Homogeneous Doping Research Articles

Significantly enhanced critical current density in nano-MgB2 grains rapidly formed at low temperature with homogeneous carbon doping

High performance MgB2 bulks using carbon-coated amorphous boron as a boron precursor were fabricated by Cu-activated sintering at low temperature (600 °C, below the Mg melting point). Dense nano-MgB2 grains with a high level of homogeneous carbon doping were formed in these MgB2 samples. This type of microstructure can provide a stronger flux pinning force, together with depressed volatility and oxidation of Mg owing to the low-temperature Cu-activated sintering, leading to a significant improvement of critical current density (Jc) in the as-prepared samples. In particular, the value of Jc for the carbon-coated (Mg1.1B2)Cu0.05 sample prepared here is even above 1 × 105 A cm−2 at 20 K, 2 T. The results herein suggest that the combination of low-temperature Cu-activated sintering and employment of carbon-coated amorphous boron as a precursor could be a promising technique for the industrial production of practical MgB2 bulks or wires with excellent Jc, as the carbon-coated amorphous boron powder can be produced commercially at low cost, while the addition of Cu is very convenient and inexpensive.

Facet‐Level Mechanistic Insights into General Homogeneous Carbon Doping for Enhanced Solar‐to‐Hydrogen Conversion

Homogeneous doping can boost solar‐to‐hydrogen conversion and therefore attracts great attention. Although a great deal of effort has been made to explore the doping–photoreactivity relationship, the doping mechanisms, especially from the perspective of crystal facets, are seldom explored. In this study, a general homogeneous carbon doping strategy is established and then serves as the doping model for a mechanistic investigation, as encouraged by its versatility in enabling homogeneous incorporation of carbon and improving solar‐to‐hydrogen conversion for typical oxides including TiO2, ZnO, and BiOCl. Using well‐defined BiOCl nanosheets of high {001} or {010} facet exposure, we clarify the homogeneous carbon doping mechanism at the level of crystal facets for the first time. This mechanism involves the initial facet‐dependent adsorption of the dopant precursor, regulated by the surface atomic structures, and the subsequent facet‐dependent diffusion of carbon dopants associated with the facet‐related arrangements of bulk atoms. This results in facet‐dependent carbon doping behavior and a dopant‐concentration‐dependent solar‐to‐hydrogen conversion property of BiOCl nanosheets. These mechanistic insights also suggest that the implantation of the dopant precursor in the shallow lattice of host nanocrystal is vital for the effective homogeneous doping. This new doping model is different from the conventional counterpart based on the organic ligands or gas molecules adsorption onto the surface of host nanocrystals, where surface doping usually occurs.

Antisite defect elimination through Mg doping in stoichiometric lithium tantalate powder synthesizedviaa wet-chemical spray-drying method

MgO-doped stoichiometric LiTaO3(MgO:SLT) is one of the most promising nonlinear materials. However, its industrial application is limited by the poor optical quality caused by the nonhomogeneous distribution of magnesium. Herein, an MgO:SLT polycrystalline powder was synthesized with a homogenous magnesium distribution by a wet-chemical spray-drying method. A comparative investigation of the coordination state of Ta ions in MgO:SLT powders synthesized by this method and by a conventional solid-state reaction method was performed by X-ray photoelectron spectroscopy. It is proved that the Ta–Li antisite was completely eliminated as a result of the homogeneous Mg doping in the SLT lattice using the wet-chemical spray-drying method. However, for MgO:LT powder produced by the solid-state reaction method, element analysis after acid treatment shows that some Mg ions did not enter the LT lattice after high-temperature calcination. Also, scanning electron microscopy and transmission electron microscopy energy dispersive spectroscopy verified that some MgO particles still exist in the as-synthesized MgO:LT powder. This synthesis method can be used for mass production of high-quality polycrystalline powders for doped crystal growth and some other doped oxide powder products with high melt point.

Creating femtosecond-laser-hyperdoped silicon with a homogeneous doping profile

Femtosecond-laser hyperdoping of sulfur in silicon typically produces a concentration gradient that results in undesirable inhomogeneous material properties. Using a mathematical model of the doping process, we design a fabrication method consisting of a sequence of laser pulses with varying sulfur concentrations in the atmosphere, which produces hyperdoped silicon with a uniform concentration depth profile. Our measurements of the evolution of the concentration profiles with each laser pulse are consistent with our mathematical model of the doping mechanism, based on classical heat and solute diffusion coupled to the far-from-equilibrium dopant incorporation. The use of optimization methods opens an avenue for creating controllable hyperdoped materials on demand.

Raman spectra of crystals LiNbO3:Zn(4.5), LiNbO3:Mg:Fe(5.01, 0.005), LiNbO3:Mg(5.1), and LiNbO3:Mg(5.3 mol %)

We have investigated the Raman spectra of strongly doped crystals that were grown from batches of different genesis. The LiNbO3:Mg(5.3 mol %) and LiNbO3:Mg:Fe(5.01, 0.005 mol %) crystals were obtained using methods of homogeneous doping and the LiNbO3:Mg(5.1 mol %) crystal was obtained using a solid-phase master addition alloy, while the LiNbO3:Zn(4.5 mol %) crystal was grown using direct doping. In the Y(ZZ){ie269-1} scattering geometry, we have revealed the occurrence of lines in the spectrum that correspond to fundamental vibrations of the A2 symmetry species, which are forbidden by the selection rules for the {ie269-2} space group. The occurrence of lines in the spectrum that correspond to fundamental vibrations of the A2 symmetry species has been explained by the existence of microstructures and clusters in strongly doped LiNbO3 crystals, which lead to significant local changes in the symmetry of the crystal.

Ionic Liquid Based Approaches to Carbon Materials Synthesis

AbstractIonic liquids (ILs) have attracted continuous interest because of their remarkable physicochemical properties, including high thermal and chemical stability, nonflammability, negligible vapor pressure, designable cation/anion pairs, electrical and ionic conductivity, low melting points, and affinity towards many compounds. These properties make ionic liquids valuable in the creation of new materials and new processes throughout almost the entire field of materials chemistry. An emerging field is the use of ionic liquids in carbon‐based nanomaterials. Initially, ionic liquids were used as self‐templating carbon sources for the generation of unusual carbon materials in which homogeneous heteroatom doping (e.g., N, B, S) can be accomplished fairly easily. Later, ionic liquids were recognized as suitable for use in the conversion of biomass into porous carbon materials, acting as both reaction media and porosity‐directing regulator. Such applications open the door towards the synthesis and accurate tuning of carbon nanostructures, for example, pore structure, morphology, heteroatom doping, and surface functionality. In addition, the hybridization of ionic liquids and nanocarbons enables the development of composites by combining the properties of the ionic liquid (e.g., ionic conductivity or catalytic activity) and those of a host (e.g., chemical or mechanical stability). Currently, although the research of this topic is rapidly expanding, the rational design and synthesis of carbon‐based materials, particularly their applications in energy storage and transformation, is still in its infancy. In this review, we focus on several aspects of ionic liquid derived carbons with the aim of shedding light on this new topic: 1) Ionic liquids as an advanced medium for carbon synthesis, 2) ionogels derived from nanocarbon, 3) ionic liquids as fluid precursors for functional carbons, and 4) ionic liquid derived carbons for heterogeneous catalysis.

Preparation of homogeneous nitrogen-doped mesoporous TiO2 spheres with enhanced visible-light photocatalysis

A series of mesoporous TiO2 spheres of nanosized crystals with high monodispersity and dimensional homogeneity are synthesized by a simple reflux method. In order to improve the visible light response and to further narrow the band bap of TiO2 sphere, nitrogen is used as the dopant in a hydrothermal method. Different from the traditional nitrogen doping in TiO2 solid sphere, the nitrogen is uniformly distributed from bulk to the surface of mesoporous TiO2 sphere, which is beneficial to the generation of the successive energy bands inside the TiO2 band gap. Interestingly, the XPS results indicate that the nitrogen doping concentration could reach 1.31% (only 0.17% for nitrogen doped solid TiO2 sphere), owing to the homogeneous doping in the mesoporous TiO2 spheres. The nitrogen doped TiO2 spheres exhibit such excellent characteristics as high specific area, relatively small particle size, pure anatase phase and excellent UV–vis absorption capacity in the range of 400–800nm, which are all beneficial to the photo-degradation of Rhodamine B under the visible light irradiation. The high photocatalytic activity could be ascribed to the homogeneous nitrogen doping on the deep adjustment of energy band of TiO2 sphere from the bulk to the surface.

Field effect in the quantum Hall regime of a high mobility graphene wire

In graphene-based electronic devices like in transistors, the field effect applied thanks to a gate electrode allows tuning the charge density in the graphene layer and passing continuously from the electron to the hole doped regime across the Dirac point. Homogeneous doping is crucial to understand electrical measurements and for the operation of future graphene-based electronic devices. However, recently theoretical and experimental studies highlighted the role of the electrostatic edge due to fringing electrostatic field lines at the graphene edges [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008); F. T. Vasko and I. V. Zozoulenko, Appl. Phys. Lett. 97, 092115 (2010)]. This effect originates from the particular geometric design of the samples. A direct consequence is a charge accumulation at the graphene edges giving a value for the density, which deviates from the simple picture of a plate capacitor and also varies along the width of the graphene sample. Entering the quantum Hall regime would, in principle, allow probing this accumulation thanks to the extreme sensitivity of this quantum effect to charge density and the charge distribution. Moreover, the presence of an additional and counter-propagating edge channel has been predicted [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008)] giving a fundamental aspect to this technological issue. In this article, we investigate this effect by tuning a high mobility graphene wire into the quantum Hall regime in which charge carriers probe the electrostatic potential at high magnetic field close to the edges. We observe a slight deviation to the linear shift of the quantum Hall plateaus with magnetic field and we study its evolution for different filling factors, which correspond to different probed regions in real space. We discuss the possible origins of this effect including an increase of the charge density towards the edges.

Depletion of superjunction power MOSFETs visualized by electron beam induced current and voltage contrast measurements

AbstractElectron Beam Induced Current (EBIC) measurements were used to produce cross sectional images of superjunction power transistors. These images show how the depletion width expands under reverse bias. Superjunctions are alternating p‐ and n‐type doped vertical columns placed between drain and source in a power transistor (Deboy et al., in: Proc. IEDM, 983–685 (1998); Lorenz et al., in: Proc. PCIM Europe, 250–258 (1998)). These columns allow a higher substrate doping of the drift region, resulting in a lower on‐state resistance while still maintaining a high breakdown voltage. When the device is reverse biased, the space charge region of the superjunction should expand symmetrically due to the homogeneous doping (in the n and p region) until the complete device depletes. The depletion process was also visualized using voltage contrast (VC) measurements. Here the secondary electron signal was detected when the device was reverse biased. We show that EBIC and VC measurements can provide valuable input for process tuning and process simulations, enabling the use of smaller dimensions and higher doping levels

Long-term air-stable n-type doped graphene by multiple lamination with polyethyleneimine

We demonstrate homogeneous and air-stable n-type doping of graphene grown by thermal chemical vapor deposition. Laminated Polyethyleneimine (PEI) layers fabricated by a simple polymer solution coating were employed. The doping stability of multiple laminated structures (graphene/PEI, PEI/graphene, and graphene/PEI/graphene) was systematically investigated. The resulting graphene/PEI/graphene laminated structure was highly stable in air. Moreover, the work function and carrier concentration of doped graphene can be tuned by altering the laminated structure.

Photoluminescence of focused ion beam implanted Er3+:Y2SiO5crystals

Erbium-doped low symmetry Y2SiO5 crystals attract a lot of attention in perspective of quantum information applications. However, only doping of the samples during growth is available up to now, which yields a quite homogeneous doping density. In the present work, we deposit Er3+-ions by the focused ion beam technique at yttrium sites with several fluences in one sample. With a photoluminescence study of these locally doped Er3+:Y2SiO5 crystals, we are able to evaluate the efficiency of the implantation process and develop it for the highest efficiency possible. We observe the dependence of ion activation after the post-implantation annealing on the fluence value. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Nanocrystalline SiC formed by annealing of a-SiC:H on Si substrates: A study of dopant interdiffusion

Nanocrystalline silicon carbide (nc-SiC) is an interesting material for electronics applications, both in its own right and as a host matrix for silicon quantum dots. When synthesized by annealing of a-SiC:H on Si substrates, interdiffusion of dopants occurs if either the a-SiC:H or the Si substrate is doped. Annealing a-SiC:H on highly boron-doped substrates at 1100 °C leads to a fairly homogeneous doping level of ≥4 × 1019 cm−3 throughout the nc-SiC film. An unexpected anomaly in secondary ion mass spectroscopy quantification is observed and a method to circumvent it is shown. The nanostructure of the nc-SiC is only weakly affected as most of the diffusion occurs after the onset of crystallization. Annealing of doped a-SiC:H on Si substrates at 1100 °C leads to strong free carrier absorption at infrared wavelengths. This is demonstrated to originate from dopants that have diffused from the a-SiC:H to the Si substrate, and a method is developed to extract from it the doping profile in the Si substrate. The detection limit of this method is estimated to be ≤6 × 1013 cm−2. Doping levels of (0.5–3.5) × 1019 cm−3 are induced at the Si substrate surface by both boron and phosphorus-doped a–SiC:H. When the Si substrate is doped opposite to the a-SiC:H p–n junctions are induced at a depth of 0.9–1.4 μm within the Si substrate for substrate resistivities of 1–10 Ω cm. Implications for different solar cell architectures are discussed. Dopant diffusion can be strongly reduced by lowering the annealing temperature to 1000 °C, albeit at the expense of reduced crystallinity.

Engineering homogeneous doping in single nanoparticle to enhance upconversion efficiency.

Upconversion nanoparticles (UCNPs) have shown considerable promises in many fields; however, their low upconversion efficiency is still the most serious limitation of their applications. Herein, we report for first time that the homogeneous doping approach based on the successive layer-by-layer method can greatly improve the efficiency of the UCNPs. The quantum yield as high as 0.89 ± 0.05% is realized for the homogeneous doping NaGdF4:Yb,Er/NaYF4 UCNPs, which is nearly 2 times higher than that of the heterogeneous doping NaGdF4:Yb,Er/NaYF4 UCNPs (0.47 ± 0.05%). The influences of spatial distributions and local relative concentrations of the dopants on the optical properties of UCNPs were investigated in the single particle level. It was found that heterogeneous doping indeed existed during the spontaneous growth process of the nanoparticles. The heterogeneous doping property can further induce many negative effects on the optical properties of UCNPs, especially the luminescent efficiency. The spatial distributions and local relative concentrations of the dopants can be well controlled by the successive layer-by-layer homogeneous doping method on the monolayer level and homogeneously distributed in the single particle level. Furthermore, by using homogeneous doping NaGdF4:Yb,Tm as initial core, the multicolor emission intensity of NaGdF4:Yb,Tm/NaGdF4:A (A = Tb(3+), Eu(3+)) core/shell nanoparticles can also exhibit 20%-30% improvement. We believe that such a homogeneous doping model can open the door to improve the upconversion optical properties by engineering the local distribution of the sensitizer, activator, host, etc., in a microcosmic and provide a track for engineering the high quality UCNPs with advanced nanostructure and optical properties.

Effect of the Active Layer on Carbon Nanotube-Based Cells for Yield Analysis

Carbon Nanotube Field Effect Transistor (CNTFET)--based technologies become more and more a concurrent alternative to Metal Oxide Semiconductor Field Effect Transistors (MOSFET) technologies. In contrast to a MOSFET technology, the active layer of a CNTFET technology is not a regular silicon film with homogeneous doping and rectangular dimensions, but an array of mostly aligned carbon nanotubes (CNTs). The quality of this active layer, which depends on various technology process parameters, is expressed by parameters such as CNT alignment and array density. These parameters affect the electrical properties of the logic cells placed on top of the active layer and hence the overall CNTFET circuit yield. Although not all parameters in CNT fabrication process can be fully controlled, designers still need to assure a very high yield of their cell layouts, that is, a high reproducibility of the electrical characteristics to achieve a reasonable manufacturing yield for the entire chip. In this work we close the gap between CNTFET process fabrication and circuit design by presenting a novel accurate model for active layers in CNTFET--based technologies. Our model enables the designers to obtain technology--dependent driver strength of the custom cell layouts under realistic conditions. The new model can also be used to extract and evaluate CNTFET Design for Manufacturing (DfM) and Design for Robustness (DfR) design rules, and provide feedback to adjust process technology parameters to achieve desirable functional yield.

Homogeneous carbon doping of magnesium diboride by high-temperature, high-pressure synthesis

We have used high-pressure, high-temperature synthesis at 1500–1700 °C and 10 MPa to create homogeneously C-substituted MgB2 from a B4C + Mg mixture. X-ray diffraction analysis showed large peak-shifts consistent with a decrease in the a lattice parameter for the B4C-derived MgB2 as compared to an undoped sample (0.033–0.037 Å, depending on the sample). Microstructural investigation showed a three-phase mixture in the B4C-derived ingots: MgB2−xCx (with 0.178 < x < 0.195), MgB2C2, and Mg. The carbon concentration determined from the lattice parameter shift (5.95 at. %) matched well with the calorimetrically derived concentration of 5.3–5.8 at. % C. Furthermore, the carbon content measured by electron probe micro-analysis was shown to be 6.2 ± 1.3 at. %. Finally, we performed bulk specific heat measurements to determine the homogeneity of C-doping in the MgB2. The width of the Tc distribution for the C-doped MgB2 was only 3–6 K with a full-width half maximum (FWHM) of 1.4 K, compared to a width of 2.5 K and a FWHM of 0.65 for an undoped sample. The consistency of these three measurements on a large-grained homogeneous material is unambiguously supportive of C-substitution.

Non-metal doping of transition metal oxides for visible-light photocatalysis

Transition metal oxides and mixed oxides are the largest group of materials for photocatalytic applications. Many highly active compounds are known from literature for environmental remediation, pollutant degradation and solar fuel generation. However, most of these oxides can only absorb UV light to perform photocatalytic reactions at their surface due to their large band gap. In this review, we present the recent progress in non-metal doping of transition metal oxides and mixed oxides, one of the major strategies to reduce the large band gap of semiconductor materials into the visible light range. We outline the advantages of this strategy compared to other band gap engineering methods, and especially stress the effect of efficient homogeneous non-metal doping on the optical, electronic and photocatalytic properties of photocatalysts, compared to surface doping and surface modification, including the effects of an open crystal structure on the efficiency of the doping process. We then present the highlights and breakthroughs of the last ten years in the research field and point out major improvements important for future applications, covering all the available non-metal doped transition metal oxides concerning photocatalytic reactions.

The search of homogeneity of LiNbO3 crystals grown of charge with different genesis

The comparative analysis of homogeneity was carried out for LiNbO3:Mg ([Mg]≥5mol%), grown of Nb2O5:Mg charge synthesized at homogeneous Mg doping during the extraction of Nb2O5 and of usual charge synthesized by adding MgO to the Nb2O5. Data obtained by optical microscopy and by Raman spectroscopy reveal that homogeneously doped crystals are more homogeneous and have better structure characteristics.

Magnetic characteristics of phase-separated CeO2:Co thin films

Herewith, we are reporting the magnetic properties of phase-separated Co-doped CeO2 films (with a Ce:Co atomic-ratio of 0.97:0.03) grown on single-crystal SrTiO3 (001) substrates. A comparison of the magnetic characteristics of these films with those of homogenously doped CeO2:Co films of the same composition illustrates the significant differences in their magnetic behavior. These behavioral characteristics provide a model for determining if the magnetic behavior observed in this, as well as in other diluted magnetic dielectric systems, is due to homogeneous doping, a mixture of doping and transition metal cluster formation, or exists purely as a result of transition metal clustering.

Excimer laser doping using highly doped silicon nanoparticles

Laser doping of crystalline Si (c-Si) using highly doped Si nanoparticles (NPs) as the dopant source is investigated. For this purpose Si NPs are deposited onto c-Si substrates from dispersion using a spin coater and subsequently laser annealed by scanning over the sample with a 248 nm line profile excimer laser. Scanning electron microscope (SEM) investigations demonstrate that the laser intensity as well as the oxide concentration in the NP thin film strongly influence the film forming properties of the annealed NPs. Substrate doping is substantiated using electrochemical capacitance voltage (ECV) measurements on realized pn-junctions. In dependence of the laser fluencies ranging from 0.81 to 2.54 J cm−2, the effective doping depth is determined to be in the range of 50 to 250 nm. The rectifying behaviour of the pn- or np-junctions is verified by current voltage measurements. A homogeneous in-plane doping distribution realized by the laser doping process is demonstrated on the µm scale by light beam induced current measurements.

Electronic structure and magnetic properties of Gd-doped and Eu-rich EuO

The effects of Gd doping and O vacancies on the magnetic interaction and Curie temperature of EuO are studied using first-principles calculations. Linear response calculations in the virtual crystal approximation show a broad maximum in the Curie temperature as a function of doping, which results from the combination of the saturating contribution from indirect exchange and a decreasing contribution from the f-d hopping mechanism. Non-Heisenberg interaction at low doping levels and its effect on the Curie temperature are examined. The electronic structure of a substitutional Gd and of an O vacancy in EuO are evaluated. When the 4f spins are disordered, the impurity state goes from single to double occupation, but correlated bound magnetic polarons are not ruled out. At higher vacancy concentrations typical for Eu-rich EuO films, the impurity states broaden into bands and remain partially filled. To go beyond the homogeneous doping picture, magnetostructural cluster expansions are constructed, which describe the modified exchange parameters near Gd dopants or O vacancies. Thermodynamic properties are studied using Monte Carlo simulations. The Curie temperature for Gd-doped EuO agrees with the results of the virtual crystal approximation and shows a maximum of about 150 K. At 3.125% vacancy concentration the Curie temperature increases to 120 K, consistent with experimental data for Eu-rich film samples.