相变存储材料Ge1Sb2Te4和Ge2Sb2Te5薄膜的结构和电学特性研究

采用射频磁控溅射方法制备了两种用于相变存储器的Ge₁Sb₂Te₄和Ge₂Sb₂Te₅相变薄膜材料,对其结构、电学输运性质和恒温下电阻随时间的变化关系进行了比较和分析.X射线衍射(XRD)和原子力显微镜(AFM)的结果表明:随着退火温度的升高,Ge₁Sb₂Te₄薄膜逐步晶化,由非晶态转变为多晶态,表面出现均匀的、 关键词： 硫系相变材料 1Sb₂Te₄')" href="#">Ge₁Sb₂Te₄ 2Sb₂Te₅')" href="#">Ge₂Sb₂Te₅     

氮掺杂Ge2Sb2Te5相变存储器的多态存储功能

通过反应溅射的方法，制备了N掺杂的Ge₂Sb₂Te₅(N-GST)薄膜，用作相变存储器的存储介质.研究表明，掺杂的N以GeN的形式存在，不仅束缚了Ge₂Sb₂Te₅ (GST)晶粒的长大也提高了GST的晶化温度和相变温度.利用N-GST薄膜的非晶态、晶态面心立方相和晶态六方相的电阻率差异，能够在同一存储单元中存储三个状态，实现相变存储器的多态存储功能. 关键词： 相变存储器 多态存储 N掺杂 2Sb₂Te₅')" href="#">Ge₂Sb₂Te₅     

应用于相变存储器的Cu-Ge3Sb2Te5薄膜的结构及相变特性研究

采用磁控溅射法制备了不同Cu含量的Cu-Ge₃Sb₂Te₅薄膜, 原位测试了薄膜电阻与温度的关系, 并利用X射线衍射仪、透射电镜、透过和拉曼光谱仪分别研究了 Cu-Ge₃Sb₂Te₅薄膜的晶体结构、微结构、禁带宽度及成键情况. 结果表明, Cu-Ge₃Sb₂Te₅薄膜的结晶温度和结晶活化能随着Cu含量的增加而增大, Cu的加入有效改善Ge₃Sb₂Te₅薄膜的热稳定性和10年数据保持力. 随着Cu含量的增加, 非晶态Cu-Ge₃Sb₂Te₅薄膜的禁带宽度逐渐减小. 同时, 拉曼峰从129 cm^-1向127 cm^-1处移动, 这是由于Cu–Te极性键振动增强的缘故. Cu-Ge₃Sb₂Te₅结晶为均匀、相互嵌套的六方Cu₂Te和Ge₂Sb₂Te₅相.     

氮掺杂Sb2Te3相变材料的脉冲激光沉积法制备与性能研究

Sb₂Te₃(ST)薄膜虽然具有高结晶速度和低结晶温度(~132℃)等优点,但由于复位电压过高,无法直接用于制备相变存储器件。文章作者尝试使用脉冲激光沉积法(PLD)制备了氮掺杂的Sb₂Te₃薄膜。用原子力显微镜(AFM)对此薄膜进行测试的结果表明,脉冲激光沉积法制备的生成态ST薄膜和氮掺杂ST薄膜表面粗糙度分别为0.12 nm和0.58 nm,表面较为平整。研究表明,PLD法制备的氮掺杂ST薄膜具有更好的组分稳定性,可显著提高薄膜低阻态电阻值,降低复位功耗,氮掺杂量在6 at%的ST薄膜的置位电压和复位电压适中,显示出较好的综合性能。     

基于Si掺杂Sb2Te3薄膜的相变存储器研究

采用磁控三靶(Si，Sb及Te)共溅射法制备了Si掺杂Sb₂Te₃薄膜，作为对比，制备了Ge₂Sb₂Te₅和Sb₂Te₃薄膜，并且采用微加工工艺制备了单元尺寸为10μm×10μm的存储器件原型来研究器件性能.研究表明，Si掺杂提高了Sb₂Te₃薄膜的晶化温度以及薄膜的晶态和非晶态电阻率，使得其非晶态与晶态电阻率之比达到10⁶，提高了器件的电阻开/关比；同Ge₂Sb₂Te₅薄膜相比，16at% Si掺杂Sb₂Te₃薄膜具有较低的熔点和更高的晶态电阻率，这有利于降低器件的RESET电流.研究还表明，采用16at% Si掺杂Sb₂Te₃薄膜作为存储介质的存储器器件原型具有记忆开关特性，可以在脉高3V、脉宽500ns的电脉冲下实现SET操作，在脉高4V、脉宽20ns的电脉冲下实现RESET操作，并能实现反复写/擦，而采用Ge₂Sb₂Te₅薄膜的相同结构的器件不能实现RESET操作. 关键词： 相变存储器 硫系化合物 2Te₃薄膜')" href="#">Si掺杂Sb₂Te₃薄膜 SET/RESET转变     

Ge2Sb2Te5非晶薄膜中超快载流子动力学的飞秒分辨反射光谱研究

利用飞秒时间分辨抽运-探测反射光谱技术研究了室温下Ge₂Sb₂Te₅非晶薄膜中载流子超快动力学及其激发能量密度依赖性.发现光激发后05 ps时间内,反射变化率降到最小值,然后开始迅速增加,在几个皮秒时间内达到大于初始反射率的新的最大值.反射率的减小量、增加量和增加速率均随激发能量密度的增大而增加.利用高密度等离子体的Auger复合及其感应的晶格加热模型较好地定量解释了反射率由最小到最大的快速变化过程,表明高密度等离子体的Auger复合加热 关键词： 抽运-探测光谱 2Sb₂Te₅非晶薄膜')" href="#">Ge₂Sb₂Te₅非晶薄膜 Auger复合 载流子动力学     

Ga30Sb70/Sb80Te20纳米复合多层薄膜的相变特性研究

采用磁控二靶(Ga₃₀Sb₇₀和Sb₈₀Te₂₀)交替溅射方法制备了新型Ga₃₀Sb₇₀/Sb₈₀Te₂₀纳米复合多层薄膜, 对多层薄膜周期中Ga₃₀Sb₇₀层厚度对相变特性的影响进行了研究. 结果表明, 多层薄膜的结晶温度可以通过周期中Ga₃₀Sb₇₀层厚度进行调节, 且随着Ga₃₀Sb₇₀层厚度的增加而升高. Ga₃₀Sb₇₀/Sb₈₀Te₂₀纳米复合多层薄膜的光学带隙随Ga₃₀Sb₇₀层厚度的增加而增大. 采用皮秒激光脉冲抽运光探测技术研究了多层薄膜的瞬态结晶动力学过程, 利用不同能量密度的皮秒激光脉冲可以实现Ga₃₀Sb₇₀/Sb₈₀Te₂₀多层薄膜非晶态和晶态的可逆转变.     

单轴压力下Ge2X2Te5(X=Sb,Bi)薄膜拓扑相变的第一性原理研究

随着拓扑绝缘体的发现, 材料拓扑物性的研究成为凝聚态物理研究的热点领域. 本文基于第一性原理计算, 研究了化合物Ge₂X₂Te₅ (X=Sb, Bi) 的块体结构和二维单层和双层薄膜结构的拓扑物性, 以及单双层薄膜在垂直方向单轴压力下的拓扑量子相变. 研究发现, A型原子序列排列的这两种化合物都是拓扑绝缘体, 其单层薄膜都是普通金属, 而双层薄膜都是拓扑金属, 单层和双层薄膜在单轴加压过程中都没有发生拓扑量子相变; 这两种化合物的B型原子序列的晶体是普通绝缘体, 其所对应的薄膜, Ge₂Sb₂Te₅单层是普通金属, 双层薄膜和Ge₂Bi₂Te₅的单层和双层薄膜均为普通绝缘体, 但是在单轴加压过程中B 型原子序列所对应的单层和双层薄膜都转变为拓扑金属.     

采用泵浦探测技术研究VO2薄膜相变特性

为研究纳秒激光作用下的VO₂薄膜的相变特性,采用泵浦-探测技术进行实验。首先,利用直流磁控溅射法制备VO₂薄膜,经X射线衍射（XRD）和原子力显微镜（AFM）分析表明样品质量较高。然后,测量VO₂薄膜在波长532 nm处的透过率随温度的变化情况,发现透过率随温度升高由32%上升到37%,与红外波段完全相反。在此基础上,选择1 064 nm泵浦光和532 nm探测光研究激光参数中能量密度和重频对VO₂薄膜相变特性的影响,同时结合ANSYS有限元软件对纳秒激光作用下VO₂薄膜的单脉冲温升情况进行分析。结果表明：VO₂薄膜在大于30 mJ/cm²的纳秒激光能量密度作用下,单脉冲温升可达相变温度,最小相变响应时间在14 ns左右。进一步提高纳秒激光能量密度,其相变响应时间略有增加但变化不大。在100 Hz以内改变纳秒激光重频对VO₂薄膜的相变响应基本无影响。VO₂薄膜的相变恢复时间随着纳秒激光能量密度的增大而呈自然指数增加,其变化过程与基底材料和纳秒激光参数密切相关。因此,可以通过优化VO₂薄膜基底材料参数提高其激光防护效果。     

Iron trichloride as oxidizer in acid slurry for chemical mechanical polishing of Ge2Sb2Te5

The effect of iron trichloride （FeC13） on chemical mechanical polishing （CMP） of Ge2Sb2Te5 （GST） film is inves- tigated in this paper. The polishing rate of GST increases from 38 nm/min to 144 nm/min when the FeC13 concentration changes from 0.01 wt% to 0.15 wt%, which is much faster than 20 nm/min for the 1 wt% H2O2-based slurry. This polish- ing rate trends are inversely correlated with the contact angle data of FeCl3-based slurry on the GST film surface. Thus, it is hypothesized that the hydrophilicity of the GST film surface is associated with the polishing rate during CMP. Atomic force microscope （AFM） and optical microscope （OM） are used to characterize the surface quality after CMP. The chemical mechanism is studied by potentiodynamic measurements such as Ecorr and Icorr to analyze chemical reaction between FeCl3 and GST surface. Finally, it is verified that slurry with FeCl3 has no influence on the electrical property of the post-CMP GST film by the resistivity-temperature （RT） tests.     

Different crystallization processes of as-deposited amorphous Ge2Sb2Te5 films on nano- and picosecond single laser pulse irradiation

The crystallization dynamics of as-deposited amorphous Ge₂Sb₂Te₅ films induced by nano- and picosecond single laser pulse irradiation is studied using in situ reflectivity measurements. Compared with nanosecond laser pulse, the typical recalescence phenomenon did not appear during the picosecond laser pulse-induced crystallization processes when the pulse fluence gradually increased from crystallization to ablation threshold. The absence of melting and recalescence phenomenon significantly decreased the crystallization time from hundreds to a few tens of nanoseconds. The role of pulse duration time scale on the crystallization process is qualitatively analyzed.     

The thickness dependence of the crystallization behavior in sandwiched amorphous Ge2Sb2Te5 thin films

The thickness dependent crystallization behavior of thin amorphous Ge₂Sb₂Te₅(GST) films sandwiched between different cladding materials has been investigated based on a thermodynamic model. It is revealed that there is a critical thickness below which the crystallization cannot occur. The critical thickness is determined by the energy difference Δγ between the crystalline GST/substrate interface energy and the amorphous GST/substrate interface energy, the melting enthalpy, and the mole volume. The calculated result is in good agreement with the experiments. Furthermore, the crystallization temperature is also affected by interface energy difference Δγ. Larger Δγ gives rise to a higher crystallization temperature, and vice versa. This impact becomes stronger as the film thickness is decreased.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Raman spectra and XPS studies of phase changes in Ge2Sb2Te5 films

Ge_2Sb_2Te_5 film was deposited by RF magnetron sputtering on Si (100) substrate. The structure of amorphous and crystalline Ge_2Sb_2Te_5 thin films was investigated using XRD, Raman spectra and XPS. XRD measurements revealed the existence of two different crystalline phases, which has a FCC structure and a hexagonal structure, respectively. The broad peak in the Raman spectra of amorphous Ge_2Sb_2Te_5 film is due to the amorphous －Te－-Te－ stretching. As the annealing temperature increases, the broad peak separates into two peaks, which indicates that the heteropolar bond in GeTe_4 and the Sb－Sb bond are connected with four Te atoms, and other units such as (TeSb) Sb－Sb (Te_2) and (Sb_2) Sb－Sb (Te_2), where some of the four Te atoms in the above formula are replaced by Sb atoms, remain in crystalline Ge_2Sb_2Te_5 thin film. And from the results of Raman spectra and XPS, higher the annealing temperature, more Te atoms bond to Ge atoms and more Sb atoms substitute Te in (Te_2) Sb－Sb (Te_2).     

Crystallization of Ge2Sb2Te5 phase—change optical disk media

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, E_a, 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.     

Si1Sb2Te3 phase change material for chalcogenide random access memory

This paper investigated phase change Si1Sb2Te3 material for application of chalcogenide random access memory. Current-voltage performance was conducted to determine threshold current of phase change from amorphous phase to polycrystalline phase. The film holds a threshold current about 0.155 mA, which is smaller than the value 0.31 mA of Ge2Sb2Te5 film. Amorphous Si1Sb2Te3 changes to face-centred-cubic structure at ~ 180℃ and changes to hexagonal structure at ~ 270℃. Annealing temperature dependent electric resistivity of Si1Sb2Te3 film was studied by four-point probe method. Data retention of the films was characterized as well.     

Effects of Si doping on the structural and electrical properties of Ge2Sb2Te5 films for phase change random access memory

The effects of Si doping on the structural and electrical properties of Ge₂Sb₂Te₅ film are studied in detail. Electrical properties and thermal stability can be improved by doping small amount of Si in the Ge₂Sb₂Te₅ film. The addition of Si in the Ge₂Sb₂Te₅ film results in the increase of both crystallization temperature and phase-transition temperature from face-centered cubic (fcc) phase to hexagonal (hex) phase, however, decreases the melting point slightly. The crystallization activation energy reaches a maximum at 4.1 at.% and then decreases with increasing dopant concentration. The electrical conduction activation energy increases with the dopant concentration, which may be attributed to the increase of strong covalent bonds in the film. The resistivity of Ge₂Sb₂Te₅ film shows a significant increase with Si doping. When doping 11.8 at.% of Si in the film, the resistivity after 460 °C annealing increases from 1 to 11 mΩ cm compared to the undoped Ge₂Sb₂Te₅ film. Current-voltage (I-V) characteristics show Si doping may increase the dynamic resistance, which is helpful to writing current reduction of phase-change random access memory.     

Crystallization of amorphous Ge2Sb2Te5 films induced by a single femtosecond laser pulse

Crystallization is achieved in amorphous Ge₂Sb₂Te₅ films upon irradiation with a single femtosecond laser pulse. Transmission electron microscopy images evidence the morphology of the crystallized spot which depends on the fluence of the femtosecond laser pulse. Fine crystalline grains are induced at low fluence, and the coarse crystalline grains are obtained at high fluence. At the damage fluence, ablation of the films occurs.     

Morphological characteristics of amorphous Ge2Sb2Te5 films after a single femtosecond laser pulse irradiation

The morphology of materials resulting from laser irradiation of the single-layer and the multilayer amorphous Ge₂Sb₂Te₅ films using 120 fs pulses at 800 nm was observed using scanning electron microscopy and atomic force microscopy. For the single-layer film, the center of the irradiated spot is depression and the border is protrusion, however, for the multilayer film, the center morphology changes from a depression to a protrusion as the increase of the energy. The crystallization threshold fluence of the single-layer and the multilayer film is 22 and 23 mJ/cm², respectively.