Field emission properties of molecular beam epitaxy grown AlGaN nanowires

We investigate the field emission properties of AlGaN nanowires, grown on GaN nanowire templates on Si(111) substrate by means of radio-frequency plasma-assisted molecular beam epitaxy technique. The field emission measurement setup was realized inside the vacuum chamber of a scanning electron microscope equipped with piezo-driven metallic probe tips working as electrodes. Current-voltage characteristics were measured by precisely positioning a tip anode above the emitting AlGaN surface, with cathode-anode separation distance varied in the range 200-1500 nm. Experimental data are discussed according to the Fowler-Nordheim theory to confirm the field emission nature of the measured current and to estimate key figures of merit, such as field enhancement factor and turn-on field. We show that field emission curves become stable after electrical conditioning, that is after the burnout of individual protruding emitters. We demonstrate field enhancement factor as high as 250 for cathode-anode separation distance of 200 nm, and that it decreases for increasing distances. Similarly, minimum turn-on field of 19 V/?m is reported for a separation distance of 800 nm. Finally, we discuss the effect of a Schottky diode in a back-to-back configuration with the field emission device, that limits the exponential growth field emitted current.