Implications of Lower Zero-Field Activation Energy of Dielectric in Al2O3/HfO2 Bi-Layer Dielectric RRAM Forming Process
Authors: Sarkar Biplab , Lee Bongmook , Misra Veena 
Published: February 2020

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 Ion/Ioff 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]. Ea
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 Ion/Ioff 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 (Ea) of Al2O3 leads to a faster filament formation.

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