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Recently, our department had the project with the small company OUT e.V. (http://www.out-ev.de/) and the glass industry leader in Germany – Interpane Glas Industrie AG ( http://www.interpane.de/) which was aimed to develop the robust Indium-free low emissivity (low-E) coatings for windows. Such coating should be chemically stable, transparent and conductive enough (TCO – Transparent Conductive Coating). When benchmarking the TCO materials (mostly oxides) great attention is paid to the mechanism of charge carriers scattering. The scattering of free electrons providing high conductivity often occurs on grain boundaries and, hence, highly crystalline materials are preferable. This, however, is usually provided by high temperatures incompatible with the industrial stages of glass manufacturing.

Our Hollow Cathode Gas Flow Sputtering (GFS) system thanks to the unique plasma conditions, allows depositing highly crystalline materials at quite low temperatures. Thus, we obtained crystalline TiO2 anatase films on glass substrates without any intentional heating. This is an outstanding result for this material.

In the low-E project noticed above, the Ta doped SnO2 films were deposited onto the single crystalline TiO2 rutile substrate with (110)-orientation (for Miller indices look at: https://web.iit.edu/sites/web/files/departments/academic-affairs/academic-resource-center/pdfs/Miller_Indices.pdf ). Pulsed deposition mode with the following core parameters was used: +100 V reverse voltage, 100 KHz frequency, 250 W power and 400°C substrate temperature.

We expected the epitaxial relation (https://en.wikipedia.org/wiki/Epitaxy) between rutile (lattice constants a = b = 4.5937 Å) and SnO2 (a = b = 4,737 Å) with the mismatch of about +3%. This small difference might however also lead to the residual compressive stresses in SnO2 film (for details see the link: https://study.com/academy/lesson/compressive-stress-definition-formula-maximum.html).

To confirm the epitaxial relation between substrate and film, XRD pole-figures are usually being acquired. Figure below represents the results of such measurements at the fixed 2q and variable j (0-360°) and c-(25-60°) angles. The scheme (c) expresses the insight of the pole-figures (a & b), confirming the epitaxial (110)-SnO2 growth onto the (110)-rutile.


Figure. Pole-figures for: a) (111)-diffraction maximum of the TiO2-rutile phase (substrate); b) (200)-diffraction maximum of the SnO2 phase (film); c) schematic imagination of the epitaxial relation between SnO2 and R-TiO2 substrate.

Thus, the hollow cathode gas flow sputtering was approved to be a feasible method for epitaxial growth of oxides. In case of SnO2 the minimal necessary substrate temperature was found to be 400°C. Crucial importance has also the pulse operation with +100 V reverse voltage.

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