Electrical properties of heavily doped polycrystalline silicon-germanium films

The electrical properties of polycrystalline silicon-germanium (poly-Si/sub 1/spl minus/x/Ge/sub x/) films with germanium mole fractions up to 0.56 doped by high-dose ion implantation are presented. The resistivity of heavily doped p-type (P/sup +/) poly-Si/sub 1/spl minus/x/Ge/sub x/ is much lower than that of comparably doped poly-Si, because higher levels of boron activation and higher hole mobilities are achieved in poly-Si/sub 1/spl minus/x/Ge/sub x/. The resistivity of heavily doped n-type (N/sup +/) poly-S/sub 1/spl minus/x/Ge/sub x/ is similar to that of comparably doped poly-Si for x<0.45; however, it is considerably higher for larger Ge mole fractions due to significant reductions in phosphorus activation. Lower temperatures (/spl sim/500/spl deg/C), as well as lower implant doses, are sufficient to achieve low resistivities in boron-implanted poly-Si/sub 1/spl minus/x/Ge/sub x/ films, compared to poly-Si films. The work function of P/sup +/ poly-Si/sub 1/spl minus/x/Ge/sub x/ decreases significantly (by up to /spl sim/0.4 Volts), whereas the work function of N/sup +/ poly-Si/sub 1/spl minus/x/Ge/sub x/ decreases only slightly, as Ge content is increased. Estimates of the energy bandgap of poly-Si/sub 1/spl minus/x/Ge/sub x/ show a reduction (relative to the bandgap of poly-Si) similar to that observed for unstrained single-crystalline Si/sub 1/spl minus/x/Ge/sub x/ for a 26% Ge film, and a reduction closer to that observed for strained single-crystalline Si/sub 1/spl minus/x/Ge/sub x/ for a 56% Ge film. The electrical properties of poly-Si/sub 1/spl minus/x/Ge/sub x/ make it a potentially favorable alternative to poly-Si for P/sup +/ gate-material applications in metal-oxide-semiconductor technologies and also for p-channel thin-film transistor applications.<
>