Band-filling Effect Research Articles

First-principles analysis of improved thermodynamic stability and mechanical properties in pseudo-binary Y[formula omitted]V[formula omitted]B2 alloys

This work explores a possibility of improving the mechanical behavior and thermodynamic stability of AlB2 -type YB2 through alloying with isostructural VB2 using first-principles calculations. The analysis derived from the cluster-expansion model suggests Y0.5V0.5B2, whose atomic configuration is represented by periodically alternating YB2/VB2 layers in the ¡0001¿ direction, is the only solution in the pseudo-binary YB2–VB2 system predicted to be stable from the thermodynamic viewpoint. By evaluating the influence of lattice dynamics on the Gibbs free energies of superlattice-structured Y0.5V0.5B2 and its constituent compounds within the quasiharmonic approximation, the thermodynamic stability, elastic properties, and hardness at a given pressure and temperature of the three diborides can be accessed. The results reveal, at a given temperature, isotropic compression of the diborides to high pressures enhances the stability of Y0.5V0.5B2 measured relative to YB2 and VB2, while raising the temperature at a given applied pressure can increasingly result in a driving force toward separation of Y0.5V0.5B2 into YB2 and VB2. The thermodynamic stabilization of superlattice-structured Y0.5V0.5B2, despite large distinctions in atomic radius and electronegativity between Y and V, can be explained in terms of band filling induced by introduction of V atoms in YB2. Within the range of temperatures (0–1200 K) and pressures (0–15 GPa) studied, the band-filling effect is found to result in significant positive deviations in the values of shear strength, stiffness, and hardness of superlattice-structured Y0.5V0.5B2 from those evaluated from its constituent compounds using the Vegard’s law, respectively, by ∼8%, ∼5%, and ∼25%, and the hardness of superlattice-structured Y0.5V0.5B2 is ∼40 GPa potentially indicating its superhard nature. These consequences strongly underline the essential impact of band filling on improvement in mechanical behavior and stability of YB2 through alloying with VB2, and also they can be served as guidance for further advancement of hard-coating technology based especially on transition-metal diborides.

Size-dependent optoelectronic characteristics of InGaN/GaN red micro-LEDs on 4-inch Si substrates: high pixel density arrays demonstration.

In this study, we explored the size-dependent optoelectronic characteristics of InGaN/GaN red micro-LEDs grown on Si substrates. We successfully demonstrated the fabrication of 4-inch wafer-scale InGaN/GaN micro-LEDs, showcasing the feasibility of large-scale production. Additionally, we presented the binary pixel display with 6 µm pitch, achieving a resolution of 4232 PPI. We also introduced a method of driving the display using PWM to linearly control and counteract the blue shift in peak wavelength caused by the QCSE and band-filling effect. Our research represents a significant milestone in the development of InGaN/GaN red micro-LEDs on Si substrates, establishing them as a key component for full-color micro-LED displays.

Ultrafast Q-boosting in semiconductor metasurfaces

Abstract All-optical tunability of semiconductor metasurfaces offers unique opportunities for novel time-varying effects, including frequency conversion and light trapping. However, the all-optical processes often induce optical absorption that fundamentally limits the possible dynamic increase of their quality factor (Q-boosting). Here, we propose and numerically demonstrate the concept of large Q-boosting in a single-material metasurface by dynamically reducing its structural anisotropy on a femtosecond timescale. This balance is achieved by excitation with a structured pump and takes advantage of the band-filling effect in a GaAs direct-bandgap semiconductor to eliminate the free-carrier-induced loss. We show that this approach allows a dynamic boosting of the resonance quality factor over orders of magnitude, only limited by the free-carrier relaxation processes. The proposed approach offers complete dynamic control over the resonance bandwidth and opens applications in frequency conversion and light trapping.

ThCr2Si2C: An Antiferromagnetic Metal with a Cr2C Square Lattice.

A quaternary compound, ThCr2Si2C, was synthesized by using the arc-melting technique. The compound adopts a tetragonal CeCr2Si2C-type crystal structure. The electronic resistivity and specific heat data exhibit metallic behavior, while the magnetic susceptibility displays a pronounced broad peak at around 370 K, indicating the antiferromagnetic phase transition. The first-principles calculations suggest A-type antiferromagnetic ordering of the Cr sublattice, which is confirmed by neutron diffraction experiments. By comparing the crystal structure of ThCr2Si2C with the isostructural Cr-based compounds, the magnetic state of Cr 3d orbital is discussed in terms of the band-filling effects and indirect spin exchange interaction.

Temperature-dependent steady-state and time-resolved photoluminescence spectroscopy of CsPbBr3 encapsulated with PbBrOH and PVA

In this work, photoluminescence (PL) intensity and stability of CsPbBr3/PbBrOH powders synthesized by one-pot method are enhanced through the encapsulation of poly (vinyl acetate) (PVA) in water. Understanding the function of PbBrOH and PVA matrix for photogenerated transition can be achieved through temperature-dependent steady-state and time-resolved PL. The enhanced PL quantum yield (65.9%) and longer lifetime indicate the strengthened structural stability and a significant reduction in nonradiative recombination of CsPbBr3. Negative thermal quenching can be observed and interpreted using a multilevel PL transition model. Through nanosecond time-resolved PL spectra under femtosecond laser excitation, it is revealed that the PL position shows a redshift (∼8 nm) with time delay due to the reduction of band-filling effect during the recombination of photoexcited carriers. PVA further encapsulates CsPbBr3/PbBrOH powders to form a flexible film, which exhibits excellent thermal and water stability due to the dual protective effects of PVA and PbBrOH. The experimental investigation demonstrates that CsPbBr3 encapsulated by PbBrOH and PVA possesses excellent luminescent and water-resistant characteristics, making it a new choice for practical applications in LEDs and screen printing.

Tunability of optical properties of InSb films developed by pulsed laser deposition

The InSb thin films were successfully prepared from binary alloy target by Pulsed Laser Deposition (PLD) onto heated Si/SiO2 substrates at various deposition temperatures (Td) ranging from room temperature to 400 °C. The evolutions of structural (XRD, XPS), microstructural (AFM, SEM) and optical (XPS, FTIR, RAMAN) properties as a function of deposition temperature were systematically taken into account. The analyzed results indicate that the films are well crystallized with Zinc Blende (ZB) structure for Td ≥ RT. Increasing Td induces significant signature of preferentially crystal orientations of (h11) accompanied by distinctive microstructural evolution from continuous fashion (Td ≤ 300 °C) to almost isolated islands feature (Td ≥ 350 °C). Derivations of crystallite sizes, lattice strain, dislocation density, Lotgering orientation factor, and root mean square roughness were correlated with Raman shift and XPS analysis, and optical properties. The developed thin films allow to observe the flexibility of optical band gap energy (from 0.18 eV to 0.50 eV), which is directly related to the quantum confinement effect coupled with the band-filling effect of Burstein–Moss shift.

Holistic Nanowire Laser Characterization as a Route to Optimal Design

AbstractNanowire lasers are sought for near‐field and on‐chip photonic applications as they provide integrable, coherent, and monochromatic radiation: the functional performance (threshold and wavelength) is dependent on both the opto‐electronic and crystallographic properties of each nanowire. However, scalable bottom‐up manufacturing techniques often suffer from inter‐nanowire variation, leading to differences in yield and performance between individual nanowires. Establishing the relationship between manufacturing controls, geometric and material properties, and the lasing performance is a crucial step toward optimisation; however, this is challenging to achieve due to the interdependance of such properties. Here, a high‐throughput correlative approach is presented to characterise over 5000 individual GaAsP/GaAs multiple quantum well nanowire lasers. Fitting the spontaneous emission provides the threshold carrier density, while coherence length measurements determine the end‐facet reflectivity. The performance is intrinsically related to the width of a single quantum well due to quantum confinement and bandfilling effects. Unexpectedly, there is no strong relationship between the properties of the lasing cavity and the threshold: instead the threshold is negatively correlated with the non‐radiative recombination lifetime of the carriers. This approach therefore provides an optimisation strategy that is not accessible through small‐scale studies.

Exciton distribution-induced efficiency droop in green microscale light-emitting diodes at cryogenic temperatures

The anomalous droop in the external quantum efficiency (EQE) induced by the localization of excitons in GaN/InGaN green micro-light-emitting diodes (micro-LEDs) has been demonstrated at temperatures ranging from 25 to 100 K. At cryogenic temperatures, the random distribution of excitons among local potential energy minima limits the radiative recombination and reduces the EQE of green micro-LEDs. As the temperature increases from 25 to 100 K, the hopping of excitons from shallow potential energy minima to the potential energy valley contributes to the enhancement of radiative recombination. The distribution of excitons among local potential energy minima at cryogenic temperatures is also affected by the current density due to the influence of Coulomb screening of the polarization field and the band-filling effect.

Origin of the Inhomogeneous Electroluminescence of GaN-Based Green Mini-LEDs Unveiled by Microscopic Hyperspectral Imaging

Although high quantum efficiency has been achieved in large-sized InGaN/GaN LEDs operating at relatively high current densities (above 35 A/cm2), the operating current density of mini-LEDs (around 1 A/cm2) is far less than that of traditional large-sized LEDs. The low external quantum efficiency (EQE) of mini-LEDs at small current densities seriously hinders their practical applications, highlighting the importance of investigating the radiative recombination mechanisms of mini-LEDs at small current densities. By using microscopic hyperspectral imaging, the cryogenic electroluminescence (EL) of GaN-based green mini-LEDs mainly originating from localized excitons was demonstrated experimentally. Based on the dependence of electron–phonon coupling on current and temperature, Coulomb screening of the polarization field weakens the electron–phonon coupling, whereas the band-filling effect enhances the coupling. Coulomb screening of the polarization field can also reduce the deviation of the localization. The EL from edge regions of the mesa adjacent to the sidewall possesses relatively higher peak energy due to the strain relaxation. Results of this work also suggest that optimization of the chromatic characteristics and efficiency can be achieved by strain engineering of GaN-based green mini-LEDs.

MOS Capacitor-Driven Silicon Modulators: A Mini Review and Comparative Analysis of Modulation Efficiency and Optical Loss

Metal-oxide-semiconductor (MOS) capacitor-driven silicon modulators have demonstrated exceeding performance in driving voltage, energy efficiency, and bandwidth through heterogeneous integration with other active materials, including III-V compound semiconductors, transparent conductive oxides (TCOs), and graphene. However, a proper quantitative comparison is lacking among various device structures due to the difference in material properties and device designs. In this article, we first briefly reviewed state-of-the-art MOS capacitor-driven silicon modulators. Following that, we modeled the modulation efficiency and optical loss of two most representative device structures by incorporating the optical properties such as plasma dispersion, band-filling, and bandgap shrinkage effects for several heterogeneously integrated materials. The comparative analysis based on simulated results shows that Ge, III-Vs, and TCOs all offer larger index modulation than Si, whereas graphene has the largest index modulation. At the end of this article, we discussed the strategy of heterogeneous integration based on device design and scalable fabrication.

Enhanced band-filling effect in halide perovskites via hydrophobic conductive linkers

To approach the theoretical efficiency of perovskite solar cells (PSCs), the defects in perovskites should be managed. Among different types of defects, halide vacancies easily form on the surface of perovskite grains (PGs), hindering perovskite stability and the charge-transport process by trapping charge carriers. In this work, oxidized black phosphorus quantum dots (O-BPQDs) are incorporated into a perovskite to resolve these issues. Oxygen atoms of the O-BPQDs interact with uncoordinated Pb (halide vacancies), forming grain interconnections. These interactions reduce halide vacancies and suppress the overall recombination kinetics. Along with defect reduction, the O-BPQDs offer an efficient charge-transport channel across individual PGs. We achieve a best power-conversion efficiency (PCE) of 22.34% for SnO2-based PSCs and of 23.1% for TiO2-based PSCs. These PSCs exhibit moisture stability in a relative humidity (RH) 40% environment comparable to 3D/2D perovskites. Our strategy provides practical applicability and versatility for PSCs to approach the theoretical PCE value.

Photogenerated carrier density dependence of ultrafast carrier dynamics in intrinsic 6H-SiC measured by optical-pump terahertz-probe spectroscopy

Ultrafast carrier dynamics and terahertz conductivity of intrinsic 6H-SiC are investigated by optical-pump terahertz-probe spectroscopy at room temperature. The amplitude of photoinduced absorption increases gradually and followed by a biexponential recovery with photogenerated carrier densities from 7.43 × 1017 to 2.81 × 1018 cm −3 at 400 nm. With the increase of photogenerated carrier density, the fast decay time decreases may be due to higher order recombination (i.e., Auger recombination), and the slow one increases is related to the band-filling effect. The evolution of complex conductivity with photogenerated carrier density at the delay time of about 50 ps can be better fitted with Drude–Smith​ model than Drude–Lorenz model. The Drude–Smith scattering time τDS and Smith​ parameter c1 increase with photogenerated carrier density implies carriers backscattering is dominant. Our analysis provides important basic data for research and improvement ultrafast semiconductor devices of intrinsic 6H-SiC.

Band-filling effects in single-crystalline oligomer models for doped PEDOT: 3,4-ethylenedioxythiophene (EDOT) dimer salt with hydrogen-bonded infinite sulfate anion chains

Band-filling modulation of single-crystalline 3,4-ethylenedioxythiophene dimer salt from the half-filled state based on hydrogen-bonded anion chain formation enhanced the conductivity.

Monolithic Multi‐Color Tunable Inorganic Light‐Emitting Diodes (Adv. Electron. Mater. 1/2022)

Blueshift Phenomenon Monolithic red, green, and blue light-emitting diodes (LEDs) can be achieved by exploiting the blueshift phenomenon, caused by the strong band-filling effect at different energy localization states. In article number 2100598, Sung-Nam Lee and colleagues design different-sized red, green, and blue LEDs by controlling the sizes of the LEDs on the same wafer, which can be utilized in micro-LED display applications.

Luminescence Properties of InGaN/GaN Green Light-Emitting Diodes with Si-Doped Graded Short-Period Superlattice.

The optical properties of InGaN/GaN green light-emitting diodes (LEDs) with an undoped graded short-period superlattice (GSL) and a Si-doped GSL (SiGSL) were investigated using photoluminescence (PL) and time-resolved PL spectroscopies. For comparison, an InGaN/GaN conventional LED (CLED) without the GSL structure was also grown. The SiGSL sample showed the strongest PL intensity and the largest PL peak energy because of band-filling effect and weakened quantum- confined stark effect (QCSE). PL decay time of SiGSL sample at 10 K was shorter than those of the CLED and GSL samples. This finding was attributed to the oscillator strength enhancement by the reduced QCSE due to the Coulomb screening by Si donors. In addition, the SiGSL sample exhibited the longest decay time at 300 K, which was ascribed to the reduced defect and dislocation density. These results indicate that insertion of the Si-doped GSL structure is an effective strategy for improving the optical properties in InGaN/GaN green LEDs.

Effects of many-body interactions on the transient optical properties of lead halide perovskites

Lead halide perovskites (LHPs) are emerging as promising candidates for use in various high-performance optoelectronic applications, yet their photophysics remains a topic of debate. Here, we theoretically investigated how the ultrafast optical properties of a few prototype LHPs are affected by many-body interactions, including the bandgap renormalization (BGR) effect, the band-filling (BF) effect, the free-carrier absorption effect, and the exciton effect, at carrier densities ranging from 1016 to 1019 cm−3. The results show that the exciton absorption becomes more obvious near the bandgap with increasing exciton energy (as the halogen component of the LHP is varied from I to Cl). Transient reflectivity results indicate that the signal has one peak below the bandgap as a result of the BGR effect at low carrier densities and one valley above the bandgap originating from the BF effect at high carrier densities. In addition, the absorbance decreases with increasing the carrier density as a result of the BF effect because the filled energy levels are observed at 2 meV above the bottom of the conduction band. The results of the present work are expected to promote the application of LHPs in solar cells, light-emitting diodes, and other photoelectric devices.

Effects of organic material on magnetoresistance in electron-doped double perovskite

The Curie temperature of electron-doped Sr2FeMoO6 can be optimized significantly due to the band-filling effect, but accompanying an almost absent low-field magnetoresistance (LFMR), which is unfavorable to applications in the magnetoresistive devices operated at room temperature. Our previous works confirmed that, a remarkable enhanced LFMR was observed in Sr2FeMoO6 by modifying the grain boundary with insulating organic small molecules (glycerin, CH2OHCHOHCH2OH). However, in this work, modifying the grain boundary strength of the La0.5Sr1.5FeMoO6 with the insulating organic macromolecules (oleic acid, CH3(CH2)7CH=CH(CH2)7COOH) or small molecules (glycerin), both of them have negligible functions on the magnetoresistance (MR) behavior in La0.5Sr1.5FeMoO6. Contrary to the glycerin-modified Sr2FeMoO6, Sr2FeMoO6/oleic acid composites do not exhibit an obviously increased MR property. Based on the above experimental results and the related works, it is proposed that, maintaining high spin polarization of the carriers at the Fermi level and improving the tunneling process across the grain boundary using the suitable organic materials are decisive factors for optimizing the MR behavior in the similar electron-doped double perovskites.

Realization and optimization of optical logic gates using bias assisted carrier-injected triple parallel microring resonators

We propose a p-i-n diode embedded parallel triple microring resonator (MRR) configuration to simultaneously realize optical OR and AND, or NAND and NOR logic gates using a bias-assisted carrier injection mechanism. The applied bias on the rings induces refractive index change in the intrinsic region through bandfilling, bandgap shrinkage and free carrier absorption effects, leading to intensity variation at the output ports of the MRR due to respective resonant wavelength shift. The optical logic gate operational outputs are represented as the light intensities at the output ports of the MRR with the wavelength of the input optical signal launched into the input port being at the resonant wavelength of the microring resonator, while the operands are represented as the bias applied onto the rings. The proposed microring resonator configuration is theoretically optimized for achieving a high contrast ratio and an optical confinement factor by optimizing the intrinsic region width, applied bias, coupling coefficient between ring-bus waveguide and lifetime of carriers in the intrinsic region.

Elucidating the photoluminescence-enhancement mechanism in a push-pull conjugated polymer induced by hot-electron injection from gold nanoparticles

Understanding the photophysical interactions between the components in organic-inorganic nanocomposites is a key factor for their efficient application in optoelectronic devices. In particular, the photophysical study of nanocomposites based on organic conjugated polymers is rare. We investigated the effect of surface plasmon resonance (SPR) of gold nanoparticles (Au NPs) on the photoluminescence (PL) property of a push-pull conjugated polymer (PBDB-T). We prepared the hybrid system by incorporating poly(3-hexylthiophene)-stabilized Au NPs (P3HT-Au NPs) into PBDB-T. The enhanced and blueshifted PL was observed in the hybrid system compared to PL in a neat PBDB-T system, indicating that the P3HT chains attached to the Au NPs suppressed charge-transfer from PBDB-T to the Au NPs and relayed the hot electrons to PBDB-T (the band-filling effect). This photophysical phenomenon limited the auto-dissociation of PBDB-T excitons. Thus, the radiative recombination of the excitons occurred more in our hybrid system than in the neat system.

The Optical Blueshift Saturation Behavior of Mg-=SUB=-x-=/SUB=-Zn-=SUB=-1-x-=/SUB=-O Films

A phenomenon of blueshift saturation was observed from cathodoluminescence (CL) spectra of MgxZn1-xO films (0.069≤x≤0.8) which were grown by pulsed laser deposition technology. By analyzing the results of composition-dependent X-ray diffraction spectra, scanning electron microscopy images, absorption spectra and CL spectra, the crystalline structural disorder was determined via Urbach energy value. Furthermore, a competition mechanism between band-filling effect and band tail states was proposed in the composition-dependent CL spectra. This competition is believed to be responsible for the composition-induced blueshift saturation often observed in the disordered alloy semiconductors. The results of this research can provide important reference in energy-band engineering.