Electron Microscope Study of Strain in InGaN Quantum Wells in GaN Nanowires *
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1184-HH01-08
Electron Microscope Study of Strain in InGaN Quantum Wells in GaN Nanowires*
R. H. Geiss, K. A. Bertness, A. Roshko, and D. T. Read National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, U.S.A. ABSTRACT Strains in GaN nanowires with InGaN quantum wells (QW) were measured from transmission electron microscope (TEM) images. The nanowires, all from a single growth run, are single crystals of the wurtzite structure that grow along the direction, and are approximately 1000 nm long and 60 nm to 130 nm wide with hexagonal cross-sections. The In concentration in the QWs ranges from 12 to 15 at %, as determined by energy dispersive spectroscopy in both the transmission and scanning electron microscopes. Fourier transform (FT) analyses of and < > lattice images of the QW region show a 4 to 10 % increase of the c-axis lattice spacing, across the full specimen width, and essentially no change in the a-axis value. The magnitude of the changes in the c-axis lattice spacing far exceeds values that would be expected by using a linear Vegard's law for GaN – InN with the measured In concentration. Therefore the increases are considered to represent tensile strains in the direction. Visual representations of the location and extent of the strained regions were produced by constructing inverse FT (IFT) images from selected regions in the FT covering the range of c-axis lattice parameters in and near the QW. The present strain values for InGaN QW in nanowires are larger than any found in the literature to date for other forms of InxGa1-xN (QW)/GaN. INTRODUCTION The interesting and technologically promising optoelectronic properties of InGaN quantum wells on GaN have been amply documented [1-3]. The role of strain in controlling the optoelectronic behavior of InGaN-GaN structures has also been well documented [4,5]. Here we report measurements of strain in and around InGaN quantum wells in GaN nanowires made by Bertness et al. [6]. This geometry is of interest both for its technological possibilities and for the insight it provides into the behavior of the InGaN-GaN material system. We describe and apply a novel computer-interactive approach to measuring and imaging local strains using high resolution TEM imaging, based on a Fourier transform (FT) technique. A number of FT methods for measuring strain by use of high resolution lattice imaging have been reported [7-10]. We suggest that our approach is much more physically intuitive and easy to use than the previously reported methods.
*Contribution of the U.S. National Institute of Standards and Technology. Not subject to copyright in the U.S.
PROCEDURES Experimental InxGa1-xN (QW)/GaN nanowires were grown on a (111) Si substrate with molecular beam epitaxy (MBE), with elemental In and Ga and an RF-plasma N2 source [6]. The substrate temperature was 820 ± 10 °C for the GaN nanowire and 530 ± 20 °C for the InxGa1-xN quantum wells (QW). The crystallographic structure of the nanowires is wurtzite, with a = 0.3189 nm and c = 0.51856 nm as determined
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