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Several Cr‐doped, low carrier density (10–20 Ω cm), (111)‐oriented wafers of Si were ion implanted at room temperature and ∼90 K with various doses of 12C, 29Si, and 120Sn. The ion energy was 380 keV except for 12C at 90 K where 150 keV was used. Infrared reflection as a function of frequency and cross‐sectional transmission electron microscopy measurements were made for both as‐implanted and thermally annealed (400 °C for 2 h) samples. The results of these measurements demonstrate the following: (i) The previously reported pair of metastable amorphous states are observed, a‐Si‐I for high‐dose as‐implanted material and a‐Si‐II for anneal‐stabilized material; (ii) interface positions and microstructural properties show good consistency between the two types of measurements; (iii) the measurements are consistent with the view that the implanted material can be a heterogeneous mixture of undamaged, damaged, and amorphous regions. By using an effective medium approximation and a damage cascade overlap model one concludes that no overlap is required for 120Sn to create amorphous zones, while a large number is necessary for a light ion, 14 for 12C implantations at room temperature; (iv) from several different approximation methods, average values for the critical amorphization energy are obtained, i.e., in units of 1021 keV/cm3, 1.4 for 120Sn, 2.0 for 29Si, and 13.0 for 12C (all for room‐temperature implantations) and 0.5 for all ions for 90 K implantations; (v) the measurements show that the annealing‐induced recrystallization behavior of incomplete or mixed amorpho- us layers is very different from that for complete or homogeneous layers which crystallized by planar epitaxial regrowth; (vi) samples were cycled between the a‐Si‐I and a‐Si‐II states and it was observed that the energy required for a‐Si‐II→a‐Si‐I is about an order of magnitude smaller than the critical amorphization energy (c‐Si→a‐Si‐I) for 29Si implantations at room temperature.