Investigating Optical Properties of Atomic Layer Deposited ZnO/TiO_x Multi-stacked Thin Films Above Mott Critical Density

The evolution of the optical properties with dopant concentration has been investigated for a series of ZnO/TiO_x multi-stacked layers having electron density exceeding the Mott critical limit of insulator-to-metal transition. These films were grown by vertically stacking multiple ZnO/TiO_x bilayers on (0001) sapphire substrates using atomic layer deposition. The films in the sparsely doped regime showed room-temperature UV photoluminescence (PL) while being transparent and heavily degenerate in nature. Optical absorption spectra of these films did not exhibit any feature of excitonic resonance, indicating a possible excitonic Mott transition in the metallic limit. The low-temperature PL spectra also support this observation that shows line-shape characteristics typical for band-to-band emission. The sharp cutoff of the PL emission at the high energy edge corresponds to the Fermi level position inside the conduction band. In contrast, the broad low-energy wing is determined by the combined density of states washed out by potential fluctuations-induced band tailing effect. A systematic blue shift of the high-energy PL edge with increasing carrier density resembles the effect of band filling that has also been considered in explaining the optical absorption spectra of the films. The results of this study demonstrate that the multi-stacked dopant incorporation scheme in atomic layer deposition could be highly useful to increase carrier concentration while minimizing disorder strength in the lattice, which eventually results in high optical quality ZnO films with tunable electrical conductivity.