$YBa_2Cu_3O_{7-x}$ thin films were prepared by metal organic chemical vapor deposition (MOCVD) using $Y(thd)_3$, $Ba(thd)_2$ and $Cu(thd)_2$ as source materials and $O_2$ as a oxidant gas.
For exact controll of cation stoichiometries in the film, the thermal properties of source materials were investigated through thermogravimetry analysis (TGA) and differential thermal analysis (DTA) and, on the basis of these results, the real sublimation characteristics of source materials in the deposition system as function of evaporation temperature, evaporation time and system pressure were investigated. The experimental results showed that the $Ba(thd)_2$ has much residual after evaporation than other source materials because of its small difference between the thermal decomposition temperature and oxidation temperature, and consequently, the evaporation rate degraded as the evaporation time exceeded 50 minute. The sublimation activation energy of source materials were so high that the delicated controll of evaporation temperature was essential for exact controll of cation stoichiometries in the films. The weight of $Y(thd)_3$ and $Ba(thd)_2$ were not lineally decreased in the early stage of evaporation because of absorbed water and/or residual solvent and/or $\beta$-diketonate which was not chemically reacted to form metal chelates. When the system pressure was decreased below 6.4 torr, the evaporation rates of source materials were increased and that of $Cu(thd)_2$ was greater than those of $Y(thd)_3$ and $Ba(thd)_2$.
The $YBa_2Cu_3O_{7-x}$ thin film was deposited on the various substrates such as $SrTiO_3$ (100), $LaAlO_3$ (100), YSZ (100), MgO (100), $Al_2O_3$ (R-plane), polycrystalline $SrTiO_3$, $SrTiO_3$/Si(100) and $Al_2O_3$/Si(100) and the microstructures and superconducting properties of these films were investigated. The experimental results showed that the film deposited on the substrate which has lower lattice misfit compared to $YBa_2Cu_3O_{7-x}$ superconducting phase had better superconducting properties and microstructure. The transition temperature (Tc) and critical current density (Jc) of $YBa_2Cu_3O_{7-x}$ films deposited on $SrTiO_3$ (100) were 90K and $\sim10^6A/cm^2$, respectively. The grain sizes of the films deposited on $SrTiO_3$ (100), $LaAlO_3$ (100), YSZ (100), and MgO (100) which were observed from TEM bright field images were about $1 \mu m,\; 0.5 \mu m,\; 0.05 \mu m$ and $0.05 \mu m$, respectively. The composition of the films were comparatively unform in the direction perpendicular to film surface and the chemical reaction between film and substrate was less occured on the $LaAlO_3$ (100) than on the $SrTiO_3$ (100) or YSZ (100).
The deposition yield of Cu cation was largest and that of Y cation was larger than that of Ba cation. The increase of Ba cation over stoichiometric value more severely deteriorated the superconducting properties and microstructure of films than those of Y cation and Cu cation. The $Y_2BaCuO_5$ phase of which decompostion rate was greatly smaller than its formation rate mainly restrained grain growth in the films.
The appropriate deposition temperature range to obtain good quality $YBa_2Cu_3O_{7-x}$ film by MOCVD was from 840$^\circ$C to 850$^\circ$C. As it could be known from Raman analysis, the oxygen contents of $YBa_2Cu_3O_{7-x}$ films deposited at lower temperature than 780$^\circ$C were close to 7, while those critical temperature became to decrease as the deposition temperature decrease below 810$^\circ$C. Therefore, it could be suggested that the deterioration of superconducting properties of films with decrease of deposition temperature was not due to insufficient oxidation of films but due to incomplete superconducting phase formation. As the oxygen partial pressure during deposition process was increased, the preferred grain growth direction which was perpendicular to the substrate surface was changed from c axis direction of $YBa_2Cu_3O_{7-x}$ phase to a axis or b axis direction of $YBa_2Cu_3O_{7-x}$ phase. The system pressure ranged from 4 to 8.8 torr had not influence on the superconducting properties of films.
The oxygen diffusion rate into the $YBa_2Cu_3O_{7-x}$ films deposited by MOCVD was so fast that the oxygen partial pressure during annealing and low temperature annealing which was necessary for complete tetragonal - orthorhombic phase transformation in the films obtained by PVD were not important. However, since the complete superconducting phase formation did not occur during deposition process in MOCVD, the high temperature annealing or slowing cooling were necessary to obtain good quality superconducting films.
When the $N_2O$ gas was used as a oxidant instead of $O_2$ gas, the superconducting phase formation was enhanced while the preferred grain growth direction and microstructure of the film did not changed.
