Field cycling behavior and breakdown mechanism of ferroelectric Al_0.78Sc_0.22N films

Abstract

The effects of field cycling of Al0.78Sc0.22N capacitors to ferroelectric poperties are investigated. In the first hundreds of switching cycles, the reduction in the switching voltage was observed, possibly due to the formation of nitrogen vacancy to facilitate the atom displacements. With futher switching cycles, fatigue effect was observed, especially for domains with low switching voltage. The leakage current analysis indicates continuous downward band bending with the number of switching cycles, effectively reducing the Schottky barrier heigh for electron. The breakdown of Al0.78Sc0.22N films is triggered by the Joule heat due to excessive leakage current. The mechanism is in contrast to conventional ferroelectric materials, where breakdown is triggred by local conductive filametns.

Effect of Substrate-RF on Sub-200 nm Al0.7Sc0.3N Thin Films

“…√ E and ln(J/E) vs. √ E, Schottky emissions result in permittivities in line with the published work in [16,17], differently than the Poole-Frenkel model, which results in unrealistic values (>30). A summary of the extracted optical permittivities is reported in the supplemental material.…”

“…In order to understand the source of the leakage, the current densities are plotted with different axis to discriminate a Schottky from Poole-Frenkel emission, as described in [38]. Despite that a linear trend is found in both ln(J/T 2 ) vs. √E and ln(J/E) vs. √E, Schottky emissions result in permittivities in line with the published work in [16,17], differently than the Poole-Frenkel model, which results in unrealistic values (>30). A summary of the extracted optical permittivities is reported in the supplemental material.…”

“…As in the leakage analysis, on which low substrate-RFs induced lower fields to transition from the two resistive states, the PUND-based measurements Furthermore, the induced positive stress is reported to increase the gaps between the polarization-dependent current emissions, resulting in larger hysteresis on the ln(J/T 2 ) vs. √ E capacitor characteristic. As described in [13,16,17], the AlScN polarity highly affects the band alignment with the metals, which results in different barrier heights and in a clear shift in the leakage current for both positive and negative bias. Differently than [16], which was based on a symmetric capacitor structure, we report a preferred orientation state for the AlScN films, i.e., they are characterized by lower losses, which corresponds to the asdeposited polarity, unchanged among all the samples.…”

“…While the slopes remains invariant, i.e., with the same optical permittivities per different states, the curve intercepts, (=ln(A × T t ) + qΦ Bn,app /(K b T)), change according to the MFM state, indicating a variation in the apparent Schottky barrier height (Φ Bn,app ) and in the effective Richardson constant (A × T t ). Such variations are quantifiable by plotting the extracted curve intercepts per different temperatures, as shown in Figures 10b and S4 for the positive-up and positive-down combinations, respectively [16,17]. Figure 10c summarizes how the lower the substrate-RF, the larger the impact of the polarization state on both Φ Bn,app and A × T t ., i.e., on the static behavior of the MFM structure.…”

“…Despite the large polarization values, if compared with other ferroelectric materials [15], the coercive field and leakage current are still too high for practical integration. While a few works addressed the latter issue [16,17], several groups demonstrated how it is possible to reduce the coercive field by acting on either the AlScN structure or on the experimental set-up. A higher Sc-content [10], higher crystallinity [18], the use of different substrates [19], and testing-temperature [20] have been proven to reduce the voltage needed to switch, while still maintaining large polarization values.…”