The electrical properties of La 2 CuO 4 /ZnO heterocontacts at different relative humidities
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Andrea Bearzotti Istituto di Elettronica dello Stato Solido (IESS), C.N.R., Via Cineto Romano 42, 00156 Roma, Italy
Masaru Miyayama Research Center for Advanced Science and Technology (RCAST), University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, Japan
Hiroaki Yanagida Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received 19 December 1994; accepted 18 May 1995)
The humidity-sensing electrical properties of heterocontacts between p-type La 2 Cu0 4 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.
I. INTRODUCTION Commercial humidity sensors currently available are based on organic polymer films and sintered porous ceramics,1 which use an ionic-type humidity-sensitive mechanism, operable at low temperatures.2 Ceramics have shown advantages in terms of thermal and chemical stability, and of mechanical strength.3 However, each of these materials presents problems that limit their use. One of the problems that must be overcome for a widespread application of chemical sensors is the surface contamination by stably adsorbed molecules, which leads to the degeneration of their performance.4 For humidity sensors, the detection of humidity is dependent on water adsorption processes. Ionic-type porous ceramics react to humidity by decreasing their impedance due to water adsorption (chemical and physical) and/or capillary condensation within pores.5 This 2286
J. Mater. Res., Vol. 10, No. 9, Sep 1995
mechanism operates at low temperatures, because water must be liquid to physisorb on the oxide surface. Because of this mechanism, the surface resistivity of porous ceramics increases during prolonged contact with humid environments b
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