Investigation of electroforming characteristics of TiO₂ based resistance switching devices

Resistance switching devices based on transition metal oxides have generated significant research interest over the last decade due to the promise they hold for non-volatile memory applications. Currently they are one of the leading candidates for replacing Flash memory technology as it nears its scaling limits. Despite many years of work and many encouraging demonstrations, the physical mechanism that drives the resistance switching phenomenon remains very poorly understood. A model based on migration of oxygen vacancies is often invoked, however direct proof of this model still remains illusive.

In this thesis, we developed a kinetic model of oxygen vacancy migration. Using this model, simulations were carried out on a 1-D device to examine the resistance switching and retention dynamics. It is found that in order to achieve fast switching (100 ns) and long retention (10 years), the vacancy migration based model requires unrealistic electric field (>10 MV/cm) and temperature (>1500 K) combinations. This situation does not change even when non-linear dependence of vacancy velocity on electric field is taken into account.

A significant portion of this thesis is focused on detailed examination of the electroforming characteristics of TiO2 based resistance switching devices. Electroforming is a step where a device is electrically stressed in order to trigger permanent changes to the oxide layer. During this process, the resistance of the device is usually lowered by 5-6 orders of magnitude. Stable resistance switching can only be obtained after performing electroforming. Most studies for this work were performed on 5 μm × 5 μm sized devices with Pt/TiO2/Pt structure. Transient pulsed method developed as part of this work allowed for precise determination of the voltage, time, and temperature combination that led to the onset of electroforming. Analysis of the transient data revealed that activation energy associated with electroforming decreases non-linearly with electric field. A vacancy migration based model cannot adequately explain this dependence. The experimental observations are better explained using a hole-injection model which asserts that onset of localized conduction is an electronic process rather than an ion migration based process.

Electroforming often produces pronounced morphological changes to the devices. These morphological changes are a strong function of the voltage pulse amplitude and width used to trigger electroforming. Electro-thermal simulations were used to correlate these changes with transient power dissipation during the electroforming process. The simulations indicate that electroforming did not produce extremely small conductive filaments (10-100 nm diameter) with very small resistance values (100-200 Ω), as it is sometimes reported in literature. Rather, the changes in the resistivity of the TiO2 layer spanned over an area as large as 8-9 μm2. Over this region, the resistivity changed gradually over 2-3 orders of magnitude. The filament(s) responsible for resistance switching can be located anywhere within this region.