Anisotropic Semiconductor Research Articles

Optical properties and polaritons of low symmetry 2D materials

Low symmetry 2D materials with intrinsic in-plane anisotropic optical properties and high tunability provide a promising platform to explore and manipulate light–matter interactions. To date, dozens of in-plane anisotropic 2D materials with diverse band structures have been discovered. They exhibit rich optical properties, indicating great potential for novel applications in optics, photonics, and optoelectronics. In this paper, we thoroughly review the anisotropic optical properties and polaritons in many kinds of low symmetry 2D materials, aiming to elicit more research interest in this field. First, the optical properties of anisotropic 2D semiconductors, including interband absorption, photoluminescence, excitons, and band structure engineering for tuning optical responses, are introduced. Then fundamentals and advances in experiments of hyperbolic polaritons in anisotropic 2D materials, including phonon, plasmon, and exciton polaritons, are discussed. Finally, a perspective on promising research directions is given.

Polaronic Effects on the Electron Raman Scattering in a Spherical Anisotropic Quantum Dot

We study the polaronic effect on the differential cross section (DCS) for an electron Raman scattering process in a spherical anisotropic semiconductor quantum dot in the presence of a homogeneous external electric field. The Fröhlich coupling of electrons with confined and surface polar optical phonon modes for resonance Raman scattering is considered. The influence of electron-phonon coupling on the energy spectrum is taken into account in the framework of perturbation theory. The emission spectra are discussed for different anisotropy cases. Resonant peaks in the spectra are found and interpreted.

High Performance Polarization-Resolved Photodetectors Based on Intrinsically Stretchable Organic Semiconductors.

Polarization-sensitive photodetectors based on anisotropic semiconductors sense both the intensity and polarization state information without extra optical components. Here, a self-powered organic photodetector (OPD) composed of intrinsically stretchable polymer donor PNTB6-Cl and non-fullerene acceptor Y6 is reported. The PNTB6-Cl:Y6 photoactive film accommodates a remarkable 100% strain without fracture, exhibiting a high optical anisotropy of 1.8 after strain alignment. The resulting OPD not only shows an impressive faint-light detection capability (high spectral responsivity of 0.45 A W-1 and high specific detectivity of 1012 Jones), but also has a high anisotropic responsivity ratio of 1.42 under the illumination of parallel and traversed polarized light. To the best of the authors' knowledge, both the detector performance and polarization features are among the best-performing OPDs and polarization-sensitive photodetectors. As a proof-of-concept, polarization-sensitive OPDs are also utilized to set up a polarimetric imaging system and full-Stokes polarimeter. This work explores the potential of highly stretchable organic semiconductors for state-of-art polarization imaging and spectroscopy applications.

Anisotropic charge trapping in phototransistors unlocks ultrasensitive polarimetry for bionic navigation

Being able to probe the polarization states of light is crucial for applications from medical diagnostics and intelligent recognition to information encryption and bio-inspired navigation. Current state-of-the-art polarimeters based on anisotropic semiconductors enable direct linear dichroism photodetection without the need for bulky and complex external optics. However, their polarization sensitivity is restricted by the inherent optical anisotropy, leading to low dichroic ratios of typically smaller than ten. Here, we unveil an effective and general strategy to achieve more than 2,000-fold enhanced polarization sensitivity by exploiting an anisotropic charge trapping effect in organic phototransistors. The polarization-dependent trapping of photogenerated charge carriers provides an anisotropic photo-induced gate bias for current amplification, which has resulted in a record-high dichroic ratio of >104, reaching over the extinction ratios of commercial polarizers. These findings further enable the demonstration of an on-chip polarizer-free bionic celestial compass for skylight-based polarization navigation. Our results offer a fundamental design principle and an effective route for the development of next-generation highly polarization-sensitive optoelectronics.

Engineering the Polarization Sensitivity in All‐2D Photodetectors Composed of Semimetal MoTe2 and Semiconductor WS2

AbstractThe polarization sensitivity, defined as the photocurrent anisotropic ratio, is of high importance for the polarization‐sensitive photodetectors that determine the resolution and contrast of captured images. Conventional polarized light detectors utilizing optical filters suffer from complex fabrication and calibration processes. 2D materials with in‐plane anisotropic structures can serve to realize the detection of polarized light, yet the polarization sensitivity still remains poor, hampering practical implementations. In this work, highly sensitive polarized photodetectors are achieved by designing an all‐2D device architecture composed of anisotropic semimetal MoTe2 and semiconductor WS2. In MoTe2/WS2 heterojunction, the built‐in electric field at the interface between a semimetal and a semiconductor yields a strong photovoltaic effect, leading to a fast (≈45 µs) and polarization‐sensitive photodetection due to the anisotropic features of MoTe2. It is shown that MoTe2/WS2/MoTe2 devices feature a polarization sensitivity of 13, a nearly five‐fold improvement by aligning the lattice orientation of side MoTe2 from cross to parallel configurations. This work offers a new degree of freedom to engineer the polarization sensitivity and photodetection performance by optimizing crystal orientation in an all‐2D architecture comprising semimetal and semiconducting 2D materials.

Simulation on different orientation hexagonal SiC-based hetero-polytype IMPATT terahertz diodes

SiC hetero-polytypes that could be fabricated by chemical vapor deposition etc. methods can be employed to design the power devices. Herein, the 〈0 0 0 1〉(〈1 1 2¯ 0〉)4H(6H)-SiC-based hetero-polytypes are adopted to construct the n+/n/i/p/p+-type impact ionization avalanche transit time diodes (IMPATTDs) and mixed tunneling avalanche transit time diodes (MITATTDs). The DC, large-signal and noise performance of the proposed diodes operating at 0.85 THz are numerically simulated based on the fundamental equations for semiconductor devices. The results indicate that, the diodes with 〈0 0 0 1〉4H(6H)-SiC in each of the i-type, n-type and p-type regions can render higher break-down voltage and conversion efficiency while lower quality factor comparing to those with 〈1 1 2¯ 0〉4H(6H)-SiC in the three regions, the latter devices involve lower noise. The diodes contained 3C-SiC show lower break-down voltage and quality factor while higher noise than those without 3C-SiC. Little difference of DC, large-signal and noise properties between IMPATTDs and MITATTDs is implied due to very weak tunneling through thick intrinsic-region. The present diodes show higher conversion efficiency comparing to the n+/n/p/p+-type devices. In addition, the fabricating feasibility of the proposed devices is discussed. This paper can guide to design the reasonable structures by using suitable anisotropic semiconductors to achieve high-performance devices.

Boron-pnictogens: Highly anisotropic two-dimensional semiconductors for nanoelectronics and optoelectronics

Two-dimensional materials open up tremendous opportunities for nanoelectronics and optoelectronics. Using first-principles density functional methods, we predict a family of two-dimensional boron-pnictogen materials. Our results show that these materials have excellent energetic, dynamical, thermal, mechanical, and chemical stabilities. The intrinsic structural anisotropy found in these materials leads to highly direction-dependent mechanical, electronic, and optical properties. They possess highly anisotropic Young's modulus and Poisson's ratio. The tensile strength under uniaxial and biaxial deformations is found to be very high for these materials. Electronically, they are all semiconductors with narrow band gaps. The band gap energies can be tuned by alloying, strain engineering, and chemical functionalization. They exhibit anisotropic and high carrier mobility. All these electronic properties make them promising candidates for nanoelectronic device applications. Using state-of-the-art $GW$- Bethe-Salpeter equation approach, taking the electron-hole effect into account, the prominent optical absorption structure with strong anisotropy in the visible light region endow the boron-pnictogen materials with great potential in optoelectronics.

Strong bulk-surface interaction dominated in-plane anisotropy of electronic structure in GaTe

Recently, intriguing physical properties have been unraveled in anisotropic layered semiconductors, in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry and thus a thickness-independent character emerges. Here, we apply high-resolution angle-resolved photoemission spectroscopy to directly image the in-plane anisotropic energy bands in monoclinic gallium telluride (GaTe). Our first-principles calculations reveal the in-plane anisotropic energy band structure of GaTe measured experimentally is dominated by a strong bulk-surface interaction rather than geometric factors, surface effect and quantum confinement effect. Furthermore, accompanied by the thickness of GaTe increasing from mono- to few-layers, the strong interlayer coupling of GaTe induces direct-indirect-direct band gap transition and the in-plane anisotropy of hole effective mass is reversed. Our results shed light on the physical origins of in-plane anisotropy of electronic structure in GaTe, providing guidance to further improving the performance of electronic and optoelectronic devices based on the layered anisotropic semiconductors.

Estimating thin-film thermal conductivity by optical pump thermoreflectance imaging and finite element analysis

We introduce a noncontact experiment method to estimate thermal conductivity of nanoscale thin films by fitting high spatial resolution thermoreflectance images of surface spot heating to a finite element simulated temperature distribution. The thin-film top surface is heated by a 1μm diameter focused, 825 nm wavelength laser spot. The surface temperature distribution in the excited sample is imaged by thermoreflectance microscopy with submicrometer spatial resolution and up to 10 mK temperature resolution. Thin-film thermal conductivity is extracted by fitting a measured surface temperature distribution to a 3D finite element temperature model. The method is demonstrated by estimating thermal conductivity for an isotropic thin-film metal (nickel, 60–260 nm) on a glass substrate. The fitted Ni thermal conductivity was 50 ± 5 W/m K, which is in good agreement with the literature. Also, we present a detailed finite element analysis for an anisotropic thin-film semiconductor sample to show how the method could be extended to estimate thermal conductivity of anisotropic thin films. Advantages of the new method are easy sample preparation (no top surface transducer film or integrated heater required), rapid in situ measurement, and application to a broad range of thin-film materials.

Kinetic specific features of singlet fission in highly anisotropic organic semiconductors.

Kinetics of singlet fission (SF) in molecular semiconductors, i.e., spontaneous splitting of the excited singlet state into a pair of triplet (T) excitons, is known to be strongly affected by geminate annihilation of created TT-pairs. In our work, we analyze in detail the specific properties of SF-kinetics in highly anisotropic molecular crystals (in which T-excitons undergo strongly anisotropic hopping migration) within the earlier proposed two-state model (TSM). This model allows for accurate treatment of the characteristic effects of anisotropic relative migration of T-excitons and TT-interaction on SF-kinetics, describing these effects within the approximation, that assumes kinetic coupling of two states: the [TT]-state of interacting TT-pairs and the [T + T]-state of freely migrating T-excitons. The TSM makes it possible to represent the TT-migration and interaction effects in terms of lattice-migration Green's functions, accurate analytical formulas that are obtained in this work. The TSM is applied to the analysis of SF-kinetics in rubrene single crystals, recently measured in a wide range of times (0.1ns < t < 104ns). The analysis enables one to obtain important information on specific properties of SF-kinetics in highly anisotropic crystals. In particular, the observed specific "hump" of SF-kinetics at intermediate times can be treated as a manifestation of the TT-coupling in the [TT]-state. It is also found that the characteristic asymptotic time-dependence of SF-kinetics (∼t-3/2) can markedly be distorted by spin relaxation in TT-pairs.

Propagation of surface plasmon polaritons at the interface of metal-free metamaterial with anisotropic semiconductor inclusions

Herein, we deal with the propagation of surface plasmon polaritons at the interface of metal-free metamaterial with anisotropic inclusions. The anisotropy effect of conventional and well-known nanostructured metamaterial combined of alternating layers is enhanced by employing anisotropic semiconductor sheets into the structure. The former provides a fertile ground for a wide range of tunability features with no needs of changing the geometry of the structure. Moreover, the current approach stands for as a perfect tool aiming to control propagation length of surface plasmon polaritons.

Bi12Rh3Cu2I5: A 3D Weak Topological Insulator with Monolayer Spacers and Independent Transport Channels

Topological insulators (TIs) are semiconductors with protected electronic surface states that allow dissipation‐free transport. TIs are envisioned as ideal materials for spintronics and quantum computing. In Bi14Rh3I9, the first weak 3D TI, topology presumably arises from stacking of the intermetallic [(Bi4Rh)3I]2+ layers, which are predicted to be 2D TIs and to possess protected edge‐states, separated by topologically trivial [Bi2I8]2− octahedra chains. In the new layered salt Bi12Rh3Cu2I5, the same intermetallic layers are separated by planar, i.e., only one atom thick, [Cu2I4]2− anions. Density functional theory (DFT)‐based calculations show that the compound is a weak 3D TI, characterized by , and that the topological gap is generated by strong spin–orbit coupling (E g,calc. ∼ 10 meV). According to a bonding analysis, the copper cations prevent strong coupling between the TI layers. The calculated surface spectral function for a finite‐slab geometry shows distinct characteristics for the two terminations of the main crystal faces ⟨001⟩, viz., [(Bi4Rh)3I]2+ and [Cu2I4]2−. Photoelectron spectroscopy data confirm the calculated band structure. In situ four‐point probe measurements indicate a highly anisotropic bulk semiconductor (E g,exp. = 28 meV) with path‐independent metallic conductivity restricted to the surface as well as temperature‐independent conductivity below 60 K.

An efficient hydrogen evolution photocatalyst of Rh@Cr2O3-loaded PbMoO4 twenty-six facet polyhedrons

Shape anisotropic semiconductors are advanced catalysts for resolving energy crises.

Strong In-Plane Anisotropic SiP2 as a IV-V 2D Semiconductor for Polarized Photodetection.

In-plane anisotropic two-dimensional (2D) materials, emerging as an intriguing type of 2D family, provide an ideal platform for designing and fabrication of optoelectronic devices. Exploring air-stable and strong in-plane anisotropic 2D materials is still challenging and promising for polarized photodetection. Herein, SiP2, a 2D IV-V semiconductor, is successfully prepared and introduced into an in-plane anisotropic 2D family. The basic characterizations combined with theoretical calculations reveal 2D SiP2 to exhibit an intrinsically low-symmetry structure, the in-plane anisotropy of phonon vibrations, and an anisotropically dispersed band structure. Moreover, the photodetector based on 2D SiP2 exhibits high performance with a high detectivity of 1012 Jones, a large light on/off ratio of 103, a low dark current of 10-13 A, and a fast response speed of 3 ms. Furthermore, 2D SiP2 demonstrates a high anisotropic photodetection with an anisotropic ratio up to 2. In addition, the polarization-sensitive photodetector presents a dichroic ratio of 1.6 due to the intrinsic linear dichroism. These good characteristics make 2D SiP2 a promising candidate as an in-plane anisotropic semiconductor for high-sensitivity and polarized optoelectronic applications.

Boundary element modeling of fractional nonlinear generalized photothermal stress wave propagation problems in FG anisotropic smart semiconductors

The main aim of this article is to develop an efficient boundary element method (BEM) modeling of the fractional nonlinear generalized photo thermal stress wave propagation problems in the context of functionally graded (FG) anisotropic smart semiconductors. Due to nonlinearity, fractional order heat conduction and strongly anisotropy of mechanical properties, the governing equations system of such problems is often very difficult to solve using classical analytical methods. Therefore, a reliable and efficient coupling scheme based on BEM was proposed to address this challenge, where, the Cartesian transformation method (CTM) has been implemented to calculate the domain integrals, and the generalized modified shift-splitting (GMSS) has been implemented for solving the linear systems arising from BEM. The calculation findings are depicted in graphical forms to display the impacts of temperature-dependent, anisotropy, piezoelectric, graded parameter and fractional parameter on the nonlinear photo thermal stress wave propagation in the considered structure. The numerical findings confirm the consistency and efficacy of the developed modeling methodology.

Dislocation blocking in elastically anisotropic semiconductor thin films

One strategy for decreasing the density of threading dislocations penetrating the surface of a heteroepitaxial semiconductor film is that in which the greater mechanical stiffness of a dislocation blocking layer acts to reduce the Peach–Koehler image forces acting on the leading segment of the half loop generated by dislocation multiplication sources at the heteroepitaxial interface situated below the blocking layer. Reducing the Peach–Koehler force, drawing the half loop to the film surface, helps prevent the two threading arms of the half loop from becoming threading dislocations once the half loop penetrates the film surface. The calculation of the Peach–Koehler force employs an analytical continuation formalism using anisotropic elasticity theory for treating dislocation image forces generated by three heteroepitaxial interfaces corresponding to the top and bottom interfaces of the blocking layer and the film surface. The system used in this calculation is that of a Ge film grown on a (001) Si substrate, using a SiGe blocking layer just below the critical thickness for dislocation generation. It is found that the dislocation blocking is favored by thinner blocking layers of greater mechanical stiffness, rather than thicker blocking layers of moderate mechanical stiffness. Specifically, for the blocking layers of composition Si0.2Ge0.8, Si0.3Ge0.7, and Si0.4Ge0.6, of thickness 50, 18, and 10 nm, respectively, it is the thinnest (and mechanically stiffest) layer (Si0.4Ge0.6, 10 nm) that brings about the greatest reduction in the Peach–Koehler force, drawing the leading segment of the half loop to the surface of the film.

Crystallographic orientation and morphology control of Sb2Se3 to sensitize TiO2 nanotube arrays for enhanced photoelectrochemical performances

Coupling the appropriate light-absorbing materials with titanium dioxide (TiO2) nanotube arrays (TNA) can significantly enhance the Performance of photoelectrochemical (PEC) devices. Anisotropic antimony selenide (Sb2Se3) is a promising photo-absorber, but optimizing its crystallographic orientation and morphology concurrently is challenging. Herein, we control the growth of Sb2Se3 utilizing the space-confined effect of TNA with varying electrodeposition strategies to regulate the nucleation position. The [211]-oriented rod-like Sb2Se3/TNA photoanode prepared by pulse electrodeposition generates a photocurrent of 3.02 mA cm−2 at 1.23 V (versus RHE) in an electrolyte of pH 7, which is ∼4.1- and ∼30.0-time as those of [230]-oriented octahedron-like Sb2Se3/TNA and bare TNA photoanodes, respectively. Photoelectron can travel easily along the vertical direction via the covalent bond of (Sb4Se6)n ribbons without the recombination loss along the van der Waals direction due to the optimized Sb2Se3 [211] orientation and 1D-1D array structure. In situ irradiation X-ray photoelectron spectroscopy is used to directly monitor photoelectron transfer from Sb2Se3 to TNA at the heterojunctions, which ensures the promoted photoelectron separation and extraction in the rod-like Sb2Se3/TNA photoanode at the positive bias. This work provides a new strategy for optimally orienting anisotropic semiconductors and a fundamental insight into their photoelectron separation for design of efficient photoanodes.

Shape-Dependent Linear Dichroism Spectra of Colloidal Semiconductor Nanocrystals.

Semiconductor nanocrystals are normally dispersed in the solvent for property studies as well as practical applications. However, rare attention has been paid to their orientation status in the colloidal solution. Herein, with the help of linear dichroism (LD) spectroscopy, we demonstrate that isotropic NCs of high symmetry (i.e., quantum dots, QDs) and anisotropic NCs (e.g., quantum rods, QRs and nanoplates, NPLs) but under diluted concentration are randomly dispersed without any preferential orientation. Meanwhile, anisotropic NCs under a high concentration can behave with some net orientation along a certain direction. For example, CdSe quantum rods (QRs) and nanoplatelets (NPLs) both show an obviously preferred orientation along the vertical direction in solution when their solution absorbances increase to certain values. An in-depth analysis of QRs' LD spectrum shows that the first excitonic transition of QRs is strongly quantumly confined while its higher-energy excitonic transitions are weakly quantumly confined. In contrast, the NPLs' LD spectrum indicates that their excitonic transitions are isotropic in the spatial space. This work provides a new viewpoint of the real status of anisotropic semiconductor NCs in solution.

Influence of Shape Anisotropy on the Emission of Low-Dimensional Semiconductors.

The emergence of precise and scalable synthetic methods for producing anisotropic semiconductor nanostructures provides opportunities to tune the photophysical properties of these particles beyond their band gaps, and to incorporate them into higher-order structures with macroscopic anisotropic responses to electric and optical fields. This perspective article discusses some of these opportunities in the context of colloidal semiconductor nanoplatelets, with a focus on the influence of confinement anisotropy on processes that dictate the emission.

Redox Photochemistry on Van Der Waals Surfaces for Reversible Doping in 2D Materials

AbstractDespite the van der Waals (vdW) surfaces are usually chemically inert, un‐destructive, scalable, and reversible redox reactions are introduced on the vdW surfaces of 2D anisotropic semiconductors ReX2 (X = S or Se) facilitated by simple photochemistry. Ultraviolet (UV) light (with humid) and laser exposure can reversibly oxidize and reduce rhenium disulfide (ReS2) and rhenium diselenide (ReSe2), respectively, yielding a pronounced doping effect with good control. Evidenced by Raman spectroscopy, dynamic force microscopy, transmission electron microscopy, and X‐ray photoelectron spectroscopy, the grafting and removal of covalently functionalized oxygen groups on the perfect vdW surfaces are confirmed. The optical and electrical properties can be thereby reversibly tunable in wide ranges. Such optical direct‐writing and rewritable capability via solvent/contaminant‐free approach for chemical doping are compelling in the coming era of 2D materials.