Atomistic understanding of two-dimensional monatomic phase-change material for non-volatile optical applications

TL;DR

This study provides an atomistic understanding of how reducing the thickness of antimony thin films affects their optical properties, revealing a 2 nm thickness limit for effective photonic applications and demonstrating reversible optical switching.

Contribution

It offers new insights into the thickness-dependent optical responses of Sb thin films and establishes a fundamental thickness limit for photonic device integration.

Findings

Optical contrast decreases with film thickness, especially below 2 nm.

A 2 nm thickness limit is identified for effective optical applications.

Reversible optical switching is demonstrated in 2 nm Sb films.

Abstract

Elemental antimony (Sb) is a promising material for phase-change memory, neuromorphic computing and nanophotonic applications, because its compositional simplicity can prevent phase segregation upon extensive programming. Scaling down the film thickness is a necessary step to prolong the lifetime of amorphous Sb, but the optical properties of Sb are also significantly altered as the thickness is reduced to a few nanometers, adding complexity to device optimization. In this work, we aim to provide atomistic understanding of the thickness-dependent optical responses in Sb thin films. As thickness decreases, both the extinction coefficient and optical contrast reduce in the near-infrared spectrum, consistent with previous optical measurements. Such thickness dependence gives rise to a bottom thickness limit of 2 nm in photonic applications, as predicted by coarse-grained device simulations. Further bonding analysis reveals a fundamentally different behavior for amorphous and crystalline Sb upon downscaling, resulting in the reduction of optical contrast. Thin film experiments are also carried out to validate our predictions. The thickness-dependent optical trend is fully demonstrated by our ellipsometric spectroscopy experiments, and the bottom thickness limit of 2 nm is confirmed by structural characterization experiments. Finally, we show that the greatly improved amorphous-phase stability of the 2 nm Sb thin film enables robust and reversible optical switching in a silicon-based waveguide device.