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Controlling a state of material between its crystalline and glassy phase has fostered many real-world applications. Nevertheless, design rules for crystallization and vitrification kinetics still lack predictive power. Here, we identify stoichiometry trends for these processes in phase change materials, i.e. along the GeTe-GeSe, GeTe-SnTe, and GeTe-Sb2Te3 pseudo-binary lines employing a pump-probe laser setup and calorimetry. We discover a clear stoichiometry dependence of crystallization speed along a line connecting regions characterized by two fundamental bonding types, metallic and covalent bonding. Increasing covalency slows down crystallization by six orders of magnitude and promotes vitrification. The stoichiometry dependence is correlated with material properties, such as the optical properties of the crystalline phase and a bond indicator, the number of electrons shared between adjacent atoms. A quantum-chemical map explains these trends and provides a blueprint to design crystallization kinetics., Tailoring the crystallization kinetics of materials is important for targeting applications. Here the authors observe a remarkable dependence of crystallization and vitrification kinetics and attribute it to systematic bonding changes for a class of materials between metallic and covalent bonding.

The controlled growth of chalcogenide nanoscaled phase change material structures can be important to facilitate integration and to enable complex architectures for phase change memory (PCM) and other microelectronic applications. Here, the growth of Sb-Te and In-Ge-Te alloys by Metal-Organic Chemical Vapour Deposition (MOCVD) on patterned substrates featured with an array of recesses (130 nm features width) was investigated. High selectivity, with preferential growth on a CoSi2 metallic layer at the recess bottom with respect to the surrounding SiO2 masking layer, was obtained, leading to a single-step fabrication of arrays of high-aspect-ratio chalcogenide nanostructures. The growth selectivity, as well as the morphology, composition and microstructure of the grown nanostructures, as a function of the different MOCVD process parameters, were investigated by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-Ray (EDX) spectroscopy, Raman spectroscopy and Fourier Transformed Infrared (FTIR) spectroscopy. Thanks to the chosen substrates, the synthesized nanostructures were also directly electrically accessible, as proved by conductive- Atomic Force Microscopy (c-AFM).

Phase change memory (PCM) is the most mature among the novel memory concepts. Embedded PCM technology can be a real breakthrough for process cost saving and performances. Nevertheless, for specific applications some improvement in high temperature data retention characteristics is needed. In this paper, we present an optimized GexSbyTez phase change material, able to guarantee code integrity after soldering thermal profile and data retention in extended temperature range. In particular, extrapolation of data retention at 10 years for temperatures higher than 150°C cell level has been demonstrated, thus enabling automotive applications. Despite the tradeoff between the SET speed and RESET data retention, competitive performances with respect to present floating gate memories have been confirmed. Finally, solid data collection based on a 4-Mb test chip integrated in a standard 90-nm CMOS technology platform has been performed. Functionality and performances are well in line with today industrial targets.

The 3ω method was employed to determine the effect of nitrogen doping (5 at.%) on the thermal conductivity of sputtered thin films of stoichiometric GeTe (a material of interest for phase change memories). It was found that nitrogen doping has a detrimental effect on the thermal conductivity of GeTe in both phases, but less markedly in the amorphous (–25%) than in the crystalline one (–40%). On the opposite, no effect could be detected on the measured thermal boundary resistance between these films and SiO2, within the experimental error. Our results agree with those obtained by molecular dynamic simulation of amorphous GeTe. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A long-standing question for avant-garde data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved x-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process; meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in PCMs.

Phase change memory (PCM) has reached the status of mature technology for stand-alone, embedded, and storage-class memory (SCM). A key requirement for these applications is stability at high temperature (T) during soldering, packaging and operation. To this aim, new materials and algorithms to improve reliability are essential. Here we demonstrate bipolar switching in PCM resulting in low-current operation and excellent high-T reliability. DC and pulsed switching characteristics are explained by ion migration in the chalcogenide layer, as supported by TEM and T-dependent studies. Excellent reliability at high T is demonstrated and explained by a physical model.

Phase change materials based on chalcogenides are key enabling technologies for optical storage, such as rewritable CD and DVD and recently also electrical nonvolatile memory, named phase change memory (PCM). In a PCM, the amorphous or crystalline phase affects the material band structure, hence the device resistance. Although phase transformation is extremely fast and repeatable, the amorphous phase suffers structural relaxation and crystallization at relatively low temperatures, which may affect the temperature stability of PCM state. To improve the time/temperature stability of the PCM, novel operation modes of the device should be identified. Here, we present bipolar switching operation of PCM, which is interpreted by ion migration in the solid state induced by elevated temperature and electric field similar to the bipolar switching in metal oxides. The temperature stability of the high resistance state is demonstrated and explained based on the local depletion of chemical species from the electrode region.

A systematic design of practicable media suitable for rewritable, ultrahigh density (>;1Tbit/sq.in.), high data rate (>;1Mbit/s/tip) scanning-probe phase-change memories is presented. The basic design requirements were met by a Si/TiN/Ge2Sb2Te5 (GST)/diamond-like carbon structure, with properly tailored electrical and thermal conductivities. Various alternatives for providing rewritability were investigated. In the first case, amorphous marks were written into a crystalline starting phase and subsequently erased by recrystallization, as in other already established phase-change memory technologies. Results imply that this approach is also appropriate for probe-based memories. However, experimentally, the successful writing of amorphous bits using scanning electrical probes has not been widely reported. In light of this, a second approach has been studied, that of writing crystalline bits in an amorphous starting matrix, with subsequent erasure by reamorphization. With conventional phase-change materials, such as continuous films of GST, this approach invariably leads to the formation of a crystalline “halo” surrounding the erased (reamorphized) region, with severe adverse consequences on the achievable density. Suppression of the “halo” was achieved using patterned media or slow-growth phase-change media, with the latter seemingly more viable.

The thermal properties of the phase-change chalcogenide alloy Ge2Sb2Te5 in its three phases (amorphous, cubic, and hexagonal) and of Si3N4 and SiO2 have been studied to obtain reliable values for device modeling. Thermal conductivity was determined, along with a quantitative estimation of the thermal resistances of the layers’ interfaces, not negligible for highly scaled devices. Electrical resistivity of the chalcogenide material has also been investigated during the phase transition by in situ measurement at constant heating rate.

A novel self-aligned μTrench-based cell architecture for phase change memory (PCM) process is presented. The low programming current and the good dimensional control of the sub-lithographic features achieved with the μTrench structure are combined with a self-aligned patterning strategy that simplify the integration process in term of alignment tolerances and of number of critical masks. The proposed architecture has been integrated in a 90 nm 128 Mb vehicle based on a pnp bipolar junction transistor for the array selection. The good active and leakage currents achieved by the purposely optimized selecting transistors combined with programming currents of 300 μA of the storage element and good distributions measured on the 128 Mb array demonstrate the suitability of the proposed architecture for the production of high-density PCM arrays at 90 nm and beyond.

Thin TiSiN flat films deposited by MOCVD have been analysed in order to evaluate their microstructure and physical properties. TEM, XRR and RBS techniques have been utilised as complementary analyses to evaluate thickness, density and composition. The main composition is Ti : N : Si = 0.9 : 1.1 : 0.14 with carbon and oxygen up to 5% as a result of organic residuals and oxidation. The analysis shows a layered structure which well correlates with deposition steps. TiN nanocrystals embedded in a SiN x -rich amorphous matrix result confined into each TiSiN deposited layer. The silane soak produces a low concentration silicon inclusion, mainly stabilising the grain boundaries and not affecting the TiN crystalline morphology. AES profiles show that the TiSiN film consists of Ti–N rich layers, separated by C and Si rich “interface” layers which are the result of the process sequence. Densification and crystallisation of the film were observed after high temperature annealing in vacuum, resulting in an electrical resistivity decrease. PECVD deposition of an oxide layer on TiSiN causes a strong oxidation and resistivity increase of the film.

Atomic migration on Phase Change Memory devices with wall architecture has been experimentally investigated and a quantitative model including electrical, thermal, and mechanical driving forces has been developed. The experimental results collected by driving the device with programming pulses with direct and reverse polarity have been accounted for. Comparison with data of atomic migration on heavily cycled cells is also provided.

The microstructure of Ni-5at%W (Ni-W) and Ni-11at%V (Ni-V) biaxially textured substrates has been investigated using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The correlation between the substrate microstructure and superconducting transport properties of YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) film grown on it has been studied on the YBCO/CeO/sub 2//Ni-W and YBCO/CeO/sub 2//NiO/Ni-V architectures. Our study has ascertained that the in-plane texture of the substrates is one of the most important factors, limiting the critical current density. The Ni-V substrate has a lower percolation area due to the larger number of twinned grains and a broader in-plane angular distribution and, as a consequence, the YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) film grown on it has a critical current density of 0.6 /spl times/ 10/sup 6/ A/cm/sup 2/, depressed by factor 2 with respect to YBCO grown on the Ni-W substrate. For the Ni-V substrate, another limiting factor is its low oxidation resistance. In contrast to Ni-V, the Ni-W substrate has a larger percolation area, mainly due to the absence of twinned grains, and a high oxidation resistance.

We report on the growth of MgB 2 thin films by means of Pulsed Laser Deposition (PLD) and Electron Beam (EB) deposition techniques. In order to develop an in-situ deposition procedure both techniques have been exploited following two approaches: the "as grown" procedure, where the superconducting phase is formed during the film growth, and no further process is performed, and the "annealing" procedure, where precursor layers are deposited and annealed in argon atmosphere. In the case of EB evaporated films, the "as grown" procedure revealed to be inadequate, because of the low reactivity of the thermally evaporated species and the high magnesium volatility. On the contrary, using PLD, the higher reactivity of the plasma species promotes the formation of the superconducting phase at deposition temperature as low as 350°C. In the "annealing" procedure, different kinds of precursor layers have been studied, in order to reduce and prevent the fast Mg evaporation at high temperature. Different annealing processes were investigated in order to promote the interdiffusion and reaction between Mg and B. The films were characterised by means of resistivity measurements and X-ray analyses. The surface morphology was observed by SEM microscopy.

A study on CeO 2 film growth on randomly oriented metallic substrate using lnclined Substrate Deposition (ISD) technique was performed in order to develop a biaxially aligned buffer layer for YBa 2 Cu 3 O 7-δ (YBCO) coated conductors. The influence of deposition parameters, as the substrate inclination angle α with respect to the CeO 2 vapor direction, deposition temperature and film thickness, on structural and morphological properties of the film was investigated. At substrate temperature between 200°C and 700°C a biaxial texture was observed for α ranging from 15° to 75°. The minimum value of the ϕ-scan full width at half maximum (FWHM) on (002) poles of about 13.5° was obtained for film 2 μm thick deposited at 200°C and α=55°. Morphological analyses on cross-sectioned samples revealed a columnar structure, typical for this deposition technique, with spaced grains and a tile like surface.

Biaxially aligned YBa2Cu3O7−δ (YBCO) thick films were deposited by pulsed laser ablation technique on cube textured non-magnetic Ni89V11 (Ni–V) substrate, using CeO2/YSZ/CeO2/NiO buffer layer architecture. The first NiO seed layer was formed by epitaxial oxidation of the Ni–V substrate. Structural analyses show typical full width at half maximum values of φ- and ω-scans less than 10° and 8°, respectively. The highest value obtained for the critical current density at 77 K and zero magnetic field was 6×105 A cm−2, which is close to that obtained for YBCO films grown on CeO2/NiO buffer layer architecture.

The epitaxial oxidation of the (0 0 1)[1 0 0] textured Ni100−xVx tapes was studied because of the practical interest of NiO as a first buffer layer for the YBCO based coated conductors. The study revealed that the oxidation of the Ni–V alloy is rather complex, the less noble V being internally oxidized, while Ni undergoes an external oxidation. Moreover, the formation of the NiVO3 and of Ni7V5O17 compounds have negative effects on the epitaxial oxidation and on the surface morphology, as well. The role of vanadium on the epitaxial oxidation of Ni–V alloy has not been fully understood yet. The optimum conditions for the epitaxial oxidation have been found to be: 700 ° C, t

Biaxial cube texturing and surface quality of the metallic substrate are key parameters for high J c coated conductors. Experiments have demonstrated that the sharp cube texture can be developed up to 11 at.% V, 12 at.% Cr and 5 at.% W alloys; the respective cube textured area fractions result to be 96%, 97% and 99%. For all the studied alloys, FWHM values of ω - and φ -scans are in the range of 5–6° and 6–8°, respectively. Microstructural analyses reveal smooth surfaces with grooves localized at the grain boundaries of twinned cube textured and misoriented grains.

A hybrid superconducting fault current limiter (HSFCL) has been developed consisting of a transformer whose secondary is connected to a series of parallel YBCO superconducting bars. The transformer is used as a coupling impedance of the protected line with a superconducting switch. We show that very high quench currents (IQ~3.000 A) may be achieved at 77 K with YBCO single domain samples grown by Bridgman, but that the transition is initiated at the electrical contacts. To overcome this problem two different strategies concerning the tuning of the critical currents of the materials have been envisaged: (i) to promote an enhanced quenched volume from the electrical contacts through the increase of the critical currents and (ii) to initiate an homogeneous quench through a decrease of the overall critical currents of the material. Several methodologies have been envisaged to enhance the critical currents of Y-123/211 composites: an optimized 211 content, a low temperature-high isostatic pressure post-processing treatment, a high oxygen pressure annealing and, finally, including Ag additives in the 123/211 composites. Post-processing processes promote the generation of (1/6)031 partial dislocations which behave as flux pinning centres. On the other hand, we will show that an enhanced spreading of the quench transition may be achieved by decreasing quench currents, either by increasing the measuring temperature or by MgO doping the MTG YBCO bars. This last approach of decreasing Tc and hence Jc appears very promising in order to develop effective elements for a SFCL.

During normal operation of Phase Change Memory (PCM) cells active materials undergo very high electrical and thermal stresses that cause a motion of the different atoms leading to composition variation which has a fundamental impact on performance and reliability. In order to address this issue we introduce here a comprehensive 3D physical model for mass transport in chalcogenide materials. In addition to the driving force for atom diffusion coming from concentration gradient and electric field, the model also accounts for the effect of temperature gradient and phase segregation. This new diffusion model is coupled with a calibrated electro-thermal-phase change model, thus providing a unified framework for the self-consistent simulation of both the electro-thermal and the phase/material change problems. The model is applied to the study of different types of PCM cells showing good agreement with experiments and demonstrating in particular the fundamental role played by the temperature profile.

The phase change active material exploration represents an important stage in order to further strengthen the know-how on the Phase Change Memory (PCM) technology. This work reports a path for PCM material exploration toward the tellurium poor region of the GeSbTe (GST) ternary compound system. Data retention enhancement is reported and associated to a factor 1.5 increase of the crystallization activation energy. A detailed description and discussion of the remarkable electrical and thermal parameters of the PCM cell as a function of GST composition is provided.

In this work, we provide a detailed electrical and physical characterization of the over-reset (OR) state in terms of amorphous volume shape/thickness, activation energy for conduction, time drift exponent, 1/f noise intensity and threshold voltage. Experimental data are compared with simulations accounting for both the transport and switching mechanisms. Comparison highlights that OR is due to a reversible modification of the active material in the amorphous volume.

Silver wires of the superconducting phase (Hg 0.8 Re 0.2 )Ba 2 Ca 2 Cu 3 O 8+ δ (Hg,Re-1223), were prepared by the powder in tube technique followed by a post annealing treatment in a sealed quartz tube. The sintering reaction was carried out inside the quartz tube using a superconducting ceramic pellet and a drop of Hg(l) as external mercury sources. We have studied the amalgamation process in order to avoid the decomposition of the Hg,Re-1223 due to the imminent amalgamation between the silver and the Hg from the superconducting phase. Different annealing treatments of the wires were studied in order to increase the superconducting properties. We have found that using additional liquid Hg as an external source the amalgamation process can be controlled. The critical current density ( J c ) values obtained were similar to the bulk specimens.

The phase-change memory (PCM) technology is considered as one of the most attractive non-volatile memory concepts for next generation data storage. It relies on the ability of a chalcogenide material belonging to the Ge–Sb–Te compound system to reversibly change its phase between two stable states, namely the poly-crystalline low-resistive state and the amorphous high-resistive state, allowing the storage of the logical bit. A careful study of the phase-change material properties in terms of the set operation performance, the program window and the electrical switching parameters as a function of composition is very attractive in order to enlarge the possible PCM application spectrum. Concerning the set performance, a crystallization kinetics based interpretation of the observed behavior measured on different Ge–Sb–Te compounds is provided, allowing a physics-based comprehension of the reset-to-set transition.

The thermal conductivity of Ge2Sb2Te5 (GST) layers, as well as the thermal boundary resistance at the interface between the GST and amorphous SiO2, were measured using a PhotoThermal Radiometry experiment. The two phase-changes of the Ge2Sb2Te5 were retrieved, starting from the amorphous and sweeping to the fcc crystalline state at 130 °C and then to the hcp crystalline state at 310 °C. The thermal conductivity resulted to be constant in the amorphous phase, whereas it evolved between the two crystalline states. The thermal boundary resistance at the GST-SiO2 interface was estimated to be higher for the hcp phase than for the amorphous and fcc ones.

The phase-change memory (PCM) technology represents one of the most attractive concepts for next generation data storage. PCM operation is based on the particular properties of a chalcogenide alloy, the ternary compound Ge 2 Sb 2 Te 5 , which is able to perform fast and reversible transitions between a crystalline, high-conductive phase and an amorphous, low-conductive one, thus enabling the binary data storage. Although the ternary alloy Ge 2 Sb 2 Te 5 is the best recognised solution to meet the device reliability and performance specifications, other alloys are being studied within the GeSbT e ternary compound system in order to investigate and to enlarge the possible spectrum of PCM applications. This work focuses both on the program parameters and on the write performances of a Sb-rich GST composition, suggesting a change in the physical properties of the PCM material and a transition from nucleation to growth-dominated crystallization mechanism, both controlled by the material composition engineering. This enables new challenging performance parameters.

International audience; The thermal conductivity of Ge2Sb2Te5 (GST) layers, as well as the thermal boundary resistance at the interface between the GST and amorphous SiO2, was measured using a photothermal radiometry experiment. The two phase changes in the Ge2Sb2Te5 were retrieved, starting from the amorphous and sweeping to the face centered cubic (fcc) crystalline state at 130 °C and then to the hexagonal crystalline phase (hcp) at 310 °C. The thermal conductivity resulted to be constant in the amorphous phase, whereas it evolved between the two crystalline states. The thermal boundary resistance at the GST-SiO2 interface was estimated to be higher for the hcp phase than for the amorphous and fcc ones.

International audience; In this article we present the first results of a quite simple phenomenological model we have developed to simulate the cationic substitutions in Nd1+xBa2-xCu3O7+x/2-d (Nd123). Although elementary concepts from Statistical Mechanics had been used in it, significant results have been obtained, as the reconstruction of the substitution region limits and their dependence on temperature. Particularly interesting is the prediction of strong temperature dependence for the minimum of the substitution parameter x.

In order to validate phase change memory (PCM) technology, the programming reliability, in terms of reading window between the programmed and erased state, must be guaranteed at array level with an error less then 1 part-per-billion. The reset distribution is significantly influenced by the quenching operation [Mantegazza D, Ielmini D, Pirovano A, Gleixner B, Lacaita A L, Varesi E, et al. Electrical characterization of anomalous cells in phase change memory arrays. IEDM Tech Dig 2006:53–56]. In this paper this phenomenon is explained in terms of PCM active material crystallization statistics. A significant spread in the crystallization times among PCM cells is detected both in the write-reset operation from the melted-amorphous state (quenching) and in the erase-set operation from the solid-amorphous state. At statistical level, a correlation between set at high and low temperatures and quenching behavior of cells is found, allowing to describe the programming distributions uniquely in terms of crystallization times statistics.

A novel self-aligned muTrench-based cell architecture for phase change memory (PCM) process is presented. The low-programming current and the good dimensional control of the sub-lithographic features achieved with the muTrench structure are combined with a self-aligned patterning strategy that simplify the integration process in term of alignment tolerances and of number of critical masks. The proposed architecture has been integrated in a 90 nm 128 Mb vehicle with programming currents of 300 muA and good distributions, demonstrating its suitability for the production of high-density PCM arrays at 90 nm and beyond.

In this work programming and retention statistics in phase change memory arrays are discussed and modeled. In particular, we provide a) an analytical, empirical model for set, reset and retention statistics and b) a physics-based retention model based on crystallization in presence of composition fluctuations at the nm scale of the active material. The models allow predicting programming/retention distributions at array level.

Phase-change memory (PCM) cell is the most promising technology as post-flash nonvolatile memory (NVM). Among the different proposed cell structures, /spl mu/trench allows to simultaneously achieve low programming currents, small cell size, easy tunability and good dimensional control. Aim of this paper is to investigate the optimization and fine tuning capability of this cell architecture. Based on theoretical analysis and experimental results, the dependence of the PCM electrical parameters on the cell structure and on the geometric dimensions is assessed. An optimized /spl mu/trench cell with a programming current of 450 /spl mu/A at 1.5 V and a set resistance of 5 k/spl Omega/ is finally presented.

A novel cell structure for chalcogenide-based non-volatile Phase-Change Memories is presented. The new /spl mu/trench approach is fully compatible with an advanced CMOS technology, is highly manufacturable and allows to optimize array density and cell performance. Programming currents of 600 /spl mu/A, endurance of 10/sup 11/ programming cycles and data retention capabilities for 10 years at 110/spl deg/C have been demonstrated. The manufacturability is proven by experimental results from multi-megabit arrays.

We report theoretical and experimental studies on the use of bulk Bi or Y materials for fault current limiters. The materials are compared in terms of required volume, recuperation time, .... The problem of the unavoidable hot spots in high Jc materials is discussed. A moderate Jc and an operation near Tc make the quench effective and non-destructive for YBCO single domain bars or meanders. Experiments carried out above 90 K are reported. YBCO doping with MgO shows good opportunities to operate at 77 K for example.