Abstract
Hafnium oxide plays an important role as a dielectric material in various thin-film electronic devices such as transistors and resistive or ferroelectric memory. The crystallographic and electronic structure of the hafnia layer often depends critically on its composition and defect structure. Here, we report two novel defect-stabilized polymorphs of substoichiometric HfO2-x with semiconducting properties that are of particular interest for resistive switching digital or analog memory devices. The thin-film samples are synthesized by molecular beam epitaxy with oxygen engineering that allows us to cover the whole range of metallic Hf with oxygen interstitials to HfO2. The crystal and defect structures, in particular of a cubic low-temperature phase c-HfO1.7 and a hexagonal phase hcp-HfO0.7 are identified by X-ray diffraction, in vacuo electron spectroscopic, and transmission electron microscopic methods. With the help of UV/Vis transmission data, we propose a consistent band structure model for the whole oxidation range involving oxygen vacancy-induced in-gap defect states. Our comprehensive study of engineered hafnia thin films has an impact on the design of resistive memory devices and can be transferred to chemically similar suboxide systems.
Highlights
Hafnium oxide was identified as an excellent candidate for several future microelectronic applications
X-ray diffraction (XRD) patterns of such thin films are labeled using closely related bulk data of various structures like high-temperature phases of HfO2, such as the tetragonal (P42/nmc) and pressure orthorhombic (Pbca) cubic (Fm-3m) or the highstructure.[10−13] Additional polymorphism in hafnia was discovered by the identification of ferroelectricity in hafnium oxide, where a polar orthorhombic phase (Pca21) is predicted.[14]
While the ferroelectric phase is mostly achieved in doped hafnium oxide, there is evidence that its stabilization is possible in pure oxygendeficient hafnium oxide.[15]
Summary
Hafnium oxide was identified as an excellent candidate for several future microelectronic applications. Electric investigations on hafnium oxide are usually conducted in microelectronic devices consisting of several layers including electrodes and their interfaces, drastically complicating the effective evaluation of specific material properties.[8,27,28] For example, it is known that even highly inert electrode materials like the popular CMOS compatible TiN are prone to interfacial oxidation (e.g., in the form of TiOxNy).[29,30] deficient hafnium oxide was shown to experience significant surface oxidation if processed in the atmosphere.[31] the nature of the band structure and type of charge carriers cannot be accessed in a typical metal−insulator−metal configuration To answer these open questions, we have performed a detailed study with the goal to correlate changes of hafnium oxide structure and material properties with oxygen deficiency.
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