Thermal Stability Study Of Weyl Semimetal WTe₂ And Its Heterostructures By Micro-Raman Scattering

Weyl semimetal Td-phase WTe₂,a novel topological matter,possesses strong spin-orbit coupling and non-trivial topological protected band structures,and thus exhibits a pronounced charge-to-spin conversion efficiency and spin-orbit torque,which is comparable with the ones in topological insulators.High efficient manipulation of magnetization can be realized by using WTe₂,therefore,Weyl semimetal WTe₂ holds great promises as a superior spin current source material.WTe₂/Ti heterostructures can well address the issue that the ferromagnetic layer with perpendicular magnetic anisotropy can not be directly prepared on WTe₂ layer.For the practical device applications,the WTe₂ and WTe₂/Ti heterostructures has to be tolerant up to～350-400℃for more than 30 min normally in the high temperature process of the established semiconductor manufacturing line（i.e.the back-end-of-line,BEOL）.Therefore,the thermal stability study of WTe₂ and WTe₂/Ti is critical for the device fabrication and performance.However,the thermal stability of WTe₂ and WTe₂/Ti interface has not been very clear yet.In this paper,the micro-Raman scattering technique is used to systematically study the thermal stability of WTe₂ flakes and the WTe₂/Ti interface at different temperatures.We find that:（1）WTe₂ flakes can still keep stable at 425℃,as the temperature increases to 450℃,the composition of WTe₂ flakes start to change;the decomposition and sublimation of Te atoms in WTe₂ was observed.Consequently,one polycrystalline Te layer was formed on top of WTe₂layer.Our results reveal that Weyl semimetal WTe₂ flakes have good thermal stability and can be compatible with the high temperature semiconductor industry process.（2）The thermal stability of the interface between WTe₂ and Ti is correlated to the thickness of WTe₂ flakes;appropriate increase of the WTe₂ thickness can lead to the improvement of thermal stability in WTe₂/Ti heterostructures.As the WTe₂ is very thin,the WTe₂（12 nm）/Ti heterostructure has started to show interfacial reaction even at the as-grown state.Moreover,high temperature annealing can cause significant interfacial reaction.After annealed at 200℃for 30 min,the interface between WTe₂（12-32 nm）and Ti changed dramatically,leading to the formation of Ti-Te interface layer.This observation is highly consistent with the high-resolution transmission electron microscopy results.Our results provide useful information for further exploring the device design based on WTe₂ and the influence of WTe₂/Ti interface on spin orbit torque effect,which will stimulate the research of low-power spintronic devices based on WTe₂.