Research On The Reliability Of High Purity Graphite Carbon Doped Sb₂Te Phase Change Thin Films And Devices

Chalcogenide devices,such as phase change memory（PCM）,memristor and optical memory,are based on the reversible phase transition between amorphous and crystalline chalcogenide materials.The chalcogenide-based compound Ge₂Sb₂Te₅（GST）is the most widely studied phase change material and has been extensively studied in both experimental and theoretical aspects.However,its poor thermal stability,low operating speed（～50 ns）,large thickness change rate during phase transition,and resistance drift have limited the widespread application of GST.In addition to GST,Sb-Te binary alloys are another important sulfur based phase change material that has attracted widespread attention from researchers in recent years.The Sb-Te binary alloy exhibits a fast crystallization rate due to its growth dominated crystallization mechanism,which can improve the operating speed of the device.However,the poor thermal stability,large grain size,and high thickness change rate of Sb-Te binary alloys limit their practical application.In recent years,researchers have focused on incorporating transition metals into Sb-Te based alloys to improve their performance.However,due to the low electronegativity of transition metals and incompatibility with complementary metal oxide semiconductor（CMOS）processes,the performance of transition metal doped Sb-Te phase change materials is still unsatisfactory.Therefore,the process control and practical application of transition metal doped Sb-Te based materials for semiconductors are difficult.This article improves the properties of Sb₂Te binary alloy by doping it with high-purity graphite C.The main results obtained are as follows:（1）A high purity graphitic carbon C-doped Sb₂Te（C-Sb₂Te）compatible with CMOS process is proposed to improve the Sb₂Te material properties.Characterization by resistance-temperature（R-T）,10-year date retention,X-ray diffraction（XRD）,X-ray reflection（XRR）,transmission electron microscopy（TEM）and X-ray photoelectron spectroscopy（XPS）illustrates that the C-Sb₂Te phase change material has excellent thermal stability,low thickness variation（0.89%）,and low resistance drift（0.0149）.The memory devices prepared based on C-Sb₂Te phase change materials exhibit fast operation speed（5 ns）along with large resistance ratio,low operation voltage（2 V）and low power consumption（6.9 p J）,and the performance of C-Sb₂Te is much better than that of conventional GST and transition metal-doped Sb-Te system phase change materials.（2）Then,theoretical calculations and finite element simulations of the C-Sb₂Te phase change material were performed,and ab initio molecular dynamics（AIMD）simulations showed that C-C bonds and C-Sb bonds and C-C chains were formed in the C-Sb₂Te structure,and the doped C atoms confined the phase change transition of the material to a small region,which restricted the Sb₂Te grain growth and changed the internal structure of the phase change material.At the same time,finite element simulation of phase change unit devices shows that changes in heating electrode size,phase change material conductivity,and thermal conductivity can all affect the heating efficiency of phase change materials,causing changes in the center temperature of phase change units（666 K-2481 K）and changing the power consumption of the device.Our study shows that the proposed C-Sb₂Te in this paper is a promising phase change material for high speed,high thermal stability,and high reliability PCM applications.