Highly conductive PdCoO₂ ultrathin films for transparent electrodes

A palladium-based delafossite metal, PdCoO₂, is one of the most conductive oxides characterized by a long mean free path,^1,2 which gives rise to hydrodynamic electron flow in ultrapure single crystals.³ The crystal structure of PdCoO₂, shown in Fig. 1(a), is a natural superlattice of highly conductive two-dimensional sheets of Pd atoms and triangular lattices of CoO₂ octahedra.⁴ The Pd ions are weakly bound with oxygens to form O–Pd–O chains. This unique anisotropic crystal structure hosts remarkably high electrical conductivity with the bulk in-plane resistivity ∼2.6 μΩ cm at 300 K, which is even lower than that of Pd metal and comparable to that of Au.⁵ Transport and optical measurements on bulk single crystals^5,6 as well as theoretical studies^7–10 have shown that the two-dimensional Pd¹⁺ layers are responsible for the high in-plane conductivity of PdCoO₂. Because of its high electrical conductivity, PdCoO₂ will be applicable to transparent electrodes when stabilized in an ultrathin film form to allow light to pass through, increasing interest in the thin-film growth of PdCoO₂. However, research on PdCoO₂ has been limited to bulk crystals^{3–6,11–17} and thick films prepared by post-annealing of amorphous precursors.¹⁸ To date, ultrathin film growth of PdCoO₂ remains unexplored in the literature.

Here, we demonstrate the thin-film growth of highly conductive PdCoO₂ on Al₂O₃ (0001) substrates. Figure 1(a) shows the crystal structure of PdCoO₂, consisting of three sets of the Pd and CoO₂ layers stacked alternately along the c-axis. The top views of the Pd and CoO₂ layers are illustrated in Figs. 1(b) and 1(c). The Pd and CoO₂ layers form triangular lattices with the in-plane lattice constants of a = b = 2.83 Å.⁴ The oxygen atoms in PdCoO₂, shown as red spheres in Fig. 1(c), form a triangular lattice with an inter-oxygen distance of 2.83 Å that equals to the in-plane lattice constants. This inter-oxygen distance in PdCoO₂ is close to that in Al₂O₃ (2.75 Å on average), suggesting Al₂O₃ (0001) an appropriate substrate for the growth of PdCoO₂ thin films. A consideration of the epitaxial relationship between PdCoO₂ and Al₂O₃ suggests two possible configurations, PdCoO₂ (A) and PdCoO₂ (B), as shown in Fig. 1(d): the orientation of oxygen triangles on the PdCoO₂ surface is in-plane upward in the domain A and is rotated by 180° in the domain B. Although the crystal structure is different between delafossite PdCoO₂ and corundum Al₂O₃, the close similarities in the triangular lattice motif of surface oxygens enable us to synthesize highly crystalline PdCoO₂ thin films.

We have prepared PdCoO₂ thin films by pulsed-laser deposition (PLD). Commercially available Al₂O₃ (0001) substrates (SHINKOSHA CO., LTD) were ultrasonically cleaned with acetone and ethanol, followed by annealing in air at 900 °C for 12 h to obtain an atomically flat surface with step-and-terrace structures. The annealed substrate was loaded into a vacuum chamber and preheated at T = 700 °C for 10 min under an oxygen partial pressure of $PO2$ = 100 mTorr. A stoichiometric PdCoO₂ target (KOJUNDO CHEMICAL LABORATORY Co., LTD) and a Pd–PdO mixed-phase target, prepared by sintering pelletized PdO powder at 1000 °C for 24 h in air, were ablated alternately by using a KrF excimer laser with a laser fluence of 2 J/cm² under the growth conditions of T = 700 °C and $PO2$ = 100 mTorr. We repeated an ablation sequence of 140 pulses at a laser repetition rate of 5 Hz for PdCoO₂ and 300 pulses at 15 Hz for Pd–PdO targets. This single cycle resulted in 0.2 nm deposition on average. We repeated the cycle to fabricate the thin films with desired thickness. The samples were cooled down to room temperature in roughly 10 min immediately after the growth. The cation composition in the films was evaluated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy-dispersive X-ray spectroscopy (EDX) using the calibration line determined by the ICP-AES of a thick (d ∼ 200 nm) sample. We note that thin films prepared with only a PdCoO₂ target suffer from significant Pd deficiencies with typical composition ratios of Pd/Co ∼ 0.6. Employing the alternate deposition with the Pd–PdO target was found to effectively improve the stoichiometry as Pd/Co = 0.9 ± 0.1. The surface morphology was measured by atomic force microscopy (AFM). Crystal structures of the PdCoO₂ thin films were characterized by X-ray diffraction (XRD). The thickness d of the samples was determined by the thickness fringes observed around the PdCoO₂ (0006) diffraction peaks. The high-resolution transmission electron microscope (HRTEM) images were collected with a JEOL EM-002B. For the electrical transport measurement, indium electrodes were mechanically soldered on the samples. The temperature dependence of the sample resistance was measured by a four-terminal method using a Quantum Design, a physical property measurement system (PPMS). The transmittance spectra were measured with a Shimadzu UV-3600 plus equipped with an integrating sphere MPC-3100.

The XRD patterns for the PdCoO₂ thin films with different thicknesses are shown in Fig. 2(a). All the film peaks are assigned to PdCoO₂ (0003n), indicating that c-axis oriented PdCoO₂ thin films are grown without any traces of impurity phases. The widths of the PdCoO₂ (0003n) peaks in the 2θ-ω scans become larger with decreasing PdCoO₂ thickness, as expected from the Laue function. In addition, clear interference thickness fringes appear around the main diffraction peaks for all the samples. We determined the film thickness denoted as the numbers in Fig. 2(a) using the periodicity of the fringes near PdCoO₂ (0006) peaks. To identify the epitaxial relationship of PdCoO₂ and Al₂O₃, we measured the XRD ϕ-scans around the PdCoO₂ (01$1¯$2) and Al₂O₃ (01$1¯$2) diffraction peaks. As shown in Fig. 2(b), the ϕ-scan diffraction patterns have a 6-fold symmetry for PdCoO₂ and a 3-fold symmetry for Al₂O₃. The peak positions in the ϕ-scans of the PdCoO₂ (01$1¯$2) are shifted by Δϕ = 30° from those of Al₂O₃ (01$1¯$2), indicating the epitaxial relationship with the same oxygen triangular configuration in Fig. 1(d), where the lattice unit of PdCoO₂ and Al₂O₃ is 30° rotated from each other. The PdCoO₂ thin films showed the 6-fold ϕ-scan diffraction pattern, despite the 3-fold symmetric crystal structure of delafossite PdCoO₂. It is thus reasonable to consider that the PdCoO₂ thin film has two domains with the in-plane orientation rotated by Δϕ = 180°. This is supported by the AFM image of the PdCoO₂ surface shown in Fig. 2(c), which detects triangular shapes aligned to two directions with one of the bases parallel to the $[11¯00]Al2O3$ direction. These triangular domains are consistent with the two kinds of crystalline orientations found in the XRD ϕ-scan and domains A and B in Fig. 1(d).

The lattice periodicity in the PdCoO₂ thin film was resolved by HRTEM. As shown in Fig. 2(d), the HRTEM image of a PdCoO₂ thin film shows periodic bright lines stacked along the PdCoO₂ [0001] direction, corresponding to the layered crystal structure of the PdCoO₂ thin film. The period of the bright lines is about 0.59 nm, which agrees well with the 1/3 unit cell height, i.e., a single stack of the Pd layer and CoO₂ octahedron. This periodicity is clearly observed in the diffraction pattern of PdCoO₂/Al₂O₃, shown in the inset of Fig. 2(d). Clear PdCoO₂ (0003n) bright spots, indicated by white arrows, are observed together with those for the Al₂O₃ substrate indicated by blue arrows. We note that the Al₂O₃ substrate showed normally forbidden (0003) and (000$3¯$⁠) spots, which might be due to the strained crystal structure with off-stoichiometric oxygen composition¹⁹ caused by annealing in the growth process. The periodic HRTEM image evidences that the atomic layers in PdCoO₂ are regularly ordered along the PdCoO₂ [0001] direction.

The thickness dependence of the room-temperature sheet conductance (1/R_s^300K) is plotted in Fig. 3(a). The 1/R_s^300K linearly increases above the thickness larger than d = 2 nm, indicating that each PdCoO₂ layer possesses the homogeneous sheet conductance. The room-temperature sheet conductance per Pd sheet, deduced from the slope of the fitting line in Fig. 3(a), is as high as 5.5 mS, which is comparable to that of the doped graphene (∼8 mS).²⁰ The existence of a roughly 1-unit-cell dead layer implies that the initial growth unit seems a whole unit cell of PdCoO₂ composed of three sequences of the Pd sheet/CoO₂ layers, rather than the 1/3 unit cell, i.e., a single sequence of the Pd sheet/CoO₂ stack. The formation of such a whole single unit cell might be a key requirement to stabilizing the structure and producing high sheet conductance. After the formation of the 1-unit-cell initial layer, the growth unit seems to become a 1/3 unit cell as evidenced by steps with the 1/3-unit-cell height in the cross-sectional height profile in Fig. 2(c). We note that a 1-unit-cell-thick dead layer should exist also in the thicker films (d > 2 nm), as indicated by the linear 1/R_s vs d dependence shifted to the positive direction in the d axis. Extrinsic scattering origins such as surface roughness and interfacial effects could be responsible for the persisting dead layer.

The temperature dependence of the sheet resistance (R_s-T curve) for d = 2.1–8.8 nm is displayed in Fig. 3(b). All the samples show a monotonic decrease of R_s upon cooling down to T ∼ 40 K with small upturns at low temperature. In comparison with polycrystalline and single-crystalline bulk studies, the residual resistance ratio (RRR) of about 2 in our films is closer to that in the polycrystalline bulk of about 4,²¹ rather than a high purity single-crystal of 400.¹⁴ The plausible origins for such upturns are either impurity in the thin films or carrier scattering at the domain boundaries. Further improvement of crystallinity in PdCoO₂ thin films will lead to an increase in 1/R_s^300K per Pd sheet toward the bulk corresponding value of ∼23 mS,¹ by reducing the effect of impurities and the domain boundaries.¹⁴ 

The optical transmittance spectra of the PdCoO₂/Al₂O₃ in Fig. 4 represent the specific features of the band structure and high optical transparency. The overall spectral shape is identical for all the thicknesses, representing high transparency in a near-infrared (NIR) region at around 1 eV. The transmittance spectra have three characteristic dips around 1.2, 2.8, and 5.3 eV (red arrows), positions of which are independent of the PdCoO₂ film thickness. As discussed for noble metals²² and correlated oxides,²³ inter-band electronic transition between a flat filled band at some different k-points and an empty band just above the Fermi surface might be the origin of the observed dip structures. The dip positions in the transmittance spectra in Fig. 4 are consistent with the flat band positions in k-space calculated by density functional theory, which is located lower than the Fermi level by E₁ ∼ 0.8-1.5; E₂ ∼ 2.3-3.0; E₃ ∼ 5.6-5.8 eV.¹⁰ The PdCoO₂ ultrathin films possess high optical transparency as shown in the inset photograph of Fig. 4. To evaluate the transmittance of the PdCoO₂ films T_film for three energy regions (three color lines in Fig. 4), the optical loss in the Al₂O₃ is subtracted as T_film = T_film+sub/T_sub. The thickness dependence of the T_film at the selected wavelengths in near-infrared (NIR), visible (Vis), and ultraviolet (UV) regions is plotted in the left inset of Fig. 4. The T_film has an approximately linear dependence on the PdCoO₂ thin film d. In particular, at the energy of 0.8 eV in the NIR region, the T_film is as high as 80% even for the thick PdCoO₂ thin films (d = 8.8 nm), demonstrating the superior optical factor as a NIR transparent conductor.

In summary, PdCoO₂ ultrathin films are successfully synthesized on Al₂O₃ (0001) substrates by PLD. High sheet conductance is obtained above d = 2.1 nm, the thickness of which is close to the c-axis lattice unit. The PdCoO₂ thin films have two domains, depending on the configurations of triangular CoO₂ lattices on an Al₂O₃ (0001) surface. Ultrathin PdCoO₂ possesses high optical transparency, particularly in the NIR region, while keeping low sheet resistance ∼100 Ω. Further attempts to reduce the low-temperature residual resistance by suppressing impurities and domain boundaries will broaden the applicable research field of PdCoO₂ thin films, for example, mesoscopic systems and heterostructures.

The authors thank Dr. S. Kuboya and Professor T. Matsuoka for the help in optical transmittance measurement, Mr. S. Ito for the HRTEM measurement, and Mr. F. Sakamoto for ICP-AES analysis. This work is a cooperative program (Proposal No. 16G0404) of the CRDAM-IMR, Tohoku University. This work is partly supported by a Grant-in-Aid for Specially Promoted Research (No. 25000003), a Grant-in-Aid for Scientific Research (A) (No. 15H02022) from the Japan Society for the Promotion of Science (JSPS), and the Mayekawa Houonkai Foundation.