Comparison of convergent beam electron diffraction and annular bright field atomic imaging for GaN polarity determinatio
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Roy H. Geiss Department of Chemistry, Colorado State University, Fort Collins, CO 80523
Igor Levin Materials Measurement Science Division, NIST, Gaithersburg, MD 20899 (Received 1 August 2016; accepted 8 November 2016)
A comparison of two electron microscopy techniques used to determine the polarity of GaN nanowires is presented. The techniques are convergent beam electron diffraction (CBED) in TEM mode and annular bright field (ABF) imaging in aberration corrected STEM mode. Both measurements were made at nominally the same locations on a variety of GaN nanowires. In all cases the two techniques gave the same polarity result. An important aspect of the study was the calibration of the CBED pattern rotation relative to the TEM image. Three different microscopes were used for CBED measurements. For all three instruments there was a substantial rotation of the diffraction pattern (120 or 180°) relative to the image, which, if unaccounted for, would have resulted in incorrect polarity determination. The study also shows that structural defects such as inversion domains can be readily identified by ABF imaging, but may escape identification by CBED. The relative advantages of the two techniques are discussed.
I. INTRODUCTION
Gallium nitride is of technological importance for a wide variety of optoelectronic applications, such as green light emitting diodes and solid state lighting. GaN nanowires (NWs) have the potential for even higher efficiencies than planar materials because of their lower defect densities. The GaN crystal structure of interest for applications is the hexagonal wurtzite phase, which is noncentro-symmetric and has either Ga or N polarity along the c-axis (see Fig. 1). The polarity causes internal electric fields and affects the electrical and optical properties as well as the material growth. Therefore, the ability to accurately determine polarity is critical for the understanding of GaN growth and for the development of GaN NWs for applications. A variety of methods have been used to determine the polarity of GaN samples, including KOH etching,1,2 resonant x-ray diffraction,3 coaxial impact collision ion scattering spectroscopy,2,4 and Kelvin probe5 or piezoresponse force6 microscopy. A disadvantage of all these methods is that they lack nanometer scale spatial resolution, which precludes them from being able to detect polarity on small scales. In addition, while these techniques work well for bulk and planar samples, they Contributing Editor: Thomas Walther a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.443
are difficult to implement on nanowire specimens due to the nanowire size and morphology. The small dimensions of the nanowires, specifically their narrow diameters with parallel side-facets, are, however, ideally suited for transmission electron microscopy (TEM) techniques. The primary method used to determine polarity of GaN nanowires in TEM has been convergent beam electron diffraction (CBED).3,7–13 CBED is a well-established technique for determining
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