1,720,982 research outputs found
Observation of the Initial Stage of 3C-SiC Heteroepitaxial Growth on the Si Nanomembrane
No full textOne considerable concern in 3C-SiC growth is the different lattice constant between the 3C-SiC and Si substrate, which causes defects and strain at the interface. Although the heteroepitaxial growth has been achieved, there have been no experimental studies on the initial process of 3C-SiC growth. In this research, we directly observe heteroepitaxial growth of 3C-SiC on the (001) Si nanomembrane (Si NM) step by step. We used in situ heating transmission electron microscopy (TEM) to study the initial growth process of 3C-SiC growth at the nanoscale in a highvacuum environment. We demonstrate the growth of 3C-SiC at the preferential (110) direction without defects. The heteroepitaxial grown 3C-SiC without defects is attributed to the bowing effect at the nanoscale to compensate for the lattice misfit. Based on these results, we proposed a new method to heteroepitaxially grow on the Si NM through in situ heating TEM study
Probing the Melting Transitions in Phase-Change Superlattices via Thin Film Nanocalorimetry
No full textPhase-change superlattices with nanometer thin sublayers are promising for low-power phase-change memory (PCM) on rigid and flexible platforms. However, the thermodynamics of the phase transition in such nanoscale superlattices remain unexplored, especially at ultrafast scanning rates, which is crucial for our fundamental understanding of superlattice-based PCM. Here, we probe the phase transition of Sb2Te3 (ST)/Ge2Sb2Te5 (GST) superlattices using nanocalorimetry with a monolayer sensitivity (∼1 Å) and a fast scanning rate (105 K/s). For a 2/1.8 nm/nm Sb2Te3/GST superlattice, we observe an endothermic melting transition with an ∼240 °C decrease in temperature and an ∼8-fold decrease in enthalpy compared to those for the melting of GST, providing key thermodynamic insights into the low-power switching of superlattice-based PCM. Nanocalorimetry measurements for Sb2Te3 alone demonstrate an intrinsic premelting similar to the unique phase transition of superlattices, thus revealing a critical role of the Sb2Te3 sublayer within our superlattices. These results advance our understanding of superlattices for energy-efficient data storage and computing. © 2023 American Chemical Society.11Nsciescopu
Understanding Interface-Controlled Resistance Drift in Superlattice Phase Change Memory
No full textResistance drift in phase change memory (PCM) reduces the accuracy of analog computing applications such as neural network inference. Recently, PCMs based on superlattice (SL) phase change layers have shown low resistance drift, however the origin of this low drift remains unexplored. Here, we uncover that resistance drift in SL-PCM based on alternating layers of Sb2Te3 and Ge2Sb2Te5 (GST) is controlled by the number of SL interfaces as well as the degree of SL intermixing. Temperature-dependent measurements reveal smaller and more stable activation energy upon annealing (thus suppressed structural relaxation) in our SL-PCM vs. control GST devices, accounting for the low resistance drift. By controlling SL interfaces, we achieve low resistance drift coefficient nu < 0.01 in these SL-PCMs, maintained after extensive cycling and at various read voltages and intervals - showing robustness required for analog computing with PCM
Dysprosium Incorporation for Phase Stabilization of Atomic-Layer-Deposited HfO2 Thin Films
No full textThe relatively low thermal stability of HfO2 films severely affects the performance of semiconductor devices. For instance, the low crystallization temperature of HfO2 (∼500 °C) leads to the formation of grain boundaries, which increases the leakage current. In this study, Dy incorporation leads to the phase transformation of HfO2 films from various directional planes to a main m(−111) plane by the crystallographic stabilization of HfO2 films, increasing the size of grains. Dy-doped HfO2 thin films with modulated doping content, prepared by plasma-enhanced atomic layer deposition (PE-ALD), are characterized by analysis of their chemical composition combined with electron microscopy and synchrotron X-ray techniques. The transformation from m(110), m(−111), m(111), m(020), and m(120) to a main m(−111) plane is observed through X-ray diffraction, which indicates that Dy plays a role for the phase stabilization of HfO2 films. The atomic-scale images of the cross section and top view obtained using an electron microscope demonstrate that the in-plane average grain size is increased by approximately 4 times due to Dy incorporation compared with that of single HfO2 films. The reduction in the area of the grain boundary of HfO2 due to Dy incorporation decreases the leakage current density of HfO2 by 1000 times and increased the breakdown strength. This result can aid future electronics by determining the effect of a dopant on the crystallographic structure of host thin-film materials. © 2023 American Chemical Society.11Nsciescopu
Unveiling the Effect of Superlattice Interfaces and Intermixing on Phase Change Memory Performance
No full textSuperlattice (SL) phase change materials have shown promise to reduce the switching current and resistance drift of phase change memory (PCM). However, the effects of internal SL interfaces and intermixing on PCM performance remain unexplored, although these are essential to understand and ensure reliable memory operation. Here, using nanometer-thin layers of Ge2Sb2Te5 and Sb2Te3 in SL-PCM, we uncover that both switching current density (J(reset)) and resistance drift coefficient (v) decrease as the SL period thickness is reduced (i.e., higher interface density); however, interface intermixing within the SL increases both. The signatures of distinct versus intermixed interfaces also show up in transmission electron microscopy, X-ray diffraction, and thermal conductivity measurements of our SL films. Combining the lessons learned, we simultaneously achieve low J(reset) & AP; 3-4 MA/ cm(2) and ultralow v & AP; 0.002 in mushroom-cell SL-PCM with similar to 110 nm bottom contact diameter, thus advancing SL-PCM for and neuromorphic applications
In situ tensile and fracture behavior of monolithic ultra-thin amorphous carbon in TEM
No full text© 2022 Elsevier LtdEven while being important components in day-to-day life and in advanced technology, the wider application of amorphous solids is limited by their brittle behavior. Although amorphous solids have been reported to show plasticity at the nanoscale, studies have so far been limited to metallic and oxide glasses. Here, we report on the tensile and fracture behavior of monolithic ultra-thin amorphous carbon (a-C) films during in situ nanomechanical testing inside a transmission electron microscope (TEM). Our results show that ultra-thin a-C films exhibit large plastic strain under uniaxial tension while retaining high strength. Beam-off tests confirm that the plasticity is not induced by electron-beam effects during testing. Consecutive cyclic tests and Raman spectra reveal that the plasticity results from an increased nanoporosity, and graphitic cluster size increases and bond/cluster alignments along the tensile direction occur and likely contributes to stiffening of the a-C film. Despite the large plastic strain, catastrophic failure still occurred accompanied by the formation of multiple shear bands, which has never been reported for amorphous carbon. This study serves as a basis for our better understanding of the mechanical behavior of amorphous solids such as ultra-thin a-C, and provides new opportunities in design of flexible electronics, mechanical nanocomponents, and nanocomposites.11Nsciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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