Abstract
In this work, analysis and simulation of all experimentally observed switching modes in hafnium oxide based resistive random access memories are carried out using a simplified electrical conduction model. To achieve switching mode variation, two metal-insulator-metal cells with identical stack combination, but varying oxygen stoichiometry of the hafnia layer, namely, stoichiometric vs highly deficient, are considered. To access the individual switching modes, the devices were subjected to a variety of cycling conditions comprising different voltage and current ranges. For modeling the device behavior, a single or two antiserially connected memdiodes (diode with memory) were utilized. In this way, successful compact simulation of unipolar, bipolar, threshold, and complementary resistive switching modes is accomplished confirming the coexistence of two switching mechanisms of opposite polarity as the basis for all observable switching phenomena in this material. We show that only calibration of the outer current–voltage loops with the memdiode model is necessary for predicting the device behavior in the defined region revealing additional information on the switching process. The correspondence of each memdiode device with the conduction characteristics of the individual top and bottom metal-oxide contacts allows one to assess the role played by each interface in the switching process separately. This identification paves the path for a future improvement of the device performance and functionality by means of appropriate interface engineering.
Highlights
Resistive random access memories (RRAMs) have been intensively studied during the last decade revealing promising characteristics such as fast switching speed in the subnanosecond region,[1] low voltage operation, low power consumption,[2] resilience toward ionizing radiation,[3] and scalability[4] below 10 nm
For the deficient stack an additional memdiode with opposite polarity (EC2 and EC3 in Fig. 3) is required for fitting the experimental curves. This distinction indicates that for the deficient stack case an additional switching phenomenon is superimposed to the observed cf8-bipolar resistive switching (BRS) behavior corresponding to the stoichiometric stack, which was found to result in the observed complementary resistive switching (CRS) [Fig. 3(f )] and combined f8 + cf8-BRS [Fig. 3(e)], respectively
If the voltage protocol is further increased with respect to that shown in Fig. 3(c), the set event in the f8-switching is followed by the reset event in the cf8-switching for the positive voltage direction, while for the negative voltage direction the set event in the cf8-switching is followed by the reset event in the f8-switching resulting in the CRS mode [see Fig. 3(f )]
Summary
Resistive random access memories (RRAMs) have been intensively studied during the last decade revealing promising characteristics such as fast switching speed in the subnanosecond region,[1] low voltage operation, low power consumption,[2] resilience toward ionizing radiation,[3] and scalability[4] below 10 nm. The impact of the oxygen content of the dielectric layer on the electronic structure can be seen by the low-loss electron energy-loss spectra depicted, which shows a clear difference between deficient and stoichiometric films. The stacks used in this work (TiN/HfO2/Pt and TiN/HfO1.5/Pt) were grown using a custom designed molecular beam epitaxy (MBE) unit enabling the tuning of the oxygen content (crystal structure) of the functional hafnia layer in the full range from metallic hafnium to highly deficient (tetragonal) to stoichiometric (monoclinic) films. The electrical characterization was performed using a Keithley Semiconductor Characterization System 4200 (SCS 4200) by applying bias to the platinum top electrode (TE) while grounding the titanium nitride back electrode (BE)
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