Research On Threshold Selector For High-density RRAM Applications

As a leading candidate for the next-generation nonvolatile memory applications,RRAM has a great advantage in terms of power consumption,capacity,speed and endurance.Due to its simple device structure,RRAM can be easily integrated in passive cross arrays to achieve the highest integratation density with the smallest cell size.However,a cross-point array suffers unavoidable cross-talk problem due to leakage current paths through neighboring unselected cells with low resistances,leading to a misreading problem.Thus,a selector element with high selectivity and low off-current in series with the memory device is essential to suppress the sneak current issue for practical high density passive crossbar array applications.This thesis mainly focuses on the research of threshold selector devices for high-density resistive memory applications.We designed and fabricated a bidirectional threshold selector and unidirectional threshold selector.By the integration of the selector with the appropriate RRAM device,a 1S-1R memory device was demonstrated,in which the undesired sneak current was significantly suppressed.The main work is summarized as follows:(1)We investigated the influence of the switching layer thickness,the device size and the top electrode material on the threshold switching characteristics in the ZrO2-based unidirectional threshold selector.We revealed the impact of active metal electrode diffusion on the device performance as the thickness of dielectric material scaled.Furthermore,the selectivity was increased highly by decreasing electrode size.In particular,the selectivity was about 2×108 when the electrode size was scaled down to 300 nm.In addition,we investigated the threshold switching characteristic of Ag/Ti/ZrO2/Pt devices with different Ti insertion thicknesses.(2)We designed and fabricated a high performance Zr02-based bidirectional threshold selector.We investigated the effects of annealing temperature and annealing time on the threshold switching characteristics.The transition from unidirectional to bidirectional threshold switching characteristics was achieved in the Ag/ZrO2/Pt stack structure wherein Ag was intentionally incorporated in the medium by the optimized annealing treatment.These incorporated Ag atoms provide sufficient silver sources for the negative threshold switching.The bidirectional threshold selector exhibited excellent threshold switching characteristics,such as low leakage(<300 pA),small threshold swing(<5 mV/dec),high selectivity(?107)and fast response time(<20 ns).In addition,we analysed the bidirectional switching mechanism by characterization analysis.(3)We designed and fabricated a unidirectional threshold selector for cross-point bipolar RRAM array.The selector exhibited rectifying characteristics in the high resistance state and the rectification ratio was as high as 103 at ± 1.5 V.By adjusting the compliance current values,a reversible conversion between volatile threshold characteristics and nonvolatile memory characteristics in the Ag/a-Si/Pt devices can be achieved.Due to the high reverse current about 9 mA at-3 V,this unidirectional threshold selector can be used as a selection element for bipolar-type RRAM.(4)In order to confirm the suppression of the selector device on the undesired sneak current,a proper bipolar RRAM device was integrated on top of the selector device and a bipolar 1S-1R memory device was demonstrated.Through the calculation of the readout margin,the storage capacity of cross-point arrays based on 1S-1R cells is dramatically improved.