| In the past few years,with the advantages of high storage density,high speed,low cost,low power consumption,and scalability,phase change memory has been recognized as one of the most promising candidates for the next-generation nonvolatile memory devices.Because the physical properties of the phase change materials directly determine the performance limit of phase change memory,the study of phase change materials based on chalcogenides become the core component of phase change memory.At present,significant progress has been made in the doping modification of several basic Te based chalcogenide material systems,such as Ge-Sb-Te,Sb-Te,and Ge-Te,etc.Meanwhile,it also faces to enormous challenges.At this point,the doping modification of chalcogenides should not continue blindly,but the phase change mechanism should be studied deeply.Up to now,mainly study on the physical properties of the phase change materials are in the electrical and thermal fields.While the investigation of crystal structure and optical properties(optical constant,optical band gap,electron transition,etc),which can reflect the change of energy band structure before and after the phase change of the materials,is very important for the analysis of the phase change mechanism and the further improvement of the material properties.In addition,the processing methods of the testing samples mainly adopt electrical impulses or rapid thermal annealing,which can make the crystallization in a nanosecond time.In this way,the study of specific structural transformation of phase change process becomes a difficult problem.In this paper,in order to study the dynamic phase change mechanism of chalcogenide materials,as well as make up for the lack of its optical properties,the variations of lattice vibration,energy band structure,and electron transition in the phase change process for chalcogenides are obtained by the means of spectroscopy methods.The main methods include Raman scattering spectra,ultraviolet-near infrared transmission spectra,and spectroscopic ellipsometry,etc.In the present paper,several innovative results are listed as below:1.Influences of W doping on the phase change behavior of Ge2Sb2Te5 have been investigated.The physical mechanism of Wx(Ge2Sb2Te5)1-x(x = 0,0.03,0.07,and 0.11)from amorphous to crystalline phase has been systematically studied.The crystalline resistances and crystallization temperatures of Wx(Ge2Sb2Te5)1-x films have been studied using in situ temperature-dependent resistance measurements.The intrinsic crystallization mechanism from amorphous to crystalline(face-centered-cubic/ hexagonal)structure has been discovered using temperature dependent Raman scattering spectra from 300 to 720 K.It was found that Wx(Ge2Sb2Te5)1-x films exhibit a more stable cubic geometry,better thermal stability of the amorphous state,and higher 10 year data retention ability than pure Ge2Sb2Te5.It could be due to the substitution of Sb atoms or vacancies by W atoms in the crystal lattice,which leads to disorder of the crystalline structure and inhibits further crystallization.Besides,W doping effects on the phase change behavior of Wx(Ge2Sb2Te5)1-x films have been investigated by variable-temperature spectroscopic ellipsometry from 210 to 660 K.The dielectric functions have been evaluated with the aid of Tauc-Lorentz and Drude dispersion models.The analysis of Tauc gap energy and partial spectral weight integral reveal the correlation between optical properties and local structural change.The order degree increment and chemical bond change from covalent to resonant should be responsible for the band gap narrowing and electronic transition enhancement during the phase change process.2.N doping effects on the crystal structure,localized states,dielectric functions,and electronic band structure for Ge Te phase change films have been investigated.The dynamic crystallization process and the role of nitrogen in Nx(Ge Te)1-x(x = 0,0.08,0.12,and 0.18)films have been studied in detail.The comparision of Rmana scattering spectra for the films before/after rapid thermal annealing revieals the change of bonding mode after crystallization.The evolutions of localized states,dielectric functions,and electronic band structure for Nx(Ge Te)1-x films as functions of temperature and N concentration have been investigated with the aid of transmittance spectra and temperature dependent spectroscopic ellipsometry experiments from 210 to 660 K.The increased Urbach absorption energy,caused by band-tail localized states,can be attributed to the increment of structure defects with N concentration,which is generated from the doped N atoms bonding with Ge atoms.The details of dynamic crystallization process and the role of nitrogen in Nx(Ge Te)1-x films have been elucidated by the abnormal behavior of interband transition energy and the evolutions of surface morphology,namely,the nitrides(AM-Ge Nx)formation before crystallization and the inhibition on the Ge Te crystallization.The dynamic crystallization process and nitrogen behavior in Nx(Ge Te)1-x films are of great significance for further study on the reliability and endurance of Nx(Ge Te)1-x-based data storage devices.3.Influences of temperature and W concentration on the optical properties,electronic structure,and microstructure of Sb2 Te phase change films have been investigated.The intermediate state of Wx(Sb2Te)1-x(x = 0,0.03,0.05,and 0.08)films between amorphous and hexagonal phases has been proposed.The optical properties,electronic structure,and microstructure of Wx(Sb2Te)1-x phase change films as functions of temperature and W concentration have been investigated by means of temperature dependent X-ray diffraction(300-600 K),Raman scattering(210-620 K),and spectroscopic ellipsometry(210-620 K)experiments.Based on the variations of the diffraction peaks,phonon modes,and dielectric functions during the temperature elevation process,the intermediate crystalline state of Wx(Sb2Te)1-x films between amorphous and hexagonal phases can be readily proposed,which is a mixture of crystalline Sb and Te.The anomalous behaviors of dielectric functions and partial spectral weight integral for crystalline films elucidate the existence of intermediate state.Furthermore,the good agreement between experimental and calculated dielectric functions of HEX Sb2 Te reveals that the first-principles calculation method can be used to make qualitative analysis in the materials with similar multilayered structures.4.Si doping effects on the phase change behaviors of Sb2 Te films have been studied.The optical properties and structural variations of Six(Sb2Te)1-x(x = 0,0.25,0.28,and 0.33)films as the function of temperature have been systematically investigated.It can be concluded that the crystallization of hexagonal phase is accelerated by Si introduction.The optical properties and structural variations of Si doped Sb2 Te films as functions of temperature(210-620 K)and Si concentration have been investigated by the means of temperature dependent Raman scattering and spectroscopic ellipsometry experiments.Based upon the changes in Raman phonon modes and dielectric functions,it can be concluded that the temperature ranges for intermediates and transition states are estimated to 150 K,120 K,90 K,and 0 K,corresponding to Sb2 Te,Si0.25(Sb2Te)0.75,Si0.28(Sb2Te)0.72,and Si0.33(Sb2Te)0.67 films,respectively.The phenomenon also can be summarized by the thermal evolutions of interband electronic transition energy and partial spectral weight integral of the phase change films.The disappearance of intermediate state for Si0.33(Sb2Te)0.67 film between amorphous and hexagonal phases can be attributed to the acceleratory crystallization of hexagonal phase by Si introduction.It illustrates that the risk of phase separation(Sb and Te)during the cyclic phasechange processes decreases with the increasing Si concentration.The enhanced crystallization behaviors can optimize the data retention ability and the long term stability of Sb2 Te by Si doping,which are important indicators for phase change materials.The performance improvement has been analyzed qualitatively from the optical perspective. |
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