Electron Field Emission from GaN Nanotip Pyramids
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Electron Field Emission from GaN Nanotip Pyramids Hock M. Ng1, Jonathan Shaw2, Aref Chowdhury1, and Nils G. Weimann1 1 Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ, U.S.A. 2 Naval Research Laboratory, Washington DC, U.S.A. ABSTRACT Electron field emission was measured from GaN nanotip pyramids formed by polarityselective chemical etching in KOH solution. The GaN samples were grown by plasma-assisted molecular beam epitaxy and consisted of regions of Ga- and N-polar GaN grown at the same time. The pyramids were formed only in the N-polar regions and have extremely sharp tips with diameters estimated to be less than 20 nm. Field emission measurements showed a characteristic Fowler-Nordheim behavior. The average turn-on field was 1.6 V/µm with a corresponding normalized field enhancement factor of about 1500. INTRODUCTION AlxGa1-xN has been proposed previously as a potential material for electron field emission devices [1]. Among the requirements for a good material candidate are large breakdown fields, resistance to radiation, ability to dope with electrons and low electron affinity. These requirements are fulfilled by the wide-bandgap III-nitrides. Common materials used in field emitters include molybdenum, silicon, diamond, and carbon nanotubes. Previously, electron field emission measurements have been made on GaN surfaces roughened by plasma treatment [2,3], polycrystalline GaN [4] as well as selectively grown GaN pyramids [5-9]. In this paper, a different approach is taken to fabricate GaN pyramids with extremely sharp tips using polarityselective chemical etching. A field emitter geometry with sharp tips is desirable due to the field enhancement effect at these points. Here, we report on the electron field emission characteristics from these GaN nanotip pyramids. EXPERIMENTAL DETAILS The GaN samples used in this study were grown on (0001) sapphire using radio frequency plasma-assisted molecular beam epitaxy (MBE). The GaN sample contains patterned stripes of Ga- and N-polarity GaN that is formed using a two-step growth. A detailed description of the growth process has been previously reported [10] and is described briefly here. First, an AlN buffer layer (20 to 30 nm) was grown at 720 °C, followed by a thin GaN (~ 50 nm) capping layer. The AlN buffer layer was found to set the polarity of the resulting GaN epilayer to be Gapolar. The sample was then removed from the growth chamber and lithographically patterned with photoresist to define the striped regions. An inductively coupled plasma with chlorine gas was then used to etch the exposed stripes down to the sapphire substrate. The subsequent growth of GaN resulted in Ga- and N-polarity material growing on the AlN/GaN and directly on the sapphire substrate, respectively. The GaN pyramids were produced by etching in a 2M KOH solution at 55 °C for 10 minutes. This process etches only the N-polar GaN regions and produces hexagonal pyramids with sharp tips. The sample is unintentionally doped n-type with an electron
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