Coherent Clustering of GdN in Epitaxial GaN:Gd Thin Film
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Coherent Clustering of GdN in Epitaxial GaN:Gd Thin Film 1
Mingjian Wu1 Steven C. Erwin2 and Achim Trampert1 Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin Germany 2 Center for Computational Materials Science, Naval Research Laboratory Washington, DC 20375, USA.
ABSTRACT We present an in-depth transmission electron microscopy (TEM) study about the character of the Gd atom distribution in epitaxial GaN:Gd thin films grown by molecular beam epitaxy. High-resolution TEM (HRTEM) imaging reveals local lattice distortions of dimensions of a few atom planes only. Geometric phase analysis of HRTEM lattice images quantifies the associated displacement field. The results are explained by means of thin coherently strained GdN clusters with platelet shape being located along the basal plane. This is consistent with the observations obtained from strain contrast dark-field TEM images. Theoretically derived structure models provided by calculations based on density functional theory are used to simulate the HRTEM contrast and to determine the corresponding displacement field for matching the experimental data. Best fit is achieved in case of a coherent GdN bi-layer cluster that conclusively reflects the energy favorable configuration. The formation of the platelet clusters is explainable in the framework of spinodal decomposition. INTRODUCTION The detection of initial stages during phase separation in GaN materials doped by rare earth (RE) elements, especially in case of low concentration, is very challenging. It has been theoretically predicted that GaN:RE have a huge miscibility gap (much less than 1 at.% dissolved in equilibrium at 1000°C [1]) meaning that the thermodynamic trend of phase separation could proceed by means of spinodal decomposition. The information about the character of the distribution of RE atoms is of great importance to understand the properties of phase stability and transition. Due to the low doping concentration, which is below the detection limit of most of the microscopic or spectroscopic methods, there are no convincing data about concentration fluctuations or cluster formation. In this paper, we present in-depth transmission electron microscopy (TEM) study of GaN:Gd samples doped with low Gd concentrations. The occurrence of small coherent platelets in the sample is demonstrated unambiguously. Comparing the displacement field mapping from the experimental image and that from various possible simulated structure models, we are able to attribute the lattice distortion to the coherent GdN cluster of bilayer thickness. EXPERIMENT The samples under investigation were grown by molecular beam epitaxy (MBE) along the GaN [0001] direction on either GaN(0001)/Al2O3 templates or SiC(0001) substrates, respectively. The GaN epilayer were intentionally doped with Gd atoms of concentrations between 5×1016 cm-3 and 3×1019 cm-3 as determined by secondary ion mass spectrometry [2]. Cross-sectional TEM samples were prepared by mechanical lapping and dimpling, followed by
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