HfxZr1–xO2 Solid Solution Nanoclusters with Size-Specific Bandgaps

Abstract

Combining zirconia and hafnia into a bimetallic oxide such as HZO (Hf_0.5Zr_0.5O₂) has attracted a lot of interest because the introduction of a novel orthorhombic phase with intrinsic polarization in HZO has led to strong ferroelectric properties. Here, we use a gas-phase aggregation technique to produce size-specific Hf_xZr_1–xO₂ (x < 1) hybrid nanoclusters (NCs), which can be precisely tuned from 4 to 14 nm in size while adjusting the Zr and Hf composition, as demonstrated by detailed characterization of their morphologies and chemical states. The crystallinity of the hybrid NCs is found to vary with the NC size obtained under specific deposition conditions, from amorphous for small NCs < 6 nm to single crystalline for 6–10 nm NCs to core–shell for NCs with higher Hf content and polycrystalline NCs with high Zr content for larger NCs > 10 nm. For the single-crystalline Hf_xZr_1–xO₂ NCs, we observe, for the first time for NCs, the special orthorhombic (Pbc2₁) structure found previously only in the HZO film prepared under extreme conditions. The measured bandgaps of these NCs are found to increase with the cluster size, in contrast to the increase in the bandgap with decreasing size generally found in NCs. The X-ray photoelectron spectra clearly show, in the Zr 3d region, components that can be attributed to oxygen vacancy defects and the substitution of Hf for Zr in the lattice. A new model involving Hf-induced electron polarization is proposed to describe the physical and electronic structures of these novel bimetallic hybrid oxide NCs. This work establishes a general formation protocol for other hybrid semiconductor NCs, while the Hf_xZr_1–xO₂ (x < 1) NCs with novel phase and polarization could provide promising electrical properties for the next-generation nonvolatile memory device applications.