Direct Gap Semiconductor Research Articles

Composition-Defined Optical Properties and the Direct-to-Indirect Transition in Core-Shell In1-xGaxP/ZnS Colloidal Quantum Dots.

Semiconductors are commonly divided into materials with direct or indirect band gaps based on the relative positions of the top of the valence band and the bottom of the conduction band in crystal momentum (k) space. It has, however, been debated if k is a useful quantum number to describe the band structure in quantum-confined nanocrystalline systems, which blur the distinction between direct and indirect gap semiconductors. In bulk III-V semiconductor alloys like In1-xGaxP, the band structure can be tuned continuously from the direct- to indirect-gap by changing the value of x. The effect of strong quantum confinement on the direct-to-indirect transition in this system has yet to be established because high-quality colloidal nanocrystal samples have remained inaccessible. Herein, we report one of the first systematic studies of ternary III-V nanocrystals by utilizing an optimized molten-salt In-to-Ga cation exchange protocol to yield bright In1-xGaxP/ZnS core-shell particles with photoluminescence quantum yields exceeding 80%. We performed two-dimensional solid-state NMR studies to assess the alloy homogeneity and the extent of surface oxidation in In1-xGaxP cores. The radiative decay lifetime for In1-xGaxP/ZnS monotonically increases with higher gallium content. Transient absorption studies on In1-xGaxP/ZnS nanocrystals demonstrate signatures of direct- and indirect-like behavior based on the presence or absence, respectively, of excitonic bleach features. Atomistic electronic structure calculations based on the semi-empirical pseudopotential model are used to calculate absorption spectra and radiative lifetimes and evaluate band-edge degeneracy; the resulting calculated electronic properties are consistent with experimental observations. By studying photoluminescence characteristics at elevated temperatures, we demonstrate that a reduced lattice mismatch at the III-V/II-VI core-shell interface can enhance the thermal stability of emission. These insights establish cation exchange in molten inorganic salts as a viable synthetic route to nontoxic, high-quality In1-xGaxP/ZnS QD emitters with desirable optoelectronic properties.

Silicon and germanium adamantane and diamantane monolayers as two-dimensional anisotropic direct-gap semiconductors

Silicon and germanium adamantane and diamantane monolayers as two-dimensional anisotropic direct-gap semiconductors

A Theoretical Investigation of the Structural, Electronic and Mechanical Properties of Pristine and Nitrogen-Terminated Carbon Nanoribbons Composed of 4–5–6–8-Membered Rings

Among the exciting recent advances in the field of carbon-based nanomaterials, the successful realization of a carbon nanoribbon composed of 4–5–6–8-membered rings (ACS Nano 2023 17, 8717) is a particularly inspiring accomplishment. In this communication motivated by the aforementioned achievement, we performed density functional theory calculations to explore the structural, electronic and mechanical properties of the pristine 4–5–6–8-membered carbon nanoribbons. Moreover, we also constructed four different nitrogen-terminated nanoribbons and analyzed their resulting physical properties. The acquired results confirm that the pristine and nitrogen-terminated nanoribbons are are thermally stable direct-gap semiconductors, with very close HSE06 band gaps between 1.12 and 1.25 eV. The elastic modulus and tensile strength of the nitrogen-free 4–5–6–8-membered nanoribbon are estimated to be remarkably high, 534 and 41 GPa, respectively. It is shown that nitrogen termination can result in noticeable declines in the tensile strength and elastic modulus to 473 and 33 GPa, respectively. This study provides useful information on the structural, thermal stability, electronic and mechanical properties of the pristine and nitrogen-terminated 4–5–6–8-membered carbon nanoribbons and suggests them as strong direct-gap semiconductors for electronics, optoelectronics and energy storage systems.

BxCyNz monolayers with full coverage of fluorine: A study of their structural, electronic and optical properties

We have investigated, using first-principles DFT calculations, the structural, electronic, and optical properties of fully fluorinated BxCyNz monolayers. The fluorinated hybrid sheets are derived from BN monolayers with a C nanodomain and from graphene monolayers with a BxNx nanodomain. The former ones display considerably stronger fluorine chemical adsorption. Phonon scattering spectrum shows that even after the deformation of the monolayers, due to the fluorination process, the structures remained mechanically stable. Ab-initio molecular dynamics (AIMD) simulations suggest the stability of fluorinated BxCyNz hybrid monolayers at high temperatures. The band structures of fluorinated hybrid sheets show that they are direct gap semiconductors with similar band gaps. Effective mass of both electron and hole presents isotropic behavior. The graphene monolayers with a BxNx nanodomain show peaks with greater intensity in the visible region, unlike from BN monolayers with a C nanodomain. The size of the nanodomain have influence in the absorption spectrum.

Computational study of III–V direct-gap semiconductors for thermoradiative cell applications

We investigate the performance of thermoradiative (TR) cells using the III–V group of semiconductors, which include GaAs, GaSb, InAs, and InP, with the aim of determining their efficiency and finding the best TR cell materials among the III–V group. The TR cells generate electricity from thermal radiation, and their efficiency is influenced by several factors such as the bandgap, temperature difference, and absorption spectrum. To create a realistic model, we incorporate sub-bandgap and heat losses in our calculations and utilize density-functional theory to determine the energy gap and optical properties of each material. Our findings suggest that the absorptivity of the material, especially when the sub-bandgap and heat losses are considered, can decrease the efficiency of TR cells. However, careful treatment of the absorptivity indicates that not all materials have the same trend of decrease in the TR cell efficiency when taking the loss mechanisms into account. We observe that GaSb exhibits the highest power density, while InP demonstrates the lowest one. Moreover, GaAs and InP exhibit relatively high efficiency without the sub-bandgap and heat losses, whereas InAs display lower efficiency without considering the losses, yet exhibit higher resistance to sub-bandgap and heat losses compared to the other materials, thus effectively becoming the best TR cell material in the III–V group of semiconductors.

Prediction and Synthesis of a Selenide Perovskite for Optoelectronics

The advent of halide perovskites in recent years has opened an avenue for redeveloping perovskite materials as semiconductors. In the quest for semiconducting perovskites, chalcogenides, which exhibit higher stability than their halide siblings and often direct band gaps for optoelectronics, have attracted more and more attention. So far, functional chalcogenide perovskites have been exclusively sulfides. Here, employing first-principles calculations and the criterion of phase stability in addition to the commonly used thermodynamic and dynamical criteria, we precisely predict the existence of LaScSe3 as a thermodynamically stable selenide perovskite, which is validated by our experimental synthesis. Combining hybrid functional and many-body quasi-particle (G0W0 and Bethe–Salpeter equation) calculations, we predict that LaScSe3 is a direct-gap semiconductor having the band gap in the green-to-blue region and capable of p- and n-type bipolar doping, potentially for optoelectronic applications.

Theoretical insights into the epitaxial growth of black arsenene enabled on GeS(001)

Black arsenene exhibits many exotic physical properties, such as Rashba spin–orbital coupling, fractional quantum Hall effect (Sheng 2021 Nature 593 56) as well as some advantages in the field of energy storage (Wu et al 2021 J. Mater. Chem. A 9 18793). High-quality and large-area BA monolayer can promote the investigations about BA and its device application. Epitaxial growth mechanism of BA is desirable. Here, based on density functional theory calculation, the epitaxial growth of BA monolayer was simulated. GeS(001) is found to be a suitable substrate for BA monolayer to epitaxially grow on. As a common isomer of arsenene, gray arsenene should be considered during the growth, because it is also energetically and thermodynamically stable in freestanding state. However, black arsenene monolayer is more energetically and thermodynamically stable than gray arsenene monolayer on GeS(001) substrate. During the growth, two arsenene atoms easily form a dimer on GeS(001), which diffuses more quickly and isotropically than arsenene monomer. In addition, the heterojunction consisted of balck arsenene and GeS(001) is an indirect gap semiconductor, but it can transform into a direct gap semiconductor with external tensile strain along zigzag direction. Remarkably, optical adsorption spectra range of BA/GeS(001) can be more abroad than that of BA and GeS(001) bilayers. The theatrical insights shed new light on some ideal substrates that can realize the epitaxial growth of high-quality simple substances of group V.

New Low-Dimensional Organic–Inorganic Lead Halide Hybrid Systems Directed by Imidazo[1,5-a]pyridinium-Based Cation or Imines: Synthesis, Structures, Non-Covalent Interactions and Optical Properties

The organic–inorganic lead halide hybrids comprising semiconducting perovskite components and organic modules have proven to be promising candidates for optoelectronic applications. The modulation of the inorganic components as optical centres by diverse organic cationic templates is under intense investigation. Herein, we successfully prepared new one-dimensional lead halide hybrid perovskites [L1]2n[Pb2Cl6]n∞·nH2O (1) and [PbBr2(L2)]n∞·0.5nH2O (2), and the dimeric complex [PbBr2(L3)]2 (3) in water media. In 1, 2-(2-hydroxyethyl)-2H-imidazo[1,5-a]pyridinium cation [L1]+ resulted from the oxidative condensation–cyclization between formaldehyde, ethanolamine and 2-pyridinecarbaldehyde (2-PCA); the polydentate Schiff base ligands L2 and L3 formed in the in situ condensation of 2-PCA and ethanolamine or ethylenediamine, respectively. The lead chloride hybrid 1 contains the previously unreported type of a [Pb2Cl6]∞ double chain constructed from three-edge- and five-edge-sharing PbCl6 octahedra, and cations forming π-bonded stacks aligned along the inorganic wires. In the crystal of 2, pairs of the double-side organically decorated [PbBr2(L2)]∞ chains built of corner-sharing PbBr3N2O octahedra arrange hydrophilic channels to host water molecules. In the solid state, the identically stacked dimers of 3 form columns parallel to the ab plane with the Pb2Br4 moieties in the column being strictly coplanar. Hirshfeld surface analysis was used to rationalize the packing patterns through hydrogen bonds of O−H···O/Cl and C−H···O/Cl types with the involvement of OH groups of [L1]+, L2 and water molecules in 1 and 2, as well as C–H∙∙∙Br hydrogen bonding in 2 and 3. The QTAIM analysis of non-covalent interactions in 1–3 was performed. According to the analysis of the solid-state UV–visible reflectance spectra by a Tauc plot, the optical band gap values of 1, 2 and 3 as direct gap semiconductors were estimated to be 3.36, 3.13 and 2.96 eV, respectively.

Effect of stress on electronic structure and optical properties of cubic Ca2Ge

In this work, the effect of stress on the electronic structure and optical properties of cubic Ca2Ge was investigated using the first-principles method. It is found that under compressive stress Ca2Ge is a direct-gap semiconductor and the band gap decreases from 0.5477 eV @ 0 GPa to 0.1025 eV @ 15 GPa till 20 GPa, at which point Ca2Ge becomes a semimetal. Under tensile stress, it is otherwise an indirect gap, and the band gap decreases to 0.3178 eV @ −10 GPa till −15 GPa where it is a metal. The dielectric constant increases exponentially under compression, while it increases irregularly but monotonically under strain. The absorption edge shifts in the low-energy direction with increasing compressive stress, while the absorption maximum of Ca2Ge shifts in the high-energy direction before Ca2Ge becomes a metal under tensile stress. The effect on absorption increases with increasing energy, and the absorption edge exhibits strong absorption characteristics under tension. Reflectivity shows that Ca2Ge is a material with low reflectivity. The effects of stress modulation on the electronic structure and optical properties are determined based on the atomic population. Under tensile stress, there is no chemical bonding. When the compressive stress increases to 10 GPa, a chemical bond is formed, and the bond population and length decrease with increasing stress.

Ensemble averages of ab initio optical, transport, and thermoelectric properties of hexagonal SixGe1−x alloys

Ge-rich hexagonal SiGe alloys have recently emerged as new direct-gap semiconductors with unprecedented potential for integration of photonics on silicon. We present a comprehensive first-principles investigation of optical, transport, and thermoelectric properties of pure and doped hexagonal ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloys based on density-functional theory calculations, the Boltzmann transport equation, and the generalized quasichemical approximation to obtain alloy averages of electronic properties. At low temperatures, phase decomposition into the hexagonal elementary crystals is thermodynamically favored, but around and above room temperature random alloys are predicted to be stable. While hexagonal Si has an indirect band gap, the gap of hexagonal Ge is direct with very weak optical transitions at the absorption edge. The alloy band gap remains direct for a Si content below 45% and the oscillator strength of the lowest optical transitions is efficiently enhanced by alloying. The optical spectra show clear trends and both absorption edges and prominent peaks can be tuned with composition. The dependence of transport coefficients on carrier concentration and temperature is similar in cubic and hexagonal alloys. However, the latter display an anisotropic response due to the reduced hexagonal symmetry. In particular, the transport mass exhibits a significant directional dependence. Seebeck coefficients and thermoelectric power factors of $n$-doped alloys show nonmonotonous variations with the Si content independently of temperature.

First-principles prediction of 1H-Na2Se monolayer: effects of external strain and point defects associated with constituent atoms

In this work, a new Na2Se monolayer in 1H-phase, with interesting properties similar to transition metal dichalcogenides (TMDs), has been predicted using first-principles calculations. Results reveal good stability and wide direct gap semiconductor nature, with an energy gap of 0.82(1.45) eV as determined by standard PBE(hybrid HSE06) functional. This two-dimensional (2D) material exhibits poor stability under compressive strain due to the sensitive ZA acoustic phonon mode. In contrast, it only becomes unstable with tensile strain from 9% due to the elongation of chemical bonds reflected in the optical E” phonon mode. Under effects of lattice tension, the direct gap character is preserved and the band gap increases nearly linearly according to increase the strain strength. In addition, the effects of point defects associated with Na and Se atoms, including vacancies, antisites, and atom adsorption are also examined. Results indicate significant magnetization induced by Na single vacancy, Se Na antisite (one Na atom substituted by one Se atom), and Na adsorption on-top of hollow (T H ) site. In these cases, feature-rich magneto-electronic properties as half-metallic and magnetic semiconductor natures are obtained. In contrast, the formation of 2Na+1Se combined vacancies as well as other defects related to Se atoms leads to an effective band gap modification, even metallization in some cases. Results introduces new promising 2D material for optoelectronic applications and proposes point defect engineering to create artificially novel features for spintronic applications.

Floquet engineering of strongly driven excitons in monolayer tungsten disulfide

Interactions of quantum materials with strong laser fields can induce exotic non-equilibrium electronic states1–6. Monolayer transition metal dichalcogenides, a new class of direct-gap semiconductors with prominent quantum confinement7, offer exceptional opportunities for the Floquet engineering of excitons, which are quasiparticle electron–hole correlated states8. Strong-field driving has the potential to achieve enhanced control of the electronic band structure and thus the possibility of opening a new realm of exciton light–matter interactions. However, a full characterization of strong-field driven exciton dynamics4,9 has been difficult. Here we use mid-infrared laser pulses below the optical bandgap to excite monolayer tungsten disulfide and demonstrate strong-field light dressing of excitons in excess of a hundred millielectronvolts. Our high-sensitivity transient absorption spectroscopy further reveals the formation of a virtual absorption feature below the 1s-exciton resonance, which we assign to a light-dressed sideband from the dark 2p-exciton state. Quantum-mechanical simulations substantiate the experimental results and enable us to retrieve real-space movies of the exciton dynamics. This study advances our understanding of the exciton dynamics in the strong-field regime, showing the possibility of harnessing ultrafast, strong-field phenomena in device applications of two-dimensional materials. The interaction of strong laser fields with tungsten disulfide leads to light-dressed Floquet replica of excitonic states, which manifest as new features in the transient absorption spectrum.

Первопринципное исследование электронных, колебательных и упругих свойств кристаллов LiInTe-=SUB=-2-=/SUB=- и LiTlTe-=SUB=-2-=/SUB=-

Using the sublattice method and density functional theory, the electronic struc-ture of a LiTlTe2 crystal with the chalcopyrite structure was studied for the first time and the equilibrium parameters of the crystal lattice a=6.7526 Å, c=13.3037 Å, u(Te)=0.2423 were calculated. It has been established that the valence bands of the LiTlTe2 crystal and its closest analogue LiInTe2 actually coincide in topol-ogy, and the LiTlTe2 crystal is a direct-gap semiconductor with a band gap of 0.63 eV and a crystal splitting of 0.04 eV. The partial contributions of the densi-ty of states are analyzed and the features of the formation of the valence and conduction bands of LiInTe2 and LiTlTe2 crystals due to the contributions of their sublattices are revealed: the structure of the valence bands of both crystals is completely determined by the interaction in the cationic tetrahedra of InTe4 and TlTe4. The vibrational modes and elastic constants are calculated, confirm-ing the stability and mechanical stability of the LiTlTe2 crystal.

The structure and optical properties of semiconductor nitrides MgSiN<sub>2</sub>, MgGeN<sub>2</sub>, ZnSiN<sub>2</sub>, ZnGeN<sub>2</sub>

Theoretical modeling within LDA, GGA, and PBE approximations was herein performed to determine the electronic band structures of MgGeN2, MgSiN2, ZnGeN2, and ZnSiN2 nitride compounds and their optical properties. It is established that the compounds with germanium are direct-gap semiconductors with the band gap values of 3.0 eV (MgGeN2) and 1.7 eV (ZnGeN2), while the silicon-based compounds are indirect-gap semiconductors with the band gap values of 4.6 eV (MgSiN2) and 3.7 eV (ZnSiN2). Optical properties analysis showed the prospects of using MgGeN2 and ZnGeN2 in optoelectronics.

Ab initio study of electronic, vibrational and elastic properties of LiInTe-=SUB=-2-=/SUB=- and LiTlTe-=SUB=-2-=/SUB=- crystals

Using the sublattice method and density functional theory, the electronic structure of a LiTlTe2 crystal with the chalcopyrite structure was studied for the first time and the equilibrium parameters of the crystal lattice a=6.7526 Angstrem, c=13.3037 Angstrem, u(Te)=0.2423 were calculated. It has been established that the valence bands of the LiTlTe2 crystal and its closest analogue LiInTe2 actually coincide in topology, and the LiTlTe2 crystal is a direct-gap semiconductor with a band gap of 0.63 eV and a crystal splitting of 0.04 eV. The partial contributions of the density of states are analyzed and the features of the formation of the valence and conduction bands of LiInTe2 and LiTlTe2 crystals due to the contributions of their sublattices are revealed: the structure of the valence bands of both crystals is completely determined by the interaction in the cationic tetrahedra of InTe4 and TlTe4. The vibrational modes and elastic constants are calculated, confirming the stability and mechanical stability of the LiTlTe2 crystal Keywords: LiInTe2, LiTlTe2, chalcopyrite, electronic structure, sublattice.

Ultrafast response solar-blind UV sensor based on ZnGa2O4 nanowire bridge arrays

In optoelectronic devices and deep UV photodetectors, there is an urgent need for solar-blind photodetectors with a fast response time. Spinel ZnGa 2 O 4 is a wide-bandgap metal-oxide direct-gap semiconductor (4.4–5.2 eV, UV band for 100–280 nm) with outstanding optical and electrical properties, making it the best choice for solar-blind photodetectors. In this work, the ZnGa 2 O 4 lateral nanowire bridge arrays were speedily manufactured on an insulating SiO 2 /Si substrate with a simplified specific low-cost CVD process with Au arrays catalyst. A thin film of ZnGa 2 O 4 nanowire bridge arrays was synthesized in 3 min, implying a high potential for the fast development of ZnGa 2 O 4 nanowire bridge arrays and their application in the future solar-blind photodetectors. Furthermore, a solar-blind photodetector using MSM (Metal-Semiconductor-Metal) ZnGa 2 O 4 bridge arrays with exceptional performance was developed here, with a rise time of similar to 0.067 s, an I dark of 0.041 nA, and a light-dark ratio 3.3 × 10 2 . Meanwhile, the response time of the nanowire arrays photodetector was 10 times faster than that of the nanowire networks photodetector prepared by the same method. Our work provided inspiration for the manufacture of high-performance solar-blind photodetectors. • ZnGa 2 O 4 nanowire bridge arrays were grown by CVD method and used to fabricate a MSM solar-blind UV photodetectors. • The growth method is fast, low cost and capable of patterned growth. • The ZnGa 2 O 4 nanowire bridge array photodetector has a larger photocurrent and a faster response speed than the nanowire photodetector grown by the same method.

Excitonic Enhancement and Excited Excitonic States in CsPbBr3 Halide Perovskite Single Crystals

Halide perovskites are novel photonics materials promising numerous applications in fields such as photovoltaics, LED light sources, microlasers, and radiation detectors. Many halide perovskites are direct-gap semiconductors, and Wannier-Mott excitons play a significant role in their optical properties near the fundamental absorption edge. The high oscillator strength of these states favors applications where efficient interaction with light is required. In this work, to study excitonic states in CsPbBr3 halide perovskite single crystals, the reflection spectroscopy at temperatures from 4 K was used. A reflection coefficient up to 70% was observed for the n=1 exciton state, followed by weak excited states of excitons with n=2 and n=3. It should be noted that the Sommerfeld enhancement factor should be considered for a correct description of the behavior of the dielectric constant, taking into account excitonic effects.

Crystal and electronic structure of a quasi-two-dimensional semiconductor Mg3Si2Te6

We report the synthesis and characterization of a Si-based ternary semiconductor Mg3Si2Te6, which exhibits a quasi-two-dimensional structure, where the trigonal Mg2Si2Te6 layers are separated by Mg ions. Ultraviolet-visible absorption spectroscopy and density functional theory calculations were performed to investigate the electronic structure. The experimentally determined direct band gap is 1.39 eV, consistent with the value of the density function theory calculations. Our results reveal that Mg3Si2Te6 is a direct gap semiconductor, which is a potential candidate for near-infrared optoelectronic devices.

MgXN2 (X = Hf/Zr) Monolayers: Auxetic Semiconductor with Highly Anisotropic Optical/Mechanical Properties and Carrier Mobility.

Two-dimensional (2D) semiconducting materials with distinct anisotropic physical properties have attracted intense interests. Herein, we show theoretical predictions that MgXN2 (X = Hf/Zr) monolayers are auxetic semiconductors with highly anisotropic electronic, optical, and mechanical properties. The density functional theory calculations coupled with a PSO algorithm (global-minimum search) suggest that both MgHfN2 (MgZrN2) monolayers exhibit orthorhombic symmetry (Pmma) and are direct-gap (indirect-gap) semiconductors with a bandgap of 2.43 eV (2.13 eV). Specifically, the MgHfN2 monolayer exhibits highly anisotropic hole mobility as well as very high electron mobility (∼104 cm2 V-1 s-1). G0W0+BSE calculations indicate that both monolayers bear notable optical anisotropy and relatively large exitonic binding energy (∼0.6 eV). In addition, both monolayers acquire remarkable mechanical anisotropy with a negative in-plane Poisson's ratio (∼-0.2) and high Young's modulus (∼260 N/m). The combination of highly anisotropic electronic, optical, and mechanical properties endows MgXN2 monolayers as potentially useful parts in multifunctional nanoelectronic devices.

Structural, electronic and optical properties of the wide band gap semiconductors KGaQ2 (Q = S, Se) and of AGaTe2 (A = K, Cs)

In this paper, we studied the structural, electronic and some optical properties of KGaQ2 (Q = S, Se) and AGaTe2 (A = K, Cs) crystals using the pseudopotential plane-wave (PP-PW) method based on density functional theory (DFT), the generalized gradient approximation (GGA) parameterized by Perdew-Burke-Ernzerhof (GGA-PBE) is used for the exchange – correlation (XC) potential. We also use the hybrid density functional (HSE06) to study the electronic structures of these materials. Our results for the equilibrium lattice constants (a, b and c), angle β are in good agreement with experiment data. The electronic structure calculation suggested that crystals are direct-gap semiconductors, employing both the Perdew–Burke–Ernzerhof (PBE) and the hybrid (HSE06) functionals. We note that the hybrid density functional improved the value of band gap, and that the studied compounds are semiconductors with wide band gaps. We have also predicted the optical properties; the refractive index, the reflection coefficient and dielectric constant on high frequencies.