Raman Signal Reveals the Rhombohedral Crystallographic Structure in Ultra-thin Layers of Bismuth Thermally Evaporated on Amorphous Substrate

TL;DR

This study shows that ultra-thin bismuth layers grown on amorphous substrates retain their rhombohedral structure as revealed by Raman spectroscopy, even at thicknesses as low as 5 nm, with minimal surface oxidation.

Contribution

It demonstrates that Raman spectroscopy can detect the rhombohedral structure in ultra-thin Bi layers, challenging previous assumptions about structural changes at reduced thicknesses.

Findings

Bi is crystalline and oriented in the (111) direction for layers over 20 nm

Rhombohedral structure persists in layers as thin as 5 nm

Minimal surface oxidation occurs under ambient conditions

Abstract

Under the challenge of growing a single bilayer of Bi oriented in the (111) crystallographic direction over amorphous substrates, we have studied different thicknesses of Bi thermally evaporated onto silicon oxide in order to shed light on the dominant atomic structures and their oxidation. We have employed atomic force microscope, X-ray diffraction, and scanning electron microscope approaches to demonstrate that Bi is crystalline and oriented in the (111) direction for thicknesses over 20 nm. Surprisingly, Raman spectroscopy indicates that the rhombohedral structure is preserved even for ultra-thin layers of Bi, down to $\sim 5$ nm. Moreover, the signals also reveal that bismuth films exposed to ambient conditions do not suffer major surface oxidation.