Optical properties and chemical reactivity of hydrogenated amorphous boron thin films

Hydrogenated boron thin films have been deposited at temperatures in the range 50–100 C with the radiofrequency plasma decomposition of diborane B2H6 diluted in hydrogen. The chemical composition of the films has been determined by elastic recoil detection and by the Castaing microprobe techniques. We have found that the as-deposited films contain 10–20 at % H and that they react with the ambient atmosphere within a few days and reach a final composition close to B0.64H0.12O0.12C0.06N0.06. The optical properties of the as-deposited films studied by spectroscopic phase-modulated ellipsometry in the range 1.7–5.0 eV are characteristic of the high optical gap semiconductors. The electron diffraction measurements performed onin situ annealed samples show that the films are amorphous up to 950 C. Infrared spectroscopy investigations performed on the as-deposited films have revealed two hydrogen bonding sites: B-H terminal bonds and B-H-B bridge bonds, along with some B-O-B groups. Upon exposure to the atmosphere, the vanishing of B-H-B and B-O-B bridge bonds and the increase of B-H and B-O-H absorption bands after 1 day, are observed.