Silicon-rich silicon nitride (SiNx) thin films are deposited at 220 ℃ on n-type monocrystalline silicon substrates by plasma enhanced chemical vapor deposition using NH3 and SiH4 as the reaction gases. The samples are annealed at temperature in a range of 500-1100 ℃ in N2 atmosphere. We analyze the crystalline states of silicon quantum dots (Si-QDs) and calculate the crystalline ratios of samples under different annealing conditions according to the Raman spectra. The crystalline ratio is less than 18% when the annealing temperature is lower than 950 ℃, when the temperature reaches 1100 ℃, the crystalline ratio is increased to 53%, which indicates that most of the Si-QDs have been converted into crystallines. Fourier transform infrared spectra are measured at room temperature to investigate the evolutions of the bonding structures within the SiN_x matrix. We find that the wavelengths of Si-N and Si-H bond shift toward higher wavelength, which manifests the formation of near stoichiometric silicon nitride. Photoluminescence generated from all samples is investigated in detail. We find five luminescence peaks, whose origins are analyzed. We conclude that the obvious green luminescence (centred at 500-550 nm) oringinates from Si-QDs and the others come from different defects in the films. The effects of sizes and distribution of Si-QDs on the shift of the luminescence peak are discussed. We acquire that the sizes of Si-QDs are in a rang from 1.6 nm to 3 nm, which have an obvious confinement effect. These results are useful for fabricating contronllable Si-QDs and achievement of luminescent devices based on Si-QDs.