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 µm². 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⁻¹ 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⁻¹ 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. (paper)