Bimetallic nanoparticles (NPs), particularly Au/Pd and Au/Pt, have attracted extensive attention due to their wide-spread application in catalysis, optoelectronics and energy recuperation.[1] Here we have attempted the fabrication of Au/Pt and Au/Pd bimetallic NPs by an energy-efficient eco-friendly microwave methodology. The microwave-assisted reactions enable considerably large product yields over conventional colloidal methods due to (a) almost two-fold increased reaction kinetics, (b) localized superheating at reaction sites and rapid rise of initial temperature.[2] Au NPs (sizes 20 ± 3 nm) are fabricated in the first step followed by the reduction of [PdCl2(NH3)2] or [K2PtCl6]in tetraethylene glycol at 180 ºC for 2 min. Controlling and understanding the atomic structure and elemental distributions of these NPs are crucial for their optimized performances. So, we address the fundamental question of the most likely arrangement of Au and Pd or Pt atoms in these bimetallic NPs prepared under similar conditions by complementary characterizations using UV-Vis spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The UV-Vis spectroscopy reveals the formation of an alloy shell. The extent of depression of the plasmon peak of Au and its blue-shift reveals substantial deposition of Pd atoms on an Au core and significant alloying in comparison to Au/Pt NPs. XRD reveals the gradual shift of the diffraction peak from the position of Au to the position of Pd or Pt with change in composition. XRD supports the formation of a thick alloy shell in these NPs. However, the TEM images reveal a very interesting result. With increase in Pt concentration, the size of the dispersed NPs decreases from 20 ± 3 nm to about 16 nm (± 1 nm) and there is evolution of a bimodal particle size distribution with small particles about 1-2 nm diameters. On the contrary, with increasing Pd concentration, the particle size of the dispersed particles increases to about 32 nm (± 1 nm). This discrepancy of particle size evolution for the two systems arises due to the differences in surface energies (Pt > Pd > Au atoms). Pt atoms tend to diffuse towards the core with the formation of Au nano-islands which eventually segregates leading to a reduction in particle size and bimodal distribution. At higher concentration of Pt, Pt and Au atoms tend to nucleate separately also contribute to the bimodal distribution. While for Au/Pd NPs, we have an Au core with an alloyed shell having higher Pd concentration. This is further supported by experimental evidence by selective etching and dissolution of Au by potassium-iodide solution. Furthermore, the Au/Pd bimetallic NPs are found to possess better catalytic activities in the reduction of 4-nitrophenol to 4-aminophenol than Au/Pt and monometallic NPs.

Phase change memory (PCM) is an emerging technology that combines the unique properties of phase change materials with the potential for novel memory devices, which can help lead to new computer architectures. Phase change materials store information in their amorphous and crystalline phases, which can be reversibly switched by the application of an external voltage. This article describes the advantages and challenges of PCM. The physical properties of phase change materials that enable data storage are described, and our current knowledge of the phase change processes is summarized. Various designs of PCM devices with their respective advantages and integration challenges are presented. The scaling limits of PCM are addressed, and its performance is compared to competing existing and emerging memory technologies. Finally, potential new applications of phase change devices such as neuromorphic computing and phase change logic are outlined.

Phase change materials can be switched rapidly and repeatedly between amorphous and crystalline phases, which differ distinctly in their optical and electrical properties. This combination of properties is utilized to store information in rewritable optical storage media and in emerging phase change memory technology. This article describes the physical properties of phase change materials such as Ge2Sb2Te5 and relates these properties to specific structural and bonding characteristics. Electrical conduction and switching, which are relevant for phase change memory operation, are explained from a physical perspective. Phase change memory device integration and technology development are discussed, including aspects of access device selection and integration.

Ion implantation of germanium and carbon ions into thin films of Ge2Sb2Te5 (GST) and GeTe was applied to modify the properties of these phase change materials. It was found that it is possible to amorphize crystalline GST and GeTe using ion implantation for optimized ion doses and energies which depend on the film thickness, ion species and capping layer. A relatively low minimum dose is required for complete amorphization as judged by the absence of diffraction peaks in x-ray diffraction (XRD) scans. It is 4–5×1013 cm−2 for germanium implantation into GST, and slightly higher (1014 cm−2) for germanium implantation into GeTe. The properties of the re-amorphized films depend on ion species, dose and energy. The re-crystallization temperature of re-amorphized GST by ion implantation is comparable or higher than as-deposited amorphous GST. Carbon implantation in particular leads to a large increase in the crystallization temperature Tx. A carbon dose of 1016 cm−2 implanted into 20 nm amorphous GST yielded a crystallization temperature of 300 ºC, much higher than the crystallization temperature of 160 ºC we recorded for as-deposited, amorphous GST. Similarly, high dose carbon implantation into amorphous GeTe leads to large increase in Tx. We recorded a shift in Tx from 195 ºC for as-deposited GeTe to 400 ºC for C-implanted GeTe. Crystalline GeTe re-amorphized by a low dose germanium ion implantation exhibits a re-crystallization temperature below Tx of as-deposited amorphous GeTe and Tx increased with the implanted Ge dose to a crystallization temperature above that of unimplanted GeTe. Ion implantation can be regarded an additional tool to create phase change materials with different and improved switching properties that cannot be achieved by conventional sputter deposition.

Ion implantation of Ge2Sb2Te5 (GST) enables localized doping of the film by using conventional lithography. Although the doped region dimensions and the doping concentration profile are defined by the opening in the mask and the ion energy, longitudinal and lateral straggling of implanted ions leads to a spread in the ions final location. Additionally, a thermal treatment such as one that induces a phase transition may lead to redistribution of the implanted dopants and further increase the spread. In this work we demonstrate doping of GST by ion implantation. Using Secondary Ion Mass Spectrometry (SIMS) we studied the as-implanted doping profiles obtain by ion implantation of carbon and silicon into GST. We also investigated by SIMS the dopant redistribution following a recrystallization annealing. The as-implanted ion profiles were found to be in fair agreement with TRIM simulation. The dopants profiles show little change after a crystallization annealing at 200°C for silicon doping and at 350°C for carbon doping.

Germanium ion implantation at an energy of 30 keV was used as a different method to re-amorphize thin films of crystalline phase change material Ge2Sb2Te5 (GST). It was found that rather low doses of 5×1013 cm-2 were sufficient to re-amorphize GST. Amorphization was determined by X-ray diffraction (XRD) as well as reflectivity measurements. Re-crystallization characteristics of ion-implantation-amorphized samples was studied using time-resolved XRD. It showed that samples re-crystallize at an increased crystallization temperature with increasing dose compared to as-deposited material. A static laser tester was applied to measure the crystallization times of material that was (1) as–deposited amorphous; (2) crystallized by annealing and re-amorphized by melt-quenching using a laser pulse; and (3) crystallized by annealing and re-amorphized by ion implantation. It was found that as-deposited amorphous and high-dose ion implanted samples had longer crystallization times while melt-quenched amorphous and low-dose ion implanted samples had shorter crystallization times.

Recent interest in phase change materials (PCMs) for non-volatile memory applications has been fueled by the promise of scalability beyond the limit of conventional DRAM and NAND flash memory [1]. However, for such solid state device applications, Ge2Sb2Te5 (GST), GeSb, and other chalcogenide PCMs require doping. Doping favorably modifies crystallization speed, crystallization temperature, and thermal stability but the chemical role of the dopant is not yet fully understood. In this work, X-ray Absorption Fine Spectroscopy (XAFS) is used to examine the chemical and structural role of nitrogen doping (N-) in as-deposited and crystalline GST thin films. The study focuses on the chemical and local bonding environment around each of the elements in the sample, in pre and post-anneal states, and at various doping concentrations. We conclude that the nitrogen dopant forms stable Ge-N bonds as deposited, which is distinct from GST bonds, and remain at the grain boundary of the crystallites such that the annealed film is comprised of crystallites with a dopant rich grain boundary.

The crystallization behavior of Ge-Sb phase change materials with variable Ge:Sb ratio X between 0.079 and 4.3 was studied using time-resolved x-ray diffraction, differential scanning calorimetry, x-ray reflectivity, optical reflectivity and resistivity vs. temperature measurements. It was found that the crystallization temperature increases with Ge content from about 130 °C for X = 0.079 to about 450 °C for X = 4.3. For low X, Sb x-ray diffraction peaks occurred during a heating ramp at lower temperature than Ge diffraction peaks. For X = 1.44 and higher, Sb and Ge peaks occurred at the same temperature. Mass density change upon crystallization and optical and electrical contrast between the phases show a maximum for the eutectic alloy with X = 0.17. The large change in materials properties with composition allows tailoring of the crystallization properties for specific application requirements.

Phase-change random-access memory (PCRAM) is a promising technology for future nonvolatile storage with the added potential for possible impact on dynamic random-access memory technologies. To be successful, however, PCRAM must be able to scale to dimensions on the order of a few tens of nanometers, considering the increasingly tiny memory cells that are projected for future technology nodes. The experiments discussed in this article directly address these scaling properties, examining both the materials themselves and the operation of nanoscale devices. One series of experiments is time-resolved x-ray diffraction studies of ultrathin films and nanostructures. Electron-beam lithography was applied to pattern thin films into nanostructures with dimensions down to 20 nm. The article also includes descriptions of prototype PCRAM devices, successfully fabricated and tested down to phase-change material cross sections of 3 nm × 20 nm. The measurements provide a clear demonstration of the excellent scaling potential offered by this technology.

We have investigated the crystallization behavior of phase change materials as a function of their thickness. Thin films of variable thickness between 1 and 50nm of the phase change materials Ge2Sb2Te5 (GST), N-doped GST (N-GST), Ge15Sb85 (GeSb), Sb2Te, and Ag and In doped Sb2Te (AIST) were deposited by magnetron sputtering, and capped in situ by a 10nm thick Al2O3 film to prevent oxidation. The crystallization behavior of the films was studied using time-resolved X-ray diffraction. For each material we observed a constant crystallization temperature Tx that was comparable to bulk values for films thicker than 10 nm, and an increased Tx when the film thickness was reduced below 10 nm. The thinnest films that showed XRD peaks were 2 nm for GST and N-GST, 1.5 nm for Sb2Te and AgIn-Sb2Te, and 1.3 nm for GeSb. The observed increase in the phase transition temperature with reduced film thickness and the fact that very thin films still show clear phase change properties are indications that Phase Change Random Access Memory technology can be scaled down to several future technology nodes.

Phase change nanodot arrays were fabricated using self-assembly diblock copolymer template PS-b-PMMA (polystyrene-poly (methyl-methacrylate)) and studied by time resolved X-ray diffraction. The size of the nanodots was less than 15nm in diameter with 40nm spacing. This method is quite flexible regarding the patterned materials, and can be used on different substrates. The crystallization behavior of small scale phase change nanodot arrays was studied for different materials, such as Ge15Sb85, Ge2Sb2Te5 and Ag and In doped Sb2Te. It was found that the nanodots had higher crystallization temperatures compared to their corresponding blanket films and crystallized over a broader temperature range.

Scaling studies have demonstrated that Phase Change Random Access Memory (PCRAM) is one of the most promising candidates for future non-volatile memory applications. The search for suitable phase change materials with optimized properties is therefore actively pursuit. In this paper, SET (crystallization) characteristics of an ultra fast switching material Ge15Sb85 in phase change memory bridge cell devices are presented. It was found that reproducible switching between two stable states with one decade resistance contrast and current pulses as short as 10 ns for SET and RESET (re-amorphization) operation is possible. Particular emphasis was placed on the difference in crystallization kinetics between the as-deposited and melt-quenched amorphous phase. Evidence is given for the existence of an electrical field as the critical parameter for threshold switching rather than a threshold voltage. For Ge15Sb85 a threshold switching field of 9MV/m was measured and it was shown that switching from the melt-quenched amorphous phase to the crystalline phase is about 600 times faster than crystallization from the as-deposited amorphous phase.

Ge2Sb2Te5 is under intense investigation for phase-change memory devices, including rewriteable DVDs where optical illumination is used to switch between the glassy and crystalline states. We investigate the influence of optical irradiation on amorphous phase. Many chalcogenides display photo-oxidation, photodarkening or photo-bleaching, but little has been reported on the Ge-Sb-Te system. Using spectroscopic ellipsometry (SE) and secondary ion mass spectrometry, we determine that the samples have a strong tendency to photo-oxidize; if this is not accounted for, then the analysis of SE data appears to show photodarkening. Other authors recently reported photodarkening in nonstoichiometric GexSb20-xTe80 [Pamukchieva et al., Proc. SPIE 5581, 608 (2004); Pamukchieva et al., J. Optoelectron. Adv. Mater 7, 1277 (2005)], but our analysis suggests that the changes were also the result of photo-oxidation. The oxide has lower value of (n, k) than Ge2Sb2Te5, and can be etched by hydrofluoric acid or water. The photo-oxidation is presumably the result of free carrier generation in the Ge2Sb2Te5. Our observation of negligible photodarkening is consistent with previous works that found less photodarkening in tellurides compared with selenides or sulfides, and that an increase in the mean coordination number, here by Ge addition, further reduces the photodarkening effect.