A method is provided for forming a rare earth (RE) element-doped silicon (Si) oxide film with nanocrystalline (nc) Si particles. The method comprises: providing a first target of Si, embedded with a first rare earth element; providing a second target of Si; co-sputtering the first and second targets; forming a Si-rich Si oxide (SRSO) film on a substrate, doped with the first rare earth element; and, annealing the rare earth element-doped SRSO film. The first target is doped with a rare earth element such as erbium (Er), ytterbium (Yb), cerium (Ce), praseodymium (Pr), or terbium (Tb). The sputtering power is in the range of about 75 to 300 watts (W). Different sputtering powers are applied to the two targets. Also, deposition can be controlled by varying the effective areas of the two targets. For example, one of the targets can be partially covered.