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

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.