Electronic Materials Processing Research Articles

Modeling of ultrafast X-ray induced magnetization dynamics in magnetic multilayer systems

In this work, we report on modeling results obtained with our recently developed simulation tool enabling nanoscopic description of electronic processes in X-ray irradiated ferromagnetic materials. With this tool, we have studied the response of Co/Pt multilayer system irradiated by an ultrafast extreme ultraviolet pulse at the M-edge of Co (photon energy ~60 eV). It was previously investigated experimentally at the FERMI free-electron-laser facility, using the magnetic small-angle X-ray scattering technique. Our simulations show that the magnetic scattering signal from cobalt decreases on femtosecond timescales due to electronic excitation, relaxation, and transport processes both in the cobalt and in the platinum layers, following the trend observed in the experimental data. The confirmation of the predominant role of electronic processes for X-ray induced demagnetization in the regime below the structural damage threshold is a step toward quantitative control and manipulation of X-ray induced magnetic processes on femtosecond timescales.

Foreword Special Issue on Low-Temperature Processing of Electronic Materials for Cutting Edge Devices

Thermal budgets hinder leading-edge ultra-scaled and novel designs, and moving to reduced temperature processing is becoming an acute impediment globally.

Physics applied to electrochemistry: Tunneling reactions

In this work, we explore how electrochemical tunneling reactions can be understood within the single-particle picture. That is, the formal approach in which band diagrams are typically utilized to understand electronic processes in solid-state materials and devices. This single-particle perspective is based upon the Gerischer–Hopfield description of electron transfer at solid–liquid interfaces. Both single and multiple electron tunneling reactions are discussed, as are related voltammetric concepts and trends. The impact of nuclear quantization on the Gerischer–Hopfield description is also addressed, as well as its compact representation of two probe electrochemical phenomena at low temperatures (often referred to as Franck–Condon blockade). In this manner, a perspective linking solid-state phenomena and tunneling electrochemical reactions is presented.

Correlation-driven machine learning for accelerated reliability assessment of solder joints in electronics

The quantity and variety of parameters involved in the failure evolutions in solder joints under a thermo-mechanical process directs the reliability assessment of electronic devices to be frustratingly slow and expensive. To tackle this challenge, we develop a novel machine learning framework for reliability assessment of solder joints in electronic systems; we propose a correlation-driven neural network model that predicts the useful lifetime based on the materials properties, device configuration, and thermal cycling variations. The results indicate a high accuracy of the prediction model in the shortest possible time. A case study will evaluate the role of solder material and the joint thickness on the reliability of electronic devices; we will illustrate that the thermal cycling variations strongly determine the type of damage evolution, i.e., the creep or fatigue, during the operation. We will also demonstrate how an optimal selection of the solder thickness balances the damage types and considerably improves the useful lifetime. The established framework will set the stage for further exploration of electronic materials processing and offer a potential roadmap for new developments of such materials.

Low temperature processing of electronic materials for cuttung edge devices

Low temperature processing of electronic materials for cuttung edge devices

Improving Quality of Material by Reducing Its Interaction with Fused Silica Container During Processing

AbstractDuring the processing of electronic and optical materials at elevated temperature, the sample has the tendency to interact with the fused silica container and form chemical bonding, the so‐called “wetting”, which makes the surface section of the sample attaching to the ampoule wall. When the sample is cooled down to room temperature after the processing, as the bulk of the sample goes through thermal contraction, the wetting area remains attached to the fused silica wall and, consequently, causes the sample to separate apart which causes cracks and other structural defects. These defects are detrimental to the electronic/optic performance of materials, especially on the applications of high‐quality compound semiconductors. It is demonstrated, in this paper, that by adding a simple procedure of hydrofluoride acid etching of empty ampoule, the wetting of the sample can be reduced during the high temperature processing of crystal growth and, as a result, the structural quality of the semiconductors can be significantly improved.

Nonadiabatic dynamics with quantum nuclei: simulating charge transfer with ring polymer surface hopping.

Investigation of many electronic processes in molecules and materials, such as charge and exciton transport, requires a computational framework that incorporates both non-adiabatic electronic effects and nuclear quantum effects, in particular at low temperatures. We have recently developed an efficient semi-empirical fewest switches surface hopping method, denoted fragment orbital-based surface hopping (FOB-SH), that was tailored towards highly efficient simulation of charge transport in molecular materials, yet with nuclei treated classically. In this work, we extend FOB-SH and include nuclear quantum effects by combining it with ring-polymer molecular dynamics (RPMD) in three different flavours: (i) RPSH with bead approximation (RPSH-BA) as suggested in Shushkov et al., J. Chem. Phys., 2012, 137, 22A549, (ii) a modification of (i) denoted RPSH with weighted bead approximation (RPSH-wBA) and (iii) the isomorphic Hamiltonian method of Tao et al., J. Chem. Phys., 2018, 148, 10237 (SH-RP-iso). We present here applications to hole transfer in a molecular dimer model and analyze detailed balance and internal consistency of all three methods and investigate the temperature and driving force dependence of the hole transfer rate. We find that RPSH-BA strongly underestimates and RPSH-wBA overestimates the exact excited state population, while SH-RP-iso gives satisfactory results. We also find that the latter predicts a flattening of the rate vs. driving force dependence in the Marcus inverted regime at low temperature, as often observed experimentally. Overall, our results suggest that FOB-SH combined with SH-RP-iso is a promising method for including zero point motion and tunneling in charge transport simulations in molecular materials and biological systems.

The Electronic Packaging Interconnect Technology Symposium (EPITS 2019)

24-25 November 2019PrefaceEPITS 2019 provides an excellent opportunity for intellectual discourse on demonstration of advances and research results in the fields of electronic packaging interconnect technology and green materials. This conference brings together all academics, researchers, engineers and scientists around the world contributing to the global debate on these fields. Topics of interest or submission include, but are not limited to;• Challenges in improving the reliability of Tin (Sn) base Pb-free solder alloys.• Challenges in implementing the Restriction of Hazardous Substances (RoHS).• Emerging interconnect and green materials technologies.• Non-solder interconnect materials at chip and package levels.• Fundamental materials behavior for electronic packaging materials and green materials.• Electromigration, thermomigration, stress-migration and mechanical effects on materials developments.• Advanced characterization methods as applied to electronic packaging technology and materials processing and technology• Whisker growth in tin, tin-based alloys and other metallic systems related to electronic packaging materials.• Developments in high temperature Pb-free solders and associated interconnects for automotive and power electronics

Recent Progress in Photonic Processing of Metal‐Oxide Transistors

AbstractOver the past few decades, significant progress has been made in the field of photonic processing of electronic materials using a variety of light sources. Several of these technologies have now been exploited in conjunction with emerging electronic materials as alternatives to conventional high‐temperature thermal annealing, offering rapid manufacturing times and compatibility with temperature‐sensitive substrate materials among other potential advantages. Herein, recent advances in photonic processing paradigms of metal‐oxide thin‐film transistors (TFTs) are presented with particular emphasis on the use of various light source technologies for the photochemical and thermochemical conversion of precursor materials or postdeposition treatment of metal oxides and their application in thin‐film electronics. The pros and cons of the different technologies are discussed in light of recent developments and prospective research in the field of modern large‐area electronics is highlighted.

Usability of Cr-Doped Alumina in Dosimetry

Dosimetry is a widespread material science field dealing with detection and quantification of ionizing radiation using electronic processes in materials. One of the main aspects that determines the performance of dosimeters is the type of defects the material contains. Crystalline lattice imperfections are formed around impurity ions, which may have a smaller or larger size, or different oxidation states compared to host ions. In this study, we show what effects Cr impurities have on the luminescent properties of alumina. Porous Al 2 O 3 : Cr microceramics synthesized using the sol-gel method showed a higher thermoluminescence response than a single crystal ruby. We have found that Cr 2 O 3 concentration of 0.2 wt% was optimal; it yielded the highest X-ray luminescence and thermostimulated luminescence readout of all studied additive concentrations added to alumina during synthesis. Our results show that Cr doped alumina could potentially be used as a promising new material for dosimetry of ionizing radiation.

(Invited) Structural Origins of Electronic Properties and Transport States in Halide Perovskites

The Asbury group has developed ultrafast infrared spectroscopy techniques to examine electronic processes in materials for photocatalysis and energy harvesting with particular focus on the influence that underlying molecular structures and structural fluctuations have on those processes. This talk will focus on the application of ultrafast infrared spectroscopy to examine the coupling of temperature-dependent lattice fluctuations on electronic transport states in perovskite photovoltaic and photocatalytic materials. The transition of photogenerated charges from free carrier states to large polarons to trapped states is directly observed through the corresponding mid-infrared transitions. Temperature dependent amplitudes of lattice fluctuations are examined through dephasing of vibrational modes and correlated with the nature of the transport states. Ligands attached to surface states provide unique insights into the nature of charge traps and help define pathways for their elimination. The ability to link underlying molecular structures with electronic properties provides unique insights that lead to new design rules in support of continued materials development efforts.

Lessons on electronic decoherence in molecules from exact modeling.

Electronic decoherence processes in molecules and materials are usually thought and modeled via schemes for the system-bath evolution in which the bath is treated either implicitly or approximately. Here we present computations of the electronic decoherence dynamics of a model many-body molecular system described by the Su-Schrieffer-Heeger Hamiltonian with Hubbard electron-electron interactions using an exact method in which both electronic and nuclear degrees of freedom are taken into account explicitly and fully quantum mechanically. To represent the electron-nuclear Hamiltonian in matrix form and propagate the dynamics, the computations employ the Jordan-Wigner transformation for the fermionic creation/annihilation operators and the discrete variable representation for the nuclear operators. The simulations offer a standard for electronic decoherence that can be used to test approximations. They also provide a useful platform to answer fundamental questions about electronic decoherence that cannot be addressed through approximate or implicit schemes. Specifically, through simulations, we isolate basic mechanisms for electronic coherence loss and demonstrate that electronic decoherence is possible even for one-dimensional nuclear bath. Furthermore, we show that (i) decreasing the mass of the bath generally leads to faster electronic decoherence; (ii) electron-electron interactions strongly affect the electronic decoherence when the electron-nuclear dynamics is not pure-dephasing; (iii) classical bath models with initial conditions sampled from the Wigner distribution accurately capture the short-time electronic decoherence dynamics; (iv) model separable initial superpositions often used to understand decoherence after photoexcitation are only relevant in experiments that employ delta-like laser pulses to initiate the dynamics. These insights can be employed to interpret and properly model coherence phenomena in molecules.

A lightweight scalable agarose-gel-synthesized thermoelectric composite

Electronic devices are now advancing beyond classical, rigid systems and moving into lighweight flexible regimes, enabling new applications such as body-wearables and ‘e-textiles’. To support this new electronic platform, composite materials that are highly conductive yet scalable, flexible, and wearable are needed. Materials with high electrical conductivity often have poor thermoelectric properties because their thermal transport is made greater by the same factors as their electronic conductivity. We demonstrate, in proof-of-principle experiments, that a novel binary composite can disrupt thermal (phononic) transport, while maintaining high electrical conductivity, thus yielding promising thermoelectric properties. Highly conductive Multi-Wall Carbon Nanotube (MWCNT) composites are combined with a low-band gap semiconductor, PbS. The work functions of the two materials are closely matched, minimizing the electrical contact resistance within the composite. Disparities in the speed of sound in MWCNTs and PbS help to inhibit phonon propagation, and boundary layer scattering at interfaces between these two materials lead to large Seebeck coefficient (> 150 μV/K) (Mott N F and Davis E A 1971 Electronic Processes in Non-crystalline Materials (Oxford: Clarendon), p 47) and a power factor as high as 10 μW/(K2 m). The overall fabrication process is not only scalable but also conformal and compatible with large-area flexible hosts including metal sheets, films, coatings, possibly arrays of fibers, textiles and fabrics. We explain the behavior of this novel thermoelectric material platform in terms of differing length scales for electrical conductivity and phononic heat transfer, and explore new material configurations for potentially lightweight and flexible thermoelectric devices that could be networked in a textile.

Protective molecular passivation of black phosphorus

Black phosphorus is a fascinating layered material, with extraordinary anisotropic mechanical, optical and electronic properties. However, the sensitivity of black phosphorus to oxygen and moisture poses significant challenges for technological applications of this unique material. Here, we report a viable solution that overcomes degradation of few-layer black phosphorus by passivating the surface with self-assembled monolayers of octadecyltrichlorosilane that provide long-term stability in ambient conditions. Importantly, we show that this treatment does not cause any undesired carrier doping of the bulk channel material, thanks to the emergent hierarchical interface structure. Our approach is compatible with conventional electronic materials processing technologies thus providing an immediate route toward practical applications in black phosphorus devices.

Capturing atomic-scale carrier dynamics with electrons

Light-driven electronic motion unfolds on times as short as the cycle period of light and on length scales as small as the distance between two neighboring atoms in a molecule. Visualizing fundamental light-matter interactions therefore requires access to attosecond and picometer dimensions. Here we report on a potential unification of electron diffraction and microscopy with attosecond technology, which could provide a full space-time access to elementary electronic processes in matter and materials. We review recent progress in ultrafast diffraction and microscopy towards temporal resolutions approaching 10fs by use of state-of-the-art microwave technology and discuss our latest findings on all-optical compression approaches for reaching sub-femtosecond, sub-optical-cycle resolution. Four-dimensional electron diffraction with attosecond-picometer resolution will access all dynamics outside the atomic core, offering an all-embracing insight into fundamental electron-nuclear dynamics of complex materials.

Controlling Polymer Solubility: Polyfluorenes with Branched Semiperfluorinated Side Chains for Polymer Light‐Emitting Diodes

AbstractA series of novel polyfluorenes, soluble exclusively in perfluorinated solvents, were prepared. The new materials were studied with regard to orthogonal processing of organic electronic materials. The desired solubility was achieved by introducing semifluorinated side chains to the fluorene monomers. Since the use of long perfluoroalkyl chains (RF) is restricted due to public health concerns, a synthetic route for polyfluorenes with short RF chains branched by aromatic units has been developed. The photophysical behavior of the resulting polymers was investigated in solution and thin films by UV/Vis absorption and photoluminescence spectroscopy. The photoluminescence quantum yields were found to be in the range of those of alkylated polyfluorenes. The electroluminescent properties were studied in single‐layer polymer light‐emitting diodes, with the new polymers as active materials, which exhibited similar characteristics to previously published single‐layer devices with polyfluorenes containing long RF. The wetting properties of different polyfluorene films containing fluorinated, polar, polyethylene glycol, or nonpolar alkyl groups were investigated by contact angle measurements.

Honoring S. R. Elliott

Honoring S. R. Elliott

Self-Assembly of Light-Harvesting Crystalline Nanosheets in Aqueous Media

A methodology leading to facile self-assembly of crystalline aromatic arrays in dilute aqueous solutions would enable efficient fabrication and processing of organic photonic and electronic materials in water. In particular, soluble 2D crystalline nanosheets may mimic the properties of photoactive thin films and self-assembled monolayers, covering large areas with ordered nanometer-thick material. We designed such solution-phase arrays using hierarchical self-assembly of amphiphilic perylene diimides in aqueous media. The assemblies were characterized by cryogenic transmission electron microscopy (cryo-TEM), revealing crystalline order and 2D morphology (confirmed by AFM studies). The order and morphology are preserved upon drying as evidenced by TEM and AFM. The 2D crystalline-like structures exhibit broadening and red-shifted absorption bands in UV-vis spectra, typical for PDI crystals and liquid crystals. Photophysical studies including femtosecond transient absorption spectroscopy reveal that two of the assemblies are superior light-harvesters due to excellent solar spectrum coverage and fast exciton transfer, in one case showing exciton diffusion comparable to solid-state crystalline systems based on perylene tetracarboxylic dianhidride (PTCDA).

Vibrational Spectroscopy of Electronic Processes in Emerging Photovoltaic Materials

Molecules affect the electronic properties of many emerging materials, ranging from organic thin film transistors and light emitting diodes for flexible displays to colloidal quantum dots (CQDs) used in solution processed photovoltaics and photodetectors. For example, the interactions of conjugated molecules not only influence morphological and charge transport properties of organic photovoltaic (OPV) materials, but they also determine the primary photophysical events leading to charge generation. Ligand-nanocrystal interactions affect the density and energetic distributions of trap states, which in turn influence minority carrier transport in CQD photovoltaics. Therefore, it is critical for scientists to understand how the underlying molecular structures and morphologies determine the electronic properties of emerging materials. Recently, chemists have used vibrational spectroscopy to study electronic processes in emerging materials, and been able to directly measure the influence molecular properties have on those processes. Time-resolved vibrational spectroscopy is uniquely positioned to examine molecular species involved in electronic processes because it combines ultrafast time resolution with measurement of the vibrational spectra of molecules. For instance, molecules at the electron donor/acceptor interfaces in OPV materials have unique vibrational features because vibrational frequencies of molecules are sensitive to their local molecular environments. Through ultrafast vibrational spectroscopy, researchers can directly examine the dynamics of charge transfer (CT) state formation and dissociation to form charge separated states specifically at donor/acceptor interfaces. Vibrational modes of ligands are also sensitive to their bonding interactions with nanocrystal surfaces, which enables chemists to directly probe the molecular nature of charge trap states in colloidal quantum dot solids. Because of the ability to connect electrical properties with the underlying molecular species, scientists can use ultrafast vibrational spectroscopy to address fundamental challenges in the development of emerging electronic materials. In this Account, we focus on two applications of vibrational spectroscopy to examine electronic processes in OPV and CQD photovoltaic materials. In the first application, we examine archetypal classes of electron acceptors in OPV materials and reveal how their molecular structures influence the dynamics and energetic barriers to CT state formation and dissociation. In the second application, we discuss the surface chemistry of ligand-nanocrystal interactions and how they impact the density and energetic distribution of charge trap states in CQD photovoltaic materials. Through direct observations of the vibrational features of ligands attached to surface trap states, we can obtain valuable insights into the nature of charge traps and begin to understand pathways for their elimination. We expect that further examination of electronic processes in materials using ultrafast vibrational spectroscopy will lead to new design rules in support of continued materials development efforts.

Active Control of Evaporative Solution Deposition by Modulated Infrared Illumination

We designed and built a highly adaptive and flexible system based on modulated infrared irradiation for the active control of evaporative material deposition suitable for solution processing of organic electronic materials. We performed systematic experiments using thick layers of a high molecular weight aqueous dispersion of poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanoparticles as well as thin layers of light-emitting polymers in an organic solvent. We identified two complementary deposition modes, where material accumulates either in the illuminated or the nonirradiated regions, depending on the molecular weight of the solute and the solution layer thickness. We developed numerical models that account for heat transfer due to infrared illumination, solvent evaporation into the gas phase, and solute redistribution in the liquid layer. The computational results agree well with the experimental observations regarding the solute depletion and accumulation behavior. © 2012 American Chemical Society.