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The conduction mechanisms of TaN/HfO x /Ni memristors are investigated in the wide temperature range from 4.2 to 400 K. The hafnium oxide layer was deposited by ion-beam sputtering deposition (IBSD) at different partial oxygen pressures for various samples, for which resistive switching was possible. This range of partial oxygen pressures corresponded to the x range from 1.78 to 1.81 The analysis and fitting of the current-voltage (I−V) curves of low- and high-resistance states (LRS and HRS, respectively) at different temperatures revealed that, at temperatures above ~70 K, the conduction is governed by the thermal generation of free carriers and space-charge-limited current (SCLC). Specifically, it is shown that trap-free and trap-mediated SCLCs govern the conduction for the LRS and HRS, respectively. At lower temperatures, the conduction in both states was found to be practically temperature-independent. At the same time, the TaN/HfO x /Ni memristor functioned properly at temperatures as low as 4.2 K. The x value is shown to influence the slope of the HRS I−V curves, and it correlates with the change in the SCLC power-law exponent (m, I ∝ V m ). TaN/HfO x /Ni memristors with low values of x ≤ 1.79 demonstrated the current power-law dependence on voltage in the HRS with m > 2 (3.0–6.5) and for the memristors with higher x values in the HRS of m ≈ 2. Within the suggested model, a local trap-rich conducting region was presumed to be formed in the oxide layer between the metal electrodes after the memristor forming process. The I−V curves modeling indicates that the LRS and HRS differ from each other by the thickness of this active region and the trap distribution. The role of oxygen vacancies and polyvacancies in HfO x , which are considered to act as charge carrier traps, is discussed.

The optical properties of the HfOx films of different chemical composition (x ≤ 2) deposited by ion beam sputtering-deposition (IBSD) method were studied. Spectral dependencies of refractive index n(λ) and extinction coefficient k(λ) were determined with ellipsometry in λ = 250–1100 nm wavelength region. The x values (i.e. [O]/[Hf] ratio) for the films were derived from x-ray photoelectron spectroscopy (XPS) data. The spectral dependences of optical constants n(λ) and k(λ) were found to undergo radical changes with x = 1.78–1.82. The films with x 1 with the metallic-like behavior of optical constants spectral dependences. The films with x > 1.82 were found to be transparent, with k = 0 and n(λ) being well approximated by a Cauchy polynomial dependence for dielectrics. Using a sample with a gradient of x, it was established that the transition from the metallic to the dielectric-like behavior of the optical constants occurs not smoothly, but is discontinuous. A sharp jump in the optical constants is observed at x ≈ 1.8. According to XPS data, the transparent films were found to consist of two components only: HfO2 and Hf4O7 suboxide. Cauchy polynomial coefficients for Hf4O7 suboxide and HfO2 were found by using the Bruggeman effective medium approximation.

Non-stoichiometric HfOx films of different chemical composition (x < 2) were fabricated by ion-beam sputtering deposition (IBSD) at room temperature. The ratio of O and Hf atoms in films x was varied by setting the O2 partial pressure in a chamber. An effect of chemical composition on the atomic structure of the films was studied by reflection high-energy electron diffraction, x-ray photoelectron spectroscopy and field emission scanning electron microscopy methods. The films were found to be amorphous, consisting only of three components: Hf-metal clusters, Hf4O7 suboxide and stoichiometric HfO2. The relative concentration of these components varies with changing x. The surface of the films contains the increased oxygen content compared to the bulk. It was found that the Hf4O7 suboxide concentration is maximal at x = 1.8. The concept of hafnium oxide film growth by the IBSD method is proposed to explain the lack of suboxides variety in the films and the instability of HfO2, when annealed at high temperature.