Hexagonal Semiconductors Research Articles

Sodium-based plasmonic waveguides with high confinement factors and ultra-low gain thresholds.

The noble metal-based hybrid plasmon mode features low loss and strong field localization, making it widely applicable in the field of nanophotonic devices. However, due to the high loss of noble metals, the gain threshold is unacceptably high, usually larger than 0.1 µm-1. Here we present a hybrid plasmonic waveguide consisting of a SiO2 layer coated Na nanowire and a hexagonal semiconductor nanowire. Based on the high performance of the proposed waveguide, the Purcell factor exceeding 120 and a confinement factor above 90% are achieved, leading to an ultra-low gain threshold of 0.02117 µm-1. In addition, the proposed waveguide exhibits an extremely low cross talk, making it highly suitable for applications in compact photonic integrated devices. The proposed waveguide may contribute to the development of low-threshold nano-lasers and promote other applications in nanophotonics.

Coherent potential approximation study of impurity effect on monolayer hexagonal boron phosphide

Impurity doping is a necessary technology for the application of semiconductor materials in microelectronic devices. The quantification of doping effects is crucial for controlling the transport properties of semiconductors. Here, taking two-dimensional (2D) hexagonal boron phosphide semiconductor as an example, we employ coherent potential approximation method to investigate the electronic properties of 2D semiconductor materials at low doping concentrations, which cannot be exploited with conventional density function theory. The results demonstrate that the positive or negative impurity potential in 2D semiconductors determines whether it is p-type or n-type doping, while the impurity potential strength decides whether it is shallow-level or deep-level doping. Impurity concentration has important impacts on not only the intensity but also the broadening of impurity peak in band gap. Importantly, we provide the operating temperature range of hexagonal boron phosphide as a semiconductor device under different impurity concentrations and impurity potentials. The methodology of this study can be applied to other 2D semiconductors, which is of great significance for quantitative research on the application of 2D semiconductors for electronic devices.

Photocatalytic degradation of acid blue 74 by Co: WO3 nanoparticles: Kinetics and response surface methodology studies

Energy-efficient sol-gel synthesis was achieved through the microwave assisted self-combustion route to produce hexagonal Tungsten oxide semiconductor (WO3). The photocatalytic activity was enhanced by doping Cobalt (Co) into the crystalline structure of the nanoparticle, which were subsequently sintered at 400 °C for an hour. The structural and morphological properties of the Co-doped WO3 were revealed using X-ray diffraction (XRD) characterization. The nanoparticles exhibited an amorphous structure before annealing, due to the short heating time during combustion synthesis. Sintering the nanoparticle transformed the nanoparticle from a monoclinic phase to orthorhombic phase structure. Additional analysis techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrum analysis (EDS).To assess the photocatalytic performance of these particles, Acid Blue 74 (AB 74) was employed in photodegradation experiments under UV light irradiation within a semi-continuous reactor.The photodegradation of dye molecules was evaluated utilizing a UV–Vis spectrophotometer, and the mineralization efficiency of the dye was determined through total organic carbon analysis (TOC). The results indicated that the dimension of the synthesized nano catalyst fell within the range of 70–120 nm, and it exhibited the ability to completely degrade a solution with an initial dye concentration of 20 ppm within 60 min. Various parameters affecting the photocatalytic reaction, including the photocatalyst dosage, initial dye concentration, pH and temperature of the dye solution were also investigated. The experiments were designed using Response surface methodology (RSM), through which a mathematical model for the dye removal process was developed.

Low‐Temperature Preparation Copper‐Doped Nickel Chloride Cathode for Thermal Battery Overcomes the Energy‐Power Trade‐Off

Nickel chloride (NiCl2) is a typical hexagonal layered semiconductor material with wide application. However, it is mainly restricted by complicated technological process within ultrahigh dehydration temperature. Utilizing copper doping, a sort of high purity and remarkable crystallinity NiCl2 is fabricated using a simple low‐temperature calcination technique. The dehydration temperature is decreased from 600 to 400 °C because the adsorbed copper ions on NiCl2 dihydrate surface can weaken NiO bond strength. Serving for thermal battery cathode, copper‐doped NiCl2 exhibits remarkable discharge ability at 500 mA cm−2, equipped with supernormal power density of 16.27 kW kg−1 and energy density of 717 Wh kg−1 simultaneously. Its energy density is increased by 28% compared to NiCl2. Copper doping optimizes thermodynamics process of discharge reaction and modifies local electronic structure of NiCl2. For copper‐doped NiCl2, the shift of Ni 3d and Cl 3p to lower energy level results in elevated redox potential, and the reduction of bandgap accelerates the carrier mobility, further promoting discharge degree. Utilizing metal ions dopant, this research surmounts the low‐temperature synthesis of NiCl2 and addresses its inferior electrochemical performance, ensuring high energy‐power output. This will expand the application scenarios of NiCl2‐based cathode materials.

Spin valve effect in CrN/GaN van der Waals heterostructures

In the pursuit of developing high-performance low-dimensional spintronic devices, two-dimensional van der Waals heterostructures comprising non-magnetic nitride and ferromagnetic materials have emerged as a crucial area of investigation. This paper proposes a novel structure that employs hexagonal two-dimensional semiconductor GaN flanked by two half-metallic two-dimensional CrN layers. The stability of the proposed structure is verified via first-principles calculations, which indicate good thermodynamic and magnetic stability. The transport characteristics of electrons in the structure are analyzed through the Band structures and density of states. Specifically, the study examines the spin polarizability of parallel and anti-parallel magnetic configurations of the CrN/GaN/CrN vdW heterostructure, and the results demonstrate a significant spin valve effect. Overall, the study establishes the potential of the CrN/GaN/CrN vdW heterostructure as a candidate for the development of innovative spintronic devices.

First principles investigations of the electronic, elastic, mechanical, anisotropic, optical, and thermoelectric performance of monoclinic SiGe semiconductors

Since crystal structure dictates the resultant physical properties of materials, titled physical features of the P21 monoclinic SiGe semiconductors, which are still unclear, have been revealed by density functional theory (DFT). The electronic band gap value with 0.49[Formula: see text]eV was found to be in the order of previously published cubic and hexagonal closed-packed SiGe semiconductors. Surprisingly, P21 monoclinic SiGe has a room temperature Seebeck coefficient of 1500[Formula: see text][Formula: see text]V/K which is higher than the reported data of both cubic and hexagonal SiGe alloys. Further, the mechanically stable P21 monoclinic phase of the SiGe displays a brittle mechanical character with clear elastic anisotropy has also been deduced. The P21 monoclinic phase of the SiGe can be also considered a good high-dielectric material and beneficial for practical applications of IR or UV goals due to its high refractive index.

Origin of the negative longitudinal piezoelectric effect and electric auxetic effect in hexagonal AIBIVCV semiconductors.

Hexagonal ABC semiconductors with a polar structure are potential candidates for piezoelectric applications. The intriguing negative longitudinal piezoelectric effect (NLPE) and electric auxetic effect (EAE) may exist in these materials, and establishing the structure-property relation provides physical insights into the underlying mechanisms responsible for these phenomena. In this work, using first-principles calculations, we investigate the piezoelectric response in a class of hexagonal AIBIVCV (A = Li, Na, and K; B = Ge and Sn; C = N, P, As, and Sb) semiconductors. We demonstrate that the quasi-layered structure with contrasting interlayer and intralayer bonding strengths plays a crucial role in the longitudinal piezoelectric response. In this class of materials, we identify 11 compounds out of the 24 candidates possessing the NLPE. We find that the NLPE tends to occur when the quasi-layered structure is pronounced. Moreover, we identify an unusual coexistence of negative longitudinal and transverse piezoelectric responses, and hence the compounds possessing the NLPE are electric auxetic materials as well. This work provides a simple guide for the search of piezoelectrics with desired responses.

The spontaneous polarization in CdS to enhance the piezo-PEC performance via phase transition stress engineering

The built-in electric field caused by piezoelectric polarization has emerged as one of the most effective strategies for photoelectrochemical (PEC) water splitting, but is challenged by low piezoelectric polarization efficiency. Herein, we firstly introduce residual stress into CdS through phase transition stress engineering for improving the spontaneous polarization of CdS with a view to revealing the mechanism of the piezoelectric PEC (piezo-PEC) performance of CdS is advanced by the spontaneous polarization. The spontaneous polarization of CdS caused via phase transition residual stress generated built-in electric field to advance carriers separation. Moreover, the spontaneous polarization of CdS is motived a greater potential under external force than hexagonal wurtzite CdS due to the existence of phase transition residual stress, which effectively improves the piezo-PEC performance of CdS. The photocurrent density of miscibility of cubic sphalerite and hexagonal wurtzite CdS (M−CdS) is 0.61 mA/cm2 at 1.23 V vs RHE, which is almost 1.79 times of hexagonal wurtzite (H-CdS). In addition, the value of M−CdS photocurrent density is reached 2.12 mA/cm2 at 1.23 V vs RHE after ultrasound, which is almost 6.2 times of H-CdS. Our detail work proves that this strategy not only stimulates the piezo-PEC performance of CdS, but also can be extended to other hexagonal wurtzite semiconductor phase transition piezoelectric fields.

Wannier Diagrams for Semiconductor Artificial Graphene

Quantum transport has been simulated in hexagonal semiconductor lattices of antidots with a period of 80 nm and short-range disorder. Wannier diagrams, i.e., DoS(n, B) maps of the density of states, where n is the electron density and B is the magnetic field strength, have been calculated for several potential modulation amplitudes comparable to or much larger than the Fermi energy. Deep dips in the maps of the density of states have the form of rays with positive, zero, and negative slopes. In addition to the fan of the rays separating the first and second, as well as the second and third Landau levels, the maps include rays that are parallel to them and are shifted in n and B by integers of the characteristic electron density n0 and the characteristic magnetic field strength B0, respectively. It has been shown that the sign and magnitude of the slope of the rays in the density of states correspond to the centers of the plateaus of quantized Hall resistances Rxy. The lattice is brightly manifested in the Rxy(n, B) maps as the replicas of the first and second plateaus in Rxy and as oscillations of Rxy between negative and positive values at a fixed magnetic field or a fixed electron density, which indicates the interchange between the hole and electron charge carriers.

Lattice Polarity Manipulation of Quasi-vdW Epitaxial GaN Films on Graphene Through Interface Atomic Configuration.

Quasi van der Waals epitaxy, a pioneering epitaxy of sp3 -hybridized semiconductor films on sp2 -hybridized 2D materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, it is reported that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity are achieved by forming CONGa(3) or COGaN(3) configurations, respectively, on artificial CO surface dangling bonds by atomic oxygen pre-irradiation on trilayer graphene. Furthermore, an aluminum nitride buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question of why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layers grown on graphene, which definitely promotes the development of novel semiconductor devices.

A feasible and low-cost green route to prepare ZnO with n or p-type conductivity by changing the parsley extract concentration

ZnO samples were prepared by an easy process based on green synthesis using three different concentrations of the parsley extract as natural reducing agents. The semiconductors were characterized structural, morphological, and opto-electronically by XRD, SEM-EDS, diffuse reflectance, and photo-response, respectively. Hexagonal structure ZnO-based semiconductors were obtained after the annealing process at 400ºC. The morphology, bandgap, and photo-response of the synthesized samples were sensitive to the extract concentrations used in this work. The incorporation of Mg traces in the bulk of ZnO produced changes in the c-lattice parameter, consequently forming native defects such as Oi and Vo. The bandgap increased as the Mg content in the ZnO increased too. The photo-response analysis suggested that the samples with 0.44 and 1.33 at% of Mg were p-type ZnO, while the sample with 2.01 at% was n-type ZnO. It likely means that the conductivity type and other optoelectronic characteristics depend on the Mg content in the ZnO samples and the formation of predominant native defects due to the interaction of Mg with the crystalline structure.

Plasmonic Modulation of Valleytronic Emission in Two‐Dimensional Transition Metal Dichalcogenides

AbstractMonolayered transition metal dichalcogenides (TMDs) are one kind of hexagonal 2D semiconductors with a direct bandgap structure. Due to the property of natural broken inversion symmetry in the lattice, the strong spin–orbit coupling of electrons in TMDs can induce degenerate levels with antiparallel spins in K and K′ valleys, which selectively respond with external light excitations. Surface plasmon resonance with efficient electromagnetic enhancement and near‐field coupling provides excellent potential opportunities to modulate valley emission of TMDs. Efforts have been devoted to investigating the interaction principles and applications of this research field. This review focuses on plasmonic modulation of valleytronic emission in TMDs with surface plasmon polaritons (SPP) and localized surface plasmons (LSP) based on different modulation principles, respectively, and discusses possible research directions for future device applications.

A computational prediction of a novel quasi hexagonal Al2SSi semiconductor monolayer

Abstract In this article, we predicted a novel two dimensional (2D) quasi hexagonal Al2SSi semiconductor monolayer by using first principles calculations based on the density functional theory (DFT). At first, it is shown that 2D Al2SSi is energetically and dynamically stable suggests its promising experimental realization. As a possible stable 2D monolayer, its electrical and optical properties were investigated. To get more insight into the effect of external strains on the electronic property of the Al2SSi monolayer, we calculated the electrical properties of the structure under different strain conditions. We found that, the electron properties of the material can be effectively modulated by strain effect. According to our calculations, 2D Al2SSi monolayer is a semiconductor with a strain tunable direct band gap 0.344 eV (1.09 eV) obtained by DFT-PBE (DFT-HSE06) level of theory. Assessing its optical properties shows that the proposed monolayer shows optical sensitivity in the visible light region as well as the high absorption in the ultraviolet (UV) portion of the electromagnetic spectrum. These results, suggests 2D Al2SSi as a promising material for using in electrical and optical nano devices.

Formation and preferential orientation of Au-free Al/Ti-based ohmic contacts on different hexagonal nitride-based heterostructures

Wide-bandgap nitride semiconductors are currently in development for high-power electronic applications. Compositional layered heterostructures of such nitrides result in a high polarization field at the interface, enabling a higher electron mobility, a higher power density, and a higher conversion efficiency. Further optimization of such GaN-based high-electron-mobility transistors can be achieved by evolving from a top AlxGa1−xN barrier toward AlN or even InyAl1−yN. An ongoing challenge in using such hexagonal nitride semiconductors is the formation of a low-resistive, Au-free, ohmic contact far below 1Ωmm. In this paper, we investigate the formation of ohmic contacts by Ti–Al–TiN-based metalization as a function of different annealing temperatures (up to 950°C), Ti–Al ratios (from 15 up to 35 at. %) and nitride barrier composition (AlxGa1−xN, GaN, AlN, and InyAl1−yN). Contacts processed on AlxGa1–x/GaN, and AlN/GaN heterostructures result in low contact resistance of, respectively, 0.30 and 0.55Ωmm, whereas the same contact stack on InyAl1−yN results in resistance values of 1.7Ωmm. The observed solid-phase reaction of such Ti–Al–TiN stacks were found to be identical for all investigated barrier compositions (e.g., AlxGa1−xN , GaN, AlN, and InyAl1−yN), including the preferential grain alignment to the epitaxial nitride layer. The best performing ohmic contacts are formed when the bottom Ti-layer is totally consumed and when an epitaxially-aligned metal layer is present, either epitaxial Al (for a contact which is relatively Al-rich and annealed to a temperature below 660°C) or ternary Ti2AlN (for a relatively Ti-rich contact annealed up to 850°C). The observation that the solid-phase reaction is identical on all investigated nitrides suggests that a further decrease of the contact resistance will be largely dependent on an optimization of the nitride barriers themselves.

Graphene‐Assisted Epitaxy of Nitrogen Lattice Polarity GaN Films on Non‐Polar Sapphire Substrates for Green Light Emitting Diodes

AbstractLattice polarity is a key point for hexagonal semiconductors such as GaN. Unfortunately, only Ga‐polarity GaN have been achieved on graphene till now. Here, the epitaxy of high quality nitrogen‐polarity GaN films on transferred graphene on non‐polar sapphire substrates by molecular beam epitaxy is reported. This success is achieved through atomic nitrogen irradiation, where CN bonds are formed in graphene and provide nucleation sites for GaN and leading to N‐polarity GaN epitaxy. The N‐polarity characteristics are confirmed by chemical etching and transmission electron microscopy measurement. Due to the higher growth temperature of InGaN at N‐polarity than that at Ga‐polarity, green light emitting diodes are fabricated on the graphene‐assisted substrate, where a large redshift of emission wavelength is observed. These results open a new avenue for the polarity modulation of III‐nitride films based on 2D materials, and also pave the way for potential application in longer wavelength light emitting devices.

Hexagonal layered group IV-VI semiconductors and derivatives: fresh blood of the 2D family.

New phases of group IV-VI semiconductors in 2D hexagonal structures are predicted and their unusual physical properties are revealed. The structures of monolayer group IV-VI semiconductors are similar to those of blue phosphorene and each unit has the same ten valence electrons. The band gap of 2D hexagonal group IV-VI semiconductors depends on both the thickness and stacking order. Atomic functionalization can induce ferromagnetism, and the Curie temperature can be tuned. Gapped Dirac fermions with zero mass are developed and this makes it exceed that of graphene. The Fermi velocity can be compared to or even above that of graphene.

Эффекты магнитного поля в экситонной генерации оптических гармоник

Mechanisms responsible for the optical harmonics generation in several classes of materials at excitonic transitions are considered. In the cubic semiconductor GaAs, magnetically induced second harmonic generation (SHG) was observed in the region of orbital quantization for the valence and conduction bands. An unusually strong third harmonic generation was detected in the region of the 1s exciton in an external magnetic field due to the exciton-polariton resonance. Magnetic field induced SHG was revealed in the hexagonal wide bandgap semiconductor ZnO at exciton resonances 1s, 2s, and 2p. Depending on the symmetry of the exciton states, the SHG mechanisms are determined by the spin and orbital Zeeman effects, and the magneto-Stark effect. Magnetically induced contribution to SHG was studied in the antiferromagnet Cr2O3 at exciton transitions in an external magnetic field.

Near-ultraviolet photodetector based on hexagonal TmFeO3 ferroelectric semiconductor thin film with photovoltaic and pyroelectric effects

The switchable ferroelectric photovoltaic (FPV) effect facilitates application of multifunctional photoelectric devices. The drawback of the FPV effect is that it generates a very low photocurrent in highly insulated ferroelectric materials. In contrast, the light-induced pyroelectric effect enhances photoelectric performance. Both effects strongly depend on the ferroelectric polarization of the material. In this study, we fabricated and characterized a near-ultraviolet photodetector consisting of a Pt/hexagonal TmFeO3/Pt heterojunction. The switchable FPV and light-induced pyroelectric effects are both observed in a hexagonal TmFeO3 ferroelectric semiconductor film. An additional potential arises from the light-induced pyroelectric effect, which strongly depends on the light intensity. The Schottky barrier height can be modulated by both the poling electric field and light-induced pyroelectric potential. Increasing the power density above the threshold leads to switchable polarization via the light-induced pyroelectric potential. The coexistence of photovoltaic and pyroelectric effects in the hexagonal TmFeO3 ferroelectric semiconductor makes it possible to develop electronic, thermal, and optical sensors as well as energy conversion devices.

Corner and side localization of electrons in irregular hexagonal semiconductor shells

We discuss the low energy electronic states in hexagonal rings. These states correspond to the transverse modes in core–shell nanowires built of III–V semiconductors which typically have a hexagonal cross section. In the case of symmetric structures the 12 lowest states (including the spin) are localized in the corners, while the next following 12 states are localized mostly on the sides. Depending on the material parameters, in particular the effective mass, the ring diameter and width, the corner and side states may be separated by a considerable energy gap, ranging from few to tens of meV. In a realistic fabrication process geometric asymmetries are unavoidable, and therefore the particles are not symmetrically distributed between all corner and side areas. Possibly, even small deformations may shift the localization of the ground state to one of the sides. The transverse states or the transitions between them may be important in transport or optical experiments. Still, up to date, there are only very few experimental investigations of the localization-dependent properties of core–shell nanowires.

Gate-tunable polariton superlens in 2D/3D heterostructures

Polaritons in polar-dielectrics and van der Waals (vdW) materials provide a channel for strong photon confinement. Precise control of their propagation could lead to deep sub-wavelength photonic devices. Here, we report negative refraction of hybrid surface phonon-hyperbolic polaritons (SPh-HP) at the interface of two-dimensional (2D) van der Waals layers such as hexagonal boron nitride (h-BN) and 3D semiconductors such as germanium and silicon carbide. These hybrid polariton modes have naturally negative group velocity arising from the intrinsic Type-I hyperbolicity of h-BN resulting in negative refraction at interfaces with positive group velocity. Using this phenomenon, we demonstrate an in-plane superlensing effect in an ultrathin (~10 nm) slab with spatial confinement of long Infrared wavelengths to below 200 nm focal spots. We further demonstrate electrical tunability of the superlens by controlling the Fermi level of graphene, thereby offering potential for miniaturized infrared to THz modulators, photodetectors as well as logic switches.