The Investigation On The Impact Mechanism Of Silicon Doping To The Characteristics Of GeSbTe Film

With the advantages of fast operation speed, high reliability and good scalability, phase change memory (PCM) is believed to be the most promising next generation nonvolatile memory. The most crucial issue need to be solved in PCM now is to reduce the power comsumption and increase the integration density. One of the major methods to cope with the issue is to improve the properties of GeSbTe(GST), which is the storage medium used in PCM now. Doping other elements into GeSbTe is a common way to improve the properties of GST. In this thesis, the thermal crystallization process, morphology and structure of silicon doped Ge2Sb2Te5(Si-GST) will be investigated and the result will be compared with pure GST. The results are listed as follows:1. GST and Si-GST with different doping concentration thin films were deposited by electron beam evaporation. Through in situ exothermal resistance measurement, it was found that the crystallization temperature and crystalline resistivity were increased with the increasing of Si doping concentration, indicating that the thermal stability of amorphous GST will be increased and the power consumption of PCRAM will be reduced by Si doping.2. I The values of crystallization activation energy and amorphous activation energy in GST and Si-GST films were estimated. The results showed that both values in GST film were increased after Si doping. The increase of crystallization activation energy suggests Si-GST requires more energy to crystallize than GST while the increase of amorphous activation energy implies the structure variation of amorphous state in Si-GST film.3. The results of in situ isothermal resistance measurements indicated the amorphous state thermal stability improves in Si-GST film. However, the crystallization rate of GST was found to be slightly slowed by Si doping through the analysis of crystallization time.4. The crystallization model of GST and Si-GST films were investigated and the results showed that the crystallization model of GST could be interpreted by the Wiener upper bound model while the model of Si-GST could be better described by the spherical crystallization model. Furthermore, the JMAK curves of GST and Si-GST showed that after Si doping, the value of Avrami coefficient lowered while the value of crystallization energy rose.5. From AFM pictures of GST and Si-GST films, grains can be observed on the surface of all the films. The numbers of grains increased with the increase of Si doping concentration and it increased dramatically in crystallized Si-GST samples. The phenomena suggest the structure of GST film is changed after SI doping and the structure variation tendency becomes more obvious after crystallization. In TEM picture of crystallized Si-GST film, the structure that the GST domains were embedded in rich Si layer was observed.6. The Raman spectra of GST and Si-GST were investigated and by comparison, the blueshift and broadening of Raman peaks in the spectrum of SI-GST can be found, indicating the crystal grains in Si-GST samples were affected by the compression stress. Further study showed that the stress should come from the compression of rich Silayer and resulted in the property variation in Si-GST film.