Minimum voltage for threshold switching in nanoscale phase-change memory

The size scaling of the threshold voltage required for the amorphous-to-crystalline transition in phase-change memory (PCM) is investigated using planar devices incorporating individual GeTe and Sb2Te3 nanowires. We show that the scaling law governing threshold switching changes from constant field to constant voltage scaling as the amorphous domain length falls below 10 nm. This crossover is a consequence of the energetic requirement for carrier multiplication through inelastic scattering processes and indicates that the size of PCM bits can be miniaturized to the true nanometer scale.     

Nanoscale ferroelectric and piezoelectric properties of Sb2S3 nanowire arrays

We report the first observation of piezoelectricity and ferroelectricity in individual Sb(2)S(3) nanowires embedded in anodic alumina templates. Switching spectroscopy-piezoresponse force microscopy (SS-PFM) measurements demonstrate that individual, c-axis-oriented Sb(2)S(3) nanowires exhibit ferroelectric as well as piezoelectric switching behavior. Sb(2)S(3) nanowires with nominal diameters of 200 and 100 nm showed d(33(eff)) values around 2 pm V(-1), while the piezo coefficient obtained for 50 nm diameter nanowires was relatively low at around 0.8 pm V(-1). A spontaneous polarization (P(s)) of approximately 1.8 μC cm(-2) was observed in the 200 and 100 nm Sb(2)S(3) nanowires, which is a 100% enhancement when compared to bulk Sb(2)S(3) and is probably due to the defect-free, single-crystalline nature of the nanowires synthesized. The 180° ferroelectric monodomains observed in Sb(2)S(3) nanowires were due to uniform polarization alignment along the polar c-axis.     

(GeTe)nBi2Te3的结构与热电性能研究

在GeTe-Bi₂Te₃赝二元系统中, (GeTe)_n(Bi₂Te₃)_m化合物往往具有较低的晶格热导率, 但其中很多组分的热电性能尚未得到系统研究。本研究通过熔融、淬火、退火结合放电等离子烧结工艺制备了一系列(GeTe)_nBi₂Te₃(n=10, 11, 12, 13, 14)单相多晶样品, 并对其相组成和热电性能进行表征和研究。掺杂Bi₂Te₃可以显著增强点缺陷声子散射, 大幅度降低材料的晶格热导率, 在723 K时, (GeTe)₁₃Bi₂Te₃样品的总热导率低至1.63 W?m ^-1?K ^-1。此外, 掺杂Bi₂Te₃和调控GeTe的相对含量, 提高了材料的载流子有效质量, 即使在较高的载流子浓度下, 样品依然保持较高的塞贝克系数和功率因子, 在723 K, (GeTe)₁₃Bi₂Te₃样品获得最大的功率因子为2.88×10 ^-3 W?m ^-1?K ^-2, 最终(GeTe)₁₃Bi₂Te₃样品在723 K获得的最大ZT值达到1.27, 较未掺杂的GeTe样品提高了16%。     

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.     

水热法制备Sb_2Te_3纳米线及其性能表征

在450K,pH值为11的条件下,以SbCl3,和Na2TeO3粉体为原料,用水热法制备了Sb2Te3纳米线。X-射线衍射(XRD)分析表明,所制的的材料为Sb2Te3,透射电镜(TEM)观察到Sb2Te3纳米线直径约为20nm,长度在200~600nm之间,并且自然平行密排成六边形纳米片状;然而在450K,pH值为13的条件下,制备出的产物则为完整的纳米片。扫描电镜(TEM)分析发现纳米线延(015)方向生长。差热测试得到Sb2Te3纳米线的熔点为727.5K。红外光谱的进一步研究表明,由Sb2Te3纳米线编织成的纳米片结构要优于整体生长成的纳米片,并就Sb2Te3纳米线的生长机理进行了初步推测。     

Catalyst-free growth of Sb2Te3 nanowires

In this study, a catalyst-free growth method was discovered to prepare the high-quality single crystal Sb₂Te₃ nanowires from the Al:Ge:Sb:Te thin films. The diameters of Sb₂Te₃ nanowires were found to be ~ 100 nm and their lengths were as great as tens of micrometers. The Al content and the annealing temperature play an important role in the growth of Sb₂Te₃ nanowires. When the Al content (> 12.4 at.%) was sufficiently contained in Al:Ge:Sb:Te film, Sb₂Te₃ nanowires were extruded spontaneously on the surface of thin film with increase in annealing temperatures. Compared with the vapor-liquid-solid method, our method has advantages of low temperature (~ 300 °C) and no impurities, such as a metal catalyst.     

Ultrafast switching in nanoscale phase-change random access memory with superlattice-like structures

Phase-change random access memory cells with superlattice-like (SLL) GeTe/Sb(2)Te(3) were demonstrated to have excellent scaling performance in terms of switching speed and operating voltage. In this study, the correlations between the cell size, switching speed and operating voltage of the SLL cells were identified and investigated. We found that small SLL cells can achieve faster switching speed and lower operating voltage compared to the large SLL cells. Fast amorphization and crystallization of 300 ps and 1 ns were achieved in the 40 nm SLL cells, respectively, both significantly faster than those observed in the Ge(2)Sb(2)Te(5) (GST) cells of the same cell size. 40 nm SLL cells were found to switch with low amorphization voltage of 0.9 V when pulse-widths of 5 ns were employed, which is much lower than the 1.6 V required by the GST cells of the same cell size. These effects can be attributed to the fast heterogeneous crystallization, low thermal conductivity and high resistivity of the SLL structures. Nanoscale PCRAM with SLL structure promises applications in high speed and low power memory devices.     

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.     

Conversion of a Bi nanowire array to an array of Bi-Bi2O3 core-shell nanowires and Bi2O3 nanotubes

A template-based heat-treatment method has been developed to convert metal nanowire arrays into arrays of metal-metal oxide core-shell nanowires and single-crystalline metal oxide nanotubes. This process is demonstrated by kinetically controlling the conversion of single-crystalline Bi nanowires to Bi-Bi(2)O(3) core-shell nanowires via a multistep, slow oxidation method, and then controlling their further conversion to a single-crystalline Bi(2)O(3) nanotube array via fast oxidation. This process can conveniently be extended to fabricate a free-standing, easily oxidized metal-metal oxide nanowire and metal oxide nanotube array, which may have future applications in nanoscale optics, electronics, and magnetics.     

磁控溅射法可控制备有序碲纳米线阵列

采用磁控溅射法在较低基底温度下(200 ℃)制备了有序碲纳米线阵列, 并利用X射线衍射、扫描电镜和透射电镜对所制备薄膜进行了相、形貌和微观结构分析。结果表明, 所制备的纳米线阵列由单晶碲纳米线组成, 单根碲纳米线具有针状形貌, 并沿[101]晶向生长, 平均直径和长度分别为100 nm和1 μm。氩气压力和基底温度均对碲纳米线阵列的形成具有重要影响, 以平衡碲原子沿[101]晶向和(101)晶面方向的扩散和生长。提出了碲纳米线阵列的生长机制, 包括吸附、结合、成核和生长等过程。     

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.     

Zn(1-x)Mg(x)Te nanowires grown by solid source molecular beam epitaxy

This paper reports on the epitaxial growth of single-crystalline ternary Zn(1-x)Mg(x)Te nanowires covering a broad compositional range of molar fraction 0≤x≤0.75. The nanowires were grown on (100), (110), and (111) GaAs substrates using a vapor-liquid-solid mechanism. Solid source molecular beam epitaxy and an Au-based nanocatalyst were used for these purposes. The composition of nanowires can be adjusted by changing the ratio of Mg to Zn molecular beam fluxes. Electron microscopy images show that the nanowires are smooth and slightly tapered. The diameters of the obtained nanowires are from?30 to 70?nm and their length is around 1?μm. X-ray diffraction analysis and transmission electron microscopy reveal that the nanowires have a zinc-blende structure throughout the whole range of obtained compositions, and have a [Formula: see text] growth axis. The Raman measurements reveal both the expected splitting and shift of phonon lines with increasing Mg content, thus proving the substitutional incorporation of Mg into metallic sites of the ZnTe lattice.     

Microstructure evolution and thermoelectric properties of Te-poor and Te-rich (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript> prepared via solidification

This paper explores in detail, the microstructures and thermoelectric properties of Te-rich and Te-poor (Bi,Sb)₂Te₃ alloys. We show that tuning the composition of ternary Bi–Sb–Te type alloys allows us to synthesize a range of microstructures containing a primary solid solution of (Bi,Sb)₂Te₃ with varying amounts of Te solid solution or a (Bi,Sb)Te compound. Te exists as a constituent of the multilayer domain while (Bi,Sb)Te appears in the thin intercellular regions of the (Bi,Sb)₂Te₃ dendritic cells. The presence of Te imparts an n-type behavior to the composite while the (Bi,Sb)₂Te₃ with a small amount of (Bi,Sb)Te exhibits p-type properties. A maximum ZT value of ≈0.4 at 425 K was achieved, opening up the possibility of using these alloys for thermoelectric device applications.     

MOCVD of Bi2Te3 and Sb2Te3 on GaAs substrates for thin-film thermoelectric applications

Metal organic chemical vapour deposition (MOCVD) has been investigated for growth of Bi2Te3 and Sb2Te3 films on (001) GaAs substrates using trimethylbismuth, triethylantimony and diisopropyltelluride as metal organic sources. The surface morphologies of Bi2Te3 and Sb2Te3 films were strongly dependent on the deposition temperatures as it varies from a step-flow growth mode to island coalescence structures depending on deposition temperature. In-plane carrier concentration and electrical Hall mobility were highly dependent on precursor ratio of VI/V and deposition temperature. By optimizing growth parameters, we could clearly observe an electrically intrinsic region of the carrier concentration over the 240 K in Bi2Te3 films. The high Seebeck coefficient (of -160 microVK(-1) for Bi2Te3 and +110 microVK(-1) for Sb2Te3 films, respectively) and good surface morphologies of these materials are promising for the fabrication of a few nm thick periodic Bi2Te3/Sb2Te3 super lattice structures for thin film thermoelectric device applications.     

Carbon nanotubes as nanoreactors for fabrication of single-crystalline Mg3N2 nanowires

Due to fast decomposition of Mg3N2 in the presence of water in the atmosphere (Mg3N2+6H2O-->3Mg(OH)2+2NH3), the synthesis of single-crystalline Mg3N2 nanowires has been a challenge. Here, we demonstrate that carbon nanotubes may serve as nanoreactors for a simple thermal reaction process resulting in the first fabrication of high-quality, large-yield, single-crystalline Mg3N2 nanowires. The Mg3N2 nanowires are homogeneously sheathed over their entire lengths with very thin graphitic carbon tubular layers, which effectively prevent their decomposition (even when the samples are put into water or exposed to atmosphere for several months). We have systematically analyzed for the first time the Mg3N2 nanomaterial by means of transmission electron microscopy (TEM), high-resolution TEM, and electron diffraction. Successful fabrication of carbon sheath protected Mg3N2 nanowires may promote further experimental studies on their crystal structures and properties.     

Si-Sb-Te materials for phase change memory applications

Si-Sb-Te materials including Te-rich Si?Sb?Te? and Si(x)Sb?Te? with different Si contents have been systemically studied with the aim of finding the most suitable Si-Sb-Te composition for phase change random access memory (PCRAM) use. Si(x)Sb?Te? shows better thermal stability than Ge?Sb?Te? or Si?Sb?Te? in that Si(x)Sb?Te? does not have serious Te separation under high annealing temperature. As Si content increases, the data retention ability of Si(x)Sb?Te? improves. The 10 years retention temperature for Si?Sb?Te? film is ~393 K, which meets the long-term data storage requirements of automotive electronics. In addition, Si richer Si(x)Sb?Te? films also show improvement on thickness change upon annealing and adhesion on SiO? substrate compared to those of Ge?Sb?Te? or Si?Sb?Te? films. However, the electrical performance of PCRAM cells based on Si(x)Sb?Te? films with x > 3.5 becomes worse in terms of stable and long-term operations. Si(x)Sb?Te? materials with 3 < x < 3.5 are proved to be suitable for PCRAM use to ensure good overall performance.     

Fabrication and optical property of single-crystalline InSb nanowire arrays

Semiconductor InSb nanowire arrays have been synthesized by the pulsed electrochemical deposition from citric acid aqueous solution into anodic alumina membranes. It was found that the InSb nanowires are of zinc-blende structure, and high filling rate and single-crystalline InSb nanowire array with right stoichiometric composition can be fabricated by proper controlling of the concentration of In and Sb ions and the PH value in the electrolyte and the pulse voltage, and the nanowires have [100] orientation with structure defects such as twins. The optical band gap has a strong blue shift with decreasing the diameter of the nanowire due to the quantum confinement effect.     

Vapor-liquid-Solid synthesis of [010]-oriented Sb2Se3 nanowires

In this article we report for the first time the synthesis of Sb2Se3 nanowires using a physical vapor-liquid-solid (VLS) process. We used microcrystals of Antimony as solid catalytic material and molten Selenium to generate the vapor source. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images show that as-obtained Sb2Se3 nanowires have diameters in the range between 20 nm and 2 microm and lengths up to 30 microm. Fringes in TEM imaging reveals that Sb2Se3 nanowires are oriented along the [010] crystallographic direction. This orientation is being reported for the first time.     

Bi-based Nanowire and Nanojunction Arrays： Fabrication and Physical Properties

This article reviews the recent developments in the fabrication and properties of one-dimensional （1D） Bi-based nanostructures, including Bi, Sb, BixSb1-x and Bi2Te3 nanowire arrays, and Bi-Bi and Bi-Sb nanojunction arrays. In this article, we present an efiective method to fabricate Bi nanowire arrays with difierent diameters in anodic alumina membrane （AAM） with a single pore size by the pulsed electrodeposition. The fabrication of the high-filling and ordered Bi1-xSbx and Bi2Te3 single crystalline nanowire arrays, the Bi nanowire metalsemiconductor homojunction and Bi-Sb nanowire metal-semiconductor heterojunction arrays by the pulsed electrodeposition are reported. The factors controlling the composition, diameter, growth rate and orientation of the nanowires are analyzed, and the growth mechanism of the nanowire and nanojunction arrays are discussed together with the study of the electrical and thermal properties of Bi-based nanowires and nanojunctions.Finally, this review is concluded with some perspectives on the research directions and focuses in the Bi-based nanomaterials fields.     

Realizing zT of 2.3 in Ge1−x−ySbxInyTe via Reducing the Phase‐Transition Temperature and Introducing Resonant Energy Doping

GeTe with rhombohedral‐to‐cubic phase transition is a promising lead‐free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon–phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge_1?xSb_xTe with optimized carrier concentration, a record‐high figure‐of‐merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase‐transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high‐density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high‐performance Pb‐free thermoelectric materials.