<title>Selective Transmission Of Thin TiN-Films</title>

Abstract

Selective transmission of thin TiN -filmsE. Valkonen, T. KarlssonDept of Solid State Physics, Institute of Technology, Uppsala University, SwedenB. KarlssonThe Alvkarleby Laboratory, Swedish State Power Board, SwedenandB.O. JohanssonDept of Physics and Measurement Technology, University of Linkoping, SwedenAbstractThe solar selective properties of thin titanium nitride films have been studied. Highquality TiN -films were made on fused silica and glass substrates by reactive sputtering oftitanium in a nitrogen atmosphere. The resulting film thicknesses ranged from 5- 120 nm.The reflectance and transmittance measurements confirm the high solar and visible trans-mission previously calculated from optical constants. The infrared reflection is lower thancalculated, but sufficiently high to make TiN a new competitor for selective transmissionapplications. Three optical parameters were determined and used to obtain the opticalconstants as well as the film thickness. The refractive index increases with decreasingthickness. The extinction coefficient is almost constant above the thickness of 12 nm, butstrongly reduced for films thinner than this.IntroductionThe optical properties of the carbides and nitrides of the fourth and fifth columntransition metals have received much attention during the last few years. The reason forthis interest is the suggestion that within this group of materials a suitable single phaseselective absorber might be foundl. Particularily for concentrating solar absorbers, thehigh melting points and the absence of inter -phase diffusion constitute interesting advant-ages with these compounds.When the reflectance curves for bulk samples of carbides and nitrides of the transitionmetals mentioned above are compared, a striking difference is observed2. The nitrides havea free electron -like behaviour with a well defined relaxation region. The comparison tometals can be carried further. Optically TiN is similar to gold in the visible and is some-times used as gold imitation in applications where mechanical strength is important. HfNand ZrN, on the other hand, ressembles silver. The corresponding carbides all have reflect-ance spectra like the transition metals and they show no extended relaxation region. Thisstriking difference between the two otherwise closely related groups of materials, has beenexplained on the basis of partial densities of states and atomic selection rules3.Attempts have been made to tailor the optical properties of bulk TiN by introducingcarbon in order to realize a single phase selective solar absorber. The results have beenessentially negative4.5 in the sense that the a- values obtained were modest. The introduc-tion of carbon in TiN shifts the reflectance edge to lower energy but smears it as well.The close similarity between the optical properties of the nitrides and the noble metalssuggests, however, that thin films of TiN, ZrN and HfN on glass should exhibit selectivethan4mti4Ztion for the same reasons as thin silver and gold films, i.e. a low n -value and lowinternal absorptance over the visible region. This suggestion was tested in a calculationwith the optical constants determined for opaque layers b. The results indicate that thesefilms offer a viable alternative to the conventional noble metal and metal -oxide films whichare well -known heat -mirror materials7.The remaining crucial question is, however, whether nitride films with these propertiescan actually be prepared. It is well -known that the potential selectivity of thin noblemetal films is strongly reduced by island formation for film thicknesses around 10 nm 8.The adhesive energy of these nitride compounds to glass is much higher than that of thenoble metals, so island formation is less likely. The formation of films, 10 nm and less,is, however, not a process which takes place in thermodynamic equilibrium, which makes theverification of the calculations an urgent task.