Electroforming-Free HfO₂:CeO₂Vertically Aligned Nanocomposite Memristors with Anisotropic Dielectric Response

Abstract

Resistive switching and anisotropic optical properties have been investigated in two-phase HfO 2 :CeO 2 nanocomposite thin films of different HfO 2 :CeO 2 ratios of 3:1, 1:1, and 1:3 on single-crystalline SrTiO 3 (001) substrates. Vertically aligned nanocomposite (VAN) thin films with CeO 2 pillars embedded in a HfO 2 matrix have been obtained using a one-step pulsed laser deposition technique. By adjusting the molar ratio of HfO 2 and CeO 2 in the films, the resistive switching effect and the anisotropic dielectric response were tuned and correlated with the density of the conducting vertical-phase boundaries. It is shown that only HfO 2 :CeO 2 VAN films of 1:1 composition give rise to a forming-free switching system as they have clear vertical boundaries that guide oxygen vacancy channels. These films show strong promise for resistive switching memory devices.

Engineering of Grain Boundaries in CeO2 Enabling Tailorable Resistive Switching Properties

“…The faceted growth of CeO 2 is often seen in the high temperature growth (>700 °C) conditions, especially when the film grows thicker and under high oxygen partial pressure. [35,44] Different from the surface morphologies of the 300 and 500 °C samples shown in Figure S1a,b, Supporting Information, the highest energy facet (001) with two orthogonal in-plane directions of the nanorod structure in the 750 °C was further confirmed by AFM, as shown in Figure S1c, Supporting Information, showing good agreement with the AFM results from the previous report on faceted CeO 2 growth. [45] The evolution of film morphology is thus clearly shown, following the structural design of our investigation, that is, the 300, 500, and 750 °C sample results in the different grain boundary morphologies referred to as "Random networks," "Columnar scaffold," and "island-like," respectively.…”

“…Such in-plane matching relationship between CeO 2 and STO was confirmed via φ-scans in our previous work as 45° peak shifts can be observed between the CeO 2 (220) and STO (110) peaks. [35] To examine the microstructures and the morphologies of all three samples further, cross-section TEM and STEM were performed, as shown in Figure 3. From Figure 3a,d,h, the CeO 2 film deposited at 300 °C consists of high density of grains with very small sizes (≈4 nm).…”

“…[13,[27][28][29] Embedding a layer of active metals or oxygen-deficient oxides provides a reservoir of oxygen vacancies which greatly reduces the energy for vacancy generation. [9,30] Extended defects such as single dislocations, [31] grain boundaries, [32,33] and phase boundaries [34,35] lower the formation energy of oxygen vacancies and lower the migration barrier along the extended defects, leading to desired switching properties. The localized switching process is possibly facilitated by such defects, as spatially resolved RS in nm scale has been demonstrated using conductive atomic force microscopy in various studies.…”

Epitaxial Growth of Aurivillius Bi3Fe2Mn2Ox Supercell Thin Films on Silicon

“…Functional complex oxides with unique functionalities have drawn much attention in recent years in the development of nanoelectronic devices. − Multiferroic oxide materials, those possessing multiple ferroic orders, e.g., exhibiting simultaneous ferromagnetism and ferroelectricity, have been an area of research for many decades. − The particular interest in these materials stems from their use in memory storage, such as spintronic tunnel junction devices, magnetoelectric random access memory, , four-state memory, and spin filters …”

Optical dielectric properties of HfO2-based films

“…Beyond the use of HfO2 as a high-k dielectric material in semiconductor industry [1][2][3][4] , the recent discovery of ferroelectricity and resistive switching in HfO2 and its compounds opens the use of this material system for further applications in microelectronics [5][6][7][8][9] . Compounds based on HfO2, including Y-doped HfO2 (HYO), Zr-doped HfO2 (HZO), and Ce-doped HfO2 (HCO), have drawn increasing attention in recent years because of the possibility to control the crystal structure and thus improve the electrical properties of HfO2 [10][11][12][13][14][15][16] . There are only a few studies on the optical response of the HfO2-based films at higher frequencies relevant for optical applications [17][18][19] .…”