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     

Mechanical stress effect on the crystallization behavior of Ge2Sb2Te5 films studied by electrical resistance measurement

The mechanical stress effect on the crystallization behaviour of Ge₂Sb₂Te₅(GST) thin films is carefully investigated by electrical resistance measurements. It is found that the crystallization temperature of GST films increases as external compressive stress is applied, while the crystallization temperature decreases under tensile stress. We also find that the uneven distribution of extrinsic stress can widen the span of transition temperature. These results clearly demonstrate that mechanical stress plays an important role during the crystallization process of GST films and may further influence the reliability and storage speed of relevant devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EXAFS study of local order in the amorphous chalcogenide semiconductor Ge2Sb2Te5

Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently switching of the a-chalcogenide Ge₂Sb₂Te₅ has been applied quite successfully to DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching provides researchers today with an active arena of technological as well as fundamental study. On the theoretical front, bond constraint theory — or BCT — provides a powerful framework for understanding the structure and properties of a-materials. Applications of BCT to switching in Ge₂Sb₂Te₅ holds the promise of finding the best composition suited for switching applications. This work presents EXAFS data that describe local bonding configurations in as-deposited Ge₂Sb₂Te₅. The data show that Ge₂Sb₂Te₅ may best be viewed as a random array of Ge₂Te₃ and Sb₂Te₃ structural units imbedded in a tissue of a-Te, 17% of which is over coordinated. In addition, a valence alternation pair defect is introduced to the model to satisfy charge conservation constraints.     

Theoretical and experimental investigations of the properties of Ge2Sb2Te5 and indium-doped Ge2Sb2Te5 phase change material

We have carried out comprehensive computational and experimental study on the face-centered cubic Ge₂Sb₂Te₅ (GST) and indium (In)-doped GST phase change materials. Structural calculations, total density of states and crystal orbital Hamilton population have been calculated using first-principle calculation. 5 at.% doping of In weakens the Ge–Te, Sb–Te and Te–Te bond lengths. In element substitutes Sb to form In–Te-like structure in the GST system. In–Te has a weaker bond strength compared with the Sb–Te bond. However, both GST and doped alloy remain in rock salt structure. It is more favorable to replace Sb with In than with any other atomic position. X-ray diffraction (XRD) analysis has been carried out on thin film of In-doped GST phase change materials. XRD graph reveals that In-doped phase change materials have rock salt structure with the formation of In₂Te₃ crystallites in the material. Temperature dependence of impedance spectra has been calculated for thin films of GST and doped material. Thickness of the as-deposited films is calculated from Swanepoel method. Absorption coefficient (α) has been calculated for amorphous and crystalline thin films of the alloys. The optical gap (indirect band gap) energy of the amorphous and crystalline thin films has also been calculated by the equation \( \alpha h\nu = \beta (h\nu - E_{\text{g }} )^{2} \) . Optical contrast (C) of pure and doped phase change materials have also been calculated. Sufficient optical contrast has been found for pure and doped phase change materials.     

Scanning tunneling microscope light emission spectra of polycrystalline Ge2Sb2Te5 and Sb2Te3

We have observed scanning tunneling microscope light emission (STM-LE) spectra of Ge₂Sb₂Te₅ and Sb₂Te₃. Although these chalcogenide alloys exhibit band gaps less than 0.5 eV, the STM-LE was observed with a narrow spectral width at a photon energy of 1.5 eV for both materials. By analyzing its bias voltage, polarity, and temperature dependencies combined with recently reported theoretical electronic structures, we concluded that the STM-LE is excited by electronic transitions taking place in the local electronic structure having a direct gap-like shape with a band gap of 1.5 eV, commonly found in the electronic structures of both materials.     

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     

Effect of indium doping on Ge2Sb2Te5 thin films for phase-change optical storage

The influence of In doping on the crystallization kinetics of Ge₂Sb₂Te₅ has been investigated using four-point-probe electrical resistance measurements, grazing incidence X-ray diffraction (XRD), X-ray reflectometry (XRR), variable incident angle spectroscopic ellipsometry, a static tester, and atomic force microscopy. For a stoichiometric Ge₂Sb₂Te₅ alloy doped with 3% In, the amorphous-to-crystalline transition is observed at 150 °C in the sheet resistance measurements. XRD reveals the formation of a predominant NaCl-type Ge₂Sb₂Te₅ phase during the amorphous-to-crystalline transition together with small amounts of crystalline In₂Te₃. Density values of 5.88±0.05 g cm^-3 and 6.22±0.05 g cm^-3 are measured by XRR for the film in the amorphous and crystalline states, respectively. Perfect erasure can be achieved by laser pulses longer than 165 ns. The retarded crystallization, as compared with the undoped Ge₂Sb₂Te₅ alloy, is attributed to the observed phase segregation. Sufficient optical contrast is exhibited and can be correlated with the large density change upon crystallization. PACS 68.55.-a; 78.20.-e; 78.66.Jg     

Dynamics of laser crystallization in amorphous Ge films

In situ time- and space-resolved transmission measurements have been used to investigate the crystal growth during laser irradiation of amorphous Ge films. For dwell times longer than 5 ms, microcrystallization processes comparable to conventional thermal annealing are observed, whereas for irradiation times shorter than 5 ms, shell like crystallites, 20 μm long, are obtained. The results from the pulsed measurements agree with corresponding experiments performed under cw-annealing conditions.     

Optical Phonon Spectrum of the Ge2Sb2Te5 Single Crystal

JETP Letters - Raman scattering by optical phonons of the Ge2Sb2Te5 single crystal grown by the Bridgman method is studied for the first time. Another method for obtaining this compound based on...     

New structural picture of the Ge2Sb2Te5 phase-change alloy

Using electron microscopy and diffraction techniques, as well as first-principles calculations, we demonstrate that as much as 35% of the total Ge atoms in the cubic phase of Ge2Sb2Te5 locate in tetrahedral environments. The Ge-vacancy interactions play a crucial stabilizing role, leading to Ge-vacancy pairs and the sharing of vacancies that clusters tetrahedral Ge into domains. The Ge2Sb2Te5 structure with coexisting octahedral and tetrahedral Ge produces optical and structural properties in good agreement with experimental data and explains the property contrast as well as the rapid transformation in this phase-change alloy.     

Study of aluminium-modified Ge2Sb2Te5 thin films for the applicability as phase-change storage device material

Electrical and optical studies have been carried out on aluminium-modified Ge₂Sb₂Te₅ thin films to check its applicability as an active material in optical and electrical memory storage devices. Five polycrystalline bulk samples were prepared with compositions: Al_x(Ge₂Sb₂Te₅)_1?x; x = 0, 0.08, 0.14, 0.21, 0.25. Amorphous thin films were deposited from the polycrystalline bulk by thermal evaporation. Temperature-dependent resistance shows the increase in crystallization temperature of Ge–Sb–Te films on aluminium addition. Activation energy for conduction, conductivity, optical band gap, coefficient of refraction and extinction coefficient are studied with respect to Al content in both amorphous and crystalline phases of Ge–Sb–Te alloy films.     

Thickness-dependent electrical and optical properties of Ge8Sb2Te11 thin films

The amorphous Ge₈Sb₂Te₁₁thin films with varying thickness are thermally deposited on well-cleaned glass substrate from its polycrystalline bulk. Absence of any sharp peak confirms the amorphous nature of deposited films. Thickness-dependent electrical and optical properties including dc-activation energy, sheet resistivity, optical band gap, band tailing parameter, etc. of Ge₈Sb₂Te₁₁thin films have been studied. The optical parameters have been calculated from transmission, reflection and absorbance data in the spectral range of 200–1100 nm. It has been found that optical band gap and band tailing parameter decreases with the increase in Ge₈Sb₂Te₁₁thin films thickness. The dc-activation energy and sheet resistivity decreases while the crystallization temperature of the amorphous Ge₈Sb₂Te₁₁ films increases with the increase in thickness of the films. The decrease of the sheet resistivity has been substantiated quantitatively using the classical size-effect theory. These results have been explained on the basis of rearrangements of defects and disorders in the amorphous chalcogenide system.     

Switching phenomena in amorphous thin films of Ge,Cd x Te y and Sb x Se y

The threshold and memory switching effects in amorphous thin films of Ge, Cd _x Te _y and Sb _x Se _y , prepared by cathodic sputtering and vacuum evaporation are investigated. The switching is dependent on the polarity and the material of the contacts. The memory effect is typical for Ge and Cd _x Te _y films whilst the threshold effect sustaining a repeating frequency of about 0·1 Hz was found with Sb _x Se _y , films. The proper switching time was 0·2–0·4 s for the samples under consideration. A considerable variance of the critical intensity of the electric field when the switching set in was observed, as well as a variance of its dependence on the temperature and the surface properties of the substrate. With Ge samples the switching depended on the total amount of energy accumulated in them. A simple model is proposed on the ground of the achieved results assuming electronic processes in the first phase of the duration of the switching (i.e. injection from electrodes into the amorphous film) and bulk thermal processes leading to the creatin of a conduction channel in the second phase.     

掺Sn的Ge2Sb2Te5相变存储薄膜的光学性质

提高存储密度和存取速率一直是光存储发展的方向。这对目前用于可擦重写存储的相变材料提出了越来越多的要求：它们既要对短波长有足够的响应,同时其相变速度也越快越好。因此,相变材料性能的改进十分重要,掺杂是提高相变材料性能的重要手段之一。用直流溅射法制备了掺杂不同量Sn的Ge2Sb2Te5相变薄膜,由热处理前后薄膜的X射线衍射（XRD）发现：薄膜发生了从非晶态到晶态的相变。研究了薄膜在250—900nm区域的反射光谱和透射光谱。结果表明：适当的Sn掺杂能大大增加热处理前后材料在短波长（300—405nm）的反射率衬比度,可见,通过Sn掺杂改良相变材料的短波长光存储性能是一种有效的途径。     

Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials

Structural details of the amorphous binary GeTe and ternary Ge₂Sb₂Te₅ (GST) phase-change materials are investigated with the aid of Raman scattering. In the case of the a-GeTe, a plethora of Raman bands have been recorded and assigned on the basis of a network structure consisting of corner- and edge-sharing tetrahedra of the type GeTe_4−nGe_n (n=0, 1, 2, 3, 4). Significant temperature-induced structural changes take place in this material even at temperatures well below the crystallization temperature. These changes tend to organize the local structure, in particular the coordination number of Ge atoms, so as to facilitate the amorphous-to-crystal transformation. The much simpler Raman spectrum of GST, characterized by one vibrational band, is accounted for by the dominance of the Sb₂T₃ component in Raman scattering; reasons about this explanation, as well as for the lack of any Te–Te bonds are briefly described.     

Characteristics of Sn-Doped Ge2Sb2Te5 Films Used for Phase-Change Memory

Sn-doped Ge2Sb2Te5 thin films deposited on Si（100）/SiO2 substrates by rf magnetron sputtering are investigated by a differential scanning calorimeter, x-ray diffraction and sheet resistance measurement. The crystallization temperatures of the 3.58 at.%, 6.92 at.% and 10.04 at.% Sn-doped Ge2Sb2Te5 thin films have decreases of 5.3, 6.1 and 0.9℃, respectively, which is beneficial to reduce the switching current for the amorphous-to-crystalline phase transition. Due to Sn-doping, the sheet resistance of crystalline Ge2Sb2Te5 thin films increases about 2-10 times, which may be useful to reduce the switching current for the amorphous-to-crystalline phase change. In addition, an obvious decreasing dispersibility for the sheet resistance of Sn-doped Ge2Sb2Te5 thin films in the crystalline state has been observed, which can play an important role in minimizing resistance difference for the phase-change memory cell element arrays.     

Current-voltage characteristics of thin Ge2Sb2Te5 films taken using a measuring circuit with a current source

The measuring circuit with a voltage generator, which is traditionally used for studying electric properties of glasslike semiconductors, is compared with an alternative version using a current generator. The results obtained in the latter case reflect the interrelation between the memory formation effect and changes in the electrical parameters more comprehensively. A new electric parameter, viz. hold voltage U _hold, which has not been described earlier, is introduced. The relation between U _hold and the memory formation process during the phase transition is established. The effect of emergence of oscillations in the current-limiting regime in the conduction channel in the film is detected.     

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.     

Design of broadband achromatic metasurface device based on phase-change material Ge2Sb2Te5

Based on the phase-change material Ge₂Sb₂Te₅ (GST), achromatic metasurface optical device in the longer-infrared wavelength is designed. With the combination of the linear phase gradient GST nanopillar and the adjustment of the crystalline fraction m value of GST, the polarization insensitive achromic metalenses and beam deflector metasurface within the longer-infrared wavelength 9.5 μm to 13 μm are realized. The design results show that the achromatic metalenses can be focused on the same focal plane within the working waveband. The simulation calculation results show that the full-width at half-maximum (FWHM) of the focusing spot reaches the diffraction limit at each wavelength. In addition, the same method is also used to design a broadband achromatic beam deflector metasurface with the same deflection angle of 19°. The method proposed in this article not only provides new ideas for the design of achromatic metasurfaces, but also provides new possibilities for the integration of optical imaging, optical coding and other related optical systems.