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 1µm2. The field emission characteristics are analysed in the framework of Fowler-Nordheim theory and we find a field enhancement factor as high as β = 556 and a minimum turn-on field E_(turn-on) = 17 V/µm for a cathode-anode separation distance d = 500 nm. We show that for increasing separation distance, E_(turn-on) increases up to about 35 V/µm and β decreases to 100 at d = 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.