Fast atomic diffusion in solid state amorphous thin films irradiated with low fluence laser pulse in UV

Full text: Thin amorphous films were irradiated with 266 nm radiation of the Nd-YAG laser. Experiments were performed on SiGe amorphous films and TiGeO amorphous films obtained by RF magnetron sputtering deposition. Low value laser fluence well below the ablation or melting threshold was used. Before and after laser irradiation, the films structure was studied by SEM, AFM and TEM methods. The TEM specimens were prepared by cross section technique. This method allows the study of structural modifications, induced by the laser pulse action in the thin surface layer corresponding to the radiation absorption depth in the film. The modifications of the surface layer atomic composition, effect which is directly related with the atomic diffusion induced by the laser pulse action, were studied using the HAADF-STEM imaging and the EDX line scan analyze technique. The SiGe amorphous films with 170 nm thickness were obtained by RF magnetron sputtering using Si:Ge ratio of 45:55. After laser pulse irradiation with 30 mJ/cm² fluence, which is well below the experimental 50 mJ/cm² value, which is the experimental threshold value for melting , the surface layer show a gradual crystallization which develop in depth to about 50 nm after 10 laser pulse irradiation. In the same time with the crystallization process, the Ge atoms concentration grows in the 10 nm film surface layer and the region under this layer is depleted in Ge, as revealed by the EDX line scan analyze. This diffusion effect takes place in solid state phase in the time of laser pulse action. Considering that the normal values of Ge diffusivities is solid state at 1000 K are of the order of magnitude of 10^-16 cm²/s, the diffusion length is of the order of several pm. However, the redistribution of Ge in the surface layer shows a real diffusion coefficient of the order of 10^-4cm²/s, which is characteristic for liquid state. A similar fast diffusion effect takes place in the case of laser pulse irradiation of TiGeO amorphous films. These films with 350 nm thickness were deposited with the Ge:TiO₂ ratio of 50:50, and their annealing at 900 K leads to formation of a nanostructurated crystalline film containing Ge(Ti)O₂ rutile, Ti(Ge)O₂ anatase and Ge cubic phases. The laser irradiation of the as deposited TiGeO amorphous film was performed perpendicular to the film surface using 100 laser pulses and a fluence of 15 mJ/cm² . After the laser irradiation the film structure remains amorphous. A wave like relief with a quasi-period of 200 nm was produced a on the film surface and the surface layer nanostructure was modified in a depth of 50 nm. An interesting effect of Ge atoms segregation in nanometric amorphous spheres takes place under the film surface. These Ge spheres have size from 5 to 20 nm and are embedded in the TiGeO amorphous matrix depleted in Ge. The surface temperature estimations lead to a maximum temperature of about 700 K, which is well below the Ge or Ge and Ti oxides melting temperature. However, the laser pulse action probably can trigger the glass transition effect, which produces a softening of the amorphous matrix and give the possibility to the surface relief appearance. Even so, the fast diffusion of Ge atoms forming the Ge amorphous nanospheres, cannot be explained only with this softening. In conclusion, the experimental facts reveal that in the case of UV pulse laser irradiation of Ge content amorphous films, using fluence values less than the melting threashold, a fast atomic diffusion effect takes place. This effect takes place in solid state phase and show diffusivity values like in the liquid state phase. (author)