Threading defect elimination in GaN nanowires
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Lei Zhang and Petros Varangis Nanocrystal Corporation, Albuquerque, New Mexico 87124 (Received 26 January 2011; accepted 31 March 2011)
This study describes the elimination of threading dislocations (TDs) in GaN nanostructures. Cross-sectional transmission electron microscopy (XTEM) analysis reveals that the nominal [0001] line direction of a TD changes when it enters a GaN nanostructure and the dislocation then terminates at a sidewall facet. It is suggested that the driving force for this process is the reduction of dislocation line energy, and for a pure-edge dislocation, this TD elimination process can be accomplished simply by dislocation climb. This mechanism is active whenever a threading defect is in close proximity to a surface. Preliminary XTEM analysis of defects in AlGaN and InGaN core–shell growth onto GaN nanostructures is also shown. Although more work is required to improve the quality of core–shell InGaN epitaxial growth, nanostructures appear to offer a route to defect-free, nonpolar GaN-based devices.
I. INTRODUCTION
The availability of a scalable process for the fabrication of uniform GaN nanowires1 has created a realistic opportunity for GaN nanowire-based devices to transition from the research laboratory into production. This process does not employ potentially contaminating metal catalysts2 and uses only high-purity metalorganic and ammonia precursors that are typical of regular metalorganic chemical vapor deposition (MOCVD). The electrical properties of GaN nanowires grown with this process were shown to be well behaved.3,4 Highly doped GaN nanowires exhibited Ohmic conduction, whereas low-doped or undoped nanowires exhibited space charge-limited conduction, and an electron mobility of 386 cm2/Vs was extracted from the conductivity data.2 The nanowires exhibited an intense photoluminescence (PL) signal,1 and lasing was observed when the nanowires were optically pumped at high intensity.5 Transmission electron microscopy (TEM) analysis shows that even though threading defects are ubiquitous in planar GaN layers, they are absent from the GaN nanowires.1,6 Homojunction7,8 and heterojunction9 GaN light-emitting diodes (LEDs) based on nanowires have been reported. Three-dimensional nanostructures presenting {1 100} sidewalls, as reported here, are especially interesting as they offer a potential route to the fabrication of defect-free, nonpolar LEDs. The elimination of TDs in GaN is important for an LED because this is expected to reduce the LED reverse bias leakage10 and to improve the longevity, especially in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.112 J. Mater. Res., Vol. 26, No. 17, Sep 14, 2011
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those devices that operate at high power.11 Furthermore, it remains to be seen if the reduction of internal quantum efficiency of LED at high current density (i.e., the “droop” issue) will be improved by reducing the TD density to zero.
II. EXPERIMENTAL DETAILS AND RESULTS
The puls
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