Regular nano-void formation on Ge films on Si using Sn ion implantation through silicon nitride caps

Germanium is known to present a peculiar lattice damage evolution under heavy mass ion irradiation. In fact, a characteristic ‘honeycomb-like’ void structure is formed once a threshold fluence is implanted (typically 5-10E1014 at/ cm2). The structure consists of a relatively regular network of columnar voids, with ~10 nm diameter and ~100 nm depth. The different diffusivity of self-interstitials and vacancies and vacancy agglomeration are thought to be the mechanisms responsible for the phenomenon. In this work we tried to use Silicon nitride (SiN) caps of different thickness to act on void formation and to prevent contamination in the nano-voids. The starting wafers had a Germanium 1.5 μm thick film CVD epitaxially deposited on (100) Si substrates. A 5E15 at/ cm2 Sn+ fluence was implanted on three different samples: no cap on Ge surface, 11 nm of SiN cap on Ge and 20 nm SiN on Ge. The implant energy was adjusted to have the same Sn (and damage) distribution in Ge despite the different SiN cap thicknesses. Sample surfaces were characterized by SEM and AFM. Cross section TEM provided information about the development in depth of the voids. SIMS and RBS were used to obtain information about final Sn and contaminant distribution and the damaged layer thickness. X-ray photoelectron spectroscopy (XPS) was used to identify contaminants and degree of oxidation of Ge and Sn atoms. Results about the obtained void geometry will be reported and formation mechanisms will be discussed.