Implications of Lower Zero-Field Activation Energy of Dielectric in Al2O3/HfO2 Bi-Layer Dielectric RRAM Forming Process

Resistive memory offers many favorable properties such as simple fabrication, higher retention, and excellent endurance for future non-volatile memory applications [1]. However, some of the physical mechanisms of these memories are not well understood, one of such phenomenon is oxygen vacancy generation which leads to the creation of the filament [2]. Filament formation process has been studied extensively for single layer dielectrics [2-4], but very few studies on filament formation in bilayer dielectric has been conducted so far. Previously, it has been reported that addition of Al₂O₃ into HfO₂ RRAM provides numerous advantages [5], one critical advantage being reduction in the forming voltage [6]. In this experiment, we studied the vacancy generation and forming properties of single layer HfO₂ and Al₂O₃/HfO₂ bilayer dielectric RRAM.

Metal-insulator-metal (MIM) capacitors with area of 100 µm x 100 µm were fabricated with either HfO₂, or bilayer Al₂O₃/HfO₂ as insulator. Tungsten was chosen as electrodes so that interfacial oxide growth can be prevented. Fig. 1 shows the forming characteristics of 5.3 nm HfO₂ dielectric and 5.3 nm HfO₂ + 1.1 nm Al₂O₃ dielectric, equivalent electric field is calculated as voltage per EOT. A gradual rise in leakage current leading to breakdown at ~5.5V was observed in only HfO₂, whereas a two-step rise in leakage current was observed in Al₂O₃/HfO₂. A fast rise was observed up to ~2V, followed by a gradual rise leading to breakdown at ~5.1V. Fig. 2 shows the forming voltage with single HfO₂ and bilayer HfO₂/Al₂O₃ dielectrics. Inset in Fig. 2 shows reset characteristics of single layer HfO₂ and bilayer dielectric. It was observed that bilayer dielectric not only provides large I_on/I_off ratio due to the incorporation of Al₂O₃ layer but also offers lower forming voltage. Forming voltage uniformity is also better in bilayer dielectric as shown in Fig. 2. All these observations make Al₂O₃/HfO₂ bilayer dielectric a superior candidate over single layer HfO₂for future RRAM applications.

To understand the origin of lower forming voltage with bi-layer dielectric, oxygen vacancy generation property of dielectric was studied. Vacancy generation in the dielectric is governed by the equation (1), Where G represents the vacancy generation rate, G₀ represents the effective vibration frequency, b represents the bond polarization factor between metal and oxygen atom, and F is the effective electric field in the oxide [6]. E_a for Al₂O₃ and HfO₂ was found to be 1.8 eV and 4.6 eV  respectively; b for Al₂O₃ and HfO₂ was found to be 19 eA⁰ and 56 eA⁰ respectively; and value of G₀ were assumed to be 2 x 10¹³ Hz and 7 x 10¹³ Hz for Al₂O₃ and HfO₂ respectively [4,7]. Simulation showed that vacancy generation is a faster process in Al₂O₃ because of lower Ea which dominates the exponential term of equation (1). Fig. 3 shows the electric field and corresponding vacancy generation rate in individual layers of bilayer dielectric. It has been reported that dielectric have excess of vacancies when value of G approaches 0 [4]. Electric field in Al₂O₃ layer is higher than in HfO₂ leading to excess number of vacancies in Al₂O₃ at ~2V, whereas breakdown of entire bi-layer stack occurs at ~5.4 V. Comparing with single layer HfO₂ dielectric, 2V corresponds to an electric field of 3.77 MV/cm and a generation rate of ~10^-30 vacancies per second, which is significantly low. Hence at ~2V, there is no additional vacancy generated in 5.3 nm HfO₂ dielectric, whereas entire Al₂O₃ layer have excess of vacancies in case of 1.1 nm Al₂O₃ + 5.3 nm HfO₂. Since every vacancy have higher electric field nearby it [4], hence presence of excess vacancies leads to faster vacancy generation in case of bilayer dielectric.

We show that the incorporation of thin Al₂O₃ layer on HfO₂ dielectric lower the forming voltage as well as large I_on/I_off ratio. It was found that the reduction in forming voltage with bi-layer dielectric is attributed to (i) low-k of Al₂O₃ leads to higher electric field, and (ii) lower zero-field activation energy (E_a) of Al₂O₃ leads to a faster filament formation.

[1] H. –S. P. Wong, Proceedings of IEEE, vol. 100, no.6, pp.1951-1970 (2012).

[2] L. Vandelli, IEEE International Memory Workshop (IMW), pp.1-4 (2011).

[3] S. Yu, Functional Metal Oxide Nanostructures. New York: Springer-Verlag, (2011).

[4] L. Vandelli, IEEE Transactions on Electron Devices, 60(5), 1754-1762 (2013).

[5] S. Yu, International Symposium on VLSI Technology, Systems and Applications (VLSI-TSA), pp. 1-2 (2011).

[6] L. Goux, Symposium on VLSI Technology (VLSIT), pp. 159-160 (2012).

[7] S. Davis, Journal of Physics: Condense Matter, vol. 23, 495401 (2011).