Epitaxial SrRuO₃/SrTiO₃(100) analyzed using x-ray photoelectron spectroscopy

A growing technological need for high performance thin film metal oxides has driven the exploration of ABO₃ perovskite material combinations. Among the materials of particular interest is SrRuO₃ (SRO). SRO has become synonymous with a group of perovskite materials referred to as highly correlated metallic oxides. While the optical properties of these materials match closely with similar semiconductors, their electronic properties are more comparable to a metal. SRO is an itinerant ferromagnetic material that has produced unusual physical properties such as the anomalous Hall effect (Ref. 1). It can be grown epitaxially on SrTiO₃, with varying degrees of epitaxial strain producing unique structure property relationships (Ref. 2).

A multitude of applications for the use of SRO has been investigated. The material has demonstrated promise for use in all-oxide magnetic tunnel junctions (Ref. 3) and other spin-electronic devices (Ref. 4). As a model material, SRO interfaces with SrTiO₃ and other perovskites has been used as a material system for simulating (Refs. 5 and 6) and studying (Ref. 7) two-dimensional electron gases. The ability of SRO to absorb heavily in the visible spectrum has made it a candidate material for heterojunction photocatalysis studies as well (Ref. 8). For a complete review of the properties of SRO, along with other applications including electrode materials, see Ref. 9.

Despite its close epitaxial match with SrTiO₃, well controlled synthesis of SrRuO₃ has proven to be a difficult challenge in the past for several reasons. The first is that several other stable crystalline materials of the same elements exist, namely, Sr₂RuO₄ and Sr₃RuO₇, meaning that deposition rates of the metals and background oxygen content may all require independent control. Bulk single crystals of both Sr₂RuO₄ and Sr₃RuO₇ have been produced using the float zone method (Ref. 10), but the same study failed to produce SrRuO₃. Epitaxial thin films of SRO have been produced using RF magnetron sputtering at elevated temperature in an argon oxygen mixture (Ref. 11); the authors of this study did not say whether the sputtering was done from a single strontium-ruthenium oxide ceramic target or cosputtered from independent Sr and Ru targets. SRO epitaxial thin films have also been produced by pulsed-laser deposition (PLD) at elevated temperature in oxygen (Ref. 12).

Host Material: Epitaxial SrRuO₃/SrTiO₃(100)

CAS Registry #: 60862-59-1

Host Material Characteristics: Homogeneous; solid; single crystal; conductor; inorganic compound

Chemical Name: Strontium ruthenate

Source: The sample was grown at Brookhaven National Laboratory by PLD.

Host Composition: SrRuO₃

Form: Epitaxial thin film

Structure: Orthorhombic, perovskite structure

History and Significance: The sample was grown at Brookhaven National Laboratory by PLD using a PVD Products PLD/MBE 2300 with a 248 nm KrF excimer laser. Deposition occurred from a single strontium-ruthenium oxide ceramic target at 750 °C in 200 mTorr of oxygen. The substrate was a single side polished SrTiO₃ (100) single crystal (MTI corporation). Cross-section transmission electron microscopy and selected area electron diffraction were performed on SrRuO₃ layers at Brookhaven National Laboratory to confirm epitaxy, similar to Ref. 13.

As Received Condition: Epitaxial thin film deposited onto SrTiO₃ (100) single crystal

Analyzed Region: Same as host material

Ex Situ Preparation/Mounting: Sample was mounted onto holder with a copper clip.

In Situ Preparation: None

Charge Control: None

Temp. During Analysis: 300 K

Pressure During Analysis: <3 × 10⁻⁷ Pa

Preanalysis Beam Exposure: 0 s

Manufacturer and Model: Kratos Axis Ultra

Analyzer Type: Spherical sector

Detector: Channeltron

Number of Detector Elements: 8

Analyzer Mode: Constant pass energy

Throughput (T = E^N): N = 0

Excitation Source Window: Not specified

Excitation Source: Al Ka monochromatic

Source Energy: 1486.6 eV

Source Strength: 210 W

Source Beam Size: 2000 × 2000 μm

Signal Mode: Multichannel direct

Incident Angle: 54°

Source to Analyzer Angle: 54°

Emission Angle: 0°

Specimen Azimuthal Angle: 45°

Acceptance Angle from Analyzer Axis: 0°

Analyzer Angular Acceptance Width: 40° × 40°

Manufacturer and Model: Kratos Minibeam I

Energy: 4000 eV

Current: 0.001 mA

Current Measurement Method: Biased stage

Sputtering Species: Ar⁺

Spot Size (unrastered): 1000 μm

Raster Size: 2000 × 2000 μm

Incident Angle: 90°

Polar Angle: 45°

Azimuthal Angle: 90°

Comment: Sputtering was performed with a differentially pumped ion gun for the calibration spectra only.

Energy Scale Correction: The binding energy was referenced to the Au 4f_7/2 at 84.0 eV.

Recommended Energy Scale Shift: −0.77 eV

Peak Shape and Background Method: Peak shape: O 1s, C 1s, Ru 4p, Sr 4s, O 2p, Sr 4p Gaussian–Lorentzian product formula GL(30) (Ref. 14); Ru 3p and Ru 3d Gaussian-Lorentzian product formula modified by an asymmetric formula (Ref. 15) A(0.33,0.5,0)SGL(30); Sr 3p A(0.3,0.6,0)SGL(30); Sr 3d A(0.33,0.43,0)SGL(30); Background: Custom three parameter Tougaard background (Ref. 16), U 4 Tougaard (B, C, D, T0 = 0) (Ref. 13) was used. O 1s: B = 325 eV², C = 320 eV², D = 560 eV². Ru 3p: B = 420 eV², C = 320 eV², D = 560 eV². C 1s: B = 375 eV², C = 320 eV², D = 560 eV². Sr 3d: B = 255 eV², C = 350 eV², D = 380 eV². Sr 4p/Ru 4s: B = 135 eV², C = 90 eV², D = 260 eV². Valence band B = 90 eV², C = 90 eV², D = 260 eV².

Quantitation Method: Quantification was done using component definitions with casaxps version 2.3.15. Sensitivity factors supplied by Kratos Analytical.

This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. This work was carried out in the Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois. D.E.B. acknowledges support from NSF (DMR 13-06822).