Lateral solid phase epitaxy of yttrium iron garnet

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Letter

Lateral solid phase epitaxy of yttrium iron garnet

Sebastian Sailler, Darius Pohl, Heike Schlörb, Bernd Rellinghaus, Andy Thomas, Sebastian T. B. Goennenwein, and Michaela Lammel

Phys. Rev. Materials 8, L020402 – Published 29 February 2024

Abstract

Solid phase epitaxy is a crystallization technique used to produce high-quality thin films. Lateral solid phase epitaxy furthermore enables the realization of nonplanar structures, which are interesting, e.g., in the field of spintronics. Here, we demonstrate lateral solid phase epitaxy of yttrium iron garnet over an artificial edge, such that the crystallization direction is perpendicular to the initial seed. We use single-crystalline garnet seed substrates partially covered by a ${SiO}_{x}$ film to study the lateral crystallization over the ${SiO}_{x}$ mesa. The yttrium iron garnet layer retains the crystal orientation of the substrate not only when in direct contact with the substrate but also across the edge on top of the ${SiO}_{x}$ mesa. By controlling the crystallization dynamics it is possible to almost completely suppress the formation of polycrystals and to enable epitaxial growth of single-crystalline yttrium iron garnet on top of mesas made from ceramic materials. From a series of annealing experiments, we extract an activation energy of $3.0 eV$ and a velocity prefactor of $6.5 \times 10^{14} nm / s$ for the lateral epitaxial crystallization along the $〈 100 〉$ direction. Our results pave the way to engineer single-crystalline nonplanar yttrium iron garnet structures with controlled crystal orientation.

Received 7 September 2023
Accepted 19 January 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.L020402

Physics Subject Headings (PhySH)

Crystal growth Crystallization

Magnetic thin films Thin films

Electron diffraction Physical deposition

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Sebastian Sailler ^1,*, Darius Pohl², Heike Schlörb³, Bernd Rellinghaus ², Andy Thomas ^3,4, Sebastian T. B. Goennenwein ¹, and Michaela Lammel ^1,†

¹Department of Physics, University of Konstanz, 78457 Konstanz, Germany
²Dresden Center for Nanoanalysis (DCN), Dresden, Center for Advancing Electronics Dresden (cfaed), TUD Dresden University of Technology, 01062 Dresden, Germany
³Leibniz Institute for Solid State and Materials Research Dresden, 01069 Dresden, Germany
⁴Institut für Festkörper- und Materialphysik (IFMP), TUD Dresden University of Technology, 01069 Dresden, Germany

^*sebastian.sailler@uni-konstanz.de
^†michaela.lammel@uni-konstanz.de

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Issue

Vol. 8, Iss. 2 — February 2024

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Images

Figure 1
Sample preparation scheme for lateral crystallization. (a) After coating the single-crystalline substrate with ${SiO}_{x}$ , (b) a 1 mm wide trench in the ${SiO}_{x}$ is defined by optical lithography and subsequent etching. (c) YIG is afterwards sputtered on top and annealed at 600– $650^{\circ} C$ , which induces (d) the crystallization at the substrate interface. (e) After vertically crystallizing, (f) the crystallization front propagates laterally, over the edge of the ${SiO}_{x}$ mesa.
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Figure 2
Observation of lateral crystallization of YIG on a patterned YAG/ ${SiO}_{x}$ substrate after annealing for $96 h$ at $600^{\circ} C$ . (a) Secondary electron (SE) image of the mesa etched into the ${SiO}_{x}$ , where the left side is elevated as sketched. First, the YIG layer on top of the YAG substrate crystallizes vertically towards the top of the sample and thereby changes from the as-deposited state (a.d.) into a single-crystalline (sc) YIG (right side of the image). Once the YIG reaches the top edge of the sample, it crystallizes laterally via LSPE from right to left onto the ${SiO}_{x}$ layer. The formation of crystalline YIG is accompanied by a change in contrast. (b) Superimposing the SE image from (a) with the EBSD mapping confirms the single-crystalline nature of the YIG layer also in the lateral crystallization regime. The color code is taken from the inverse pole figure in the out-of-plane direction and confirms the epitaxial growth in the crystal direction given by the substrate. The TEM investigation in (c) further verifies the lateral solid phase epitaxy of YIG on top of the ${SiO}_{x}$ . Directly next to the sharp edge of the ${SiO}_{x}$ mesa a slight rotation in the YIG crystal orientation can be seen, which transfers to the single-crystalline YIG on top of the ${SiO}_{x}$ , as it grows epitaxially from the YIG seed. To visualize this rotation, the fast Fourier transforms of two TEM images of YIG crystallized laterally on a ${SiO}_{x}$ mesa, one near the mesa edge and one near the end of the crystallization front, are depicted in (d) and (e), respectively. Here, the zone axis is along $[1 \bar{1} 0]$ , and two reflections along (110) and (112) are marked.
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Figure 3
Lateral crystallization velocities for YIG at $600^{\circ} C$ on two different seed substrates. After the sample was annealed for $24 h$ the lateral crystallization distance was extracted from multiple SEM images of different areas over the mesa edge, and the sample was annealed again. The average lateral crystallization with the standard deviation is depicted over the annealing time. A linear fit to the data was used to extract the lateral crystallization rate and the delay before the onset of lateral crystallization. While YIG starts to grow earlier on YAG, it shows a slower rate of $10.3 nm / h$ compared to the $21.5 nm / h$ on GGG.
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Figure 4
Crystallization velocities as a function of temperature along different crystal directions on a GGG substrate. A semilogarithmic plot over the inverse temperature yields the activation energy $E_{A}$ as well as the maximal rates $v_{0}$ . For YIG an activation energy of $E_{A}$ = $3.0 eV$ is found. The crystallization velocity depends only marginally on the direction and is $v_{0} (〈 100 〉$ ) = $6.5 \times 10^{14} nm / s, v_{0} (〈 110 〉$ ) = $6.5 \times 10^{14} nm / s$ , and $v_{0} (〈 112 〉$ ) = $6.9 \times 10^{14} nm / s$ .
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