Abstract
The transition between Ta2O5 and TaO2 governs resistive switching in tantalum oxide-based resistive random access memory. Despite its importance, the Ta2O5–TaO2 transition is scarcely described in the literature, in part because the tantalum oxide layer in devices is amorphous, which makes it difficult to characterize. In this paper, we use first-principles calculations to construct the convex hull of the amorphous Ta2O5−x system for 0 ≤ x ≤ 1 and show that oxygen deficiency in tantalum oxide leads to phase-separation into Ta2O5 and TaO2. In addition, our work challenges the conventional interpretation of X-ray Photoelectron Spectroscopy (XPS) spectra of the Ta 4f orbitals. Specifically, we find that TaO2 exhibits both the Ta4+ peak associated with TaO2 and the Ta5+ peak normally associated with Ta2O5. While our simulated Ta2O5 peak originates from a narrow range of oxidation states, the TaO2 peak comes from disproportionated Ta atoms with Bader charges ranging from +3 to +1, the lowest of which are well below Ta atoms in crystalline TaO. Finally, we demonstrate that the XPS blueshift of around 1 eV observed experimentally in amorphous Ta2O5 with respect to crystalline Ta2O5 comes from both the presence of under-coordinated Ta atoms and longer Ta–O bond distances in the amorphous system. Our simulated XPS analysis shows that amorphous XPS spectra may be more complex than previously thought, and hence, caution should be applied when assigning XPS peaks to oxidation states.
Highlights
Resistive switching devices constitute an important research topic within the general area of random access memory (RAM) technology.1,2 Since Chua proposed the “memristor”—memory resistor—in 1971,3 researchers have demonstrated memory resistive properties in metal/metal-oxide/metal devices4,5 using semiconducting and insulating transition metal oxides such as TiO2,4 HfO2,6,7 and Ta2O58 as the active layer
Our calculations show a clear division into high-energy and low-energy stoichiometries, and we expect some degree of similarity between the three low-energy stoichiometries
We examine the connection between the Ta coordination numbers, binding energies (BEs), and Bader charges
Summary
Resistive switching devices constitute an important research topic within the general area of random access memory (RAM) technology. Since Chua proposed the “memristor”—memory resistor—in 1971,3 researchers have demonstrated memory resistive properties in metal/metal-oxide/metal devices using semiconducting and insulating transition metal oxides such as TiO2,4 HfO2,6,7 and Ta2O58 as the active layer. Resistive switching devices constitute an important research topic within the general area of random access memory (RAM) technology.. Since Chua proposed the “memristor”—memory resistor—in 1971,3 researchers have demonstrated memory resistive properties in metal/metal-oxide/metal devices using semiconducting and insulating transition metal oxides such as TiO2,4 HfO2,6,7 and Ta2O58 as the active layer. The switching mechanism in these layers is initiated by applying a large electric field across the device. This leads to the creation and subsequent migration of oxygen vacancy (vO) defects and eventually to the growth of nanoscale filaments of oxygen-deficient and conductive regions through the device.. Tantalum pentoxide (Ta2O5) is one of the key candidate materials for the switching layer of memristive switching devices. In a direct comparison to TiO2, which was the first material to be linked to memristive switching, Ta2O5 shows five times greater ionic mobility and, by extension, may yield greater switching speed and lower power consumption. a switching endurance exceeding 1012 cycles has been demonstrated without device breakdown.
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