Two kinds of electroluminescent ZnS (Cu, Mn, Cl) vacuum-deposited thin film, heat-treated and untreated, both being operative under dc and ac excitation conditions, are investigated with regard to their chemical and electrical properties. When the excitation is by dc supply, the intensity and stability of luminescence depend on the polarity of applied voltage: with untreated films, the emission is stronger when the polarity of electrode (aluminum) is negative than when it is positive; with heat-treated films, this relation is reversed. In general, untreated films are unstable when the polarity of electrode is positive and their luminescence accompanies sparks. When the excita-tion is by ac supply of low frequency, the above characteristics are also observed, suggesting that the mechanism of luminescence by dc excitation and that by low frequency ac excitation are the same. From the observation of dc luminescence, the emission is inferred to occur near the anode. In dc current vs. voltage relation, there are cases in which the films show a negative characteristic at a certain voltage, beyond which the emission increases rapidly, and raising and lowering of voltage gives a hysteresis loop. From these obtained results, a conclusion is drawn that the luminescence is caused by the influx of carriers through heterojunctions found between aluminum electrode or NESA electrode and ZnS phosphor base.
It is possible to make highly crystallized Ge films by zone-melting of Ge films evaporated in vacuum on tungsten plates. Two kinds of zone-melting method, the micro-zone-melting method by electron beam scanning and the filamental-zone-melting method with a linear heater, are used. As well known, vacuum evaporated Ge films are always of p-type even though n-type Ge is used as a source, but it is only when they are highly crystallized by the zone-melting that they become n-type. Those highly crystallized films withstand for a while the action of HF-HNO3 etchant adhering firmly to the substrate with ohmic contact. The rectifying characteristic of a composte of tungsten plate electrode-crystallized n-type Ge film-evaporated p-type Ge film-and melt-bonded indium electrode is similar to that of an ordinary p-n junction and the breakdown voltage is about 20 volts.
BaTiO3 thin films are prepared on various substrates (rock salt cleavage face, glass and platinum plate) by evaporation in vacuo (10-5 Torr) or in the argon atomosphere (10-1 Torr). Evaporation is carried out in an ordinary vacuum evaporation unit with molybdenum or tantalum heating source. When powdered BaTiO3 was evaporated, X-ray and electron diffraction methods revealed that the evaporant was decomposed to BaO and TiO2. It is observed that BaO turns out to be BaO2 or α-BaCO3 in air at room temperature and that as the temperature of the substrate is raised, crystal of BaTiO3 is formed in the film. The single crystal pattern of BaTiO3 is observed by electron diffraction method when the evaporation rate is fast and the substrate temperature is high. A sort of epitaxial growth of BaTiO3 crystal is observed on a cleavage face of rock salt by the flashing evaporation method at higher temperatures. The higher the substrate temperature and the faster the evaporation rate, the easier is the crystal-lization of BaTiO3.
Resistivity, Hall effect and magnetoresistance of evaporated bismuth film are measured at temperatures between 77 and 293°K in a steady magnetic field of flux density up to 16000 gauss. From the experimental results, the concentrations of conduction electrons and holes and their mobilities are calculated by the use of a two band model. The concentration of electrons is found to be nearly equal to that of holes. It is larger in the film than in the bulk; it increases with the decrease of thickness. On the other hand, the mobilities of conduction electrons and holes in the film are much smaller than in the bulk. In films thinner than 1000Å, the mobility increases with the rise of temperature. The effective mean free paths are calculated from the carrier concentrations and mobilities. They are found to be much smaller in the film than in the bulk; they decrease with the decrease of thickness. The ratio d/l∞ (d is the thickness and l∞, the mean free path determined by the internal scattering) becomes always larger than l in the thickness range between 300 and 18000Å. This means that the size effect by the diffuse scattering at surface is difficult to detect, and whether the scattering at surface is diffuse or specular remains unascertained. It is concluded that the thickness dependence of the resistivity of the evaporated bismuth film is mainly caused by the change of the mean free path, l∞, and of carrier concentrations with thick-ness. The negative temperature coefficient of the resistivity observed is due to the difference in temperature dependence of the mobility between the bulk and the thin film.
The results of investigation on some essential problems that are encountered in the process of preparing thin film superconducting circuits by vacuum evaporation are reported. (1) The deposi-tion rate of Sn film must be higher than 150Å/sec., otherwise the superconducting characteristics -the transition temperature, the electrical resistivity at the normal state, and the critical current of the film-are seriously affected. (2) As to the SiO film used for insulation, its thickness needs to be more than 1000Å to ensure its effectiveness if Sn film is to be deposited on it after it was exposed to air. But if the deposition of Sn film is to be made without the SiO film being exposed to air, thickness of more than 2500Å is necessary. (3) The adhesion of Pb terminal to glass substrate is poor; the terminal is likely to peel off at liquid helium temperature. Use of Cr film in between makes Pb film and base firmly bonded. (4) Pb-Pb contacts made by deposition of Pb over Pb film that was once exposed to air do not become s, .iperconductive. (5) The superconduct-ing transition temperature of the Sn film deposited directly on glass substrate is different from that of the bulk, owing to the difference in shrinkage between Sn film and glass at low tempera-ture. Use of SiO film between Sn film and glass removes this effect. From the above, it is concluded that the deposition of all the films should be made continu-ously in vacuum without interruption. A continuously operated multi-layer evaporator capable of producing thin film superconducting circuits is devised.