Photoluminescence dynamics and quantum yield of intrinsically conductive ZnO from atomic layer deposition

Zinc oxide (ZnO) thin films deposited by atomic layer deposition (ALD) and a (0001)-oriented bulk-ZnO single-crystal are compared by ultrafast time-resolved and spectral photoluminescence spectroscopy as well as by luminescence quantum yield (QY) measurements. While the ALD-ZnO is intrinsically conductive, bulk-ZnO is electrically rather insulating. Nevertheless, PL spectra of both materials reveal similarities: A peak in the near-UV originating from inter-band transitions and a defect peak in the green-yellow spectral region associated to deep trap states. We investigate the dynamics and efficiency of these luminescence emissions to understand the interdependency of defects and the yet insufficiently understood conductivity mechanism of ALD-ZnO. Spectrally, a PL blueshift of the trap-mediated peak is observed for the transition from poor to good conductivity, which occurs with increasing ALD-deposition temperature. The quantum yield of ALD-ZnO is shown to be ~ 18-times lower and the luminescence lifetime of the UV-peak is ~ 14-times shorter compared to bulk-ZnO. We conclude that these properties are related to a significantly higher density of non-radiative defects, which might also represent electrically active scattering centers. These results indicate a new direction for further optimization of ALD-ZnO towards indium tin oxide (ITO)-compatible electrical conductivities by reducing or passivating these defects.