Defect states of amorphous Si probed by the diffusion and solubility of Cu

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Abstract
The diffusivity and solubility of Cu impurities have been measured in different structural states of amorphous Si (a-Si) formed by MeV Si implantation. The 2.2-μm-thick a-Si layers were first annealed (structurally relaxed) at 500 °C and then implanted with 200 keV Cu ions, returning a 300-nm-thick surface layer to the as-implanted state. After diffusion at temperatures in the range 150–270 °C, we observe solute partitioning at a sharp phase boundary between the annealed and Cu-implanted layers, the partition coefficient being as large as 8.2±1.3. The diffusion coefficient in annealed a-Si is 2–5 times larger than in as-implanted a-Si, with activation energies of 1.39±0.15 and 1.25±0.04 eV, respectively. The data show quite strikingly the role which defects can play in the a-Si structure.