Fabrication and field emission properties of vertical, tapered GaN nanowires etched via phosphoric acid
Barbara Kazanowska, Keshab Sapkota, Ping Lu, A. Alec Talin, Ezra Bussmann, Taisuke Ohta, Brendan Gunning, K. S. Jones, George T. Wang
Abstract
Abstract The controlled fabrication of vertical, tapered, and high-aspect ratio GaN nanowires via a two-step top-down process consisting of an inductively coupled plasma reactive ion etch followed by a hot, 85% H 3 PO 4 crystallographic wet etch is explored. The vertical nanowires are oriented in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">[</mml:mo> <mml:mn>0001</mml:mn> <mml:mo stretchy="false">]</mml:mo> </mml:math> direction and are bound by sidewalls comprising of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">{</mml:mo> <mml:mn>33</mml:mn> <mml:mover accent="true"> <mml:mn>6</mml:mn> <mml:mo>¯</mml:mo> </mml:mover> <mml:mn>2</mml:mn> <mml:mo stretchy="false">}</mml:mo> </mml:math> semipolar planes which are at a 12° angle from the [0001] axis. High temperature H 3 PO 4 etching between 60 °C and 95 °C result in smooth semipolar faceting with no visible micro-faceting, whereas a 50 °C etch reveals a micro-faceted etch evolution. High-angle annular dark-field scanning transmission electron microscopy imaging confirms nanowire tip dimensions down to 8–12 nanometers. The activation energy associated with the etch process is 0.90 ± 0.09 eV, which is consistent with a reaction-rate limited dissolution process. The exposure of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">{</mml:mo> <mml:mn>33</mml:mn> <mml:mover accent="true"> <mml:mn>6</mml:mn> <mml:mo>¯</mml:mo> </mml:mover> <mml:mn>2</mml:mn> <mml:mo stretchy="false">}</mml:mo> </mml:math> type planes is consistent with etching barrier index calculations. The field emission properties of the nanowires were investigated via a nanoprobe in a scanning electron microscope as well as by a vacuum field emission electron microscope. The measurements show a gap size dependent turn-on voltage, with a maximum current of 33 nA and turn-on field of 1.92 V nm −1 for a 50 nm gap, and uniform emission across the array.