Study of the conduction mechanism of La₂CuO₄-ZnO heterocontacts at different relative humidities

Heterocontacts between p-type La₂CuO₄ and n-type ZnO semiconductors have been prepared by mechanically pressing sintered discs of the two oxides. Their humidity-sensing electrical properties are evaluated using d.c. and a.c. measurements at various relative humidity (rh) values. The current-voltage (I-V) curves, measured in dry and wet N₂ or O₂, are typical of p-n diodes at all rh values. The current versus rh sensitivity of the heterocontacts is bias dependent. The influence of the original adsorbates on the rh sensitivity is proved. Higher rh sensitivities of the heterocontacts are measured in oxygen, where a larger number of interface states is present. Electrochemical impedance spectroscopy (EIS) measurements, carried out in dry and wet air, show that the higher the rh, the higher the interface capacitance and the lower the interface resistance. EIS spectra are dependent on the d.c. bias applied, which mainly affects the resistence. At 90% rh, a distribution of capacitances with different relaxation time is found, which may be caused by the electrolysis of water molecules at p-n junction sites.