Nanoparticles of Si-alloys were grown at room temperature in a RF-PECVD process from mixtures of SiH4, CH4 and NH3. Quasi-monodisperse nanoparticles were produced in conditions of fast particle development. Among the different conditions for particle formation, we chose moderate low pressure (below 100Pa) and RF-power lower than 200W in all cases. The particle size was controlled through the modulation of the RF power supplied to the discharge. A post-thermal treatment at 800-900°C was used to eliminate the hydrogen of the particles and to stabilize them against environmental oxidation. To study the thermal crystallization of the particles, various thermal-treatments were used. The annealing of these particles at 1400°C for one hour under Ar or N2 led to different results depending on the composition of the samples as revealed by HRTEM and electron-diffraction analysis: SiC particles showed a marked internal crystallization; SiCN particles developed nanocrystals embedded in their amorphous matrix; Si particles crystallized in diamond-like silicon when annealed in Ar, whereas long whiskers of the α-Si3N4 phase, 50-200nm wide, grew from the particles in N2.