ELECTRICAL-PROPERTIES OF HEAVILY-DOPED POLYCRYSTALLINE SILICON-GERMANIUM FILMS

The electrical properties of polycrystalline silicon-germanium (poly-Si1-xGex) 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-Si1-xGex is much lower than that of comparably doped poly-Si, because higher levels of boron activation and higher bole mobilities are achieved in poly-Si1-xGex. The resistivity of heavily doped n-type (N+) poly-Si1-xGex 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 (approximately 500-degrees-C), as well as lower implant doses, are sufficient to achieve low resistivities in boron-implanted poly-Si1-xGex films, compared to poly-Si films. The work function of P+ poly-Si1-xGex decreases significantly (by up to approximately 0.4 Volts), whereas the work function of N+ poly-Si1-xGex decreases only slightly, as Ge content is increased. Estimates of the energy bandgap of poly-Si1-xGex show a reduction (relative to the bandgap of poly-Si) similar to that observed for unstrained single-crystalline Si1-xGex for a 26% Ge film, and a reduction closer to that observed for strained single-crystalline Si1-xGex for a 56% Ge film. The electrical properties of poly-Si1-xGex 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.