Study of phase separation in an InGaN alloy by electron energy loss spectroscopy in an aberration corrected monochromate
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erre Ruterana CIMAP, UMR 6252, CNRS-ENSICAEN-CEA-UCBN, 14050 Caen, Cedex, France
Paolo Longo Gatan, Warrendale, PA 15086, USA
Toshihiro Aoki LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, AZ 85287, USA (Received 1 August 2016; accepted 8 November 2016)
Phase separation of InxGa1xN into Ga-rich and In-rich regions has been studied by electron energy-loss spectroscopy (EELS) in a monochromated, aberration corrected scanning transmission electron microscope (STEM). We analyze the full spectral information contained in EELS of InGaN, combining for the first time studies of high-energy and low-energy ionization edges, plasmon, and valence losses. Elemental maps of the N K, In M4,5 and Ga L2,3 edges recorded by spectrum imaging at 100 kV reveal sub-nm fluctuations of the local indium content. The low energetic edges of Ga M4,5 and In N4,5 partially overlap with the plasmon peaks. Both have been fitted iteratively to a linear superimposition of reference spectra for GaN, InN, and InGaN, providing a direct measurement of phase separation at the nm-scale. Bandgap measurements are limited in real space by scattering delocalization rather than the electron beam size to ;10 nm for small bandgaps, and their energetic accuracy by the method of fitting the onset of the joint density of states rather than energy resolution. For an In0.62Ga0.38N thin film we show that phase separation occurs on several length scales.
I. INTRODUCTION
Optoelectronic devices based on ternary InGaN alloys can cover a wide range of optical emission and absorption wavelengths, from near ultraviolet (GaN) to infrared (InN). However, InGaN growth is complex and a number of phenomena such as ordering1 crystallographic defects2 as well as phase separation3–6 may significantly influence the performance of light emitting devices (LEDs). Phase separation of InxGa1xN alloys into Ga-rich and In-rich regions was first predicted by Ho and Stringfellow7 and later observed by a number of research groups, particularly for InGaN samples of medium high In content grown at high temperatures. As the indium concentration controls the optical properties of InGaN, both wavelength and efficiency of optical emission, it is important to quantify any degree of phase separation in an InGaN thin film. Matsuoka et al.8 reported the growth of InGaN alloys by low temperature (500 °C) metalorganic Contributing Editor: Eric A. Stach a) Address all correspondence to this author. e-mail: t.walther@sheffield.ac.uk DOI: 10.1557/jmr.2016.447
chemical vapor deposition (MOCVD) with In concentration up to 42%, while other studies9–12 indicated that growth of InGaN thin films and InxGa1xN/InyGa1yN heterostructures by MOCVD at temperatures between 700 and 800 °C may maximally achieve x 5 30%. Previously, a reliable quantification of the degree of phase separation was only possible by Rutherford backscattering spectrometry (RBS). Due to the radiation sensitivity of InGaN to beam damage by 200 keV electrons, as observed by O’Neill et al.13 and later Smeeton et al.,14 high
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