Pyroelectric Coefficient Optimization Through Grain Size Control in (Ba, Sr)TiO 3 Thin Films
- PDF / 1,240,500 Bytes
- 6 Pages / 391.5 x 607.5 pts Page_size
- 79 Downloads / 206 Views
Mat. Res. Soc. Symp. Proc. Vol. 623 © 2000 Materials Research Society
stress increases this modified slope, one could, in a limited way, increase a film's bolometric sensitivity by increasing the film's tensile stress. Pertsev et al.'s theory only considers crystals with a single domain. Yet, domain structures of 900 twins have been predicted and observed in ferroelectric thin films. Indeed, the same two-dimensional tensile stress which can be accommodated by an in-plane orthorhombic distortion can also be accommodated by in-plane, 900 twinned tetragonal distortions.7 Further, the tetragonally distorted unit cell volume is larger than the pseudocubic orthorhombic unit cell.8 This suggests that the tetragonal transition may be preferred in films under greater stress. In this case, the phase transition should be accompanied by 9a large dielectric peak since in-plane tetragonal transitions display such peaks in bulk crystals. Yet, this transition may not appear in thin films due to the small columnar grain size. The number of 90' domains has been observed to diminish significantly in bulk ceramics as the grain size is reduced below 1 Am.' 0 In the case of columnar grains, it is the column diameter which would be the limiting factor for the formation of an in-plane domain structure. Thus, the
proposed in-plane, 900 twinned tetragonal phase transition may only appear in large-grained films under large tensile stress. In the materials system of Bal-xSrxTiO 3 thin films deposited on SrTiO 3 (STO), tensile thermal mismatch stresses will be generated. By depositing films of various grain sizes, we have attempted to create a film with a large grain size and in-plane tensile stress. In this way, we hoped to observe the in-plane tetragonal phase transition and its associated increase in bolometric sensitivity. EXPERIMENT Thin Film Growth Ba0 .2Sr 0 .8TiO 3/La0. 5 Sr 0o 5CoO 3 (BST/LSCO) thin films were pulsed laser deposited onto single crystal (001) exact-cut STO substrates in a 300 mTorr oxygen ambient. Ablation occurred via a 248 nm KrF excimer laser with an energy density of 2.4 J/cmn. The substrate temperature was set to 680 TC while the target to substrate distance was approximately 5 cm. The LSCO layers were deposited at a laser pulse repetition rate of 5 Hz while the BST layers were grown at 1, 5, and 20 Hz. Total pulse count was varied to achieve a BST thickness of 4000 A. Characterization Film stress was calculated by measuring substrate bending both pre- and post-deposition using a Tencor Flexus FLX2320 laser interferometer stress gauge. Surface morphology was examined using a LEO Gemini 1550 field-emission scanning electron microscope (SEM). Film compositions were calculated from Rutherford backscattering spectroscopy (RBS) measurements utilizing a 1.95 MeV He+ beam with a silicon detector positioned at 1650 with respect to the beam. X-ray diffraction (XRD) 0-20 scans were performed on a Siemens 4000 high resolution Cu K0 x-ray diffractometer, equipped with a 4-circle goniometer and a 4-crystal Ge monochrometer.
Data Loading...
