5,701 research outputs found
Sn-Doping Enhanced Ultrahigh Mobility In<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Se Phototransistor
No full textTwo-dimensional ternary
materials are attracting widespread interest
because of the additional degree of freedom available to tailor the
material property for a specific application. An In1–xSnxSe phototransistor
possessing tunable ultrahigh mobility by Sn-doping engineering is
demonstrated in this study. A striking feature of In1–xSnxSe flakes is the reduction
in the oxide phase compared to undoped InSe, which is validated by
spectroscopic analyses. Moreover, first-principles density functional
calculations performed for the In1–xSnxSe crystal system reveal the same
effective mass when doped with Sn atoms. Hence, because of an increased
lifetime owing to the enhanced crystal quality, the carriers in In1–xSnxSe
have higher mobility than in InSe. The internally boosted electrical
properties of In1–xSnxSe exhibit ultrahigh mobility of 2560 ± 240
cm2 V–1 s–1 by suppressing
the interfacial traps with substrate modification and channel encapsulation.
As a phototransistor, the ultrathin In1–xSnxSe flakes are highly sensitive
with a detectivity of 1014 Jones. It possesses a large
photoresponsivity and photogain (Vg =
40 V) as high as 3 × 105 A W–1 and
0.5 × 106, respectively. The obtained results outperform
all previously reported performances of InSe-based devices. Thus,
the doping-engineered In1–xSnxSe-layered semiconductor finds a potential
application in optoelectronics and meets the demand for faster electronic
technology
Decagonal Sn clathrate on d-Al-Ni-Co
No full textDecagonal quasiperiodic ordering of Sn thin films on d-Al-Ni-Co is shown based on scanning tunneling
microscopy (STM), low-energy electron diffraction, and density functional theory (DFT). Interestingly, the
decagonal structural correlations are partially retained even up to a large film thickness of 10 nm grown at
165 ± 10 K. The nucleation centers called “Sn white flowers” identified by STM at submonolayer thickness are
recognized as valid patches of the decagonal clathrate structure with low adsorption energies of these motifs. Due
to the excellent lattice matching (to within 1%) between columns of Sn dodecahedra in the clathrate structure
and pentagonal motifs at the d-Al-Ni-Co surface, the interfacial energy favors clathrate over the competing Sn
crystalline forms. DFT study of the Sn/Al-Ni-Co composite model shows good mechanical stability, as shown
by the work of separation of Sn from Al-Ni-Co slab that is comparable to the clathrate self-separation energy.
The relaxed surface terminations of the R2T4 clathrate approximant are in self-similarity correspondence with
the motifs observed in the STM images from monolayer to the thickest Sn film.This article is published as Singh, Vipin Kumar, Eva Pospíšilová, Marek Mihalkovič, Marian Krajčí, Pramod Bhakuni, Shuvam Sarkar, Katariina Pussi et al. "Decagonal Sn clathrate on d-Al-Ni-Co." Physical Review B 107, no. 4 (2023): 045410.
DOI: 10.1103/PhysRevB.107.045410.
Copyright 2023 American Physical Society.
Posted with permission.
DOE Contract Number(s): AC02-07CH1135
Ordering of organic molecules on templated surfaces
No full textThis thesis describes the controlled growth of molecular nanostructures using modified metallic and semiconductor surfaces. The Ag/Si(lll)-(root3 x root3),the Sn/Cu(100) surface alloy system and the Bi/Si(100) nanolines and (2xn) surfaces were all investigated as suitable substrates for the controlled growth of pentacene, (C22H14) or trimesic acid, (C6H3(COOH)3) organic molecules. The following techniques were used in this study; Scanning Tunnelling Microscopy (STM), Low Energy Electron Diffraction (LEED), Normal Incident X-Ray Standing Waves (NIXSW) and Temperature Programmed Desorption (TPD).
The room temperature growth and ordering of trimesic acid on the AgfSi(ll1)-(root3 x root3) surface was investigated. An oblique unit cell was determined and a model proposed for the highly ordered close-packed domains.
The discovery of a new submonolayer phase on Sn/Cu(100) and the re-examined known phase are discussed. New models for these reconstructions are proposed. Adsorption of trimesic acid at room temperature on the clean substrate the lowest Sn coverage phase were studied. Two new Sn coverage dependent structures were discovered and bonding schemes in upright and flat orientations are discussed.
BifSi(100)-(2xn) surface was exploited as a template for the ordered growth of pentacene, which exhibited orientation specific adsorption. The Bi/Si(100)-(2xn) single domain surface created on vicinal silicon was used to test the suitable of Daresbury 4.2 beamline for NIXSW Imaging experiments and the quality of the results are discussed
Whisker Growth from Electrodeposited Sn Coatings - Developing Materials Science and Mechanics Based Insights
No full textPure Sn and Sn-alloys are widely used in electrical and microelectronic devices as protective layer to prevent oxidation of Cu conductors and also as a component of Pb-free, Sn-based solders. Sn coatings, typically 0.5-10 μm thick, deposited on substrates, e.g., Cu, brass, etc., are prone to spontaneous growth (i.e., without any external stimuli) of Sn whiskers under ambient conditions. The growth of whiskers from Sn plating has caused numerous failures in micro-electronic devices, mainly due to short-circuiting, leading to failure of components or devices. Whisker growth is, thus especially very critical in aviation, space and defines applications, where the electronic components are designed for longer life span. Furthermore, due to miniaturization of electronic devices, the spacing between adjacent conductors or interconnects can be as small as a few hundred nanometres to a few micrometres, making them more prone to whisker induced short-circuiting. Minor alloying of Sn with Pub was the principle way for mitigating the whisker growth in Sn plated components; however, due to the recent worldwide acceptance of European Union’s Restriction of Hazardous Substances (RoHS) act, enforcing Pub-free manufacturing, whisker growth has re-emerged as a reliability issue in Pub-free solders and the Sn plating finishes.
Even after decades of research, a universal whisker growth mechanism and hence effective mitigation technique is still not available in the public domain. This is mainly due to the fact that large number of factors that affect the whisker growth directly or indirectly, making it difficult to devise an experimental procedure, which allows studying effect of one factor at a time while keeping other factors constant. Although many mechanistic models for Sn whispering have been proposed in the past, the experimental evidences to support them are lacking. For example, recrystallization of whisker grain was proposed by various researchers; however, a direct observation confirming whisker grain is indeed a recrystallized grain has never been reported.
Nevertheless, it is well understood that whisker growth is a form of stress relaxation process and diffusion plays important role in the formation of whiskers. Since Sn is extremely anisotropic with tetragonal crystal structure, the stress state of Sn coatings, as well as the diffusion needed for mass transport of atoms, varies drastically depending upon the direction of interest. Therefore, it is important to study the role of crystallographic texture (both macroscopic and microscopic) on whisker propensity by systematically varying the crystallographic texture of Sn coating while keeping thickness, grain size, substrate material, and post-deposition storage conditions the same. Better understanding of role of macro- and micro- texture is very crucial before any whispering mechanism can be proposed. Furthermore, recent studies indicate that role of stresses in Sn coatings driving whisker growth is not fully understood. It is generally accepted that compressive stress in Sn coating is the main factor that drives the whisker growth. However, whiskers were also observed when Sn coating was under tensile stress, making the role of stress controversial. Again, the stresses in Sn have multiple origins and need a systematic approach to understand their origin, quantify them and then relate it to whisker growth. Such systematic approach was never adopted in previous works. Hence, the current thesis aims to address the role of macro- and micro- crystallographic texture, stress regeneration mechanism, nature (i.e., magnitude and sign) of stress and stress gradient in the Sn coatings via systematic variation of texture, post-deposition storage conditions and substrate composition, including deposition of an interlayer in between Sn coating and the brass or Cu substrate.
Whisker growth was studied from electro-deposited Sn coatings. The deposition parameters were optimized for producing different thickness and grain orientations. X-Ray diffraction (XRD) techniques were used to extract macro-texture of the coatings. The macro-texture measurement using XRD and micro-texture measurement using electron backscatter diffraction (EBSD) showed the same dominant and the second dominant orientations. It was observed that current density and deposition temperature, which are the two main electro-deposition parameters, significantly influence the crystallographic orientation of the grains. Thus, the global or macro-texture can be manipulated by changing the deposition parameters systematically. It was observed that whisker propensity increases drastically by growth of low index planes, such as (100) and (110), during deposition. Hence, proper selection of deposition parameters that lead to growth of high index planes can be used to suppress the whisker growth.
Furthermore, micro-texture surrounding whisker grain was studied using EBSD technique by observing the same set of grains surrounding a whisker grain before and after whispering. Orientation imaging microscopy (OIM) maps of several whisker regions clearly indicate that whiskers preferentially grow from low index planes, such as (100), etc. Furthermore, using orientation dependent stiffness mapping (in-plane and out-of-plane), it was noticed that whiskers preferentially grew from regions of soft oriented grains (low modulus) surrounded by hard orientations. In addition, grain boundary disorientation analysis revealed presence of high fraction of high angle grain boundaries (HAGBs) in the vicinity of whisker grain. It was observed that overall fraction of HAGBs in the whispering region was 0.7 while the fraction of HAGBs surrounding and leading to whisker grain was 0.85. In addition, it was observed that whisker grew from pre-existing grain and not from the recrystallized grain. Also, grain boundary sliding was not observed as a pre-requisite for whisker growth in Sn coatings on brass substrate.
The local stress field around the whisker grain also plays a crucial role in whisker growth. Therefore, local stress field around whisker site was simulated using crystal plasticity simulation by incorporating grid resolved spatial description of orientation in terms of Euler’s angles. The crystal plasticity model included slip systems of Sn and other material parameters, such as anisotropic elastic stiffness constants, critical resolved shear stresses for different slip systems, etc. Thus, the slip in individual grain was accounted following homogenization to maintain compatibility at grain boundaries. The simulated stress field shows that both in-plane and out-of-plane stresses were highly inhomogeneous without any unique condition around whisker grain. It has been observed that high compressive hydrostatic stresses develop in the vicinity of the whisker grain, while whisker grain is slightly tensile. Therefore, the gradient of hydrostatic stress around the whisker suggests whisker growth is mainly controlled by vacancy transport phenomenon.
The stress in Sn coatings may originate from many factors, such as residual stress inherent to electro-deposition, diffusion of substrate atoms (Cu, Zn, etc.) into the coating, formation of interfacial intermetallic compound (IMC) layer, segregation of impurities at Sn grain boundaries, formation of surface oxide layer, and coefficient of thermal expansion (CTE) mismatch between in Sn and substrate as well as between differently orientated grains of Sn. Therefore, it is important to understand the dominant stress regeneration mechanism responsible for whisker growth. To identify dominant mechanism, which can continuously regenerate the compressive stress in Sn, samples deposited under fixed electro-deposition conditions were exposed to different post-deposition storage conditions, such as isothermal aging at room temperature, 50 °C, 150 °C, and thermal cycling from -25 to 85 °C with and without hold time at the highest temperature. It has been observed that Cu6Sn5 IMC growth due to the inter-diffusion of Cu and Sn atoms is the dominant mechanism responsible for whisker growth. Both growth kinetics and morphology of IMC have a significant impact on whisker growth. The role of CTE mismatch in regenerating compressive stresses in Sn coatings on brass substrate for whisker growth is highly limited.
The substrate composition as well as the under layer metallization affects the inter-diffusion between Sn and the substrate atoms and therefore IMC growth, which is mainly responsible for whisker growth in Sn coatings on brass or Cu substrates. The effects of substrate composition on whisker growth was studied by using pure Cu, brass (65 wt. % Cu 35 wt. % Zn) and Ni (bulk and electro-deposited under layer) as substrate. Whisker growth was more rapid if brass substrate was used instead of pure Cu. Whiskers were not observed when Sn was deposited on either bulk Ni or when Ni under layer was electro-deposited on brass or Cu substrates prior to Sn deposition. Ni under layer effectively stops the diffusion of Cu into Sn, thus avoiding the growth of Cu6Sn5 (which places Sn coatings under compressive stress). Thus, it is clear that continuous formation of Cu6Sn5 at the interface provides the long-term driving force for whisker growth.
Since the whisker growth is a stress driven phenomenon, it is important to understand the stress evolution in Sn coatings. Stress state of the Sn coatings was studied using custom-built laser curvature set-up with multi-beam optical stress sensor (MOSS). This allowed monitoring of curvature change of the coating-substrate system in real time and the bulk average stress was calculated using Stoney’s equation. For multi-layer system such as Sn deposited on pre-deposited Ni under layer on brass substrate modified Stoney’s equation was used. In case of Sn deposited on brass without any under layer, it is known that the Cu6Sn5 IMC do not form a continuous layer at the interface between Sn and substrate under aging at ambient conditions, therefore, the curvature change due to IMC can be neglected. In addition, glancing angle X-ray diffraction was employed to analyse stress in the top surface region of the coating. The variation of glancing angle allowed probing strain at different penetration depths. Both the bulk stress and the stress in only near surface region evolve with time. The residual bulk stresses in Sn coatings are tensile immediately after deposition. The residual stresses relax very quickly upon room temperature aging and become compressive. The bulk of Sn coatings on brass substrate progressively become more compressive upon continued aging. However, stresses in Sn coatings deposited on brass substrate with Ni under layer saturate quickly at low compressive stress. Surprisingly, stress in the top-most region of Sn coating measured using XRD evolve differently. The surface of Sn coating deposited on brass substrate is compressive
initially and progressively become more tensile (less compressive), while the initial compressive stress in the sample with Ni under layer saturated at a higher compressive stress than the bulk stress value recorded from curvature measurement. Therefore, the surface of the Sn coatings with Ni under layer is always more compressive than the bulk stress in the Sn coating. Therefore, a negative stress gradient for the diffusion of Sn atoms towards surface is never established and whiskers do not grow in these Sn coatings. Interestingly, through thickness voids are observed in the Sn coatings on Ni. Contrarily, in Sn coatings without Ni under layer after 170 h of aging, the surface stress becomes more tensile than the bulk of the Sn coating, favouring continuous migration of atoms from the highly compressed region near Cu6Sn5 IMC layer to the stress-free whisker root. Aforementioned observation indicates the crucial role of negative stress gradient in the mass transport of atoms required for whispering.
The importance of stress and stress gradient was further studied by analysing the effect of externally imposing stress and stress gradient on whisker growth. The stresses were applied using a three-point bend setup. It has been observed that externally applied stress accelerates the whisker growth. This is mainly because applied stress alters the diffusion kinetics and growth of Cu6Sn5 IMC at the interface. However, the coating under tensile stress shows more whisker growth as compared to the coating under high compressive stress. This is attributed to the fact the coating under tensile stress is under higher negative stress gradient. Therefore, it is proposed that out-of-plane stress gradient is more important rather than the sign and the magnitude of stress in determining the propensity of whisker growth in Sn coatings
A trio of gamma-ray burst supernovae : GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu
No full textWe present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu. For GRB 130215A/SN 2013ez, we also present optical spectroscopy at t − t0 = 16.1 d, which covers rest-frame 3000–6250 Å. Based on Fe ii λ5169 and Si ii λ6355, our spectrum indicates an unusually low expansion velocity of ~4000–6350 km s-1, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A/SN 2013fu, we used our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we took advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Mészáros (2001, ApJ, 552, L35), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P = 12 ms and a magnetic field of B = 1.1 × 1015 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs
A Study of Intermetallics in Cu-Sn system and Development of Sn-Zn Based Lead Free Solders
No full textIn the electronic industry Pb-Sn solder is a very important material but Pb is toxic and has adverse effects on the environment and human beings. Due to the harmful effects of the Pb the use of the Pb-Sn solder alloys are being avoided and new Pb-free solder alloys are being used for electronic applications. This study is mainly based on the intermetallics that are formed in the Cu-Sn system and development of Sn-Zn based lead free solders. The aim was to understand the solidification of Cu-Sn alloys, the various intermetallics formed and their morphology. Thermal analysis of Sn-Zn based lead free alloys and their wetting characteristics has been analysed. The Pb-Sn solders are being replaced by the Sn-Cu, Sn-Ag, Sn-Ag-Cu, Sn-Zn and Sn-Zn-Bi alloys. Here the intermetallics formed between Cu-Sn during solidification of the molten solder on the Cu electrical contacts are also studied. The intermetallics are brittle in nature and this leads to fracture of the solder. In order to understand the Sn-Cu or Sn-Zn based lead free solder alloys it is essential to is understand the solidification of the molten alloys. This is why a few compositions of Cu-Sn alloy are selected and the various intermetallics formed during the solidification of the molten Cu-Sn alloys are analyzed. At high temperatures diffusion of Cu and Sn increases and as a result intermetallics of Cu-Sn are formed. The common intermetallics formed are Cu3Sn and Cu6Sn5. Sn-8.8Zn and Sn-8Zn-3Bi solder alloys has been developed. DSC analysis of these alloys has carried out to determine their melting points. It has been also observed that the addition of Bi increases the wettability and decreases the melting point of these alloys
Effect of Ag on Sn-Cu Lead Free Solders
No full textLead free solders are expected to replace the traditional Sn-Pb alloys due to environmental concern. The Sn-Cu lead free solder alloys are found to be a potential alternative to the Sn-Pb alloys compared to other solders. Eutectic Sn-0.7Cu (wt.%) solder has been used for interconnecting and packaging electronic component due to the good wettability between the Sn-Cu solder and the Cu substrates. Three compositions Sn-0.7Cu, Sn-1Cu and Sn-2Cu and three compositions containing Ag, Sn-2Ag-0.7Cu, Sn-2.5Ag-0.7Cu and Sn-4.5Ag-0.7Cu were considered here for the study. Ag was added to the eutectic Sn-0.7Cu composition in order to decrease the melting temperature of the eutectic alloy and to enhance the mechanical properties of the alloy such as hardness. But the amount of Ag was more increases hardness was decreases. The wettability of the Sn-Cu solder on the Cu substrate was also enhanced by the addition of Ag. Structure and morphology of the solder alloys were analyzed using a SEM, XRD and EDX. The microstructural observation reveals the formation of â-Sn matrix and presence of intermetallic phases like Cu6Sn5 and Ag3Sn. Furnace cooling was employed for solidifying the lead free solder alloys. Thermal analysis of the solder alloys were done with the help of a differential scanning calorimeter (DSC). Trace additions of Ag have been found to significantly reduce the melting temperature of these alloys
Evolution of the Type IIb SN 2011fu
No full textThe UBVRI photometric follow-up of SN 2011fu has been initiated a few days after the explosion, shows a rise followed by steep decay in all bands and shares properties very similar to that seen in case of SN 1993J, with a possible detection of the adiabatic cooling phase at very early epochs. The spectral modeling performed with SYNOW suggests that the early-phase line velocities for H and Fe ii features were ~ 16000 km s−1 and ~ 14000 km s−1, respectively. Studies of rare class of type IIb SNe are important to understand the evolution of the possible progenitors of core-collapse SNe in more details
tardis-sn/tardis: TARDIS v2023.11.05
No full text<p>This release has been created automatically by the TARDIS continuous delivery pipeline.</p>
<p>A complete list of changes for this release is available at <a href="https://github.com/tardis-sn/tardis/blob/master/CHANGELOG.md">CHANGELOG.md</a>.</p>
tardis-sn/tardis: TARDIS v2023.10.20
No full text<p>This release has been created automatically by the TARDIS continuous delivery pipeline.</p>
<p>A complete list of changes for this release is available at <a href="https://github.com/tardis-sn/tardis/blob/master/CHANGELOG.md">CHANGELOG.md</a>.</p>
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