Exciton localization mechanisms in wurtzite/zinc-blende GaAs nanowires

We investigate the emission properties of excitons in GaAs nanowires containing quantum disks formed by structural alternation between the zinc-blende and wurtzite phases, by means of temperature-dependent photoluminescence. At 10 K the emission from an ensemble of disks is distributed in a band of full width at half maximum ∼30 meV, whereas the emission linewidth for a single disk is 700 μeV. While the disk ensemble emission exhibits an S-shaped temperature dependence, the emission from single quantum disks follows the temperature dependence of the band gap over the whole temperature range. This indicates that intradisk exciton localization on impurities is negligible and that increasing the temperature induces a transfer of excitons from narrow to thick disks along the length of the wires. Our observations of the emission linewidth for single crystal-phase quantum disks show a scattering rate of excitons with acoustic phonons eight times larger than the values usually reported for (Al,Ga)As/GaAs quantum wells. This large scattering rate demonstrates that the electron effective mass in wurtzite GaAs is much heavier than in zinc-blende GaAs and is evidence of coupling between the 7 and 8 conduction bands of wurtzite GaAs.