Electrical Properties and Nanoresistive Switching of Ni-HfO2-Si Capacitors
Authors: García Héctor , Gonzalez Mireia , Vaca Cesar , Castán Helena , Dueñas Salvador , Campabadal. Francesca , Miranda Enrique , Bailon Luis 
Published: February 2020

Resistive random access memory (ReRAM) devices based on HfO2, in which a conducting filament (CF) that acts as a circuit breaker/switch between the electrodes, are studied intensively because of its high compatibility with CMOS process beyond the 22 nm node [1]. The breaking and forming of a CF occurs over a nanometer-scale region in a timespan of nanoseconds [2]. In order to explore their potential applications, it is imperative to investigate the cryogenic performance of these devices [3]. In this work, the electrical characterization of Ni/HfO2/Si ReRAM devices is carried out. Unipolar resistive switching (RS) loops are reported in the whole temperature range studied (77 - 473 K).

The MOS devices were fabricated on (100) n-type CZ silicon wafers with resistivity in the range (7-13) mµÙ·cm. The 20 nm-thick HfO2 layers were deposited by atomic layer deposition (ALD). The top Ni electrode was deposited by magnetron sputtering. A complete electrical characterization with focus on the oxide and oxide-semiconductor quality assessment was performed in a cryogenic system with liquid nitrogen. Fig. 1 shows the Poole-Frenkel fitting of I-V curves at several temperatures. Data fit well especially in a limited high E-field range [4, 5], and indicates that the main conduction is bulk limited, associated with the field enhanced thermal excitation of charge carriers from traps. The theoretical value of field-lowering coefficient, βPF, of HfO2 is 1.5∙10-5 eV m1/2V-1/2 [6]. The experimental values are reasonably in agreement with the theoretical one. In Fig. 2 current-temperature dependence for several bias voltages is shown. In the 150-300 K temperature range the relationship between ln(I) and 1/kBT is clearly lineal, and the slopes of the lines do not vary appreciably with the applied voltage, indicating that the conduction takes place through an activated process having a single activation energy, ΔEσ. The activation energy values obtained are similar to those previously reported for HfO2[7]. As the temperature decreases below 155 K the current becomes temperature independent. This can be attributed either to the presence of the competing emission mechanisms or to the device self-heating [8].

After the study of pristine samples, the MIS devices were electroformed by DC reverse bias sweeping from 0 to 13 V with a current compliance of 0.1 mA. Electroforming causes a current-limited oxide breakdown, and a metallic filament is created [9]. So, devices are taken to the low resistance state (LRS). A cycle is completed when the CF is broken by applying a reset voltage and samples switch to the high resistance state (HRS). I-V cycles were recorded using an HP4155B semiconductor parameter analyzer in the voltage sweep mode with current compliance of 0.1 mA for the HRS state and 100 mA for the LRS state (Fig. 3). A whole discussion of all the experimental results will be given at the conference.

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Figure 1