Indirect Band Gap Semiconductor Research Articles

Investigation on elastic, optical and thermoelectric properties of 2D MgX (X= O, S, Se, Te) materials under DFT framework

The first principles study of two-dimensional magnesium (Mg) chalcogenides (X = O, S, Se, Te) in square lattice (s-MgX) and hexagonal phases (h-MgX) is investigated for the first time. Except for h-MgSe and h-MgTe, all the square lattice and hexagonal structures in MgX compounds are dynamically stable, agreeing to phonon dispersion calculations. The electronic band structure and projected density of states of s- and h-MgX materials provided insight into the essence of electronic properties of these compounds. All s- and h-MgX compounds are found to be indirect wide band gap semiconductors, according to calculations using PBE and HSE06 functionals. The effective mass, mobility and relaxation time of electrons and holes carriers from the band structure of s- and h-MgX are examined to acquire a better intuition into these materials. Along the zigzag direction, h-MgTe has a largest mobility as well as relaxation time of 78104.92 cm2 V−1s−1 and 64385.56 fs, respectively in the entire MgX series. Additionally, we examined their mechanical stability through elastic characteristics, and the derived elastic parameters and polar graphs of Young's modulus and Poisson's ratio verifying their mechanical stability. In application of parallel and perpendicular field polarizations, the optical properties of s- and h-MgX are examined. The thermoelectric properties of the complete s- and h-MgX series are examined for the temperature range from 50 K to 800 K. The results of the present study reveal that s-MgO and h-MgS are the better thermoelectric materials in the considered series. Finally, since these compounds are mostly UV-active, they may find useful applications in UV-protectant and UV-photodetectors materials.

Electronic and optical characteristics of Silicane/GeAs van der Waals heterostructures: Effects of external electric field and biaxial strain: A first-principles study

The van der Waals heterostructure offers unusual physical properties to design novel structures for efficient optoelectronic applications. To enhance the low mobility of monolayer SiH and broaden the light absorption coefficient and range of monolayer SiH, a lattice-matched SiH/GeAs vdWh has been constructed. At the same time, the basic electronic and optical properties were investigated by first-principles calculations. The SiH/GeAs vdWh with six different structural configurations were compared, and the most stable configuration model H2 has a type-II band alignment with an indirect bandgap semiconductor value of 1.54 eV. It is associated with a strong charge transfer. Moreover, SiH/GeAs vdWh possesses anisotropic transport properties with high electron mobility along the y-direction. Interestingly, both external electric field and biaxial strain can adjust band offsets and bandgaps values. After applying vertical electric fields, The SiH/GeAs vdWh has a mutual transformation from type-I (symmetric) to type-II(staggered) to type-III(broken) band alignment, and metal-semiconductor phase transitions occurred at −0.35 and 0.15 V/Å. Besides, it's found that band alignment transition from type-II to type-I emerged under the in-plane biaxial strain of 2% and transformed indirect into direct bandgap. More importantly, tensile biaxial strain can enhance the visible-light absorption while sustaining type-II band alignment, which is favorable for photocatalysis of visible light. Our results indicate that the SiH/GeAs vdWh's moderate bandgap, excellent transport properties and optical performance may have potential applications in the field of nanoelectronic and optoelectronic devices.

Reversal of Band-Ordering Leads to High Hole Mobility in Strained p-type Scandium Nitride.

Low hole mobility of nitride semiconductors is a significant impediment to realizing their high-efficiency device applications. Scandium nitride (ScN), an emerging rocksalt indirect band gap semiconductor, suffers from low hole mobility. Utilizing the ab initio Boltzmann transport formalism including spin-orbit coupling, here we show the dominating role of ionized impurity scattering in reducing the hole mobility in ScN thin films. We suggest a route to increase the hole mobility by reversing band ordering through strain engineering. Our calculation shows that the biaxial tensile strain in ScN lifts the split-off hole band above the heavy hole and light hole bands, leading to a lower hole-effective mass and increasing mobility. Along with the impurity scattering, the Fröhlich interaction also plays a vital role in the carrier scattering mechanism due to the polar nature of ScN. Increased hole mobility in ScN will lead to higher efficiencies in thermoelectric, plasmonics, and neuromorphic computing devices.

Investigations on the thermoelectric and thermodynamic properties of quaternary coinage metal HgSBr

We present the findings of a thorough first-principles analysis of physical parameters related to the ground state, elastic, electronic, optical, thermodynamic, and transport properties of the quaternary coinage metal-based compound CuHgSBr using the WIEN2k package. The computed equilibrium lattice parameters align well with their experimental equivalents, providing strong support for the validity of the findings. We performed numerical and computational calculations to estimate the elastic constants for the orthorhombic structure with space group Pbam. The band structure analysis of CuHgSBr reveals an indirect band gap semiconductor of 0.76 eV, classifying it as a p-type semiconductor. We also calculated the optical properties within the energy range of 0–13.56 eV. Moreover, we investigated the effective mass, exciton binding energy, and exciton Bohr radius, which indicated that CuHgSBr exhibits a weak exciton binding energy and belongs to the Mott-Wannier type exciton category. Using the Boltzmann transport theory, along with the constant relaxation time and Slack equations, we determined the thermoelectric properties and lattice thermal conductivity of CuHgSBr. Notably, the figure of merit at 800 K is calculated to be 0.54, which is encouraging for potential thermoelectric applications. The comprehensive research study we conducted provides valuable insights for experimental research across multiple physical properties, as the material is being theoretically examined for the first time in this full prospectus.

The stability and electronic properties of hydrogenated Janus MSiGeZ4 (M = Sc-Zn, Y–Ag, Hf–Au; Z = N, P) monolayers: A first-principles study

The stability and electronic properties of the hydrogenated Janus MSiGeZ4 (M = Sc-Zn, Y–Ag, Hf–Au; Z = N, P) monolayers are predicted by the first-principles calculations. There are 12 energetically stable Janus MSiGeZ4 monolayers were selected from 104 candidates. Accordingly, 24 kinds of the semi/full hydrogenated Janus MSiGeZ4 (H-MSGZ) monolayers are predicted to be stable because of the suitable formation, binding and adsorption energy. Then, the electronic properties of semi-hydrogenated and fully hydrogenated Janus MSiGeZ4 were investigated. Based on the analysis of the band structure of the H-MSGZ monolayers, the forbidden band gaps decrease with the hydrogenation degree. Both the intrinsic and hydrogenated Janus MSiGeZ4 monolayers are indirect band gap semiconductors. Among the 12 Janus MSiGeZ4 monolayers, the intrinsic ZnSiGeN4 has the largest band gap of 2.20 eV. Comparing the hydrogenation processes, the band gap of ZnSiGeN4 decreases most dramatical, which change from 2.20 eV (intrinsic ZnSiGeN4) to 0.57 eV (semi-ZnSiGeN4) to 0 eV (full-ZnSiGeN4). Based on density of states analysis, there are many new doping peaks near the Fermi level after the hydrogenation process, and the positions of the peaks are changed, inducing the band gap decrease. The valence band edges of H-MSGZ monolayers are mainly contributed to the hybrid between TM d and N/P p states, while the conduction band edges are mainly contributed by TM d state. Then, the work functions are calculated and analyzed. In the process of hydrogenation, the greatest change of work function is ScSiGeN4, which changes from 6.67 eV (intrinsic ScSiGeN4) to 6.33 eV (semi-ScSiGeN4) and finally to 2.04 eV (full-ScSiGeN4). Overall, the work functions of Janus MSiGeZ4 showed a gradual decrease after hydrogenation because of the hydrogenation reduces the built-in electric field between the H atom and the MSiGeZ4 monolayer surface. Therefore, the hydrogenation can be effective in changing the properties of the material, and imply that H-MSGZ monolayers should have potential for applications in novel electronics, piezoelectric photovoltaics, and photocatalysts.

Evolutionary Algorithm-Based Crystal Structure Prediction of CuxZnyOz Ternary Oxides.

Binary zinc(II) oxide (ZnO) and copper(II) oxide (CuO) are used in a number of applications, including optoelectronic and semiconductor applications. However, no crystal structures have been reported for ternary Cu-Zn-O oxides. In that context, we investigated the structural characteristics and thermodynamics of CuxZnyOz ternary oxides to map their experimental feasibility. We combined evolutionary crystal structure prediction and quantum chemical methods to investigate potential CuxZnyOz ternary oxides. The USPEX algorithm and density functional theory were used to screen over 4000 crystal structures with different stoichiometries. When comparing compositions with non-magnetic CuI ions, magnetic CuII ions, and mixed CuI-CuII compositions, the magnetic Cu2Zn2O4 system is thermodynamically the most favorable. At ambient pressures, the thermodynamically most favorable ternary crystal structure is still 2.8 kJ/mol per atom higher in Gibbs free energy compared to experimentally known binary phases. The results suggest that thermodynamics of the hypothetical CuxZnyOz ternary oxides should also be evaluated at high pressures. The predicted ternary materials are indirect band gap semiconductors.

First-principles calculation of electronic structure, chemical bonding and optical properties of β-AgBiS2

We investigated the structural, electronic, chemical bonding, and optical properties of β-AgBiS2 crystal by using the Perdew-Burke-Ernzerhof (PBE) functional and the hybrid functional Heyd Scuseria Ernzerhof (HSE) within the DFT formalism. The electronic band structures obtained by both methods indicate that β-AgBiS2 is an indirect band gap semiconductor with band gap of 0.571 and 1.025 eV, respectively. The electron density difference and Mulliken overlap population show that the Ag-S bonds and Bi-S bonds are both ionic bonds. The calculated optical absorption spectrum prove that β-AgBiS2 is a promising material for solar photovoltaic conversion

On the excitonic effects of the 1T and 1O[formula omitted] phases of PdS2, PdSe2, and PdSSe monolayers

We report the excitonic effects in the 1T and 1OT phases of PdS2, PdSe2, and PdSSe monolayers. Using fully relativistic norm-conserving non-local PBE pseudopotentials and including the spin–orbit (SOC) coupling, the 1T geometries are indirect band gap semiconductors; conversely, the 1OT structures are semimetallic. The band gaps were corrected using the HSE06 hybrid functional, making all structures semiconductors. Maximally localized Wannier functions were utilized to fit the bands of all six structures. The excitonic band structures were obtained using the Bethe–Salpeter equation (BSE), and the absorption coefficients were estimated at both the BSE and the independent particle approximation (IPA) levels. While the 1OT monolayers are more inclined to absorb light polarized in the y-direction, the 1T structures absorb light equally, independent of the polarization direction. We perceive that the 1T phase is the most promising candidate for photovoltaic devices; in particular, the 1T PdSSe system possesses a maximum power conversion efficiency (PCE) of 29.31% if light trapping techniques are used to enhance absorbance. Amongst the 1OT materials, PdS2 features the highest PCE of 8.38%. Our results constitute the first step to encourage experimental studies towards the fabrication of Pd-based regular and Janus transition metal dichalcogenides photoabsorbers and give hints of how to increase the PCE of such materials.

The absorption of transition metal atoms in g-C6N6 nanoribbon induces narrow band gap semiconductor with magnetism

The g-C6N6 is a semiconductor material, the effect of this material used in electronic and microelectronic devices is far from reaching the expected effect. To improve the overall performance of g-C6N6, this study decided to cut g-C6N6 for nanoribbons. It was found that g-C6N6 nanoribbons could not be directly used in electronic devices and microelectronic devices through first principles calculation. In order to change its properties, it was decided to adsorb two different transition metal atoms Fe and Mn in the g-C6N6 nanoribbons, respectively. The results show that g-C6N6 nanoribbons have magnetic properties after adsorbing transition metal atoms, and the two structures have spin polarization phenomena after adsorption of Fe and Mn atoms. Their energy band, density of states (DOS) and the charge density after the adsorption of transition metal atoms were analyzed. The results show that g-C6N6 nanoribbons becomes an indirect narrow band gap semiconductor by adsorbing different transition metal atoms, which improves the performance of g-C6N6 nanoribbons. Through computational molecular dynamics simulation, g-C6N6 nanoribbons can exist stably in a real environment. Our research provides theoretical support for the application of such materials in spintronics devices and optoelectronic devices.

Structural phase stability, elastic properties, and electronic structure of pressure-induced CaC2: A first-principles study

This work investigates the structural phase transition of CaC2 material under 0–150 GPa pressure by using first-principles calculation. The enthalpy difference results suggest that the C2/c structure transforms into the Cmcm structure at 0.08 GPa, while the C2/m structure transforms into the Cmcm structure at 0.11 GPa. The Cmcm structure transforms into the Immm structure at 13.70 GPa and the P6/mmm structure at 110.65 GPa. Moreover, the enthalpy values of C2/c, C2/m, I4/mmm, and Cmcm are very close between 0 and 0.11 GPa, indicating that these four structures may coexist in this pressure range. The change of volume with pressure shows that the volume decreases with the increased pressure, especially it decreases sharply at the pressure transition point. In addition, the results show that the elastic modulus of different structures increases linearly with the increase of pressure in their respective pressure stable ranges, but it will increase sharply at the pressure transition point, indicating that CaC2 exhibits better stiffness under high pressure than under low pressure. Finally, the change of the electronic structure of CaC2 with pressure is also analyzed, the results show that C2/c and C2/m are indirect band gap semiconductor structures between 0–0.11 GPa, but the band gap becomes 0 ⅇV at the first pressure transition point 0.11 GPa, and it is always 0 ⅇV in the pressure range of 0.11–150 GPa, which means that it becomes a metal phase. This work will provide a valuable reference for the experiment and application of CaC2 under high pressure.

Prediction of structural, elastic, electronic and thermoelectric properties of the Rb3CuO and Rb3AgO anti-perovskites

The Structural, elastic and electronic and thermoelectric properties of anti-perovskites Rb3AgO and Rb3CuO compounds have been investigated using the full potential linearised augmented plane waves (FP-LAPW) method based on the density functional theory (DFT) implemented in the wien2k code. The analysis of the structural and elastic properties of Rb3AgO and Rb3CuO antiperovskites compounds were performed with mean of the generalized gradient potential approximation developed by Perdew–Burke–Ernzerhof (GGA-PBE) while the electronic and thermoelectric properties have been calculated using the GGA-PBE + TBmBJ approximation. In all calculations the spin-orbit coupling correction is tested which revealed it insignificant effect on the electronic properties and the related properties. These materials are found to be indirect narrow band gap semiconductors and exhibit good thermoelectric performance with a figure of merit ZTe ∼1. On the other hand, investigation of the phonon dispersion and the formation energy indicate the possible synthesize of these materials. In the absence of other theoretical and experimental results for Rb3AgO and Rb3CuO, this work is considered as a first prediction.

Structural stability and physical properties of MAX phases M2SX (M=Sc, Y, X=B, C, N) via first-principles calculations

The structural, mechanical, lattice-dynamic, anisotropic, electronic and thermal properties of M2SX (M=Sc, Y; X=B, C, N) are investigated based on density functional theory. The calculated results indicate that all the phases satisfy the thermodynamic, mechanical and dynamic stability criteria. The mechanical properties are in good agreement with the reported values, and the results show that Sc2SN exhibits the highest bulk modulus B (145.7 GPa), shear modulus (103.0 GPa) and Young’s modulus E (250.0 GPa) with brittle behavior. The elastic anisotropy of M2SX indicates that Sc2SC is the most isotropic among the 6 phases. The electronic structure reveals that Sc2SC and Y2SC are indirect-bandgap semiconductors with 0.927 eV and 1.260 eV bandgap, and the other phases exhibit metallic characteristics. The Debye temperature, lattice thermal conductivity, minimum thermal conductivity, heat capacity and entropy have also been calculated for M2SX phases. The tendency for lattice thermal conductivity in high temperature: K lat (M2SN) > K lat (M2SC) > K lat (M2SB). All the present calculated data will provide useful guidance for development and research on the novel S-based MAX phases in the future.

TiO2‐Based Hybrid Nanoscale Light Source

AbstractThe titanium dioxide nanosphere (TiO2 NS) is a moderate refractive index (MRI) nanostructure which can support Mie resonances such as magnetic dipole resonance (MDR), electric dipole resonance (EDR), etc. However, TiO2 is a wide indirect band gap (3.2 eV) semiconductor, and TiO2 NSs cannot be excited to emit luminescence by near‐infrared (NIR, 770 nm–800 nm) femtosecond (fs) lasers. In this paper, a MDR‐induced nanoscale plasmon source is conceived to provide a large number of hot electrons, which can be used to light up TiO2 NSs. The effect of different metals (Titanium‐Ti, Palladium‐Pd, Gold‐Au) as the MDR‐induced nanoscale plasmon sources on the optical response of TiO2‐based hybrid NSs is studied. This work provides a new strategy for the application of TiO2‐based hybrid NSs as integrated efficient nanoscale white light sources.

Idealizing Tauc Plot for Accurate Bandgap Determination of Semiconductor with Ultraviolet-Visible Spectroscopy: A Case Study for Cubic Boron Arsenide.

The Tauc plot is widely used to determine the bandgap of semiconductors, but the actual plot often exhibits significant baseline absorption below the expected bandgap, leading to bandgap discrepancies from two different extrapolations. In this work, we first discuss the origin of baseline absorption and show that both extrapolation methods can produce significant errors by simulating Tauc plots with varying levels of baseline absorption. We then propose and experimentally verify a new method that idealizes the absorption spectrum by removing its baseline before constructing the Tauc plot. Finally, we apply this new method to cubic boron arsenide (c-BAs), resolve its bandgap discrepancies, and obtain a converging bandgap of 1.835 eV based on both previous and new transmission spectra. The method is applicable to both indirect and direct bandgap semiconductors with absorption spectrum measured via transmission or diffuse reflectance, which will become essential to obtain accurate values of their bandgaps.

Highly efficient and stable Ra2LaNbO6 double perovskite for energy conversion device applications

Using first-principles calculations, in this piece of work, authors have investigated the physical properties of Ra2LaNbO6 double perovskite by employing the linearized augmented plane wave (LAPW) method. Structural and electronic properties are determined by using LDA, GGA (WC and PBE), LDA + mBJ, and GGA + mBJ potentials. We have found that Ra2LaNbO6 is an indirect band gap (Eg = 2.4 eV) semiconductor. Its elastic and thermodynamic parameters demonstrate its stability. Its optical study indicates that this material opens the door to its applications in optical devices such as photodetectors, solar cells, superlenses, optical fibers, filters, electromagnetic shielding devices, photovoltaic devices, etc. This material is very good for its practical implementation in thermoelectric devices as both p- and n-type material and extends the interest of experimentalists for further investigations. Thus, Ra2LaNbO6 is found thermodynamically stable and identified as a potential candidate for photovoltaic and thermoelectric devices.

Strain Modulation of Electronic Properties in Monolayer SnP2S6 and GeP2S6

In recent years, two-dimensional (2D) materials have attracted significant attention due to their distinctive properties, including exceptional mechanical flexibility and tunable electronic properties. Via the first-principles calculation, we investigate the effect of strain on the electronic properties of monolayer SnP2S6 and GeP2S6. We find that monolayer SnP2S6 is an indirect bandgap semiconductor, while monolayer GeP2S6 is a direct bandgap semiconductor. Notably, under uniform biaxial strains, SnP2S6 undergoes an indirect-to-direct bandgap transition at 4.0% biaxial compressive strains, while GeP2S6 exhibits a direct-to-indirect transition at 2.0% biaxial tensile strain. The changes in the conduction band edge can be attributed to the high-symmetry point Γ being more sensitive to strain than K. Thus, the relocation of the conduction band and valence band edges in monolayer SnP2S6 and GeP2S6 induces a direct-to-indirect and indirect-to-direct bandgap transition, respectively. Consequently, the strain is an effective band engineering scheme which is crucial for the design and development of next-generation nanoelectronic and optoelectronic devices.

C2O Nanotubes with Negative Strain Energies and Improvements of Thermoelectric Properties via N-Doping Predicted from First-Principle Calculations

In this study, the geometric structure, strain energy, and electronic properties of armchair C2O nanotubes (A_C2ONTs(n,n)) and zigzag C2O nanotubes (Z_C2ONTs(n,0)) are studied in detail. The results show that these nanotubes behave as special shapes; the section of Z_C2ONTs(n,0) along the axial direction is an n-edge type, and the section tends to be round with an increase in n while the boundary of section along the axial direction for A_C2ONTs(n,n) fluctuates. With an increase in n, the fluctuation disappears gradually, and the section also tends to be round. Compared with C2O nanosheets (g_C2O), C2O nanotubes have similar or even smaller strain energy, when the tube diameter is greater than or equal to 15 Å, the strain energy begins to show a negative value, and the negative value tends to be stable as the pipe diameter increases. Z_C2ONTs start to show negative strain energies from n = 8 and A_C2ONTs from n = 12, indicating their higher stabilities relative to the g_C2O sheets. The calculation of the electronic band structure shows that Z_C2ONTs are an indirect band gap semiconductor, and A_C2ONT is a direct band gap semiconductor. The band gap value of Z_C2ONTs first increases and then stabilizes with the diameters of the nanotubes, and the band gap value of A_C2ONTs decreases with the increase in the nanotubes and tends to the band gap value of the g_C2O sheet. In addition, the electronic properties and thermoelectric properties of C2ONTs(n=4,6,8) before and after N-doping were also studied. We found that N-doping changed the electronic and thermoelectric properties of C2ONTs. It reduced the nanotube band gap value and significantly improved the thermoelectric figures of merit of Z_C2ONTs(n=4,6,8) and A_C2ONT(4,4), which comprised an effective strategy to improve the thermoelectric figure of merit of nanotubes. The results showed that the C2ONTs had potential as thermoelectric materials after N-doping, which provided important guidance for designing low-dimensional g_C2O nanostructures.

Ab-initio study about the electronic structure, optical, and transport properties of novel AIn2O4 (A = Ca, Sr, and Na) materials

Here, using first-principles calculations within the framework of density functional theory, we reported results relating to the structural stability, electronic, optical, and thermoelectric properties of AIn2O4 (A = Ca, Sr, and Na) spinel oxides. Among the three materials CaIn2O4, and NaIn2O4 have a direct bandgap semiconductor nature, whereas the SrIn2O4 shows an indirect bandgap semiconductor nature. These materials’ broad energy bandgaps reveal that the bonds present are strongly covalent in nature. The results of the band structures are also strongly supported by the calculated density of states for the three materials, which also validates their semiconducting nature. Our calculated density of states plots shows an overall similarity trend, indicating that the top of the valence bands in the CaIn2O4 materials originate primarily from the p-states and for SrIn2O4, and NaIn2O4 are due to the s-states of the oxygen anions. Additionally, the linear optical constants like the complex dielectric function, the refractive index, the electron energy loss function, the absorption coefficient, and the reflectivity spectra of these novel spinel oxides are computed and examined in detail for their possible applications in optoelectronic devices. The thermoelectric transport parameters were also calculated, and the findings obtained are presented in depth, indicating that these materials are suitable for thermoelectric device applications. Essentially, the present effort must assist the progress of discrete and integrated semiconductor device applications.

Structural, electronic and optical properties of Li3BN2 under pressure: a first-principles calculation

In this article, by using the first-principles calculations based on density functional theory (DFT), we investigate the structural, electronic and optical properties of α-Li3BN2 under pressure up to 10 GPa. The obtained results indicate that the value of V/V0 basically decreases linearly with the increase of pressure. By increasing the pressure, α-Li3BN2 remains an indirect band gap semi-conductor. Although, the band gap of the α-Li3BN2 increases with pressure. Besides, the optical spectra exhibit anisotropy in the x and z directions and a small blue shift under pressure.

High-throughput calculation screening for new silicon allotropes with monoclinic symmetry.

A total of 87 new monoclinic silicon allotropes are systematically scanned by a random strategy combined with group and graph theory and high-throughput calculations. The new allotropes include 13 with a direct or quasi-direct band gap and 12 with metallic characteristics, and the rest are indirect band gap semiconductors. More than 30 of these novel monoclinic Si allotropes show bulk moduli greater than or equal to 80 GPa, and three of them show even greater bulk moduli than diamond Si. Only two of the new Si allotropes show a greater shear modulus than diamond Si. The crystal structures, stability (elastic constants, phonon spectra), mechanical properties, electronic properties, effective carrier masses and optical properties of all 87 Si monoclinic allotropes are studied in detail. The electron effective masses ml of five of the new allotropes are smaller than that of diamond Si. All of these novel monoclinic Si allotropes show strong absorption in the visible spectral region. Taken together with their electronic band gap structures, this makes them promising materials for photovoltaic applications. These investigations greatly enrich the current knowledge of the structure and electronic properties of silicon allotropes.