Thin sputter deposited gold ﬁlms on In 2 O 3 :Sn, SnO 2 :In, TiO 2 and glass: Optical, electrical and structural effects P.C. Lansåker a,n , P. Petersson b , G.A. Niklasson a , C.G. Granqvist a a Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden b Tandem Laboratory, Uppsala University, P.O. Box 533, SE-751 21 Uppsala, Sweden article info Article history: Received 13 February 2013 Received in revised form 26 June 2013 Accepted 27 June 2013 Keywords: Gold ﬁlm Sputter deposition Optical properties Electrical properties Nanostructure Transparent conductor abstract Thin gold ﬁlms are promising transparent conductors with many actual and potential uses in “green” technologies, transparent electronics, etc. These applications require different substrate materials, and hence it is important to understand the role of the substrate on Au thin ﬁlm growth. Such effects have been studied in this work wherein Au ﬁlms—ranging from island structures, via large scale coalescence into meandering metal networks, to thin homogenous layers—were deposited by DC magnetron sputtering onto glass substrates and In 2 O 3 :Sn (ITO), SnO 2 :In and TiO 2 base layers backed by glass. Optical, electrical and structural properties were recorded for ﬁlms deposited onto unheated substrates. We found distinct and characteristic differences in Au growth on the various backings. Thus ITO and SnO 2 :In base layers yielded gold ﬁlms with island features remaining to larger thicknesses than for deposition directly onto glass, and the sheet resistance was lower for gold deposition onto SnO 2 :In and ITO only when the gold ﬁlms were less than 5 nm in thickness. Our results highlight the complexity of substrates' inﬂuence on thin ﬁlm formation. & 2013 Published by Elsevier B.V. 1. Introduction Materials combining optical transmittance with electrical con- ductivity are essential for many current and forthcoming technol- ogies, such as for photovoltaics, light emitting devices and electrochromic smart windows [1,2]; other applications deal with transparent electronics and display devices [3–5], etc. The trans- parent conductors (TCs) can be of several different kinds and include thin metal ﬁlms, heavily doped wide band gap oxide semiconductors, nanowire composites based on carbon nanotubes or metals, graphene, and certain organics; overviews over this rapidly evolving ﬁeld—in its full extent or partly—can be found in recent literature [1,6,7]. Oxide-based TCs have been investigated for decades with strong emphasis on materials such as In 2 O 3 :Sn (ITO), SnO 2 :F, ZnO:Al (AZO), ZnO:Ga (GZO) and ZnO:In (IZO) [8,9]. They are able to combine an electrical resistivity of  10 –4 Ωcm with a luminous absorption no larger than a few percent in  300 nm-thick ﬁlms. The properties of ITO, AZO and GZO, in particular, are excellent with regard to transparent electronics and other applications that are driven by appeal more than cost, but the required deposition times make the oxide-based TCs unattractive for large-area, low- cost applications such as those in energy-related technology. Furthermore, the most widely used ITO ﬁlms are plagued by the high price of In (not withstanding its relative abundance [10]) and, possibly, its newly discovered health issues [11–13]. Metal-based TCs are free from several of the limitations of the mentioned oxides, and ﬁlms of, especially, the coinage metals Cu, Ag and Au can give the same electrical conductivity as the oxides but at a thickness of  10 nm, implying that the deposition time and cost can be a tiny fraction of those for making an oxide ﬁlm with comparable properties. The metals show some optical reﬂec- tance, though, so that high luminous transmittance hinges on efﬁcient antireﬂection treatment, and in practice the metal ﬁlms must be embedded between high-refractive-index layers. Com- monly used antireﬂection coatings include ITO, AZO, GZO, IZO and their undoped base oxides, which seemingly brings back the problems with long deposition times mentioned above. This is not necessarily the case, though, and recent advances [14] in high- rate deposition of plasma polymerized organic coatings can relieve the metal-based TCs of the problems of oxide-based antireﬂection and increase their viability with regard to energy related applications. The discussion on TCs and “green” technologies points at the importance of understanding the growth of coinage metal ﬁlms on different types of substrates. This is a subject with a venerable history [15] and the scientiﬁc literature is huge and impenetrable in its entirety; key aspects are covered in numerous books [16–22]. From a more practical and applications-oriented point of view, there have been a very large number of studies on TCs with Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.solmat.2013.06.051 n Corresponding author. Tel.: +46 184177783. E-mail address: pia.lansaker@angstrom.uu.se (P.C. Lansåker). Solar Energy Materials & Solar Cells 117 (2013) 462–470