Nanostructure formation on Germanium by ion irradiation

This thesis work is focused on the investigation of a peculiar phenomenon observed in germanium: the formation of a regular network of columnar nanovoids induced by heavy ion and high fluence irradiation at room temperature. This phenomenon can represent a possible way to produce wide nanostructured areas on semiconductor surfaces by a well-established semiconductor technology process such as ion implantation. However, the formation mechanism of this regular network of Ge columnar nanovoids is still under debate. Therefore, the work has been focused on the investigation of the formation mechanisms and on the possible strategies to control the geometry and the composition of these structures, in order to exploit the results for possible technological applications. In particular, ion implantation was carried out using Sn+ ions with the double aim of creating Ge1-xSnx nanostructures and following the depth distribution of the impinging ions. Furthermore, ion implantation through ultra-thin (10-20 nm) films of silicon nitride (SiNx) was investigated as a possible way to impact on nanovoid formation kinetics, prevent ambient contaminations and prevent Sn out-diffusion upon thermal treatments. Firstly, low temperature Sn+ implants were carried out in order to define a recipe to prepare Ge1-xSnx alloy: Ge1-xSnx alloy films with thickness of 15-30 nm were obtained by implanting Sn+ in Ge at liquid nitrogen temperature and subsequent thermal annealing (600 °C for 10 s). High Sn substitutionality, no relevant diffusion, limited surface segregation and excellent crystallinity were achieved, a tin concentration of x=6-7 at.% was reached. Secondly, Ge nanostructures were prepared by high fluence ion implantation at room temperature and then morphologically and chemically characterized, determining that the obtained nanostructures are constituted by Sn-rich Ge. Nanovoids developed under the SiNx film, with reduced oxygen contamination. The first stages of nanovoid formation were observed for samples with and without the SiNx layer. The SiNx layer seems to induce a retarded nanovoid nucleation in terms of threshold fluence, without hindering nanovoid growth. The experimental data were interpreted on the basis of the vacancy clustering theory. SRIM simulations were performed to compare the distributions of point defects and implanted ions at different conditions in the SiNx/Ge stack. These helped to show that the depth distribution of energy deposition is the relevant parameter. Moreover, it was highlighted that both the redistribution in depth of the SiNx atoms and the implanted Sn+ contribute to a lowering of the Ge concentration causing the formation of a layer where nanovoid nucleation does not occur. Taking into account the ion mixing effect including the introduction of Sn, threshold value of the deposited energy was found. The thermal treatments investigated for the Ge1-xSnx alloy thin films were applied on nanostructured samples, causing a dramatic deformation of the nanovoids probably due to a melting temperature decreased by the presence of tin. The investigation of possible technological applications of Ge nanostructures was carried out, in particular in thermoelectric applications, in lithium ion batteries and gas sensors. Several samples were designed and ad-hoc substrates were produced.