Nonorthogonal twinning in thin film oxide perovskites

K. P. Fahey, B. M. Clemens, L. A. Wills
1995 Applied Physics Letters  
We have observed nonorthogonal twinning in thin film oxide perovskites. This twinning geometry is equivalent to a 60°rotation of the twinned crystallites about the ͓111͔ direction in an ͓001͔ oriented film. This twinning is not detectable by the standard x-ray tests for film quality ͑a radial -2 scan and a scan at a chi of 45°͒. In this study we observe this twinning in films of PbZr x Ti 1Ϫx O 3 , BaTiO 3 , and SrRuO 3 grown by sputtering and MOCVD on MgO and SrTiO 3 . These twins are observed
more » ... in films deposited on a substrate with which there is a large lattice mismatch and in films that are deposited in an oxygen deficient environment. We propose that these twins result from plastic deformation associated with misfit accommodation and discuss the twin geometry and the nature of the twin interface. © 1995 American Institute of Physics. Thin film oxide perovskites have many potential applications based upon their ferroelectric, 1,2 optical, 3 and other properties. Many of these applications may require single crystal or highly oriented thin films. It has, however, proven difficult to obtain thin films that have the same properties as the materials in bulk form. In order to understand and control the properties of thin films, it is necessary to understand their structure, particularly the presence and nature of defects particular to thin films. In this letter we report a twin structure that has been investigated in bulk perovskites 4 but has not been reported previously in thin film perovskite materials. We find that these twins consistently occur in cases of relatively large substrate-film lattice mismatch, or in cases where the films are oxygen deficient. This twinning is not detected by the standard x-ray diffraction characterization that is usually performed to establish epitaxy and phase purity, and we believe that the occurrence of this defect structure may be widespread and partially responsible for nonbulklike film properties. Highly oriented films of SrRuO 3 and BaTiO 3 on both SrTiO 3 and MgO substrates were grown using off-axis rf magnetron sputter deposition in a chamber that has been described previously. 5,6 The base pressure was less than 5 ϫ10 Ϫ8 Torr, and films were deposited at 600 to 650°C in an atmosphere of Ar and O 2 , with ratios varying from 1.5 to 1.0. After a deposition of 1 to 2 h, the samples were cooled at 2°C per second in an atmosphere of O 2 at a pressure of 80 Torr. In addition, we have studied PbZrTiO 3 films grown by MOCVD 7 and thin ͑100 to 200 Å͒ films of SrRuO 3 grown by dc magnetron sputtering with arc reduction ͑SPARC-LE͒. The various films have been investigated using highresolution four-circle synchrotron x-ray diffraction at Stanford Synchrotron Laboratory beamline 7-2, and on a Phillips MRD tube source x-ray diffractometer. A variety of diffraction geometries were employed including symmetric -2 scans and scans, where the sample is rotated about the surface normal by an angle , with the scattering vector held fixed at the Bragg condition for a family of reflections, oriented at an angle relative to the surface normal. For all these films, x-ray diffraction in symmetric reflection geometry showed only the ͑00l͒ type peaks, indicating that the c axis is perpendicular to the film plane. The scan of the ͕101͖-type peaks at 45°to the surface normal showed the four fold in-plane symmetry of these films, indicating the highly aligned nature of these films. Typical results of this scan for a sputter deposited SrRuO 3 film on MgO are shown Fig. 1͑a͒ . These measurements, which are typically performed to determine the crystalline quality and epitaxy of perovskite films, showed no indication of twinning. A more thorough exploration of reciprocal space diffraction features shows that many of these films are not singly oriented. For example, Fig. 1͑b͒ shows a ͕101͖ phi scan at ϭ76.8°for the same SrRuO 3 film. This scan has a set of eight peaks over the 360° domain. This is not consistent with a single ͑001͒ orientation, which would have no ͕110͖type peaks at this value of , and would show no more than four peaks in any phi scan. By examination of these and other features, we can determine that there are a set of four twins in addition to the primary ͑001͒ orientation. These twins are rotated by 60°about the four ͓111͔ poles which are at ϭ54.73°. This geometric relationship is conveniently viewed in a stereographic projection shown in Fig. 2, showing the position of the ͕110͖-type peaks from the twinned crystallites. In addition to finding the peaks that are shown, we found several other sets of peaks to verify that we are indeed seeing these 60°rotated twins. These additional phi scans showed eight ͕100͖-type peaks at a of 48.63°, and eight ͕111͖-type peaks at a of 80.17°. Furthermore, there were only four ͕111͖ peaks at ϭ54.73°, as is consistent with twinning about this direction. Table I gives a summary of the systems that were investigated in this study. The first samples were 600, 300, and 100 Å films of SrRuO 3 on MgO. This system has a large lattice mismatch ͑6.7%͒, so the biaxial compressive stress in the plane of the film is expected to be rather high. The 60°t winning about the ͓111͔ axes is observed in all three of these films. Several heterostructures grown on MgO were also ex-a͒ Electronic
doi:10.1063/1.114614 fatcat:3f2ym5tt5zcdtasx6tjpayqxgm