Composition Dependence of Electrical Resistivity of Magnesium-cobalt Films During Hydridation and Dehydridation
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Composition Dependence of Electrical Resistivity of Magnesium-cobalt Films During Hydridation and Dehydridation Yiu Bun Chan, and Chung Wo Ong Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China, People's Republic of ABSTRACT Palladium (Pd)-coated magnesium-cobalt (Mg-Co) films were prepared by co-sputtering at various ratios of the sputtering power of Mg to that of Co (PMg/PCo). The elemental composition of the films varies coherently with PMg/PCo. Films of higher Mg contents are of stronger interest. Their structures are more disordered even when just a relatively small amount of Co is added. The respective elemental contents are not uniform along depth, but more Mg aggregate near the surface region. A substantial volume fraction of Mg oxide is present. Though of structural complications, a Mg-rich film shows stronger and faster resistivity (ρ) response when reacting with hydrogen. Soaking an as-deposited Mg-rich film in 15% H2 (in argon) gives a huge change of ρ by 40 times, but it is mainly due to some irreversible structural change when a freshly prepared sample is first brought into contact with H2. The change of ρ in subsequent hydridation-dehydridation cycles is about 50%, while the drift of the baseline is less severe. Corich films give much weaker resistivity response to H2. INTRODUCTION In 1996, Huiberts et al. reported that yttrium and lanthanum films can react with hydrogen (H2) and be switched between a metallic state and a semiconducting oxide state [1], with substantial change in electrical resistivity (ρ). Such a phenomenon is considered to be useful in H-sensing technology. In a subsequent stage of study, many rare-earth were found to possess similar electrical switching properties [2-4]. A further improvement was introduced by adding magnesium (Mg) in these metals to enlarge the average optical band gap and enhance the electrical response. This idea was verified to be valid for many rare earth metals. To lower the material cost, a more recent development is to mix Mg with some transition metals, like nickel (Ni), iron (Fe) and cobalt (Co), instead of the rare earth metals. Ni-Mg system is most widely studied among the three. It is shown to give a strong resistivity switching effect with a typical response time around 30 s [5, 6] when reacting with hydrogen. However, Mg-Co system is less studied though it is of similar scientific and practical interests [7-11]. In this study, we performed detailed investigations on the structure and elemental composition of Mg-Co films against the relative contents of the two metal constituents. In particular, the depth profiles of elemental composition and chemical states of the two metals were analyzed, which are correlated with the observed changes of electrical resistivity (ρ) of the films when reacting with hydrogen.
EXPERIMENT MgCo films were deposited by co-sputtering of Mg and Co (99.95%). The sputtering chamber was first pumped down to a background pressure 10
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