Yasuji Kumagai

Hall Mobility of Evaporated Tellurium Films

The Hall mobility of evaporated tellurium films was measured as a function of the temperature ranging from 77 to 300 K, and was compared with the theoretical values based on bulk single crystal theories. The measured Hall mobility was about one-tenth of the theoretical value. This deviation was accounted for by considering the potential barrier scattering model due to the grain boundaries of the polycrystalline films proposed by Petritz.

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.

Grain growth of evaporated Te films on a heated and cooled substrate

The effect of the substrate temperatures in a wide range between −150 and 270 °C on the grain size, crystal orientation, Hall mobility, and the carrier concentration of evaporated Te films has been investigated. The grain size and the corresponding Hall mobility were observed to increase remarkably with increasing substrate temperature above 50 °C and with decreasing temperature below 50 °C, contrary to the expectation. The carrier concentration decreased with increasing substrate temperature. The influence of Au nucleation centers on the crystallinity and electronic properties were also investigated. An x−ray diffractometer study indicated c−axis texture to occur with improving grain growth.

Effect of Au nucleation centers and deposition rate on crystallinity and electronic properties of evaporated Te films.

The effect of Au nucleation sites, deposition rates, and the film thickness on the crystallinity and electronic properties of evaporated Te films has been investigated. It has been found that to obtain large−grain Te films there exists an optimum conditoin in the coating ratio of Au nucleation sites and the film thickness of Te, and that a fast deposition rate of Te results generally in large−grain Te films with preferred orientation. The best films obtained have shown a grain size of 20−30 μ diameter with the c axis parallel to the substrate and a Hall mobility of 170 cm2 V−1 sec−1.The effect of Au nucleation sites, deposition rates, and the film thickness on the crystallinity and electronic properties of evaporated Te films has been investigated. It has been found that to obtain large−grain Te films there exists an optimum conditoin in the coating ratio of Au nucleation sites and the film thickness of Te, and that a fast deposition rate of Te results generally in large−grain Te films with preferred orientation. The best films obtained have shown a grain size of 20−30 μ diameter with the c axis parallel to the substrate and a Hall mobility of 170 cm2 V−1 sec−1.

Epitaxial and Amorphous-Crystalline Phase Transition Growth of Evaporated Te Films

The structure and growth behavior of Te films evaporated on mica and glass substrate have been investigated as a function of substrate temperature. A marked difference in grain structure, grain size, surface smoothness, effect of deposition rate and also effect of artificially predeposited metal nucleation centers was observed between Te films deposited above and below 50°C. From optical microscopic observation and from the measurement of electric conductivity changes in Te films during and after deposition, it is shown that at temperatures above 50°C the Te vapor condenses in a crystalline film epitaxially when a mica substrate is used, while at temperatures below 50°C it condenses in a form of amorphous film and then a sudden change from amorphous to crystalline phase takes place a short time after the deposition is completed.

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.

Annealing Effect in Tellurium Films

Using electron microscopy, X ray diffraction techniques and Hall effect measurement, the annealing effect on the crystallinity and electronic properties of evaporated Te films was investigated as a function of annealing time and temperature. A very short-period (5 min) annealing in Ar gas at 320°C was found to be effective in improving both the crystallinity and electronic properties of small-grain Te films. The average grain size increased from 0.1 µm to 0.6 µm and the corresponding changes in the Hall mobility and carrier concentration were from 10 to 160 cm2V-1sec-1 and from 4×1018 to 6×1017 cm-3, respectively. Preferred orientation with the c axis parallel to the substrate also occurred during the course of recrystallization due to annealing at 320°C.

Preparation of High Mobility Tellurium Films

Deposition of Te on a glass substrate at high temperature is generally difficult because of the high vapor pressure of the material. We found that when a substrate glass is predeposited with Au islands previous to Te evaporation, it is possible to deposit Te films at much more elevated temperature because Au islands prevent re-evaporation of Te. Te films thus obtained show a large Hall mobility and a low carrier concentration.

Tunnel Current through a Thin Al2O3 Film with Nonuniform Thickness

In order to clarify the origin of the discrepancy between theoretical and experimental values of tunneling current through a thin Al2O3 layer, nonuniformity of the thickness of the oxide layer is taken into account in the determination of the theoretical current of an Al–Al2O3–Al thin‐film tunnel emitter. The nonuniformity mentioned above is expressed by a modified Poissons distribution function with respect to the oxide thickness s=nq, where q is a definite thickness unit and n is an integer. The above‐cited distribution is discontinued by some finite integer κ, by which the distribution function below a thickness s=κq is discarded so that the calculated tunneling current approaches the experimental value. Numerical calculation of the current through the oxide layer with such nonuniformity of thickness is worked out with regard to two samples prepared anodically by using an electrolyte of 3% ammonium tartrate, and the predicted voltage‐current characteristics are actually obtained. Thus, such a scheme c...

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.