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

Abstract

The evolution of the optical properties with dopant concentration has been investigated for a series of ZnO/TiOx 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/TiOx 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...