  The accurate control of the crystal phase in III-V semiconductor nanowires
(NWs) is an important milestone for device applications. In this work, we
present a method to select and maintain the wurtzite (WZ) crystal phase in
self-assisted NWs. By choosing a specific regime where the NW growth process is
a self-regulated system, the main experimental parameter to select the
zinc-blende (ZB) or WZ phase is the V/III flux ratio. The latter can be
monitored by changing the As flux, and drives the system toward a stationary
regime when the wetting angle of the Ga droplet falls in a target interval,
typically in the 90{\deg} - 125{\deg} range for the WZ phase growth. The
analysis of the in situ RHEED evolution, high-resolution scanning transmission
electron microscopy (HRSTEM), dark field transmission electron microscopy
(DF-TEM), and photoluminescence (PL) data all confirm the control of an
extended few micrometers long pure WZ segment obtained by MBE growth of
self-assisted GaAs NWs with a V/III flux ratio of 4.0.
