Growth characteristics and physicochemical properties of nanoporous hafnium oxide layers prepared by anodic oxidation of Hf

Abstract

Well defined nanostructures of doped or undoped HfO2 with unique physicochemical properties are crucial for applications in electronics, optoelectronics, sensors, and biomaterials. In this context, we have studied the fabrication of nanoporous anodic hafnium oxide (AHO) thin layers in ethylene glycol-based electrolytes containing NH4F. Although some earlier works have described the formation of nanoporous/nanotubular AHO layers in F− containing electrolytes, the key factors affecting the porosity, pore diameter, and thickness of synthesized materials have yet to be systematically investigated. We explored the feasibility of tuning unordered nanopores to ordered nanopores by varying the NH4F concentration in the electrolyte. We demonstrated that at constant anodizing voltage and time (30 or 60 V, 30 min), the mean porosity for the ordered porous AHO layer increases with NH4F concentration up to 0.10 M. We investigated the exclusive effects of anodizing time and NH4F concentration on the mean pore diameter, porosity, and thickness of AHO layer. The maximum mean porosity of 24.5 %, pore diameter of 39.8 nm, and the layer thickness of 24.6 μm were observed for AHO layers produced for 60 min at 60 V. Post-treatment thermal studies revealed the nanoporous AHO layer remained structurally stable up to 500 °C. X-ray diffraction studies showed a significant enhancement in crystallinity for thermally treated AHO layers. The composition and chemical environment of core elements at the top and bottom of AHO layer were thoroughly investigated using X-ray photoelectron spectroscopy (XPS). Furthermore, we elucidated the rate limiting step of AHO growth by applying growth models.

Keywords

• Anodization
• Nanoporous hafnium oxide
• Thin layers
• Growth models