Strain-Free Growth of Single-Crystal Ag Thin Films via Monolayer-Thick Interface Formation

In heteroepitaxy for metal films, energetic and geometric constraints associated with the surface, interface, and strain energy have restricted the achievable size of a single crystal grain in thin film geometry. Extensive research has aimed to address this issue toward the defect- and strain-free heteroepitaxy to a wafer scale. Despite the significant progress, the occurrence of structural defects inside the growing film has never been completely avoided due to the presence of slight residual strain, and this imperfection usually results in undulated film surface. Furthermore, the allowable range of lattice mismatch in the previous techniques for high-quality heteroepitaxy is usually limited by less than 4%. Since a metal film growing on a typical substrate can usually be subjected to more than 10% misfit strain, metal film's defect- and strain-free growth in the large misfit regime is challenging with current epitaxial techniques. To break down the limiting barrier toward the epitaxial growth of metal film, we used our atomic sputtering epitaxy (ASE) technique that allows depositing atoms on a substrate with single-atom precision (Fig. 1a) [1]. With this, we grew a wafer-scale ultra-flat Cu(111) film on Al2O3(0001) substrate only with monoatomic step edges. The Ag thin films were subsequently grown on the Cu(111) buffer layer via the same sputtering condition. In the Ag(111)/Cu(111) heteroepitaxy, the lattice misfit between the two materials is 13%, which seems far beyond the viable strain range for the epitaxial growth. However, we found that once the template buffer layer can be grown with the atomically flat surface, the condition of extended atomic distance mismatch (EADM) between the two materials can be considered an indicator of epitaxial growth feasibility. The value of EADM for Ag(111)/Cu(111) combination is only estimated as low as ∼0.3%, probably a small misfit enough to realize the epitaxial growth of Ag film. Indeed, we have succeeded in growing a defect-free Ag single crystalline film with an atomically flat surface to a wafer scale. Remarkably, atomic resolution scanning transmission microscopy revealed that the misfit strain is fully accommodated just in a monolayer-thick Ag interface layer, acting as a perfect strain absorber by regulating in-plane displacements of Ag atoms, and the subsequent Ag lattice starting from the second layer is recovered to values of the single crystal counterpart (Fig. 1b and c). The unveiled growth mechanism provides an important implication for scaling up metal thin film heteroepitaxy without altering structural properties. Owing to the strain-free growth, the Ag films were revealed to be atomically flat with occasional monoatomic steps, which would be ideal for a material platform for plasmonics or optical reflector because the surface flatness is a crucial factor required for exploiting the intrinsic optical properties [2]. Atomic distance analysis for the abrupt interfacial structure of the Ag/Cu heterostructure. a, Cross-sectional ADF-STEM image of the Ag/Cu heterointerface with the orientation relation of (111)Ag[-110]Ag//(111)Cu[1-10]Cu. b, Profile of projected atomic distances (PADs) in each atomic row measured across the interface. The PAD values of the Ag (dAg-Ag) and Cu (dCu-Cu) atoms in the bulk structures are 2.51 and 2.23 Å, respectively. c, Histograms of the measured PADs for comparison between each material contacted layer and bulk layers. Periodic in-plane atomic displacements for strain accommodation on Ag first layer. a, The experimental and simulated STEM images for the interfacial structure which contains Ag atomic displacements. b, Comparison of the intensity profile between simulated and experimental Ag atomic displacement (yellow ovals on a). c, Atomic-scale experimental abrupt interfacial STEM image with periodic Ag displacements on the Ag first layer. Overlaps of Ag and Cu atomic sites determined by 2D Gaussian fits.