Growth of strontium ruthenate films by hybrid molecular beam epitaxy

We report on the growth of epitaxial Sr2RuO4 films using a hybrid molecular beam epitaxy approach in which a volatile precursor containing RuO4 is used to supply ruthenium and oxygen. The use of the precursor overcomes a number of issues encountered in traditional MBE that uses elemental metal sources. Phase-pure, epitaxial thin films of Sr2RuO4 are obtained. At high substrate temperatures, growth proceeds in a layer-by-layer mode with intensity oscillations observed in reflection high-energy electron diffraction. Films are of high structural quality, as documented by x-ray diffraction, atomic force microscopy, and transmission electron microscopy. The method should be suitable for the growth of other complex oxides containing ruthenium, opening up opportunities to investigate thin films that host rich exotic ground states.


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The discovery of unusual ground states in the layered ruthenates, such as p-wave superconductivity [1,2] and nematic order [3], has led to significant interest in the growth of highquality thin films of these materials. For example, Sr 2 RuO 4 is an unconventional, spin triplet superconductor [4], whose superconducting transition temperature is highly sensitive to strain [5].
Thin films of Sr 2 RuO 4 would allow for advancing the understanding of unconventional forms of superconductivity through the application of epitaxial strain [6] and are needed for applications in topological and hybrid superconducting devices. A remarkable success of pulsed laser deposition (PLD) is the growth of superconducting Sr 2 RuO 4 films [7,8], because even minute defect concentrations destroy the superconductivity completely [9].
Molecular beam epitaxy (MBE) has been key in the development of high quality semiconductor thin film heterostructures [10] and has produced high performance complex oxide thin films [11]. MBE of the ruthenates is, however, challenging because of the extremely low volatility of Ru [12], requiring the use of electron beam assisted evaporation [13,14]. Electron beam sources suffer from inherent flux variations and require in-situ monitoring and real time feedback to continuously adjust the flux [14], which makes stoichiometry control difficult. Other challenges include practical limits on the oxygen pressure/fluxes that can be used and flux instabilities in the presence of oxygen [15].
An alternative approach is hybrid MBE, which uses a combination of solid and gas sources to supply the metals [16,17]. In particular, metalorganic and other gaseous sources that have much higher vapor pressures and already contain metals bonded to oxygen, and whose fluxes can be precisely controlled, have been shown to successfully addresses all of the challenges in MBE of perovskite titanates [17][18][19], zirconates [20], and vanadates [21], resulting in high quality thin films having record properties [22,23].
Here, we demonstrate the growth of epitaxial Sr 2 RuO 4 films using hybrid MBE. Strontium is evaporated from an elemental metal source while a volatile ruthenium tetroxide (RuO 4 )containing precursor is used to supply pre-oxidized Ru [24]. The RuO 4 precursor has been used to synthesize Ru, RuO 2 , and SrRuO 3 films [24][25][26] during growth. Films were grown by co-deposition. Strontium was supplied from a high-purity elemental source contained in a low-temperature effusion cell. Ruthenium was supplied in the form of the RuO 4 -containing precursor [24]. This precursor was packed with fluorinated ethers to stabilize the highly reactive RuO 4 molecules, which comprised about 10% by weight of the mixture. The precursor cylinder was at room temperature and no carrier gas was used, as the vapor pressure of RuO 4 is sufficiently high for delivery to the growth chamber. The precursor mixture was delivered to the growth chamber via a gas inlet system using a capacitive manometer and a linear leak valve to monitor and control the flow rate. It was introduced to the growth chamber through a gas injector heated to 50 °C. Additional oxygen was supplied using a plasma source operated at a power of 300 W with a background pressure of ~ 3×10 -6 Torr during growth. Film growth and structure were monitored in-situ using reflection high-energy electron diffraction 5 (RHEED), ex-situ via x-ray diffraction (XRD), atomic-force microscopy (AFM), and transmission electron microscopy (TEM). Room temperature electrical measurement were performed in four point probe (Van der Pauw) geometry using Au/Ti ohmic contacts.
The film stoichiometry was varied by changing the Sr flux via the effusion cell temperature and keeping all the other fluxes constant. Similar to hybrid MBE of SrTiO 3 [29], post-growth RHEED patterns (Fig. 1) were found to be sensitive to the film stoichiometry. The stoichiometry was also evident in the electrical properties, as the room temperature resistivities of the Sr-deficient, stoichiometric, and Sr-rich films were 509 µWcm, 230 µWcm, and 643 µWcm, respectively. In contrast, the out-of-plane lattice parameter (12.74 Å) was found to be independent of stoichiometry. Thus, the best indicators of stoichiometric Sr 2 RuO 4 films were high intensity and streakiness of the RHEED pattern, prominent thickness fringes in XRD, and low resistivity.
In addition to the flux ratios during growth, high substrate temperatures were essential.
Films grown at substrate temperatures of less than 920 °C (thermocouple reading) showed no oscillations in RHEED during growth and were highly resistive. The use of higher substrate temperatures during growth also resulted in a substantial decrease in the room temperature resistivity, from 1100 µΩcm for films grown at 920 °C to 230 µΩcm for films grown at 950 °C.
Furthermore, the highest substrate temperatures (950 °C) used in this study were found to promote layer-by-layer growth (see below). All results shown here are from films grown at 950 °C. Fig. 2(a), indicated layer-by-layer growth mode.

RHEED intensity oscillations, shown in
This growth mode allows for excellent control over layer thickness and is a signature of sufficiently high adatom mobility on the growing film surface. The initial behavior of the RHEED intensity may be a result of the change in symmetry from the cubic substrate to the tetragonal film, as described previously for PLD-grown films [7]. The period of the intensity oscillations corresponded to a growth rate of 16 nm/hr, with two intensity oscillations per Sr 2 RuO 4 unit cell, as has also been found by others [30]. AFM [ Fig. 2(b)] confirmed the smooth film surface with features that are on order of the unit cell height (12.74 Å).
A wide angle XRD scan is shown in Fig. 3(a). The 002, 004, 006, 008, and 0010 Bragg reflections of Sr 2 RuO 4 are all present. The lattice spacing is consistent with an out-of-plane lattice constant of 12.74 Å, matching the known value for the tetragonal unit cell of Sr 2 RuO 4 (space group I4/mmm [31]). Thus, the films are phase-pure Sr 2 RuO 4 . This is significant, as mixed-phase films, containing perovskite SrRuO 3 and the n = 2 Ruddlesden-Popper phase Sr 3 Ru 2 O 7 , are also possible [32]. Figure 3