Design Of Semiconductors Research Articles

Thermodynamic stability of defects in hybrid MoS2/InAs heterostructures

Vertical van der Waals heterostructures created from two chemically different two-dimensional (2D) materials have made significant headway in the current electronic manufacturing landscape since they demonstrate enhanced attributes. Recently, among the wide variety of possible van der Waals heterostructures, a 2D molybdenum disulfide (MoS2) thin film deposited on a 3D indium arsenide (InAs) semiconductor substrate has demonstrated promising properties. To realize the true potential of these hybrid heterostructures, it is imperative to understand their structural stability, as well as the thermodynamic stabilities of various defects at the heterointerface. We report first principles-based electronic structure calculations to study MoS2(100)/InAs(111) heterostructures with two different terminations of the substrate. We present relative stabilities of arsenic-terminated and indium-terminated heterostructures, and shed light on the thermodynamic stabilities of arsenic, indium, and sulfur vacancy defects at the heterointerface. Atomic-scale structure and electronic structure of defects is detailed along with changes in band gaps due to inclusion of vacancy defects. In general, defects are found to be energetically favorable to occur at the interface as compared to the bulk. We offer new insights into the fundamental role of interfaces in hybrid van der Waals heterostructures and their influence on defects, with potential implications for future design of next-generation semiconductors.

Gold-embedded fabrication of three-dimensional ordered macroporous TiO2 via in-situ pyrolysis for enhanced photocatalysis

Based on the three-dimensional ordered macroporous titanium dioxide (3DOM-TiO2), a series of gold-embedded 3DOM-TiO2 (nAu@3DOM-TiO2) with controlled Au loading amount (n = 0.24, 1.2, 2.4, 4.8, 12, and 24 wt%) was successfully fabricated via in-situ pyrolysis for the first time. Compared with the surface-modified Au/3DOM-TiO2 by traditional impregnation–reduction route, this body-doped Au@3DOM-TiO2 structure exhibited smaller particle size and better dispersive property which are important for photocatalysis. Therefore, the photocatalytic activities of nAu@3DOM-TiO2 have been tested and the optimized photocatalyst of 4.8Au@3DOM-TiO2 was identified. A possible mechanism of Au@3DOM-TiO2 for the enhanced photoactivity was well discussed. The present work demonstrated a unique “gold-embedded” interface with potential advantages of highly dispersed, anti-aggregation, easy recovery, and enhanced catalytic performance, and that this approach could provide an ideal platform for the further design of noble metal-doped semiconductors.

GE SiC Semiconductor Device Operation at Extreme Temperatures

Abstract GE Research has been working on the design, packaging, and testing of SiC Power semiconductors devices at junction temperatures up to 500°C for the past 5 years. Intrinsically, SiC power devices are able to endure and operate reliably at harsh environments. Limiting factors to packaged devices’ operation at high temperature are the contact metallization and packaging. While testing bare die power devices, probe contact resistance was found to be an issue for resistance measurements at very high temperatures. On the backside contact, traditional die attach solders have operational temperature limits. Using insulated metal substrates, sintered die attach, and wedge bond interconnect, GE Research tested its power devices up to 500°C junction temperature. GE Research also demonstrated various junction termination dielectric stackups which can block up to 1000V at 500°C junction and show improvements over traditional organic dielectric options (e.g. Epoxy, Silicone, Polyimide).

Embedment of Multiple Transition Metal Impurities into WS2 Monolayer for Bandstructure Modulation

AbstractBand structure by design in 2D layered semiconductors is highly desirable, with the goal to acquire the electronic properties of interest through the engineering of chemical composition, structure, defect, stacking, or doping. For atomically thin transition metal dichalcogenides, substitutional doping with more than one single type of transition metals is the task for which no feasible approach is proposed. Here, the growth of WS2 monolayer is shown codoped with multiple kinds of transition metal impurities via chemical vapor deposition controlled in a diffusion‐limited mode. Multielement embedment of Cr, Fe, Nb, and Mo into the host lattice is exemplified. Abundant impurity states thus generate in the bandgap of the resultant WS2 and provide a robust switch of charging/discharging states upon sweep of an electric filed. A profound memory window exists in the transfer curves of doped WS2 field‐effect transistors, forming the basis of binary states for robust nonvolatile memory. The doping technique presented in this work brings one step closer to the rational design of 2D semiconductors with desired electronic properties.

The Prospect of Dimensionality in Porous Semiconductors.

With the advent of silicon-based semiconductors, a plethora of previously unknown technologies became possible. The development of lightweight low-dimensional organic semiconductors followed soon after. However, the efficient charge/electron transfers enabled by the non-porous 3D structure of silicon is rather challenging to be realized by their (metal-)organic counterparts. Nevertheless, the demand for lighter, more efficient semiconductors is steadily increasing resulting in a growing interest in (metal-)organic semiconductors. These novel materials are faced with a variety of challenges originating from their chemical design, their packing and crystallinity. Although the effect of molecular design is quite well understood, the influence of dimensionality and the associated change in properties (porosity, packing, conjugation) is still an uncharted area in (metal-)organic semiconductors, yet highly important for their practical utilization. In this Minireview, an overview on the design and synthesis of porous semiconductors, with a particular emphasis on organic semiconductors, is presented and the influence of dimensionality is discussed.

A comparative DFT study of tetracyanoquinodimethane and its difluoro and tetrafluoro analogs

The optimized molecular structures, harmonic vibrational wavenumbers and corresponding vibrational assignments, frontier molecular orbitals and UV data of 7,7,8,8-tetracyanoquinodimethane, 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane were computed using quantum mechanical code. Calculations were carried out at Becke-3-Lee-Yang-Parr (B3LYP) functional with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) using the 6–311++G (d,p) basis set. The theoretical results were successfully compared with some of the available experimental data. The influence of fluorine on the structural, vibrational and electronic properties were investigated. The addition of fluorine reduces both the electrical and optical band gaps. The findings of this research can be useful for analogs of the molecules studied which have potential applications in the design of organic semiconductors for electronic devise applications.

Drop test analysis of ball grid array package using finite element method

Proper design of semiconductors is the most essential step in ensuring electronic product reliability during manufacturing. The adoption of design optimization approach enhances the performance and reliability of the semiconductor package, hence, minimizes product failure. Semiconductor reliability especially in the application to automobiles is very crucial. High product failure tolerance in semiconductors in automobiles is required due to problems such as defects and malfunctions that are directly linked to casualty accidents. Such high tolerance requires keen quality control up to the semiconductor solder component level. This study aims to do a sensitivity analysis of the solder balls’ material properties and its effect on the stresses experienced by the semiconductor component with respect to its reliability using drop test. A drop test analysis was simulated wherein a shorter distance of 5 mm between the package and platform was implemented to make the simulation time faster. The density and modulus of elasticity of the solder ball material were identified as the independent factors while the stresses experienced by the BGA package during the drop test is the dependent variable. The results have shown that the maximum stress for all runs was found in the same area in the package. Changing the modulus of elasticity showed greater effect on the impact stress compared to the alteration of material density.

Nanoporous Cubic Silicon Carbide Photoanodes for Enhanced Solar Water Splitting.

Cubic silicon carbide (3C-SiC) is a promising photoelectrode material for solar water splitting due to its relatively small band gap (2.36 eV) and its ideal energy band positions that straddle the water redox potentials. However, despite various coupled oxygen-evolution-reaction (OER) cocatalysts, it commonly exhibits a much smaller photocurrent (<∼1 mA cm–2) than the expected value (8 mA cm–2) from its band gap under AM1.5G 100 mW cm–2 illumination. Here, we show that a short carrier diffusion length with respect to the large light penetration depth in 3C-SiC significantly limits the charge separation, thus resulting in a small photocurrent. To overcome this drawback, this work demonstrates a facile anodization method to fabricate nanoporous 3C-SiC photoanodes coupled with Ni:FeOOH cocatalyst that evidently improve the solar water splitting performance. The optimized nanoporous 3C-SiC shows a high photocurrent density of 2.30 mA cm–2 at 1.23 V versus reversible hydrogen electrode (VRHE) under AM1.5G 100 mW cm–2 illumination, which is 3.3 times higher than that of its planar counterpart (0.69 mA cm–2 at 1.23 VRHE). We further demonstrate that the optimized nanoporous photoanode exhibits an enhanced light-harvesting efficiency (LHE) of over 93%, a high charge-separation efficiency (Φsep) of 38%, and a high charge-injection efficiency (Φox) of 91% for water oxidation at 1.23 VRHE, which are significantly outperforming those its planar counterpart (LHE = 78%, Φsep = 28%, and Φox = 53% at 1.23 VRHE). All of these properties of nanoporous 3C-SiC enable a synergetic enhancement of solar water splitting performance. This work also brings insights into the design of other indirect band gap semiconductors for solar energy conversion.

Monte carlo optimization of multi-layer semiconductor material

At present, the demand for various types of semiconductor devices is increasing, and under the current situation of energy shortage, the improvement of the energy efficiency of semiconductors is the top priority for development of science and technology. For this purpose, this article establishes a physical model and mathematical optimization model for multi-layer semiconductors based on Monte Carlo Simulation. The developed model and corresponding equations are derived from thermodynamic theory and principles, and the computer programming is set up to get the optimal design of multilayer semiconductors through Monte Carlo simulation to achieve higher energy efficiency.

Structure-Property Relationships in Bithiophenes with Hydrogen-Bonded Substituents.

The use of crystal engineering to control the supramolecular arrangement of π-conjugated molecules in the solid-state is of considerable interest for the development of novel organic electronic materials. In this study, we investigated the effect of combining of two types of supramolecular interaction with different geometric requirements, amide hydrogen bonding and π-interactions, on the π-overlap between calamitic π-conjugated cores. To this end, we prepared two series of bithiophene diesters and diamides with methylene, ethylene, or propylene spacers between the bithiophene core and the functional groups in their terminal substituents. The hydrogen-bonded bithiophene diamides showed significantly denser packing of the bithiophene cores than the diesters and other known α,ω-disubstituted bithiophenes. The bithiophene packing density reach a maximum in the bithiophene diamide with an ethylene spacer, which had the smallest longitudinal bithiophene displacement and infinite 1D arrays of electronically conjugated, parallel, and almost linear N-H⋅⋅⋅O=C hydrogen bonds. The synergistic hydrogen bonding and π-interactions were attributed to the favorable conformation mechanics of the ethylene spacer and resulted in H-type spectroscopic aggregates in solid-state absorption spectroscopy. These results demonstrate that the optoelectronic properties of π-conjugated materials in the solid-state may be tailored systematically by side-chain engineering, and hence that this approach has significant potential for the design of organic and polymer semiconductors.

U‐Shaped Heteroacenes Embedded with Heavy Chalcogen Atoms: Unique Bilayer Self‐Organization of Crooked π‐Cores Enabling Efficient Charge Transport

AbstractOrganic semiconductors are indispensable functional materials for next‐generation printed electronics, and significant progress has been made in the development of high‐mobility organic semiconductors in recent years. Most of these molecules rely on the use of linearly π‐extended polycyclic heteroaromatic systems, referred to as heteroacenes, as the functional core. Here, to expand the scope of the rational design of organic semiconductors, a new series of U‐shaped heteroacenes that features unprecedented “U” geometric configuration together with systematic chalcogen modification is elaborated. This U‐shaped molecular geometry promotes the formation of supramolecular bilayer lamellar assemblies, in which the U‐shaped molecules are packed into a head‐to‐head arrangement via the multiple intermolecular chalcogen interactions. Importantly, the incorporation of heavier chalcogen atoms (selenium and tellurium) instead of sulfur atoms can systematically modulate the key electronic properties of materials by influencing the molecular geometry, frontier orbital energy levels, and noncovalent intermolecular interactions. Consequently, anisotropic, high hole mobilities of up to 3.8 cm2 V−1 s−1 can be attained in thin‐film organic field‐effect transistors based on the selenium‐embedded U‐shaped heteroacene. In this study, the relationship between the molecular structures, supramolecular self‐organization, and charge transport properties among these U‐shaped heteroacenes are investigated from computational and experimental perspectives.

Origin of abnormal thermal conductivity in group III-V boron compound semiconductors

Over the past half-century, according to Moore’s law, the sizes of transistors continue shrinking, and the integrated circuits have approached to their physical limits, which puts forward higher requirements for the thermal dissipation capacity of material. Revealing the physical mechanisms of heat conduction in semiconductors is important for thermal managements of devices. Experimentally, it was found that boron arsenide has a very high thermal conductivity compared with diamond, and boron arsenide has lattice constant close to silicon’s lattice constant, which can be heterogeneously integrated into silicon to solve the thermal management problem. However, group III-V boron compounds show abnormal thermal conductivities: the thermal conductivity of boron arsenide is significantly higher than that of boron phosphide and boron antimonide. Here, we use the first-principles calculation and the Boltzmann transport equation to study the thermal conductivity properties of the group III-V boron compounds. Comparison between the IV and III-V semiconductors shows that the high thermal conductivity of boron arsenide is due mainly to the existence of a large frequency gap between the acoustic and the optical branches. The energy sum of two acoustic phonons is less than energy of one optical phonon, which cannot meet the energy conservation requirements of three-phonon scattering, and then seriously restrict the probability of scattering of three phonons. The high thermal conductivity of diamond is due mainly to its great acoustic phonon group velocity. Although the boron phosphide also has a relatively large acoustic phonon group velocity, the frequency gap is relatively small, which cannot effectively suppress the three-phonon scattering, so the thermal conductivity of boron phosphide is less than that of boron arsenide. Although the frequency gap of boron antimonide is similar to that of boron arsenide, the thermal conductivity of boron antimonide is lower than that of boron arsenide due to its smaller acoustic phonon group velocity and larger coupling matrix element. The research provides a new insight into the design of semiconductor materials with high thermal conductivities.

Pressure effects on the metallization and dielectric properties of GaP.

In situ impedance measurement, resistivity measurements and first-principles calculations have been performed to investigate the effect of high pressure (up to 30.2 GPa) on the metallization and dielectric properties of GaP. It is found that the carrier transport process changes from mixed grain and grain boundary conduction to pure grain conduction at 5.8 GPa, and due to pressure-induced structural phase transition, the resistance drops drastically by three orders of magnitude at 25.5 GPa. Temperature dependence of resistivity measurements and band structure calculations suggest the occurrence of a semiconductor-metal transition. Combining differential charge density and dielectric analysis, it is observed that the electron localization is weakened, which leads to increased polarization and larger relative permittivity in the zb structure. After the phase transition, both the polarization and the relative permittivity decrease. Pressure increases the complex dielectric constant and dielectric loss factor, due to the increase in relaxation polarization and the scattering effect of carriers. Moreover, by comparing the high-pressure behavior of GaP, GaAs and GaSb, the changes in the electronic structure and electric transport process caused by the phase transition can be understood, which can enable us to better understand the metallization behavior and dielectric properties of Ga-based III-V family semiconductors under pressure, and stimulate the design and modification of other related group III-V semiconductors for optoelectronic devices and sensors.

Zeolitic octahedral niobium oxide with microchannels of seven-membered rings for photocatalytic H2 evolution from saline water.

Zeolitic octahedral metal oxides (ZOMOs) are fully inorganic crystalline materials, mostly containing transition metals, and possess a defined framework as well as regular hollow channels. Therefore, they have significant application potential in many fields, particularly catalysis. Herein, we report the synthesis and characterization of zeolitic octahedral niobium oxides (ZOMO-NbOx) and the framework was made of {NbOx} polyhedra ({NbO6} octahedron and {Nb6O27} pentagon units). Microchannels consisting of 7-membered rings with ∼0.4 nm diameter were realized, and rarely reported plural {Nb6O27} units including dimers, trimers, and tetramers were discovered. Owing to the continuous hollow microporous structure that provides a large surface area as well as a shortened transfer path for charge carriers from bulk to the surface, considerably enhanced activities beyond pristine Nb2O5 were achieved towards photocatalytic H2 evolution from (saline) water. The results show a practical case of solo-Nb-based zeolitic materials, which expand the ZOMO family and provide further insights into the design and synthesis of efficient zeolitic semiconductors for artificial photocatalysis.

Rational design of organic semiconductors with low internal reorganization energies for hole and electron transport: position effect of aza-substitution in phenalenyl derivatives.

Amphoteric-redox phenalenyl radical (PLY) is a suitable candidate used to design ambipolar organic materials. Because the singly occupied nonbonding molecular orbital (NBMO) of PLY has a perfect local nonbonding character, its internal reorganization energy (λ) for transporting holes (λ+) or electrons (λ-) is known to be small. Herein, PLY is employed to study the position effect of the aza group on the λ. By adding or extracting an electron from the NBMO, the bond length alterations can be minute. Therefore, the PLY derivatives are also an excellent candidate to study the contributions from the bond angle alterations to the λ. Substituting the aza groups at the β- or α-positions of PLY shows two different trends. When consecutively substituting the aza group at the three β-positions of PLY, the λs are consistently decreased. Contrarily, a series of double functionalization of aza groups at the four α-positions of PLY, the λs are increased. It is because the local bonding or antibonding character in frontier orbitals (FMO) is observed in α2N-PLY and α4N-PLY. As the FMOs of the three β-substituted PLYs and α6N-PLY have perfect local nonbonding character, we found the bond angle alterations are the main contributors of λ. The λs for most aza-PLYs were smaller than 100 meV. Thus, we propose a design rule for substituting aza groups on the parent molecules with strong local nonbonding character in their FMOs. Based on the adiabatic ionization potential and electron affinity, two π-extended PLY derivatives with small λ were recommended for fabricating air-stable ambipolar OFET.

Design of one-dimensional organic semiconductors with high intrinsic electron mobilities: lessons from computation

A bottom-up design strategy for one-dimensional organic semiconductors with high intrinsic electron mobilities is proposed.

FB-ECDA: Fragment-based Electronic Coupling Decomposition Analysis for Organic Amorphous Semiconductors.

We present a fragment-based decomposition analysis tool (FB-ECDA) for the electronic coupling of charge transfer processes. This tool provides insight on the sophisticated relationship between molecular packing, electronic coupling, and the molecular transport network present in organic amorphous semiconductors. On the basis of atomic orbitals, FB-ECDA decomposes the total electronic coupling into individual electronic coupling terms arising from each molecular building blocks in a straightforward manner. The usefulness of this approach is demonstrated by revealing the structure-packing-property relationships for two series of molecules differing by the number of arm substituents and acene core length. Overall, we provide insight on the design of organic semiconductors exhibiting efficient charge transport network through achieving a subtle balance between molecular packing and electronic structure. We expect FB-ECDA to be a valuable tool for understanding sophisticated charge transfer processes in amorphous systems and guiding the rational design of organic semiconductors.

Characterization of α-phenyl-4′-(diphenylamino)stilbene single crystal and its anisotropic conductivity

Presently, organic-semiconductor electronics suffer from low performance owing to their amorphous nature. Improving their crystallinity, and thus the charge transport properties is a crucial endeavor. Accordingly, this study aims to elucidate the conductivity mechanism of a novel triphenylamine-derivative single crystal. We successfully synthesized an α-phenyl-4′-(diphenylamino)stilbene (TPA) single crystal via a solution method employing the solubility and supersolubility curves of TPA in the tetrahydrofuran (THF) solvent. Its crystal structure was determined via single-crystal X-ray diffraction (SCXRD), which revealed that the obtained single crystal contains not only TPA but also THF. The molecular ratio of TPA to THF was found to be 2:0.74. We further analyzed the hole transport property in three directions of the single crystal, and the anisotropic conductivity was observed. Based on the results of the observed molecular packing and charge transport property, we confirm that the observed anisotropic conductivity is determined based on the orientation of the triphenylamine unit in each direction of the TPA single crystal. Overall, the findings can help in guiding the design of high-mobility organic semiconductors for realizing more effective organic electronic devices.

Construction of simple and low-cost acceptors for efficient non-fullerene organic solar cells

The flexibility in structural design of organic semiconductors endows organic solar cells (OSCs) not only great function-tunabilities, but also high potential toward practical application. In this work, four simple and low-cost non-fullerene acceptors with fluorene or carbazole as central cores, 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c] thiophen-4-ylidene)malononitrile (TC) as terminal groups, and thiophene or furan as linkers, named DTC-T-F, DTC-F-F, DTC-T-C and DTC-F-C, are developed through twostep synthesis, and their photophysical properties, electrochemical behavior and photovoltaic performance are systematically and comparatively studied. The results revealed that fluorene-based acceptors exhibited superior photophysical properties and morphology characteristics than carbazole-based counterparts, and thiophene is more suitable as bridging groups. Combining the advantages of both, the BHJ-OSC based on PTB7-Th:DTC-T-F blend film showed a high PCE of 8.8%, with a V oc of 0.78 V, a J sc of 17.46 mA cm −2 , and an FF of 0.65, which is the highest value in the PTB7-Th and fluorene-based acceptors coupled devices, implying its potential application. Four simple and low-cost non-fullerene acceptors were synthesized and systematic comparative investigated, in which the DTC-T-F based BHJ-OSC showed a high PCE of 8.8%. • Four simple, low-cost non-fullerene acceptors were synthesized and systematic comparative investigated. • Both electron-donating donors and bridging groups have direct effects on the properties of non-fullerene acceptors. • PTB7-Th:DTC-T-F based OSC showed a high PCE of 8.8%, which is the highest value in the fluorene-based acceptors devices.

Thiophene-Bridged Donor-Acceptor sp2 -Carbon-Linked 2D Conjugated Polymers as Photocathodes for Water Reduction.

Photoelectrochemical (PEC) water reduction, converting solar energy into environmentally friendly hydrogen fuel, requires delicate design and synthesis of semiconductors with appropriate bandgaps, suitable energy levels of the frontier orbitals, and high intrinsic charge mobility. In this work, the synthesis of a novel bithiophene-bridged donor-acceptor-based 2D sp2 -carbon-linked conjugated polymer (2D CCP) is demonstrated. The Knoevenagel polymerization between the electron-accepting building block 2,3,8,9,14,15-hexa(4-formylphenyl) diquinoxalino[2,3-a:2',3'-c]phenazine (HATN-6CHO) and the first electron-donating linker 2,2'-([2,2'-bithiophene]-5,5'-diyl)diacetonitrile (ThDAN) provides the 2D CCP-HATNThDAN (2D CCP-Th). Compared with the corresponding biphenyl-bridged 2D CCP-HATN-BDAN (2D CCP-BD), the bithiophene-based 2D CCP-Th exhibits a wide light-harvesting range (up to 674nm), a optical energy gap (2.04eV), and highest energy occupied molecular orbital-lowest unoccupied molecular orbital distributions for facilitated charge transfer, which make 2D CCP-Th a promising candidate for PEC water reduction. As a result, 2D CCP-Th presents a superb H2 -evolution photocurrent density up to ≈7.9 µA cm-2 at 0V versus reversible hydrogen electrode, which is superior to the reported 2D covalent organic frameworks and most carbon nitride materials (0.09-6.0 µA cm-2 ). Density functional theory calculations identify the thiophene units and cyano substituents at the vinylene linkage as active sites for the evolution of H2 .