# Atomistic Understanding of 2D Monatomic Phase‐Change Material for Non‐Volatile Optical Applications

## TL;DR

This paper explores how reducing the thickness of antimony films affects their optical properties, finding a practical limit of 2 nm for photonic applications.

## Contribution

The study provides an atomistic understanding of thickness-dependent optical responses in antimony thin films for photonic applications.

## Key findings

- As thickness decreases, extinction coefficient and optical contrast reduce in the near-infrared spectrum.
- A practical thickness limit of 2 nm is established for antimony thin films in photonic applications.
- Reduced optical contrast results from different behaviors of amorphous and crystalline antimony upon downscaling.

## Abstract

Elemental antimony (Sb) is a promising material for phase‐change memory, neuromorphic computing, and nanophotonic applications, because its compositional simplicity prevents phase segregation upon extensive programming. Scaling down the film thickness is a necessary step to prolong the amorphous‐state lifetime. However, the optical properties of Sb are significantly altered as the thickness is reduced to a few nanometers, adding complexity to device optimization. In this work, an atomistic understanding of the thickness‐dependent optical responses is provided in Sb thin films. As thickness decreases, both the extinction coefficient and optical contrast are reduced in the near‐infrared spectrum, consistent with previous optical measurements. Such thickness dependence establishes a practical thickness limit of 2 nm, as predicted by coarse‐grained device simulations. 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 demonstrated by ellipsometric spectroscopy experiments, and the bottom thickness limit of 2 nm is confirmed by structural characterization experiments. Finally, it is shown 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.

Antimony is a promising monatomic phase‐change material. Scaling down the film thickness is necessary to prolong the amorphous‐state lifetime, but it alters the optical properties. The combined computational and experimental study shows that, as thickness decreases, the extinction coefficient and optical contrast are reduced in the near‐infrared spectrum, establishing a practical thickness limit of 2 nm for photonic applications.