Epitaxial $\mathrm{SrTi}{\mathrm{O}}_{3}$ film on silicon with narrow rocking curve despite huge defect density

Epitaxial $SrTi O_{3}$ film on silicon with narrow rocking curve despite huge defect density

Z. Wang, B. H. Goodge, D. J. Baek, M. J. Zachman, X. Huang, X. Bai, C. M. Brooks, H. Paik, A. B. Mei, J. D. Brock, J-P. Maria, L. F. Kourkoutis, and D. G. Schlom

Phys. Rev. Materials 3, 073403 – Published 29 July 2019

Abstract

The structural perfection and defect microstructure of epitaxial (001) $SrTi O_{3}$ films grown on (001) Si was assessed by a combination of x-ray diffraction and scanning transmission electron microscopy. Conditions were identified that yield 002 $SrTi O_{3}$ rocking curves with full width at half-maximum below 0.03° for films ranging from 2 to 300 nm thick, but this is because this particular peak is insensitive to the $\sim 8 \times 10^{11} c m^{- 2}$ density of threading dislocations with pure edge character and extended defects containing dislocations and out-of-phase boundaries. Our results show that one narrow rocking curve peak is insufficient to characterize the structural perfection of epitaxial films.

Received 20 February 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.073403

Physics Subject Headings (PhySH)

Crystal growth High-resolution transmission electron microscopy Molecular beam epitaxy Reflection high-energy electron diffraction X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Z. Wang^1,2, B. H. Goodge^1,3, D. J. Baek⁴, M. J. Zachman^1,*, X. Huang⁵, X. Bai¹, C. M. Brooks², H. Paik^2,6, A. B. Mei², J. D. Brock^1,5, J-P. Maria⁷, L. F. Kourkoutis^1,3, and D. G. Schlom^2,3,†

¹School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
²Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
³Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
⁴School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA
⁵Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York 14853, USA
⁶Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), Cornell University, Ithaca, New York 14853, USA
⁷Penn State University, Department of Materials Science and Engineering, University Park, Pennsylvania 16802, USA

^*Present address: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
^†Author to whom all correspondence should be addressed: schlom@cornell.edu

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Vol. 3, Iss. 7 — July 2019

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Figure 1
(a) Schematic drawing of the growth pathway for the epitaxy-by-periodic-annealing method of the initial 5 unit-cells of $SrTi O_{3}$ on silicon. $T_{sub}$ is the substrate temperature and $P_{O_{2}}$ is the oxygen partial pressure. Different colors indicate different stages. Note the difference of the $O_{2}$ introduction ( $O_{2}$ input) step between the first 2.5 unit-cells and the second 2.5 unit-cells of $SrTi O_{3}$ growth. (b) A set of actual growth parameters recorded during the growth of 5 unit-cells of $SrTi O_{3}$ on silicon. P is the background oxygen partial pressure measured by the chamber ion gauge. $T_{sub}$ is the substrate thermocouple temperature.
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Figure 2
RHEED images at different stages during the growth of the 20-nm-thick $SrTi O_{3}$ film on silicon. A half a monolayer of strontium was deposited at ∼800 °C, followed by another half a monolayer of strontium deposited at ∼300 °C. (a) The as-grown and (c) annealed first 2.5 unit-cells of $SrTi O_{3}$ viewed along the [100] azimuth of (001) $SrTi O_{3}$ . (b) The as-grown and (d) annealed first 2.5 unit-cells of $SrTi O_{3}$ viewed along the [110] azimuth of (001) $SrTi O_{3}$ . RHEED patterns after the growth of the 20-nm-thick $SrTi O_{3}$ film viewed along (e) the [100] azimuth and (f) the [110] azimuth of (001) $SrTi O_{3}$ .
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Figure 3
XRD using (a)–(d), (f), (g) laboratory, and (e) sychrotron diffractometers of two $SrTi O_{3}$ films grown on silicon in this study. (a) θ-2θ scan of the 20-nm-thick $SrTi O_{3}$ on silicon shows only 00l Bragg reflections, which in combination with the RHEED images indicates an epitaxial $SrTi O_{3}$ film on silicon. Asterisks indicate peaks arising from the silicon substrate. (b) Rocking curve of the 002 $SrTi O_{3}$ peak of the 20-nm-thick $SrTi O_{3}$ on silicon has a FWHM of ∼0.0054°. (c) Rocking curves around the 002 $SrTi O_{3}$ peak of the 10-nm-thick $SrTi O_{3}$ on silicon, along with (d) its corresponding RSM exhibit peak broadening consistent with the rocking curve. The broadening in the θ-2θ scan (a) is caused by the finite thickness of the film, while the confinement along the ω direction shows low mosaic spread in the out-of-plane direction. For the RSM, the broadening along the $Q_{z}$ direction is due to the finite thickness of the film, while the narrow width in the $Q_{x}$ direction indicates the low mosaic spread of the $SrTi O_{3}$ film in this direction. (e) The RSM of the 10-nm-thick $SrTi O_{3}$ on silicon with clear thickness fringes measured at the G2 station of CHESS indicates that the narrow intensity spread along the $Q_{x}$ direction and the broadening along the $Q_{z}$ direction are consistent with the lab XRD measurement. For the 10-nm-thick and 20-nm-thick samples that are shown in Fig. 2 and Fig. 3, we first deposited half a monolayer of strontium at high temperature (>= 700 °C), and then deposited another half a monolayer of strontium at 300 °C. Normally we only deposit half a monolayer of strontium once at a certain temperature, before the growth of the $SrTi O_{3}$ film. (f) Rocking curves of the 103 and 002 $SrTi O_{3}$ peaks of the 300-nm-thick $SrTi O_{3}$ film on silicon. (g) XRD pole figure measurements sampling the 202 Si and 101 $SrTi O_{3}$ family of peaks of the 20-nm-thick $SrTi O_{3}$ on silicon. The intensity color scale is logarithmic.
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Figure 4
(a) The rocking curve FWHMs of 20-nm-thick films of $SrTi O_{3}$ on silicon as a function of the temperature ( $T_{strontium deposition}$ ) at which the half a monolayer of strontium was deposited show that 20-nm-thick $SrTi O_{3}$ films with narrow rocking curves can be grown when the half a monolayer of strontium is deposited anywhere in the ∼200 to 800 °C range. Rocking curves of the 002 $SrTi O_{3}$ peak were measured. All data points are from 20-nm-thick $SrTi O_{3}$ films grown on silicon with θ-2θ scans showing only 00l Bragg reflections. (b) FWHMs of $SrTi O_{3}$ films of different thicknesses on silicon show that from ∼2 to ∼300 nm, our growth method yields films with narrow FWHM. The data points of this work are represented with red stars. The purple data points and blue data points are from Refs. [6] and [54], respectively. (c) Raw data of the rocking curves of the 002 $SrTi O_{3}$ peak of $SrTi O_{3}$ films of different thicknesses. (d) Out-of-plane lattice parameter of the $SrTi O_{3}$ film on silicon as a function of film thickness showing that the $SrTi O_{3}$ film on silicon relaxes quickly above a film thickness of about 5 unit-cells. (e) The FWHM of the 101 $SrTi O_{3}$ peak in ϕ (measured in a triple-axis geometry) for $SrTi O_{3}$ films on silicon as a function of film thickness indicates that the $SrTi O_{3}$ films on silicon possess a relatively large in-plane rotational disorder, and the in-plane mosaicity spread decreases as the film thickness increases.
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Figure 5
(a) STEM images of a 10-nm-thick $SrTi O_{3}$ film on silicon. The interfacial region has an amorphous $Si O_{2}$ layer. Strontium (orange), titanium (blue), and silicon (red) atoms are overlaid to show the schematic atomic structure. (b) Plan-view STEM image of a 20-nm-thick $SrTi O_{3}$ film on silicon showing a variety of both localized and branching dislocations with a density of $\sim 8 \times 10^{11} c m^{- 2}$ , between which exist crystallographically defect-free regions with area on the order of $\sim {(20 nm)}^{2}$ . (c) Atomic resolution STEM image of a branching defect like those seen in (b), with an out-of-phase boundary highlighted by yellow arrows marking rows of bright strontium atoms offset on either side of the fault. (d) Atomic resolution STEM image of a localized edge dislocation like those seen in (b) with a Burgers circuit shown in yellow.
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Physical Review Materials

Epitaxial SrTiO3 film on silicon with narrow rocking curve despite huge defect density

Z. Wang, B. H. Goodge, D. J. Baek, M. J. Zachman, X. Huang, X. Bai, C. M. Brooks, H. Paik, A. B. Mei, J. D. Brock, J-P. Maria, L. F. Kourkoutis, and D. G. Schlom

Phys. Rev. Materials 3, 073403 – Published 29 July 2019

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Epitaxial $SrTi O_{3}$ film on silicon with narrow rocking curve despite huge defect density