Study On Preparation And Performance Of GeSe-based Ovonic Threshold Switch

Three-dimensional phase change memory(3D PCM)adopts a cross-stacking method to ensure excellent storage performance while greatly improving storage density,becoming the next-generation non-volatile memory closest to large-scale industrialization.However,as the scale of the array continues to increase,the current flowing through the half-selected cells becomes larger and larger,bringing problems such as serious information misinterpretation,misoperation,and increased circuit power consumption.Traditional transistors are difficult to realize three-dimensional stacking due to their three-terminal structure and preparation process.New types of two-terminal selectors must be developed to suppress the array leakage current,among which Ovonic Threshold Switch(OTS)selectors have become the best choice for integration with PCM due to their excellent characteristics such as high driving capability and high thermal stability.However,the most mature OTS selectors(represented by Ge Se As Si)generally have problems such as excessive threshold voltage,complex material components,and especially containing the toxic element As.In this paper,we focus on Ge Se based OTS selector with simple material components and nodoping of toxic elements.Through first-principle calculation and device fabrication,optimized performance of Ge Se-based OTS selector without As is obtained.The main results obtained in the paper are as follows:(1)The optimal "self-heating" structure device is designed,and the device preparatio n process is investigated.The sputtering coating parameters,small-hole array layout of electron beam lithography,and top electrode array layout of UV lithography were studied in detail.At the same time,a complete process flow was designed,laying a solid foundation for subsequent device preparation and performance analysis.(2)First-principles calculations are used to study the changes in the local structure and electronic density of states of the material caused by Ge Se components and doping,and the corresponding OTS performance change laws are analyzed.It is found that when there are more Ge atoms in Ge Se material,the number of tetrahedra inside the material is large,which can improve the thermal stability of the material;the simple element C doping can play a role similar to As doping,both of which can increase the tetrahedral configuration inside the material and thus improve the thermal stability of the material,but excessive C doping will deteriorate the material properties;if doping with As,a large number of interband states will be introduced,which will increase the leakage current of the device,while doping with a small amount of Si can improve the mobility bandgap and reduce the leakage current of the device.(3)The performance of the GeSe based OTS selector was optimized from three aspects,including preparation process,component control and simple element doping.The effect laws of the thickness,component ratio,feature size,and a small amount of Si doping on the device performance of the Ge Se based OTS selector were systematically analyzed.The Ge-Se-Si-based OTS selector with simple,non-toxic composition and greatly reduced threshold voltage(down to 1.25V)was designed and optimized,which solved the problem of excessive threshold voltage of the Se-based OTS selector.It's also found that a small amount of Si doping can reduce the leakage current of the device by up to about 200 times(23.8p A).Combined with the analysis of the first-principles calculation results,it can be seen that Si doping reduces the leakage current by increasing the mobility band gap.At the same time,the doped Si atoms and Se atoms form bonds,which increases the number of defects formed by Ge atoms,thereby lowering the threshold voltage of the device.