Dislocations as quantum wires: Buffer leakage in AlGaN/GaN heterostructures
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Judith G. Reynolds Department of Physics, North Carolina State University, Raleigh, North Carolina 27695
Antonio Crespo, James K. Gillespie, and Kelson D. Chabak Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, Ohio 45433
Robert F. Davis Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 (Received 14 November 2012; accepted 18 March 2013)
Buffer leakage in aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure transistors is recognized as an issue that has deleterious consequences on device performance for high-power, high-frequency transistors and has been related to the presence of uncharged threading screw dislocations. In this study, we demonstrate that measurements of buffer leakage in AlGaN/GaN heterostructures grown on bulk gallium nitride (GaN) substrates are consistent with a mechanism based on the concept of dislocations acting as quantum wires in series with unintentional silicon (Si) impurity incorporation at the bulk GaN substrate/GaN buffer interface. The number of electronic channels N deduced from the leakage data using Landauer’s formula for the quantum resistance of N electronic channels is consistent with the number of dislocations along the ohmic contact pads determined from panchromatic cathodoluminescence and x-ray diffraction measurements of the dislocation density. This mechanism is consistent with Shockley’s suggestion that dislocations can act as one-dimensional conductors due to the presence of edge states along the dislocation core.
I. INTRODUCTION
Buffer leakage in aluminum gallium nitride/gallium nitride (AlGaN/GaN) high electron mobility transistor structures (HEMTs) is recognized to be a serious problem and have deleterious consequences on devices for highspeed, high-power applications.1–5 These devices must support high voltages with small current flow for minimizing power dissipation in the blocking mode of operation. On the basis of these investigations, since the critical electric field for breakdown in GaN-based materials is high, we can attribute some aspects of reduced breakdown voltage and enhanced leakage to the short-circuiting caused by threading dislocations in the nominally undoped GaN buffer layers. Since these investigations clearly demonstrated that dislocations play an important role in leakage and breakdown in these structures, it is important to characterize further the nature of conduction associated with dislocations. More than 50 years ago, Shockley6 suggested that dislocations could be “one-dimensional degenerate electron-gas conductors.” He based this postulate on the potential existence of a band of one-dimensional edge states along the core of the dislocation that are partially filled. In a suba)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.76 J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
sequent article, Shockley7 proposed a row of impurities along the extra half plane of atoms associated with a dislocation th
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