BYU Physics and Astronomy

Chemical Vapor Deposition (CVD) is a versatile and efficient technique for thin film deposition in the microelectronics, electro-optics, tool and protective coatings industries. This technology has been adapted by several groups to the preparation of optical thin films. We report on several CVD thin films stacks for photothermal solar energy conversion which combine promising spectral selectivity and durability at elevated operating temperatures.

Coatings of ZrB2 and TiB2 for photothermal solar absorber applications were prepared using chemical vapor deposition (CVD) techniques. Oxidation tests suggest a maximum temperature limit for air exposure of 600 K for TiB2 and 800 K for ZrB2. Both materials exhibit innate spectral selectivity with an emittance at 375 K ranging from 0.06 to 0.09, a solar absorptance for ZrB2 ranging from 0.67 to 0.77 and a solar absorptance for TiB2 ranging from 0.46 to 0.59 ZrB2 has better solar selectivity and more desirable oxidation behavior than TiB2. A 0.071 μm antireflection coating of Si3N4 deposited onto the ZrB2 coating leads to an increase in absorptance from 0.77 to 0.93, while the emittance remains unchanged.

Chemical vapor deposited (CVD) amorphous silicon alloyed with carbon or nitrogen (¿-Si:X, X=C or N) to retard high temperature crystallization is a promising absorber material for photothermal solar energy conversion. Films are prepared by decomposing silane containing gas mixures, a technique which is known to incorporate hydrogen into ¿-Si in some cases. Using the 16.45 MeV resonance of the 1H(19F,¿¿)16O reaction we made the first measurements of the hydrogen incorporation in CVD a-Si:X films (X=C,N). We have made three observations. First, the incorporation efficiency of hydrogen into CVD a-Si increases by a factor of twenty as the carbon content increases from 0 to 35 atomic percent which indicates that previous studies of multicomponent systems may need to be reevaluated since this enhancement in incorporation efficiency involves hydrogen--a key alloyant in a-Si. Second, the quantity of hydrogen incorporated increases at a greater than linear rate as a function of carbon content which implies that the presence of hydrogen in the films is not accidental but is a necessary part of film growth. Third, the hydrogen content of a-Si decreases to almost zero after high temperature anneal which may help explain reported shift in optical constants.