Molecular-Beam Epitaxy of SrMoO₃ Films with Record Low Electrical Resistivity

<u>Molecular-beam epitaxy of SrMoO₃ films with record low electrical resistivity</u><br/><u>Anna S. Park^1,2,*</u>, Vivek Anil^3,*, Matthew R. Barone^1,2, Brendan D. Faeth², Tobias Schwaigert^1,2, Kyle M. Shen^3,4, Darrell G. Schlom^1,2,4,5<br/><br/><i>¹</i><i>Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA</i><br/><i>²</i><i>Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), Cornell University, Ithaca, New York 14853</i><br/><i>³</i><i>Department of Physics, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA</i><br/><i>⁴</i><i>Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA</i><br/><i>⁵</i><i>Leibniz-Institut für Kristallzüchtung, Max-Born-Str. </i><i>2, 12489 Berlin, Germany</i><br/>*authors contributed equally<br/><br/>SrMoO₃ is the most conducting perovskite oxide (~5.1 at room temperature),^[1] about 40 times more conductive than SrRuO₃. This makes it an attractive material as a bottom electrode for perovskite heterostructures, particularly for the high-<i>K</i> dielectric Ba<i>_x</i>Sr_1-<i>x</i>TiO_3.^[2] Unfortunately, the synthesis of molybdates by traditional MBE is difficult due to the low vapor pressure of molybdenum.^[3] In contrast to elemental molybdenum, its oxide MoO₃ has a very high vapor pressure, which makes it a suitable candidate for a variant of MBE where a molecular beam of a metal oxide rather than its elemental counterpart is used.^[4] One challenge of using MoO₃ as a source, however, is its tendency to reduce to non-volatile MoO₂ in an ultrahigh vacuum environment. This challenge was recently circumvented by injecting a steady flow of oxygen directly into the crucible, enabling the growth of SrMoO₃ films using the stable flux of MoO₃ molecules emanating from a MoO₃ source in conjunction with the flux of strontium atoms emanating from a strontium source, both of which were containing within MBE effusion cells.^[3] The properties of oxide conductors often depend strongly on composition, where off stoichiometry can increase the room-temperature resistivity and dramatically decrease the residual resistivity ratio . This disorder is presumably responsible for the lowest room-temperature resistivity values obtained to date for SrMoO₃ films grown by conventional MBE and pulsed-laser deposition being 24 and , respectively,^[^2,5] which is a factor of 5 higher than the best SrMnO₃ single crystals.^[1] Here we report the growth of SrMoO₃ in an adsorption-controlled regime where thermodynamics automatically controls film composition. To achieve this adsorption-controlled regime, substrate temperatures above 1100 °C are needed, which are unattainable in conventional oxide MBE systems. At the PARADIM thin film facility, we can reach substrate temperatures up to 2000 °C with a CO₂-laser substrate heater, allowing us to capitalize on the volatility of SrO at ~1100 °C. The resulting phase-pure epitaxial SrMoO₃ thin films are characterized by narrow rocking curves, room-temperature resistivities under and residual resistivity ratios higher than the best SrMoO₃ single crystal.^[1] We additionally map the band structure of these high-quality SrMoO₃ films with angle-resolved photoemission spectroscopy.<br/><br/>[1] I. Nagai, <i>et al</i>. <i>Appl. Phys. Lett.</i> <b>87</b>, 024105 (2005).<br/>[2] H. Takatsu, <i>et al</i>., <i>J. Cryst. Growth</i> <b>543</b>, 125685 (2020).<br/>[3] P. Saig, <i>et al</i>., <i>APL Mater.</i> <b>7</b>, 051107 (2019).<br/>[4] K. Atkinson, <i>et al</i>., <i>APL Mater.</i> <b>8</b>, 081110 (2020).<br/>[5] T. Kuznetsova, <i>et al</i>., <i>J. Vac. Sci. Technol. A</i> <b>41</b> (2023).<br/>[6] A. Radetinac, <i>et al</i>., <i>Appl. Phys. Lett.</i> <b>105</b>, 114108 (2014)