Band Gap Semiconductor Research Articles

Metal-Oxide Heterojunction Optoelectronic Synapse and Multilevel Memory Devices Enabled by Broad Spectral Photocarrier Modulation.

Broad spectral response and high photoelectric conversion efficiency are key milestones for realizing multifunctional, low-power optoelectronic devices such as artificial synapse and reconfigurable memory devices. Nevertheless, the wide bandgap and narrow spectral response of metal-oxide semiconductors are problematic for efficient metal-oxide optoelectronic devices such as photonic synapse and optical memory devices. Here, a simple titania (TiO2 )/indium-gallium-zinc-oxide (IGZO) heterojunction structure is proposed for efficient multifunctional optoelectronic devices, enabling widen spectral response range and high photoresponsivity. By overlaying a TiO2 film on IGZO, the light absorption range extends to red light, along with enhanced photoresponsivity in the full visible light region. By implementing the TiO2 /IGZO heterojunction structure, various synaptic behaviors are successfully emulated such as short-term memory/long-term memory and paired pulse facilitation. Also, the TiO2 /IGZO synaptic transistor exhibits a recognition rate up to 90.3% in recognizing handwritten digit images. Moreover, by regulating the photocarrier dynamics and retention behavior using gate-bias modulation, a reconfigurable multilevel (≥8 states) memory is demonstrated using visible light.

Improvement of the electronic, mechanical and optical properties of cubic As-doped BN alloy for energy harvesting applications

The electronic structure, elastic and optical properties with wide energy band gap (Eg) in mixed crystal semiconductors containing As-doped BN are reported. Density functional calculations (DFT) were performed for comprehensive detection and understanding of structural, optoelectronic, and elastic properties for mixed semiconductors in the cubic phase. When applying Vegard’s law for As, BN, and mixed alloys based on BN1−xAsx, where x = 0, 0.25, 0.5, 0.75, and 1, we observed that the distortion of the lattice constants was slight to different condensations. Calculations of Eg values found that the values decreased with an increase in (x) condensation. The bulk modulus, and other elastic parameters were computed and confirmed that the alloys are mechanically stable. The calculations of dielectric Ɛ(ω), refractive index n(ω), higher absorption α(ω) peaks, and other coefficients confirm that BN1−xAsx alloys are favourable for practical photovoltaics. The results confirm that mixed semiconductors have great interest in next-generation optoelectronic devices. This work provides a route for optoelectronic applications.

Defects in WS2 monolayer calculated with a nonlocal functional: any difference from GGA?

Density functional theory (DFT) with generalised gradient approximation (GGA) functionals is commonly used to predict defect properties in 2D transition metal dichalcogenides (TMDs). Since GGA functionals often underestimate band gaps of semiconductors and incorrectly describe the character of electron localisation in defects and their level positions within the band gap, it is important to assess the accuracy of these predictions. To this end, we used the non-local density functional Perdew—Burke—Ernzerhof (PBE)0-TC-LRC to calculate the properties of a wide range of intrinsic defects in monolayer WS2. The properties, such as geometry, in-gap states, charge transition levels, electronic structure and the electron/hole localisation of the lowest formation energy defects are discussed in detail. They are broadly similar to those predicted by the GGA PBE functional, but exhibit numerous quantitative differences caused by the degree of electron and hole localisation in charged states. For some anti-site defects, more significant differences are seen, with both changes in defect geometries (differences of up to 0.5 Å) as well as defect level positions within the band gap of WS2. This work provides an insight into the performance of functionals chosen for future DFT calculations of TMDs with respect to the desired defect properties.

Ohmic Behavior in Metal Contacts to n/p-Type Transition-Metal Dichalcogenides: Schottky versus Tunneling Barrier Trade-off

High contact resistance (RC) between 3D metallic conductors and single-layer 2D semiconductors poses major challenges toward their integration in nanoscale electronic devices. While in experiments the large RC values can be partly due to defects, ab initio simulations suggest that, even in defect-free structures, the interaction between metal and semiconductor orbitals can induce gap states that pin the Fermi level in the semiconductor band gap, increase the Schottky barrier height (SBH), and thus degrade the contact resistance. In this paper, we investigate, by using an in-house-developed ab initio transport methodology that combines density functional theory and nonequilibrium Green’s function (NEGF) transport calculations, the physical properties and electrical resistance of several options for n-type top metal contacts to monolayer MoS2, even in the presence of buffer layers, and for p-type contacts to monolayer WSe2. The delicate interplay between the SBH and tunneling barrier thickness is quantitatively analyzed, confirming the excellent properties of the Bi–MoS2 system as an n-type ohmic contact. Moreover, simulation results supported by literature experiments suggest that the Au–WSe2 system is a promising candidate for p-type ohmic contacts. Finally, our analysis also reveals that a small modulation of a few angstroms of the distance between the (semi)metal and the transition-metal dichalcogenide (TMD) leads to large variations of RC. This could help to explain the scattering of RC values experimentally reported in the literature because different metal deposition techniques can result in small changes of the metal-to-TMD distance besides affecting the density of possible defects.

Thermodynamic limits and performance optimization of nighttime thermoradiative energy conversion systems with non-idealities

To address the challenge of the growing global energy crisis and the greenhouse effect, employing the thermoradiative effect of photodiodes to harvest radiative energy from colder outer space is considered a promising approach for direct nighttime renewable energy generation. By considering the coupled effect of the atmosphere, ambient, and non-radiative losses, a thermodynamic model of nighttime thermoradiative diodes (NTRDs) is developed. The fundamental performance limit for the achievable electric power density and device optimal designs of NTRDs are theoretically investigated based on Shockley-Queisser analysis. The modeling predicts that an ideal (nonideal) optimized NTRD can generate a maximum power density of 1.5 (0.12) mW/cm2 when operated at room temperature with an external luminescent efficiency of 100% (10%). The calculated results indicate that the maximal electric power generation of NTRDs is strongly influenced by the semiconductor bandgap and the external luminescent efficiency of the diode, as well as the ambient temperature and humidity. This work offers insights for optimal device designs of NTRDs, thus paving the way toward designing high-performance nighttime thermoradiative power generation systems.

Density-functional quantum computations on bandgap engineering and tuning of optoelectronic properties of MgH2 via Mo doping: Prospects and potential for clean energy hydrogen-storage fuel and optoelectronic applications

With the increasing depletion of conventional energy sources and their detrimental environmental hazards, it is imperative to search for sustainable alternative clean energy sources. In the recent decades, hydrogen has emerged as potential source of clean energy. One of the potential alternatives to achieve the objective is the designing and characterization of materials for hydrogen-storage energy applications. In this regard, metal-bearing hydrides are the most promising candidates. For instance, magnesium-bearing hydrides are the focus of current research work owing to high hydrogen capacity of 7.6 wt%. In this paper, we first time report density functional-based quantum theoretical analysis to explore the potential of Mo-doped magnesium hydrides MgH2:Mo for optoelectronic and hydrogen-storage applications. For the quantum computations of the required optoelectronics and energy storage properties, we employed all-electron methods within generalized gradient approximation (GGA). Besides applying GGA approximation to account for the electronic correlated effects, we employed the Hubbard potential U (= 4 eV) for onsite repulsive Coulomb force. We predict that 10% doping by weight of Mo into MgH2 suppresses its insulating band gap of 4.9 eV to semiconducting band gap of order 3.15 eV for spin up and 0.15 eV for spin down. As such the doping of Mo can tune the the bandgap, structural, electronic and optoelectronic properties of MgH2 considerably for potential applications.

Singlet Fission in Perylene Monoimide Single Crystals and Polycrystalline Films

Singlet fission (SF) is a spin-allowed process in which a photogenerated singlet exciton down-converts into two triplet excitons. Perylene-3,4-dicarboximide (PMI) has singlet and triplet state energies of 2.4 and 1.1 eV, respectively; thus making SF slightly exoergic and providing triplet excitons that have sufficient energy to raise the efficiency of single-junction solar cells by reducing thermalization losses from hot excitons formed when absorbed photons have energies higher than the semiconductor bandgap. However, PMI SF in the solid state has not been studied previously. Here, we show that 2,5-diphenyl-N-(2-ethylhexyl)perylene-3,4-dicarboximide (dp-PMI) crystallizes into a slip-stacked intermolecular morphology favorable for SF. Transient absorption microscopy and spectroscopy show that dp-PMI SF occurs in ≤50 ps in both single crystals and polycrystalline thin films with a triplet yield of 150 ± 20%. Ultrafast SF in the solid state, the high triplet yield, and its photostability make dp-PMI an attractive candidate for SF-enhanced solar cells.

Trace Cu+-dominated band structure engineering in CuxIn0.25ZnSy for promoting photocatalytic H2 evolution

As an attractive semiconductor photocatalyst, (CuInS2)x-(ZnS)y has been intensively studied in photocatalysis, due to its unique layered structure and stability. Here, we synthesized a series of CuxIn0.25ZnSy photocatalysts with different trace Cu+-dominated ratios. The results show that doping with Cu+ ions leads to an increase in the valence state of In and the formation of a distorted S structure, simultaneously inducing a decrease in the semiconductor bandgap. When the doping amount of Cu+ ions is 0.04 atomic ratio to Zn, the optimized Cu0.04In0.25ZnSy photocatalyst with a bandgap of 2.16 eV shows the highest catalytic hydrogen evolution activity (191.4 μmol.h−1). Subsequently, among the common cocatalysts, Rh loaded Cu0.04In0.25ZnSy gives the highest activity of 1189.8 μmol·h−1, corresponding to an apparent quantum efficiency of 49.11 % at 420 nm. Moreover, the internal mechanism of photogenerated carrier transfer between semiconductors and different cocatalysts is analyzed by the band bending phenomenon.

Self-screening corrections beyond the random-phase approximation: Applications to band gaps of semiconductors

The self-screening error in the random-phase approximation (RPA) and the $GW$ approximation (GWA) is a well-known issue and has received attention in recent years with several methods for a correction being proposed. We here apply two of these, a self-screening and a so-called "self-polarization" correction scheme, to model calculations to examine their applicability. We also apply an explicit self-screening correction to \textit{ab-initio} calculations of real materials. We find indications for the self-polarization scheme to be the more appropriate choice of correction for localized states, and additionally we observe that it suffers from causality violations in the strongly correlated regime. The self-screening correction used in this work on the other hand significantly improves the description in more delocalized states. It provides a notable reduction in the remaining GWA error when calculating the band gaps of several semiconductors, indicating a physical explanation for a part of the remaining discrepancy in one-shot $GW$ compared to experiment, while leaving the localized semicore $d$ states mostly unaffected.

Prediction of electronic structural properties of semi-Heusler CoHfZ (Z: Sb, Sn)

Co-based half-Heusler alloys CoHfZ (Z: Sb, Sn) are investigated under density functional formalism by using the Quantum Espresso and BoltzTrap packages. Unit cell relaxation and ground state energy optimization are carried out for different possible phases considered for MgAgAs-type crystal structure for these alloys. The equilibrium lattice parameter is obtained and then the density of states and band structural calculations are performed. An indirect semiconducting band gap is found for both the spin-up and spin-down charge carriers with a gap of 1.12 eV and 0.66 eV for CoHfSb and CoHfSn, respectively. For CoHfSn alloy, a few bands are found to just cross the Fermi level suggesting the p-type semiconducting behaviour for this alloy. A maximum Seebeck coefficient of 110 μV/K is estimated for CoHfSn half-Heusler alloy.

Cesium-metalloid halide perovskites MBX3 (M1+= Cs; B2+= Si, Ge, Sn, Pb; X–= Cl, Br, I) as semiconductor photovoltaic materials for sustainable renewable-energy applications

With the dawning of 21st century, governments faced three urgent challenges, global economic crisis, energy crisis and global warming. So, the research goals have directed on developing novel renewable-energy technologies as suitable alternative sources of the traditional energy that addresses these problems. Photovoltaic based solar cells technology gives sustainable solutions and depends on inorganic materials with specific properties. Among this family, halide perovskites (MBX3) have been investigated during the last five years. Besides studying their unique properties as flexible structures, high stability, tunable semiconductor band-gap (Eg 2.50 eV), high charge-carrier mobility and large optical absorption, research also seek for promising and multifaceted electroptical applications that give an amazing power efficiency (~24.0 %) in photovoltaic technology. The current challenge is to synthesis MBX3 materials provide suitable properties, include notable chemical stability at high temperatures, high electrical power efficiency, broad emission and tunable semiconducting Eg. Motivated by the site substitution effect, we extended this concept to build a series of cesium-metalloid MBX3 (M1+ = Cs; B2+ = Si, Ge, Sn, Pb; X– = Cl, Br, I), and investigate their structural, stability and optoelectronic properties. We expect these investigations will provide inspiration for an innovation of such MBX3 materials in photovoltaic applications.

Investigation and modeling of trap states in ambipolar organic field-effect transistor

This work presents a compact analytical model for charge carrier transport in an ambipolar OFET (a-OFET) based on trap-limited conduction (TLC). The model accounts for tail traps in the band gap of the organic semiconductor (OSC) in order to analyse the transistor behaviour in above threshold regime. For the sub-threshold operation of ambipolar OFET, both deep and interface trap states are accounted for, thus accurately describing the transistor behaviour comprising of drift and diffusion elements respectively. A detailed description of surface potential and field-effect mobility for both the electron and hole carriers, in consequence to these trap states has been presented. Individual current components, obtained as a result of modeling tail, deep and interface trap states are combined using the hyperbolic tangent transition function and are in good agreement with the available experimental data. • This work presents modeling of ambipolar OFET using trap-limited conduction (TLC) approach. • Presented work includes modeling of ambipolar current in both above and below threshold regime, by considering tail, deep and interface acceptor and donor-like states in organic semiconductor. • Subthreshold modeling includes both drift and diffusion components. • Dependence of the mobility of electrons and holes on the characteristic temperatures of the distributions of both tail and deep acceptor and donor-like states has been studied. • The model accurately describes the transfer characteristics of ambipolar OFET.

High-low Kelvin probe force spectroscopy for measuring the interface state density.

The recently proposed high-low Kelvin probe force microscopy (KPFM) enables evaluation of the effects of semiconductor interface states with high spatial resolution using high and low AC bias frequencies compared with the cutoff frequency of the carrier transfer between the interface and bulk states. Information on the energy spectrum of the interface state density is important for actual semiconductor device evaluation, and there is a need to develop a method for obtaining such physical quantities. Here, we propose high-low Kelvin probe force spectroscopy (high-low KPFS), an electrostatic force spectroscopy method using high- and low-frequency AC bias voltages to measure the interface state density inside semiconductors. We derive an analytical expression for the electrostatic forces between a tip and a semiconductor sample in the accumulation, depletion, and inversion regions, taking into account the charge transfer between the bulk and interface states in semiconductors. We show that the analysis of electrostatic forces in the depletion region at high- and low-frequency AC bias voltages provides information about the interface state density in the semiconductor bandgap. As a preliminary experiment, high-low KPFS measurements were performed on ion-implanted silicon surfaces to confirm the dependence of the electrostatic force on the frequency of the AC bias voltage and obtain the interface state density.

Investigation of Aluminum Arsenide Honeycomb Monolayer via Density Functional Theory

First principle calculations are performed to theoretically predict the physical properties of hexagonal aluminium arsenide planar and buckled monolayers. The structural characteristics showed that the buckled parameter is about 0.32 A°. Cohesive energies have favourable values and it indicates the fabrication possibility. Phonon dispersion properties indicated that the planar aluminium arsenic monolayers are dynamically unstable, while the buckled is less dynamically unstable. The elastic constant parameters achieved the required characteristics of stable hexagonal monolayer structures. The study of electronic band structure prefers to indirect semiconductor band gaps, and the density of states showed strong orbital hybridization in the conduction band. Planar structure has isotropic light electron effective mass and anisotropic heavy hole effective mass. The buckled structure has isotropic light electron effective mass and isotropic heavy hole effective mass. The absorption spectra have high absorption coefficient in various visible and ultraviolet wavelength. The absorption coefficient levels off at about direct and indirect band gaps.

Overcoming energy mismatch of metal oxide semiconductor catalysts for CO2 reduction with triboelectric plasma

The activities of semiconductor-based catalytic reactions are limited by mismatch between energy bandgaps of semiconductors and redox potentials of inert small molecules, especially for stable molecules, such as CO2. Herein, a triboelectric plasma catalytic system using metal oxide catalysts has been developed, which reduces CO2 to CO at room temperature and atmospheric pressure. Among the various metal oxide semiconductors, TiO2 catalyst exhibited the best activity, reaching a production rate of 0.14mmol·gcat−1·h−1, and an energy efficiency of 5.3 % for the conversion of electrical to chemical energy. In triboelectric plasma, CO2 molecules were pre-activated to form the transient CO2− anions, and the limiting step of electron transfer between TiO2 and CO2 was avoided, overcoming the energy mismatch between catalyst and CO2. The energy barrier of CO2 dissociation was markedly reduced to 0.18 eV. This work provides an effective strategy to overcome the energy bandgap restriction inherent in semiconductor-based catalytic reactions.

The structural, magnetic, and pressure-induced elastic predictions of ZrPd2O4 oxide spinel via GGA, GGA+mBJ, and GGA+U approximations

According to the electronic, structural and magnetic results, ZrPd2O4 oxide spinel was obtained as a magnetic semiconductor material with both up and down spin characters. The different calculation approximations applied have affected the semiconductor band gaps. The largest difference in band gaps was obtained when using the GGA + mBJ approach. The U values applied according to the Coulomb interaction parameters increased the band gaps and equilibrium lattice parameters. While the equilibrium lattice parameter was obtained as 9.16 Å in the GGA approach, the lattice parameter values for U = 1, 2, 3, and 4 eV values were obtained as 9.17 Å, 9.18 Å, 9.19 Å and 9.20 Å, respectively. The energetically most stable state was obtained in the GGA approach. The magnetic moment value of the ZrPd2O4 spinel was calculated as 8.00 µB/cell. The magnetic moment values at the equilibrium lattice point of the Pd, Zr, and O atoms were obtained as 1.301 µB/f.u, 0.0754 µB/f.u. and 0.273 µB/f.u, respectively. ZrPd2O4 oxide spinel is resistant to mechanical deformations according to its elastic properties. The Debye temperature was obtained as 417 K at 0 GPa.

Intense blue emission from one-pot synthesized quaternary CsZnxPb1-xBr3 perovskite quantum dots

Suitable ions incorporation into CsPbX3 perovskite nanocrystals can improve the luminescence efficiency, chemical stability, and more importantly modify the emission bands from semiconductor band gap change, for various applications. In this research, high content Zn2+ was introduced into CsPbBr3 quantum dots through a simple one-pot synthesis method. A new intense blue emission centered at 440 nm was observed from heavy Zn alloyed nanocrystals, for increased semiconductor band gap, along with green emission at 510 nm from CsPbBr3 quantum dots, when the ratio of ZnBr2/PbBr2 precursor was larger than unity. The appearance of Zn 3d peaks and the decreasing of bonding energy of Pb 4f peaks from X-ray photoelectron spectroscopy spectra indicated that Zn2+ ions were incorporated into CsPbBr3 quantum dots to form CsZnxPb1-xBr3 alloyed quantum dots. The elements quantitative analysis showed that partial Zn2+ ions in precursors were involved in the synthesis of the quantum dots, and the Zn alloyed perovskite nanocrystals could be described as CsPbBr3, CsZn0.43Pb0.57Br3, CsZn0.54Pb0.46Br3, CsZn0.68Pb0.32Br3 respectively when the ZnBr2/PbBr2 precursor ratios were 0:1, 1:1, 3:1 and 5:1. The X-ray diffraction patterns confirmed that the incorporation of small radius Zn2+ ions into crystals led to crystal shrinkage, instead of crystal phase change. The transmission electron microscope images of the nanocrystals showed cubic geometries and reduced average sizes from 23.3 nm to 17.6 nm with Zn2+ content increase, resulting in the blue shift of emission bands. The energy dispersive spectroscopy elemental mapping of nanocrystals also confirmed the incorporation of Zn2+ ions into the nanocrystals.

Machine learning-aided band gap prediction of semiconductors with low concentration doping.

The important physical quantities of materials, such as band gap, have been predicted efficiently with the help of machine learning (ML), in addition to the widely used experimental or computational methods. Under this scheme, by combining density functional theory (DFT) calculations and ML predictive models, the band gaps of doped semiconductors with normal doping concentrations are predicted successfully. Our present work provides a solution to the problem of how to obtain the band gaps of semiconductors doped with extremely low concentrations, which are important cases for some device designs. The structures were constructed by configuration screening with a symmetric criterion, and three-dimensional spatial structural variation was mapped to one-dimensional features, which are the key steps for the ML predictive model. The biggest error in the predicted band gaps of dilute nitride-doped GaAs by ML models is not more than 10%, compared with values obtained from DFT. Considering the limitation of material data, a few-shot learning method was further adopted to check the performance of the predictive models. The performance of the ML models was validated using data out of training and testing datasets. Our method will efficiently accelerate the prediction of physical properties of semiconductors with extremely low-concentration doping.

Research progress of bulk photovoltaic effect in two-dimensional materials

The bulk photovoltaic effect is a second-order nonlinear photoelectric response, which refers to a phenomenon that non-centrosymmetric structural material generates a steady-state photocurrent under uniform light irradiation. The bulk photovoltaic effect has attracted widespread attention due to its open-circuit voltage is not limited by the semiconductor bandgap and power conversion efficiency breaks through the Shockley-Queisser limit. In addition, the bulk photovoltaic effect is closely related to the quantum geometric properties (such as Berry curvature and quantum metric) of solids, thus making it an effective means to study crystal polarization, orbital magnetization, and quantum Hall effects. Two-dimensional (2D) materials are rich in electrical, optical, magnetic, topological properties and their interactions, which can effectively improve the performances of bulk photovoltaic devices (such as expanding response range of bulk photovoltaic effect) and have important research value for exploring basic physical problems. This paper reviews the development process of bulk photovoltaic effect and its physical mechanism. The research progress of bulk photovoltaic effect in 2D materials is discussed in detail, including single component 2D materials, 2D material stacking engineering (such as 2D material homojunctions and heterojunctions), and other factors (such as magnetic field, strain engineering) to generate or regulate the bulk photovoltaic effect response. Finally, the development prospect of two-dimensional bulk photovoltaic effect is prospected.

EPR spin trapping of nucleophilic and radical reactions at colloidal metal chalcogenide quantum dot surfaces.

The participation of the surfaces of colloidal semiconductor nanocrystal quantum dots (QDs) in QD-mediated photocatalytic reactions is an important factor that distinguishes QDs from other photosensitizers (e.g. transition metal complexes or organic dyes). Here, we probe nucleophilic and radical reactivity of surface sulfides and selenides of metal chalcogenide (CdSe, CdS, ZnSe, and PbS) QDs using chemical reactions and NMR spectroscopy. Additionally, the high sensitivity of EPR spectroscopy is adapted to study these surface-centered reactions through the use of spin traps like 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) under photoexcitation and thermal conditions. We demonstrate that DMPO likely adds to CdSe QD surfaces under thermal conditions by a nucleophilic mechanism in which the surface chalcogenides add to the double bond, followed by further oxidation of the surface-bound product. In contrast, CdS QDs more readily form surface sulfur-centered radicals that can perform reactions including alkene isomerization. These results indicate that QD surfaces should be an important consideration for the design of photocatalysis beyond simply tuning QD semiconductor band gaps.