Study On Optical/Electrical Properties Of Chalcogenide Phase-change Materials SbTe/GeSbTe And Their Data Storage Mechanism Based On C-AFM

With the rapid development of information,the requirements for density and capacity of data storage are also rising.Among various types of semiconductor memory,Phase Change Random Access Memory(PCRAM)is considered to be the most likely candidates to replace SRAM,DRAM and FLASH and become the pioneer of the commercial products in the next generation of semiconductor memory device.To improve the PCRAM performance,one of the main research directions is to reduce the current and power consumption,including the improvement of materials and device structure.For the Phase change materials,we must study the optical and electrical properties of the most popular phase change materials SbTe/GeSbTe,including their crystallization rate,crystallization temperature,thermal stability and nano-scale crystallization mechanism,conduction mechanism to meet the demand of future phase-change memory for fast write/erase speed,excellent data retention,low switching energy,good cyclability and scalability to nanometer cell sizes.About the improvement of the device structure,owing to the superparamagnetic effect,the diffraction effect of light and the minimum lithographic dimensions,it is difficult tofurther improve the storage density of the traditional magnetic storage,optical storage,and semiconductor memory.Due to the nano-scale information storage,the data storage technology based on SPM(Scanning Probe Microscopy)is expected to become a new generation of ultra-high density storage technology.C-AFM(Conductive Atomic Force Memory)was used in chalcogenide phase-change data storage to achieve the ultra-high density storage of GeSbTe.The article include the following aspects:(1)Firstly,the development and application prospects of the semiconductor memory were introduced.With the performance comparison of memory,PCRAM shows excellent advantages in non-volatile random access memory.The characteristics,working principle,storage medium and development trends of PCRAM were described in detail.Next,the research development of memory was discussed.One is improvement of the traditional semiconductor memory or optical storage technology;the other is data storage based on SPM technology.And the data storage principle,development status and trends of each memory technology are also discussed.(2)SbTe thin films were prepared by electron beam evaporation and electrochemical deposition method,the composition,structure,electrical and optical properties of films were investigated.SbTe thin films were prepared by electron beam evaporation.X-ray photo-electron spectroscopy(XPS)was used to estimate the atomic ratio of SbTe thin film;The results of electrical resistivity measurements of the thin films show that with the increasing annealing temperature,phase transition occurs,the crystallization temperature is at about 75?,and the crystallization activation energy of film was calculated by resistance measurement on high temperature probe station;Stoichiometric Sb2Te3 thin films were prepared by electrochemical deposition,and X-ray Diffraction(XRD),Scanning Electron Microscope(SEM),Ultraviolet-Visible-Infrared(UV-VIS-IR)spectrophotometer were used to characterize the SbTe films.The thermal stability can be increased by doping with other elements.(3)GeSbTe films were prepared by Electron beam evaporation.XRD,Atomic Force Microscope(AFM)were used to analyze structure and morphology of films,With the temperature increases,GeSbTe thin film exhibits a two-step phase transition:first from amorphous to fcc(face-centered cubic)phase state,finally to hex(hexagonal)phase state,and the grain size of thin films and the RMS surface roughness increase with increasing temperature.The resistivity of GeSbTe thin films with temperature were measured on high temperature probe station,the resistance of GeSbTe thin films has two sharp decreases at temperature of 150? and 220?,respectively,and there are 2-3 orders of magnitude difference in resistance between crystalline and amorphous state,when the heating rate is lowered,the crystallization temperature of films decreased.The activation energy of crystallization and conductivity of GeSbTe films were investigated;by measuring the time-dependent resistance of the GeSbTe films,the incubation time of the film,the data retention time and other parameters were calculated.I-V characteristic of GeSbTe was also been measured.The results show that GeSbTe materials meet the requirements of PCRAM devices.The JMAK(Johnson-Mehl-Avrami-Kolmogorov)kinetics was optimized by classical nucleation theory,the relation between the volume fraction and time was investigated and a good fit with the experimental data were found.GeSbTe films with different thicknesses were annealed under different temperatures,the transmittance and reflectance of spectroscopy of thin films were measured by UV-VIS-IR spectrophotometer,and the effects of film thickness and annealing temperature on the light absorption coefficient and optical band gap of thin films were discussed.We found that the absorption coefficient and optical band gap of fcc crystalline GeSbTe thin films are gernerally consistent with that of hex crystalline state,and significantly higher than the amorphous state,the film thickness does almost not change the absorption coefficient and optical band gap of films.(4)After analyzing the phase change mechanism of chalcogenide material as well as the storage mechanism based on the storage media,C-AFM was used to explore the mechanism of data write and read processes;In the I-V measurements by C-AFM,when the voltage applied on sample exceeded threshold switching voltage,the threshold switching occurred and conductive nanofilaments were generated in amorphous matrix which can be detected by the local enhanced conductive current signals.Take advantage of this I-V characteristics,C-AFM was used to fabricated nano-sized crystalline recorded dot array on surface of GeSbTe film;through the observation of the morphology and current of the nano-array with C-AFM simultaneously,the current changes between the crystalline spots and amorphous substrate were detected,and the effects of times of "Spectroscopy" on nano-array were investigated.C-AFM results show that as the times of I-V spectroscopies increased,the morphology of crystallized spots changed from bump to pit,the sizes of conductive nanofilaments and detected current signals increased.These results can be attributed to that the energy induced by Joule heating dissipated to surrounding films increases with increasing times of I-V spectroscopies.