Tunable Dielectric Thin Films for HTS Microwave Applications
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Our measured temperature dependencies of c, and tan5 are similar to those reported by other groups.2- 5 We observe an increase of both of these quantities with decreasing temperature until a temperature between 20 and 60 K is reached, at which point 6, and tanS reach a maximum and then begin to decrease. The initial increase has been attributed to softening of the soft optical phonon mode, 6,7 and the subsequent decrease, seen only in thin films, may be due to the existence of a random internal electric field. 8 We occasionally observe another loss peak occurring at approximately 100 K; this peak is in addition to the ubiquitous lower-temperature loss peak. For our films with higher dielectric constant values, however, this higher-temperature loss peak is usually absent. These peaks in tan4(T) are variously ascribed to phase transitions 9 though there is strong evidence that they are due to thermally-activated defect modes. 10 Effect of growth conditions All films discussed in this paper were grown by laser ablation. The films were deposited in an oxygen ambient using either ceramic or single-crystal targets. A variety of substrates were employed and will be discussed in the next section. The excimer2 laser was operated at 248 nm with a typical repetition rate of 10 Hz and a fluence of I J/cm . We have observed a strong dependence of the film properties on substrate temperature and background oxygen pressure. As shown in Figure 1, the out-of-plane lattice constant of the films as measured by x-ray diffraction depends rather strongly on oxygen pressure during growth. The large scatter in the data is due to a variety of substrate temperatures (740 to 850 'C) and film thicknesses (0.1 to 4 jim). Nevertheless, there is a general tendency for the lattice constant to peak around 100 to 200 mTorr and to decrease for higher and lower oxygen pressures. We note that the trend we observe is similar to that reported by some other groups.1 1 The reason for this dependence is not obvious, but is probably related to the complex dynamics of the ablated plume and the ensuing behavior of the adatoms upon arrival at the substrate. Note that most of our films do not possess the lattice constant of bulk single-crystal STO; this observation may be expected due to the stress imposed by the substrate. However, we also find that the films with maximum dielectric constants have a lattice constant well above that of single-crystal STO. Thus an increase in the thin-film dielectric constant of the films does not imply that they are becoming more "singlecrystal-like," at least in terms of the c-axis lattice parameter, which does not appear to be a reliable indicator of the film properties. 3.925
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