Diffusion of oxygen in amorphous tantalum oxide

The switching in resistive random-access memory (RRAM) relies on the migration of ions in sputtered amorphous transition metal oxides to both create and annihilate conductive filaments. A comprehensive understanding of ionic diffusion in these materials is therefore critical for optimizing future RRAM devices. While predicting diffusion barriers in crystalline materials is straightforward, the complex energy landscape in disordered materials makes analysis of the effective diffusion barrier inherently more difficult. Using classical molecular dynamics, we examine oxygen diffusion at various temperatures in amorphous tantala $\left({\mathrm{Ta}}_2{\mathrm{O}}_5\right)$, a leading material for RRAM applications. We compare the predicted activation energies for self-diffusion to those from isotope tracer diffusion experiments. We find a diffusion activation energy of 1.55–1.60 eV, which is higher than that measured in isotopic diffusion experiments $\left(1.2\pm 0.1\mathrm{eV}\right)$ on amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$. However, the predicted values are comparable to the activation energy $\left(1.60\pm 0.18\mathrm{eV}\right)$ determined from secondary ion mass spectroscopy analysis of isotopic diffusion experiments of sintered L-${\mathrm{Ta}}_2{\mathrm{O}}_5$ samples. We find that this difference could be due to issues of deposited film porosity, film stoichiometry, or else a preference for the empirical potentials to form nanocrystalline regions.

Figure 1

Mean square displacement versus time for oxygen in amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ for temperatures ranging from 1500 to 2000 K.

Figure 2

Amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ comparing oxygen diffusion coefficients for small supercell (black squares: 1512 atoms) and large supercell (blue diamonds: 6048 atoms) for 1 ns.

Figure 3

Log of the oxygen diffusion coefficient (${\text{cm}}^2$/s) versus inverse temperature for 10 ns NPT (black square) and NVT (blue triangle) molecular dynamic runs for ${\mathrm{Ta}}_2{\mathrm{O}}_5$.

Figure 4

Arrhenius plot to determine activation energy for amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ for 10 ns NVT (blue triangles) and NPT runs (black squares).

Figure 5

Average radial distribution function calculated for 2 ns intervals in the 10 ns NVT simulation run is shown for (a) $T=1500$ K and (b) $T=2250$ K.

Figure 6

The number of bonds with a specific coordination are plotted as a function of time for molecular dynamic simulations at (a) 800, (b) 1800, and (c) 2250 K. The five, six, and seven coordinated Ta atoms are shown in red, teal, and purple, respectively.