Sb2Te3 Phase Research Articles

Evidence for Ge2Sb2Te5 phase formation upon crystallization of Ge-doped Sb2Te3 phase change films

In this work, we investigated the phase transition characteristics of Ge-doped Sb2Te3 phase change films prepared by the magnetron sputtering method. During the crystallization process of Ge-doped Sb2Te3 films, we observed a metastable phase and identified its crystal structure as the face-centered cubic Ge2Sb2Te5, a previously uncharacterized phenomenon. The Te phase precipitated upon crystallization of Sb2Te3 films, while the Te phase disappeared and the Ge2Sb2Te5 phase appeared upon crystallization of Ge-doped Sb2Te3 films, resulting in the coexistence of Ge2Sb2Te5 and Sb2Te3 phases in the single-layer Ge-doped Sb2Te3 films. The appearance of the Ge2Sb2Te5 phase resulted from the formation of Ge–Te bonds due to the combination of the precipitated Te phase with the incorporated Ge. Furthermore, the formation of the Ge2Sb2Te5 phase was mainly responsible for the increased crystalline resistance and crystallization temperature of Sb2Te3 films. The surface roughness of Ge-doped Sb2Te3 films was significantly reduced, exhibiting good potential for improving the contact performance between the films and electrodes. Our findings contribute to enhancing the understanding of Ge-doped Sb2Te3 films and their role in phase change memory technology.

Understanding the microstructure evolution of carbon-doped Sb2Te3 phase change material for high thermal stability memory application

The Sb2Te3 phase change material shows a growth-dominated crystallization mechanism with fast phase transition but poor thermal stability of the amorphous state. This work investigated the effects of carbon doping on the thermal stability, microstructure, and electrical properties of the Sb2Te3 material. The 10-year data retention temperature of the material increased to ∼147.3 °C and the size of the grains was limited to ∼10 nm by carbon doping. The formation of the C cluster upon crystallization was found at the grain boundaries, which was accelerated as the temperature increased due to the break of the Sb–C bonds. The memory device based on the carbon-doped Sb2Te3 material exhibited a switching speed of 15 ns and an endurance of ∼105 cycles with a resistance ratio of more than two orders of magnitude. This work suggests that the carbon-doped Sb2Te3 material is a promising candidate for memory applications that require high thermal stability, fast speed, and high endurance.

Revealing the crystallization dynamics of Sb–Te phase change materials by large-scale simulations

Using an efficient and accurate machine learning potential, large-scale crystallization dynamics of Sb–Te phase change materials are achieved.

Strong atom capture ability and pinning effect by doping an element with large electronegativity difference in Sb2Te3 phase change material

The ultimate goal of memory researchers and developers is to devise a universal memory to replace all memories in complex hierarchies of memory systems. Phase change memory is considered by the International Semiconductor Industry Association as the most likely emerging storage technology to achieve this ambitious goal due to its excellent performance. However, the relatively slow operation speed and poor cycle endurance are stumbling blocks for this purpose. Here, we propose a new doping strategy to construct stable bonding by introducing atoms with large electronegativity differences from the matrix Te atoms enhancing the capture ability and the pinning effect to Te atoms, which facilitates fast nucleation and suppresses migration of Te atoms to achieve significant performance improvement. Using ab initio molecular dynamics simulations, we explore and compare the atomic structures, chemical bonding, and dynamics properties of Hf doped Sb2Te3 (HfST), La doped Sb2Te3 (LaST), Rb doped Sb2Te3 (RbST) and Sr doped Sb2Te3 (SrST). In the XST (X refers to Hf, Rb, La and Sr atoms) systems, the increase of the more stable ring structure fluctuation and the stronger atom capture capability of rings enable more and faster formation of nuclei during crystallization. Higher Peierls-like distortions suggest better thermal stability. The motility of Te atoms is suppressed due to the pinning effect. High coordination of atoms and the concentration reduction of the lone-pair electron and void enable small density difference. Our results show that fast universal memory with good endurance can be achieved by the construction of strong bonding by doping atoms with large electronegativity differences, providing an important guidance for experiments and industries.

Dielectric functions evolution and electronic bandgap manipulation by silicon doping for Sb2Te3 phase change films: Temperature dependent spectroscopic ellipsometry study

The dielectric function evolution and electronic bandgap manipulation by silicon (Si) doping for Sb2Te3 phase change films have been investigated by temperature dependent spectroscopic ellipsometry measurements. During the phase change from amorphous to rhombohedral structures, the significant contrast of electronic band structure for pure Sb2Te3 and Si-doped Sb2Te3 (SST) films as functions of temperature (210–620 K) and Si concentration (0%–12%) has been systematically studied by analyzing the dielectric functions, Tauc gap energy, and partial spectral weight integral. The distinct differences can be mainly attributed to the increment of structure order degree, originated from the change of local bonding arrangement. Based upon the evolutions of Tauc gap energy and partial spectral weight integral with increasing temperature for all four samples, it can be concluded that Si doping can inhibit the crystallization of amorphous films and accelerate the phase change process by serving as nanoscale heaters, which is helpful in improving the thermal stability of amorphous films. The elevated crystallization temperature and phase change rate by Si doping contribute to the dependability and endurance for SST-based phase change memory. The present data provide an important direction on the physical mechanism investigation of Si doping Sb2Te3 by optical techniques.

Zaykovite, Rh3Se4, a new mineral from the Kazan placer, South Urals, Russia

AbstractZaykovite, ideally Rh3Se4, is a new mineral, the first natural rhodium selenide. It was discovered in the assemblages of platinum-group minerals from the Kazan gold placer, South Urals, Russia. The mineral occurs as crystals up to 40 μm in size within the grains of Pt3Fe alloy, in association with unnamed Pd–Sb–Te phase and Au–Pd alloy. In reflected light, zaykovite has a grey colour with bluish-greenish tint; it shows weak bireflectance and anisotropy. Reflectance values [Rmax/Rmin(%) for COM approved wavelengths (nm)] are: 30.1/29.3(470), 32.2/31.0(546), 33.4/32.0(589) and 35.1/33.7(650). The chemical composition corresponds to the empirical formula (Rh2.26Pt0.46Ir0.25Ru0.01Pd0.01Fe0.01)Σ3.00(Se2.77S1.21Te0.02)Σ4.00Zaykovite is monoclinic, space groupC2/m,a= 10.877(1),b= 11.192(1),c= 6.4796(6) Å, β = 108.887(2)°,V= 746.3(1) Å3,Z= 6 andDcalc= 8.32 g cm–1. The crystal structure has been solved and refined toR1= 0.016 based on 858 unique observed reflections. The strongest lines of the powder X-ray diffraction pattern [d(Å), (I), (hkl)] are: 5.43(37)($\bar{1}$11), 3.275(75)(310), 3.199(100)($\bar{1}$31), 3.061(87)(002), 2.568(62)(400), 2.545(41)(041), 3.413(34)($\bar{2}$41) and 1.697(34)(441). Zaykovite is a Se analogue of kingstonite, Rh3S4. A continuous series of solid solutions between kingstonite and zaykovite was encountered in the samples from the Kazan placer. The possible sources of this unique Rh–Se mineralisation in the South Urals could be serpentinised dunite–harzburgite or gabbro–clinopyroxenite–dunite complexes in the vicinity.

Nanostructured Sb2Te3 films composited with Bi2S3 for p–n conduction type conversion

The crystallization of p-type Sb2Te3 material can be tuned deterministically through compositing with n-type Bi2S3. The underlying changes in conduction from p-type to n-type are due to the crystallization changes induced by the composited Bi2S3 and the simultaneous changes in crystalline phase type. Bi2S3 nanoprecipitate formation and crystallization transition could effectively stimulate the p–n conduction transition. Upon annealing, pure Sb2Te3 partially crystallizes from the as-deposited state, which exhibits continuous crystallization during annealing. The formation of the p-type Sb2Te3 phase and the Sb2O3 phase in pure Sb2Te3 films through oxidation is significantly suppressed through compositing with excess Bi2S3. Moreover, the crystallization temperatures of the Sb2Te3–Bi2S3 films exceed 200 ℃, and the Seebeck coefficients and carrier concentrations of the samples are larger than those of pure Sb2Te3. The X-ray diffraction and Raman spectroscopy characterization of the crystalline thin films reveals a local change of the bonding arrangement around Sb atoms during crystallization. Our results suggest that Sb2Te3–Bi2S3 can serve as a promising candidate for thermoelectric applications based on n-type conduction.

Crystallization kinetics and thermodynamics of an Ag–In–Sb–Te phase change material using complementary in situ microscopic techniques

Crystallization kinetics and thermodynamics of an Ag–In–Sb–Te phase change material using complementary in situ microscopic techniques

High optical/color contrast of Sb2Te thin film and its structural origin

This work has investigated the optical/color contrast of Sb2Te phase change material and its structural volution. It is found that Sb2Te film possesses high optical/color contrast owing to its high discrepancy of refractive index and extinction coefficient between as-deposited and crystalline states. Raman spectra reveal that the enhanced Sb–Te bonds within two-dimensional Sb2Te3 plane is favorable for the high contrast of Sb2Te. The ab initio calculations further reveal that Sb2Te as-deposited phase has a complicated local motif owing to the excess Sb atoms. Moreover, analysis of the electronic structure of Sb2Te shows that the degree of electronic delocalization may in turn result in the large optical/color contrast of Sb2Te film. This work may provide a useful reference for future color display applications.

Excellent thermal stability attributed to Cr dopant in Sb2Te phase change material

Herein, Cr was selected as the dopant to improve the performance of Sb2Te in phase change random access memory (PCRAM). The thermal properties, crystal structure and chemical bonding state of the as-prepared Cr0.39Sb2Te (CST) were investigated. The CST containing 11.5At% Cr-dopant exhibited excellent performances including high crystallization temperature (218.6℃), good data retention (10 years @136.5 ± 1.0℃) and high stability with low-density change rate (2.8%). The switching speed up to 10 ns and the endurance nearly 1 × 105 cycles were obtained for the CST based PCRAM cells. All these findings indicated that the CST material is a potential candidate for universal memory devices.

Microstructure and morphology related electrical characterization and dielectric relaxation studies of nanocrystalline Sb2Te3 synthesized by mechanical alloying

This article explores a rapid and facile mechanical alloying method for synthesizing nanocrystalline Sb2Te3 at room temperatures with the least number of ingredients under an inert atmosphere. XRD, FESEM, and TEM have characterized the structure and microstructure. The rhombohedral Sb2Te3 phase formation without any impurity has been ascertained after 3 h of milling. A minimal size-dependent band gap is revealed from the FTIR spectra. The ionic conductivities measured in the temperature range 300–473 K increases with rising temperature, confirming the semi-metallic character of samples. The impedance spectra analysis reveals the electrical relaxation process and temperature-dependent transport properties. The non–Debye type behavior is confirmed from the Cole-Cole plot. The frequency-dependent ac conductivity obeys the universal Jonscher power law, and a small polaron transaction is used to explain the temperature dependence of the frequency exponent. A reduced value of dielectric loss factor at higher frequencies makes the material applicable in higher frequencies.

Melt solidification rate-dependent structural and thermoelectric properties of Sb2Te3/Te nanocomposites

Polycrystalline Sb2Te3/Te nanocomposites were prepared using the melt-quenching process, wherein different quenching mediums such as alkaline ice water, ice water, normal water, liquid nitrogen, and air-cooling were used. The existence of both Sb2Te3 and Te phases was revealed by various structural characterizations. The rate of melt solidification has a significant influence on the composition, microstructure, and thermoelectric transport properties of Sb2Te3/Te nanocomposites. The moderate cooling rate (normal water and liquid nitrogen cooling) of melt solidification showed a higher figure of merit (zT) value of ~0.25 at 423 K due to higher defect density. The different zT values observed for different quenching mediums and the correlation between melt solidification rate and the structural and thermoelectric transport properties were discussed in this paper.

Effect of microstructure on thermoelectric conversion efficiency in metastable δ-phase AgSbTe2

Herein, the effect of AgSbTe2 microstructure on the thermoelectric conversion efficiency of an AgSbTe2-based alloy was studied. The as-sintered sample exhibited material separation into a multiphase mixture of cubic Sb-rich AgSbTe2 (Ag21Sb28Te51), monoclinic Ag2Te with an inhomogeneous distribution, and an Sb-rich zone. The samples were subjected to heat treatment for 24 h at 523, 573, and 673 K. The results indicated that AgSbTe2 was metastable against Ag2Te and Sb2Te3 at 573 K. The accelerated kinetics induced the separation of AgSbTe2 into less-Sb-rich δ’-AgSbTe2 and Sb2Te3 with sub-µm sizes. The observations also revealed that AgSbTe2 was stable against Ag2Te and Sb2Te3 at 673 K. Micrometer-scale Ag2Te precipitates and rhombohedral Sb2Te3 phases were dissolved in the main matrix. Furthermore, single-phase δ-AgSbTe2 was stabilized with nanoscale precipitates of Ag2Te and Sb-rich nanodots. The thermoelectric transport properties of the heat-treated samples were investigated. The results indicated that the thermoelectric performance of the single-phase metastable structure with nanoscale precipitates was superior to that of the multiphase structure. Multiphase AgSbTe2 exhibited a low thermal conductivity; however, the effective Seebeck coefficient was balanced by the presence of multiple phases. This resulted in a low thermoelectric power factor. Metastable single-phase AgSbTe2, with Ag2Te and Sb-rich nanodots, exhibited a high Seebeck coefficient with a slightly low p-type conductivity. This resulted in a high thermoelectric power factor, and the presence of nanoscale precipitates lowered the thermal conductivity. It was concluded that the thermoelectric performance of metastable single-phase δ-AgSbTe2 was superior to that of multiphase AgSbTe2.

First-principles study of the liquid and amorphous phases of Sb2Te phase change memory material

We have investigated the local structure of liquid and amorphous phases of Sb2Te phase change memory material by the means of density functional theory-molecular dynamics simulations. The models of liquid and amorphous states were generated by quenching from the melt. The results show that the local environment of liquid Sb2Te is a mixed bonding geometry, where the average coordination numbers (CNs) of Sb and Te atoms are 4.93 and 4.23, respectively. Compared with crystalline state, there are more Sb–Sb bonds (∼53%) and less Sb–Te bonds (∼42%) with the presence of Te–Te bonds (∼5%) in liquid Sb2Te. Therefore, the formation of homopolar bonds and the breaking of heteropolar bonds are important structural transformations in melt process. For amorphous Sb2Te, the average CNs of Sb and Te atoms are 4.54 and 3.57, respectively. They are mostly in an octahedral environment, similar to the case in crystalline phase. The fractions of Sb–Sb, Te–Te, and Sb–Te bonds are ∼52%, ∼2%, and ∼46%, respectively. Thus, the increase in the fraction of octahedron accompanied with the decrease in average CN is the major structural changes in quenching process. Furthermore, the octahedral geometry in both the crystalline and amorphous Sb2Te increases the local structural similarity, facilitating the rapid low-energy crystallization.

Obtaining glasses in the extremely crystallizing Ge–Sb–Te phase change material

Using thermal co-evaporation techniques, we show that various glassy compositions can be obtained along the GexSbxTe100-2x join in the ternary Ge–Sb–Te system which is known to display dramatic crystallization tendencies. Earlier attempts to produce bulk glasses have been limited to Sb-poor compositions close to the eutectic GeTe6. Results indicate a weak variation of Tg with composition x and also a thermal stability that is weak for most systems, and especially for compositions close to the domain where Ge2Sb2Te5 can be formed. Our results are put in perspective with other network-forming chalcogenides and the Tg variation suggests the preferential formation of Te–Sb–Te, at variance with isochemical compounds such as Ge–Sb–Se. Data are discussed within the framework of topological based approaches which not only indicate that optimal glass formation is achieved for compositions satisfying the Maxwell stability criterion but also predict a flexible to rigid transition at x=8.5%. Most of our glasses could be formed around this composition.

Pt-Sb2Te as high speed phase-change materials with excellent thermal stability

Phase change memory (PCM) has been regarded as one of the most promising candidates for the next-generation nonvolatile memory. In this paper, we propose PtSb2Te (PST) phase change material for phase change memory. The doping of Pt improves the crystallization temperature and Ten-year data-retention temperature of Sb2Te to 180 °C, 192 °C, 204 °C and 117 °C,123 °C,137 °C, and refines the grain to about 10 nm. At the same time, the density change of Sb2Te film after phase transition is reduced to 4.19% due to the addition of Pt. There are no other new phases formed in PST film except hexagonal Sb2Te phase. For PST-based phase change memory cell, only 10 ns electrical pulse is required to complete the reversible operation with a Reset voltage lower than 4.3 V. At the same time, the number of cycle operations of the memory cell exceeds 105 and it has a lower resistance drift coefficient as 0.019.

Rhenium doped Sb2Te phase change material with ultrahigh thermal stability and high speed

Sb2Te material is an important base material for phase change memory or optical data storage, but the poor amorphous stability of Sb2Te severely limits its application. In this work, we proposed a superhard and infusible Rhenium (Re) doped Sb2Te (RST) phase change material to improve the properties of Sb2Te. The data retention of RST was remarkably increased to 152 °C@10 years, which is much higher than those of traditional Ge2Sb2Te5 (87 °C@10 years) and Sb2Te (56 °C@10 years). The operation speed of the RST based device is as fast as 10 ns and the resistance ratio is more than 102 times. The density change of RST (4.2%) is smaller than those of Ge2Sb2Te5 (6.5%) and Sb2Te (5.7%). Furthermore, the operation voltage and power consumption of RST based device are much lower than those of Ge2Sb2Te5 and Sb2Te based devices. The results indicate that the RST is a promising material for high performance phase change memory or optical data storage applications.

Reversible switching in bicontinuous structure for phase change random access memory application

SiSbTe phase change materials (PCMs) have excellent thermal stabilities. Their properties and microstructures are strongly affected by their Si content. Si3.3Sb2Te3 (SST) gives the best electrical performance, at Si contents of around 40%. Here, use of a combination of an advanced three-dimensional (3D) tomography technique and transmission electron microscopy clearly showed that a crystallized SST film has a uniform equiaxed-structure in 3D space, and consists of a reversible Sb2Te3 (ST) phase and an amorphous (a-) Si phase, which are well nested with each other. The a-Si nest localizes structure switching and diffusion of the host element in the nano-area. The most innovative aspect is significant retention of the metastable face-centered cubic (f-) ST phase, even above 370 °C, in this bicontinuous system. Specifically, the a-Si frame is stable and the ST phase switches between a- and f-structures under external stimulation. This promotes faster SET speed and low-power RESET consumption. Our results give new insights into PCM systems. They suggest that bicontinuous structures are potential candidates for use in phase-change random access memory devices, especially in automotive electronics applications that require a high data retention ability.

Multiple phase transitions in Sc doped Sb2Te3 amorphous nanocomposites under high pressure

Subnanosecond switching speed from an amorphous state with stable crystal precursors to the crystalline state was recently achieved in amorphous Sc-doped Sb2Te3 (a-SST) phase change materials (PCMs), which is about two orders of magnitude faster than that in the well-studied Ge2Sb2Te5 and Ge1Sb2Te4 PCMs. However, the phase change mechanism and phase stability of a-SST remain unknown. Here, we prepared amorphous Sc0.3Sb2Te3 nanocomposites within a minute amount of face-centered-cubic (FCC) type nanograins embedded in the amorphous matrix. Using in situ high-pressure synchrotron X-ray diffraction, we found that nanograins were frustrated under high pressure and gradually dissolved into the matrix around 11.0 GPa. Beyond 11.0 GPa, the a-SST matrix transformed into a uniform high density metallic like amorphous state with a five orders of magnitude drop in electric resistivity compared to the pristine materials. When further compressed to 23.97 GPa, the high density amorphous (HDA) phase switched into a body-centered-cubic (BCC) high-pressure structure, a different phase from the ambient pressure crystalline structure. Upon decompression back to ambient pressure, a pure amorphous phase was sustained. The present study provides additional insight into the phase change mechanism of amorphous nanocomposites.

Investigation on thermal stability of vanadium-doped Sb2Te phase change material

Transition metal elements doping can greatly improve the usability of Sb–Te for phase change memory. In this paper, we focused on the thermal stability of transition vanadium (V)-doped Sb2Te (VSb2Te) films with different V concentration which were prepared by co-sputtering. The effects of V doping on the crystallization and microstructure properties of Sb2Te were studied by temperature-dependent resistance measurements, in-situ heating TEM and HRTEM observation, respectively. With increasing V content, the crystallization is inhibited and the data retention is obviously enhanced, and the grain size shrinks significantly. Ab initio molecular dynamics simulation was used to explain the mechanism of the improved thermal stability and the results show that the diffusion of Sb and Te atoms are suppressed by doped V, which may account for the high thermal stability of VSb2Te.