Broadening mechanisms of donor-bound exciton photoluminescence in Ga-doped ZnO nanowires

Donor spins in ZnO NWs have promise for quantum information (QI) applications due to high crystalline quality, narrow excitonic luminescence linewidths, and a direct bandgap of this material. It is important to understand the processes that can lead to inhomogeneous broadening of the excitonic transitions for realization of QI devices. We investigate the effect of Ga dopant concentration on the low temperature photoluminescence (PL) of Ga-doped ZnO nanowires. Spectrometer-resolution-limited donor-bound exciton (D0X) PL lines are observed at low concentrations with linewidths of around 0.1 meV. A clear increase in the Ga D0X line is observed as trace amounts of Ga are added. Above a certain concentration threshold, we observe a strong increase in the lateral growth coupled with a significant tail on the low energy side of the D0X emission, which scales linearly with dopant precursor concentration. We have analyzed this behavior using different models, including a model based on a bound exciton wavefunction overlap with neigbhouring donors and a Stark effect model due to random charged impurities. We rule out both of these models based on PL excitation spectroscopy measurements and show that a simple exponential model of the Urbach form gives the best fit and points to disorder in the more heavily doped shells.