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$mu m^2$. The field emission characteristics are analyzed in the framework of Fowler-Nordheim theory and we find a field enhancement factor as high as $beta$ = 556 and a minimum turn-on field $E_{turn-on}$ = 17 V/$mu$m 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/$mu$m and $beta$ 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.
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