  GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of
both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth
conditions, we find that optical emission of such NWs occurs either
predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This
result is consistent with the assumption that the band gap energy of wurtzite
GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and
WZ segments along the wire axis establish a type II band alignment, where
electrons captured within the ZB segments recombine with holes of the
neighboring WZ segments. Thus, the corresponding transition energy depends on
the degree of confinement of the electrons, and transition energies exceeding
E(g,ZB) are possible for very thin ZB segments. At low temperatures, the
incorporation of carbon acceptors plays a major role in determining the
spectral profile as these can effectively bind holes in the ZB segments. From
cathodoluminescence measurements of single GaAs NWs performed at room
temperature, we deduce a lower bound of 55 meV for the difference
E(g,WZ)-E(g,ZB).
