Physical Vapor Deposition Synthesis Of Two-dimensional GeTe Flake And Its Phase Transition Study

GeTe is a IV-VI narrow-bandgap semiconductor,which possess excellent performance and broad application prospects in many hotspot fields such as phase change memory,thermoelectricity and ferroelectricity.As an important component of the phase change materials,GeTe can reversibly transit between amorphous phase with high resistance and crystalline phase with low resistance,thus showing great potential in application of phase change memory.Compared to bulk materials,GeTe in nanoscale possess superior thermal stability and erase/write speed,and meet the current trend of high-density storage as well.However,to the best of our knowledge,the synthesis of two-dimensional single crystal GeTe has not hitherto been reported,and its application in the field of phase change memory is still remaining in blank.Therefore,it is of great significance to explore the synthetic method of two-dimensional single-crystal GeTe flakes and investigate corresponding phase change property.In this work,we for the first time synthesized high-quality two-dimensional singlecrystal GeTe flakes and discovered a novel reversible bi-stable crystalline-crystalline phase transition.The main research contents are as follows:1.This work for the first time synthesized non-centrosymmetric two-dimensional singlecrystal R3 m GeTe flakes.With the assistance of confined space,we have successfully synthesized high-quality two-dimensional single-crystal GeTe flakes by physical vapor deposition method.SEM,TEM and Raman proved that the synthesized two-dimensional GeTe flakes are assigned to be rhombohedral(R3m)phase with high purity and good crystal quality.The vibration modes of peaks have been verified by angle-resolved Raman spectra.Second harmonic generation(SHG)measurement confirmed the non-centrosymmetric structure and second-order susceptibility.The azimuthal angle dependence of the SHG intensity was also measured to further certificate that the symmetry group of our GeTe crystals is the same of R3 m.2.For the first time,we discovered a novel reversible bi-stable crystalline-crystalline phase transition in GeTe flakes in ambient environment.The transition of the initial R3 m phase to the final t-R3 m phase was verified by investigating the significant structural,optical and electrical changes of GeTe flakes,which were detected by TEM,Raman,SHG and electric resistance under various temperatures.The reversible bi-stable crystalline-crystalline phase transition in GeTe flakes was realized under the stimulation of pulsed voltage,corresponding Raman,SHG and electrical tests were applied to characterize significant optical and electrical changes deriving from the crystalline-crystalline phase transition.