Copper–nickel oxide thin film library reactively co-sputtered from a metallic sectioned cathode

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Cu–Ni sectioned cathode made up of two hemicycles of each of the metals was used for reactive co-sputtering of a thin film combinatorial library of Cu–Ni oxides covering a total compositional spread of 63 at.%. The thickness profiling of the library showed a nonuniform film thickness with a maximum region shifted toward the Cu side of the cathode. The presence of CuO, Cu2O, NiO, and metallic Cu–Ni alloys was identified during the scanning x-ray diffraction investigations along the compositional spread. A distinct structural zone was defined between Cu–14 at.% Ni and Cu–19 at.% Ni, where the scanning electron microscopy investigations showed a higher surface porosity combined with smaller grain sizes. This zone corresponds to the maximum film thickness region and correlates well with the position of the maximum work function of the Cu–Ni oxide films as mapped using a scanning Kelvin probe. During local corrosion studies focused on Cu dissolution, an improved corrosion resistance was identified in the Ni rich side of the compositional spread. I. INTRODUCTION

Copper and nickel as well as their oxides have a natural affinity to each other. This is seen not only in the joint abundance of the two elements in ores but also in a number of well established technical applications of their alloys. In 1906, a patent was granted to Monell1 for the invention of a copper-nickel alloy with extreme corrosion resistance. This alloy is produced still today, interestingly, in the direct course from the copper and nickel containing ores rather than from pure metals. It is an important alloy that can withstand even fluor, fluoride, and hydrofluoric acid containing environments. A second, also very important application is the production of Konstantan (constantan). This is a trade name of the German company ThyssenKrupp VDM GmbH that consists of 55% Cu, 44% Ni, and 1% Mn, showing a relatively high electrical resistance that is almost temperature independent.2 While most of the metal oxides are insulators or n-type semiconductors, CuO and NiO are p-type semiconductors.3 Although the band gap of NiO (3.5 eV) is almost twice as large as that of CuO (1.7 eV), its absolute position is the same. This makes combinations of their oxides highly interesting for photochemical applications.4,5 Thin film oxide preparation using reactive sputtering represents an important method used in the current developments of many low- and large-scale applications. The advantage of obtaining an oxide film using a metallic target was already exploited more than 10 years ago in some of a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.336 148

J. Mater. Res., Vol. 29, No. 1, Jan 14, 2014

the first studies of the reactive formation of NiO by DC sputtering.6 At that time, it was clearly proven that using a reactive route via an O plasma, a cubic NiO thin film is formed, which has tuneable electrical resistivity as a function of the O content.7 Later on, the relationship between the preferential orientation of the NiO films and thei