Epitaxial Aluminum Nanofilms for Next-Generation Quantum Technologies: A Materials-to-Applications Survey from Taiwan

Abstract

Aluminum (Al) is the most critical material in the burgeoning field of superconducting quantum computing due to its self-limiting native oxide and reliable Josephson effect. However, material-induced decoherence remains a major bottleneck currently limits the scalability of quantum processors. As highlighted in seminal roadmaps, dielectric loss from amorphous interfaces and structural defects in standard evaporated Al films are the primary sources of noise. While current international efforts focus on engineering-level mitigation (e.g., circuit design), this review proposes a materials-level solution through the lens of research conducted in Taiwan. We will focus on the paradigm shift enabled by molecular beam epitaxy (MBE) growth of ultrahigh-quality Al nanofilms. Unlike traditional “dirty-limit” superconductors where transition temperature (Tc) increases are linked to disorder, researchers in Taiwan have demonstrated a doubling of Tc (reaching ≈ 2.4 K) in the ultraclean limit. We will discuss the underlying physics, such as surface phonon softening and quantum size effects, and why these clean-limit films are superior for practical applications. Notably, in situ Al2O3 passivation ensures long-term environmental stability, enabling MBE-grown-Al-based resonators to maintain internal quality factors exceeding 106 even after extended air exposure. Current literature frequently reviews atomic layer deposition (ALD) or disordered “granular aluminum” for high kinetic inductance. However, there is a significant gap in surveying epitaxial, wafer-scale Al for applied devices. Our review aims to provide a balanced global survey, showcasing how the synergy between MBE expertise and low-temperature physics in Taiwan provides a viable technological pathway to address the materials challenges of the quantum era.