Work function of in situ heavily P- or B-doped poly-Si1-x-yGexCy films were investigated. The poly-Si1-x-yGexCy films were deposited on thermally oxidized Si(1 0 0) at 550 degreesC using an ultraclean hot-wall low-pressure chemical vapor deposition system in a SiH4-GeH4-SiH3CH3-dopnat (PH3 or B2H6) gas mixture, and the gate-to-substrate work function difference was estimated from flat-band voltage determined by high-frequency capacitance-voltage measurement. With increasing Ge and C fractions, the work function of B-doped poly-Si1-x-yGexCy film decreases, although that of P-doped poly-Si1-x-yGexCy scarcely changes. X-ray diffraction results show that the lattice constant increases with the C fraction, in other words, C is incorporated at interstitial site in the film at least at 550 degreesC. It is suggested that the change in the work function of doped poly-Si1-x-yGexCy is universally explained by the change in the lattice constant as function of the Ge and C fractions. (C) 2003 Elsevier Science B.V. All rights reserved.