Ovonic Threshold Switching Selection Device Based on Chalcogenide ZnTe for Cross-Point ReRAM Device

Cross-point array structure has attracted increasing attention for high-density ReRAM, owing to the development of ReRAM with promising scalability. Cross-point array of only resistors(1R) cannot function because of sneak current through unselected devices, which can be reduced by integrating resistive memory elements with bidirectional nonlinear selection devices(1S1R). Namely, the high value of K_w in ON states, which is the writing current @ V_write / the current @ 1/2V_write, was required in cross-point ReRAM memory industry.

 In this study, the high current density non-linear IV curve of Ovonic threshold switching(OTS) selection device based on Chalcogenide ZnTe was shown and the ALD HfO₂ switching device has the linear properties of ON states and the compliance current of 100uA. In 1S1R unit cells based on ZnTe OTS selection device of low off current (< 400nA) and HfO₂ switching device, excellent non-linear characteristics of ON states such as K_w of 522, K_r (reading current @V_read / current  @ 1/2V_read) of 1430 and on/off ratio of 43 were shown. OTS behaviors with ZnTe film thickness, device area and annealing temperatures were investigated. Initial current during electro-forming process was decreased but OTS characteristics were not changed with increasing ZnTe thickness and decreasing device area.  Initial current in electro-forming process and OTS characteristics at annealing temperature of below 200°C was not changed, but initial current and off current was rapidly increased and threshold voltage (V_th) in OTS was decreased at above 250°C because of Te diffusion in ZnTe film. To suppress the diffusion of Te atoms, the reactive N₂during deposition which can improve the thermal stability of the chalcogenide glass was performed. And chemical compositions of ZnTe thin films were analyzed by Rutherford backscattering spectrometry(RBS). Also, crystallinity as a function of annealing temperature was investigated by Raman spectroscopy, X-ray diffraction (XRD) and plan-view transmission electron microscopy (TEM).