Structural peculiarities of $\varepsilon$-Fe$_2$O$_3$ / GaN epitaxial layers unveiled by high-resolution transmission electron microscopy and neutron reflectometry

TL;DR

This study investigates the structural features of epitaxial $ ext{ε}$-Fe$_2$O$_3$ films on GaN substrates using advanced microscopy and neutron reflectometry, revealing a low-density transition layer at the interface crucial for device applications.

Contribution

The paper provides the first detailed structural analysis of $ ext{ε}$-Fe$_2$O$_3$ / GaN interfaces, combining multiple high-resolution techniques to understand interface properties.

Findings

Identification of a nanometer-thick low-density transition layer at the interface.

Detailed characterization of the nanoscale columnar structure of $ ext{ε}$-Fe$_2$O$_3$ films.

Insights into the properties of the interface relevant for heterostructure device development.

Abstract

The present paper is dedicated to the structural study of crystallographic peculiarities appearing in epitaxial films of metastable epsilon iron oxide ($\varepsilon$-Fe$_2$O$_3$) grown by pulsed laser deposition onto a semiconductor GaN (0001) substrate. The columnar structure of the nanoscale $\varepsilon$-Fe$_2$O$_3$ films has been for the first time investigated using high resolution electron microscopy (HRTEM) direct space technique complemented by reciprocal space methods of high-energy electron diffraction and color-enhanced HRTEM image Fourier filtering. The study of $\varepsilon$-Fe$_2$O$_3$ / GaN interface formation has been further expanded by carrying out a depth resolved analysis of density and chemical composition by neutron reflectometry and energy-dispersive X-ray spectroscopy. The obtained results shed light onto the properties and the origin of the enigmatic few-nanometer thick low density transition layer residing at the $\varepsilon$-Fe$_2$O$_3$ / GaN interface. A detailed knowledge of the properties of this layer is believed to be highly important for the development of $\varepsilon$-Fe$_2$O$_3$ / GaN heterostructures that can potentially become part of the iron-oxide based ferroic-on-semiconductor devices with room temperature magneto-electric coupling.