Ion Implantation and Synthesis of Materials - Studium

136 10 Crystallization and Regrowth of Amorphous Si1.41.2E cT meltFree Energy (eV)1.00.80.60.40.2E(V -2 )E(V - )xE(V )E(V+)E vE v0 200 400 600 800 1000 1200 1400 1600 1800T (K)Fig. 10.8. Variations of the donor and acceptor ionization levels of the single vacancy in Sias a function of temperature (from Van Vechten 1980)−x EFEx EC0.37 EV−⎛ − ⎞ ⎛ − − ⎞[ V ] = [ V ]exp ⎜⎟= [ V ]exp⎜ ⎟⎝ kT ⎠⎝ kT ⎠x ⎛ EG− 0.37 ⎞ x ⎛0.54⎞= [ V ]exp ⎜ ⎟ = [ V ]exp⎜ 0.07⎟⎝ kT ⎠⎝ ⎠x= 2022[ V ]From the above example, [V − ] is about 2,000 times larger than [V x ] at 823 K,which explains in part the enhanced regrowth that amorphous Si experiences whenit is doped with P. As one would anticipate, the growth rate is not increased byimplantation of both B and P at equal concentrations. In this case, the implantedlayer has equal concentrations of donors and acceptors, and the Fermi level is nearthe center of the energy gap.Implantation of oxygen ions tends to decrease the growth rate. If a sufficientlyhigh concentration of oxygen is implanted, the growth rate will be slowed enoughso that the remaining amorphous material recrystallizes in the form of apolycrystalline layer. The electrical characteristics of this polycrystalline layer aremarkedly inferior to those of the epitaxial regrown layer.