Advances in Complex Oxide Quantum Materials Through New Approaches to Molecular Beam Epitaxy

For many years, complex oxides were primarily synthesized via pulsed laser deposition (PLD) from stoichiometric ceramic targets. This approach was first developed as a means to examine superconducting oxides, including bismuthates and cuprates[11]. It was initially thought to be a straightforward way to produce stoichiometric films, which is non-trivial for complex materials that do not exhibit an epitaxial growth window. Beyond superconducting materials, key breakthroughs came in the form of multiferroic BiFeO₃ thin films[3], interfacial 2D electron gases (2DEGs) in LaAlO₃/SrTiO₃ [12], and numerous other systems. Over time, however, it became clear that material transfer was not uniformly stoichiometric and that growth conditions in PLD had significant impacts on resulting material properties [13, 14]. While PLD is still commonly employed for research in oxide films, including recent advances in superconducting nickelates [15], more and more groups have begun employing molecular beam epitaxy as a way to grow films using thermalized adatoms that can be carefully controlled from elemental sources.

Initial efforts in molecular beam epitaxy (MBE) of complex oxides focused primarily on 3d transition metals, beginning circa 1990. SrTiO₃ films were a primary focus for technique development in the emerging field since they can be grown homoepitaxially [16] and are also of interest for their prospects as high-k gate oxides in silicon devices [17]. Additionally, efforts in cuprate film synthesis reflected the primary motivations of the era in condensed matter physics to better understand the high T_c superconducting materials [18]. Over time, numerous other materials of interest in the 3d block generated attention, including nickelates [19, 20], manganites [21, 22, 23], ferrites [24, 25, 26], vanadates [27], cobaltates [28, 29], and chromites [30, 31, 32, 33]. Additional efforts on band-insulating LaAlO₃ synthesis by MBE helped to resolve long-standing controversies in the field regarding the 2D electron gas at the LaAlO₃/SrTiO₃ interface [34, 35, 36]. These works showed that stoichiometric films did not yield conductive interfaces or built-in electric fields, clarifying that the polar catastrophe model did not govern the behavior [37]. As the field has come to better understand the 3d perovskite oxides, however, new challenges have emerged, with shifts in the field towards quantum materials that often involve refractory 4d or 5d transition metals on the B site. Because refractory elements generally cannot be grown from an effusion cell due to the extreme temperatures required for thermal evaporation, this has led to significant efforts to find alternative approaches for delivery of these cations.

One avenue to delivery of metals that are challenging to evaporate in a conventional effusion cell is the use of a metalorganic precursor, many of which are now ubiquitous thanks to the widespread use of atomic layer deposition (ALD). While this approach is not new, dating back to the growth of metalorganic MBE (MOMBE) in the 1980s [45], it has only been commonly applied in oxide MBE since circa 2007 [5, 46]. By redesigning an MBE chamber to accommodate a heated gas injector and developing a pressure-regulated feedback loop, metalorganic precursors can be delivered into an ultra-high vacuum environment. This approach has commonly been referred to as hybrid MBE, because the A site cations in a perovskite are still delivered from an effusion cell, while the B site cations are delivered through a metalorganic MBE source. For extensive details on the history of the approach, see the work of Brahlek et al [5]. Materials that have been synthesized include SrTiO₃ [46, 47, 48], rare-earth titanates [49, 50, 51], SrVO₃ [52, 53], BaSnO₃ [54, 55], and SrNbO₃ [56], among others.

Among the greatest accomplishments of hybrid MBE has been the demonstration of materials with some of the highest quality on record. SrTiO₃ films have been shown to exhibit an epitaxial growth window for cation stoichiometry [47], analogous to what is seen in III-V materials grown by MBE. This has led to the highest reported carrier mobilities on record for SrTiO₃ films [48]. While it is still not entirely clear what surface chemistry produces the stoichiometric growth window, it is apparent that the titanium tetra-isopropoxide (TTIP) precursor preferentially adheres to a surface with a SrO termination and is more likely to desorb from a TiO₂-terminated surface [5, 57]. Stoichiometric growth windows have also been reported for SrVO₃ [53] and BaSnO₃ films [54].

Because stoichiometric control of complex oxide films has historically been such a challenge in conventional MBE, the availability of a growth window for these materials gives hybrid MBE a great deal of promise for quantum materials further down the periodic table. Perhaps the biggest challenge to date has been in the determination of suitable metalorganic precursors that span the constituent elements in the transition metal block. Difficulties related to the vapor pressure of the precursor, its melting temperature, and the radicals that are released after dissociation on the film surface all play a significant role in this selection. Specifically, ligands cannot contain elements that will contaminate a UHV chamber, such as Cl, F, or P, which are sometimes used in ALD processes. While most precursors contain some variation on hydrocarbon ligands that contain C, H, and O, in some cases a precursor that contains nitrogen-based ligands has proven to be more suitable, as in the case of SrNbO₃ growth using a (t-butylimido)tris(diethylamino) niobium (C₁₆H₃₉N₄Nb) precursor [56]. An analogous precursor exists for Ta, offering a route to the synthesis of tantalate films as well.

One alternative that overcomes the challenges of some liquid precursors that require pressure regulation has been to use a solid metalorganic source material. Groups have demonstrated this approach for Pt, Ru, and Ir using acetylacetonate (acac) ligands [58, 59, 60]. These precursors are solid at room temperature and evaporate above 100 °C, making them viable in low temperature effusion cells. Several examples of this new technique will be discussed below in the sections on iridate and ruthenate films.

Recent advances in the growth of wide bandgap semiconductors such as Ga₂O₃ [61] have led to spillover into the complex oxide film community and have generated new approaches for obtaining high quality films. New ideas have been applied to MBE growth of binary and ternary oxide semiconductors by using metal-oxide sources rather than elemental metals. This approach relies on difference in vapor pressures between metallic sources and oxide sources. In many cases, a binary oxide of the form A_xO_y will produce a higher flux of metal adatoms at the same effusion cell temperature than an elemental metallic source. This is particularly important for refractory elements that are difficult to evaporate from an effusion cell, such as 4d and 5d transition metals. Thermodynamic calculations by Adkison et al [62] showed that much of the periodic table is accessible through suboxide MBE, including low vapor-pressure transition metal elements such as Zr via ZrO, Nb via NbO₂, Mo via MoO₃, Ru via RuO₂, Hf via HfO, Ta via TaO₂, W via WO₃, Ir via IrO₂, and Pt via Pt₂O. To date, suboxide sources have been employed for the growth of MoO₃ films [63] and WO₃ films [64] by MBE, and there have been limited reports of the use of suboxide MBE for transition metal perovskite oxides. We highlight the most prominent example, KTaO₃ [65], below and note that oxides such as SrMoO₃ [66], BaSnO₃ [67] and Ba_n+1In_nO_2.5n+1 [68] have been successfully synthesized. This method has also been employed to catalyze surface reactions that results in higher growth rates [69]. As the oxide field moves towards a greater emphasis on quantum materials that employ 4d and 5d transition metals, we expect that suboxide MBE will fill a significant gap in the current synthesis capabilities.

One additional approach to the delivery of refractory elements has emerged over the past 3 years that offers significant promise for the delivery of refractory elements in ultraclean conditions. Using a continuous-wave thermal infrared laser, refractory elements can be evaporated at high deposition rates, comparable to those attained by electron-beam evaporation [70, 71, 72, 73]. With this approach, the surface of a refractory element can be heated to the sublimation or evaporation temperature of the material while the rest of the material remains solid. Such a technique bridges the gap between electron-beam evaporation, which is difficult to control with sufficient precision for stoichiometric MBE growth, and effusion cells, which are not able to achieve sufficiently high temperatures. In particular, binary oxides of refractory 4d and 5d transition metals have been produced using this approach, including Nb, Hf, Zr, Ru, and Mo [72]. Ongoing efforts in this area have focused on the development of careful temperature control such that laser power can be controlled via a feedback loop to maintain constant flux that is comparable to what can be achieved via an effusion cell. If appropriate control systems and growth chamber designs can be engineered, thermal laser MBE has a bright future for oxide and other quantum materials systems.

In addition, the same thermal laser heating approach has been shown to produce pristine substrate surfaces through in situ annealing to  1500 °C immediately before film growth [74]. This approach to sample heating in MBE can potentially remove the need for chemical pre-treatments for substrates and reduce outgassing from SiC heaters in oxide MBE systems.

Additional developments in material processing techniques have provided a route to grow high-quality oxide films. Recently, it was demonstrated that kinetics of noble metals may be utilized to grow films in a fashion similar to adsorption control [75]. In such a method, excess amount of a transition metal species can diffuse into the substrate through the film, while the film retains its stoichiometry. This proof-of-concept method was demonstrated in the growth of CuCrO₂, wherein excess Cu diffuses into an Al₂O₃ substrate due to its high diffusion kinetics. It was demonstrated that the CuCrO₂ film also acts as a filter that remains stoichiometric, yet helps excess Cu to diffuse through the substrate. Questions still remain on the nature of the diffused atoms, and whether a topotactic reaction occurs around the interface, but this method is likely to be useful in the case of materials involving other noble species where adsorption control is generally not feasible. Further studies on this behavior is likely to answer many details that leads to this behavior.

A new method also combines laser in MBE. In this case, dubbed the hybrid PLD, pulsed laser sources, which are typically used in PLD, are used to to ablate binary oxide targets and generate reasonable fluxes of one species of atoms while traditional effusion cells are used for secondary species. This was recently demonstrated in the adsorption-controlled growth of KTaO₃ [76]. In this instance, a ceramic Ta₂O₅ target is ablated by a pulsed laser while a conventional effusion cell is used to generate excess K flux. It should be noted that the critical difference between this technique and thermal laser method is that in this method, flux is generated from a target is being ablated, not heated as in thermal laser MBE. However, the ability to use ceramic targets may expand the possibility to use numerous types of low vapor pressure oxides as the molecular source.

Finally, another development in epitaxial film growth is the concept of remote epitaxy [77, 78]. In this method, the film is grown on a substrate that has been convered by an atomically thin 2D material template. It was found that even though the substrate is covered by a layer of a 2D material, the film is commensurate with the substrate lattice. This method has been applied mainly to obtain free-standing oxide films, such that the film properties are not influenced by the substrate. However, one main challenge for oxide growth lies in the potential destruction of the 2D layer in activated oxygen or elevated temperatures. Hybrid MBE is therefore a promising avenue for growth of oxide films by remote epitaxy, because for growing materials such as SrTiO₃, the oxygen ligands in the precursor supply the necessary oxygen and the use of an additional oxygen source can be circumvented [79]. The use of these membranes is now being investigated for various applications, including twisted bilayer structures [80].

The potential and the development of oxide MBE can be traced to the growth of cuprate thin films [89]. Precise control of doping and oxidation are some of the strong reasons that MBE is used to grow superconducting films and heterostructures. Extensive studies have been carried out on MBE-grown cuprates, such as La_xSr_1-xCuO₄ (LSCO), Bi₂Sr₂CuO_6+x (BSSCO), YBa₂Cu₃O_7-x (YBCO) and variants [90, 91], often involving custom-designed MBE systems that allows fine-tuning of various subtle growth parameters [92]. There are two main challenges in the growth on superconducting cuprates: oxidation and stoichiometry control. The issue with oxidation can be easily circumvented by using gas sources such as ozone or oxygen plasma. The more challenging issue arises from proper stoichiometry control. Superconductivity is due to the 2D copper oxide layers and since the properties of the films are highly dependent on doping, precise flux control is critical. Multiple in-situ methods such as a beam flux monitor, QCM, RHEED oscillations, and atomic absorption spectroscopy-based calibration provide high confidence in stoichiometry control of these materials.

Recent progress on MBE-grown cuprates as been exemplified by heterostructures involving La₂CuO₄ [93] and YBa₂Cu₃O_7-x [94]. In heterostructures of Sr₂CuO₄ and La₂CuO₄, STEM and EELS was used to obtain the spatial information on different CuO planes to help understand the localization of holes in superconducting heterostructures [95]. Further progress has also been made in the infinite-layer cuprates such as SrCuO₂. This phase of the cuprates are generally not as well studied as the others, since the thermodynamic stability is a big issue. Recently, detailed thermodynamic studies on the growth of the infinite layer cuprates was carried out, and plasma-assisted MBE was employed to successfully grow superconducting Ca_1-xSr_xCuO₂ [96]. Other groups have also been able to synthesize infinite layer cuprates using ozone [97, 98].

The ease of doping and thickness control has resulted in ultra-high quality films and hetrerostructures allowing the investigation of the nature of the dimensionality of copper oxide [99, 100, 101]. For more details on the synthesis-related details, we refer to recent review articles on the growth of cuprates using layer-by-layer MBE [102, 103].

As a final note, recent theories on twisted layers of cuprates have predicted novel states such as topological superconductivity [104, 105], majorana modes [106] and pair density waves [107]. Experiments are already ongoing on exfoliated layers of BSSCO [108] and the renewed interest on this old material system is certainly exciting from a synthesis perspective.

Rare-earth nickelate thin films with the chemical formula RNiO₃ have been explored extensively over the past two decades due to the metallic nature of LaNiO₃ and the metal-insulator transition and antiferromagnetism present when other rare-earth elements are substituted, such as Nd, Sm, and Pr. For a review on the physics of perovskite nickelates with the wide range of phenomena they exhibit, readers are referred to the work of Catalano et al [109]. Much of this work was ultimately motivated by the analogy of the nickelates to cuprate superconductors, where the Ni¹⁺ ion is isoelectronic to the Cu²⁺ ion.

In general, perovskite nickelate thin films are a challenge to synthesize by MBE with ideal oxygen stoichiometry, due to the lower oxidation potential of late 3d transition metal ions such as Ni. Groups have found success with synthesis of LaNiO₃ using distilled ozone[110] or oxygen plasma sources to enhance oxidation potential. Annealing after each unit cell of growth in the materials has also been shown to enhance the oxidation of the materials [41, 111]. Various other in situ synchrotron studies during growth have focused on structural recovery of epitaxial films during the initial growth stages on mixed-termination substrates [112, 113]. However, MBE-grown films have provided excellent platforms for studies of nickelate physics, including the effects of surface termination [19] and thickness [110] on the metallicity of LaNiO₃. These results suggest that surface effects, due to the combined contributions of octahedral distortions and surface chemistry, play a significant role in transport in nickelate films. In the case of the highest quality LaNiO₃ films grown via MBE, evidence of antiferromagnetism has been seen below 1 K, with a strong dependence on film disorder in the form of cation off-stoichiometry ($\sim$2%) and differing oxidation conditions [114]. Given the similarities between nickelate and cuprate physics, significant efforts were undertaken to use interfaces to engineer orbital polarization in LaNiO₃ to drive the material closer to the cuprate electronic configuration [115, 20]. Collectively, our understanding of nickelate physics has been significantly advanced through careful studies of MBE-grown films.

A significant breakthrough within the nickelate research field came with the first observation of superconductivity in nickelate films grown by PLD [15]. In this work, Li et al reported that a layered phase of Nd_0.8Sr_0.2NiO₂ could be produced via chemical reduction of Nd_0.8Sr_0.2NiO₃ using a topotactic phase transition induced by annealing in CaH₂ powder in a tube furnace. This layered phase is analogous to the SrCuO₂ phase in the cuprates, but is unstable in atmospheric conditions and required a SrTiO₃ capping layer in the initial work [15]. Further work has been performed on PLD-grown infinite-layer nickelates with this structure, as is discussed in recent reviews by Nomura and Arita[116] and Bernardini et al [117]. Notably, a recent work in PLD nickelate films argued that hydrogen incorporation into the films during the CaH₂ annealing step was responsible for the superconductivity [118]. This result raises important questions as to the effects of the synthesis process, particularly the annealing step. Efforts to explore alternative synthesis avenues to achieve the same effective Ni charge state as the infinite-layer structure are critical to understand the mechanisms of the superconductivity in nickelate films.

Two particularly noteworthy approaches have been pursued through MBE film growth in superconducting nickelates. In the first case, Pan et al. synthesized layered Ruddelsden-Popper (RP) nickelates that are analogous to the layered cuprate superconductors [119]. Nd₆Ni₅O₁₆ films were grown by MBE and then reduced to Nd₆Ni₅O₁₂ using the same CaH₂ reduction process. These films have an average Ni formal charge of Ni^1.2+, producing a 3$d^{8.8}$ electronic configuration that matches the configuration for the layered cuprates. Such layered materials would be a significant challenge to grow by PLD, but by alternating between NdO and NiO₂ layers via shuttering the group was able to stabilize the RP phase. After reduction, the films showed maximum T_c values of $\sim$13 K, with a broad superconducting transition that does not reach zero resistance until $\sim$1 K. These results are notable for the lack of a SrTiO₃ capping layer due to the greater atmospheric stability of the RP phase in comparison to the infinite-layer phase. Also notable is the broad phase transition in comparison to the infinite-layer structures. Given the complexity of the RP phase and the propensity of such films to form stacking faults that reduce the uniformity of the layered structure [120, 121], it suggests that structural inhomogeneities play a significant role in the superconducting behavior of the materials. Additionally, hydrogen may also be present within the films thanks to the CaH₂ reduction process.

Recent studies have particularly focused on reduction of nickelate films without CaH₂ using metallic aluminum capping layers [122, 123]. In these works, the authors grew epitaxial nickelate films followed by Al capping layers without air exposure. The Al film is highly reducing to the underlying nickelate material, driving the valence from the stoichiometric perovskite or Ruddelsden-Popper phase to the reduced phases. In the case of Ruddelsden-Popper films with the initial stoichiometry of Nd₆Ni₅O₁₆, the reduced films did not exhibit superconductivity in the Nd₆Ni₅O₁₂ phase [123], contrary to the reports for MBE-grown films synthesized via CaH₂ reduction [119]. Interestingly, infinite-layer Nd_1-xEu_xNiO₂ films synthesized in this manner were superconducting and exhibit T_c values of 21 K for $x=0.25$ [122], higher than any previous reports for MBE or PLD-grown films. Transport data for these films is shown in Figure 4. Clearly, there are still numerous questions left to address in regards to the origins of superconductivity in nickelate materials and the role of hydrogen, but MBE film growth has proven to be quite valuable thus far.

Superconductivity was first discovered in KTaO₃ surfaces by field-effect induced doping [124], and work over the past few years examining tantalate films and heterostructures has also shown the analogy of KTaO₃ to SrTiO₃ in terms of its tunability at epitaxial interfaces. These studies were first inspired by the work of Liu et al. on EuO/KTaO₃ heterostructures grown by MBE on KTaO₃ (111) substrates, where superconductivity with T_c of $\sim$1.5 K was reported [125]. This superconducting phase transition was attributed to electron doping of the KTaO₃ crystal due to creation of oxygen vacancies by the overgrowth of EuO by MBE and LaAlO₃ by PLD. Subsequent work by the same group showed that the critical temperature could be tuned electrostatically by varying the carrier concentration in the interfacial 2DEG and explained how phonon-mediated pairing drove the superconductivity at the (111)-oriented interfaces but not in (100)-type interfaces [126]. Other works have employed magnanite capping layers grown by MBE on KTaO₃ (111) substrates to achieve similar superconducting behavior [127, 128]. These films showed that the higher spin-orbit coupling in KTaO₃ in comparison to the analogous superconducting titanate interfaces led to more complex dependence on the in-plane direction of current flow and direction of applied magnetic field [127]. In general, the promise of superconducting materials that exhibit strong spin-orbit coupling such as KTaO₃ leaves a great deal of room for future exploration.

One of the biggest challenges in the field is the need to develop ways to synthesize tantalate films by MBE. In the works from the preceding paragraph, KTaO₃ single crystal substrates were used for film growth and only non-refractory materials were deposited by MBE. While others have grown KTaO₃ by PLD [129], only recently have attempts to synthesize the material by MBE been reported [65]. In that work, K, which has an extremely high vapor pressure at near-ambient conditions in elemental form, was delivered from an effusion cell that contained an In-K alloy. Meanwhile, Ta was delivered in the form of TaO₂ by suboxide MBE from a Ta₂O₅ source and as Ta metal from an e-beam source. A STEM image from a film grown from the suboxide source is shown in Figure 5. This is an important demonstration of suboxide MBE and is the first step towards more complex tantalate heterostructures that can be grown by MBE.

The perovskite BaPb_1-xBi_xO₃ was the second oxide that was shown to be a superconductor, exhibiting a relatively high T_C of about 13 K [86]. Further research led to the discovery of superconductivity in Ba_1-xK_xSbO₃ (with a lower T_C) [130] and Ba_1-xK_xBiO₃ with a higher T_C of over 30 K [131, 132].

Studies of MBE-grown thin films of these materials soon followed [133] and more efforts were made to investigate potential superconductivity in heterostructures involving the bismuthates [134]. Recently, superconductors based on group III and IV metals have gained a new interest, driven by the possibility of topological phenomena due to their high spin-orbit coupling. First principles calculations has also predicted a topological insulator state in doped BaBiO₃ surface [135], showing the possibility for complex phases to exist in this system. Research on this system involving thin films and heterostructures is still ongoing [136, 137] and new insights on the interplay of strong spin-orbit coupling, superconductivity and symmetry breaking have been found. It should be pointed out that one challenge in the growth and heterostructuring of the bismuthates also lies in the charge disproportionation of Bi. Since Bi cannot stabilize in Bi⁺⁴ state, it forms Ba₂Bi⁺³Bi⁺⁵O₆ structure involving distortions, octahedral tilts, and a charge density wave [138].

Although superconductivity was discovered in Sb-doped BaPbO₃, the first report on only Sb based material was reported only recently, with T_C reaching 15 K [139]. A charge density wave was also found, similar to the bismuthates. These old, yet relatively unexplored systems provide incentives and new avenues for novel properties and phases in a non transition metal oxide.

Real space topology, namely topological spin textures (skyrmions and its variants) have been widely investigated, especially in the spintronics community [162]. For recent review on these topological spin textures, see Junquera et al [163] and Tokura et al [164]. Generally, inversion symmetry needs to be broken for skyrmions to exist, and the antisymmetric Dzyaloshinskii-Moriya interaction is invoked in explaining many features of skyrmion-like behavior in SrRuO₃ films and heterostructures. Various numerical and experimental methods are being used to predict or measure the effects of this interaction [165].

Transport based measurements, namely involving the topological Hall effect, have claimed to prove the existence of skyrmions in various oxides [166, 167, 168, 169]. SrRuO₃ is an exemplary oxide that is frequently cited in literature for hosting skyrmions, with many experiments linking the topological Hall effect to the existence of skyrmions. However, recent experiments using engineered films and heterostructures, many using MBE, show that factors such as sample inhomogeneity and competing anomalous Hall effect and Berry phases are responsible for these hump-like features [170, 171, 172, 173, 174, 175]. Therefore, careful assessment of material properties is required and these features in anomalous Hall effect may not be necessarily related to topological Hall effect caused by skyrmions or similar textures.

MBE-grown SrRuO₃ films have been used for investigating multiple physical phenomena. High residual resistivity ratio (RRR), which is tied to the higher quality and lower defects, has been achieved for films grown using e-beam evaporated MBE [38] and hybrid metalorganic MBE [175]. As a ferromagnetic conductor with high spin-orbit coupling, SrRuO₃ has been investigated from spintronic and topological perspectives, and a recent review by Gu et al provides an good overview [149].

Because of the unusual p-wave spin-triplet superconductivity believed to be present in the Ruddelsden-Popper Sr₂RuO₄ phase[176, 177], these materials have also been an area of significant interest for epitaxial films for many years as possible candidates for topological quantum computation [8]. Because of the sensitivity of spin-triplet superconductors to impurities and defects, efforts to achieve superconducting films via PLD took many years with few reports of success and T_c values up to 1.0 K [178, 179]. Subsequent work using MBE via electron-beam evaporation [180, 181] of Ru later produced films with T_c values up to 1.8 K on NdGaO₃ substrates [180]. This work clarified the importance of both film purity and choice of substrate to yield superconducting films. Recent work in metal-organic MBE using a Ru(acac)₃ solid precursor demonstrated that superconductivity could also be achieved via this novel method, with a reported T_c of 0.8 K [182]. A comparison of the critical temperatures for various film growth techniques, RRR values, and substrates is shown in Figure 6. Further work with thermal laser MBE to synthesize superconducting films from a metallic Ru source is also ongoing [183].

Pyrochlore and honeycomb lattice based phases of iridates, along with heterostructures, have been predicted to host novel electronic states [145, 154]. Most of these materials have been studied in bulk single-crystal form and thin films are slowly being developed. Other phases such as Sr₂IrO₄ has also been predicted to be a superconductor, although no experimental verification has been realized. Similarly, SrIrO₃-based (hetero)structures have been predicted to be topological insulators [188].

Thin films have been fabricated using various methods, and we would like to refer to some recent reviews and articles that focus on iridate thin films [189, 190]. Heterostructures based on iridates and other transition metal oxides are being actively investigated that allows for fine tuning of interfacial effects. Particularly, SrIrO₃ is interfaced with magnetic complex oxides such as LaMnO₃ and SrRuO₃ that may result in interfacial topological textures [191, 192, 193, 194]. At the interface of this high spin-orbit coupled material, charge transfer, combined with symmetry breaking and hybridization effects, may allow the development of novel phases. Sharp interfaces require the growth of defect-free, high-quality films and new methods are actively being investigated to grow these materials.

The extremely low vapor pressure of Ir is the main bottleneck for MBE growth of the iridates, and e-beam evaporation of Ir was typically used [39, 195]. The challenge in using e-beam evaporator is flux stability and source degradation. To overcome those issues, recent developments in hybrid MBE [196] has provided new routes. So far, MBE has been used for the growth of the binary oxide IrO₂ [195] and the Ruddlesden-Popper phases SrIrO₃ and Sr₂IrO₄ [39, 195, 196, 197]. Heterostructures involving SrIrO₃ and SrRuO₃ were also investigated, and the role of charge transfer was investigated using ARPES [198]. Their work showed that Ir donates a fraction of an electron to Ru at the interface, changing the Fermi surface of the materials. ARPES measurements of these samples are shown in Figure 7.

A challenge in MBE synthesis of iridates lies with the pyrochlores. Other methods such as PLD has been used to grow many pyrochlore iridates and signatures of topological phases have been found in these films [190]. It is thus appealing to investigate how pyrochlores grow, and determine the potential of heterostructures involving these materials.