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Estimate of the Crystallization Kinetics in Stoichiometry Compositions Films of Ge:Sb:Te  [PDF]
Carlos Virgilio Rivera Rodríguez, Eduardo Morales Sanchez, Jesús González Hernández, Evgen Prokhorov, Juan Mu?oz Salda?a, Gerardo Trapaga Martínez
Journal of Surface Engineered Materials and Advanced Technology (JSEMAT) , 2012, DOI: 10.4236/jsemat.2012.21008
Abstract: The aim of this work is to compare the isothermal crystallization kinetic in the films along GeTe-Sb2Te3 line with composition Ge2Sb2Te5, Ge1Sb2Te4 Ge1Sb4Te7 and Ge4Sb1Te5 using mainly Johnson–Mehl–Avrami-Kolmogorov (JMAK) model. Results obtained have shown different crystallization mechanism in the investigated films. In Ge2Sb2Te5 and Ge1Sb2Te4 films the analysis of the kinetic results (Avrani coefficient) showed that at the beginning of crystallization a metastable phase appeared with the Ge1Sb4Te7 composition, this is followed by the nucleation and growth of the stable fcc phase up to full crystallization. In contrast Ge4Sb1Te5 and Ge1Sb4Te7 films show diffusion control growing from small dimension grains with decreasing nucleation rate.
Crystallization of Ge2Sb2Te5 phase-change optical disk media
Crystallization of Ge2Sb2Te5 phase—change optical disk media

Liu Bo,Ruan Hao,Gan Fu-Xi,

中国物理 B , 2002,
Abstract: In this paper, the crystallization behaviour of amorphous Ge2Sb2Te5 thin films is investigated using differential scanning calorimetry), x-ray diffraction and optical transmissivity measurements. It is indicated that only the amorphous phase to face-centred-cubic phase transformation occurs during laser annealing of the normal phase-change structure, which is a benefit for raising the phase-change optical disk's carrier-to-noise ratio (CNR). For amorphous Ge2Sb2Te5 thin films, the crystallization temperature is about 200℃ and the melting temperature is 546.87℃. The activation energy for the crystallization, Ea, is 2.25eV. The crystallization dynamics for Ge2Sb2Te5 thin films obeys the law of nucleation and growth reaction. The sputtered Ge2Sb2Te5 films were initialized by an initializer unit. The initialization conditions have a great effect on the reflectivity contrast of the Ge2Sb2Te5 phase-change optical disk.
Crystallization Kinetics of Ge-Sb-Te-O Phase-Change Thin Films

GU Si-Peng,HOU Li-Song,

无机材料学报 , 2002,
Abstract: Ge-Sb-Te and Ge-Sb-Te-O thin films were prepared by RF-sputtering. XRD spectra of the films in as-deposited and heat-treated states show that the films changed from amorphous to crystalline states due to heat-treatment. By using DSC data of the amorphous film materials, measuring the peak temperature of crystallization at different heating rates, the activation energies and frequency factors were calculated. The experimental results show that sample Ge-Sb-Te-O has a higher value of activation energy than sample Ge-Sb-Te, so oxygen-doping can improve the crystallization rate of Ge-Sb-Te phase-change material.
Atomistic origin of doping-enhanced rapid crystallization in Ag-doped Ge-Sb-Te alloys: a joint experimental and theoretical study  [PDF]
B. Prasai,M. E. Kordesch,D. A. Drabold,G. Chen
Physics , 2013, DOI: 10.1002/pssb.201349150
Abstract: We have applied extended X-ray absorption fine structure (EXAFS) analyses and ab-initio molecular dynamics (AIMD) simulations to study the atomic structure of Ag-doped (up to 42%) Ge1Sb2Te4 alloys. Analysis of the models that are consistent with the EXAFS experiment reveals that the Ge environment is significantly modified by Ag doping whereas those of Sb and Te are barely affected (except for high Ag concentrations), and suggests that Ag prefers bonding with Te to Ge or Sb. Doping with Ag promotes the conversion of tetrahedral Ge sites to octahedral Ge sites and enhances the speed of crystallization of Ge-Sb-Te (GST) alloys as evidenced directly from the MD simulations. Our study shed light on the atomistic mechanism of rapid crystallization of GST alloys enhanced by Ag doping.
Laser-induced Crystallization Behavior of the Sputtered Ge2Sb2Te5 Film


无机材料学报 , 2002,
Abstract: The crystallization behavior of sputtered Ge2Sb2Te5 films initialized by initializer unit was studied by using XRD. It is indicated that only the amorphous phase to FCC phase transformation occurs during laser annealing of the normal phase-change structure, which is benefit for raising the phase-change optical disc's signal-to-noise ratio. The phase transformation from FCC to HCP doesn't occur, which occurs during the heat-induced phase-change process. The initialization power and velocity affect the Ge2Sb2Tes film's crystallization fraction.
Crystallization Mechanism and Course of the Ge2Sb2Te5 Thin Films under Focused Pulse Laser

WEI Jing-Song,YUAN Hao,CAN Fu-X,

无机材料学报 , 2002,
Abstract: The relationship between the reflectivity of the Ge2Sb2Te5 thin films, which are as-deposited and melt-quenched states separately, and the pulsed laser time was investigated by the focused pulse laser, and it was found that the reflectivity changes for the two kinds of samples are different. On the basis of the principle of the droplet formation in the gas-liquid system and the basic ideas of the statistical physics, the crystallization mechanism and course of the Ge2Sb2Te5 thin films, which are in as-deposited and melt-quenched states separately, were analyzed and studied in detail. The crystalline nucleus can't form if Ge2Sb2Te5 is in below-saturation or saturation state, different size crystalline nucleus can form if it is in super-saturation. While only the crystalline nucleus which are more than the critical size can grow into crystalline. In addition, the stress, decreasing the crystallization energy barrier and increasing the super-saturation degree of the amorphous Ge2Sb2Te5, is the leading factor causing the different relationships of reflectivity change with pulsed laser width between the as-deposited and melt-quenched Ge2Sb2Te5 thin films. At the same time, on the basis of the analytical results, the reflectivity change characters and laws of the two kinds of the samples under the focused pulse laser were explained in detail.
Study of Ge2Sb2Te5 Film for Nonvolatile Memory Medium
Study of Ge_2Sb_2Te_5 Film for Nonvolatile Memory Medium

Baowei QIAO,Yunfeng LAI,Jie FENG,Yun LING,Yinyin LIN,Ting''''ao TANG,Bingchu CAI,Bomy CHEN,

材料科学技术学报 , 2005,
Abstract: The amorphous Ge2Sb2Te5 film with stoichiometric compositions was deposited by co-sputtering of separate Ge,Sb, and Te targets on SiO2/Si (100) wafer in ultrahigh vacuum magnetron sputtering apparatus. The crystallization behavior of amorphous Ge2Sb2Te5 film was investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). With an increase of annealing temperature, the amorphous Ge2Sb2Te5 film undergoes a two-step crystallization process that it first crystallizes in face-centered-cubic (fcc) crystal structure and finally fcc structure changes to hexagonal (hex) structure. Activation energy values of 3.636±0.137and 1.579±0.005 eV correspond to the crystallization and structural transformation processes, respectively. From annealing temperature dependence of the film resistivity, it is determined th at the first steep decrease of the resistivity corresponds to crystallization while the second one is primarily caused by structural transformation from "fcc"to "hex" and growth of the crystal grains. Current-voltage (I-V) characteristics of the device with 40 nm-thick Ge2Sb2Te5 film show that the Ge2Sb2Te5 film with nanometer order thickness is still applicable for memory medium of nonvolatile phase change memory.
Direct ab-initio molecular dynamic study of ultrafast phase change in Ag-alloyed Ge$_{2}$Sb$_{2}$Te$_{5}$  [PDF]
B. Prasai,G. Chen,D. A. Drabold
Physics , 2012, DOI: 10.1063/1.4789877
Abstract: We employed ab-initio molecular dynamics to directly simulate the effects of Ag alloying ($\sim5%$ Ag concentration) on the phase change properties of Ge$_{2}$Sb$_{2}$Te$_{5}$. The short range order is preserved, whereas a slight improvement in the chemical order is observed. A slight decrease in the fraction of tetrahedral Ge (sp$^{3}$ bonding) is reflected in the reduction of the optical band gap and in the increased dielectric constant. Simulations of the amorphous to crystalline phase change cycle revealed the fact that the crystallization speed in Ag$_{0.5}$Ge$_{2}$Sb$_{2}$Te$_{5}$ is no less than that in Ge$_{2}$Sb$_{2}$Te$_{5}$. Moreover, the smaller density difference and the larger energy difference between the two phases of Ag$_{0.5}$Ge$_{2}$Sb$_{2}$Te$_{5}$ (compared to Ge$_{2}$Sb$_{2}$Te$_{5}$) suggest a smaller residual stress in devices due to phase transition and improved thermal stability for Ag$_{0.5}$Ge$_{2}$Sb$_{2}$Te$_{5}$. The potential viability of this material suggests the need for a wide exploration of alternative phase change memory materials.
Multiple-state storage capability of nitrogen-doped Ge2Sb2Te5 film for phase change memory

Lai Yun-Feng,Feng Jie,Qiao Bao-Wei,Ling Yun,Lin Yin-Yin,Tang Ting-Ao,Cai Bing-Chu,Chen Bang-Ming,
,冯 洁,乔保卫,凌 云,林殷茵,汤庭鳌,蔡炳初,陈邦明

物理学报 , 2006,
Abstract: Nitrogen-doped Ge2Sb2Te5 (N-GST) film for phase change memory was prepared by reactive sputtering. Testing results show that doped nitrogen combines with Ge to form GeN, which not only restrains crystal grain growth but also increases the crystallization temperature and phase transformation temperature of Ge2Sb2Te5 (GST). Phase change memory (PCM) with N-GST can realize three-state storage in one PCM cell by using the resistivity difference between the amorphous state, the face centered cubic phase and the hexagonal phase of GST.
Optical Constants of Laser-Induced Crystalline Ge2Sb2Te5 Phase-Change Media

Liu Bo,Ruan Hao,Gan Fuxi,

半导体学报 , 2002,
Abstract: The effects of crystallization fraction on the op ti cal constants of Ge_2Sb_2Te_5 phase-change films are studied using spectros copic ellipsometer and spectrometer.With fixed initialization velocity,the refra ctive index and transmissivity decrease while the extinction coefficient increas es when the laser power increases.With fixed initialization laser power,the refr active index and transmissivity increase and the extinction coefficient decrease s when the initialization velocity increases.The change between amorphous phase and crystalline phase,the changes in microstructure (including change in bonding state) and the internal residual stresses in film are the main factors that aff ect the complex refractive index of Ge_2Sb_2Te_5 phase-change film.The refle ction spectra of the Ge_2Sb_2Te_5 film in CD-RW (compact disk rewritable) ph ase-change optical disk at amorphous and crystalline states are also measured.
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