Optical and thermal properties of amorphous hydrogenated carbon thin films

Optical and thermal properties of amorphous hydrogenated carbon (a-C:H) films deposited by plasma enhanced chemical vapor deposition (PECVD) and magnetron sputtering are reported. The deposition parameters were varied to produce a-C:H films with effective optical bandgaps from 0.3 eV to 2.3 eV. The optical properties of the a-C:H films deposited on glass substrates were determined from analysis of variable angle of incidence spectroscopic ellipsometry (VASE$\sp\circler)$ and transmittance data. An empirical parameterization for the a-C:H optical constants as a function of effective bandgap was developed based on the measured spectra, and this single parameter model was used to determine the optical constants and thickness of films deposited on silicon substrates. Forouhi and Bloomer optical parameters calculated from the measured optical constant spectra are tabulated. This model was modified to allow non-parabolic bands, and the resulting model was fit to the measured spectra to determine the energy dependence of the joint density of states function for each film. The thermal conductivities of fourteen a-C:H films on 5 mm thick silicon substrates were measured by photothermal deflection (PTD) experiments. The measurements were performed using mechanical chopping or photoacoustic modulation of the heating beam, with the sample in air or immersed in de-ionized water. An exact model for prediction of temperature distributions and PTD signals for multilayers heated by axially symmetric sources is presented, and FORTRAN programs to perform nonlinear multiple regression fits and simulated annealing fits to experimental PTD data using this model are described. Thermal conductivities of a-C:H films with bandgaps ranging from 1.2 to 2.3 eV were determined, and an exponential decrease of thermal conductivity with increasing bandgap is observed. The thermal conductivity for a given bandgap was also seen to decrease with decreasing film thickness