TaO x films with controlled ratio of Ta4+ and Ta5+ atoms were prepared at different hydrogen concentrations in plasma. As shown by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, the chemical state of Ta4+ corresponds to oxygen vacancies in the TaO x film. Electrophysical studies of the metal–dielectric–metal structures revealed an increase in the leakage current by four orders of magnitude as the hydrogen concentration in the plasma was increased from 7 to 70%, which is due to an increase in the concentration of oxygen vacancies in TaO x . A test structure of a resistive memory cell was made on the basis of the nonstoichiometric TaO x obtained. It withstood more than 106 rewriting cycles. The suggested atomic layer deposition process shows promise for solving one of the main problems of resistive memory: extension of its working life.

Recently proposed novel neural network hardware designs imply the use of memristors as electronic synapses in 3D cross-bar architecture. Atomic layer deposition (ALD) is the most feasible technique to fabricate such arrays. In this work, we present the results of the detailed investigation of the gradual resistive switching (memristive) effect in nanometer thick fully ALD grown TiN/HfO2/TiN stacks. The modelling of the I-V curves confirms interface limited trap-assisted-tunneling mechanism along the oxygen vacancies in HfO2 in all conduction states. The resistivity of the stack is found to critically depend upon the distance from the interface to the first trap in HfO2. The memristive properties of ALD grown TiN/HfO2/TiN devices are correlated with the demonstrated neuromorphic functionalities, such as long-term potentiation/depression and spike-timing dependent plasticity, thus indicating their potential as electronic synapses in neuromorphic hardware.

Ta2O5-based metal-insulator-metal stacks for resistive random access memory were grown by atomic layer deposition technique only with the emphasis on different top metal-oxide interface engineering. The impact of top TiN electrode growth and NH3 treatment on dielectric chemical and electrical properties was discussed. In addition the TiN/Ta2O5/Al2O3/TiN stack with bilayer dielectric was grown and studied too. According to in vacuo XPS analysis at top interface both TiN/Ta2O5/TiN and TiN/Ta2O5 (NH3-treated)/TiN stacks comprise the TaOxNy, interlayer which is twice thicker in the case of stack with NH3 treatment (similar to 1.3 nm) in comparison with untreated one (similar to 0.7 nm). In vacuo XPS analysis also showed that 2 nm Al2O3 insert between Ta2O5 and top TiN electrode allowed to completely block formation of TaOxNy interlayer at TiN/Ta2O5/Al2O3/TiN stack. As a result it was found that TiN/Ta2O5/TiN demonstrated gradual and rather slow (similar to 10(-3) s) character of resistive switching while the switching at stack with bilayer Ta2O5/Al2O3 dielectric is much more abrupt, faster and it reveals more than one order of magnitude higher endurance. (C) 2015 Elsevier B.V. All rights reserved.

Atomic‐layer deposition (ALD) technique in combination with in vacuo X‐ray photoelectron spectroscopy (XPS) analysis has been successfully employed to obtain fully ALD‐grown planar TiN/HfO2/TiN metal–insulator–metal structures for resistive random access memory (ReRAM) memory elements. In vacuo XPS analysis of ALD‐grown TiN/HfO2/TiN stacks reveals the presence of the ultrathin oxidized layers consisting of TiON (∼0.5 nm) and TiO2 (∼0.6 nm) at the bottom TiN/HfO2 interface (i); the nonoxidized TiN at the top HfO2/TiN interface (ii); the oxygen deficiency in the HfO2 layer does not exceed the XPS detection limit (iii). Electroformed ALD TiN/HfO2/TiN stacks reveal both conventional bipolar and complementary types of resistive switching. In the complementary resistive switching regime, each programming sequence is terminated by a reset operation, leaving the TiN/HfO2/TiN stack in a high‐resistance state. The observed feature can avoid detrimental leaky paths during successive reading operation, which is useful in the passive ReRAM arrays without a selector element. The bipolar regime of resistive switching is found to reveal the gradual character of the SET and RESET switching processes. Long‐term potentiation and depression tests performed on ALD‐grown TiN/HfO2/TiN stacks indicate that they can be used as electronic synapse devices for the implementation of emerging neuromorphic computation systems.

A nonvolatile memory cell prototype based on the effect of resistive switching in thin Hf x Al1 − x O y oxide films grown by atomic layer deposition (ALD) with a Al depth-graded profile has been developed. This prototype imitates the arrangement of memory cells between the metallization layers of integral circuits. The obtained results are much superior to the parameters of conventional flash memory cells in increasing the reset speed and decreasing the reset voltage.

Разработан прототип ячеек энергонезависимой памяти на эффекте резистивного переключения в оксидных пленках HfxAl1 - xOy с переменным (по глубине) содержанием Al, выращенных методом атомно-слоевого осаждения. Данный прототип моделирует размещение ячеек памяти между слоями металлизации интегральных схем. В части повышения скорости перезаписи и снижения напряжения перезаписи полученные результаты существенно превосходят параметры традиционной флэш-памяти.

The multilevel resistive switching effect in Pt/HfxAl1−xOy/TiN stacks has been studied by hard X-ray photoelectron spectroscopy (HAXPES). Atomic layer deposition was used to grow HfxAl1−xOy films with graded Al depth profile, where the engineered Al concentration across the film facilitates the desired oxygen vacancies' profile. The method to derive the electrical potential profile across dielectric from the HAXPES spectra is proposed. By combining the information on the chemical state at both interfaces with the extracted potential distribution, a qualitative model for the resistive switching effect in the Pt/HfxAl1−xOy/TiN metal–insulator–metal stack is proposed. According to this model, the conductive filaments controlling the resistivity are formed by the electric field driven condensation of the charged oxygen vacancies in the oxide.

In this work, HfxAl1-xOy, films with graded Al depth profile are grown by atomic layer deposition (ALD). HfxAl1-xOy based Pt/HfxAl1-xO/TiN stacks exhibit multilevel resistive switching behavior as revealed by I-V measurements. The distribution of the charged oxygen vacancies across as grown graded HfxAl1-xOy layer on Pt underlayer is evident from the X-ray photoemission spectroscopy analysis. The resistance switching in the graded HfxAl1-xOy layer is described in terms of the redistribution of the charged oxygen vacancies following the external biasing of the opposite polarities. (C) 2013 Elsevier B.V. All rights reserved.