Obliquely deposited tantalum germanium multilayers: electron transport and optical properties

Abstract

Thin film superlattices of amorphous Tantalum/Germanium alloys separated by pure Germanium layers and monolayers of TaGe have been fabricated by oblique-angle thermal evaporation. The evaporation angle between the source and the substrate normal varied for different superlattice samples between 61° and 77°. Scanning-Electron Microscopy investigations confirmed that this technique yields a microstructure of columns inclined towards the evaporation source. This structure can be understood on the basis of a geometrical shadowing effect. Optical transmittance measurements in the deposition plane were performed over the spectral range from 400nm to 1200nm, different polarisations and for both, superlattice and monolayer alloy samples. P-polarised light (E-vector in the incident plane) with incident angles symmetrical to the film normal result in a transmittance of up to 50 % higher along the column orientation than across it. This angular selective transmittance was not observed for s-polarised light. The reflectivity was also found to be symmetrical to the film normal. A qualitative explanation for this behaviour is given in form of altered thin-film equations. Resistance measurements have been performed on superlattice samples produced under different evaporation angles and deposition times. An anisotropy in the film plane confirms column coalescence in the plane perpendicular to the deposition plane. The resistivity increases with thinner TaGe layers as a consequence of shorter deposition time for the conducting layer. A strong increase in the resistivity with increasing evaporation angle is found. This resistance has to be explained in terms of a weak localisation theory. Lower dimensional behaviour of the electron paths and an inhomogeneous morphology for the films prepared with the highest angles are believed to be responsible for the sensitivity of the system to the evaporation angle.