Large Direct Band Gap Research Articles

PbGa4S7: a wide-gap nonlinear optical material

PbGa4S7possesses a large powder second harmonic generation response and a large direct band gap of 3.08 eV.

Analysis of electronic structure of boron nitride nanotubes with different positions of intrinsic impurities

The pristine boron nitride nanotubes have a large direct band gap around 5 eV. This band gap can be engineered by doping. We investigate electronic structure of the doped hexagonal boron nitride (5,5) nanotubes using the linearized augmented cylindrical wave method. In particular, this work focuses on systematical study of the band gap and the density of states around the Fermi-level when the nanotubes are doped by intrinsic impurities of two substitutional boron atoms in a super cell and a comparative analysis of the relative stability of three structures studied here. This corresponds to 3.3% of impurity concentration. We calculate 29 configurations of the nanotubes with different positions of the intrinsic impurities in the nanotube. The band gap and density of states around the Fermi level show strong dependence on the relative positions of the impurity atoms. The two defect sub bands called Dπ(B) appear in the band gap of the pristine nanotube. The doped nanotubes possess p-type semiconductor properties with the band gap of 1.3-1.9 eV.

Spectral properties of Dirac electron system

We present the extension of our previous G0W0 approach to the two-dimensional system of massless Dirac electrons interacting via the long-range Coulomb interaction. We determine the one-particle spectral function taking into account only a partially filled linear band above the Dirac point. The electron self-energy is calculated, paying particular attention to the contribution coming from the collective plasmon mode through the dynamically screened Coulomb electron–electron interaction. The obtained results show a dispersing feature in the spectral function and a low energy quasi-particle with the renormalized spectral weight. We expect the results obtained to be qualitatively in agreement with spectral properties of monolayer of MoS2 which has the large direct band gap between the valence and conduction bands. We also discuss the influence of the underlying substrate on the obtained results.

Coupling of plasmon and dipolar modes in a monolayer of MoS2

The dielectric function of a monolayer of molybdenum disulfide is investigated within the model of two-band metal using the random phase approximation (RPA). The large direct bandgap in a monolayer of MoS 2 leads to the appearance of the interband dipolar mode which couples to the intraband plasmon mode. This results in two hybridized modes. The renormalized dipolar mode is close to its bare value, while coupling suppresses the plasmon mode. The obtained modes are responsible for the appearance of dispersing peaks in the energy-loss function.

PA-MBE Grown p-n (p-ZnO:(As+Sb)/<i>n</i>-GaN) and p-i-n (p-ZnO:As/HfO<sub>2</sub>/n-GaN) Heterojunctions as a Highly Selective UV Detectors

Zinc oxide is a promising candidate for application in UV photodetectors due to the large direct band gap and the high absorption coefficient in the UV. The high quality p-n and p-i-n structures consist of single or dual acceptor doped ZnO:(As,Sb) films grown by MBE, thin HfO2 layer grown by ALD method and n-type GaN templates. The As and Sb concentrations is 1020 cm-3.The maximum forward-to-reverse current ratio IF/IR in the obtained p-n diodes is of about 105 at ±4 V and in the case of p-i-n diodes is of about 106, which are very good results for this type of heterojunctions. The UV photodetectors are highly selective. The maximum of the detection wavelength was found at about 365 nm (FWHM of the photocurrent peak is ~17 nm). In the case of p-i-n detectors, the maximum of detection was found at 376, 360, and 341 nm. Additionally, it is possible to control the detection range by the applied reverse voltage. The dark to light current ratio in both cases is ~104.

Defect Driven Emission from ZnO Nano Rods Synthesized by Fast Microwave Irradiation Method for Optoelectronic Applications

ABSTRACTBecause of its large direct band gap of 3.37 eV and high exciton binding energy (∼60 meV), which can lead to efficient excitonic emission at room temperature and above, ZnO nanostructures in the würtzite polymorph are an ideal choice for electronic and optoelectronic applications. Some of the important parameters in this regard are free carrier concentration, doping compensation, minority carrier lifetime, and luminescence efficiency, which are directly or indirectly related to the defects that, in turn, depend on the method of synthesis. We report the synthesis of undoped ZnO nanorods through microwave irradiation of an aqueous solution of zinc acetate dehydrate [Zn(CH3COO)2. 2H2O] and KOH, with zinc acetate dihydrate acting as both the precursor to ZnO and as a self-capping agent. Upon exposure of the solution to microwaves in a domestic oven, ZnO nanorods 1.5 µm -3 µm and 80 nm in diameter are formed in minutes. The ZnO structures have been characterised in detail by X-ray diffraction (XRD), selective area electron diffraction (SAED) and high-resolution scanning and transmission microscopy, which reveal that each nanorod is single-crystalline. Optical characteristics of the nanorods were investigated through photoluminescence (PL) and cathodoluminescence (CL). These measurements reveal that defect state-induced emission is prominent, with a broad greenish yellow emission. CL measurements made on a number of individual nanorods at different accelerating voltages for the electrons show CL intensity increases with increasing accelerating voltage. A red shift is observed in the CL spectra as the accelerating voltage is raised, implying that emission due to oxygen vacancies dominates under these conditions and that interstitial sites can be controlled with the accelerating voltage of the electron beam. Time-resolved fluorescence (TRFL) measurements yield a life time (τ) of 9.9 picoseconds, indicating that ZnO nanorods synthesized by the present process are excellent candidates for optoelectronic devices.

From the ZnO Hollow Cage Clusters to ZnO Nanoporous Phases: A First-Principles Bottom-Up Prediction

A family of ZnkOk (k = 12, 16) cluster-assembled solid phases with novel structures and properties has been characterized utilizing a bottom-up approach with density functional calculations. Geometries, stabilities, equation of states, phase transitions, and electronic properties of these ZnO polymorphs have been systematically investigated. First-principles molecular dynamics (FPMD) study of the two selected building blocks, Zn12O12 and Zn16O16, with hollow cage structure and large HOMO–LUMO gap shows that both of them are thermodynamically stable enough to survive up to at least 500 K. Via the coalescence of building blocks, we find that the Zn12O12 cages are able to form eight stable phases by four types of Zn12O12–Zn12O12 interactions, and the Zn16O16 cages can bind into three phases by the Zn16O16–Zn16O16 links of H′, C′, and S′. Among these phases, six ones are reported for the first time. This has greatly extended the family of ZnO nanoporous phases. Notably, some of these phases are even more stable than the synthesized metastable rocksalt ZnO polymorph. The hollow cage structure of the corresponding building block ZnkOk is well preserved in all of them, which leads to their low-density nanoporous and high flexibility features. In addition the electronic integrity (wide-energy gap) of the individual ZnkOk is also retained. Our calculation reveals that they are all semiconductor with a large direct or indirect band gap. The insights obtained in this work are likely to be general in II–VI semiconductor compounds and will be helpful for extending the range of properties and applications of ZnO materials.

Plasmons and screening in a monolayer of MoS2

We investigate the dynamical dielectric function of a monolayer of molybdenum disulfide within the random phase approximation. While in graphene damping of plasmons is caused by interband transitions, due to the large direct band gap in monolayer MoS$_2$ collective charge excitations enter the intraband electron hole continuum similar to the situation in two-dimensional electron and hole gases. Since there is no electron-hole symmetry in MoS$_2$, the plasmon energies in p- and n-doped samples clearly differ. The breaking of spin degeneracy caused by the large intrinsic spin-orbit interaction leads to a beating of Friedel oscillations for sufficiently large carrier concentrations, for holes as well as for electrons.

Electronic and Spectroscopic Study of the GaAs nitridation – Electronic Characterization Associated

Gallium nitride is one of the III-V semiconductors the most promising in many application areas. Indeed, because of its large direct bandgap (3,4 eV), it can be dedicated to both optoelectronic applications as transistors achieve hyper frequency. It allows the manufacture of components stable at high temperatures and high frequencies. In this work we are interested in the nitriding of gallium arsenide substrates of n-type in order to obtain a thin layer of GaN. The samples were chemically cleaned immediately introduced into the chamber ultra-high vacuum. They then undergo ionic cleaning in situ by argon ions. Spectrometric analyzes do show the absence of any impurities from the surface, in fact, it only detects the presence of gallium and arsenic. Before proceeding to the nitriding of our structures, deposition of gallium is conducted for 20 minutes to send then simultaneously nitrogen and gallium. Analyzes were performed elaborate structures using X-ray photoelectron spectroscopy (XPS). Deposition of mercury (Hg) has been developed on the structures, we have then made an electrical characterization I-V to see the effect of nitriding on the resulting structure.

Achieving direct band gap in germanium through integration of Sn alloying and external strain

GeSn is predicted to exhibit an indirect to direct band gap transition at alloy Sn composition of 6.5% and biaxial strain effects are investigated in order to further optimize GeSn band structure for optoelectronics and high speed electronic devices. A theoretical model has been developed based on the nonlocal empirical pseudopotential method to determine the electronic band structure of germanium tin (GeSn) alloys. Modifications to the virtual crystal potential accounting for disorder induced potential fluctuations are incorporated to reproduce the large direct band gap bowing observed in GeSn alloys.

Tunable band gap in half-fluorinated bilayer graphene under biaxial strains

Opening and tuning of the band gap of bilayer graphene (BLG) is of particular importance for its utilization in nanoelectronics. We presented here the electronic structures of two types of stoichiometrically half-fluorinated BLGs (i.e. C2Fs) as well as those under biaxial compressive and tensile strains. Our results reveal that both C2Fs are semiconductor with large direct band gaps in their unstrained configurations. Under biaxial compressive strains, the band gaps of both C2Fs can be reduced. However, by applying biaxial tensile strains, both C2Fs undergo a direct-to-indirect band gap transition. Electronic nature of the strain-tuned band gaps has been discussed.

Synthesis of monocrystalline zinc oxide microrods by wet chemical method for light confinement applications

The growth of Zinc Oxide (ZnO) microrods by wet chemical method, under prolonged treatment of zinc nitrate for 22h in low temperature (80°C) with hexamine is presented. The purpose was to obtain large microrods that can act as laser resonators with emission in the blue region. The electron microscopy and X-ray diffraction observations reveal that the ZnO microrods are single crystalline. We have done various optical characterizations in order to assess the suitability of the rods in photonic applications. Studies include optical absorption, fluorescence and photon backscattering. Room temperature optical measurements reveal that they have a large direct band gap of 3.7eV and an emission at the visible and ultraviolet region and have excitation wavelength dependence. The backscattering studies clearly reveal the photon confinement effects, which is the first test of the material's self lasing action.

Laser-Thinning of MoS2: On Demand Generation of a Single-Layer Semiconductor

Single-layer MoS2 is an attractive semiconducting analogue of graphene that combines high mechanical flexibility with a large direct bandgap of 1.8 eV. On the other hand, bulk MoS2 is an indirect bandgap semiconductor similar to silicon, with a gap of 1.2 eV, and therefore deterministic preparation of single MoS2 layers is a crucial step toward exploiting the large direct bandgap of monolayer MoS2 in electronic, optoelectronic, and photovoltaic applications. Although mechanical and chemical exfoliation methods can be used to obtain high quality MoS2 single layers, the lack of control in the thickness, shape, size, and position of the flakes limits their usefulness. Here we present a technique for controllably thinning multilayered MoS2 down to a single-layer two-dimensional crystal using a laser. We generate single layers in arbitrary shapes and patterns with feature sizes down to 200 nm and show that the resulting two-dimensional crystals have optical and electronic properties comparable to that of pristine exfoliated MoS2 single layers.

Optical properties of LFZ grown β-Ga2O3:Eu3+ fibres

Due to their relevance for electronic and optoelectronic applications, transparent conductive oxides (TCO) have been extensively studied in the last decades. Among them, monoclinic β-Ga2O3 is well known by its large direct bandgap of ∼4.9eV being considered a deep UV TCO suitable for operation in short wavelength optoelectronic devices. The wide bandgap of β-Ga2O3 is also appropriate for the incorporation of several electronic energy levels such as those associated with the intra-4fn configuration of rare earth ions. Among these, Eu3+ ions (4f6) are widely used as a red emitting probes both in organic and inorganic compounds. In this work, undoped and Eu2O3 doped (0.1 and 3.0mol%) Ga2O3 crystalline fibres were grown by the laser floating zone approach. All fibres were found to stabilize in the monoclinic β-Ga2O3 structure while for the heavily doped fibres the X-ray diffraction patterns show, in addition a cubic europium gallium garnet phase, Eu3Ga5O12. The spectroscopic properties of the undoped and Eu doped fibres were analysed by Raman spectroscopy, low temperature photoluminescence (PL) and photoluminescence excitation (PLE). The Eu3+ luminescence is mainly originated in the garnet, from where different europium site locations can be inferred. The spectral analysis indicates that at least one of the centres corresponds to Eu3+ ions in dodecahedral site symmetry. For the lightly doped samples, the spectral shape and intensity ratio of the 5D0→7FJ transitions is totally different from those on Eu3Ga5O12, suggesting that the emitting ions are placed in low symmetry sites in the β-Ga2O3 host.

Band Gap Shift and the Optical Nonlinear Absorption of Sputtered ZnO-TiO2 Films

ZnO-TiO2 composite films with different Zn/Ti atomic ratios were prepared with radio frequency reactive sputtering method. The Zn percentage composition (f(Zn)) dependent optical band gap and optical nonlinear absorption were investigated using the transmittance spectrum and the Z-scan technique, respectively. The results showed that composite films with f(Zn) in the range of 23.5%-88.3% are poor crystallized and their optical properties are anomalous which exhibit adjustable optical band gap and large optical nonlinear absorption. The optical absorption edge shifted to the blue wavelength direction with the increasing of f(Zn) and reached the minimum value of 285 nm for the sample with f(Zn) = 70.5%, which has the largest direct band gap of 4.30 eV. Further increasing of f(Zn) resulted in the red-shift of the optical absorption edge. The maximum optical nonlinear absorption coefficient of 1.5 x 10(3) cm/GW was also obtained for the same sample with f(Zn) = 70.5%, which is more than 40 times larger than those of pure TiO2 and ZnO films.

Synthesis of colloidal SnSe quantum dots by electron beam irradiation

Water-soluble orthorhombic colloidal SnSe quantum dots with an average diameter of 4 nm were successfully prepared by a novel irradiation route using an electronic accelerator as a radiation source and hexadecyl trimethyl ammonium bromide (CTAB) as a surfactant. The quantum dots exhibit a large direct bandgap of 3.89 eV, greatly blue shifted compared with that of bulk SnSe (1.0 eV) due to the quantum confinement effect. The quantum dots show blue photoluminescence at ∼420 nm. The influence of CTAB on the growth of the quantum dots was investigated and a possible reaction/growth mechanism was proposed.

Crystal engineering using functionalized adamantane

We performed a first-principles investigation on the structural, electronic andoptical properties of crystals made of chemically functionalized adamantanemolecules. Several molecular building blocks, formed by boron and nitrogensubstitutional functionalizations, were considered to build zinc blende andwurtzite crystals, and the resulting structures presented large bulk moduli andcohesive energies, wide and direct bandgaps, and low dielectric constants (low-κ materials). Those properties provide stability for such structures up to room temperature,superior to those of typical molecular crystals. This indicates a possible road map forcrystal engineering using functionalized diamondoids, with potential applications rangingfrom space filling between conducting wires in nanodevices to nano-electromechanicalsystems.

ZnO-MgZnO Quantum-Dot Semiconductor Optical Amplifiers

Abstract: ZnO quantum dot (QD) semiconductor optical amplifier (SOA) is studied theoretically using non-Markovian gain model. SOA performance is then studied by rate equations model. These QD-SOAs have shown to have High gain, low noise figure, and polarization insensitivity. This article appraised patent and patent application related to semiconductor optical amplifier. Keywords: Quantum dot (QD), ZnO, semiconductor optical amplifier (SOA), non-Markovian gain, small-signal gain, rate equations, ground state (GS), excited state (ES), noise figure. 1. INTRODUCTION Zinc oxide (ZnO) has been identified as a promising material for optoelectronics applications ranging from ultra-violet (UV) to red wavelengths due to its large fundamental direct bandgap, high thermal conductivity, high luminous efficiency and mechanical and chemical robustness. ZnO also has an advantage of a large exciton binding energy about 60meV, which assures more efficient excitonic emis-sion at higher temperatures [1, 2]. Various wide-gap semi-conducting materials have been adopted to construct ZnO-related heterostructures. Mg

Band Structures of Narrow Zigzag Silicon Carbon Nanoribbons

A first-principles investigation on the electronic properties of the silicon carbon nanoribbons (SiC NRs) having zigzag-shaped edges passivated by hydrogen with widths up to 1.7 nm is presented. It is found that, for the zigzag SiC NRs wider than 0.6 nm, the ground state is a ferrimagnetic state that has two different direct band gaps for the spin-up and the spin-down channels near the X point, and thus it is semiconducting instead of half metallic. We also found that the ferromagnetic (FM) state is metallic and has higher energy than the ferrimagnetic state for very narrow zigzag SiC NRs, though the energies of the FM metallic state and the ferrimagnetic semiconducting state are almost equivalent as the width increases. Furthermore, we found that the energies of nonmagnetic semiconducting states that have only one direct band gap were much higher than those of the FM and the ferrimagnetic states. On the other hand, the zigzag SiC NRs narrower than 0.6 nm have only a nonmagnetic semiconducting state that has a large direct band gap of 0.74 eV at the X point. In addition, we found that the Fermi energy of the zigzag SiC NR is susceptible to k-point sampling points in the Brillouin zone integration; when k-point sampling points are not large enough, it can lead to falsity half-metallic ground state. Thus, a properly large k-point sampling should be used in the Brillouin zone integration.

Particle Dynamics in a Chemical Vapor Deposition Reactor: A Multiscale Approach

Compared to other semiconductors, zinc sulfide has a large direct band gap which makes it useful in a broad range of optical applications which demand high quality zinc sulfide films which are produced through the chemical vapor deposition method. In order to achieve high quality films with low defects, a better understanding of the deposition process is necessary. A common cause of defects in the deposited film is due to the fact that the morphology of adducts in the gas phase is different from that of the deposited film. These adducts affect the deposition efficiency and film quality. This paper strives to understand the impact of adducts through a multiscale modeling approach which is used to explain the possible link between the cluster size and the morphological defects on the deposited film. This understanding can be used within a simulation framework to optimize reaction conditions that minimize film defects.