The evolution and dynamics of damage accumulation due to ion beam implantation studied by X-ray diffraction

In the last years, the development of XFEL facilities have stimulated the discussions on the application of highly intense, ultrafast and/or coherent X-ray pulses for nonconventional in-situ studies in order to improve the understanding of fundamental and atomic-level ultrafast, processes like athermal materials modification by intense fs- or ps-laser or ion pulses generating collision cascades. A predictive understanding of such processes on the performance of advanced (nano) materials on their fabrication and live time would have a strong influence on the development of new materials and technologies. We will discuss the layout of a potential experiment at the European XFEL that could it make possible to probe the cascade dynamics of individual cascades in the (sub-)picosecond temporal regime.

Today, with the present x-ray sources it is almost impossible to monitor a collision cascade in-situ. However, the accumulation of damage and the diffusion of defects in implanted species are much slower processes and can be studied in-situ already today.

Ion beam modification of materials consists usually of two steps (i) the ion implantation and (ii) a subsequent thermal treatment. The first step is characterized by the continuous formation and relaxation of collision cascades over the irradiation time leading to a super-saturation of different types of defects (vacancies, self-interstitials, clusters, etc.) and the resulting kinetic processes like defect diffusion, the build-up of strain etc., on the time scales ranging from ns up to minutes and even hours. The second step is directed towards a reduction of radiation damage formed by ion implantation. It is determined by kinetic processes whose duration is mainly given by the length of the thermal treatment, such as damage recovery, recrystallization of amorphous regions, migration and/or clustering or segregation of point defects and dopant atoms. As a result the material undergoes a strong modification that determines the way how its properties of are changed.
An in-situ ion beam implantation experiment was set up at ROBL/MRH at ESRF. For this purpose an ion gas source with a maximum acceleration voltage of 5keV was mounted on a sputtering chamber. To guarantee a sufficient volume damage the ion energy was further raised to 20keV. Si (001) samples were irradiated at room temperature using He+ at an ion flux of about 1013ions/cm2s. In-situ/in-operando subsequent reciprocal space maps (RSM) were measured to study the evolution of the implanted layer. The time resolution for one RSM as shown in figure 1 was 100ms resulting an acquisition time for one map of below 1min. The crystal truncation rod vanished within the first seconds of He-ion bombardment. In the following the Si (004) reflection broadens, forming a layer peak that gives clearly a hint of increasing strain in the material. After 50 minutes (>1016ions/cm2) a steady-state that corresponds to a heavily damaged or amorphized Si layer was reached.