聚焦脉冲激光作用下Ge2Sb2Te5薄膜晶化过程及机理

采用聚焦脉冲激光研究了Ge2Sb2Te5薄膜在沉积和激光淬火两种非晶态下反射率与激光脉冲宽度变化的关系，发现沉积态的Ge2Sb2Te5薄膜在晶化触发阶段内的反射率随激光脉冲宽度增加而减小，经过激光淬火的非晶态Ge2Sb2Te5薄膜在晶化触发阶段内的反射率随激光脉冲宽度增加而变化平缓。本文借用气－液体系中过饱和度分析液滴形成的原理，从统计物理学角度详细研究了两种非晶态Ge2Sb2Te5薄膜在脉冲激光作用下的晶化过程及机理，结果表明，当Ge2Sb2Te5的非晶态程度处于未饱和或饱和状态时不形成晶核；当Ge2Sb2Te5的非晶态程度处于过饱和状态时，此时的Ge2Sb2Te5为亚稳态，可能形成大小不等的晶核，但只有半径大于临界晶核尺寸时才可能长大成晶粒，而应力降低晶化能垒，增加非晶态Ge2Sb2Te5的过饱和度是导致沉积态与激光淬火态的Ge2Sb2Te5薄膜在晶化触发阶段内反射率随激光脉冲宽度变化规律不一致的根本原因，并据此解释了Ge2Sb2Te5薄膜在这两种状态下的反射率随激光脉冲宽度的变化特点及规律。     

非晶Ag11In12Te26Sb51薄膜的结晶行为

采用初始化仪使非Ag11In12Te26Sb51薄膜结晶，利用差分扫描量热仪、X射线衍射和光学透过率的测量研究了非晶Ag11In12Te26Sb51薄膜的结晶行为，结果表明，非晶Ag11In12Te26Sb51膜结晶温度约为210℃，熔化温度为481.7℃，结晶活化能Ea=2.07eV/atom；Ag11In23Te26Sb51膜的结晶力学遵循成核和生长机理；在激光致相变过程中可能出现的晶相有AgSbTe2、AhInTe2、Sb和Ag2Te等相；Ag11In12Te26Sb51薄膜的结晶程度受初始化功率和转速的影响。     

Sn掺杂Ge-Sb-Te相变薄膜的晶化特性

用磁控溅射法制备了掺杂Sn的Ge2Sb2Te5相变材料薄膜,研究了Sn含量对结晶性能的影响.薄膜的X射线衍射(XRD)表明,热处理使薄膜发生了从非晶态到晶态的相变,并出现Sn-Te相.通过示差扫描量热(DSC)实验测出在不同加热速率下非晶态薄膜粉末的结晶峰温度,计算了材料的结晶活化能.根据结晶动力学分析和结晶活化能数据,掺杂Sn后的Ge-Sb-Te具有更高的结晶速率,用于光存储时将具有更高的擦除速度.     

激光致溅射沉积Ge2Sb2Te5薄膜的结晶行为研究

利用ＸＲＤ研究了激光致溅射沉积Ｇｅ_２Ｓｂ_２Ｔｅ_５薄膜的结晶行为;研究发现,与热致相变不同的是,激光致相变只发生从非晶态到ＦＣＣ晶态结构的转变,从ＦＣＣ到ＨＣＰ的结构转变不再发生,这有利于提高相变光盘的信噪比．Ｇｅ_２Ｓｂ_２Ｔｅ_５薄膜的结晶程度受初始化功率和转速的影响．     

基于Sn掺杂Ge2Sb2Te5的相变存储器器件单元存储性能

采用0.18μm标准工艺制备出基于Sn掺杂Ge2Sb2Te5相变材料的相变存储器器件单元,利用自行设计搭建的电学测试系统研究了其存储性能.结果表明:Sn的掺杂没有改变Ge2Sb2Te5的相变特性,其相变阚值电压和阈值电流分别为1.6V和25μA;实现了器件单元的非晶态(高阻)与晶态(低阻)之间的可逆相变过程;器件单元中相变材料结晶所需电流最低为1.78mA(电流宽度固定为100ns)、结晶时间大于80ns(电流高度固定为3mA);相变材料非晶化脉冲电流宽度为30ns时,所需电流大于3.3mA;与Ge2Sb2Te5相比,Sn的掺杂降低了SET操作的脉冲电流宽度,提高了结晶速度,有利于提高相变存储器的存储速度.     

热致VO2薄膜在激光防护方面的应用研究

VO2是一种固态热致变色材料,它的晶态结构可以在半导体-金属-绝缘体之间可逆转变。介绍了VO2的相变特性,研究了氧化钒的相变机理。根据激光对光电探测器的损伤机理,研究了氧化钒薄膜对激光的智能防护,并对其进行了计算和分析。结果表明,当一束激光照射VO2薄膜防护的探测器时,VO2薄膜可以在激光对探测器造成损伤之前完成相变,从而保护探测器,因此可用于探测器针对强激光的智能防护。     

Ge2Sb2Te5材料与非挥发相转变存储器单元器件特性

对Ge2Sb2Te5材料的结构、形貌和电学特性进行了表征,将材料应用于不挥发存储单元器件中并研究了器件性能。研究了退火温度对薄膜电阻率的影响,发现在从高阻向低阻状态转变的过程中,电阻率下降的趋势发生变化,形成拐点,分析表明这是由于在拐点处结构由面心立方向密排六方结构转变所致;对不同厚度Ge2Sb2Te5薄膜的电阻率进行了分析,结果表明当厚度薄于70nm时,电阻率随厚度显著上升而迁移率下降,材料晶态电学性能的测量显示,材料有正电阻温度系数并以空穴导电;测量了Ge2Sb2Fe5非挥发相转变存储器单元的I-V曲线,发现有阈值特性,在晶态时电学特性呈欧姆特性,非晶态时I-V低场为线性关系,电场较高时呈指数关系。     

Ge2Sb2Te5相变存储器导电机制研究

本文研究了Ge2 Sb2 Te5相变存储器的导电机制.本文考虑了实验中观察到的场致激活能的非线性下降以及结晶化后跳跃间距变大带来的影响,提出了一种有效导带底偏移模型,并在此基础上建立了修正的电流模型.计算结果表明,激活能随电压增加呈双曲余切的下降趋势,符合测量结果.该模型还包含了温度效应,结果表明跳跃间距与温度成反比....     

射频磁控溅射纳米二氧化钒薄膜的电致相变特性

采用射频磁控溅射方法和热处理工艺制备了二氧化钒（VO2）薄膜,并制作了金属钨／VO2／金属钨三明治结构,通过改变金属钨／VO2／金属钨三明治结构中VO2薄膜与金属钨电极的接触面积,研究了VO2薄膜的电致相变特性．采用x射线衍射仪（XRD）、扫描电子显微镜（SEM）、四探针和半导体参数测试仪对VO2薄膜的结晶取向、表面形貌、方块电阻和,I-V特性进行了测试．实验结果表明,所制备的VO2薄膜为具有热致相变特性的单一组分VO2纳米薄膜,在热激励下,薄膜的方块电阻相变幅度达到2个数量级;在电压的激励下,VO2薄膜与金属钨的接触面积为12μm×12um时,电流发生跳变的阁值电压为9．4V,随着接触面积的减小,闽值电压也逐渐降低．     

VO2的相变原理及影响相变的因素

VO2是一种热致变色材料,在T1=68℃时发生从低温的单斜相向高温四方相转变,同时伴随着光、电、磁性能的突变,这些优异特性使其具有好的应用前景.本文综述了VO2相变过程中结构和能带的变化特征及其影响相变的因素,这对其应用具有重要的理论及实际意义.     

Study of Ge2Sb2Te5 Film for Nonvolatile Memory Medium

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±0.137 and 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 that the first steep decrease of the resistivity corresponds to crystallization while the second one is primarily caused by structural transformation from     

Effect of Initialization Technical Parameters on Optical Absorption of Ge2Sb2Te5 Thin Films

The optical absorption properties of phase-change optical recording thin films subjected to various initialization conditions were investigated. The effects of initialization power and velocity on optical constants of the Ge2Sb2Te5 thin films were also studied. The energy gap of Ge2Sb2Te5 thin films subjected to various initialization conditions was also obtained. It was found that the optical energy gap of the Ge2Sb2Te5 thin films increased with either increasing initialization laser power or decreasing initialization velocity, with peak of 0.908 eV at laser power of 1000 mW or initialization velocity of 4.0 m/s, but the continued increasing initialization laser power or decreasing initialization velocity resulted in the decrease of the optical energy gap. The change of the optical energy gap was discussed on the basis of amorphous crystalline transformation.     

Crystallization and amorphization studies of a Ge(2)Sb(2.3)Te(5) thin-film sample under pulsed laser irradiation

We present the results of crystallization and amorphization studies on a thin-film sample of Ge(2)Sb(2.3)Te(5), encapsulated in a quadrilayer stack as in the media of phase-change optical disk data storage. The study was conducted on a two-laser static tester in which one laser, operating in pulsed mode, writes either amorphous marks on a crystalline film or crystalline marks on an amorphous film. The second laser, operating at low power in the cw mode, simultaneously monitors the progress of mark formation in terms of the variations of reflectivity both during the write pulse and in the subsequent cooling period. In addition to investigating some of the expected features associated with crystallization and amorphization, we noted certain curious phenomena during the mark-formation process. For example, at low-power pulsed illumination, which is insufficient to trigger the phase transition, there is a slight change in the reflectivity of the sample. This is believed to be caused by a reversible change in the complex refractive index of the Ge(2)Sb(2.3)Te(5) film in the course of heating above the ambient temperature. We also observed that the mark-formation process may continue for as long as 1 mus beyond the end of the write laser pulse. This effect is especially pronounced during amorphous mark formation under high-power, long-pulse illumination.     

Microscopic origin of the fast crystallization ability of Ge-Sb-Te phase-change memory materials

Ge-Sb-Te materials are used in optical DVDs and non-volatile electronic memories (phase-change random-access memory). In both, data storage is effected by fast, reversible phase changes between crystalline and amorphous states. Despite much experimental and theoretical effort to understand the phase-change mechanism, the detailed atomistic changes involved are still unknown. Here, we describe for the first time how the entire write/erase cycle for the Ge(2)Sb(2)Te(5) composition can be reproduced using ab initio molecular-dynamics simulations. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rocksalt structure, form during melt cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge(2)Sb(2)Te(5). As this simulation procedure is general, the microscopic insight provided on crystal nucleation should open up new ways to develop superior phase-change memory materials, for example, faster nucleation, different compositions, doping levels and so on.     

Three distinct optical-switching states in phase-change materials containing impurities:From physical origin to material design

Ge2Sb2Tes is the most widely utilized chalcogenide phase-change material for non-volatile photonic applications,which undergoes amorphous-cubic and cubic-hexagonal phase transition under external excitations.However,the cubic-hexagonal optical contrast is negligible,only the amorphous-cubic phase transition of Ge2Sb2Te5 is available.This limits the optical switching states of traditional active dis-plays and absorbers to two.We find that increasing structural disorder difference of cubic-hexagonal can increase optical contrast close to the level of amorphous-cubic.Therefore,an amorphous-cubic-hexagonal phase transition with high optical contrast is realized.Using this phase transition,we have developed display and absorber with three distinct switching states,improving the switching perfor-mance by 50％.Through the combination of first-principle calculations and experiments,we reveal that the key to increasing structural disorder difference of amorphous,cubic and hexagonal phases is to intro-duce small interstitial impurities(like N)in Ge2Sb2Tes,rather than large substitutional impurities(like Ag)previously thought.This is explained by the formation energy and lattice distortion.Based on the impurity atomic radius,interstitial site radius and formation energy,C and B are also potential suit-able impurities.In addition,introducing interstitial impurities into phase-change materials with van der Waals gaps in stable phase such as GeSb4Te7,GeSb2Te4,Ge3Sb2Te6,Sb2Te3 will produce high optical con-trast amorphous-metastable-stable phase transition.This research not only reveals the important role of interstitial impurities in increasing the optical contrast between metastable-stable phases,but also proposes varieties of candidate matrices and impurities.This provides new phase-change materials and design methods for non-volatile optical devices with multi-switching states.     

Spectroscopic investigation on phase transitions for Ge2Sb2Te5 in a wide photon energy and high temperature region

We investigated the electronic properties of phase-change material Ge₂Sb₂Te₅ (GST) films using spectroscopic ellipsometry in a wide photon energy and high temperature region. Apart from the charge carrier response, the totality of optical conductivity spectra for three phases of GST films, i.e., amorphous (AM), face-centered-cubic (FCC), and hexagonal (HEX), is quite similar, composed of two interband transitions in visible and UV regions. From optical analysis in a wide photon energy region up to 8.7 eV, we found that the intensity as well as the position of the interband transition in the visible region changes significantly as the phase of GST films turns from the amorphous to the crystalline phase, which is consistent with previous theoretical studies. In high temperature measurements above room temperature for the three phases of GST films, we found that the change of optical response for the AM phase of GST film occurs abruptly through two successive phase transitions near 150 °C and 270 °C, while the optical spectra of the FCC phase shows a change only near 270 °C. In contrast to the two above-mentioned cases, a slight change in optical spectra is observed for the HEX phase with the increasing temperature. From the measured optical spectra, we derived the temperature dependence of optical bandgap for the three phases, which are closely correlated to the change of the transport property for the GST films.     

Role of hydrogen in Sb film deposition and characterization of Sb and GexSby films deposited by cyclic plasma enhanced chemical vapor deposition using metal-organic precursors

To meet increasing demands for chemical vapor deposition methods for high performance phase-change memory, cyclic plasma enhanced chemical vapor deposition of Sb and Ge_xSb_y phase-change films and characterization of their properties were performed. Two cycle sequences were designed to investigate the role of hydrogen gas as a reduction gas during Sb film deposition. Hydrogen gas was not introduced into the reaction chamber during the purge step in cycle sequence A and was introduced during the purge step for cycle sequence B. The role of hydrogen gas was investigated by comparing the results obtained from these two cycle sequences and was concluded to exert an effect by a combination of precursor decomposition, surface maintenance as a hydrogen termination agent, and surface etching. These roles of hydrogen gas are discussed through consideration of changes in deposition rates, the oxygen concentration on the surface of the Sb film, and observations of film surface morphology. Based on these results, Ge_xSb_y phase-change films were deposited with an adequate flow rate of hydrogen gas. The Ge and Sb composition of the film was controlled with the designed cycle sequences. A strong oxygen affinity for Ge was observed during the X-ray photoelectron spectroscopy analysis of Sb 3d, Sb 4d, and Ge 3d orbitals. Based on the XPS results, the ratios of Ge to Sb were calculated to be Ge_0.32Sb_0.68, Ge_0.38Sb_0.62, Ge_0.44Sb_0.56, Ge_0.51Sb_0.49 and Ge_0.67Sb_0.33 for the G1S7, G1S3, G1S2, G1S1, and G2S1 cycles, respectively. Crystal structures of Sb, Ge, and the GeSb metastable phase were observed with various Ge_xSb_y film compositions. Sb crystallinity decreased with respect to Ge crystallinity by increasing the Ge fraction. A current-voltage curve was introduced, and an electro-switching phenomenon was clearly generated at a typical voltage, V_th. V_th values increased in conjunction with an increased proportion of Ge. The Sb crystallinity decrease and V_th increase were explained via the bonding characteristics of each element.     

Interfacial phase-change memory

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.