Perovskite SrIrO3 films and its heterostructures are very promising, yet less researched, avenues to explore interesting physics originating from the interplay between strong spin–orbit coupling and electron correlations. Elemental iridium is a commonly used source for molecular beam epitaxy (MBE) synthesis of SrIrO3 films. However, elemental iridium is extremely difficult to oxidize and evaporate while maintaining an ultra-high vacuum and a long mean free path. Here, we calculated a thermodynamic phase diagram to highlight these synthesis challenges for phase-pure SrIrO3 and other iridium-based oxides. We addressed these challenges using a novel solid-source metal-organic MBE approach that rests on the idea of modifying the metal-source chemistry. Phase-pure, single-crystalline, coherent, epitaxial (001)pc SrIrO3 films on (001) SrTiO3 substrate were grown. Films demonstrated semi-metallic behavior, Kondo scattering, and weak antilocalization. Our synthesis approach has the potential to facilitate research involving iridate heterostructures by enabling their atomically precise syntheses.

Iridates host a plethora of interesting physics originating from the interplay between strong spin–orbit coupling (SOC) and electron correlations. Theoretical and experimental studies reveal the existence of various exotic properties such as Weyl semimetal behavior1 and quantum criticality2 in pyrochlore iridates; superconductivity3 and Dirac semimetal behavior4 in Ruddlesden–Popper (RP) iridates. In RP iridates, Srn+1IrnO3n+1 (where n = 1, 2, 3, …, ∞), SOC splits the t2g band into fully-filled Jeff = 3/2 and half-filled Jeff = 1/2 bands. Electron correlations can further split the half-filled Jeff = 1/2 band into Lower Hubbard Band (LHB) and Upper Hubbard Band (UHB). The gap between UHB and LHB in these materials dictates their electronic properties – making Sr2IrO4 and Sr3Ir2O7 insulating, and SrIrO3 semi-metallic.5,6

Bulk SrIrO3 can stabilize in two different crystal structures: monoclinic and orthorhombic. Perovskite orthorhombic SrIrO3 has Pbnm symmetry with a = 5.60 Å, b = 5.58 Å, and c = 7.89 Å.7 This structure can be treated as a pseudo-cubic structure with a ≈ b ≈ c ≈ 3.95 Å. Perovskite orthorhombic SrIrO3 has been difficult to realize in bulk crystals due to the very high pressures (>4 GPa) required to synthesize it.7 Only polycrystals of perovskite SrIrO3 are reported in the literature,4,8 and there are no reports of bulk single-crystal perovskite SrIrO3 to-date. In thin films, however, the perovskite phase readily forms at ambient pressure owing to dimensionality and epitaxial constraints.9 Note that all the Miller indices in this article are for pseudo-cubic SrIrO3, unless stated otherwise.

Thin films of SrIrO3 have gained significant interest in recent years owing to their heterostructures showing topological Hall effect,10,11 anomalous Hall effect,12 and persistent metallicity in ultrathin films.13 Strain-driven modulation of the electronic properties of SrIrO314–16 and its large catalytic activity toward oxygen evolution reaction17 further make it a very promising material to pursue. However, despite such interesting prospects, studying the physical properties of this material has been limited by challenges associated with growing high-quality epitaxial thin films. Although various film synthesis routes, such as chemical vapor deposition,18 pulsed laser deposition,19,20 and sputtering21 have been adopted to grow this material, molecular beam epitaxy (MBE)22–24 is a low-energy method that has been shown to have excellent control over stoichiometry, thickness, and defect density. MBE, however, has its own challenges for iridates growth. Two of these most pressing issues are: (1) stubbornness of iridium toward oxidation and (2) extremely low vapor pressure of iridium. Conventionally, ozone-assisted MBE has been employed to overcome the issue of oxidation,22–24 as ozone is highly reactive—a small amount can oxidize the stubborn metal while maintaining the ultra-high vacuum required for a large mean free path of the source materials. The highly reactive ozone, however, can be a potential safety concern and can cause flux instability by oxidizing other source materials. Furthermore, to address the issue of low vapor pressure of iridium, electron-beam (e-beam) evaporator has been used,22 as it can heat Ir to an extremely high temperature, achieving the vapor pressure required for MBE growth. E-beam evaporators, however, have high setup cost and can potentially cause flux instability due to their localized nature of heating, which might result in non-stoichiometric films.

In this paper, we address the issue of difficult evaporation and oxidation of iridium using a novel growth technique,25 which is an ozone-free, low-temperature process that replaces the pure iridium metal precursor with a more volatile metal-organic solid precursor that has iridium in a pre-oxidized state Ir3+ and contains extra oxygen atoms in the organic ligands. We demonstrate this technique can grow single-phase, epitaxial, high-quality SrIrO3 thin films using a conventional effusion cell and oxygen plasma, eliminating the need for e-beam evaporator and ozone altogether. Structural and electrical transport results of the films are discussed in detail. Temperature-dependent electrical transport and magnetotransport studies reveal semi-metallic behavior and Kondo scattering at low temperatures. This is the first ever report of growth of SrIrO3 using oxygen plasma as an oxidant.

Perovskite SrIrO3(001)pc films were grown on single-crystalline SrTiO3(001) substrate (CrysTec GmbH, Germany) using an oxide MBE system (EVO 50, Scienta Omicron, Germany). Prior to growth, the substrate was cleaned in acetone, methanol, and isopropanol for 5 min each, in that order, before loading in the load lock. In the load lock, it was heated at 200 °C for 2 h to remove any adsorbed contaminants, such as moisture. A more rigorous cleaning using oxygen plasma was performed in the growth chamber at 650 °C for 20 min to minimize interface contamination. Sr metal precursor (99.99%, Sigma-Aldrich) heated at 462 °C using a low temperature effusion cell (MBE-Komponenten, Germany) was used to supply Sr. Oxygen was supplied using a radio-frequency oxygen plasma source (Mantis, UK), equipped with charge deflection plates, operated at 300 W and 8 × 10−6 Torr background pressure. Ir was supplied using an organometallic precursor iridium acetylacetonate [Ir(acac)3] (99.9%, American Elements), heated at 164 °C using a low temperature effusion cell (E-Science, USA). Co-deposition was employed to grow the films for 2 h at a substrate temperature of 650 °C measured by a thermocouple. The films were cooled in the presence of oxygen plasma until the substrate temperature reached below 200 °C to minimize oxygen loss from the films and the substrate.

Reflection high-energy electron diffraction (RHEED, STAIB instruments, USA) images were taken in situ along the [100]pc direction. The high-resolution x-ray diffraction (HRXRD) 2θω coupled scans, the grazing incidence x-ray reflectivity (GIXR) scan, and the reciprocal space map (RSM) were obtained using Rigaku Smartlab XE (Rigaku Corporation, Japan) using Cu Kα radiation at 45 kV and 40 mA. GIXR data was fitted using GenX software to obtain film thickness.26 The out-of-plane lattice parameter was calculated from the 2θ position of the (002)pc SrIrO3 peak obtained in HRXRD. The in-plane lattice parameter was calculated from RSM. The chemical properties of the film were determined using x-ray photoelectron spectroscopy (XPS, Physical Electronics VersaProbe III) with a monochromatic Al Kα x-ray source (1486.6 eV), an energy step size of 0.1 eV, and a pass energy of 55 eV. An argon gas cluster ion beam (Ar-GCIB) with an energy of 1 keV at an incidence angle of 30° was used to clean the surface for 60 s prior to the XPS scan.

Temperature-dependent resistance, magnetoresistance, and Hall resistance were measured with the Van der Pauw technique using aluminum as ohmic wire-bonded contacts in a physical property measurement system (Dynacool, Quantum Design, USA). Temperature-dependent resistance was measured from 1.8 to 300 K, with a warming rate of 3 K/min. Thickness extracted from GIXR was used to convert the sheet resistance to electrical resistivity (ρ). Magnetoresistance was measured along [100]pc and [010]pc crystalline axes with applied magnetic field out-of-plane. Little or no anisotropy in the magnetoresistance was observed for the two crystalline directions. Hall resistance is measured in-plane with applied field out-of-plane. Note that a current of 50 µA was used for all these measurements.

Thermodynamics of MBE (TOMBE) phase diagrams are helpful in predicting the temperature and oxygen pressure required to grow stoichiometric films.27Figure 1(a) is the TOMBE phase diagram for Sr–Ir–O system showing various regions at different partial pressures of oxygen and growth temperatures, where a particular phase of strontium iridate is thermodynamically favored. This diagram was calculated following the same approach discussed in Ref. 27. Details of construction of the phase diagram can be found in the supplementary material. Figure 1(a) clearly shows that iridium is difficult to oxidize. For example, at a growth temperature of 650 °C, impractically high oxygen pressure exceeding 106 Torr may be required to grow SrIrO3 in adsorption-controlled growth mode. An adsorption-controlled growth mode is where any excess iridium is oxidized to IrO2(g) or IrO3(g) and desorbs from the film surface, leaving behind stoichiometric SrIrO3. At lower oxygen pressures, iridium is partially oxidized to Ir(s) or IrO2(s), and as a result, phase-pure SrIrO3 is difficult to grow. Figure 1(b) compares the vapor pressure of elemental Sr, Ir, and the solid organometallic precursor Ir(acac)3. It shows that elemental iridium needs to be heated to a high temperature (>2000 °C) to obtain vapor pressures required for MBE growth. Ir(acac)3, on the other hand, provides equivalent vapor pressure at temperatures <200 °C. Figure 1(c) illustrates the schematic of our solid-source metal-organic MBE approach, in which we co-supply Sr, oxygen plasma, and Ir(acac)3. The molecular structure of this precursor is shown in the top right panel of Fig. 1(c). It contains iridium in a pre-oxidized state as Ir3+, and contains extra oxygen in the ligands, adding to its easier oxidation. The bottom right panel in Fig. 1(c) shows the crystal structure of the grown film.

FIG. 1.

(a) The TOMBE phase diagram of Sr–Ir–O system. Highlighted region is the adsorption-controlled growth window of phase-pure, stoichiometric SrIrO3. (b) Vapor pressure comparison of the elemental metals and chemical precursor for SrIrO3 growth. Solid lines are data taken from Refs. 3537, and dashed lines are linear extrapolations. (c) Schematic of the solid-source metal-organic MBE technique along with the molecular structure of the precursor (top right) and the crystal structure of the film (bottom right).

FIG. 1.

(a) The TOMBE phase diagram of Sr–Ir–O system. Highlighted region is the adsorption-controlled growth window of phase-pure, stoichiometric SrIrO3. (b) Vapor pressure comparison of the elemental metals and chemical precursor for SrIrO3 growth. Solid lines are data taken from Refs. 3537, and dashed lines are linear extrapolations. (c) Schematic of the solid-source metal-organic MBE technique along with the molecular structure of the precursor (top right) and the crystal structure of the film (bottom right).

Close modal

Figures 2(a) and 2(b) show the on-axis HRXRD scans for a representative SrIrO3/SrTiO3 film (−1.1% in-plane biaxial epitaxial strain). Figure 2(a) confirms the films are phase-pure, single-crystalline, and (001)pc oriented. The RHEED images in the supplementary material (see Fig. S1) show a streaky pattern, indicating that the films are epitaxial. Note that the figure also shows secondary diffraction streaks highlighted by a red box, highlighting 1/2 order surface reconstruction.22 The presence of Laue oscillations in Fig. 2(b) suggests that the films have good crystalline quality and a smooth film–substrate interface. The film thickness extracted from these oscillations is 32 nm. The out-of-plane lattice parameter calculated from the 2θ position of the (002)pc SrIrO3 peak is 3.99 Å. Using Poisson’s ratio of 0.3, since the elasticity tensor for SrIrO3 is not available in the literature, the out-of-plane lattice parameter for fully strained films on SrTiO3(001) can be calculated to be 3.97 Å. This observed expansion in the out-of-plane lattice parameter can possibly be due to oxygen vacancies or the cation nonstoichiometry in the film.28 However, since elasticity tensor for SrIrO3 is not known, it is difficult to ascertain whether the out-of-plane lattice parameter is indeed expanded or as expected. Figure 2(c) shows the GIXR scan and the fit to data using GenX. Film thickness extracted from the reflectivity oscillations is 35 nm, which is comparable to the thickness calculated from the Laue oscillations in the HRXRD scan. Moreover, the Kiessig fringes in Fig. 2(c) persist up to 2θ = ∼7°, indicating a smooth film-substrate interface. Figure 2(d) shows the RSM scan around the (1̄03) SrTiO peak, revealing that the films are fully strained in-plane and epitaxial. No impurity peaks other than Sr, Ir, and O were observed in the XPS scan (see the supplementary material, Fig. S2). It is worthwhile to note that even though an organometallic precursor was used to grow these films, there are no organic impurities that can be attributed to the precursor, at least within the measurement limit of the XPS.

FIG. 2.

(a) HRXRD scan of SrIrO3 film grown on SrTiO3(001) substrate at 650 °C substrate temperature for 2 h. (b) HRXRD scan around the (002) substrate peak showing Laue oscillations. (c) GIXR scan of the film. Inset shows the film/substrate schematic. (d) RSM scan around the (1̄03) SrTiO3 peak showing a fully coherent film.

FIG. 2.

(a) HRXRD scan of SrIrO3 film grown on SrTiO3(001) substrate at 650 °C substrate temperature for 2 h. (b) HRXRD scan around the (002) substrate peak showing Laue oscillations. (c) GIXR scan of the film. Inset shows the film/substrate schematic. (d) RSM scan around the (1̄03) SrTiO3 peak showing a fully coherent film.

Close modal

Temperature-dependent resistivity of the film is shown in Fig. 3(a). The resistivity drops with decreasing temperature from 300 to 140 K with an upturn at T < 140 K. In the literature, this upturn in resistivity has been observed at lower temperatures (∼20–50 K) and has been attributed to sample disorder and weak antilocalization in SrIrO3.29,30 The inset in Fig. 3(a) shows the resistivity normalized with respect to resistivity at 300 K. It may be noted that the normalized resistivity stays close to 1 for the entire range of measured temperatures, indicating a semi-metallic behavior. A small kink in resistivity was noted around 105 K, possibly due to antiferrodistortive transition of the SrTiO3 substrate. The kink is not quite apparent in the resistivity data and can be more clearly seen in the resistivity derivative data plotted in supplementary material Fig. S3. Observation of this kink, despite our substrate being non-conducting, further suggests that the films have good epitaxial relation with the substrate and any structural changes in the substrate are reflected in the resistivity measurements of the films. Figure 3(b) presents the resistivity data on a logarithmic temperature scale. The resistivity at low temperatures is saturating, hinting toward a Kondo-like behavior. To verify the presence of Kondo scattering, resistivity data was fitted to an empirical Kondo model described in Eq. (1).31,32 The solid line in Fig. 3(b) shows the resulting fit,

ρT=ρo+ρKT/TK221/s1+1s+ATBqlnT.
(1)

In Eq. (1), ρo (=275 µΩ ⋅ cm) is the residual resistivity. ρK (=352 µΩ·cm) is the Kondo-resistivity at zero temperature. TK (=147 K) and s (=0.43) are the Kondo temperature and the impurity spin parameter, respectively. The power-law term, ATB (where A = 0.1, and B = 1.44), indicates resistivity contribution from electron–electron or electron–phonon scattering. The q ln(T) term is added to account for non-saturating resistivity upturn at low temperature likely due to weak antilocalization (WAL),33 the presence of which is also supported by fits to the magnetoconductance data below 10 K as shown in the supplementary material (see Fig. S4). The fitted value of q is 6.18 µΩ ⋅ cm, indicating that the contribution from WAL is small and the scattering mechanism is predominantly Kondo. The Kondo behavior is not only present in the representative film we show here but is also present in all the other films that were grown at slightly different growth conditions, as can be seen in Fig. S5 of the supplementary material.

FIG. 3.

(a) Temperature-dependent resistivity along the edge of the film showing an upturn below 140 K. Inset shows that normalized resistivity stays close to 1, indicating semi-metallic behavior. (b) Resistivity on log temperature scale along with the Kondo fit [Eq. (1)]. (c) In-plane magnetoresistance and (d) Hall resistance of the film for applied field out-of-plane.

FIG. 3.

(a) Temperature-dependent resistivity along the edge of the film showing an upturn below 140 K. Inset shows that normalized resistivity stays close to 1, indicating semi-metallic behavior. (b) Resistivity on log temperature scale along with the Kondo fit [Eq. (1)]. (c) In-plane magnetoresistance and (d) Hall resistance of the film for applied field out-of-plane.

Close modal

To the best of our knowledge, this is the first report on the presence of Kondo scattering in SrIrO3. We hypothesize that the origin of the Kondo scattering could be oxygen vacancies or non-negligible cation non-stoichiometry.34 It could also explain why we observe the expanded out-of-plane lattice parameter. A detailed study, however, is required to probe the origin of Kondo scattering. Figure 3(c) shows the magnetoresistance (MR) of the film at different temperatures. In other reports, low temperature MR is shown to be dominated by weak antilocalization—a sharp positive MR at low applied fields and then decreasing as the field is further increased.29,30 On the contrary, we observe positive MR at all temperatures and all applied fields, consistent with Kondo scattering being the dominant scattering mechanism in our films. Figure 3(d) shows the Hall resistance of the film at different temperatures. The Hall coefficient is negative, indicating that transport is electron dominated, in line with other reports.15 No hysteresis in MR or Hall data is observed, hinting that strong magnetic order is absent.

In summary, we demonstrated successful ozone-free growth of epitaxial SrIrO3 films using a novel solid-source metal-organic MBE approach. Our films showed semi-metallicity with the presence of anomalous Kondo scattering. We believe our novel synthesis approach paves a pathway to MBE synthesis of other difficult-to-evaporate and difficult-to-oxidize metal oxides with the same ease and control as that of III–V MBE. Although the origin of Kondo behavior is yet to be established, the observation is intriguing and may guide new theoretical and experimental studies investigating its mechanism in these strong SOC materials.

See the supplementary material at (URL) for details of thermodynamic calculations, RHEED of SrIrO3 films, for the XPS, and for transport analysis.

This work was supported by the Air Force Office of Scientific Research (AFOSR) through Grant Nos. FA9550-21-1-0025 and FA9550-21-0460 and from the NSF through the MRSEC program under Award No. DMR-2011401. R.C. also acknowledges partial support from the U.S. Department of Energy (DOE) through Grant No. DE-SC0020211. Parts of this work were carried out at the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC program under Award No. DMR-2011401. Substrate preparation was carried out at the Minnesota Nano Center, which is supported by the NSF through the National Nanotechnology Coordinated Infrastructure under Award No. ECCS-2025124.

The authors have no conflicts to disclose.

Rashmi Choudhary: Formal analysis (lead); Investigation (lead); Methodology (lead). Sreejith Nair: Conceptualization (equal). Zhifei Yang: Data curation (supporting); Investigation (supporting). Dooyong Lee: Data curation (supporting); Formal analysis (supporting); Writing – review & editing (equal). Bharat Jalan: Conceptualization (lead); Funding acquisition (lead); Investigation (lead); Project administration (lead); Supervision (lead); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material and from the corresponding authors upon reasonable request.

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