Sumio Okuyama

Hydrogen Gas Sensing Using a Pd-Coated Cantilever

A cantilever consisting of a thin glass plate coated with an evaporated Pd thin film can be utilized as a hydrogen gas sensor because the Pd film expands upon absorption of hydrogen, resulting in the bending of the free end of the cantilever. A comparison has been made between the response obtained experimentally from 100–1000-nm-thick Pd-coated glass plate cantilevers in the presence of 0.1–600 Torr hydrogen and the response calculated from the data of the Pd lattice expansion due to hydrogen absorption and the elasticity theory. It is shown that the Pd-coated cantilever can detect hydrogen at least from 0.1 to 10 Torr with good reproducibility. The use of a Pd–Ag alloy film instead of a pure Pd film was found to be effective both in enhancing the hydrogen sensitivity at low hydrogen pressures and in extending the hydrogen responsivity up to 600 Torr.

Observation of HCl- and HF-Treated GaAs Surfaces by Measuring Contact Angles of Water Droplets

A GaAs surface chemically treated with hydrochloric acid (HCl) or hydrofluoric acid (HF) has been characterized by measurements of the contact angle between the surface and a water droplet on it in air. The contact angle measured from the GaAs samples dipped into water shows the formation of a thin surface layer on the GaAs surface and the subsequent removal of the layer upon chemical treatment with HCl or HF solution. Annealing in nitrogen atmosphere is shown to be effective in stabilizing the surface against water.

Current vs voltage characteristics of Al-Al2O3-Pd tunnel junction hydrogen sensor

The conduction mechanism of an Al–Al2O3–Pd MIM (metal-insulator-metal) junction which can operate as a hydrogen gas sensor was investigated using the logarithmic derivative of the current curve, Fowler-Nordheim plot and temperature dependence of the junction current. Al–Al2O3–Pd junctions showed current-voltage characteristics typical of tunneling conduction at voltages above 3 V. The barrier height of the Pd-Al2O3 contact obtained from the position of the logarithmic derivative maximum of the current was 3.4 eV in vacuum, and decreased to 2.7 eV upon introduction of 40 Pa hydrogen. On the other hand, the barrier height of the Al–Al2O3 contact was 1.8 eV in vacuum and was unchanged even when 40 Pa of hydrogen gas was introduced.

Improved Response Time of Al-Al2O3-Pd Tunnel Diode Hydrogen Gas Sensor.

An Al–Al2O3–Pd tunnel diode acts as a sensitive hydrogen gas sensor. A serious drawback, however, of this hydrogen gas sensor operating at room temperature was a very long response time, typically 30 min. To overcome this disadvantage, the Pd tunnel diode was operated at elevated temperatures, resulting in a marked reduction of the response time, less than 1 min at 83°C. A microheater attached to the back surface of the substrate could be an efficient power source to heat the hydrogen sensor to the required temperature.

Pd/Ni-Al2O3-Al Tunnel Diode as High-Concentration-Hydrogen Gas Sensor.

Pd/Ni–Al2O3–Al tunnel diodes were fabricated on a glass substrate for detection of high-concentration hydrogen gas. For Pd–Al2O3–Al diodes, the output signals were saturated at hydrogen partial pressures higher than approximately 0.1 Torr, whereas a Pd/Ni(22%)–Al2O3–Al diode showed output signals even at 50 Torr hydrogen partial pressure at room temperature. The concentration of hydrogen detectable with the Pd/Ni alloy metal–insulator–metal (MIM) diodes increased with an increase in the Ni content. The change in the barrier height at the Pd/Ni–Al2O3 interface upon introduction of hydrogen gas was also measured for various Ni contents in Pd/Ni alloys by means of the logarithmic derivative of the tunnel current and was found to be associated with the hydrogen sensitivity of the Pd/Ni alloy MIM junctions.

Hydrofluoric-Treated GaAs Surfaces Analyzed by Contact Angle Measurement and Auger Electron Spectroscopy

A GaAs surface chemically treated with hydrofluoric (HF) acid was characterized using measurement of the contact angle between the GaAs surface and a water droplet on it and through Auger electron spectroscopy (AES). The surface oxide layer formed on GaAs during planar etching was removed by dipping it into HF solution. As the dipping time increased, a thicker arsenic layer was formed on the GaAs surface, which prevented the surface oxidation of the substrate. This is reflected in an increase in the contact angle. When the dipping time becomes very long (180 min), the amount of surface oxygen increases probably due to the roughened surface of the GaAs substrate, resulting in a decrease in the contact angle.

Electrochemical Detection of Defects in Ge / GaAs Structures by an Anodic Dissolution Method under Illumination

Detection of crystal defects in Ge/GaAs structure has been studied by an electrochemical etching under illumination. Germanium film grown by the plasma-assisted epitaxy method with hydrogen on a semi-insulating GaAs substrate is anodically dissolved in a NaOH solution. Hillocks connected with the structural defect are then detected and their densities are estimated. Results obtained suggest that the defects in the Ge film originate from plasma ion bombardment, related to those in the GaAs substrate

Characterization of Pure Water-Treated GaAs Surfaces by Measuring Contact Angles of Water Droplets

A GaAs surface treated with pure water was characterized by using contact angle measurement. Auger electron spectroscopy, and atomic force microscopy. The contact angle was found to be dependent on treatment conditions such as treatment time, agitation of pure water, dissolved oxygen concentration in pure water, and illumination. The contact angle of a water droplet on GaAs surface treated with agitated deionized pure water was found to increase with increasing treatment time, indicating that the hydrophobicity of the surface increased. The agitation of pure water strongly influenced the oxygen concentration and the roughness of the treated GaAs surface, and hence, the contact angle of a water droplet on it, t.

Hydrogen-induced light emission from an organic electroluminescent device

An organic electroluminescent device consisting of ITO/aromatic amine/tris(8–quinolinolato)aluminum(Alq3)/Pd was fabricated. The light emission was found to occur when hydrogen gas was present in the ambience and ceased when hydrogen was removed from the Pd electrode. The light emission induced by hydrogen gas was attributed to the work function lowering of the Pd electrode at the Pd–Alq3 interface by adsorption of hydrogen.