Absence of Lateral Composition Fluctuations in Aberration-corrected STEM Images of an InGaN Quantum Well at Low Dose
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Absence of Lateral Composition Fluctuations in Aberration-corrected STEM Images of an InGaN Quantum Well at Low Dose Andrew B. Yankovich1, A. V. Kvit1, X. Li2, F. Zhang2, V. Avrutin2, H.Y. Liu2, N. Izyumskaya2, Ü. Özgür2, H. Morkoç2, and P. M. Voyles1 1 Department of Materials Science and Engineering, University of Wisconsin - Madison, Madison, WI 53706 U.S.A. 2 Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284 U.S.A. ABSTRACT By using aberration corrected scanning transmission electron microscopy we have found no small scale lateral In composition fluctuations exist in the In0.15Ga0.85N active region of a light emitting diode. Images were acquired at 2% of the electron dose known to create electron beam damage, so the acquired images reflect the intrinsic structure of the InGaN active region. Position averaged convergent beam electron diffraction reveals the local sample thickness where images were acquired is 4.8 nm, eliminating the possibility that the absence of composition variation was observed due to projection through a thick sample. In addition, 2-3 atomic layer steps were observed in the top surface of In0.08Ga0.92N layers and the In0.15Ga0.85N active layers, providing a possible mechanism for lateral carrier confinement. INTRODUCTION The development of III – nitride semiconductor devices is partially motivated by the desire for high power, short wavelength, and high temperature optoelectronic devices.1,2 InxGa1xN based light emitting diodes (LEDs) have extraordinary internal quantum efficiencies above 80% despite the high density of threading dislocations originating from the sapphire substrate GaN interface. Threading dislocation densities of 107-108 cm-2 are typical in state of the art devices despite extensive efforts to reduce them.3 The high densities of dislocations should quench luminescence completely if those dislocations were hosts of effective nonradiative recombination centers. One proposed solution to this discrepancy is the presence of small-scale lateral In composition fluctuations within the high In concentration active regions of the LED devices that could confine carriers away from threading dislocations and prevent nonradiative recombination.4–7 These In composition fluctuations in the plane of the active region could be created due to a possible miscibility gap in the GaN – InGaN system.8 Evidence for the existence of lateral In composition fluctuations has been obtained using various electron microscope imaging and microanalysis techniques,4,7,9,10 but later studies performed by the Cambridge group show that these composition fluctuations could be induced by TEM radiation damage in InGaN layers that initially show no fluctuations.11 The same group later proposed that active region width variations could be a cause of lateral carrier confinement away from threading dislocations in InGaN based LEDs.12 In this work, we use aberration-corrected Z-contrast scanning transmission electron microscopy (STEM) imaging at an electron dose that
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