Amorphous-cubic-hexagonal Phase Transition, Electrical And Optical Properties Of Doped Germanium Antimony Tellurium Films

Chalcogenide phase-change materials are materials that exist in at least two structurally distinct solid phases,an amorphous and one?or more?crystalline phases.These materials are able to be switched repeatedly and reversibly between the two phases with totally different optical and electrical properties because of the large differences in structure.Owing to these unique properties,chalcogenide phase-change materials have been developed for data storage,such as phase change memory devices,blu-ray discs,DVD and CD.These traditional devices with the two-level data storage would not meet the requirements of high storage density in the big data era any more.And new storage technology must be developed.Studies have shown that high temperature annealing will induce secondary transformation of Ge₂Sb₂Te₅ thin films from amorphous through cubic to hexagonal phases.If electrical/optical property contrast among the three phases are used,three-state storage devices will be invented,thereby improving the storage density.However,it's still exists a lot of deficiencies:?1?For the electrical storage:Ge₂Sb₂Te₅ films exhibit three states,amorphous phase with high resistance,cubic phase with intermediate resistance and hexagonal phase with low resistance,respectively.However,the temperature span of intermediate resistance state for Ge₂Sb₂Te₅ films is rather narrow,and the rate of resistance changing with temperature is too high.These make the control of the resistance state very difficult and the resolution for the data collection rather low.?2?For the optical storage:Ge₂Sb₂Te₅ films exhibit three states,amorphous phase with low reflectivity,cubic phase with intermediate reflectivity and hexagonal phase with high reflectivity,respectively.However,Ge₂Sb₂Te₅ thin film has a low contrast?⁸%?in reflectivity between the cubic and hexagonal phases,and it is not satisfactory for commercial standard?¹5%?.In addition,it's unclear that the physical mechanism of the apparent difference in reflectivity between cubic and hexagonal.With the above these questions in mind,we prepared the N-Ge₂Sb₂Te₅ and Ag-Ge₂Sb₂Te₅ films via magnetron sputtering method using the discharge gas of Ar and N₂,and carried out four aspects of studies by employing the Tauc-Lorentz mode,first-principle calculations in combination of the experimental measurements,namely,Energy-dispersive X-ray spectroscopy?EDS?,Gracing Incident X-ray Diffraction?GIXRD?,High Resolution Transmission Electron Microscope?HRTEM?,Four-probe measurement system,Fourier-Transform Infrared Spectroscopy?FTIR?and ultraviolet-visible-near-infrared?UV-vis-NIR?spectrometer.The main contents and results of this thesis are summarized as following:?1?Improved three-level electrical storage properties of Ge₂Sb₂Te₅ films by nitrogen doping.Proper nitrogen doping is an effective way to improve the property of three-level data storage for Ge₂Sb₂Te₅ films.The nitrogen doping in Ge₂Sb₂Te₅films not only prolongs the temperature span of intermediate resistance state,but also creates a quasi-platform of resistance.This is attributed to the improvement in thermal stability of both face centered cubic phase and band gap.In addition,the N doping increases the temperature of insulator-metal transition due to the formation of Ge-N bonds,reducing the dielectric coefficient and hence increasing the Urbach tail.These results suggest that doping renders an effective avenue to modulate resistance state and insulator-metal transition,which opens up a new door in preparing chalcogenide phase-change films with three-level data storage.?2?Effect of nitrogen and silver doping on three-level optical storage of Ge₂Sb₂Te₅ films.The appropriate nitrogen and silver doping will significantly improve the reflectivity contrast of Ge₂Sb₂Te₅ film between cubic and hexagonal.However,when more nitrogen and silver were introduced,the reflectivity contrast of the Ge₂Sb₂Te₅ thin film decreased.This phenomenon is attributed to the evolution of chemical bonding and the increase of disorder degree.The reflectivity contrast of the N-Ge₂Sb₂Te₅ films meets commercial requirements at 450 nm,650 nm,and 780 nm.However,the reflectivity contrast of Ag-Ge₂Sb₂Te₅ films meets commercial requirements only at 650 nm and 780 nm.