P.E. Troyan1, M.A. Sviridenko2

1,2 Tomsk State University of Control Systems and Radioelectronics (Tomsk, Russia)

The paper considers the influence of the lower electrode microrelief and dielectric layer thickness on the value of electrical strength and electric field for the formation of thin film metal-dielectric-metal structures as a memristor-type memory element.

The study was performed on MDM structures with aluminum electrodes and SiO2 dielectric films. The range of SiO2 layer thickness varied from 10 nm to 100 nm.

Micro-relief refers to the degree of micro-roughening of the lower electrode, associated with the technological process of obtaining the aluminum film and having micro-roughenings significantly larger than at the atomic level.

Metallic films produced by magnetron sputtering have microroughnesses uniformly distributed over the surface. Films obtained at a substrate temperature of T = 300 K are characterized by microroughness, often cone-shaped height of 10 nm to 100 nm. Films obtained at substrate temperature T = 600 K have a block structure. The size of the blocks ranges from 0.5 nm to 0.8 nm.

The experimental samples were subjected to breakdown at constant voltage. The data for the first breakdown of MDM structures are given in the paper. For subsequent breakdowns, the breakdown voltage and electrical strength increase. The electrical hardening effect is observed on the films studied if the bottom electrodes are obtained by magnetron sputtering and thermal evaporation at a substrate temperature of 600 K. For structures with bottom electrodes obtained at a substrate temperature of 300 K, the hardening effect appears at dielectric thicknesses starting from 40 nm, but the electrical strength decreases to 2.7 MV/sm at dielectric thicknesses less than 10 nm, which contradicts the laws of the electrical hardening phenomenon. Such behavior can be explained by the fact that the value of the electrical strength of MDM structures is determined not only by the dielectric material and the technology of its production, but also by the microrelief of the lower electrode.

Previously, it was experimentally established that the range of dielectric thicknesses at which the electrical molding process takes place is limited to dielectric thicknesses of 10 nm and 100 nm. The lower boundary is determined by the fact that when the dielectric thickness is less than 10 nm, a significant number of shorted MDM structures appear, while large currents flow through the non-shorted structures and the volt-ampere characteristic of structures with non-volatile memory is not observed. In this case breakdown of MDM structures becomes more probable.

The upper limit is determined by the fact that at such thicknesses (≈100 nm) Еf is close to Еpr and electric breakdown of MDM-structure is more probable than molding process. The best conditions for molding occur when the electric field amplification factor will be minimal.

This study has shown that the breakdown process and electrical forming of MDM systems depend significantly on the ratio of the geometrical dimensions of the microroughnesses on the bottom electrode and the thickness of the dielectric layer. For the first time, a deviation from the laws of the effect of electrical hardening was found for structures with a bottom electrode obtained by thermal evaporation of aluminum in a vacuum onto a glass substrate at room temperature (T = 300 K) in the region of dielectric thickness less than 10 nm.

It is shown that for MDM structures with different degrees of microfine lower electrode polarity effect (dependence of Epr on the polarity of the applied voltage) is almost absent if the lower electrode is "smooth" and is not pronounced if the lower electrode is sprayed at substrate temperature T = 600 K.

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