Band Gap Offsets Research Articles

Unlocking the electronic, optical and transport properties of semiconductor coupled quantum dots using first principles methods

Semiconductor coupled quantum dots provide a unique opportunity for tuning band gaps by tailoring band offsets, making them ideal for photovoltaic and other applications. Here, we have studied stability, trends in the band gap, band offsets, and optical properties for a series of coupled quantum dots comprised of II–VI semiconductors using a hybrid functional method. We have shown how the quantum confinement and interfacial strain considerably affect the band gap and band offsets for these heterostructures at the nanoscale. We show that the trend in band offsets obtained from our first-principles electronic structure calculations agrees with that obtained from the method of average electrostatic potential. It is found that a common anion rule for band offset is followed for these heterostructures at the nanoscale. Further, the calculated optical absorption spectra for these coupled quantum dots reveal that absorption peaks lie in the ultra-violet (UV) region, whereas absorption edges are in the visible region. In addition to electronic and optical properties, we have also explored transport properties for two representative coupled quantum dots, either having common cations or common anions, which revealed asymmetric nature in current–voltage characteristics. Therefore, these semiconductor coupled quantum dots may be useful for photovoltaic, light-emitting diode, and optoelectronic devices.

Boosted Electronic, NLO and Absorption Characteristics for Quercetin and Taxifolin; Comparative Experimental and DFT Studies

In the present study, a considerable, reproducible, and eco-friendly biological synthesis of Ag nanoparticles using Mangifera indica leaf extract as a reductant is documented. The spectroscopic characteristics of synthesized Ag nanoparticles are described by both UV-Vis and FT-IR techniques. The bandgap offsets, reactivity, and NLO properties for two flavonoids, quercetin, and taxifolin, are examined using the DFT approach. Also, a detailed comparative analysis for HOMO-LUMO interactions among quercetin and taxifolin is discussed. Results show that quercetin and taxifolin possess dipole moment (DM=4.79, 3.99 Debye) and bandgap offset (2.59, 2.98 eV). Both molecules are promising candidates as window layers for solar cells and memory switch devices. In addition, hyperpolarizability calculations show that quercetin NLO response is higher than taxifolin, which sets a revolutionary recall for NLO manufacture upgrade. Moreover, NBO and UV-Vis absorption characteristics are reported as well.

Boosted Electronic, NLO and Absorption Characteristics for Quercetin and Taxifolin: Comparative Experimental and DFT Studies

In the present study, a considerable, reproducible, and eco-friendly biological synthesis of Ag nanoparticles using Mangifera indica leaf extract as a reductant is documented. The spectroscopic characteristics of synthesized Ag nanoparticles are described by both UV-Vis and FT-IR techniques. The bandgap offsets, reactivity, and NLO properties for two flavonoids, quercetin, and taxifolin, are examined using the DFT approach. Also, a detailed comparative analysis for HOMO-LUMO interactions among quercetin and taxifolin is discussed. Results show that quercetin and taxifolin possess dipole moment (DM=4.79, 3.99 Debye) and bandgap offset (2.59, 2.98 eV). Both molecules are promising candidates as window layers for solar cells and memory switch devices. In addition, hyperpolarizability calculations show that quercetin NLO response is higher than taxifolin, which sets a revolutionary recall for NLO manufacture upgrade. Moreover, NBO and UV-Vis absorption characteristics are reported as well.

Multifunctional Integrated Compartment Systems for Incompatible Cascade Reactions Based on Onion-Like Photonic Spheres.

One of the central aims of synthetic biology and metabolic engineering is to mimic the integrality of eukaryotic cells to construct a multifunctional compartment system to perform multistep incompatible cascade reactions in a one-pot, controlled, and selective fashion. The key challenge is how to address the coexistence of antagonistic reagents and to incorporate these functionalities into an integrated system in a smart and efficient way. A novel strategy called "iterative etching-grafting" is proposed here based on monodispersed photonic spheres (PSs) prepared by microfluidics, which constructs a universal platform for incompatible cascade reactions. As a proof of concept, we spatiotemporally regulated the degree of etching of PSs, then grafted precursory groups of acid and base onto PSs, and incorporated a photocleavage method, which were capable of compartmentalizing the acid and base inside PSs. Utilizing the band-gap offsets of PSs could track the progress of cascade reactions in situ, and grafting various charged polymers on the surface of the pores by surface-initiated atom transfer radical polymerization (SI-ATRP) achieved the selectivity of the substrates, which flexibly constructed a multifunctional and integrated acid-base photonic multicompartment system (PMCS). The created PMCS shows excellent catalytic performance, convenient monitoring, and efficient substrate selectivity in the deacetalization-Knoevenagel cascade reaction. Furthermore, two types of electrophile/nucleophile PMCSs have also been accessibly constructed, demonstrating the facile generation of other incompatible systems with the versatility as well as the advancement and extensibility of the developed strategy.

Stable C2N/h-BN van der Waals heterostructure: flexibly tunable electronic and optic properties

Monolayer C2N has been successfully synthesized. To explore the enhancement of its stability and the expansion of its potential applications in electronics and optoelectronics, a vertical van der Waals heterostructure is constructed by C2N monolayer integrated with h-BN monolayer. The first-principles calculations based on the density functional theory show that this structure possesses a type-II band alignment with a smaller direct band gap and lager band offsets, suggesting that photo-generated electron–hole pairs can be well spatially separated, and thus an excellent photoelectric and photovoltaic material can be obtained. Also shown is that the electronic properties of such a heterostructure can be effectively regulated by a vertical strain and external electric field. For example, under compressive strain or forward electric field, its band gap can be significantly reduced to enhance light-excitation electron transition further, meanwhile the feature of direct band-gap and large band offsets is always well-preserved. Furthermore, it is found that the intrinsic heterostructure holds a wide optic adsorption range and large adsorption coefficient, and the applied compressive strain or a positive electric field can lead to a wide and high main absorption peak across the near-infrared, visible light, and ultraviolet region, implying that a tuned heterostructure has more promising applications in optoelectronics.

Excellent properties of type-II van der Waals Janus-XM2X’/MX heterojunctions toward solar cell utilization

Constructing van der Waals (vdW) heterojunctions is an effective strategy for band engineering of two dimensional (2D) materials. The XM2X’/MX (M = Al, Ga, In; X = S, Se, Te) heterostructures are investigated by first-principles calculation. Five XM2X’/MX with less than 3% of lattice mismatch are found to be direct-band-gap type-II band structures with moderate band gap from 1.08 to 1.73 eV in HSE06 method, high optical absorption coefficient (∼105 cm−1) and broad absorption spectrum, which facilitates effective utilization of the solar visible light and separation of photo-generated electron-hole pairs. Intriguingly, the band gap and band offsets are tunable to achieve high power conversion efficiency by external electric field and vertical strain. These results indicate potential applications of the XM2X’/MX in solar energy conversion.

Characterization of CdS/Sb2Te3 micro/nano-interfaces

In this study, transparent micro/nano heterojunction structures are designed and characterized. The X-ray diffraction, optical and impedance spectroscopy techniques are employed for this purpose. In the design, a CdS film of thickness of 0.5 μm was used as the micro layer. The nano-layer is composed of 20 or 50 nm thick Sb2Te3 film. The analysis of the experimental data has shown that the CdS films exhibit a Major hexagonal and minor orthorhombic phases. While the grain size of the microlayer decreases, the strain, defect density and stacking faults increases when the nano-layer is co-evaporated. In addition, the absorption of the CdS is improved by ∼4 times and by ∼8 times in the visible and infrared ranges of light. The dielectric constant of CdS is also increased by this method. The calculated changes in the energy band gap and energy band offsets are found to be sufficient to allow using the micro/nano heterojunction device in optoelectronic and microwave technologies. The recombination mechanisms of the device that were discovered through the analysis of the photocurrent was observed to be mostly dominated by the trap-assisted recombination at high light powers. On the other hand, while the Yb/CdS/Sb2Te3 anisotype device performed well as a low pass microwave filter, the hybrid Au/CdS/Sb2Te3 device behaved as high pass filter in the frequency domain of 10–1800 MHz.

Band alignments of O3-based and H2O-based amorphous LaAlO3 films on silicon by atomic layer deposition

Amorphous LaAlO3 films were grown on p-type Si substrate by atomic layer deposition using O3 and H2O as the oxygen source, respectively. Band alignments of LaAlO3 films were analyzed by X-ray photoelectron spectroscopy measurements using the photoemission-based method. Extra hydroxyl groups and C and N-related impurities were detected in the H2O-based LaAlO3 film. As a result, the O3-based LaAlO3 dielectric gains higher band gap and band offsets than those of the H2O-based dielectric. Consequently, for the O3-based film, the leakage current of more than one order of magnitude less than that of H2O-based LaAlO3 film was obtained. All the results indicate that O3 is a more appropriate oxidant for the deposition of LaAlO3 dielectric.

First-principles predictions of electronic properties of GaAs1-x-yPyBix and GaAs1-x-yPyBix-based heterojunctions

Significant efficiency droop is a major concern for light-emitting diodes and laser diodes operating at high current density. Recent study has suggested that heavily Bi-alloyed GaAs can decrease the non-radiative Auger recombination and therefore alleviate the efficiency droop. Using density functional theory, we studied a newly fabricated quaternary alloy, GaAs1-x-yPyBix, which can host significant amounts of Bi, through calculations of its band gap, spin-orbit splitting, and band offsets with GaAs. We found that the band gap changes of GaAs1-x-yPyBix relative to GaAs are determined mainly by the local structural changes around P and Bi atoms rather than their electronic structure differences. To obtain alloy with lower Auger recombination than GaAs bulk, we identified the necessary constraints on the compositions of P and Bi. Finally, we demonstrated that GaAs/GaAs1-x-yPyBix heterojunctions with potentially low Auger recombination can exhibit small lattice mismatch and large enough band offsets for strong carrier confinement. This work shows that the electronic properties of GaAs1-x-yPyBix are potentially suitable for high-power infrared light-emitting diodes and laser diodes with improved efficiency.

Composition dependent interfacial thermal stability, band alignment and electrical properties of Hf1−xTixO2/Si gate stacks

The optical properties, interface chemistry and band alignment of Hf1−xTixO2 (x=0.03, 0.08, 0.12 and 0.20) high-k gate dielectric thin films, deposited by RF sputtering on Si substrate, have been systematically investigated. The effect of TiO2 incorporation on the interfacial chemical structure and energy-band discontinuities has been investigated by using X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–vis). It has been found that the band gap and band offsets of the Hf1−xTixO2 thin film decrease with the increase of TiO2 concentration. Meanwhile, the obtained band offsets are all over 1eV. Thin film capacitors fabricated with the MOS configuration of Al/Hf1−xTixO2/n-Si/Al exhibits excellent electrical properties with low interface state density, hysteresis voltage and low leakage current density. The suitable band gap, symmetrical band offsets relative to Si and prominent electrical properties render sputtering-derived Hf1−xTixO2 with 9% TiO2 films as promising candidates for high-k gate dielectrics.

Effect of Pre-annealing to Blocking Oxide on the Performance of Dual Trapping-layer Engineered Charge Trapping Memory

The effect of pre-annealing to blocking oxide on the performance of trapping layer engineered charge trapping flash memory was investigated in this work. Compared to the devices fabricated by conventional process, the devices with pre-annealing treatment exhibit larger memory window, faster program speed, and significantly improved data retention. The enhancement of memory performance and reliability can be possible from the improvement of blocking oxide quality and the change of band-gap and band offsets in memory gate stack. The findings provide a useful guide for CTM process optimization.

Bandgap engineering of colloidal zinc oxysulfide via lattice substitution with sulfur

Zinc oxysulfide nanocrystals with zinc blende phase are synthesized through a wet-chemical method. An affirmation of the crystal structure, elemental homogeneity and phase transformation is obtained by X-ray diffraction and authenticated by electron micrographic studies. Theoretical observations have strongly supported the thermodynamic solubility limit for its (30%) formation. An anomalous bandgap bowing with modulation in bandgap from 3.74 eV (ZnO) to 3.93 eV (ZnS) was observed with a minimum bandgap of 2.7 eV. Tunable bandgap and a wide range of visible emission ascertain it as a potential material for optoelectronic and solar cell applications due to its large bandgap offsets.

Investigation of band offsets and direct current leakage properties of nitrogen doped epitaxial Gd2O3 thin films on Si

Dielectric properties of epitaxial Gd2O3 thin films grown on Si have been found to improve significantly by incorporation of suitable dopants. However, in order to achieve optimum electrical properties from such doped oxides, it is important to understand the correlation between doping and the electronic structure of the material. In the present article, we report about the effect of nitrogen doping on the electronic structure and room temperature dc leakage properties of epitaxial Gd2O3 thin films. Epitaxial Gd2O3:N thin films were grown on p-type Si (111) substrates by solid source molecular beam epitaxy technique using molecular N2O as the nitridation agent. First investigations confirmed the presence of substitutional N in the Gd2O3:N layers. Incorporation of nitrogen did not affect the structural quality of the oxide layers. X ray photoelectron spectroscopy investigations revealed band gap narrowing in epitaxial Gd2O3 due to nitrogen doping, which leads to reduction in the valence band offset of the Gd2O3:N layers with Si. DC leakage current measured at room temperature revealed that despite reduction in the band gap and valence band offsets due to N doping, the Gd2O3:N layers remain sufficiently insulating. A significant reduction of the leakage current densities in the Gd2O3:N layers with increasing nitrogen content suggests that doping of epitaxial Gd2O3 thin films with nitrogen can be an effective route to eliminate the adverse effects of the oxygen vacancy induced defects in the oxide layers.

Numerical study of metal oxide hetero-junction solar cells with defects and interface states

Further to our previous work on ideal metal oxide (MO) hetero-junction solar cells, a systematic simulation has been carried out to investigate the effects of defects and interface states on the cells. Two structures of the window/absorber (WA) and window/absorber/voltage-enhancer (WAV) were modelled with defect concentration, defect energy level, interface state (ISt) density and ISt energy level as parameters. The simulation showed that the defects in the window layer and the voltage-enhancer layer have very limited effects on the performance of the cells, but those in the absorption layer have profound effects on the cell performance. The interface states at the W/A interface have a limited effect on the performance even for a density up to 1013 cm−2, while those at the A/V interface cause the solar cell to deteriorate severely even at a low density of lower than 1 × 1011 cm−2. It also showed that the back surface field (BSF) induced by band gap off-set in the WAV structure loses its function when defects with a modest concentration exist in the absorption layer and does not improve the open voltage at all.

Temperature dependence of chemical states and band alignments in ultrathin HfOxNy/Si gate stacks

The chemical states and band alignments of HfOxNy films on Si substrates as a function of post-thermal treatment were systematically investigated. Analysis of N1s spectra reveals that there are four components in N chemical states, which are assigned to the N–Hf, N ≡ Si, Si2 = N–O and N–O bonds. The N–O bond is very unstable upon annealing as compared with the N–Hf, N ≡ Si and Si2 = N–O bonds. The relationship between band alignments and chemical states in the HfOxNy/Si stack upon annealing is also studied. It is found that the conduction-band offset shows no obvious change upon annealing, while the band gap and the valence-band offset increase with the increase in the annealing temperature, which is mainly due to the reduction in the N–O bond during the annealing process. With the values of band gap and band offsets of the HfOxNy film, the energy-band alignments of the HfOxNy/Si stack as a function of post-deposition annealing were constructed.

Thermoelectric Performance of ZnO:Al/ZnO:(Al,In) Quantum Well Multilayer Structures as a Function of Indium Composition and Band-Gap Offset at High Operating Temperatures

This study investigates the thermoelectric (TE) power factor for radiofrequency (RF)-sputtered n-type ZnO:Al/ZnO:(Al,In) multilayer quantum wells (QW) and the dependence on band-gap offset at high operating temperatures. The structures are 50 periods of 10-nm-thick barrier and well layers. The TE power factors for all films are promising (>20 × 10−4 W/m2 K); however, a decline in electronic transport is evident for each of the doped wells over a specific range of operating temperatures. This supports the need for optimum doping levels for a specific operating temperature range; e.g., 2 at.%, 5 at.%, and 8 at.% indium are optimum at 300 K to 600 K, 600 K to 725 K, and >725 K, respectively. A model has also been developed to calculate the QW electronic transport as a function of temperature and band-gap offset, which is subsequently related to doping concentration. This model confirms a loss in electrical conductivity due to charge carriers gaining enough kinetic energy to overcome the QW barrier, which results in multilayer hopping across well–barrier interfaces. In addition, film microstructure and interface roughness have been determined by x-ray and spectroscopy techniques.

Band Gap and Band Offset with Silicon of Amorphous High-k LaGdO3 Thin Films

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STRAIN INDUCED ACTIVE LAYER DESIGN OF GaN-THz QUANTUM CASCADE LASERS

GaN -based pseudomorphic heterostructures with their demonstrated superior thermal performance suggest an alternative to the standard GaAs -based technology to realize high power lasing at THz frequencies. A larger electron effective mass in GaN based system results in the energy levels lying deeper within the quantum well compared to its GaAs counterpart resulting in longer carrier lifetimes assisting transitions required for THz radiation while reducing tunneling current. However, the presence of spontaneous and piezoelectric polarization and dependence of bandgap and band offsets on structural and bias induced strain reduces carrier lifetime.

Electronic Structure of Cerium Oxide Gate Dielectric Grown by Plasma-Enhanced Atomic Layer Deposition

We have systematically investigated the electronic structure of CeO2 dielectric by plasma enhanced atomic layer deposition (PE-ALD). The CeO2 films were deposited by using novel tris(isopropyl-cyclopentadienyl)cerium [Ce(iPrCp)3] precursor and O2 plasma. The energy band gap (3.48 eV), Fermi level (4.71 eV), and band offsets of PE-ALD CeO2 were elaborately determined by combining spectroscopic ellipsometry and ultraviolet photoemission spectroscopy (UPS) techniques. In addition, based on the experimental results of Fowler-Nordheim(F-N) tunneling and Poole-Frenkel(P-F) conduction for Al/PE-ALD CeO2/p-Si, the increased interfacial layer and reduced trapped oxide densities were found to be correlated to the increase in effective barrier height (0.89, 1.18 and 1.26 eV) and decrease in decreased trap energy levels (0.47, 0.26 and 0.18 eV) with increasing O2 annealing temperature (400, 600 and 800°C), respectively. These discrepancies might be attributed to the thin film characteristics and oxygen vacancies in the CeO2 dielectrics, depending on the deposition techniques and post deposition annealing processes.

Band alignment of Hf–Zr oxides on Al 2O 3/GeO 2/Ge stacks

Band gap and band offsets of hafnium and zirconium oxides were investigated on Ge substrates with a fixed Al 2O 3/GeO 2 passivation layer. Chemical properties and band alignment with Ge were determined by X-ray photoelectron spectroscopy; characterization of structure and morphology was done using transmission electron microscopy. All samples investigated were found to be chemically suitable for Ge advanced applications and with sufficient energy barriers for holes and electrons for the use of Ge as a future channel material.