Synthesis of Superconducting Infinite-Layer Nickelate Thin Films by Aluminium Sputtering Deposition

After decades of research, a cuprate analog displaying superconductivity was finally found within the nickelate family. In 2019, H. Hwang’s group reported superconductivity in thin films of hole-doped infinite-layer (IL) nickelates (Nd_1-xSr_xNiO₂, x=dopant concentration) with a critical temperature T_C ≈10-15 K [1]. This remarkable discovery has then sparked an entire new realm of research, raising several outstanding questions regarding whether nickelates are true analogs of cuprates or belong to a distinct family of unconventional superconductors.
However, progress in this field is slowed by significant challenges in materials synthesis and the limited number of research groups capable of producing high quality superconducting samples. The primary difficulty lies in the intricate topotactic reduction process required to selectively remove all the apical oxygens from the initial parent perovskite phase to achieve the superconducing infinite-layer phase. The reduction is typically performed through an ex situ complex chemical process using CaH₂ as the reducing agent. This method not only suffers from reproducibility issues but also often results in samples with poor surface crystallinity, which hinders the use of surface-sensitive techniques. Recently, two different in situ reduction methods have been proposed that improve this aspect: i) by depositing a metal overlayer using molecular beam epitaxy technique [2,3] and ii) by atomic hydrogen bombardment [4]. However, these techniques remain limited in accessibility for many research groups, emphasizing the need for realiable and simpler methods to facilitate the synthesis of superconducting infinite-layer nickelates.
In this work, we demonstrate a simple alternative route to synthesize high quality superconducting infinite-layer Pr_0.8Sr_0.2NiO₂ (PSNO₂) thin films. Using a more accesible technique such as direct current (DC) magnetron sputtering, we deposit a thin aluminum metal overlayer that pumps the apical oxygen atoms from the perovskite thin films via an efficient redox reaction, resulting in the complete transformation of the nickelates thin films into the superconducting IL phase. We present the systematic optimization of the aluminum deposition parameters and compare the superconducting properties of samples reduced through in situ Al deposition with those exposed to air prior to Al reduction (ex situ). By using this method, we obtain high quality superconducting infinite-layer PSNO₂ thin films with a maximum superconductivity transition T_C (onset) of 17 K, consistent with the optimum value reported for this compound. This simple synthesis route, much more accessible than existing methods and compatible with surface-sensitive techniques, offers better control and reproducibility over the topotactic transformation, opening new opportunities to gain insights into the physics of superconductivity in nickelates.