Metal-Semiconductor Transition on the Surface and in the Bulk of Europium Hydride Thin Film
M. Knor, R. Nowakowski and R. Duś
Institute of Physical Chemistry, Polish Academy of Sciences, M. Kasprzaka 44/52, 01-224 Warszawa, Poland
Received: April 17, 2012; in final form August 30, 2012
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Thin europium films (20-50 nm thick) on a glass substrate were transformed into EuHx (0 < x < 2) by interaction with H2 introduced into the reactor in successive calibrated doses. By measuring the pressure, the hydrogen uptake (H/Eu) was determined at every step of the reaction. In situ monitoring of bulk properties (electrical resistance R(H/Eu), relative transparency to light T(H/Eu)/-T0 and (H/Eu) dependent light transparency spectrum) confirms metal-semiconductor transition at room temperature. Both the electrical resistance and optical transparency of the film strongly increase with hydrogen concentration as a consequence of the resulting increase of the content of semiconducting dihydride. Moreover, the course of work function changes ΔΦ(H/Eu) indicates inversion: of the charge-transfer direction on the surface. The transition at room temperature from positively to negatively polarized hydrogen adsorbate was observed in situ during hydrogen uptake. As a result, the work function at equilibrium state varies with hydrogen content from +18 to -18 mV with respect to pure metal film, reflecting the change of "mirror potential" generated on the surface due to the accumulation of hydrogen adsorbates in the subsurface region.
DOI: 10.12693/APhysPolA.122.698
PACS numbers: 67.63.Gh, 73.61.-r, 78.66.-w, 73.20.-r