Thin Film Growth of High Tc Superconductors by Microwave Plasma Assisted Reactive Evaporation
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THIN FILM GROWTH OF HIGH Tc SUPERCONDUCTORS BY MICROWAVE PLASMA ASSISTED REACTIVE EVAPORATION
Akira Tukamoto, Masahiko Hiratani, Toshiyuki Aida, Yoshinobu Tarutani and Kazumasa Takagi Central Research Laboratory,Hitachi Ltd., Kokubunji, Tokyo 185, Japan
ABSTRACT The oxidizing activity of an oxygen plasma is investigated in terms of the oxygen content of HoBa 2 Cu 3 O. thin films. The thermodynamically stable region of HoBa 2 Cu 3O. and the region where the HoBa2Cu2 O- films are formed are also determined both in molecular oxygen and in an oxygen plasma. The oxidizing activity of an oxygen plasma is equivalent to that of molecular oxygen at three orders of magnitude higher pressure. The oxygen plasma expands the thermodynamic stability limit toward higher temperatures and the oxidizing activity limit toward lower oxygen pressures. In addition, the interfacial reaction between an oxide superconductor and silicon is investigated. The oxygen ions in HoBa 2 Cu 3 0 film diffuse into Si even at room temperature, probably enhanced by the strong affinity of silicon for oxygen ions. [NTRODUCTION Since the discovery of high Tc superconductors, thin films have been prepared by various methods. Evaporation is performed in a higher vacuum than that used for sputtering, laser ablation, and chemical vapor deposition. Evaporation is thought to be the best technique for fabricating electronic devices which require a pure atmosphere during growth. The evaporation method usually uses activated oxygen, like oxygen plasma, oxygen ion beams [1-5], and highly reactive gases such as ozone, N2 0, and NO2 [6,7]. These highly reactive sources compensate for the extremely low oxygen partial pressure in a high vacuum. We have studied the thin film growth of oxide superconductors by microwave plasma (ECR plasma) assisted reactive evaporation from the viewpoint of using them in electronic devices [1,8]. The distinctive features of ECR plasma are its high plasma density, low ion energy, and a discharge pressure lower than that of an RF plasma which easily combines the ECR plasma with the evaporation [1,9]. There are some essential techniques to be developed, though, and problems to be overcome. These include; i) low temperature growth to suppress interdiffusion between the superconductor and other materials, ii) establishing the conditions for ideal epitaxial growth to make a smooth interface without lattice mismatch, mis-orientation, or pin-holes, and iii) how to assure sufficient oxidation as well as good crystallinity needed to bring about superior superconducting properties. In this paper, the oxidizing activity of oxygen plasma, the effects of oxygen plasma on the phase diagram of HoBa 2 Cu 3 O,, and the growth region of HoBazCu 3O. films are investigated. The interfacial reaction between HoBazCu3O. and silicon and the heteroepitaxial growth of HoBa 2 Cu3 on Lai. sBa 1 . sCu3O, are 2O also reported. EXPERIMENTAL Figure 1 shows the configuration of the reactive evaporation system we used. The metal sources are evaporated from each Knudsen ce
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