Self-assembly of ordered wurtzite/rock salt heterostructures—A new view on phase separation in MgxZn1−xO

Abstract

This version is available at https://strathprints.strath.ac.uk/54019/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output. Transmission electron microscopy reveals that these effects are caused by phase separation leading to the formation of a vertically ordered rock salt/wurtzite heterostructures. To explain these observations we suggest a phase separation epitaxy model that considers this process being initiated by the formation of a cubic (Mg,Zn)Al 2 O 4 spinel layer at the interface to the sapphire substrate, acting as a planar seed for the epitaxial precipitation of rock salt-type Mg x Zn 1-x O. The equilibrium fraction x of magnesium in the resulting wurtzite (rock salt) layers is approximately 0.15 (0.85), independent of the MgO content of the as-grown layer and determined by the annealing temperature. This model is confirmed by photoluminescence analysis of the resulting layer systems after different annealing temperatures. In addition we show that the thermal annealing process 2 results in a significant reduction in the density of edge-and screw-type dislocations, providing the possibility to fabricate high quality templates for quasi-homoepitaxial growth.3

Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

“…In conclusion, we have shown with a spectroscopic high-pressure approach that phase segregation can be probed in as grown thin films of phase separated Mg 0.3 Zn 0.7 O even for small embedded volumes not detected by x-ray diffraction and not accessible by transmission electron microscopy except with annealed samples [14]. We have solved the controversy about the lowenergy absorption tail usually observed overlapping with the main absorption edge for x > 0.3.…”

“…1 (c)]. According to Gries et al [14] the presence of phase separation in Mg x Zn 1−x O would give rise to the appearance of segregated wurtzite phase with less Mg content. This would result into the appearance of a second PL peak at lower energies.…”

“…ZnO crystallizes in the hexagonal wurtzitetype structure while MgO crystallizes in the cubic rocksalt-type structure. Therefore, once the solubility limit is reached in wurtzite-type Mg x Zn 1−x O solid solution, ∼ 4% in bulk [12] and ∼ 30% or ∼ 50% in thin films depending on the growth method [8,13], phase separation appears [14] and both rock-salt and wurtzite type phases coexist. A lot of effort has been put on trying to reach the highest incorporation limit of Mg 2+ in phase pure wurzite-type Mg x Zn 1−x O.…”

“…Therefore, the origin of the observed low-energy absorption tail remains unclear. The study of Gries et al [14] has success- * Electronic address: javier.ruiz-fuertes@uv.es fully employed transmission electron microscopy (TEM) on thermally annealed Mg 0.3 Zn 0.7 O thin films grown by molecular beam epitaxy (MBE) to investigate the microscopic effect of having phase separation. They found that the coexistence of the wurtzite and rock-salt phases in Mg x Zn 1−x O due to phase separation gives rise to the existence of an secondary wurtzite-type phase, with a reduced Mg content of x ≈ 0.15 determined by TEM energy dispersive x-ray spectroscopy (EDX).…”

“…The volume of this segretated wutzite phase was too low to be detected by XRD and it was embedded in the thin film preventing selective access with TEM. For this reason, Gries et al [14] had to employ a buffer layer of MgO/ZnO to promote the growth of a segregated wurtzite phase in the Mg x Zn 1−x O thin film, after annealing the sample at 950…”