, the formation of a relatively regular nanostructured network of columnar voids with ~20 nm diameter and ~100 nm depth occurs. The formation mechanism of these structures seems to be strongly dependent on the different vacancy and interstitial mobility in Ge and clustering of vacancy [2]. In this work, Ge (1.5 µm thick film epitaxial on Si(100) or bulk) was implanted with 5-100x1014 Sn+/cm2. A protective layer (SiNx) with different thickness (0, 10 nm, 20 nm) was deposited on Ge to prevent contamination of the nanostructures and to act on voids formation, to modify their size, dimension and symmetries [3]. The implantation energies were tuned in order to obtain the same Sn distribution in samples with the same dose but different protective layer thickness. Our goal is the formation of nanostructures with Ge1-xSnx walls through thermal treatment of our samples, in order to combine the peculiar properties of Ge1-xSnx alloy (tunable bandgap[4], high electron and hole mobility[5]) with the Ge nanostructures. Electronic microscopy (TEM cross sections and SEM plan views) and AFM measurements allowed the morphologic characterization of our samples. SAD measurements gave information on the crystallization degree. Moreover SIMS and RBS measurements provided information on Sn distribution and damaged layer thickness. XPS characterization was used to investigate the Ge oxidation degree as a function of the air exposition time and the identification of other contaminants. [1] I. Wilson, J. Appl. Phys. 53(3), 1698, 1982 [2] N.G. Rudawski and K.C. Jones, J. Mater. Res. 28(13), 1633, 2013 [3] T. Janssens et al, J. Vac. Sci. technol. B 24(1), 2006, 510 [4] G. He and H.A. Atwater, Phys. Rev. Lett., 79, (2007), 1937. [5] J.D. Sau and M.L. Cohen, Phys. Rev. B, 75, (2007), 045208.