The effects of P doping on photoluminescence (PL) properties of Si1-xGex alloy nanocrystals (nc-Si1-xGex) in SiO2 thin films were studied. P doping drastically decreases the electron spin resonance (ESR) signals that are assigned to the Si and Ge dangling bonds at the interfaces between nc-Si1-xGex and SiO2 matrices (Si and Ge Pb centers). With increasing P concentration, the signal from the Ge Pb centers are first quenched, and then the signal from the Si Pb centers start to be quenched. The quenching of the ESR signals is accompanied by a drastic enhancement of the PL intensity. The PL intensity has a maximum at a certain P concentration, which depends on the Si:Ge ratio. By further increasing the P concentration, the PL intensity becomes weaker. In this P concentration range, optical absorption emerges due to the intravalley transition of free electrons generated by the P doping. The observation of the free-electron absorption provides direct evidence that carriers in nanometer-sized Si1-xGex alloy crystals can be controlled by impurity doping. © 2001 American Institute of Physics.