Emergence of metallic surface states and negative differential conductance in thin $\beta$-FeSi$_2$ films on Si(001)

TL;DR

This study reveals metallic surface states and negative differential conductance in thin $eta$-FeSi$_2$ films on Si(001), combining experimental STM/STS data with DFT calculations to elucidate surface electronic properties.

Contribution

It provides detailed experimental and theoretical analysis of surface states in $eta$-FeSi$_2$ films, confirming metallic states and energy gaps through STM, STS, and DFT.

Findings

Metallic surface states confirmed by STS and DFT.

Fermi level pinned by surface state in the bulk band gap.

Negative differential conductance observed at 0.45 eV above Fermi level.

Abstract

The electronic properties of the surface of $\beta$-FeSi$_2$ have been debated for a long while. We studied the surface states of $\beta$-FeSi$_2$ films grown on Si(001) substrates using scanning tunnelling microscopy (STM) and spectroscopy (STS), with the aid of density functional theory (DFT) calculations. STM simulations using the surface model proposed by Romanyuk et al. [Phys. Rev. B 90, 155305 (2014)] reproduce the detailed features of experimental STM images. The result of STS showed metallic surface states in accordance with theoretical predictions. The Fermi level was pinned by a surface state that appeared in the bulk band gap of the $\beta$-FeSi$_2$ film, irrespective of the polarity of the substrate. We also observed negative differential conductance at $\sim$0.45 eV above the Fermi level in STS measurements performed at 4.5 K, reflecting the presence of an energy gap in the unoccupied surface states of $\beta$-FeSi$_2$.