Field emission from AlGaN nanowires with low turn-on field

We fabricate AlGaN nanowires by molecular beam epitaxy and we investigate their field emission properties by means of an experimental setup using nano-manipulated tungsten tips as electrodes, inside a scanning electron microscope. The tip-shaped anode gives access to local properties, and allows collecting electrons emitted from areas as small as 1 m2. The field emission characteristics are analysed in the framework of Fowler-Nordheim theory and we find a field enhancement factor as high as ? = 556 and a minimum turn-on field = 17 V m-1 for a cathode-anode separation distance = 500 nm. We show that for increasing separation distance, increases up to about 35 V m-1 and ? decreases to ~100 at = 1600 nm. We also demonstrate the time stability of the field emission current from AlGaN nanowires for several minutes. Finally, we explain the observation of modified slope of the Fowler-Nordheim plots at low fields in terms of non-homogeneous field enhancement factors due to the presence of protruding emitters.