Conductive bridge random access memory (CBRAM) has been regarded as a promising candidate for the next-generation nonvolatile memory technology. Even with the great performance of CBRAM, the global generation and overinjection of cations after much repetitive switching cannot be prevented. The overinjection of cations into an electrolyte layer causes high-resistancestate resistance (R HRS ) degradation, on/off ratio reduction, and eventual switching failure. It also degrades the switching uniformity. In this work, a Cu-cone-structure-embedded TiN/TiO 2 /Cu cone/TiN device is fabricated to alleviate the problems of Cu-based CBRAM, mentioned above. The fabrication method of the device, which is useful for laboratory scale experiment, is developed, and its superior switching performance and reliability compared with the conventional planar device. The insertion of the Cu cone structure allows the placement of only a limited amount of cation source in each cell, and the embedded conical structure also concentrates the applied electric field, which enables filament growth control. Furthermore, the concentrated field localizes the resistive switching on the tip area of the cone structure, which makes the effective switching area about tens of nanometers even for the much larger area of the entire electrode (several µm 2 ).
The retention behavior of a HfO2 resistive switching memory device with a diameter of 28 nm and an ultra-thin (1 nm) HfO2 layer as the switching layer was examined. Ta and TiN served as the oxygen vacancy (VO) supplying the top and inert bottom electrodes, respectively. Unlike the retention failure phenomenon reported in other thicker oxide-based resistance switching memory devices, the current of both the low and high resistance states suddenly increased at a certain time, causing retention failure. Through the retention tests of the devices in different resistance states, it was concluded that the involvement of the reset step induced the retention failure. The pristine device contained a high portion of VO-rich region and the location of the border between the VO-rich and VO-free regions played the critical role in governing the retention performance. During the reset step, this borderline moves towards the Ta electrode, but moves back to the original location during the retention period, which eventually induces the reconnection of the disconnected conducting filament (in a high resistance state) or strengthens the connected weak portion (low resistance state). The activation energy for the retention failure mechanism was 0.15 eV, which is related to the ionization of neutral VO to ionized VO.
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