Anisotropic Semiconductor Research Articles

Angle-resolved polarization Raman spectroscopy of β-Ga2O3 and their application in sensitive solar-blind photodetection

Polarization-sensitive photodetection has promising prospects for civilian and military applications based on anisotropic semiconductors. However, it is greatly limited due to the lack of valid materials as well as the terrible linear dichroism ratio. In this Letter, a metal–oxide–semiconductor β-Ga2O3 with strong anisotropic property is proposed for highly efficient polarizing detection, which can potentially overcome these limitations. Angle-resolved polarization Raman spectroscopy was performed to confirm excellent anisotropic phonon vibration. Unique narrow solar-blind polarization-sensitive photo-absorption (240–270 nm) can be observed, which can be attributed to the natural anisotropy, referring in particular to the polarization-resolved absorption in the surround of the bandgap of β-Ga2O3. Benefiting from the structural anisotropy, the polarization-sensitive photodetector exhibits an excellent linear dichroic ratio of ∼1.8. Moreover, obvious color change is observed under different polarized angles, providing great potential in polarization imaging. With these advantages, we anticipated that this research will pave avenues for the fabrication of polarization-sensitive solar-blind UV photodetectors.

Piezoelectricity in NbOI2 for piezotronics and nanogenerators

2-dimensional (2D) piezoelectric materials have gained significant attention due to their potential applications in flexible energy harvesting and storage devices. Recently, niobium oxide dihalides NbOI2 stands out as a multifunctional anisotropic semiconductor family with an exceptionally high lateral piezoelectric constant (~21.8 pm/V), making it a promising candidate for energy conversion applications. Here we report the experimental observation of anisotropic in-plane piezoelectricity in multilayer NbOI2. Current-voltage relationships reveal a significant piezotronic effect in two typical crystalline orientations. Additionally, cyclic tensile and release experiments demonstrate an intrinsic current output of up to 140 pA when subjected to a tensile strain of 0.51%. A flexible piezoelectric nanogenerator prototype is demonstrated on the human finger and wrist, which opens up new avenues for the development of wearable electronic devices and provides valuable insights for further exploration in this field.

Multi-dimensional optical information acquisition based on a misaligned unipolar barrier photodetector

Acquiring multi-dimensional optical information, such as intensity, spectrum, polarization, and phase, can significantly enhance the performance of photodetectors. Incorporating these dimensions allows for improved image contrast, enhanced recognition capabilities, reduced interference, and better adaptation to complex environments. However, the challenge lies in obtaining these dimensions on a single photodetector. Here we propose a misaligned unipolar barrier photodetector based on van der Waals heterojunction to address this issue. This structure enables spectral detection by switching between two absorbing layers with different cut-off wavelengths for dual-band detection. For polarization detection, anisotropic semiconductors like black phosphorus and black arsenic phosphorus inherently possess polarization-detection capabilities without additional complex elements. By manipulating the crystal direction of these materials during heterojunction fabrication, the device becomes sensitive to incident light at different polarization angles. This research showcases the potential of the misaligned unipolar barrier photodetector in capturing multi-dimensional optical information, paving the way for next-generation photodetectors.

Surface-Originated Weak Confinement in Tetrahedral Indium Arsenide Quantum Dots.

While the shape-dependent quantum confinement (QC) effect in anisotropic semiconductor nanocrystals has been extensively studied, the QC in facet-specified polyhedral quantum dots (QDs) remains underexplored. Recently, tetrahedral nanocrystals have gained prominence in III-V nanocrystal synthesis. In our study, we successfully synthesized well-faceted tetrahedral InAs QDs with a first excitonic absorption extending up to 1700 nm. We observed an unconventional sizing curve, indicating weaker confinement than for equivalently volumed spherical QDs. The (111) surface states of InAs QDs persist at the conduction band minimum state even after ligand passivation with a significantly reduced band gap, which places tetrahedral QDs at lower energies in the sizing curve. Consequently, films composed of tetrahedral QDs demonstrate an extended photoresponse into the short-wave infrared region, compared to isovolume spherical QD films.

Insights into the formation of CdSe nanoplatelets using a flow reactor.

In recent years, the anisotropic semiconductor nanoplatelets (NPLs) gained interest due to their unique optical properties, which depend primarily on their thickness. However, the formation mechanism behind the zinc blende CdSe NPLs remains unclear. Several theories were presented and discussed, but a concrete mechanism has not yet been found with evidence. Here, we want to present a synthesis of CdSe NPLs in a flow reactor with a liquid precursor, enabling in situ absorbance measurements. The flow reactor allows for more control in obtaining early-stage synthesis samples, which were ex situ examined with optical spectroscopy, transmission electron microscopy, as well as small-angle and powder X-ray diffraction. Our results show that CdSe magic size clusters (MSCs) formed prior to the formation of CdSe NPLs, indicating that these CdSe MSCs are necessary for the initial CdSe NPLs growth.

Hyperbolic exciton polaritons in a van der Waals magnet

Exciton polaritons are quasiparticles of photons coupled strongly to bound electron-hole pairs, manifesting as an anti-crossing light dispersion near an exciton resonance. Highly anisotropic semiconductors with opposite-signed permittivities along different crystal axes are predicted to host exotic modes inside the anti-crossing called hyperbolic exciton polaritons (HEPs), which confine light subdiffractionally with enhanced density of states. Here, we show observational evidence of steady-state HEPs in the van der Waals magnet chromium sulfide bromide (CrSBr) using a cryogenic near-infrared near-field microscope. At low temperatures, in the magnetically-ordered state, anisotropic exciton resonances sharpen, driving the permittivity negative along one crystal axis and enabling HEP propagation. We characterize HEP momentum and losses in CrSBr, also demonstrating coupling to excitonic sidebands and enhancement by magnetic order: which boosts exciton spectral weight via wavefunction delocalization. Our findings open new pathways to nanoscale manipulation of excitons and light, including routes to magnetic, nonlocal, and quantum polaritonics.

The Study of the Electrical Properties of Anisotropic Semiconductors in the Laboratory Workshop of the University Course in Physics

The purpose of the article is to develop a theoretically and experimentally substantiated specific laboratory work for studying the anisotropy of the electrical properties of semiconductors within the framework of the curricula of higher educational institutions, for such areas of training as “Physics”, “Technical Physics”, “Radiophysics” and “Electronics and Nanoelectronics”. The relevance of the problem is due to the desire of domestic electronics for technological sovereignty and high requirements for the training of qualified personnel for the production of structures of solid-state functional electronics. Based on the solution of the boundary electrodynamic problem, an expression is obtained for the electric potential in the region of a thin rectangular semiconductor sample with a tensor character of conductivity. The results of this work is the development of an original laboratory setup for the demonstration and practical study of the anisotropy of the electrical properties of semiconductor crystals. The proposed technique makes it possible to determine the main electrokinetic parameters of an anisotropic semiconductor – specific conductivity, concentration and Hall mobility of the main charge carriers. A schematic diagram of the installation and formulas for calculating the errors of the measured quantities are proposed. The results obtained may also be of scientific interest in the study of anisotropic semiconductor materials in laboratory conditions.

2D semiconductor nanosheets for solar photocatalysis

AbstractIn the advancing world of graphene, highly anisotropic 2D semiconductor nanosheets, notable for their nanometer‐scale thickness, have emerged as a leading innovation, displaying immense potential in the exploration of renewable and clean energy production. These have garnered significant attention from researchers. The nanosheets are marked by their extraordinary electronic, optical, and chemical attributes, positioning them as attractive foundational components for heterogeneous photocatalysts. This review diligently summarizes both the seminal work and ongoing developments pertaining to 2D semiconductor nanosheets and their application to solar energy within the context of heterogeneous photocatalysis. We begin by detailing the distinctive properties of 2D semiconductor nanosheets, concentrating on their pivotal roles in augmenting photocatalytic efficiency, and explaining the intrinsic mechanisms that govern the migration rate of photogenerated carriers on the material's surface. Subsequently, we delineate the methods employed to synthesize typical 2D semiconductor nanosheets. In alignment with the overarching objective of expanding light absorption capacity and accelerating charge transfer, we also examine the current research on the hybridization techniques involving 2D materials of varied dimensions, as well as their deployment in diverse photocatalytic applications. We conclude by identifying promising avenues and potential challenges that await further exploration in this burgeoning field.

Dimensionality crossover for moiré excitons in twisted bilayers of anisotropic two-dimensional semiconductors

Dimensionality crossover for moiré excitons in twisted bilayers of anisotropic two-dimensional semiconductors

Realizing Unassisted Photo-Charging of Zinc-Air Batteries by Anisotropic Charge Separation in Photoelectrodes.

Solar rechargeable zinc-air battery is a promising approach for capturing and storing intermittent solar energy through photoelectrochemical reactions. However, unassisted photo-charging of zinc-air batteries is challenging due to suboptimal carrier accumulation on photoelectrodes, resulting in sluggish reaction kinetics. Here, unassisted photo-charging of zinc-air battery is achieved by investigating anisotropic photogenerated charge separation on a series of representative semiconductors (ZnIn2 S4 , TiO2 , and In2 O3 ), among which the exceptional anisotropic charge separation on a ZnIn2 S4 photoelectrode is revealed based on anisotropic charge diffusion capabilities. The charge separation is facet-dependent, which is observed using Kelvin probe force microscopy, verifying a cause-and-effect relationship between the photo-charge accumulation on photoelectrodes and their photo-charging performance in zinc-air batteries. This work achieves an unassisted photo-charging current density of 1.9mAcm-2 with a light-to-chemical energy conversion efficiency of 1.45%, highlighting the importance of anisotropic semiconductors for unassisted photo-charging of zinc-air batteries via efficient photogenerated charge separation.

Mathematical Modelling of the Electric Field in Anisotropic Semiconductors during Hall Measurements

Mathematical Modelling of the Electric Field in Anisotropic Semiconductors during Hall Measurements

Features of the Electrical Properties of Limited Diffusion Semiconductors in Contact Measurements

The purpose of this work is to study the features of the electrical properties of bounded semiconductors, as well as the effect of the inhomogeneous distribution of impurities in anisotropic semiconductors on the structure of electric fields with standard methods of contact measurements.Diffusion and ion-doped semiconductor structures, as well as composite structures with misalignment of film and substrate lattices, are currently a promising class of materials in micro- and nanoelectronics. The introduction of an impurity into a semiconductor crystal sample changes its electrical properties, which makes it possible to reduce the size of individual components of the chip, while maintaining or increasing their power.Methods. The results of the work are based on the known dependences of the distributions of impurities during diffusion alloying. To calculate the potential distribution, the Fourier method was used, which made it possible to solve the Poisson Equation with Neumann boundary conditions without approximations. The mathematical package MathCad was used to analyze the obtained expressions characterizing electric fields in inhomogeneous semiconductors.Results. A technique for solving the boundary value problem for the potential in bounded diffusion semiconductors is presented. Expressions in the form of series of analytical functions for the distribution of electric potential in conducting anisotropic structures are presented. The constructed models make it possible to determine and qualitatively describe the distributions of electric fields during probe measurements and to investigate their structure in diffusion semiconductors. Conclusion. On the basis of computer modeling, the importance of taking into account the parameters of inhomogeneity of conductivity is shown. The effect of concentration of current density lines in diffusion semiconductors is shown and quantified.

Two-dimensional monoelement nanosheets produced from bulk crystals for potassium-ion storage

Two-dimensional (2D) nanomaterials, particularly anisotropic monoelemental semiconductors, have attracted considerable attention for optoelectronic and electronic applications. Herein, a general substrate-free solution process is presented to prepare 2D non-layered selenium (Se)/tellurium (Te) and layered antimony (Sb) from bulk crystals. As a representative anode for K+ storage, the 2D Sb/mesoporous carbon (CMK-3) composite exhibits a high capacity up to 390.1 mAh/g at 50 mA g−1 and ultra-high retention of 98.9 % after 200 cycles, demonstrating that the reduced particle size and expanded interlayer distance effectively shorten the ionic diffusion pathway for fast kinetics.

Optical polarization perturbed by shear strains of ultrasonic bulk waves in anisotropic semiconductors: Multiphysics modeling and optoacoustic validation

Characterization of lattice properties of monocrystalline semiconductors (MS) has been rapidly advanced. Of particular interest is the use of shear strains induced by optoacoustic-bulk-waves. However, this technique has been hindered owing to the lack of quantitative correlations between optoacoustic-bulk-waves-induced shear strains and anisotropic photoelasticity of MS. Motivated by this, a multiphysics model is developed to interrogate the coupling phenomena and interaction between optical polarization and shear strains in MS. With the model, perturbation to the polarization of a monochromatic laser beam, upon interacting with optoacoustic waves in MS, is scrutinized quantitatively. Experimental results are in agreement with those from the model, both revealing the polarization perturbed by shear strains quantitatively depends on the crystal orientation and crystal-structure-related symmetry, which are jointly governed by mechanical/photoelastic/optical anisotropies of MS. The approach has paved a new way for selectively acquiring high-sensitivity shear components of optoacoustic-ultrasonic-waves for in situ, high-definition characterization of anisotropic MS.

Silicon and germanium adamantane and diamantane monolayers as two-dimensional anisotropic direct-gap semiconductors

Silicon and germanium adamantane and diamantane monolayers as two-dimensional anisotropic direct-gap semiconductors

In-plane anisotropic photoelectric property of α -In2Se3 based phototransistor

Contributing to excellent photoelectric property, the tunable bandgap and intercorrelated in-plane and out-of-plane ferroelectric polarization simultaneously, α-In2Se3 has great potential in the applications of optoelectronic devices and photo-controlled devices, like memories, sensors, and synapses. However, little attention is paid to the in-plane anisotropic photoelectric property of α-In2Se3, which may restrict its competitiveness in application of designing and fabrication of optoelectronic devices and photo-controlled devices. Herein, multi-layered α-In2Se3 based phototransistors with eight terminals are prepared, and its in-plane anisotropic photodetection is investigated. By comparing the dark current (Idark), photocurrent (Iph), responsivity (R), external quantum efficiency (EQE), and specific detectivity (D*), in-plane anisotropic photoelectric property of multi-layered α-In2Se3 is demonstrated, and the Idark, Iph, R, EQE, and D* anisotropic ratios are up to 163.76, 480.59, 480.59, 480.59, and 58.8, respectively. The carrier mobility and the in-plane ferroelectric polarization are the two main factors determining the in-plane anisotropic photoelectric property. The excellent in-plane anisotropic photoelectric property makes α-In2Se3 a promising candidate as an in-plane anisotropic semiconductor for high-sensitivity optoelectronic and photo-controlled applications.

Efficient GW calculations in two dimensional materials through a stochastic integration of the screened potential

Many-body perturbation theory methods, such as the G0W0 approximation, are able to accurately predict quasiparticle (QP) properties of several classes of materials. However, the calculation of the QP band structure of two-dimensional (2D) semiconductors is known to require a very dense BZ sampling, due to the sharp q-dependence of the dielectric matrix in the long-wavelength limit (q → 0). In this work, we show how the convergence of the QP corrections of 2D semiconductors with respect to the BZ sampling can be drastically improved, by combining a Monte Carlo integration with an interpolation scheme able to represent the screened potential between the calculated grid points. The method has been validated by computing the band gap of three different prototype monolayer materials: a transition metal dichalcogenide (MoS2), a wide band gap insulator (hBN) and an anisotropic semiconductor (phosphorene). The proposed scheme shows that the convergence of the gap for these three materials up to 50meV is achieved by using k-point grids comparable to those needed by DFT calculations, while keeping the grid uniform.

Two-dimensional single crystal monoclinic gallium telluride on silicon substrate via transformation of epitaxial hexagonal phase

Van der Waals (vdW) epitaxial growth of large-area and stable two-dimensional (2D) materials of high structural quality on crystalline substrates is crucial for the development of novel device technologies. 2D gallium monochalcogenides with low in-plane symmetry stand out among the layered semiconductor materials family for next-generation optoelectronic and energy conversion applications. Here, we demonstrate the formation of large-area, single crystal and optically active 2D monoclinic gallium telluride (m-GaTe) on silicon substrate via rapid thermal annealing induced phase transformation of vdW epitaxial metastable hexagonal gallium telluride (h-GaTe). Stabilization of multilayer h-GaTe on Si occurs due to the role of the first layer symmetry together with efficient GaTe surface passivation. Moreover, we show that the phase transformation of h-GaTe to m-GaTe is accompanied by the strain relaxation between Si substrate and GaTe. This work opens the way to the fabrication of single-crystal 2D anisotropic semiconductors on standard crystalline wafers that are difficult to be obtained by epitaxial methods.

Quasi-One-Dimensional ZrS3 Nanoflakes for Broadband and Polarized Photodetection with High Tuning Flexibility.

Two-dimensional (2D) layered materials with low crystal symmetries have exhibited unique anisotropic physical properties. Here, we report systematic studies on the photoresponse of field effect transistors (FETs) fabricated using quasi-one-dimensional ZrS3 nanoflakes. The as-fabricated phototransistors exhibit a broadband photocurrent response from ultraviolet to visible regions, where the responsivity and detectivity can be enhanced via additional gate voltages. Furthermore, benefiting from the strong in-plane anisotropy of ZrS3, we observe a gate-voltage and illumination wavelength-dependent polarized photocurrent response, while its sub-millisecond-time response speed is also polarization-dependent. Our results demonstrate the flexible tunability of photodetectors based on anisotropic layered semiconductors, which substantially broadens the application of low symmetry layered materials in polarization-sensitive optoelectronic devices.

2D janus niobium oxydihalide NbOXY: Multifunctional piezoelectric semiconductor for electronics, photonics, sensing and sustainable energy applications

Two-dimensional (2D) niobium oxydihalide NbOI2 has been recently demonstrated as an excellent in-plane piezoelectric and nonlinear optical material. Here we show that Janus niobium oxydihalide, NbOXY (X, Y = Cl, Br, I and X ≠ Y), is a multifunctional anisotropic semiconductor family with exceptional piezoelectric, electronic, photocatalytic and optical properties. NbOXY are stable and flexible monolayers with band gap around the visible light regime of ∼1.9 eV. The carrier mobility of NbOXY exhibits an exceptionally strong anisotoropy ratio of ∼400, which is larger than most of the anisotropic 2D semiconductors. The exceptionally large deformation potential of NbOXYmonolayers leads to strong piezoresistive effect useful for strain sensing application. Inversion symmetry breaking in Janus NbOXY generates sizable out-of-plane d31 piezoelectric response while still retaining a strong in-plane piezoelectricity. Remarkably, NbOXY exhibits an additional out-of-plane piezoelectric response, d32 as large as 0.55 pm/V. G0W0-BSE calculation further reveals the strong linear optical dichroism of NbOXY in the visible-to-ultraviolet regime. The optical absorption peaks of 14 ∼ 18% in the deep UV regime (5 ∼ 6 eV) outperform the vast majority of other 2D materials. The high carrier mobility, strong optical absorption, sizable built-in electric field and band alignment compatible with overall water splitting further suggest the strengths of NbOXY in solar-to-hydrogen conversion. We further propose a directional stress sensing device to demonstrate how the out-of-plane piezoelectricity can be harnessed for functional device applications. Our findings unveil NbOXY as an exceptional multifunctional 2D semiconductor for flexible electronics, optoelectronics, UV photonics, piezoelectronics and sustainable energy applications.