Semiconductor properties of passive films formed on sputter-deposited Fe–18Cr alloy thin films with various additive elements

The semiconductor properties of passive films formed on an Fe–18Cr alloy with various additive elements in a borate buffer solution were studied using electrochemical impedance spectroscopy and photoelectrochemical response. The photocurrent was plotted as a photoelectrochemical action spectrum that could be separated into two components, which were mainly derived from Cr oxide (Eg = 3.4 eV) and Cr hydroxide (Eg = 2.4 eV). The band gap energy, Eg; of each component was almost constant, independent of the species and amount of additive elements. The photoelectrochemical response showed positive photocurrent for most potential in the passive region, which indicates that the passive film behaved as an n-type semiconductor. In addition, the Mott–Schottky plot of the capacitance showed positive slope, which also means that the passive films behaved as an n-type semiconductor. The donor density of the passive films, which can be estimated by the Mott–Schottky plots, changes depending on the film formation potentials and a variety of additives.