The electrical properties of La2CuO4/ZnO heterocontacts at different relative humidities

The humidity-sensing electrical properties of heterocontacts between p-type La2CuO4 and n-type ZnO semiconductors, and of the single oxides, as a comparison, were studied. The heterocontacts was prepared by mechanically pressing sintered disks of the two oxides. The electrical characterization of the heterocontacts was carried out using dc and ac measurements at various relative humidity (RH) values, in order to evaluate the sensing mechanisms and the electrical properties of these p-n junctions. Their humidity sensitivity was explained in terms of the variation of the barrier height at the p-n junctions, due to the saturation of the original interface states by physisorbed water, which leads to the release of trapped electrons, resulting in an increase in the forward current. The higher the number of interface states, the higher the RH-sensitivity of the heterocontacts. Electrochemical impedance spectroscopy (EIS) measurements showed, at 90% RH, a distribution of capacitances with different relaxation times, which may be caused by the electrolysis of water molecules at p-n junction sites. For their use as humidity sensors, they showed a response of 4 orders of magnitude in the whole RH range tested, and a fast response time. The response of the heterocontacts was bias-dependent, tunable by externally applied electric field. They also have stand-by capability and a self-cleaning mechanism, which allow them to be described as intelligent materials.