Suboxide Molecular Beam Epitaxy—A new Thin Film Growth PARADIM

This paper employs a newly established growth method—suboxide molecular-beam epitaxy (S-MBE) [1]—which enables a drastic enhancement in the growth rates of Ga₂O₃ and related materials to over 1 μm h⁻¹ in an adsorption-controlled regime, combined with excellent crystallinity. Using a Ga + Ga₂O₃ mixture with an oxygen mole fraction of x(O) = 0.4 as an MBE source, kinetic limits are overcome that had previously hampered the adsorption-controlled growth of Ga₂O₃ by MBE. Here, the user achieved growth rates up to 1.6 μm h⁻¹ and 1.5 μm h⁻¹ for Ga₂O₃/Al₂O₃ and Ga₂O₃/Ga₂O₃ structures, respectively.

Using S-MBE, allows the growth of phase-pure, smooth, and high-purity homoepitaxial Ga₂O₃ films that are thicker than 4.5 μm and that exhibit very high crystalline quality at unparalleled low growth temperature. With the enhanced quality and high growth rate of S-MBE, we anticipate a significant improvement to vertical Ga₂O₃-based devices. We describe and demonstrate how this growth method can be applied to a wide range of oxides. With respect to growth rates and crystalline quality, S-MBE rivals leading synthesis methods currently used for the production of Ga₂O₃-based devices.

[1]   K.M. Adkison, S.-L. Shang, B.J. Bocklund, D. Klimm, D.G. Schlom, and Z.-K. Liu, "Suitability of Binary   Oxides for Molecular-Beam Epitaxy Source Materials: A Comprehensive Thermodynamic Analysis,"     APL Mater.8, 081110 (2020), DOI 10.1063/5.0013159—a PARADIM publication selected as journal cover for APL Materials, Issue August 2020.

Importance of the Achievement: Suboxide MBE is a new variant of MBE that provides advantages for the synthesis of many oxygen-containing compounds. In this paper comprehensive calculations were made for binary oxides of the entire periodic table—128 binary oxides in all—plus an additional 27 two-phase mixtures of binary oxides with their elements. These calculations established the promise of this approach, at least from a thermodynamic perspective. 16 binary oxides were determined to evaporate nearly congruently. Another 24 binary oxides were identified that can provide molecular beams of suboxide species. Note that in contrast to conventional MBE where the molecular beams are elemental, in suboxide MBE the molecular beams are pre-oxidized. Having the species arrive to the growth surface pre-oxidized in the desired oxidation state can be very useful. It provides a means to navigate reaction pathways. By way of example, consider trying to make compounds containing tin in the 2+ oxidation state. Achieving Sn²⁺ by conventional MBE involves trying to oxidize the incident molecular beam of elemental tin just enough to become Sn²⁺ and not so much that it becomes Sn⁴⁺. To date, this has never been achieved. One either sees tin metal, Sn⁴⁺, or a mixture of the two when trying to oxidize tin with an oxidant in MBE growth.

Being able to supply SnO as a pure, pre-oxidized molecular beam has advantages. Indeed, it was exactly this suboxide MBE route that was used by PARADIM user Professor Chang-Beom Eom from the University of Wisconsin-Madison to realize the antiperovskite Sr₃SnO (see PARADIM Highlight #28). Controlling oxidation is key to the formation of Sr₃SnO and by providing a molecular beam of SnO, the oxidation can be controlled. Another suboxide molecular beam of interest is GeO. It was by providing a molecular beam of GeO that PARADIM user Professor John Heron from the University of Michigan was able to navigate reaction pathways to realize epitaxial films of the rutile polymorph of GeO₂ that he desired (see PARADIM Highlight #32). Suboxide MBE has enabled never before synthesized epitaxial films of Sn²⁺ materials to be realized, BaSnO₃ films to be grown with the highest mobility in the world, VO₂ films with ultra-sharp metal-insulator-transitions, and additional challenging electronic materials.

Figure 3: Approved PARADIM proposals utilizing suboxide MBE.