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Ge2Sb2Te5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与HCP的结构转变不再发生，这有利于提高相变光盘的信噪比。Ge2Sb2Te5薄膜的结晶程度受初始化功率和转速的影响。     

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.     

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.     

Highly scalable non-volatile and ultra-low-power phase-change nanowire memory

The search for a universal memory storage device that combines rapid read and write speeds, high storage density and non-volatility is driving the exploration of new materials in nanostructured form. Phase-change materials, which can be reversibly switched between amorphous and crystalline states, are promising in this respect, but top-down processing of these materials into nanostructures often damages their useful properties. Self-assembled nanowire-based phase-change material memory devices offer an attractive solution owing to their sub-lithographic sizes and unique geometry, coupled with the facile etch-free processes with which they can be fabricated. Here, we explore the effects of nanoscaling on the memory-storage capability of self-assembled Ge2Sb2Te5 nanowires, an important phase-change material. Our measurements of write-current amplitude, switching speed, endurance and data retention time in these devices show that such nanowires are promising building blocks for non-volatile scalable memory and may represent the ultimate size limit in exploring current-induced phase transition in nanoscale systems.     

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.     

Understanding the phase-change mechanism of rewritable optical media

Present-day multimedia strongly rely on rewritable phase-change optical memories. We demonstrate that, different from the current consensus, Ge(2)Sb(2)Te(5), the material of choice in DVD-RAM, does not possess the rocksalt structure but more likely consists of well-defined rigid building blocks that are randomly oriented in space consistent with cubic symmetry. Laser-induced amorphization results in drastic shortening of covalent bonds and a decrease in the mean-square relative displacement, demonstrating a substantial increase in the degree of short-range ordering, in sharp contrast to the amorphization of typical covalently bonded solids. This novel order-disorder transition is due to an umbrella-flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this unique two-state nature of the transformation that ensures fast DVD performance and repeatable switching over ten million cycles.     

Thickness dependence of the amorphous-cubic and cubic-hexagonal phase transition temperatures of GeSbTe thin films on silicon nitride

The crystallization temperature of GeSbTe thin films with thicknesses between 11 and 87 nm on silicon nitride was studied through resistance versus temperature measurements. The amorphous-cubic phase transition occurs at ~ 150 °C for all films thicknesses, whereas the cubic-hexagonal phase transition temperature increases with film thickness, from ~ 200 °C for the 20 nm film to ~ 250 °C for the 87 nm film. The cubic-hexagonal transition occurs gradually for the 11 nm film. Implications for phase-change memory devices are discussed.     

相变光盘记录介质Sb-Se系和Ge-Sb-Te系合金组织及结构

按照配方熔炼制成Sb-Se系和Ge-Sb-Te系合金,对其组织结构进行了观察测试,研究结果表明,制备的合金组织比较均匀,X射线衍射结果表明,Sb-Se系合金1＃样品有Sb析出,2＃样品有Se析出,符合化学计量比的3＃样品全部形成Sb2Se3共晶体。Ge-Sb-Te系合金5＃样品有Sb析出,而符合化学计量比的4＃样品形成GeSb2Te4共晶体。     

Investigation of the crystallization process and crystal structure of Si-incorporated GeSb phase-change films

The effect of Si incorporation on the crystallization process and crystal structure of Te-free Sb-rich GeSb was investigated in this study. Si concentrations were controlled to 0, 5.1, 9.3, and 12.8 at.% by controlling the sputtering power of the GeSb alloy target (20:80 at.% for Ge:Sb) and Si target. After film deposition, the crystallization process and crystal structure were investigated. Crystallization temperature increased from 320 to 400 °C and the overall crystallinity was decreased with increasing Si concentration. These were analyzed by sheet resistance measurements after thermal annealing and optical contrast measurements by optical static testing. Glass transition temperatures were calculated and increased from 240 to 285 °C with increasing Si concentration. Considering the proportional relation between the glass transition temperature and crystallization temperature, it is thought that more energy is required for crystallization with increased Si concentration. A study of the crystallization process kinetics was conducted by applying the Johnson–Mehl–Avrami model to the optical static test results, which were carried out under a pseudo-isothermal process. The Avrami coefficient was 4.10 and decreased to 3.18 when the crystallization was generated with increased Si concentration from 0 to 12.8 at.%. Therefore, crystallization speed was thought to decrease with increased Si concentration. Based on the results of crystal structure analysis by XRD and HRTEM, the crystal structure of our Sb-rich GeSb PCM was revealed to be a typical Sb structure, i.e., an A7 hexagonal structure with lattice parameters of a = 4.26 Å and c = 11.45 Å. No crystal phase of Ge or Si was observed and no evidence of the structure change in Sb crystals due to Ge or Si incorporation was observed.     

Ga doping induced structural and optical modification in $$\hbox {Ge}_{2}\hbox {Sb}_{2}\hbox {Te}_{5}$$ Ge 2 Sb 2 Te 5 thin films

\(\text {Ge}_2\text {Sb}_2\text {Te}_5\) (GST) is considered a promising candidate for next-generation data storage devices due to its unique property of non-volatility and low power consumption. In present work, the bulk alloys and thin films of (\(\text {Ge}_2\text {Sb}_2\text {Te}_5\))\(_{100-x}\text {Ga}_x\) (x = 0, 3, and 10) are prepared using melt quenching and thermal deposition method, respectively. The effect of Ga doping on host composition is investigated by analyzing X-ray diffraction patterns and field emission scanning electron microscope images. From obtained results, it is found that all doped thin films retained the amorphous nature and exhibited uniform and smooth morphology. In Raman spectra, the appearance of a new peak in 10% Ga-doped GST thin film indicated an alteration in the atomic arrangement of host lattice. Transmission spectra revealed the highly transparent nature of all deposited thin films in the near-infrared region. The optical band gap of Ga-doped GST thin film is lower than that of the pure GST thin film which can be correlated with an increase in band tailing, attributed to an increase in localized defect states in the band gap. Due to the pronounced electronegativity difference between the Ga and Te element, new Ga–Te bonds with a higher number of wrong bonds (Ge–Ge, Sb–Sb, and Ge–Sb) are expected to thermally stabilize the amorphous phase. Such results predict the better performance of Ga-doped GST composition for better performance of phase-change random access memory.

     

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.     

Density functional calculations on the mechanical properties of nitrogen or oxygen doped crystalline Ge2Sb2Te5

The mechanical properties of pure and doped crystalline Ge2Sb2Te5 were investigated by using density functional calculations. Nitrogen or oxygen was added at either the interstitial or substitutional sites of cubic Ge2Sb2Te5. The lattice parameter, elastic stiffness and related moduli were investigated from the viewpoint of the doping concentration, dopant species, dopant states and film direction. The effect of the doping concentration was more dominant than those of the dopant species and their states on the non-directionality properties, such as the bulk modulus and lattice parameter. It turned out that Ge2Sb2Te5 became slightly more rigid as the doping concentration of nitrogen or oxygen increased. On the other hand, the effect of the film direction on the directional properties, such as the biaxial modulus of the Ge2Sb2Te5 film, was found to be more predominant than that of doping. The biaxial modulus of the (001) oriented film was calculated to be much higher than those of the other films, indicating that the (001) film is the most vulnerable to thermal stress.     

High-resolution transmission electron microscopy study of electrically-driven reversible phase change in ge2sb2te5 nanowires

By combining high-resolution transmission electron microscopy (HRTEM) characterization and electrical measurements on a unique device platform, we study the reversible electrically-driven phase-change characteristics of self-assembled Ge(2)Sb(2)Te(5) nanowires. Detailed HRTEM analyses are used to correlate and understand the effect of full and intermediate structural transformations on the measured electrical properties of the nanowire devices. The study demonstrates that our unique approach has the potential to provide new information regarding the dynamic structural and electrical states of phase-change materials at the nanoscale, which will aid the design of future phase-change memory devices.     

Low-consumption phase change material with good data retention selected from SbxTe

Various compositions of SbxTe phase-change films have been investigated by means of in situ temperature-dependent resistance measurement. Although crystallization temperature of Sb3Te is about 536 K, a little lower than those of Sb2Te and Sb4Te, it has a better data retention performance than the other two compositions. X-ray diffraction patterns imply that poly-crystalline structures of Sb-Te alloys are all hexagonal, and phase separation appears in SbTe3, Sb2Te, Sb3Te and Sb4Te alloys. Sb2Te and Sb3Te have been selected for phase-change random access memory devices. RESET voltage for Sb3Te is 1.2 V when the voltage pulse width is 50 ns, 0.3 V lower than that for Sb2Te. By using the voltage pulse width of 200 ns, Sb3Te needs 1.8 V and 4 V to complete SET-RESET performance, 0.4 V and 0.5 V lower than Sb2Te respectively. It indicates that Sb3Te is more suitable for the application of phase-change random access memory.     

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.     

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.     

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。     

Metal organic chemical vapor deposition of phase change Ge1Sb2Te4 nanowires

The self-assembly of Ge(1)Sb(2)Te(4) nanowires (NWs) for phase change memories application was achieved by metal organic chemical vapor deposition, catalyzed by Au nanoislands in a narrow range of temperatures and deposition pressures. In the optimized conditions of 400 °C, 50 mbar, the NWs are Ge(1)Sb(2)Te(4) single hexagonal crystals. Phase change memory switching was reversibly induced by nanosecond current pulses through metal-contacted NWs with threshold voltage of about 1.35 V.     

Size-dependent surface-induced heterogeneous nucleation driven phase-change in Ge2Sb2Te5 nanowires

By combining electron microscopy and size-dependent electrical measurements, we demonstrate surface-induced heterogeneous nucleation-dominant mechanism for recrystallization of amorphous phase-change Ge2Sb2Te5 nanowires. Heterogeneous nucleation theory quantitatively predicts the nucleation rates that vary by 5 orders of magnitude from 190 to 20 nm lengthscales. Our work demonstrates that increasing the surface-to-volume ratio of nanowires has two effects: lowering of the activation energy barrier due to phonon instability and providing nucleation sites for recrystallization. The systematic study of the effect of surface in phase-change behavior is critical for understanding nanoscale phase-transitions and design of nonvolatile memory devices.     

Amorphous structure melt-quenched from defective Ge2Sb2Te5

Ge₂Sb₂Te₅ (GST) is a technologically important phase-change material for data storage, where the fast reversible phase transition between crystalline and amorphous states is used for recording information. The encoding is achieved by the large contrast in physical properties between the two states. Ge vacancies (V_Ge) and Sb antisite atoms (Sb_Te) are primary point defects in crystalline GST. The effect of V_Ge and Sb_Te on the atomic arrangements in amorphous GST is unknown, which, however, has significant effects on the performance of GST. In this work, by means of ab initio molecular dynamics calculations, the atomic arrangements in amorphous ideal and defective GST have been investigated. The results show that the amorphous structure of GST with Sb_Te (St-GST) or with V_Ge (V-GST) has the same cubic framework and close chemical ordering to ideal GST, and hence similar fast reversible phase transition behavior is expected in the defective phases. Furthermore, the presence of Sb_Te or V_Ge in the crystalline phase will result in much more Ge atoms in a tetrahedral geometry as well as in a fourfold octahedral environment in the amorphous state. Especially in V-GST, around 36 % Ge atoms occupy a fourfold octahedral geometry. As fourfold octahedral Ge atoms can enhance the large contrast in physical properties between the crystalline and amorphous phases, introducing Ge vacancies would be an efficient way to improve the performance of GST phase-change materials.