OPTICAL PROPERTIES OF CHEMICAL VAPOR DEPOSITED MOLYBDENUM THIN FILMS

The reflectance of thin metal films is often less than the reflectance of highly pure, polished samples of the bulk metal. Structural and compositional modifications from the bulk which are observed within thin films are generally blamed for this discrepancy. Few studies, however, have determined which modifications are responsible for the anomalous reflectance of metallic films. This study has investigated reasons for differences between the infrared reflectance of chemical vapor deposited (CVD) molybdenum (Mo) thin films and polished, bulk Mo. Molybdenum was chosen for this investigation because substantial differences of up to 22% have been observed between the infrared reflectance of thin film and bulk Mo. In addition, an understanding of the anomalous reflectance of thin film Mo will lead to the capability of reproducibly depositing Mo thin films with bulk-like reflectance. This will benefit applications of thin film Mo which depend on both the thermal stability and high infrared reflectance of Mo.; Impurities, grain boundaries, and anomalous crystal structure were identified as those modifications from the bulk which could potentially reduce the infrared reflectance of thin film Mo by several per cent. The composition, grain size, and crystal structure of all films were therefore determined by Auger spectroscopy, electron microscopy, and X-ray diffraction, respectively. The results of these measurements were related to the infrared reflectance of the films.; An analysis involving three approaches was used to separate the influence on reflectance of each modification from the bulk. The first approach used the versatility of CVD to deposit films with similar composition and crystal structure, but different grain size. Since these films exhibited similar infrared reflectance, grain boundary scattering in CVD Mo was shown to have little influence on the infrared optical properties. The second approach used the Drude model to determine the dielectric function of a CVD Mo film containing nearly 20 atomic per cent carbon and 20 per cent oxygen as a function of the Drude relaxation time ((tau)) and carrier concentration (n). The (tau) of this film was roughly equal to the (tau) of the bulk, while the n of the film was equivalent to only 2% of the n of the bulk. The low carrier concentration in the film can be explained by the removal of carriers from the metallic bands and their localization in Mo-C and Mo-O bonds. Bonds between Mo and oxygen were found to be more detrimental to the infrared reflectance than Mo-C bonds. This result was derived by depositing several films with similar grain size and crystal structure, but with various compositions. A third approach revealed the influence of interband transitions on the infrared reflectance of CVD Mo by comparing the experimental reflectance to the reflectance predicted by the Drude model in the infrared. The occurrence of interband transitions, which is dependent on crystal structure, had a larger effect on the infrared reflectance of the CVD film with a depletion of carriers than in the bulk. Finally, some of this analysis has been extended to physical vapor deposited Mo films.; During the course of this investigation, the most reflective Mo surface yet reported in the literature was produced by exposing a CVD Mo sample to a post-deposition anneal. The high reflectance and high temperature stability of this film allow for applications of CVD Mo in both photothermal solar energy conversion and high power laser technology. The versatility of CVD in adjusting the composition and structure of Mo films resulted in another film of interest in photo-thermal conversion--black Mo. These uses of CVD Mo establish the feasibility of using CVD to expand applications of thin film optical devices at high temperatures.