Hydrogenated amorphous carbon (a-C:H) films were deposited in a plasma-enhanced chemical vapor deposition (PECVD) system. The substrate temperature at deposition was found to have significant effects on the film stoichiometry, sp(2) phase, and optical properties. Raman spectroscopy reveals an increase in sp(2)-bonded carbon and a continual structure ordering of the sp(2) phase with increasing substrate temperature at deposition. Thermal desorption spectroscopy analysis revealed that the onset temperature of CH4 effusion of PECVD a-C: H films increase with increasing substrate temperatures, implicating enhanced structural stability via elevating the substrate temperature at deposition. The extinction coefficient k measured from spectroscopic ellipsometry gradually increases with increasing substrate temperature at deposition, due possibly to the graphitization effect which decreases the optical gap resulting in higher k. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3497025]