Strained relaxation in buried SrRuO3 layer in (Ca1-xSrx) (Zr1-xRux)O3/SrRuO3/SrTiO3 System

A novel relaxation phenomenon occurs in buried SrRuO3 layers in strained (Ca1-xSrx) (Zr1-xRux)O3/SrRuO3/SrTiO3 (001) thin film system. The lightly strained SrRuO3 buried layer is initially clamped by the SrTiO3 substrate. After a heavily strained (Ca1-xSrx) (Zr1-xRux)O3 overlayer is deposited, localized strain relaxation develops in the buried layer. This is manifested by a crosshatch pattern of 〈100〉 corrugations on the surface, due to the slip of 〈100〉 {100} threading dislocations. The phenomenon can be controlled by tuning the growth kinetics and strain energy of the overlayer. Comments Copyright American Physical Society. Reprinted from Applied Physics Letters, Volume 89, Issue 3, Article 031905, July 2006, 5 pages. Publisher URL: http://dx.doi.org/10.1063/1.2221900 This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/111 Strain relaxation in buried SrRuO3 layer in „Ca1−xSrx...„Zr1−xRux...O3/SrRuO3/SrTiO3 system Soo Gil Kim, Yudi Wang, and I-Wei Chen Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received 17 April 2006; accepted 25 May 2006; published online 18 July 2006 A novel relaxation phenomenon occurs in buried SrRuO3 layers in strained Ca1−xSrx Zr1−xRux O3/SrRuO3/SrTiO3 001 thin film system. The lightly strained SrRuO3 buried layer is initially clamped by the SrTiO3 substrate. After a heavily strained Ca1−xSrx Zr1−xRux O3 overlayer is deposited, localized strain relaxation develops in the buried layer. This is manifested by a crosshatch pattern of 100 corrugations on the surface, due to the slip of 110 110 threading dislocations. The phenomenon can be controlled by tuning the growth kinetics and strain energy of the overlayer. © 2006 American Institute of Physics. DOI: 10.1063/1.2221900 Thin films grown on lattice-mismatched substrates can undergo strain relaxation by generating threading and misfit dislocations. Subsequently, film surfaces develop a crosshatch topography reflecting dislocation slip traces undergoing diffusional smoothing. In 100 III-V semiconductor layers on 100 substrates e.g., InGaAs/GaAs, SiGe/Si , 110 dislocations on 111 slip planes leave 110 surface traces arranged in a 90° crosshatch grid pattern. Crosshatch morphology has also been reported for SrRuO3 SRO films deposited on 100 oriented SrTiO3 STO substrates misfit strain m=0.64% , but only in thick films 320 nm after postdeposition annealing 8 h at 650 °C . Here we report the first observation of crosshatch development which relaxes a lightly strained buried layer SRO burdened by a heavily strained overlayer SRO-alloyed CaZrO3 CZO . The relaxation occurs via 110 110 dislocations. Multilayer CZO 1−x SRO x /SRO/STO films were grown on 001 STO substrates by laser ablation deposition using a KrF laser =248 nm emitting 200 mJ pulses. The substrates with a miscut angle 0.3° were prepared per Refs. 10 and 11 to provide TiO2-terminated surfaces with steps of a unit cell height 0.4 nm . A 20 nm STO layer was first deposited at 700 °C in 100 mTorr O2 which grew in a step-flow manner. Next, SRO was grown under the same condition to a thickness b of 30 nm, which was too thin to cause strain relaxation during either deposition or postdeposition annealing e.g., 650 °C for 1 h . Finally, an overlayer of Ca1−xSrx Zr1−xRux O3 of various thicknesses o and compositions x was deposited at various temperatures T and O2 pressures P to impart additional strain energy to the Ca1−xSrx Zr1−xRux O3/SRO/STO system. The film thickness, lattice parameters, and full width at half maximum FWHM of the crystal orientations e.g., scan of 002 reflection of SRO were determined by a four-circle x-ray diffractometer Bruker-AXS D8 using a Cu K source. The surface morphology was observed by atomic force microscopy AFM . The as-grown SRO film has a step-and-terrace structure Fig. 1 a . Its out-of-plane lattice parameter cb , 0.3954 nm determined from Fig. 2 a , is larger than the stress-free lattice parameter 0.3930 nm indicating a state of in-plane compressive strain b set to match the substrate STO lattice parameter 0.3905 nm . Despite the strain, the FWHM of SRO 002 plane, 0.05°, is only slightly higher than that of the substrate 0.03° . So the SRO film was probably clamped and not yet relaxed. The subsequent overlayer deposition may take either a two-dimensional 2D or a threedimensional 3D island-growth mode. Under the latter condition e.g., P 10−2 Torr, T=650 °C , the buried SRO film a Electronic mail: [email protected] FIG. 1. Color online AFM images of a SrRuO3 on SrTiO3 001 substrate, with additional Ca0.93Sr0.07 Zr0.93Ru0.07 O3 overlayer deposited at b 650 °C/10 mTorr to 20 nm, c 650 °C/1 mTorr to 20 nm, d 650 °C/1 mTorr to 10 nm, and e 650 °C/1 mTorr to 30 nm, or with f additional 20 nm CaZrO3 overlayer deposited at 650 °C/10 −6 Torr. APPLIED PHYSICS LETTERS 89, 031905 2006 0003-6951/2006/89 3 /031905/3/$23.00 © 2006 American Institute of Physics 89, 031905-1 Downloaded 27 Nov 2006 to 130.91.116.168. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp typically showed a little change in cb Figs. 2 b and 2 c , and the surface morphology remained flat Fig. 1 b . In contrast, under the former condition e.g., P 10−3 Torr, T =650 °C , the buried film often showed a cb reduction Figs. 2 d and 2 e , together with the development of a crosshatch surface pattern Fig. 1 c . This suggests that crosshatch is associated with the 2D growth of a coherent overlayer which strains the system to the point of triggering relaxation in the buried layer, whereas the 3D growth of an overlayer has little effect. To further substantiate the above claim, we have investigated the overlayer deposition under a wide range of conditions x, o, T, and P , and documented the crosshatch observation, growth modes, and the strains in the overlayer and buried layer. Since the in-plane strain of the overlayer o is related to the out-of-plane strain, o =−2 o / 1− , where is the Poisson’s ratio, we can evaluate o by o = co /coo −1, where co is the out-of-plane lattice parameter of the overlayer, and coo its stress-free value coo nm =0.4012 1−x +0.3930x, given CZO lattice parameter =0.4012 nm . The correlations to o o of the overlayer are shown in Fig. 3 for P, T, and the cb and FWHM of the buried layer. In all cases, it is apparent that a crosshatch never develops in 3D growth, b o o 0.4 nm is required for crosshatch to form in 2D growth, and c both cb and FWHM of the buried layer are constant for o o 0.4 nm, but cb decreases and FWHM increases for o o 0.4 nm. A subset of the latter correlation is shown for o variation in Fig. 2 inset; similar observations were also made when P, T, or x were varied. A causal relation between crosshatch formation, growth mode, and the o o of the overlayer is thus established. Several points are noteworthy. First, the 2D/3D growth mode transition typically took place at high P and low T Figs. 3 a and 3 b , corresponding to a relatively low kinetic energy of incoming atoms and low thermal energy of adatoms, respectively. Since in 3D growth the side surfaces of islands are free of constraint, the overlayer can elastically relax despite clamping by the buried layer beneath, thus adding little driving force for strain relaxation. Second, as crosshatch develops at lower P and higher T, its grid spacing l decreases with o o ; e.g., as o increases from 10 to 30 nm o o from 0.239 to 0.611 nm l decreases from 218 to 120 nm Fig. 1 d and 1 e indicating more strain relaxation in thicker film. Third, although coo increases with decreasing x, the excessive mismatch of CZO m =2.67% films cannot be supported by the overlayer in coherent growth, resulting in a 2D/3D growth transition. Fig. 1 f , where o of the CZO layer is only 1%. Lastly, despite the preponderance of crosshatch formation in the present study, it was possible to grow a highly strained crosshatchfree overlayer e.g., x=0.1, o=20 nm, and o o =0.39 nm by using relatively low T and high P e.g., 625 °C/10−3 Torr or 600 °C/10−4 Torr , conditions at the border of 2D/3D growth mode yet giving relatively smooth surfaces. According to x-ray diffraction, the crosshatch grid aligns along 100 Fig. 1 c . This is consistent with the operation of 110 110 dislocations, a dominant slip system in perovskites, depicted in Fig. 4 inset with a 010 slip trace and a dislocation with a Burgers vector b of 0.552 nm. Using the method of Freund and assuming a uniform shear modulus in the entire system, we can write the driving force G on threading dislocation advance as G = 2 b o o + b b sin sin 1 +