Thin-film growth of ABO2 delafossites has recently attracted significant attention due to its attractive transport properties and potential applications. A fundamental requirement for achieving high-quality thin films is the availability of lattice matching substrates and chemical compatibility. However, there are still many obstacles to achieving high-quality thin films. Here, we report a process to further engineer a template ABO2 delafossite structure by solid-phase epitaxy of CuAlO2 on the surface of a commercial sapphire substrate, which offers a promising route to growing high-quality epitaxial thin films. The starting reagents involve a layer of polycrystalline Cu2O deposited on a c-Al2O3 substrate by pulsed laser deposition (PLD). Subsequent thermal treatment activates a solid-state interface reaction between the film and substrate, producing a CuAlO2 thin film. The reaction temperature and dwell time parameters were optimized in this study to prepare a phase diagram for CuAlO2 samples without phase impurities. This method provides an essential stepping-stone toward the approachability of a lattice matching template (i.e., substrate-buffer layer) for ABO2 heterostructures. An example of successful epitaxial growth of highly conducting PdCrO2 is also demonstrated by using a CuAlO2 buffer layer.
Delafossite oxides with the chemical formula ABO2 are materials with fascinating physical properties due to the alternate layer stackings of A-cation layers with BO2 layers.1–5 Depending on the A- and B-cations, physical properties can range from ultraclean metals (i.e., A = Pd and Pt) or transparent semiconductors (i.e., A = Ag and Cu and B = Al, Cr, Fe, Ga, Y, and In) to complex magnetic ground states.2–5 To explore their distinctive properties, honing the material architecture as thin films will have great potential in designing heterostructures by combining different ABO2 materials, which currently are in uncharted territory. One major drawback for ABO2 thin film research is the lack of commercially available lattice-matched substrates; currently no known delafossite substrates are available. Current substrates that have been used for ABO2 growth include triangular substrates, including c-Al2O3, β-Ga2O3, (111) SrTiO3, (111) MgO, and (001) 4H–SiC.4–14 The structural and chemical dissimilarity and huge lattice mismatch between the available substrates and delafossite thin films limit the successful epitaxy and film quality. Therefore, advances are required to enable novel delafossite-based materials and their applications for oxide electronics. We recently reported the vital role of Cu-based delafossite buffer layers in growing Pd-based delafossites on commercially available single-crystal substrates.9,10 Such progress offers a step toward advanced materials, enabling the deliberate fabrication of artificial heterostructures, with enhanced electronic and magnetic properties.
Template engineering or buffer layers have been commonly used in various oxide heterostructure syntheses (i.e., perovskites) to achieve epitaxial film growth.15–17 Even in the thin film growth of delafossite, a templated CuCrO2 layer significantly reduced the formation of impurity phases.10 Compared to the CuCrO2 template, CuAlO2 is an ideal system due to its nonmagnetic and transparent properties. In this study, we focus on fabricating the CuAlO2 structure on the surface of the commercially available c-Al2O3 substrate through solid-phase epitaxy.
In literature, Cu–Al–O phase diagrams were reported by mixing starting reagents of Cu2O or CuO with Al2O3, followed by thermal treatments.18–21 Earlier reports also showed that CuAlO2 thin film layers could be formed at the interface when a layer of Cu2O is chemically reacted with Al2O3.12–14,22 There have been reports of a two-step process to form CuAlO2: one used Cu evaporated on an Al2O3 substrate (CuO is the starting precursor),12 and another sputtered a Cu2O layer on a sapphire substrate and covered with an Al2O3 layer (Al2O3|Cu2O|sapphire).13,14 By using Cu|sapphire, it was observed that the formation of CuAlO2 also included unreacted CuO, and in the sandwich method (Al2O3|Cu2O|sapphire), the top reaction with the Al2O3 film would not lead to epitaxy, indicating that Cu2O reacts with the substrate interface to produce the epitaxial structure.12–14 Here, we investigate how one can create such an epitaxial template of CuAlO2 on Al2O3 substrates with minor defects to be explicitly used for ABO2 film growth thereon. We start with the following chemical reaction, as shown in Eq. (1):
We used a “bake-method” technique to fabricate the CuAlO2 surface layer.12–14,23 By characterizing the films with x-ray diffraction (XRD) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurements, we confirmed the growth of the CuAlO2 structure on c-Al2O3 substrates. Moreover, we demonstrate the epitaxial growth of PdCrO2 on a templated CuAlO2 on a c-Al2O3 substrate. PdCrO2 films were reported in earlier studies,10,11 where PLD-grown films had a rate-limiting effect, causing thicker films (>33 nm) to form impurity phases. Utilizing the templated CuAlO2 structure on c-Al2O3 has negated the thickness limitation. It displays enhanced structure (no secondary phases observed) and similar physical properties as measured by temperature-dependent resistivity in contrast to other PdCrO2 films.
A polycrystalline Cu2O target was sintered by solid-state reaction and used for PLD. c-Al2O3 substrates were cleaned by rinsing in acetone, isopropanol, and deionized water for 10 min each, then were annealed at 1000 °C for 1 h in a box furnace. Substrates were then ready to be used for deposition. The Cu2O thin films were grown on the prepared substrates by PLD using a KrF excimer laser (λ = 248 nm). Substrate growth temperature and oxygen partial pressure during the polycrystalline Cu2O growth were 700 °C and 10 mTorr at a laser repetition rate of 5 Hz. Thin films were cooled to room temperature under the growth pressure. For post-growth treatment, the Cu2O films were placed in an alumina boat by capping with a bare Al2O3 substrate to protect the surface and suppress the evaporation of Cu2O.13 The furnace temperature, T, and dwell times, t, were varied in this study to find the best thermal treatment conditions. The samples were annealed with different reaction temperatures ranging from 1000 to 1300 °C, and dwell times ranged from 30 min to 5 h. Atomic force microscopy (AFM) was used to scan the surface morphology in each step. XRD was used to measure wide-angle diffraction patterns during each cycle process. HAADF-STEM experiments were performed using a Nion UltraSTEM200 operated at 200 kV. The microscope is equipped with a cold-field emission gun and a corrector of third- and fifth-order aberration for sub-Angstrom resolution. The convergence half-angle of 30 mrad was used, and the inner angle of the HAADF-STEM was ∼65 mrad. Resistivity was measured as a function of temperature with a Physical Properties Measurement System (PPMS) for a PdCrO2 film on a CuAlO2/c-Al2O3 substrate.
Figure 1(a) displays our approach to forming epitaxial CuAlO2 through solid-phase epitaxy. First, we discuss the two-step process in Fig. 1 by (1) depositing Cu2O by PLD and then (2) annealing at a high temperature (>1000 °C) to activate the solid-state interface reaction between Cu2O (film) and Al2O3 (substrate), depicted by the colors in blue and yellow, respectively. CuAlO2 begins to form during the solid-state reaction, as depicted by the green color. As the simple cartoon picture demonstrates, when undergoing a dwell time at a high temperature, all of the Cu2O (blue) eventually reacted with Al2O3 (yellow), forming CuAlO2 (green). AFM micrographs of a bare Al2O3, a PLD deposited Cu2O polycrystalline film (8 nm in thickness), and a CuAlO2 film after the solid-state reaction at 1100 °C for 1 h are also shown in Fig. 1(b). Remarkably, the surface of a CuAlO2 thin film is atomically flat with a well-defined step-and-terrace structure. The average step height is ∼5.26 Å, around one-third of the lattice parameter of the CuAlO2 unit cell stacked with three monolayers (c = 16.958 Å). This finding suggests that the surface roughness consists of a CuAlO2 monolayer. We further noted that the surface of CuAlO2 thin film becomes rough when a thicker Cu2O is used, which will be discussed later.
Figure 2(a) shows an as-grown film of 800 nm thick Cu2O with XRD peaks revealing the polycrystalline phase used to make the sample measured in Figs. 2(b)–2(e). Figure 2(b) displays the 0003L peaks confirming the CuAlO2 phase without the CuO2 phase prepared using the reaction temperature at 1100 °C with a dwell time of 1 h. The full width at half maximum (FWHM) of the rocking curve of the 0006 film peak in Fig. 2(c) represents the high crystallinity. The other reaction temperatures, shown in Fig. S1 in the supplementary material, show that at ∼1000 °C Cu2O films transformed to the CuAl2O4 phase, and above 1200 °C, the deterioration was confirmed by vanishing the film peaks. To add, Fig. S2 shows XRD scans of the phase evolution performed to find the optimal condition as a function of the dwell time at the annealing temperature of 1100 °C ranging from 30 min to 5 h. Dwell time greater than 3 h showed other impurity peaks with overall film quality deteriorated, confirmed by vanishing the film peaks at dwell times greater than 4 h. Rocking curves of the 0006 film peak were measured as shown in Fig. 2(c), and the corresponding FWHM was analyzed and shown in Fig. 2(d) for the samples annealed at 1100 °C with varying dwell times.
Under the optimum condition, CuAlO2 films are oriented along the c axis, indicating that the films are stabilized by exposing a (0001)-oriented surface or forming a (0001)-oriented interface. Figure 2(e) shows a HAADF-STEM image of a CuAlO2 thin film formed at the optimum annealing condition (T = 1100 °C and t = 1 h). Notably, a sharp interface is observed between the CuAlO2 film and c-Al2O3 substrate with atomic-scale reconstructions at the interface. Specifically, an extra AlOx [denoted by the yellow arrow in Fig. 2(e)] layer, different from the Al2O3 layers, was formed at the substrate surface. Then, AlO2 and Cu sublayers are stacked alternatively to constitute the delafossite phase; the first AlO2 sublayer of the film is denoted by the green arrow in Fig. 2(e). Such a characteristic atomic structure must lower the interfacial energy between the thin film and substrate, which may enable the formation of atomically flat CuAlO2 thin films oriented along the  direction.
A phase diagram is constructed in Fig. 3 for the solid-state reaction of 800 nm Cu2O polycrystalline films on c-Al2O3 substrates used under different temperatures and dwell times. The data points are from samples analyzed by XRD. Cu2O and CuO are shown in the phase diagram, as reported elsewhere22, demonstrated by in situ high-temperature XRD scans of a Cu2O film on Al2O3, which oxidizes into CuO at 300 °C. Based on the rocking curve FWHM of the 0006 peak of CuAlO2 thin films and the phase diagram, the optimal growth regime resides at 1000 < T < 1200 °C and 0.5 < t < 3 h.
Although XRD patterns of the thin films grown under the optimal conditions show single-phase CuAlO2, we find that its surface morphology highly depends on the thickness of the pre-deposited chemical reagent Cu2O. To understand how the thickness of a Cu2O film affects the interfacial surface chemistry, Fig. S3 displays AFM micrographs when varying the thickness of Cu2O films (note that due to the rough and grainy surface, it is hard to determine the film thickness precisely. Thus, we approximately estimate the nominal thickness to be around 8, 80, 130, and 270 nm). The AFM taken after the reaction from Fig. S3(a) was shown in Fig. 1(b), where the surface of CuAlO2 maintains flat step terraces. Due to the thickness limitation, we find that the thinner layer of Cu2O used resulted in undetectable film peaks in our XRD measurements. However, the surface morphology characterized by AFM showed flat step terraces. The difference in the number of pulses deposited by PLD plays a role in CuAlO2 film growth. As the Cu2O film thickness is increased to ∼80 nm, the steps become wavy, as shown in Fig. S4. We also note that shiny specks on the surface, which are not detected in XRD, are often observed in thick CuAlO2 thin films. We further note that a large portion of Cu2O films was re-evaporated without participating in the reaction on the surface; so, the final CuAlO2 thickness is much smaller. For about 8 nm Cu2O, we have observed the formation of only a 1–2 unit cell thick CuAlO2 film. The loss of Cu layers was also observed previously. Thus, this requires further study to better understand the underlying mechanism for the disappearance of Cu.10
The obtained CuAlO2 thin films with an atomically flat surface can be utilized as a template for the growth of metallic delafossite. A PdCrO2 thin film was grown by PLD on a templated CuAlO2 under the optimal growth condition for PdCrO2 (T = 650 °C, PO = 200 mTorr).10 CuAlO2 provides a small lattice mismatch of −2.4% and offers the structural similarity between PdCrO2 and CuAlO2. These conditions play a crucial role in stabilizing a good-quality PdCrO2 thin film. More importantly, the templated CuAlO2 layer enables the growth of very thick films (≈70 nm). We noted that such thick PdCrO2 films could not be grown previously because many impurities started to form above a certain thickness (∼30 nm). Thick films of delafossite would allow opportunities for various experiments, such as neutron scattering, magnetization, and specific heat, that can provide ways to deepen our understanding of delafossite thin films and heterostructures.
Figure 4(a) shows an XRD pattern for a 70 nm thick PdCrO2 thin film where only delafossite 0003L peaks are resolved. By comparing the FWHM of the rocking curve with our previous PdCrO2 grown on CuCrO2,10 we found that the quality of our film was poorer (Δω ∼1.4°). We attribute this to the intrinsic nature of the solid-phase epitaxy and the unavoidable exposure of our sample to air for thermal annealing in a furnace. To add, we observed thicker PdCrO2 grown on CuAlO2 resulted in an increase in FWHM. Temperature-dependent resistivity of the 70 nm thick PdCrO2 film showed a metallic behavior with a residual resistivity ratio, RRR ≈ 1.4, as shown in Fig. 4(b). The RRR value is comparable to PdCrO2 films grown on c-Al2O3 (18 nm),11 β-Ga2O3 (19 nm),11 and CuCrO2 buffer grown on c-Al2O3 (33 nm),10 while we admit that there is room to improve as compared to the bulk value.24 By taking the derivative of the resistivity curve as a function of the temperature and applying a Gaussian fit to the peak, we find the transition temperature TN ≈ 35 K, which corresponds to the antiferromagnetic transition in PdCrO2.25,26 Figure 4(c) shows a HAADF-STEM image of a PdCrO2 thin film grown on an ultrathin CuAlO2 template. Multiple AlOx layers (denoted by the green and yellow arrow) were observed below the CrO2 sublayer, consistent with a pristine CuAlO2 template film grown on c-Al2O3 [see Fig. 2(e)]. In HAADF-STEM, the scattering intensity is approximately proportional to the square of the atomic number. Therefore, Cu layers should be observed in HAADF-STEM images at the template layers, but there is no bright layer between the AlOx and CrO2 sublayers. In other words, there is no clear sign of Cu atomic columns at the template layers. We noted that the absence of Cu in the templated layer was also observed in previous results when using CuCrO2 as the buffer layer.10 This phenomenon is not yet fully understood, but we think that the Cu atoms most likely diffused into the film’s interior. Nevertheless, these findings suggest that the templated CuAlO2 plays a significant role in alleviating the lattice mismatch and structural dissimilarity, which may enable high-quality delafossite film growth.
We have used the solid-phase epitaxy approach to form single-phase CuAlO2 as a template layer for delafossite growth on a sapphire substrate. Since large-scale delafossite substrates are not available, the demonstrated formation of the CuAlO2 delafossite structure is of considerable topical interest for epitaxial growth of other metallic or magnetic delafossites. We have shown the optimal reaction temperature and dwell time to obtain single-phase CuAlO2 from our structural characterizations. Furthermore, we have successfully grown a 70 nm thick PdCrO2 thin film on this templated substrate, establishing a viable example for successfully synthesizing high-quality PdCrO2 films, which are otherwise difficult to grow epitaxially. We hope that this process will be a stepping-stone to drive further research in delafossite thin films and heterostructures.
See the supplementary material for additional XRD and AFM data of the samples studied.
This work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. J.M.O. acknowledges support in part for the data analysis from BrainLink program funded by the Ministry of Science and ICT through the National Research Foundation of Korea (Grant No. 2022H1D3A3A01077468) and National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (Grant No. 2022R1F1A1074425).
Conflict of Interest
The authors have no conflicts to disclose.
All authors contributed equally to this work.
Amanda Huon: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Jong Mok Ok: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Sangmoon Yoon: Formal analysis (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Andrew R. Lupini: Formal analysis (supporting); Writing – review & editing (equal). Ho Nyung Lee: Funding acquisition (lead); Project administration (lead); Resources (lead); Supervision (lead); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding author upon reasonable request.