The Texture Evolution And Optical-electrical Characteristics Of Multi-level Phase Change Memory Materials

Phase change memory(PCM)has been proved to be a promising memory with non-volatile,high recyclability and fast phase change speed.While with the increasing demand for computer and storage,the development of device design and the materials optimization are under constant research.The phase change materials are based on chalcogenides nanomaterials.The data storage of PCM relies on the resistivity or reflectivity contrast between their amorphous and crystalline states.The phase change and microstructure evolution of phase change materials lead to significant differences in their electrical and optical properties.The composition,electrode layers and superlattice have great influence on the grain growth,local atomic arrangement and texture evolution of the phase change thin films,thus would affect the optical reflectivity and conductivity of PCM and achieve multi-level storage.Therefore,researching the microstructure and properties of phase change materials are important.In this study,we investigated the microstructure and phase transformation mechanism of classical Ge2Sb2Te5(GST)and noval multi-level Ga-Sb phase change thin films,as well as the enhancement mechanism of PCM with elements doping and superlattice structure.Firstly,the texture evolution of GST thin films was studied,the texture components of cube {100}<001>and rotation cube {100}<011>are present in metastable state fcc GST thin films,which was deposited on Si(100)substrate,crystallographic orientation was triggered by the minimization of lattice mismatch strain energy.In addition,{110} fiber texture formed in GST/SiO2 thin films,{0001}texture existed in hcp structure GST with the increase of annealing temperature.Thus the orientation of substrate has an influence on the microstructure of GST.Moreover,with the addition of Ti electrode layer,Ti atoms diffused into the interface of GST/Ti thin films and formed Te2Ti phase when annealed at the temperature higher than 400?.A smaller unbalanced stress(GST/Ti:0.039 N/m2,GST:0.29 N/m2)and more uniform grains distribution of GST/Ti thin films were proved,which contributed to higher hardness value of GST/Ti.Secondly,the influence of Sn doping on stability and optical reflectivity of GST was studied.The replacement of Ge by Sn in Ge-Te bonds of GST structure leads to alterations in bonding states,thus the crystallization temperature and crystallization activation energy of GST-Sn were increased(from 2.48 eV to 3.57 eV).Meanwihle,the conductivity activation energy(E?)of GST-Sn decreased from 0.395 eV to 0.226 eV with the addition fo Sn.A smaller grain boundary density is beneficial for electron tunneling.Thus E? values varied with thin films grain sizes due to the grain boundary space effect and the temperature stability of the conductivity was improved.In addition,an increase in reflectivity was found in crystalline GST-Sn thin films from?74%to?81%across the whole wavelength range.Therefore,a model of texture and optical reflectivity of GST-Sn was proposed.The basal {0001} texture facilitates the high crystalline optical reflectivity of GST thin films thanks to their six-fold rotational symmetry in the hcp structure.Then,we discussed the structure and characteristics of a novel binary Ga-Sb phase change materials.The thickness change of Ga45Sb55(0.4%)during crystallization is much less than that of GST and the crystallization activation energy of Ga15Sb85 increased to 5.96 eV.The mechanical properties of Ga-Sb films were significantly enhanced,and the effect of grain size was the main influencing factor for the increase of their hardness value.Thus the structure and thermal stability of Ga-Sb thin films were improved.Therefore,combining with the excellent properties of Ga-Sb thin films,the superlattice-like[GaSb(x nm)/GST(12nm)]n system was newly proposed in this work.It is also demonstrated that Ga elements diffusion occurred and the crystal structure in GaSb layers was disordered with crystallization,the multilayers structure was still distinguishable,Moreover,the threshold voltage and power consumption of[GaSb(6 nm)/GST(12 nm)]6 and[GaSb(6nm)/GST(12 nm)]3 are estimated to be 0.77 V and 2.8×10-7 W respectively,which is two orders of magnitude lower than GST.Moveover,[GaSb(xnm)/GST(12 nm)]n system shows super-hardness effect,their hardness values increased tripled than that of GST.The hardness enhancement mechanism was also discussed and the critical thickness of GST layer was calculated,the interfaces are coherent without dislocation threading in this critical condition.Finally,considering the application foundation and device fabrication of Ge2Sb2Te5 systems,the microstructure and stability of GaSb doped GST films were studied.It was proved that no addtional phase appeared in the GST-GaSb,while the binding energy of each element was altered.The phase transition process of GST-GaSb was observed by HRTEM,in which the stacking order of atomic layer was changed due to the slipping of[111]crystal faces of fcc structure.According to the first principle calculation,it was found that the doped Ga atoms would occupy the position of Te atoms in GST structure,the band gap was reduced and the metallic characteristic of GST-GaSb would be enhanced.The Ga d level electrons in GST-GaSb had a significant contribution to their valence band DOS.In addition,this study proved that the GaSb doping of low contents(3.4 at.%,5.6 at.%,7.0 at.%)could also improve the mechanical properties and thermal stability of GST significantly.The crystallization temperature of GST-GaSb increased to 216?,and the amorphous hardness of GST-GaSb increased to 4.16 GPa.Therefore,low contents doping of GaSb is beneficial to improve their reliability and achieve significant enhancement effect of GST phase change materials.