We report the possibility of employment of low temperature (≤330 °C) plasma-enhanced atomic layer deposition for the formation of both electrodes and hafnium-oxide based ferroelectric in the metal-insulator-metal structures. The structural and ferroelectric properties of La doped HfO2-based layers and its evolution with the change of both La content (2.1, 3.7 and 5.8 at. %) and the temperature of the rapid thermal processing (550–750 °C) were investigated in detail. Ferroelectric properties emerged only for 2.1 and 3.7 at. % of La due to the structural changes caused by the given doping levels. Ferroelectric properties were also found to depend strongly on annealing temperature, with the most robust ferroelectric response for lowest La concentration and intermediate 650 °C annealing temperature. The long term wake-up effect and such promising endurance characteristics as 3 × 108 switches by bipolar voltage cycles with 30 μs duration and ± 3 MV/cm amplitude without any decrease of remnant polarization value were demonstrated.
The plasma-enhanced atomic layer deposition (PEALD) process using Ta(OCH) as a Ta precursor and plasma-activated hydrogen as a reactant for the deposition of TaO films with a controllable concentration of oxygen vacancies (V) is reported herein. The V concentration control was achieved by varying the hydrogen volume fraction of the hydrogen-argon mixture in the plasma, allowing the control of the leakage current density in the tantalum oxide films within the range of 5 orders of magnitude compared with the TaO film grown via thermal ALD using the identical Ta precursor and HO. Temperature-dependent current-voltage measurements combined with Poole-Frenkel emission modeling demonstrated that the bulk trap depth decreases with the increasing hydrogen volume fraction, which could be attributed to the increase of the V concentration. The possible chemical change in the PEALD TaO films grown under different hydrogen volume fractions was confirmed by the in situ X-ray photoelectron spectroscopy (XPS) measurements of the Ta 4f core and valence band spectra. The comparison of the XPS-measured nonstoichiometry and the secondary ion mass spectrometry analysis of the hydrogen content allowed this study to conclude that the nonstoichiometry is largely related to the formation of Ta-V sites rather than of Ta-H sites. Such oxygen-deficient TaO layers were studied for application as an oxygen-deficient layer in a resistance switching random access memory stack (TaO/TaO) where the actual switching occurred within the stoichiometric TaO layer. The bilayer memory stack showed reliable resistance switching up to ∼10 switching cycles, whereas the single-layer TaO memory showed only several hundred switching cycles.
Oxygen‐deficient TaOx layers are grown by a radical‐enhanced atomic layer deposition (REALD) process on a chemically active bottom electrode (Ta) to create electroforming‐free resistance switching random access memory (ReRAM) cells. When the top electrode is Pt, the Ta/TaOx/Pt device shows a completely electroforming‐free behavior with a pulse‐switching induced endurance cycle of up to 6 × 106 cycles. This is due to the abundantly present oxygen vacancies within the TaOx layer, which eliminate the need to create further oxygen vacancies during the electroforming cycles in other types of oxide‐based ReRAM cells. The adoption of the Pt top electrode contributes to the suppression of the leakage current, making the switching reliable. When the top electrode is changed to the more production‐compatible Ru, which is also grown by another REALD, the devices show very mild electroforming behavior with an increased leakage current. Due to these slight decreases in the electrical performance, the switching endurance can be confirmed only up to 6 × 104 cycles with the help of an incremental step pulse programming technique.
Herein, Ta2O5/Ta interface‐based TiN/Ta2O5/Ta and TiN/HfO2/Ta2O5/Ta resistance switched (RS) stacks are investigated. The stacks reveal area‐dependent RS behavior indicating nonfilamentary (homogeneous) current transport. The nonfilamentary nature of the stacks provides high reproducibility of current–voltage hysteresis loops and the absence of electroforming. It is demonstrated that the nature of the current hysteresis in single Ta2O5‐based stacks obeys space‐charge‐limited conduction, and the space charge responsible for this behavior is formed by filled/emptied traps at the Ta/Ta2O5 interface. Due to the high potential barrier at the TiN/HfO2 interface, as measured using X‐ray photoelectron spectroscopy, sufficiently thick HfO2 (≈4 nm) blocks the trapping/detrapping process, thereby reducing the current hysteresis. The evaluated current mechanism results in a high rectification ratio of the TiN/Ta2O5/Ta device of ≈1.6 × 104. However, relatively short retention is inherent to the observed switching mechanism.
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