In situ spectroscopic ellipsometry was used to analyze hafnium diboride thin films deposited by chemical vapor deposition from the single-source precursor Hf(BH4)4. By modeling the film optical constants with a Drude-Lorentz model, the film thickness, surface roughness, and electrical resistivity were measured in situ. The calculated resistivity for amorphous films deposited at low temperature ranged from 340 to 760 μΩ cm. These values are within 25% of those measured ex situ with a four-point probe, indicating the validity of the optical model. By modeling the real-time data in terms of film thickness and surface roughness, the film nucleation and growth morphology were determined as a function of substrate type, substrate temperature, and precursor pressure. The data show that at low precursor pressures (∼10-6 Torr) and at low substrate temperatures (≪300 °C), the onset of growth is delayed on both Si and SiO2 surfaces due to the difficulty of nucleation. A higher substrate temperature or precursor pressure reduces this delay. At low temperatures the film morphology is a sensitive function of the precursor pressure because site-blocking effects change the reaction probability; the authors show that the morphology of newly grown film can be reversibly transformed from dense smooth to rough columnar by decreasing the precursor pressure.