Wurtzite phase control for self-assisted GaAs nanowires grown by molecular beam epitaxy.

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.