On the metal distribution in the system GeTe−Sb2Te3

The structures of GeSb₂Te₄, Ge₂Sb₂Te₅ and GeSb₄Te₇ are not determined completely by means of classical X-ray or electron diffraction studies. We have measured the Mössbauer parameters of¹²¹Sb in these compounds as well as in their binary constituent Sb₂Te₃ as an attempt to improve our knowledge on the question.     

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.     

Pressure-induced site-selective disordering of Ge2Sb2Te5: a new insight into phase-change optical recording

We demonstrate that , the material of choice in phase-change optical recording (such as DVD-RAM), can be rendered amorphous by the application of hydrostatic pressure. It is argued that this structural change is due to a very strong second-nearest-neighbor Te-Te interaction that determines the long-range order in the metastable cubic phase of and also to the presence of vacancies. This newly discovered phenomenon suggests that pressure is an important factor for the formation of the amorphous phase which opens new insight into the mechanism of phase-change optical recording.     

High Thermoelectric Properties of PbTe Doped with Bi2Te3 and Sb2Te3

The composition-dependent thermoelectric properties of lead telluride (PbTe) doped with bismuth telluride (Bi2Te3), antimony telluride (Sb2Te3) and (BiSb)2Te3 have been studied at room temperature. All the sampies exhibit small thermal conductivity. The figures of merit, 7.63, 1.03 and 8.97 x 10-4, have been obtained in PbTe with these dopants, respectively. These values are several times higher than those of PbTe containing other dopants with small grain sizes. The high thermoelectric performance is explained by electronic topological transition induced by alloying. The results indicate that these dopants are effective to enhance the thermoelectric performance of PbTe.     

Local atomic structure in molten Si3Sb2Te3 phase change material

By means of ab initio molecular dynamics calculations, we have studied the local structures of liquid and amorphous Si₃Sb₂Te₃. The results show that all the constitute elements in liquid Si₃Sb₂Te₃ are octahedrally coordinated. While in amorphous state, Sb and Te atoms are mainly octahedrally coordinated and Si atoms are mainly tetrahedrally coordinated. In both states, Si is mainly homo-bonded by Si. Finally, we proposed a phase separation model for liquid and amorphous Si₃Sb₂Te₃, which is responsible for the good performance of Si₃Sb₂Te₃ alloy as a phase change material.     

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.     

Tunneling through narrow-gap semiconductor Sb2Te3 barrier

Tunnel experiments have been performed on Au/Sb₂Te₃/Al tunnel junctions to study elastic interelectrode tunneling through the small energy gap of a narrow-gap semiconductor. Tunnel conductance exhibited narrow width conductance peak at zero bias voltage. This behaviour is in accordance with the result of the theoretically calculated tunnel conductance, in which the nonparabolic dispersion relation within the energy gap of the narrow-gap semiconductor used as a tunnel barrier in a metal/narrow-gap semiconductor/metal tunnel structure is included. And some interesting structures are also observed in the conductance curves.     

High pressure transport characteristics of Bi2Te3, Sb2Te3, and BiSbTe3

This paper presents ambient and high pressure measurements of transport properties of the Bi₂Te₃–Sb₂Te₃ series of materials. The electrical resistivity, thermal conductivity, and Seebeck coefficient have been measured on both end compounds and the direct solid solution of the two at pressure up to 10 GPa. An additional discussion involving the high pressure structure will be presented. From this, it was determined that these materials undergo at least two structural phase transitions between 0 and 20 GPa and a discussion is presented regarding this and the changes in the transport properties.     

Plasma screening of the fundamental absorption edge in crystals of Bi2Te3-Sb2Te3 solid solutions with more than 80 mol % Sb2Te3

The regularities of changes in the optical properties of crystals of Bi₂Te₃-Sb₂Te₃ solid solutions in the range of the effects caused by free-carrier plasma oscillations at a variation in the ratio of Bi₂Te₃ and Sb₂Te₃ components are investigated. It is established that, when the Sb₂Te₃ content in a solid solution exceeds 80 mol %, the fundamental absorption edge undergoes plasma screening. Doping a Sb₂Te₃ crystal with tin increases the free-carrier concentration and plasma frequencies, thus enhancing the screening effect, whereas introduction of 0.1 mol % selenium reduced the plasma frequency and thus removes the fundamental-absorption-edge screening.     

Mixing kinetics and write-once optical recording characteristics of Sb/Se bilayer films

Bilayer Sb/Se films are irradiated with 12 ns pulses from an ArF laser (extended areas) and from a focused Ar⁺ laser (micron-sized areas). Real-time reflectivity measurements are used to determine if the process occurs within the solid or liquid phase and the transformation time, in addition to measure the optical contrast and the medium sensitivity. Transmission electron microscopy is used to analyze the structure of the transformed areas and the medium resolution. The results show that mixing is initiated by preferential melting at the grain boundaries and an amorphous phase is produced upon irradiation at high energy densities. Finally, the characteristics of the mixing process in Sb/Se films as a write-once optical recording mechanism are discussed in terms of the sensitivity and resolution of the recorded spots and the time required for recording.     

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×1Oμm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500 ns的电脉冲下实现SET操作,在脉高4 V、脉宽20 ns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.     

Electric quadrupole interaction of111Cd in the system Sb1−xMx (M=Ag,Cd,In,Sn), Sb2Te3, Bi2Te3 and In2Te3

We have performed TDPAC-measurements to investigate the static quadrupole interaction of₁₁₁Cd in the classic semimetal Sb. The coupling constant depends on the concentration of small amounts of metal admixtures. The only exception is the system Sb_1–xAg_x. The temperature dependence of the efg in the narrow gap semiconductors Sb₂Te₃ and Bi₂Te₃ is similar to that one in Te. In contrast to these results the efg in the III–VI-semiconductor In₂Te₃ is temperature independent.     

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     

Application of bond constraint theory to the switchable optical memory material Ge2Sb2Te5

A new extended x-ray-absorption fine structure spectroscopy study of local bonding identifies for the first time significant concentrations of Ge-Ge bonds in amorphous Ge2Sb2Te5. The study provides a new understanding of the local molecular structure of this phase-change material. Application of bond constraint theory indicates that the amorphous phase is an ideal network structure in which the average number of constraints per atom equals the network dimensionality. Analysis within this framework imparts new and significant insights concerning the nature of the reversible optically driven amorphous-crystalline phase transition of Ge2Sb2Te5.     

A tunneling spectroscopy study of the temperature dependence of the forbidden band in Bi2Te3 and Sb2Te3

Tunneling measurements of dI/dV, d ² I/dV ², and d ³ I/dV ³ were formed along the C ₃ axis (normally to layers) for Bi₂Te₃ and Sb₂Te₃ layered semiconductors in the temperature range 4.2<T>29 5 K. Temperature dependences of the forbidden band energy E _g were obtained. The forbidden band energy in Bi₂Te₃ was 0.20 eV at room temperature and increased to 0.24 eV at T=4.2 K. The E _g value for Sb₂Te₃ was 0.25 eV at 295 K and 0.26 eV at 4.2 K. The distance between the top of the higher valence band of light holes and the top of the valence band of heavy holes situated lower was found to be ΔE _V≈19 meV in Bi₂Te₃; this distance was independent of temperature. The conduction bands of Bi₂Te₃ and Sb₂Te₃ each contain two extrema with distances between them of ΔE _c≈25 and 30 meV, respectively.     

Microstructure of thermoelectric (Bi0.15Sb0.85)2Te3 film

The film of thermoelectric ternary p-type (Bi_0.15Sb_0.85)₂Te₃ was deposited on polyimide foil substrate at 168 °C using direct-current magnetron sputtering. Microstructural investigations of the film were performed by electron microscopy techniques. SEM observations showed that the film surface consisted of large-sized particulates with small-sized particles and also mound-like crystal agglomerates in some areas. Chemical composition of the film was analyzed using energy-dispersive X-ray spectrometer (EDS). It has been observed that the EDS results were in an agreement with nominal composition for the film. Detailed microstructural investigations were carried out using transmission electron microscopy (TEM). TEM images and selected area electron diffraction patterns showed that the film has randomly oriented polycrystalline grain structure. High-resolution TEM images indicated that the microstructure of film also contained nano-crystal structure, smaller than 10 nm.     

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.     

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.