Swift heavy ion irradiation of semiconducting materials - defect production, phase transformation and annealing

Abstract

Swift heavy ion irradiation causes several effects on the target material. This doctoral thesis is about two effects, namely track formation and ion beam induced annealing.Tetrahedral amorphous carbon, a material consisting of 80% sp3 bound carbon, is known to form sp2 rich filaments after swift heavy ion irradiation along the ions path (i.e. 1 GeV U-ions). This phase transformation causes a change in conductivity, the ion tracks with a diameter of 8 nm form conductive nanowires embedded in an insulating matrix. The aim of this thesis was increasing the conductivity of the tracks and retaining the insulating properties of the surrounding matrix. Two approaches were made to achieve this. First, the ta-C samples were doped with nitrogen, boron, iron or copper during deposition. The concentration was kept below 2 at% to avoid sp2 bond formation. It is expected that these dopants enhance conductivity by increasing the amount of localized states at the Fermi level. Second, as irradiation with a higher electronic energy loss forms more conductive tracks, the electronic energy loss during irradiation was increased. The electronic energy loss of 1 GeV U ions in ta-C is about 40 keV/nm. The ta-C samples were also irradiated with molecules, here 30 MeV C60 molecules. The electronic energy loss in ta-C, which can be calculated as the sum of the electronic loss of each atom, equals 72 keV/nm. The conductivity of the ion tracks in ta-C was analyzed using two methods, atomic force microscopy (AFM) with a conductive cantilever and macroscopic transport measurements at low temperatures.Doping of ta-C increases track conductivity. The best results were obtained after copper doping. All other dopants also increase the matrix conductivity, thus, the conductivity contrast decreases. Irradiation with C60 molecules also results in very conductive tracks. It was also found that tracks created by ion irradiation show a broad distribution in conductivity, visible in the AFM-images. After cluster irradiation, all tracks show the same conductivity.The other effect of swift heavy ion irradiation analyzed in this work is ion beam induced annealing. In principle, ion beam induced annealing has some advantages compared to furnace annealing, like only locally heated for a short time span (1 ps), sample decomposition and dopant diffusion can be suppressed. A possible ion beam induced annealing effect was analyzed on three different semiconductors, gallium nitride, diamond and silicon carbide. These materials were irradiated with a variety of ions and energies. For GaN, the irradiation was performed at room temperature; the other materials were irradiated at elevated temperatures. The samples were either implanted with Mg-ions (GaN) or Ar-ions (diamond, SiC) for damage production. The crystal quality before and after irradiation was monitored by luminescence analysis.An annealing effect was observed for low fluence (3 x 1013 Mg-ions/cm2) implanted GaN after irradiation with 578 MeV Cr-ions (8 keV/nm) with a low fluence (5 x 1011 ions/cm2). The intensity ratio of the near band-edge to the intensity of the pristine blue band was found to be an empirical figure of merit for crystal quality. However, Mg-related luminescence could not be recorded. An annealing effect comparable to the effect found for GaN could not be found for diamond. Nonetheless, the luminescence spectra show that unimplanted diamond is unaffected by swift heavy ion irradiation. SiC is damaged further during swift heavy ion irradiation. These results can support the feasibility of ion beam induced annealing under certain preconditions.