Non-centrosymmetric Semiconductors Research Articles

Strongly enhanced shift current at exciton resonances in a noncentrosymmetric wide-gap semiconductor

Excitons are fundamental quasiparticles that are ubiquitous in photoexcited semiconductors and insulators. Despite causing a sharp and strong photoabsorption near the interband absorption edge, charge-neutral excitons do not yield photocurrent in conventional photovoltaic processes unless dissociated into free charge carriers. Here, we experimentally demonstrate that excitons can directly contribute to photocurrent generation through a nonlinear light−matter interaction in a noncentrosymmetric semiconductor CuI. Epitaxial thin films of CuI exhibit a substantial enhancement of photocurrent at exciton resonance energies even below the bandgap. From the light polarization dependence, this photocurrent is identified to be shift current, a nonlinear photocurrent driven by the change in the geometric Berry phase of electron wave functions upon the optical transition. The shift current at the exciton resonance is much larger than that induced above the band gap by free electron−hole excitation, and their signs are opposite. First-principles calculations elucidate that the sign and magnitude of the exciton shift current are strongly dependent on the strain in the thin film. The present study reveals the crucial role of excitons in enhancing the shift current magnitude and its strain sensitivity, and will open an unprecedented route for efficient manipulation of nonlinear optical effects.

Induced Circularly Polarized Luminescence and Exciton Fine Structure Splitting in Magnetic-Doped Chiral Perovskites.

Magnetic impurity doping in semiconductors has emerged as an important strategy to endow exotic photophysical and magnetic properties. While most reported hosts are centrosymmetric semiconductors, doping magnetic ions into a noncentrosymmetric chiral semiconductor can offer additional control of photonic and spin polarization. In this work, we synthesized a Mn2+-doped chiral two-dimensional (2D) perovskite, Mn2+:(R-MPA)2PbBr4 (R-MPA+ = R-methyl phenethylammonium). We found that the optical activity of chiral 2D perovskites is enhanced with an increased concentration of Mn2+ ions. Additionally, efficient energy transfer from the chiral host to the Mn2+ dopants is observed. This energy transfer process gives rise to circularly polarized luminescence from the excited state of Mn2+ (4T1 → 6A1), exhibiting a photoluminescence quantum yield up to 24% and a dissymmetry factor of 11%. The exciton fine structures of undoped and Mn2+-doped (R-MPA)2PbBr4 are further studied through magnetic circular dichroism (MCD) spectroscopy. Our analysis shows that chiral organic cations lead to an exciton fine structure splitting energy as large as 5.0 meV, and the splitting is further increased upon Mn2+ doping. Our results reveal the strong impacts of molecular chirality and magnetic dopants on the exciton structures of halide perovskites.

Magnetic phase diagram of Ge1–x–y(SnxMny)Te multiferroic semiconductors: Coexistence of ferromagnetic and cluster glass ordering

Noncentrosymmetric IV-VI semiconductors doped with magnetic ions offer exciting possibilities such as carrier mediated ferromagnetism, ferroelectric-controlled spin texture and integration of semiconducting features with ferromagnetic order. Also, coupling between more than one ferroic orders and giant Rashba spin splitting can be studied to develop materials for spintronic applications. In that context, we report the structural and magnetic results of polar α–GeTe doped with Sn and Mn from x = 0.185–0.841 and y = 0.02–0.086, respectively. The magnetic results of Ge1–x–y(SnxMny)Te (GSMT) crystals identify Mn-clustering effect with scaling parameter, R = 0.033 for x ≈ 0.2 and y = 0.06. The excessive Sn ions are assumed to drive the inception of short range ferromagnetic clusters. For the crystals revealing glassy magnetic behavior, the irreversibility temperature, Tirr shifts at high dc magnetic field which is well described by de Almeida-Thouless equation, δTirr ∝ HΦ/2 yielding Φ values of 1.55 and 1.7 that validates the formation of Mn clusters. The spin relaxation time, τ0 ∼ 10–9 s, activation energy, Ea/kB ∼ 5 TF where TF is freezing temperature and Vogel-Fulcher temperature, T0 ∼ TF also signify intracluster interactions. Also, the Mn-hole magnetic exchange constant, Jpd drops from 0.24 eV for the samples with smaller Sn content of x ≈ 0.2 to 0.16 eV for Sn rich alloy with x ≈ 0.8. Consequently, we present a magnetic phase diagram for GSMT bulk crystals as a function of Mn ions.

Reversal of spin-polarization near the Fermi level of the Rashba semiconductor BiTeCl

Spin–orbit coupling forms the physical basis for quantum materials with non-trivial topology and potential spintronics applications. The Rashba interaction is a textbook model of spin–orbit interactions, with charge carriers undergoing linear, isotropic spin-splitting in momentum space. Recently, non-centrosymmetric semiconductors in the family BiTeX (X = Cl, Br, I) have been identified as exemplary Rashba materials due to the strong splitting of their bulk bands, yet a detailed investigation of their spin textures, and their relationships to local crystal symmetry, is currently lacking. We perform high-efficiency spin-resolved photoemission spectroscopy to directly image the spin texture of surface states of BiTeCl, and we find dramatic deviations from idealized behavior, including a reversal of the spin-polarization near the Fermi level. We show that this behavior can be described by higher-order contributions to the canonical Rashba model with the surface states localized to individual trilayers of the crystal. Due to the prominence of these effects near the Fermi level, they should have a strong impact on the spin-dependent transport of carriers.

Synthesis of chiral high-entropy sulfides for non-linear optical applications

Non-centrosymmetric semiconductors may exhibit non-linear optical (NLO) properties.

Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric α-GeTe(111)

Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, α-GeTe(111) is a non-centrosymmetric ferroelectric semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of α-GeTe(111) using time and angle-resolved photoemission spectroscopy. Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage, namely an increase corresponding to a 13% enhancement of the lattice distortion. This opens the route for the control of the ferroelectric polarization in α-GeTe(111) and ferroelectric semiconducting materials in quantum heterostructures.

Enhanced Third Generation Semiconductor Material-Based Solar Cell Efficiency by Piezo-Phototronic Effect

By applying the outward uniform strain on the non-centrosymmetric piezoelectric semiconductor, the polarization charges on the material surface are induced. Polarization charges are often generated within the crystals provided that the applied strain is non-uniform. The strain applied has an effect on electronic transport and can be utilized to modulate the properties of the material. The effect of multiway coupling between piezoelectricity, semiconductor transport properties, and photoexcitation results in piezo-phototronic effects. Recent studies have shown the piezoelectric and semiconductor properties of third-generation semiconductors have been used in photodetectors, LEDs, and nanogenerators. The third-generation piezoelectric semiconductor can be used in high-performance photovoltaic cells. A third-generation piezo-phototronic solar cell material is theoretically explored in this manuscript on the basis of a GaN metal-semiconductor interaction. This study aims to determine the effects of piezoelectric polarization on the electrical performance characteristics of this solar cell material. Performance parameters such as Power Conversion Efficiency, Fill Factors, I-V Characteristics, Open Circuit Voltage, and Maximum Output Power have been evaluated. The piezophototronic effect can enhance the open-circuit current voltage by 5.5 percent with an externally applied strain by 0.9 percent. The study will open a new window for the next generation of high-performance piezo-phototronic effects.

Hot carrier dynamics of BiTeI with large Rashba spin splitting.

We present a time-resolved ultrafast optical spectroscopy study on BiTeI, a noncentrosymmetric semiconductor with large spin–orbit splitting. By tuning the pump photon energy, hot carriers can be excited into different energy bands, and the hot carriers decay dynamics are measured. The hot carriers excited by an 1.544 eV photon induce a positive differential reflectivity following a single exponential decay, while the hot carriers excited by an 1.651 eV photon show a negative reflectivity following two exponential decays, i.e., the hot carriers excited by 1.544 eV and 1.651 eV photons show different decay dynamics. We also investigate hot carrier dynamics in each Rashba splitting band at the 1.544 eV and 1.651 eV photon pump, and there is no difference in hot carrier decay between the left and right Rashba splitting bands for both cases.

Synthesis, structure, linear and nonlinear optical properties of noncentrosymmetric quaternary diamond-like semiconductors, Cu2ZnGeSe4 (CZGSe) and the novel Cu4ZnGe2Se7

In this study, two quaternary diamond-like semiconductors (DLSs) in the Cu-Zn-Ge-Se system, the known Cu2ZnGeSe4 (also referred to as CZGSe) and the new Cu4ZnGe2Se7, are compared in terms of their crystal structures, electronic structures and physicochemical properties. Both compounds were prepared by high-temperature, solid-state synthesis at 800 °C. Single crystal X-ray diffraction was used to determine the structure of Cu4ZnGe2Se7. The structures of both Cu2ZnGeSe4 and Cu4ZnGe2Se7 can be considered as derivatives of cubic diamond. Cu4ZnGe2Se7, with a reduced symmetry due to the cation ordering pattern, adopts the Cu4NiSi2S7 structure type, with space group C2. The corner-sharing tetrahedra in Cu4ZnGe2Se7 are slightly distorted because the charge for some of the S2- anions is not compensated by the first-nearest-neighbor cations. Rietveld refinements using synchrotron X-ray powder diffraction data were used to assess the phase purity of the samples and confirm the bulk structural behavior. Diffuse reflectance UV/Vis/NIR spectroscopy shows that Cu2ZnGeSe4 and Cu4ZnGe2Se7 have direct optical bandgaps of 1.38 and 0.91 eV, respectively. Electronic structure calculations implementing density functional theory confirm the direct bandgap for Cu4ZnGe2Se7, with a calculated value of 0.62 eV. Both compounds are air stable, thermally stable up to relatively high temperatures, undergo phase transitions and have wide windows of optical clarity. The Kurtz-Perry powder technique was used to determine the second harmonic generation (SHG) responses using a commercial AgGaSe2 standard. While Cu4ZnGe2Se7 displays a weak SHG response, Cu2ZnGeSe4 exhibits a response that is greater than several benchmark materials, with a large SHG coefficient, χ(2), of 43 ± 6 pm/V at λ = 2900 nm.

Two-Dimensional Materials with Giant Optical Nonlinearities near the Theoretical Upper Limit.

Nonlinear optical (NLO) phenomena such as harmonic generation and Kerr and Pockels effects are of great technological importance for lasers, frequency converters, modulators, switches, etc. Recently, two-dimensional (2D) materials have drawn significant attention due to their strong and peculiar NLO properties. Here, we describe an efficient first-principles workflow for calculating the quadratic optical response and apply it to 375 non-centrosymmetric semiconductor monolayers from the Computational 2D Materials Database (C2DB). Sorting the nonresonant nonlinearities with respect to bandgap Eg reveals an upper limit proportional to Eg-4, which is neatly explained by two distinct generic models. We identify multiple promising candidates with giant nonlinearities and bandgaps ranging from 0.4 to 5 eV, some of which approach the theoretical upper limit and greatly outperform known materials. Our comprehensive library of ab initio NLO spectra for all 375 monolayers is freely available via the C2DB Web site.

Modeling of surface-induced second-harmonic generation from multilayer structures by the transfer matrix method.

We analytically and numerically investigate surface second-harmonic generation (SHG) from a stack of dielectric layers. We develop a theoretical formalism based on the transfer matrix method for the calculation of the surface-driven second-harmonic radiation from multilayer structures and elaborate it for the case of ultrathin dielectric layers using a power series expansion to derive the effective surface nonlinear tensor for the whole stack. We show that for deeply subwavelength thicknesses of the layers the surface responses from all interfaces can efficiently sum up, leading to largely enhanced efficiency of SHG. As a result, such surface-driven nonlinearity can become comparable to the bulk nonlinearity in noncentrosymmetric semiconductors and can yield high performance for nonlinear nanophotonic applications.

Antiferromagnetism-induced second-order nonlinear optical responses of centrosymmetric bilayer CrI3

Antiferromagnetism (AF) in AB′-stacked centrosymmetric bilayer (BL) CrI3 breaks both spatial inversion (P) and time-reversal (T) symmetries but maintains the combined PTsymmetry, thus inducing novel second-order nonlinear optical (NLO) responses such as second-harmonic generation (SHG), linear electric-optic effect (LEO) and bulk photovoltaic effect (BPVE). In this work, we calculate AF-induced NLO responses of the BL CrI3 based on the density functional theory with the generalized gradient approximation (GGA) plus onsite Coulomb correlation (U), i.e., the GGA+U method. Interestingly, we find that the magnetic SHG, LEO and photocurrent in the AF BL CrI3 are huge, being comparable or even larger than that of the well-known nonmagnetic noncentrosymmetric semiconductors. For example, the calculated SHG coefficients are in the same order of magnitude as that of MoS2 monolayer (ML), the most promising 2D material for NLO devices. The calculated LEO coefficients are almost three times larger than that of MoS2 ML. The calculated NLO photocurrent in the CrI3BL is among the largest values predicted so far for the BPVE materials. On the other hand, unlike nonmagnetic semiconductors, the NLO responses in the AF BL CrI3 are nonreciprocal and also switchable by rotating magnetization direction. Therefore, our interesting findings indicate that the AF BL CrI3 will not only provide a valuable platform for exploring new physics of low-dimensional magnetism but also have promising applications in magnetic NLO and LEO devices such as frequency conversion, electro-optical switches, and light signal modulators as well as high energy conversion efficiency photovoltaic solar cells.

Far-field polarization signatures of surface optical nonlinearity in noncentrosymmetric semiconductors

We analyse possibilities to quantitatively evaluate the surface second-order optical nonlinearity in noncentrosymmetric materials based on polarization-resolved analysis of far-field radiation patterns of second-harmonic generation. We analytically demonstrate that for plane-wave illumination the contribution to the second-harmonic signal from the surface of a nonlinear medium exhibits different polarization properties and angular dependencies compared to the contribution from the bulk. In view of this, we optimize the illumination geometry in order to enable the most efficient separation and comparison of both nonlinearities. Furthermore, we consider the illumination of an AlGaAs slab by a tightly-focused linearly-polarized Gaussian beam as an alternative measurement geometry. It is found that the reliable separation of the surface nonlinearity contribution as well as a wide range of detectable values can be achieved with this geometry as well.

Flexoelectronics of centrosymmetric semiconductors

Interface engineering by local polarization using piezoelectric1-4, pyroelectric5,6 and ferroelectric7-9 effects has attracted considerable attention as a promising approach for tunable electronics/optoelectronics, human-machine interfacing and artificial intelligence. However, this approach has mainly been applied to non-centrosymmetric semiconductors, such as wurtzite-structured ZnO and GaN, limiting its practical applications. Here we demonstrate an electronic regulation mechanism, the flexoelectronics, which is applicable to any semiconductor type, expanding flexoelectricity10-13 to conventional semiconductors such as Si, Ge and GaAs. The inner-crystal polarization potential generated by the flexoelectric field serving as a 'gate' can be used to modulate the metal-semiconductor interface Schottky barrier and further tune charge-carrier transport. We observe a giant flexoelectronic effect in bulk centrosymmetric semiconductors of Si, TiO2 and Nb-SrTiO3 with high strain sensitivity (>2,650), largely outperforming state-of-the-art Si-nanowire strain sensors and even piezoresistive, piezoelectric and ferroelectric nanodevices14. The effect can be used to mechanically switch the electronics in the nanoscale with fast response (<4 ms) and high resolution (~0.78 nm). This opens up the possibility of realizing strain-modulated electronics in centrosymmetric semiconductors, paving the way for local polarization field-controlled electronics and high-performance electromechanical applications.

1D nanowires of non-centrosymmetric molecular semiconductors grown by physical vapor deposition

Understanding how dipolar, non-centrosymmetric organic semiconductors self-assemble, nucleate, and crystallize is integral for designing new molecular solids with unique physical properties and light-matter interactions.

Piezotronic and piezo-phototronic devices based on the third generation semiconductors

<p indent=0mm>Compared with the first-generation semiconductors (such as silicon, germanium) and the second-generation semiconductors (such as gallium arsenide, indium antimonide), the third-generation semiconductor materials represented by silicon carbide (SiC), zinc oxide (ZnO), gallium nitride (GaN) and cadmium sulfide (CdS) usually possess wider band gap, higher thermal conductivity, bigger electron saturation rate and better radiation resistance properties, and thus draw intensive attentions in high temperature and high frequency applications in recent years. Most of the third-generation semiconductors are wurtzite structures, which have piezoelectric effects due to their lack of symmetry in certain directions. This feature serves as a good bridge of transferring mechanical stress signals between the flexible semiconductor electronic devices and the surrounding environment or the host (e.g., the human body). Conventional piezoelectric effects are mainly found in barium titanate and lead zirconate titanate type perovskite materials, but such materials do not have semiconductor properties, thus limiting their use in electronics and optoelectronic devices. Our group has pioneered a new research field, i.e., piezoelectric nanogenerator, by utilizing third-generation semiconductor nanowires (such as ZnO nanowires) under a dynamic force. The piezoelectric potential generated by the dynamic strain on the nanowire can drive electrons to flow in the external load circuit, which is the basic principle of the piezoelectric nanogenerator. Piezoelectric nanogenerator based on ZnO nanowires was firstly proposed in 2006. When a uniform strain is applied on a non-centrosymmetric semiconductor, a piezo-potential will be induced in the semiconductor, accompanied with static piezoelectric charges distributed on the surfaces. This piezoelectric effect is commonly existed in the third generation semiconductors. The strain-induced piezo-potential and piezoelectric polarization charges can also statically and/or dynamically tune the transportation and photoelectric processes of carriers at the interfaces or junctions. The three ways coupling among piezoelectric, photoexcitation and semiconducting properties coined two new research fields, that is, the piezotronics and piezo-phototronics. In order to systematically explain the coupling properties of piezoelectric and semiconductor transport properties in such materials, our group first coined the term piezotronics in a paper published in 2007. The piezotronics is about the modulation of the metal-semiconductor interface barrier via piezoelectric potential acting as a “gate” voltage, which has been used to explain the transistor behavior observed in metal-ZnO-metal structures and the strain-gated diode effect of metal-ZnO structures. The basic core of piezo-phototronics is to regulate the generation, separation, transport and/or recombination of photogenerated carriers at interfaces or heterojunctions by piezoelectric polarization charges, which was first proposed by our group in 2010. The performance of many optoelectronic devices can be effectively enhanced by piezo-phototronic effect. Over the past decade, these two areas have received extensive attentions and made great progresses in basic science and device applications. The combinations of piezotronics and piezo-phototronics with the current popular electronics, optoelectronics and spintronics have great significances, which will bring in revolutionized impacts for future sensor networks, artificial intelligence, micro-nano energy and human-machine interaction applications. This paper gives a brief review of the experimental progress in device application in these two fields in recent years, and looks forward to the future development of these two subjects.

Structural Determination and Nonlinear Optical Properties of New 1T‴-Type MoS2 Compound.

Noncentrosymmetric MoS2 semiconductors (1H, 3R) possess not only novel electronic structures of spin-orbit coupling (SOC) and valley polarization but also remarkable nonlinear optical effects. A more interesting noncentrosymmetric structure, the so-called 1T‴-MoS2 layers, was predicted to be built up from [MoS6] octahedral motifs by theoreticians, but the bulk 1T‴ MoS2 or its single crystal structure has not been reported yet. Here, we have successfully harvested 1T‴ MoS2 single crystals by a topochemical method. The new layered structure is determined from single-crystal X-ray diffraction. The crystal crystallizes in space group P31m with a cell of a = b = 5.580(2) Å and c = 5.957(2) Å, which is a √3 a × √3 a superstructure of 1T MoS2 with corner-sharing Mo3 triangular trimers observed by the STEM. 1T‴ MoS2 is verified to be semiconducting and possesses a band gap of 0.65 eV, different from metallic nature of 1T or 1T' MoS2. More surprisingly, the 1T‴ MoS2 does show strong optical second-harmonic generation signals. This work provides the first layered noncentrosymmetric semiconductor of edge-sharing MoS6 octahedra for the research of nonlinear optics.

High spectral resolution second harmonic generation microspectroscopy at thin layer interfaces with broadband continuum pulses

We demonstrate an effective microspectroscopy technique by tracing the dispersion of second order nonlinear optical susceptibility χ(2) in single atomic layer materials. The experimental method relies on the detection of single-shot second harmonic (SH) spectra from the materials and the subsequent data normalization. The key point in our study is that we used a broadband (˜350 nm) near-infrared femtosecond continuum pulses generated at high repetition rates in a photonic crystal fiber with superior spatial quality and stable spectral power density. This is opposite to the point-by-point laser tuning method in SH generation spectroscopy that was applied extensively in the past and has shown limited precision in obtaining χ(2) dispersion. The continuum pulse technique produces spectral resolution better than 2 meV (<0.3 nm at 450 nm) and shows low (<5–6% rms) signal detection noise allowing the detection of subtle features in the χ(2) spectrum at room temperatures. Fine sub-structure features within the main peak of χ(2) spectra indicate the impact of broadened resonances due to exciton transitions in the single layer materials.• Tailored continuum pulses are used to generate second harmonic signal in non-centrosymmetric semiconductors.• SHG spectrum carries fingerprints of the bandstructure around the direct gap states.• The technique produces fine spectral resolution and much better signal-to-noise ratio compared to point-by-point wavelength tuning methods.

Piezotronics and piezo-phototronics with third-generation semiconductors

Abstract

Femtosecond two-photon absorption spectroscopy of copper indium sulfide quantum dots: A structure-optical properties relationship

We have interpreted the two-photon absorption spectrum of water-soluble copper indium sulfide (CIS) QDs with stoichiometry 0.18 (Cu), 0.42 (In), and 2 (S) and an average diameter of approximately 2.6 nm. For that, we employed the wavelength-tunable femtosecond Z-scan technique and the parabolic effective-mass approximation model, in which the excitonic transition energies were phenomenologically corrected due to the stoichiometry of the nanocrystal. This model considers a conduction band and three valence sub-bands allowing excitonic transitions via centrosymmetric (Δl = ±1, where l is the angular momentum of the absorbing state) and non-centrosymmetric (Δl = 0) channels. In such case, this became relevant because the CIS QDs with chalcopyrite crystalline structure is a non-centrosymmetric semiconductor. Thus, our experimental results pointed out two 2 PA allowed bands located at 715 nm (2hv = 3.47 eV) and 625 nm (2hv = 3.97 eV) with cross sections of (6.3 ± 1.0) x 102 GM and (4.5 ± 0.7) x 102 GM, respectively. According to the theoretical model, these 2 PA bands can be ascribed to the 1P1/2(h3) → 1S3/2(e) (lower energy band) and 1P1/2(hheavy) → 1S3/2(e) (90%)/(10%)1P1/2(hsplit-off) → 1P3/2(e) (higher energy band) excitonic transitions. A good agreement (magnitude and spectral position) between the experimental and theoretical data were obtained. However, our experimental data suggest that the higher-energy 2 PA band may have other contributions due to the mixing between the heavy- and the light-hole bands, which the effective mass model does not take into consideration.