Research On Conductive-bridge Threshold Switching Selectors Based On Chalcogenides

Emerging resistive memory devices are proposed to supple or replace DRAM and NAND which confronts performance and cost gap and also the scaling wall,thus to promote the performances of computing system.Phase change memory(PCM)arises wide attention owing to the scalable structure and industrial maturity.Particularly,3D PCM of high integration density is highly expected by the market.The main technical challenge is to address the sneak path crosstalk,which requires 3D stackable two-terminal selectors.Among different types of selectors,conductive-bridge threshold switching(CBTS)selector is a strong competitor owing to the large selectivity,low leakage and simple material.However,the driving current of CBTS selectors is generally not enough for PCM operating.For example,a PCM cell of 80 nm critical dimension requires 8 MA/cm² driving current density(J_ON)in 4F² array,which is far beyond the typical 0.5 MA/cm² J_ON of CBTS selectors.This paper performed systematical study on the CBTS switching mechanism from the aspects of large-current induced device failure and the influence of conducting filament(CF)size and quantity on the current driving.Based on that,three optimizing schemes on device performances were proposed and demonstrated,including accelerating CF rapture,confining the CF size and increasing the CF quantity.The main results are as following:Aiming at the CBTS device failure owing to irremovable CF under high current,this paper proposed a selector based on Ag electrode of high diffusing coefficient and GeSe switching layer of abundant defects,which accelerates the CF rupture to improve current driving.The fabricated Pt/Ag/GeSe/Pt selector realized high driving current(200?A)and large selectivity(10⁹).Three selectors adopting electrodes of different diffusing coefficient and oxidation potential were fabricated as comparison,indicating that electode of high diffusing coefficient helps on improving driving current.Meanwhile,the characterization of three selectors with switching layers of different bandgap demonstrates the tunability of switching layer bandgap on selectivity.Considering that the thermal stability has always been a shortage of chalcogenides,GeSe layer is deposited on different substrates and tested under different temperature while keeping for different time,showing a tolerance of 550oC.Aiming at the CBTS device failure caused by CF overgrowth under large current,this paper proposed a selector based on metal sulfur cation layer Cu S and chalcognide switching layer GeSe,which confines CF size to further improve driving current.The fabricated Ti W/Cu S/GeSe/Pt selector successfully increases the driving current to 600?A and possesses1.25×10⁹ selectivity at the same time.The driving current of the device is sufficient for PCM operating and broke the current dilemma of CBTS selectors when the device was reported.As a result,the high tunability of CF size on device driving current was proved.Further,reliability tests were performed to verify the industrial applicability.The current driving is improved when device scales down to 100 nm diameter.Meanwhile,the device is well behaved under thermal and leakage stress.Also,the device shows uniformity from device to device and cycle to cycle.For the issue that the switching speed of CBTS selectors is slow,the influences of writing pulse amplitude and duration on switching speed were studied,and a method of realizing high-speed switching by applying shaped writing pulse is proposed.Aiming at the issue that the driving current of CBTS selectors decreases drastically as the device scaling,resulting in a low J_ON(<0.5 MA/cm²),this paper proposed a selector consisting of super-ionic cation layer Cu₂S and chalcogenide switching layer GeSe,which increases the CF quantity to improve J_ON.Combining CF thermal effect and Cu⁺subphase migration,multiple localized Cu⁺sources are naturally formed,facilitating the growth of many CFs.The fabricated Pt/Cu₂S/GeSe/Pt devices of 250 nm size demonstrated a J_ON of 10MA/cm²,increasing the J_ON of CBTS selectors by more than 10 times.Also,the J_ON of 10MA/cm² enables the selector to be integrated in 4F² PCM array.Meanwhile,the device possesses the highest selectivity of 10¹⁰ and overcomes the tradeoff between high current and cycling.The analysis of the failure process of the device shows that the device can switch off more easily under high current and massive cycles than other reported devices.According to the TEM observation,densely distributed Cu⁺sources with a diameter of 20 nm and an interval of 20 nm are formed as expected.Multiple CFs grow from the cation sources to deliver current in parallel,thereby increasing the J_ON.The CFs can naturally form in the devices,thus the process includes only simple film deposition,making the selector industrially practical.