EFFECT OF THE SENSOR STRUCTURE ON THE STABILITY OF GA(2)O(3) SENSORS FOR REDUCING GASES

Gas sensors based on semiconducting polycrystalline Ga2O3 thin films c an be used either for sensing oxygen (T greater-than-or-equal-to 900-d egrees-C) or reducing gases (T < 900-degrees-C), depending on the oper ating temperature. Whereas the stability of these sensors under purely oxidizing conditions has been proven, up to now there have been no sy stematic investigations on their material stability in the presence of reducing gases. The following study deals with sensor films on Al2O3 and BeO substrates; the sensors are equipped with interdigital electro des made of sputtered platinum or gold thin films. Tests were carried out at the operating conditions of the Ga2O3 hydrogen sensor (600-degr ees-C, 1% H-2 in synthetic air), with the gas atmosphere remaining the same but at elevated temperatures, and also under purely reducing con ditions (5% H-2 in N2). Ga2O3 sensor films on a BeO substrate are stab le under the operating conditions of hydrogen sensors, namely a temper ature of 600-degrees-C (H-2 in air), even under sustained exposure to hydrogen. No degradations have been found due to changes in the Ga2O3 film even at operating temperatures up to 900-degrees-C. Generally spe aking, films on Al2O3 substrate have poorer stability characteristics. In using gold electrodes, there is a limit with respect to thermal st ressability of gold contacts at about 800-degrees-C. When used in a pu rely reducing atmosphere, these Ga2O3 thin film sensors are stable so long as the oxygen-defect equilibrium of the Ga2O3 crystal lattice is frozen (T < 700-degrees-C). Upon starting bulk-defect diffusion, oxyge n depletion occurs in the lattice, which in the case of sensors on a B eO substrate leads to an increase in conductivity that is reversible b y subsequent oxidation, while in the case of sensors on Al2O3 it leads to the destruction of the film. At the usual operating temperatures t herefore, H-2 sensors can be operated in both oxidizing and reducing a tmospheres.