A Damage Model for Disordered Structures in Ion Irradiated Silicon
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A Damage Model for Disordered Structures in Ion Irradiated Silicon Ju-Yin Cheng1 and J. Murray Gibson2 Department of Materials Science and Engineering, University of Illinois, 104 S. Goodwin Avenue, Urbana, IL 61801, U.S.A. 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. 1
ABSTRACT Medium-range order has been observed in ion-implanted amorphous silicon, suggesting a paracrystalline structure for this material. The origin of a paracrystalline structure may be due to an energy spike phenomenon. To evaluate the influence of energy spikes on a particular process, we have attempted to calculate the characteristic energy in a spike. However, the observed depth dependence of amorphous structures in as-implanted silicon is puzzling. To explain this, we simulated the depth distribution of cascade events in a particular energy range. We found a great increase of point defect concentration and cascade events as the depth increases. This result could explain the experimental depth dependence.
INTRODUCTION Ion-implanted amorphous silicon has been long believed to be a continuous random network, typically with a short range order. This short range order is commonly measured by conventional diffraction techniques, such as X-ray diffraction. Since diffraction techniques have their limit of ability to detect higher-order atomic correlations, new techniques are needed. We have demonstrated a new technique, fluctuation electron microscopy, to probe order beyond the short-range length scale in amorphous materials, mainly in medium-range order (∼13 nm) [1-3]. Using fluctuation microscopy, a pronounced medium-range order and the effect of annealing were observed in ion-implanted silicon. These results are consistent with previous results on sputtered hydrogenated amorphous silicon and evaporated amorphous germanium. The medium-range order in amorphous silicon can be interpreted by a paracrystalline model [4]. In the paracrystalline model, small crystalline grains are interconnected with a disordered matrix under a large strain field, so that it is hard to identify the medium-range order in conventional diffraction experiments. In addition, annealing results in a dramatic effect on as-implanted structures. Less medium-range order was found in annealed states. This result is due to the increase of disorder during annealing which corresponds to heat evolution. An example of the heat evolution is given by calorimetric measurements [5,6]. Ion beam amorphization in most metals and semiconductors is attributed to point defect evolution. Since the time scale of ballistic phases is less than picoseconds, computer simulations such as molecular dynamic methods barely predict the kinetic behavior of point defects, and the detailed dynamics in the early stage of a displacement process is still unclear. O14.1.1/R9.1.1
Our primary question regarding the effect of ion irradiation in silicon is why a paracrystalline structure is generated during implantation. Instinctively, the origin of paracrystallinity i
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