Maximilian Dr. Fleischer

H2-induced changes in electrical conductance of β-Ga2O3 thin-film systems

H2-induced changes of electrical conductivity in polycrystalline, undoped β-Ga2O3 thin films in the temperature range of 400–650° C are described. The sheet conductance of these films depends reversibly, according to a power law σ□ ∼ p1/3, on the partial pressure of hydrogen in the ambient atmosphere of the Ga2O3 film. A bulk vacancy mechanism is excluded by experiments and it is shown that the interaction is based on a surface effect. Changes in conductance are discussed to result from the formation of an accumulation layer due to chemisorption on the grain surfaces. Typical coverages are determined to be approximately 10−4 ML for pH2=0.05 bar and T=600° C. A possible explanation of the σ□ ∼ p1/3 power law is provided.

Effect of the sensor structure on the stability of Ga2O3 sensors for reducing gases

Abstract Gas sensors based on semiconducting polycrystalline Ga 2 O 3 thin films can be used either for sensing oxygen ( T ⩾900 °C) or reducing gases ( T 2 O 3 and BeO substrates; the sensors are equipped with interdigital electrodes made of sputtered platinum or gold thin films. Tests were carried out at the operating conditions of the Ga 2 O 3 hydrogen sensor (600 °C, 1% H 2 in synthetic air), with the gas atmosphere remaining the same but at elevated temperatures, and also under purely reducing conditions (5% H 2 in N 2 ). Ga 2 O 3 sensor films on a BeO substrate are stable under the operating conditions of hydrogen sensors, namely a temperature of 600 °C (H 2 in air), even under sustained exposure to hydrogen. No degradations have been found due to changes in the Ga 2 O 3 film even at operating temperatures up to 900 °C. Generally speaking, films on Al 2 O 3 substrate have poorer stability characteristics. In using gold electrodes, there is a limit with respect to thermal stressability of gold contacts at about 800 °C. When used in a purely reducing atmosphere, these Ga 2 O 3 thin film sensors are stable so long as the oxygen-defect equilibrium of the Ga 2 O 3 crystal lattice is frozen ( T 2 O 3 it leads to the destruction of the film. At the usual operating temperatures therefore, H 2 sensors can be operated in both oxidizing and reducing atmospheres.

Effect of coadsorption of reducing gases on the conductivity of β-Ga2O3 thin films in the presence of O2

Abstract The electrical conductivity of sputtered 2 μm thick films of the oxide semiconductor α-Ga2O3 was studied in function of the partial pressure of CO or H2 above the solid and also in the presence of CO+O2, H2+O2 and CO+H2 mixtures. The concentrations of the examined gases varied in the range 0.01–1 vol.% in nitrogen carrier gas, while the experiments were carried out at 823, 873 and 923 K. Under the influence of H2 and CO the conductivity of the samples increased with the partial pressures of the gases according to the relations: lg σ∼ (1/n)lgp co and lg σ∼(1/m)lgp H 2, where n and m are greater than unity. As a second step the effect of coadsorption of CO+O2 and H2+O2 on the conductivity was investigated. The β-Ga2O3 sensors, connected electrically by Pt electrodes, manifested a moderate catalytic activity. Due to this property the conductivity of the solid changed steeply in the vicinity of the stoichiometric composition of the gas mixtures. In the third step the coadsorption of CO+H2 was studied. In function of the CO partial pressure the conductivity of the solid passed through a minimum. The shape of the experimental curves was attributed to the formation of certain organic intermediate surface compounds.

Coadsorption and cross sensitivity on high temperature semiconducting metal oxides: water effect on the coadsorption process☆

Abstract The operation of high temperature oxide semiconductor gas sensors is strongly influenced by their surface properties and the processes taking place in the topmost atomic layers such as adsorption, coadsorption, gas—gas interactions and the catalytic processes on them. From our experimental and theoretical results a resistance limit was calculated; owing to the measured resistance it is possible to estimate when the surface phenomena dominate. According to our results the effect of water on the resistance is the result of at least two processes. The adsorption of molecular water is fast and results in a donor effect while the parallel formation of surface OH groups is relatively slow and results in an acceptor effect. When the temperature is increased the OH formation becomes the dominant process and at a definite temperature its influence passes a maximum. Our results prove that on β-Ga2O3 and n-SrTiO3 the adsorption and coadsorption processes are strongly influenced by the presence of water. For both substances the effects are very similar. We suggest that similar mechanisms take place on every oxide semiconductor.