Showing papers on "Doping published in 2020"

Bandgap engineering of two-dimensional semiconductor materials

Abstract: Semiconductors are the basis of many vital technologies such as electronics, computing, communications, optoelectronics, and sensing. Modern semiconductor technology can trace its origins to the invention of the point contact transistor in 1947. This demonstration paved the way for the development of discrete and integrated semiconductor devices and circuits that has helped to build a modern society where semiconductors are ubiquitous components of everyday life. A key property that determines the semiconductor electrical and optical properties is the bandgap. Beyond graphene, recently discovered two-dimensional (2D) materials possess semiconducting bandgaps ranging from the terahertz and mid-infrared in bilayer graphene and black phosphorus, visible in transition metal dichalcogenides, to the ultraviolet in hexagonal boron nitride. In particular, these 2D materials were demonstrated to exhibit highly tunable bandgaps, achieved via the control of layers number, heterostructuring, strain engineering, chemical doping, alloying, intercalation, substrate engineering, as well as an external electric field. We provide a review of the basic physical principles of these various techniques on the engineering of quasi-particle and optical bandgaps, their bandgap tunability, potentials and limitations in practical realization in future 2D device technologies.

Recent progress on the electronic structure, defect, and doping properties of Ga2O3

Abstract: Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor that has attracted a large amount of interest due to its ultra-large bandgap of 4.8 eV, a high breakdown field of 8 MV/cm, and high thermal stability. These properties enable Ga2O3 a promising material for a large range of applications, such as high power electronic devices and solar-blind ultraviolet (UV) photodetectors. In the past few years, a significant process has been made for the growth of high-quality bulk crystals and thin films and device optimizations for power electronics and solar blind UV detection. However, many challenges remain, including the difficulty in p-type doping, a large density of unintentional electron carriers and defects/impurities, and issues with the device process (contact, dielectrics, and surface passivation), and so on. The purpose of this article is to provide a timely review on the fundamental understanding of the semiconductor physics and chemistry of Ga2O3 in terms of electronic band structures, optical properties, and chemistry of defects and impurity doping. Recent progress and perspectives on epitaxial thin film growth, chemical and physical properties of defects and impurities, p-type doping, and ternary alloys with In2O3 and Al2O3 will be discussed.

Evaluation of the Efficiency of Detection and Capture of Manganese in Aqueous Solutions of FeCeOx Nanocomposites Doped with Nb2O5.

Abstract: The main purpose of this work is to study the effectiveness of using FeCeOx nanocomposites doped with Nb2O5 for the purification of aqueous solutions from manganese. X-ray diffraction, energy–dispersive analysis, scanning electron microscopy, vibrational magnetic spectroscopy, and mossbauer spectroscopy were used as research methods. It is shown that an increase in the dopant concentration leads to the transformation of the shape of nanoparticles from spherical to cubic and rhombic, followed by an increase in the size of the nanoparticles. The spherical shape of the nanoparticles is characteristic of a structure consisting of a mixture of two phases of hematite (Fe2O3) and cerium oxide CeO2. The cubic shape of nanoparticles is typical for spinel-type FeNbO4 structures, the phase contribution of which increases with increasing dopant concentration. It is shown that doping leads not only to a decrease in the concentration of manganese in model solutions, but also to an increase in the efficiency of adsorption from 11% to 75%.

Wide Band Gap Chalcogenide Semiconductors.

Abstract: Wide band gap semiconductors are essential for today's electronic devices and energy applications because of their high optical transparency, controllable carrier concentration, and tunable electrical conductivity. The most intensively investigated wide band gap semiconductors are transparent conductive oxides (TCOs), such as tin-doped indium oxide (ITO) and amorphous In-Ga-Zn-O (IGZO), used in displays and solar cells, carbides (e.g., SiC) and nitrides (e.g., GaN) used in power electronics, and emerging halides (e.g., γ-CuI) and 2D electronic materials (e.g., graphene) used in various optoelectronic devices. Compared to these prominent materials families, chalcogen-based (Ch = S, Se, Te) wide band gap semiconductors are less heavily investigated but stand out because of their propensity for p-type doping, high mobilities, high valence band positions (i.e., low ionization potentials), and broad applications in electronic devices such as CdTe solar cells. This manuscript provides a review of wide band gap chalcogenide semiconductors. First, we outline general materials design parameters of high performing transparent semiconductors, as well as the theoretical and experimental underpinnings of the corresponding research methods. We proceed to summarize progress in wide band gap (EG > 2 eV) chalcogenide materials-namely, II-VI MCh binaries, CuMCh2 chalcopyrites, Cu3MCh4 sulvanites, mixed-anion layered CuMCh(O,F), and 2D materials-and discuss computational predictions of potential new candidates in this family, highlighting their optical and electrical properties. We finally review applications-for example, photovoltaic and photoelectrochemical solar cells, transistors, and light emitting diodes-that employ wide band gap chalcogenides as either an active or passive layer. By examining, categorizing, and discussing prospective directions in wide band gap chalcogenides, this Review aims to inspire continued research on this emerging class of transparent semiconductors and thereby enable future innovations for optoelectronic devices.

Band structure engineering and defect control of Ta3N5 for efficient photoelectrochemical water oxidation

Abstract: Ta3N5 is a promising photoanode material with a theoretical maximum solar conversion efficiency of 15.9% for photoelectrochemical water splitting. However, the highest applied bias photon-to-current efficiency achieved so far is only 2.72%. To bridge the efficiency gap, effective carrier management strategies for Ta3N5 photoanodes should be developed. Here, we propose to use gradient Mg doping for band structure engineering and defect control of Ta3N5. The gradient Mg doping profile in Ta3N5 induces a gradient of the band edge energetics, which greatly enhances the charge separation efficiency. Furthermore, defect-related recombination is significantly suppressed due to the passivation effect of Mg dopants on deep-level defects and, more importantly, the matching of the gradient Mg doping profile with the distribution of defects within Ta3N5. As a result, a photoanode based on the gradient Mg-doped Ta3N5 delivers a low onset potential of 0.4 V versus that of a reversible hydrogen electrode and a high applied bias photon-to-current efficiency of 3.25 ± 0.05%. Despite the efforts to tune their properties, the efficiency of tantalum nitride photoanodes falls short of the theoretical value. Here, a gradient Mg doping strategy is introduced to engineer tantalum nitride’s band structure and control its defects, leading to an applied bias photon-to-current efficiency of 3.25%.

Lanthanide doping in metal halide perovskite nanocrystals: spectral shifting, quantum cutting and optoelectronic applications

Abstract: Lanthanides have been widely explored as optically active dopants in inorganic crystal lattices, which are often insulating in nature. Doping trivalent lanthanide (Ln3+) into traditional semiconductor nanocrystals, such as CdSe, is challenging because of their tetrahedral coordination. Interestingly, CsPbX3 (X = Cl, Br, I) perovskite nanocrystals provide the octahedral coordination suitable for Ln3+ doping. Over the last two years, tremendous success has been achieved in doping Ln3+ into CsPbX3 nanocrystals, combining the excellent optoelectronic properties of the host with the f-f electronic transitions of the dopants. For example, the efficient quantum cutting phenomenon in Yb3+-doped CsPb(Cl,Br)3 nanocrystals yields a photoluminescence quantum yield close to 200%. Other approaches of Ln3+ doping and codoping have enabled promising proof-of-principle demonstration of solid-state lighting and solar photovoltaics. In this perspective article, we highlight the salient features of the material design (including doping in Pb-free perovskites), optical properties and potential optoelectronic applications of lanthanide-doped metal halide perovskite nanocrystals. While review articles on doping different metal ions into perovskite nanocrystals are present, the present review-type article is solely dedicated to lanthanide-doped metal halide perovskite nanocrystals. Methods for integrating lanthanide materials into light-emitting devices to improve their performance have been reviewed by scientists from India and China. Semiconductors create light when the energy of a high-energy electron is converted to a single photon. Conventionally, the minimum energy of this photon is determined by an intrinsic material property known as the bandgap. Wasim Mir from the Indian Institute of Science Education and Research Pune, and colleagues, summarize developments in using lanthanide ions to enable the generation of lower-energy light. Lanthanide ions provide an energy “step” within the bandgap that means the electron is converted to two low energy photons rather than a single high energy one. The authors review how embedding lanthanum ions within nanoparticles made from so-called metal-halide perovskites makes them easier to integrate into common optoelectronic semiconductors such as silicon. Metal halide perovskites are extraordinary defect-tolerant semiconductors. A unique structural aspect of perovskites is the octahedral coordination for (B-site) metal ions, unlike other semiconductors that exhibit tetrahedral coordination. This octahedral coordination helped to achieve lanthanide doping in halide perovskite nanocrystals in 2017. Fundamental understanding of material design, luminescence and quantum cutting phenomena in lanthanides (with focus on Yb3+) doped in CsPbX3 (X = Cl, Br, I) and Cs2AgInCl6 nanocrystals are reported. Subsequently, these doped systems are applied for solar energy harvesting and lighting in both visible and near infrared region. This perspective article summarizes everything important that has happened so far in field and discusses about the future research directions.

Point defects in Ga2O3

Abstract: In the field of high-power electronics, gallium oxide (Ga2O3) is attracting attention due to its wide bandgap and ability to be doped n-type. Point defects, including vacancies, impurities, and dopants, play important roles in optimizing device performance. This tutorial discusses the fundamental properties of point defects in monoclinic β-Ga2O3 and the methods employed to study them. Oxygen vacancies are deep donors that do not cause n-type conductivity but may compensate acceptors. Gallium vacancies are deep acceptors that can be partially passivated by hydrogen. Substitutional magnesium is a promising acceptor that produces a semi-insulating material and also forms a complex with hydrogen. Calcium and iron also have deep acceptor levels. Iridium deep donors are introduced into crystals grown from a melt in an Ir crucible. Other defects are introduced by irradiation with energetic particles such as neutrons or protons. In addition to altering the electronic properties, defects give rise to UV/visible emission bands in photoluminescence and cathodoluminescence spectra.

Low-loss composite photonic platform based on 2D semiconductor monolayers

Abstract: The optical properties of transition metal dichalcogenides (TMDs) are known to change dramatically with doping near their excitonic resonances. However, little is known about the effect of doping on the optical properties of TMDs at wavelengths far from these resonances, where the material is transparent and therefore could be leveraged in photonic circuits. We demonstrate the strong electrorefractive response of monolayer tungsten disulfide (WS2) at near-infrared wavelengths (deep in the transparency regime) by integrating it on silicon nitride photonic structures to enhance the light–matter interaction with the monolayer. We show that the doping-induced phase change relative to the change in absorption (|∆n/∆k|) is ~125, which is significantly higher than the |∆n/∆k| observed in materials commonly employed for silicon photonic modulators, including Si and III–V on Si, while accompanied by negligible insertion loss. Strong electrorefractive effects in semiconductor transition metal dichalcogenides (TMDs) at near-infrared wavelengths, where the TMDs are transparent, are observed and used to demonstrate photonic devices based on a composite SiN–TMD platform with large phase modulation, minimal induced loss and low electrical power consumption.

Enhanced carrier separation and increased electron density in 2D heavily N-doped ZnIn2S4 for photocatalytic hydrogen production

Abstract: Element doping is an effective approach to modify photocatalysts for boosting solar H2 evolution, especially anion doping. However, there still exists controversy regarding the role of heavy doping in photocatalysis. Herein, 2D heavily N-doped ZnIn2S4 is reported based on an in-depth perspective on the role that N heteroatoms play in improving the photocatalytic properties. The electron dynamics was investigated via femtosecond transient absorption spectroscopy which revealed that the introduction of N doping in ZnIn2S4 provides a strong electron aggregation ability for improving the efficiency of charge separation. Meanwhile, the increased valence band width and the elevated conduction band, respectively, promote the mobility of holes and provide more reductive photogenerated electrons. As a result, 2D heavily N-doped ZnIn2S4 exhibited superior photocatalytic hydrogen production compared to the pristine ZnIn2S4. The optimal hydrogen evolution rate is 11 086 μmol g−1 h−1 under visible light irradiation. This work contributes to the improvement of photocatalytic performance in 2D ZnIn2S4 and provides an in-depth understanding of heteroatom doped photocatalysts.

Interfacial Molecular Doping of Metal Halide Perovskites for Highly Efficient Solar Cells.

Abstract: Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way Here, a small molecule of 4,4',4″,4″'-(pyrazine-2,3,5,6-tetrayl) tetrakis (N,N-bis(4-methoxyphenyl) aniline) (PT-TPA) which can effectively p-dope the surface of FAx MA1- x PbI3 (FA: HC(NH2 )2 ; MA: CH3 NH3 ) perovskite films is reported The intermolecular charge transfer property of PT-TPA forms a stabilized resonance structure to accept electrons from perovskites The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT-TPA After applying PT-TPA into perovskite solar cells, the doping-induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there The optimized devices demonstrate an increased photovoltage from 112 to 117 V and an efficiency of 234% from photocurrent scanning with a stabilized efficiency of 229% The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques

Doped TiO2: the effect of doping elements on photocatalytic activity

Abstract: Doping of TiO2 with various elements increases its photocatalytic activity due to the formation of new energy levels near the conduction band. Photocatalysis involving titanium dioxide is a heterogeneous process in which the surface of the catalyst plays an important role. The structural properties of TiO2 are influenced by the synthesis method, the doping method, and the dopants. In this work, we compare different doping elements for improving the photocatalytic activity of titanium dioxide, which was synthesized by the sol–gel method. In the doping method, low-temperature underwater plasma was used. Al, Cu, Mo, and W acting as electrodes were chosen as doping elements. The obtained samples were characterized by various techniques. The incorporation of elements leads to the distortion of the TiO2 crystal lattice, thus changing its surface characteristics, and to a decrease in the band gap. The introduction of aluminum and copper increases the photocatalytic activity to 70% while doping with Mo and W increases the activity to 96% upon visible light irradiation for 60 minutes. Explanations of the effect of various doping elements on the photocatalytic activity of titanium dioxide are presented.

Enabling room temperature ferromagnetism in monolayer MoS 2 via in situ iron-doping

Abstract: Two-dimensional semiconductors, including transition metal dichalcogenides, are of interest in electronics and photonics but remain nonmagnetic in their intrinsic form. Previous efforts to form two-dimensional dilute magnetic semiconductors utilized extrinsic doping techniques or bulk crystal growth, detrimentally affecting uniformity, scalability, or Curie temperature. Here, we demonstrate an in situ substitutional doping of Fe atoms into MoS2 monolayers in the chemical vapor deposition growth. The iron atoms substitute molybdenum sites in MoS2 crystals, as confirmed by transmission electron microscopy and Raman signatures. We uncover an Fe-related spectral transition of Fe:MoS2 monolayers that appears at 2.28 eV above the pristine bandgap and displays pronounced ferromagnetic hysteresis. The microscopic origin is further corroborated by density functional theory calculations of dipole-allowed transitions in Fe:MoS2. Using spatially integrating magnetization measurements and spatially resolving nitrogen-vacancy center magnetometry, we show that Fe:MoS2 monolayers remain magnetized even at ambient conditions, manifesting ferromagnetism at room temperature. Ferromagnetism with a Curie temperature above room temperature in 2D materials is highly desirable for practical spintronics applications. Here, the authors demonstrate such phenomenon in monolayer MoS2 via in situ iron-doping and measured local magnetic field strength up to 0.5 ± 0.1 mT.

P-type Doping in Large-Area Monolayer MoS2 by Chemical Vapor Deposition.

Abstract: Molybdenum disulfide (MoS2) with excellent properties has been widely reported in recent years. However, it is a great challenge to achieve p-type conductivity in MoS2 because of its native stubborn n-type conductivity. Substitutional transition metal doping has been proved to be an effective approach to tune their intrinsic properties and enhance device performance. Herein, we report the growth of Nb-doping large-area monolayer MoS2 by a one-step salt-assisted chemical vapor deposition method. Electrical measurements indicate that Nb doping suppresses n-type conductivity in MoS2 and shows an ambipolar transport behavior after annealing under the sulfur atmosphere, which highlights the p-type doping effect via Nb, corresponding to the density functional theory calculations with Fermi-level shifting to valence band maximum. This work provides a promising approach of two-dimensional materials in electronic and optoelectronic applications.

A thermally activated and highly miscible dopant for n-type organic thermoelectrics.

Abstract: N-doping plays an irreplaceable role in controlling the electron concentration of organic semiconductors thus to improve performance of organic semiconductor devices However, compared with many mature p-doping methods, n-doping of organic semiconductor is still of challenges In particular, dopant stability/processability, counterion-semiconductor immiscibility and doping induced microstructure non-uniformity have restricted the application of n-doping in high-performance devices Here, we report a computer-assisted screening approach to rationally design of a triaminomethane-type dopant, which exhibit extremely high stability and strong hydride donating property due to its thermally activated doping mechanism This triaminomethane derivative shows excellent counterion-semiconductor miscibility (counter cations stay with the polymer side chains), high doping efficiency and uniformity By using triaminomethane, we realize a record n-type conductivity of up to 21 S cm−1 and power factors as high as 51 μW m−1 K−2 even in films with thicknesses over 10 μm, and we demonstrate the first reported all-polymer thermoelectric generator Realizing efficient n-doping in organic thermoelectrics remains a challenge due to dopant-semiconductor immiscibility, poor dopant stability and low doping efficiency Here, the authors use computer-assisted screening to develop n-dopants for thermoelectric polymers that show record power factors

Tuning Electrical Conductance of MoS2 Monolayers through Substitutional Doping

Abstract: Tuning electrical conductivity of semiconducting materials through substitutional doping is crucial for fabricating functional devices. This, however, has not been fully realized in two-dimensional (2D) materials due to the difficulty of homogeneously controlling the dopant concentrations and the lack of systematic study of the net impact of substitutional dopants separate from that of the unintentional doping from the device fabrication processes. Here, we grow wafer-scale, continuous MoS2 monolayers with tunable concentrations of Nb and Re and fabricate devices using a polymer-free approach to study the direct electrical impact of substitutional dopants in MoS2 monolayers. In particular, the electrical conductivity of Nb doped MoS2 in the absence of electrostatic gating is reproducibly tuned over 7 orders of magnitude by controlling the Nb concentration. Our study further indicates that the dopant carriers do not fully ionize in the 2D limit, unlike in their three-dimensional analogues, which is explained by weaker charge screening and impurity band conduction. Moreover, we show that the dopants are stable, which enables the doped films to be processed as independent building blocks that can be used as electrodes for functional circuitry.

Highly Efficient Blue-Emitting CsPbBr 3 Perovskite Nanocrystals through Neodymium Doping.

Abstract: Colloidal CsPbX3 (X = Br, Cl, and I) perovskite nanocrystals exhibit tunable bandgaps over the entire visible spectrum and high photoluminescence quantum yields in the green and red regions. However, the lack of highly efficient blue-emitting perovskite nanocrystals limits their development for optoelectronic applications. Herein, neodymium (III) (Nd3+) doped CsPbBr3 nanocrystals are prepared through the ligand-assisted reprecipitation method at room temperature with tunable photoemission from green to deep blue. A blue-emitting nanocrystal with a central wavelength at 459 nm, an exceptionally high photoluminescence quantum yield of 90%, and a spectral width of 19 nm is achieved. First principles calculations reveal that the increase in photoluminescence quantum yield upon doping is driven by an enhancement of the exciton binding energy due to increased electron and hole effective masses and an increase in oscillator strength due to shortening of the Pb-Br bond. Putting these results together, an all-perovskite white light-emitting diode is successfully fabricated, demonstrating that B-site composition engineering is a reliable strategy to further exploit the perovskite family for wider optoelectronic applications.

Highly conductive and transparent gallium doped zinc oxide thin films via chemical vapor deposition.

Abstract: Degenerately doped ZnO is seen as a potential substitute to the ubiquitous and expensive Sn doped In2O3 as a transparent electrode in optoelectronic devices. Here, highly conductive and transparent Ga doped ZnO thin films were grown via aerosol assisted chemical vapor deposition. The lowest resistivity (7.8 × 10-4 Ω.cm) and highest carrier concentration (4.23 × 1020 cm-3) ever reported for AACVD grown ZnO: Ga was achieved due to using oxygen poor growth conditions enabled by diethylzinc and triethylgallium precursors.

High-Mobility Semiconducting Two-Dimensional Conjugated Covalent Organic Frameworks with p-Type Doping

Abstract: Two-dimensional conjugated covalent organic frameworks (2D c-COFs) are emerging as a unique class of semiconducting 2D conjugated polymers for (opto)electronics and energy storage. Doping is one of the common, reliable strategies to control the charge carrier transport properties, but the precise mechanism underlying COF doping has remained largely unexplored. Here we demonstrate molecular iodine doping of a metal-phthalocyanine-based pyrazine-linked 2D c-COF. The resultant 2D c-COF ZnPc-pz-I2 maintains its structural integrity and displays enhanced conductivity by 3 orders of magnitude, which is the result of elevated carrier concentrations. Remarkably, Hall effect measurements reveal enhanced carrier mobility reaching ∼22 cm2 V-1 s-1 for ZnPc-pz-I2, which represents a record value for 2D c-COFs in both the direct-current and alternating-current limits. This unique transport phenomenon with largely increased mobility upon doping can be traced to increased scattering time for free charge carriers, indicating that scattering mechanisms limiting the mobility are mitigated by doping. Our work provides a guideline on how to assess doping effects in COFs and highlights the potential of 2D c-COFs to display high conductivities and mobilities toward novel (opto)electronic devices.

Ground-state electron transfer in all-polymer donor-acceptor heterojunctions

Abstract: Doping of organic semiconductors is crucial for the operation of organic (opto)electronic and electrochemical devices. Typically, this is achieved by adding heterogeneous dopant molecules to the polymer bulk, often resulting in poor stability and performance due to dopant sublimation or aggregation. In small-molecule donor–acceptor systems, charge transfer can yield high and stable electrical conductivities, an approach not yet explored in all-conjugated polymer systems. Here, we report ground-state electron transfer in all-polymer donor–acceptor heterojunctions. Combining low-ionization-energy polymers with high-electron-affinity counterparts yields conducting interfaces with resistivity values five to six orders of magnitude lower than the separate single-layer polymers. The large decrease in resistivity originates from two parallel quasi-two-dimensional electron and hole distributions reaching a concentration of ∼1013 cm–2. Furthermore, we transfer the concept to three-dimensional bulk heterojunctions, displaying exceptional thermal stability due to the absence of molecular dopants. Our findings hold promise for electro-active composites of potential use in, for example, thermoelectrics and wearable electronics. Doping through spontaneous electron transfer between donor and acceptor polymers is obtained by selecting organic semiconductors with suitable electron affinity and ionization energy, achieving high conductivity in blends and bilayer configuration.