Steady Multi-Level Phase Change Memory Based On Ge-Ga-Sb Material

Phase change memory(PCM)is a promising candidate for the next-generation data storage and computing technologies.Multi-level cell(MLC)technology,one of the most competitive advantages of PCM,increases the storage capacity of PCMs exponentially without changing the physical volume,and thus the cost per bit of data is remarkably reduced.At the same time,MLC is also one of the necessary characteristics of phase change memory used in new computing technologies.However,the MLC PCM faces a series of instability problems such as narrow operating window,process variation and resistance drift in practical applications.To this end,Ge-Ga-Sb has been proposed for the application of MLC PCM.Its excellent multi-value characteristics solve the above-mentioned instability problem.The specific research process is as follows:(1)MLC characteristics of Ge-Ga-Sb.In this paper,Ge₁₅Ga₂₅Sb₆₀(GGS)thin film and 250 nm pillar structure PCM cells based on GGS are prepared by micro-nano processing technology.In the R-T test of the thin film and the electrical test of the phase change memory device,GGS exhibited three resistance states,namely,high resistance state(HRS),middle resistance state(MRS)and low resistance state(LRS).(2)MLC stability of GGS.In the electrical test process,the GGS-based PCM cells showed a large resistance ratio(>5)between neighboring levels,a large voltage operating window(>1.2 V),and a low resistance drift coefficient(0.025,1/4 of Ge₂Sb₂Te₅)and high crystallization temperature(300?).The above features can effectively solve the aforementioned instability problem in the practical application of multi-value storage.(3)MLC mechanism analysis of GGS.XRD results show that the crystal structures of Ge-Ga-Sb in MRS and LRS are both rhombohedral structures,indicating that the MLC characteristics are not caused by multiple crystal structures.Atom-probe tomography(APT)shows that the LRS has nano-scale element aggregation.Transmission electron microscopy shows that HRS of GGS is amorphous and the elements are uniformly distributed.The MRS of GGS is a polycrystal composed of small-sized(<10 nm)crystal grains,and the elements are equally distributed;while LRS is a polycrystalline composed of large-sized crystal grains(>40 nm)with inhomogeneous distribution of elements.Through the selected area electron diffraction analysis,the crystal structures of HRS and LRS are both rhombohedral structures,which are consistent with the XRD test results.In summary,the three resistance states correspond to a homogenous amorphous phase,a homogenous polycrystalline phase,and a heterogeneous polycrystalline phase,respectively.(4)Ge-Ga-Sb material design.By changing the Ga content,Ge-Ga-Sb can be used in different scenarios,such as multi-value storage applications and high-temperature applications.