Session A55: Extreme Heteroepitaxy and Novel Growth Methods Including Hybrid MBE and Laser Thermal Evaporation

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Integrating ferroelectric materials onto Si is a critical part of the design new photonic devices such as optical modulators and switches. However, a difficult question to answer thus far has been how to optimize the crystalline orientation in these materials. One interesting material is Barium Titanate (BTO), because it exhibits the largest known Pockels effect which is the modulation of the refractive index by an electric field. However, many applications and device geometries require BTO with the polarization pointing in plane with the substrate, but so far it has proven difficult to optimize BTO to maximize in plane domains. Here, we present a method for the growth of single crystal BTO films that are entirely in plane and strain tunable. In this study, we use extensive X-ray characterization, TEM imaging and direct measurement of the Pockels coefficient in these films to show that the films are entirely in plane oriented BTO. This provides a method for high quality in plane oriented BTO to be directly integrated on Silicon.   

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Germanium-containing oxides such as rutile GeO₂ are earning attention owing to their ultra-wide band gap for high power device application. Here, we present the use of germanium tetraisopropoxide (GTIP) as a precursor for hybrid MBE of Ge-containing compounds. We use Ge_xSn_1-xO₂ as a model system for this new synthesis method. Using a combination of high-resolution X-ray diffraction and X-ray photoelectron spectroscopy to characterize these films, we successfully grow epitaxial rutile Ge_xSn_1-xO₂ on TiO₂ (001) substrates up to x = 0.5, above which the films become amorphous at low substrate temperatures. No Ge-rich films grow at higher temperatures likely due to desorption of Ge-based suboxides. These results confirm the viability of the GTIP precursor in the growth of germanium containing oxides by hybrid MBE, and may thus pave the road to achieving high-quality perovskite germanates thin films.   

Live

Rare-earth titanates RTiO₃ (R = La… Lu or Y) are prototypical Mott-Hubbard insulators and are of great interest for studying the correlations among electronic, magnetic, and orbital orders. Among them, DyTiO₃ (DTO) has the highest Curie temperature (T_c) of 60 K, as well as typically high coercive field and remanent magnetization. Nevertheless, due to its highly distorted structure, the growth of high-quality DTO films remains challenging. In this study, we demonstrate the first hybrid molecular beam epitaxy growth of DTO films on LaAlO₃(001) and DyScO₃(110) substrates. Films grown at optimized stoichiometric conditions show clear ferromagnetism with bulk-like T_c = 60 K. Spectroscopic analysis confirms these DTO films as nominally stoichiometric. By systematically tuning the Dy flux, we find that small deviations in cation stoichiometry lead to simultaneous disappearance of film peaks in X-ray diffraction and ferromagnetism. Based on these results, we discuss the correlation between stoichiometry, crystal structure, and ferromagnetic order in DTO films, providing critical insight into the structure-property relationship in DTO and other rare-earth titanates.   

Live

The development of superior ultra-wide bandgap (UWBG) semiconductors is crucial for next-generation high-power devices. This talk establishes SrSnO₃ (SSO) as an outstanding UWBG semiconductor. We show SSO thin-film growth with controlled doping and present detailed transport measurements supplemented by first-principles calculations of e-ph interactions and charge transport. We discuss details of the MBE growth, strain relaxation and electronic transport properties as a function of temperature, chemical doping and strain. Our results shed light on the transport behavior and mobility-limiting mechanisms in SSO, and demonstrate that Nd-doped SSO films achieve the best of both worlds – carrier densities as high as 2 × 10²⁰ cm^-3 together with a high room temperature mobility of ~70 cm²V^-1s^-1. These figures of merit place perovskite oxides, for the first time, on par with well-established UWBG semiconductors.   

Live

A fascinating consequence of electron correlation is the emergence of a wide range of quantum phases including superconductivity in perovskite oxides such as cuprates and ruthenates. In particular, ruthenates are difficult to grow in molecular beam epitaxy (MBE) due to the ultra-low vapor pressures of Ru. Additionally, Ru has high electronegativity which further makes it difficult for achieving complete oxidation, Ru to Ru⁴⁺ states under standard MBE conditions. The former issue has been dealt by supplying Ru using an e-beam source whereas the latter oxidation issue is addressed by using highly reactive ozone. However, the use of e-beam and ozone has adverse consequences on the stability of Ru beam-flux, which is critical for obtaining electronic-grade materials. Here we show a new hybrid MBE approach that circumvents these issues by supplying a metal-organic compound containing Ru for the growth of RuO₂. Highly perfect RuO₂ films as evident from rocking curve and electrical measurements were obtained down to a substrate temperature of 300°C. Bulk-like room temperature resistivity of 35 μΩcm was achieved for films of only 10 nm. The effect on the electrical properties of film thickness, orientation, strain, and defects such as oxygen vacancies will be discussed.   

Live

Functional oxide heterostructures grown using molecular beam epitaxy (MBE) can have atomically abrupt interfaces, enabling the systematic study of their intrinsic properties. In particular, heterostructures using epitaxial SrRuO₃ as metallic electrodes for electronic devices have been widely studied. SrRuO₃ exhibits itinerant ferromagnetism at low temperatures and topological behavior which are of significant interest in magnetic devices. The conventional MBE growth of SrRuO₃ has been challenging due to the low vapor pressure of Ru, requiring electron beam evaporation. Ru also has a higher electronegativity than most B-site cations in ABO₃-type compounds, requiring stronger oxidants like ozone to form Ru⁴⁺. The conventional process is, therefore, prone to flux instabilities and demands highly reactive oxidants during growth. Here, we report a novel MBE approach with scalable growth rates for the growth of (110)_O SrRuO₃ on (001) SrTiO₃. Initial growth optimization yielded atomically smooth, epitaxial, phase pure SrRuO₃ films with a rms roughness of ~230 pm. Films were conductive with room temperature resistivities of ~200 µΩ cm. We will discuss the effect of substrate temperature and Sr/Ru flux ratio on stoichiometry, surface morphology, carrier density and magnetic properties.   

Live

Since the discovery of superconductivity in (La,Ba)₂CuO₄ in 1986, there has been much work in studying structurally similar materials such as layered ruthenates. Sr₂RuO₄ is an unconventional superconductor with T_c = ~ 1.7 K that is highly sensitive to defects and strain. In this talk, we will present a novel molecular beam epitaxy (MBE) growth strategy for the growth of high quality Sr₂RuO₄ films that utilizes an effusion cell for Sr, a chemical precursor for Ru, and a radio-frequency (RF) plasma to supply oxygen. Films were grown at 900 °C. Reflection high energy electron diffraction (RHEED) showed streaky patterns consistent with smooth film morphology. High resolution X-ray diffraction of Sr₂RuO₄ on LSAT (001) yielded a bulk-like lattice parameter. Room temperature electronic transport yielded a resistivity value of 73 µΩ-cm. We will discuss a detailed structure - strain - transport property relationship of Sr₂RuO₄ films grown using our novel MBE approach.   

Live

The film-substrate interface crucially determines the structural quality of the following layers. The substrate needs to provide a chemically uniform structure with a specific surface reconstruction as a functional template for the nucleation of the first epitaxial layer. In contrast to compound semiconductor MBE, the temperature range to reach an adsorption-desorption equilibrium is out of reach for most established oxide substrate heater technologies. Therefore, ex-situ chemical treatments are often performed to obtain a desired substrate surface stoichiometry. We demonstrate that by using a ~10 µm laser substrate heating system that we have developed, typical oxide substrates can be heated with high precision to any temperature below their melting point. This allows the termination of oxide substrates by selectively evaporating the volatile components of the substrate surface [1], thereby foregoing the need for chemical etching. In addition, such a laser heating enables the deposition of virtually any material in a thermal laser epitaxy [2] setup, which uses exclusively lasers as power sources, both for substrate heating and the continuous thermal evaporation from individual, high-purity sources.
[1] https://doi.org/10.1063/5.0008324
[2] https://doi.org/10.1063/1.5111678   

Live

The growth of oxide heterointerfaces demands a precise control of the deposition parameters and a high-purity of the source materials. Thermal laser evaporation (TLE) is a promising candidate for the deposition of high-purity oxide heterostructures. In the presentation we reveal that TLE is well suited for the growth of high-quality oxide films, reporting on the growth and characterization of a spectrum of binary transition metal oxides films deposited by TLE. These films were grown by laser-evaporating metallic source materials under oxidizing conditions and deposition on unheated Si (100) substrates. Our results reveal the potential of TLE for the growth of oxide heterostructures using almost any element of the periodic table.   

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Engineered heterostructures formed by complex oxide materials are a rich source of emergent phenomena and technological applications. In the quest for new functionality, a vastly unexplored avenue is interfacing oxide perovskites with materials having dissimilar crystallochemical properties. Here, a unique class of heterointerfaces based on nitride antiperovskite and oxide perovskite materials is engineered as a new direction for materials design. Atomically sharp interfaces between nitride antiperovskite Mn 3 GaN with noncollinear spin structure and oxide perovskites (La _0.3 Sr _0.7 )(Al _0.65 Ta _0.35 )O ₃ (LSAT) and SrTiO ₃ are fabricated, and atomic resolution imaging / spectroscopic techniques and first-principle calculations, are used to determine the atomic-scale structure, composition, and bonding at the interface. The epitaxial antiperovskite / perovskite heterointerface is mediated by a coherent interfacial monolayer that interpolates between the two anti-structures. These results represent a major step for the development of functional antiperovskite/perovskite heterostructures, combining their unique characteristics such as topological properties for ultra low power applications. We also discuss a heterostructure application exploiting charge to spin conversion arising from the noncollinear spin structure.   

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Line defects such as dislocations in heteroepitaxial films can have many undesirable consequences to the films’ electronic properties including carrier compensation and lower carrier mobilities. In this talk, we will discuss the remote epitaxy approach for the growth of SrTiO₃ (STO) films on different substrates using hybrid molecular beam epitaxy (MBE). The use of titanium isopropoxide (TTIP) as a source of both titanium and oxygen allowed the growth of epitaxial STO without oxidation of the graphene layer. STO could be exfoliated from the substrate as a freestanding membrane and transferred on to a flexible polymer substrate. We will discuss how the graphene interlayer effects the structural quality and strain relaxation, and its implications for defect management in heteroepitaxial thin films.   

Live

Complex-oxide heterostructures are a leading example of quantum-matter heterostructures that open a new arena of solid-state physics. For the scientific development of this field and for a range of potential applications, the growth of high-purity heterostructures is required. We have developed a new thin-film deposition technique that is especially suited to the growth of oxide heterostructures with atomic precision. Thermal laser epitaxy uses chemical elements as sources which are evaporated with continuous-wave lasers. The lasers’ virtually arbitrary power density allows for the evaporation of almost all elements of the periodic table in the same setup. We demonstrate this by showing elemental metal films of a large range of elements; from low-vapour-pressure elements like W and Ta to high-vapour-pressure elements like S and Bi. We will discuss the benefits of thermal laser epitaxy for high-purity deposition of complex-oxide materials and heterostructures with almost all elements from the periodic table.   

Live

In oxide epitaxy, the growth temperature and background oxygen partial pressure are considered as the most critical factors that control the phase stability of an oxide thin film. Here, we report an unusual case wherein diffusion of oxygen vacancies from the substrate overpowers the growth temperature and oxygen partial pressure to deterministically influence the phase stability of pulsed laser deposition grown Bi₂WO₆ thin film. We show that when grown on an oxygen-deficient SrTiO₃ substrate, the Bi₂WO₆ film exhibits a mixture of (001) and (100)/(010)-oriented domains alongside (001)-oriented impurity WO₃ phases. Oxygen annealing of the substrate or using a few monolayer-thick SrRuO₃ as the blocking layer for oxygen vacancy diffusion enables growing high-quality single-crystalline Bi₂WO₆ (001) thin film exhibiting an atomically smooth film surface with step-terrace structure. We propose that large oxide-ion conductivity of Bi₂WO₆ facilitates diffusion of oxygen vacancies from the substrate during the film growth, accelerating the evaporation of volatile Bismuth (Bi), which hinders the epitaxial growth. Our work provides a general guideline for high-quality thin film growth of Aurivillius compounds and other oxide-ion conductors containing volatile elements.