In situ real-time spectroscopic ellipsometry (RTSE) provides detailed information on the evolution of the structural and optical properties of Si:H films during growth [Collins et al., Appl. Surf. Sci. 154-155, 217 (2000)]. We have used in situ RTSE to characterize the morphology and crystallinity of hot-wire chemical vapor deposition (HWCVD) Si:H films grown on native-oxide covered crystal silicon substrates as a function of substrate temperature T-s, hydrogen dilution R=[H-2/SiH4], and film thickness d(b). Transitions from one mode of film growth to another are correlated with abrupt changes in the magnitude of the surface roughness during film growth. The degree of crystallinity of the film can be determined from the shape of the dielectric spectrum. We have studied the growth parameter space consisting of R from 0 to 14, T-s of 250 and 500 degreesC, and d(b) from 0 to I mum. We have mapped out the crystallinity versus R, T-s and d(b) based on our analysis of the RTSE data. The transition between a-Si:H and muc-Si:H for growth on crystalline silicon substrates is near the R=3 to R=4 dividing line and is also a strong function of film thickness. Initial coalescence of purely muc-Si:H material does not occur until R greater than or equal to 10. These results have been corroborated using Raman scattering and atomic force microscopy to characterize the crystallinity and surface morphology of the films.