Epitaxial integration and properties of SrRuO 3 on silicon

We report the integration of SrRuO 3 , one of the most widely used oxide electrode materials in functional oxide heterostructures, with silicon using molecular-beam epitaxy and an SrTiO 3 buffer layer. The resulting SrRuO 3 ﬁlm has a rocking curve full width at half maximum of 0.01 ◦ , a resistivity at room temperature of 250 µ Ω cm, a residual resistivity ratio ( ρ 300 K (cid:14) ρ 4 K ) of 11, and a paramagnetic-to-ferromagnetic transition temperature of ∼ 160 K. These structural, electrical, and magnetic properties compare favorably to the best reported values for SrRuO 3 ﬁlms on silicon and rival those of epitaxial SrRuO 3 ﬁlms produced directly on SrTiO 3 single crystals by thin ﬁlm growth techniques other than molecular-beam epitaxy. These high quality SrRuO 3 ﬁlms with metallic conductivity

We report the integration of SrRuO 3 , one of the most widely used oxide electrode materials in functional oxide heterostructures, with silicon using molecular-beam epitaxy and an SrTiO 3 buffer layer. The resulting SrRuO 3 film has a rocking curve full width at half maximum of 0.01 • , a resistivity at room temperature of 250 µΩ cm, a residual resistivity ratio (ρ 300 K ρ 4 K ) of 11, and a paramagnetic-to-ferromagnetic transition temperature of ∼160 K. These structural, electrical, and magnetic properties compare favorably to the best reported values for SrRuO 3 films on silicon and rival those of epitaxial SrRuO 3 films produced directly on SrTiO 3 single crystals by thin film growth techniques other than molecular-beam epitaxy. These high quality SrRuO 3 films with metallic conductivity on silicon are relevant to integrating multi-functional oxides with the workhorse of semiconductor technology, silicon. © 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5041940 SrRuO 3 is one of the most widely used oxide electrode materials in epitaxial oxide heterostructures and related devices. 1 This utility arises from its excellent thermochemical stability, 2,3 high conductivity at room temperature, and especially because of its close lattice match (SrRuO 3 has a pseudocubic lattice parameter of 3.93 Å) with many functional perovskite oxides. These include multiferroics such as BiFeO 3 (pseudocubic lattice parameter of 3.96 Å), 4 ferroelectrics such as BaTiO 3 (in-plane lattice parameter of 3.99 Å), 5 superconductors such as YBa 2 Cu 3 O 7-x (in-plane pseudotetragonal lattice parameter of 3.85 Å), 6 and piezoelectrics such as Pb(Zr,Ti)O 3 (in-plane lattice parameter 3.905-4.14 Å). 7,8 SrRuO 3 is often employed in ferroelectric devices, 9 superconducting multilayers, 10 Josephson junctions, 11 electro-optic and magneto-optic devices, 12 Schottky junctions, 13 ferroelectric tunnel junctions, 14 magnetocaloric devices, 15 resistivity switching devices, 14,16 magnetoelectric devices, 17,18 photovoltaic devices, 19 and optoelectronic devices. 20 In condensed matter physics, SrRuO 3 also plays an active role in moderately correlated materials physics due to its unusual itinerant ferromagnetism as a 4d transition metal oxide. The transport properties of SrRuO 3 also draw great attention, including its Fermi liquid behavior at low temperature 21 and bad metallic behavior at high temperature. 22 Recently, heterostructures involving SrRuO 3 layers have been shown to exhibit the topological Hall effect 23 and the inverse spin Hall effect, 24 indicating its potential for spintronic applications.
Integration of SrRuO 3 with silicon, the backbone of the electronics industry, is critical for leveraging the extensive existing infrastructure for large-scale semiconductor manufacturing. This will enable the widespread use of SrRuO 3 -based multi-functional oxide heterostructures for a wide range of applications.
Unfortunately, directly integrating epitaxial SrRuO 3 on silicon is difficult as the formation of an amorphous SiO 2 layer in the oxidative environment during the growth of SrRuO 3 can impede epitaxial growth, resulting in polycrystalline SrRuO 3 films. 25 Polycrystalline SrRuO 3 precludes the epitaxial integration of functional oxide thin films on top of the SrRuO 3 electrode and with it a loss of the optimal properties that epitaxial heterostructures often provide for complex oxide integration.
To achieve epitaxial SrRuO 3 on silicon, various buffer layers that can be epitaxially grown on silicon have been introduced, including yttria-stabilized zirconia (YSZ), 9,26 SrO, 27,28 SrTiO 3 ,29,30 and SrTiO 3 on TiN. 31 Although epitaxy of SrRuO 3 on silicon can be realized via these buffer layers, the quality of the SrRuO 3 films on silicon still cannot compete with typical SrRuO 3 films grown on single-crystal oxide substrates, in terms of both structural perfection [as evaluated by the width of the rocking curve (ω scan) of x-ray diffraction (XRD)] and electrical transport characteristics [as assessed by the residual resistivity ratio (RRR = ρ 300 K ρ 4 K )]. For example, the highest RRR reported for SrRuO 3 films on silicon is ∼3, 31 indicating significant room for improvement in the transport properties of SrRuO 3 on silicon.
Various growth techniques have been utilized for the growth of SrRuO 3 on conventional singlecrystal oxide substrates. These include 90 • off-axis sputtering, 32,33 pulsed-laser deposition (PLD), 10 reactive evaporation, 34-36 molecular-beam epitaxy (MBE), 37,38 metal-organic chemical vapor deposition, 39 and chemical solution deposition. 40 For the integration of epitaxial SrRuO 3 with silicon, usually more than one growth technique is involved due to the step for growing the epitaxial buffer layer. For example, in the study of Park et al., 41 the SrTiO 3 buffer layer on silicon was grown by MBE while the SrRuO 3 film was subsequently deposited by off-axis sputtering. Compared with a combination of multiple growth techniques, which typically involves an air exposure of the buffer layer during the transfer of the sample from one growth chamber to the other (assuming that the two growth chambers are not connected under vacuum), an individual growth method for both the buffer layer and the SrRuO 3 film can avoid exposing the buffer layer surface to air and is preferred for the preparation of epitaxial heterostructures. To our knowledge, there is no report of MBE-grown epitaxial SrRuO 3 films on silicon although SrRuO 3 films of very high quality can be grown on single-crystal oxide substrates by MBE. 38,42 Here we report the in situ integration of SrRuO 3 thin films on SrTiO 3 -buffered (001) Si via MBE. By in situ, we mean that the SrRuO 3 film was grown on an SrTiO 3 film on silicon without removing, etching, or post-annealing the SrTiO 3 /(001) Si stack outside of vacuum after the SrTiO 3 growth on silicon. The resulting films have the highest structural, transport, and magnetic properties among all SrRuO 3 films on silicon reported to date; 31,41 these properties are comparable to those of SrRuO 3 films grown directly on perovskite single crystals by thin film growth techniques other than MBE. 10,32,33,[43][44][45][46] Both the SrTiO 3 and SrRuO 3 films were grown in a Veeco Gen10 dual-chamber MBE system on 2 commercial silicon wafers (p-type, boron doped, and resistivity >10 Ω cm). The base pressure of the chamber was in the upper 10 −9 Torr range. Both growth chambers are equipped with in situ reflection high-energy electron diffraction (RHEED) systems for monitoring the growth of the SrTiO 3 and SrRuO 3 layers. Substrate temperature is monitored by using either a thermocouple for temperatures below 500 • C or an optical pyrometer with a measurement wavelength of 980 nm for temperatures above 500 • C. Prior to film growth, the silicon substrate was cleaned ex situ in an ultraviolet ozone cleaner for 20 min to remove organic contaminants from the surface of the substrate. Molecular beams of strontium, titanium, and ruthenium were generated from elemental sources using a conventional low-temperature effusion cell, a Ti-Ball TM , 47 and an electron-beam evaporator, respectively.
The SrTiO 3 layer was formed in the first growth chamber by the epitaxy-by-periodic-annealing method [48][49][50] for its first 2 nm (5 unit cells) and then with a high temperature codeposition (strontium, titanium, and oxygen all supplied simultaneously) growth step at a substrate growth temperature of 580 • C to achieve a total SrTiO 3 film thickness of 14 nm. The stoichiometry of SrTiO 3 was calibrated using shuttered RHEED oscillations, and the growth rate of SrTiO 3 was determined by the RHEED oscillations that occurred when an SrTiO 3 film was codeposited. 51 For each period of the epitaxyby-periodic-annealing stage, a 2.5 unit-cell-thick layer of SrTiO 3 was first codeposited at 300 • C under an oxygen partial pressure of ∼5 × 10 −8 Torr. Then the substrate temperature was raised to 580 • C in vacuum for the annealing stage to enhance the crystalline quality of the as-grown SrTiO 3 film. During the high temperature codeposition step, the oxygen partial pressure was maintained at ∼(5-8) × 10 −8 Torr. The growth of the SrTiO 3 layer on silicon is described in detail elsewhere. 50 The 14 nm thick SrTiO 3 layer exhibits a rocking curve with a full width at half maximum (FWHM) of 0.01 • for the SrTiO 3 001 peak, indicating that the SrTiO 3 buffer layer is of high crystalline quality. This SrTiO 3 buffer layer serves as an excellent template for the epitaxial growth of SrRuO 3 , not only due to the small lattice mismatch [∼0.64% lattice mismatch for (001) Fig. S1 of the supplementary material. 49 Accordingly, we first deposited a submonolayer amount of SrO to neutralize the surface before the growth of the SrRuO 3 film. The SrRuO 3 film was grown under adsorption-controlled growth conditions. 42 Unlike the growth of SrTiO 3 , which needs careful calibration to provide 1:1 matched fluxes of strontium and titanium 51 to yield a stoichiometric SrTiO 3 film, 53 the stoichiometry of the SrRuO 3 film grown by adsorption-controlled growth is ensured by providing an excess ruthenium flux to the growing film and exploiting thermodynamics to precisely desorb the excess ruthenium in the form RuO x (g). 42 We grew the SrRuO 3 film at a substrate temperature of 660-700 • C (measured using the optical pyrometer) and an oxidant (a mixture of ∼10% O 3 + 90% O 2 ) background pressure of 1 × 10 −6 Torr. After growth, the film was cooled down under a chamber background pressure of ∼2 × 10 −7 Torr of the same oxidant (a mixture of ∼10% O 3 + 90% O 2 ) until the substrate temperature reached ∼150 • C.
The RHEED patterns of the 14 nm thick SrRuO 3 film along the [100] p and [110] p azimuths are shown in Figs. 1(c) and 1(d), respectively. These figures show that the SrRuO 3 film is also epitaxial and smooth. The surface morphology of the heterostructure was further examined via ex situ atomic force microscopy (AFM) using an Asylum Research MFP-3D in tapping mode, as is shown in Fig. 1(e). The rms roughness of the heterostructure is ∼8 Å, which is consistent with the streaky RHEED patterns of the SrRuO 3 film. A height histogram of the AFM image is shown in Fig. S2 of the supplementary material; it exhibits a Gaussian distribution of step heights.
The epitaxial nature and the crystalline quality of the heterostructure were further assessed ex situ by XRD with both Rigaku SmartLab and PANalytical X'Pert four-circle x-ray diffractometers utilizing Cu K α1 radiation. Figure 2(a) shows the XRD θ-2θ scan of the same heterostructure characterized in Fig. 1. The appearance of only 00 reflections indicates that the heterostructure is epitaxial and phase-pure. The intense Bragg peaks reflect the high structural perfection of the perovskite SrTiO 3 buffer layer and the SrRuO 3 film. The thickness fringes indicate that the interfaces of the heterostructure are smooth. Using a Nelson-Riley fit, the out-of-plane lattice parameter of the SrRuO 3 film is found to be 3.935 ± 0.005 Å, which manifests that the SrRuO 3 film is relaxed on the 14 nm thick SrTiO 3 film on silicon. This might originate from the large thermal expansion difference between SrRuO 3 (averaging 1.03 × 10 −5 K −1 between 150 • C and 800 • C) 54 and silicon (averaging 3.7 × 10 −6 K −1 between room temperature and 720 • C). 55 Even though a commensurate film of (001) p SrRuO 3 is compressively strained to (001) SrTiO 3 , the tensile strain induced by the thermal expansion difference to the underlying silicon substrate during the cool-down process can make the lattice parameter of the SrRuO 3 film relax to its bulk value.
The rocking curves of both the SrTiO 3 001 and the SrRuO 3 001 p peaks were measured, together with that of the Si 004 peak. Figure 2(b) shows that the FWHM of the SrTiO 3 001 peak is 0.01 • ; this FWHM is comparable to single crystal SrTiO 3 substrates. 56 With a FWHM of the SrRuO 3 001 p peak of 0.01 • , the 14 nm thick SrRuO 3 film on SrTiO 3 on silicon has the narrowest rocking curve ever reported for SrRuO 3 films on silicon; 28,41 this FWHM is comparable to most SrRuO 3 films grown on single-crystal oxide substrates. 34,38,43,44 Representative rocking curve FWHM values of SrRuO 3 films reported in the literature are summarized in comparison with our result for SrRuO 3 on silicon in Fig. S3(a) of the supplementary material. The in-plane orientation relationship between the film and the silicon substrate was confirmed with a φ scan: (001) SrTiO Fig. 2(c). The resistivity (ρ) vs. temperature (T ) of the same sample was measured in a standard fourprobe van der Pauw geometry with wire-bonded contacts made using aluminum wire in a Quantum Design physical property measurement system (PPMS). The result is shown in Fig. 3(a). The RRR is ∼11, which is the largest RRR reported for SrRuO 3 films on silicon; 31,57 it is comparable to the RRR values of SrRuO 3 films grown on single-crystal oxide substrates by PLD [43][44][45] but is inferior to those of SrRuO 3 films grown on single-crystal oxide substrates by MBE. 42 A general comparison of the RRRs of SrRuO 3 films in the literature is summarized in Fig. S3(b) of the supplementary material.
The linear relationship between resistivity and T 2 for temperatures below 10 K [ Fig. 3(c)] is consistent with the Fermi liquid behavior observed in SrRuO 3 films grown on single-crystal SrTiO 3 substrates by reactive evaporation. 21 There is a clear kink observed at ∼160 K in Fig. 3(a), indicating the change in the scattering rate due to the paramagnetic-to-ferromagnetic transition. The paramagnetic-to-ferromagnetic transition temperature is approximately given by the temperature at which the derivative of the temperature-dependent resistivity is maximal, as is shown in Fig. 3(b). The transition temperature of ∼160 K is close to that of bulk SrRuO 3 single crystals, 58 which indicates that the 14 nm thick SrRuO 3 film is relaxed on the SrTiO 3 -buffered silicon. A comparison of the Curie temperatures of SrRuO 3 films in the literature is summarized in Fig. S3(c) of the supplementary material.
The magnetic properties of the same sample were measured with a superconducting quantum interference device (SQUID) from Quantum Design. The sample was cooled under a 0.1 T field, and the in-plane (along [100] p ) and out-of-plane magnetization was measured as a function of temperature. The result is shown in Fig. 4(a). The in-plane and out-of-plane magnetic hysteresis loops measured at 10 K are shown in Fig. 4(b). Both loops show similar hysteresis with a large squareness (a ratio between the remanent and saturation magnetization) implying strong magnetocrystalline anisotropy of the SrRuO 3 film. The in-plane and out-of-plane saturation magnetization at 10 K is ∼0.75 µ B and ∼0.61 µ B per ruthenium atom, respectively. These values are again comparable to the results from SrRuO 3 films grown directly on SrTiO 3 single crystals 59,60 and are among the highest for SrRuO 3 films on silicon. 31 The SrRuO 3 /SrTiO 3 and SrTiO 3 /Si interfaces in the same sample were examined by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) using a Titan microscope operated at 300 keV. As is shown in Fig. 5(a), the interface between the 14 nm thick SrRuO 3 layer and the 14 nm thick SrTiO 3 layer is abrupt on the atomic scale. White, cyan, and green circles indicate strontium, ruthenium, and titanium atoms, respectively. Figures S4(a) and S4(b) of the supplementary material show the microstructure and thickness uniformity of the same sample at lower magnification. These HAADF-STEM images indicate that the SrTiO 3 buffer layer and the SrRuO 3 film both exhibit a high degree of crystalline perfection and that the sample is uniform over a large scale. There is an amorphous SiO 2 layer between the SrTiO 3 film and the silicon substrate, which originates from the diffusion of oxygen through the SrTiO 3 layer during the growth of either the SrTiO 3 or the SrRuO 3 film. This amorphous layer is typical for epitaxial SrTiO 3 films grown on silicon and is seen in other related studies. [61][62][63][64] Note that despite the high crystalline perfection and electrical characteristics, opportunities remain to further improve the quality of SrRuO 3 films on SrTiO 3 -buffered silicon. For example, the FWHM of the φ scan is relatively large, indicating a considerable amount of in-plane mosaic spread of the SrRuO 3 film. Also, SrRuO 3 samples with less surface roughness are needed for applications where interfaces are critical. Finally, the temperatures used for the deposition of high quality SrRuO 3 films (in our study as well as in the work by others on single-crystal perovskite substrates) are too high to be compatible with underlying complementary metal-oxide-semiconductor (CMOS) circuitry.
In summary, we have integrated SrRuO 3 films on SrTiO 3 -buffered silicon with a film quality similar to SrRuO 3 films grown on single-crystal oxide substrates via thin film growth techniques other than MBE. This integration paves the way toward integrating multi-functional devices of recordperformance on the workhorse of semiconductor technology, silicon.