Electrical properties of heavily doped polycrystalline silicon-germanium films

The electrical properties of polycrystalline silicon-germanium (poly-Si_1-xGe_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⁺) poly-Si_1-x Ge_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_1-xGe_x. The resistivity of heavily doped n-type (N⁺) poly-S_1-xGe_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 (~500°C), as well as lower implant doses, are sufficient to achieve low resistivities in boron-implanted poly-Si_1-xGe_x films, compared to poly-Si films. The work function of P⁺ poly-Si_1-xGe_x decreases significantly (by up to ~0.4 Volts), whereas the work function of N⁺ poly-Si_1-xGe_x decreases only slightly, as Ge content is increased. Estimates of the energy bandgap of poly-Si_1-xGe_x show a reduction (relative to the bandgap of poly-Si) similar to that observed for unstrained single-crystalline Si_1-xGe_x for a 26% Ge film, and a reduction closer to that observed for strained single-crystalline Si _1-xGe_x for a 56% Ge film. The electrical properties of poly-Si_1-xGe_x make it a potentially favorable alternative to poly-Si for P⁺ gate-material applications in metal-oxide-semiconductor technologies and also for p-channel thin-film transistor applications