Reststrahlen band and optical bandgaps in semiconducting CrN films

TL;DR

This study provides a detailed optical characterization of CrN(111) films, revealing their bandgaps, Reststrahlen band, and ionic nature through spectroscopic ellipsometry across a broad spectral range.

Contribution

It offers the first comprehensive optical constants and phonon-related properties of CrN films grown on different substrates, enhancing understanding of their electronic and vibrational characteristics.

Findings

Identified interband transitions at ~0.35 and 0.60 eV.

Detected a Reststrahlen band at ~403 and 629 cm$^{-1}$.

Estimated static permittivity of about 39.

Abstract

We present a comprehensive optical characterization of 200-nm-thick CrN(111) films grown simultaneously on Al$_2$O$_3$(0001) and AlN/Al$_2$O$_3$(0001) using plasma-assisted molecular beam epitaxy. Spectroscopic ellipsometry, spanning the far-infrared to ultraviolet range (0.04 - 5.5 eV), is conducted at room temperature to determine the optical constants $n$ and $k$ of the films. Spectral fits reveal two interband transitions at approximately 0.35 and 0.60 eV. In the infrared range, the ellipsometry data also reveals a pronounced Reststrahlen band stemming from transversal and longitudinal optical phonons at approximately 403 and 629 cm$^{-1}$, respectively. The relative static and high-frequency permittivities are estimated to be about 39 and 15, respectively. A Born effective charge of approximately 2.7, extracted from the far-infrared region, indicates that CrN is partially ionic.