Spatial Distribution Of Dopants Research Articles

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy.

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.

Concentration Phase Separation of Substitution-Doped Atoms in TMDCs Monolayer.

The density and spatial distribution of substituted dopants affect the transition metal dichalcogenides (TMDCs) materials properties. Previous studies have demonstrated that the density of dopants in TMDCs increases with the amount of doping, and the phenomenon of doping concentration difference between the nucleation center and the edge is observed, but the spatial distribution law of doping atoms has not been carefully studied. Here, it is demonstrated that the spatial distribution of dopants changes at high doping concentrations. The spontaneous formation of an interface with a steep doping concentration change is named concentration phase separation (CPS). The difference in the spatial distribution of dopants on both sides of the interface can be identified by an optical microscope. This is consistent with the results of spectral analysis and microstructure characterization of scanning transmission electron microscope. According to the calculation results of density functional theory, the chemical potential has two relatively stable energies as the doping concentration increases, which leads to the spontaneous formation of CPS. Understanding the abnormal phenomena is important for the design of TMDCs devices. This work has great significance in the establishment and improvement of the doping theory and the design of the doping process for 2D materials.

Engineering Orthogonal Upconversion through Selective Excitation in a Single Nanoparticle

AbstractOrthogonal upconversion with color‐switchable emissions under different excitation conditions provides new chances to diverse frontier applications of luminescent materials. However, the previous orthogonal upconversion systems compulsorily require the precise control of spatial distributions of dopants and host compositions to avoid spectral cross‐talk, greatly limiting their synthesis and application. Herein, a conceptual design is presented to realize the orthogonal upconversion by selectively activating sensitizers and activators in nanostructure. In detail, the 915 nm laser, instead of the mostly used 980 nm, is adopted to activate the sensitizer Yb3+ because of the broad 2F5/2←2F7/2 absorption band; while it is non‐responsive to Er3+, consequently avoiding the spectral cross‐talk due to the simultaneous activation of both Er3+ and Yb3+ under 980 nm irradiation as shown in the previous works. Such a selective excitation strategy is able to realize the red/green, blue/red, and blue/green orthogonal upconversion in a single nanoparticle by simply altering the excitation wavelength from 915 to 1530 nm. More interestingly, it further allows for full‐color output by temporal control of the upconversion dynamics. The findings suggest a facile but effective approach to the design of smart luminescent materials, showing great potential in multi‐level anti‐counterfeiting, information security and biological applications.

Visualizing In‐Plane Junctions in Nitrogen‐Doped Graphene

AbstractControlling the spatial distribution of dopants in graphene is the gateway to the realization of graphene‐based electronic components. Here, it is shown that a submonolayer of self‐assembled physisorbed molecules can be used as a resist during a post‐synthesis nitrogen doping process to realize a nanopatterning of nitrogen dopants in graphene. The resulting formation of domains with different nitrogen concentrations allows obtaining n–n’ and p–n junctions in graphene. A scanning tunneling microscopy is used to measure the electronic properties of the junctions at the atomic scale and reveal their intrinsic width that is found to be ≈7 nm corresponding to a sharp junction regime.

Formation of Highly Doped Nanostripes in 2D Transition Metal Dichalcogenides via a Dislocation Climb Mechanism.

Doping of materials beyond the dopant solubility limit remains a challenge, especially when spatially nonuniform doping is required. In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but full control over spatial distribution of dopants remains a challenge. A post-growth doping of single layers of WSe2 is performed by adding transition metal (TM) atoms in a two-step process, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. Remarkably, an extremely high density (6.4-15%) of various types of impurity atoms is achieved. The dopants are revealed to be largely confined within nanostripes embedded in the otherwise pristine WSe2 . Density functional theory calculations show that the dislocations assist the incorporation of the dopant during their climb and give rise to stripes of TM dopant atoms. This work demonstrates a possible spatially controllable doping strategy to achieve the desired local electronic, magnetic, and optical properties in 2D materials.

The role of the shell in core-shell-structured La-doped NaTaO3 photocatalysts.

NaTaO3, a semiconductor with a perovskite structure, has long been known as a highly active photocatalyst for overall water splitting when appropriately doped with La cations. A profound understanding of the surface feature and why and how it may control the water splitting activity is critical because redox reactions take place at the surface. One surface feature characteristic of La-doped NaTaO3 is a La-rich layer (shell) capping La-poor bulk (core). In this study, we investigate the role of the shell in core-shell-structured La-doped NaTaO3 through systematic chemical etching with an aqueous HF solution. We find that the La-rich shell plays a role in electron-hole recombination, electron mobility and water splitting activity. The shallow electron traps populating the La-rich shell trap the photoexcited electrons, decreasing their mobility. The shallowly trapped electrons remain reactive and are readily available on the surface to be extracted by the cocatalysts for the reduction reaction evolving H2. The presently employed chemical etching method also confirms the presence of a La concentration gradient in the core that regulates the steady-state electron population and water splitting activity. Here, we successfully reveal the nanoarchitecture-photoactivity relationship of core-shell-structured La-doped NaTaO3 that thereby allows tuning of the surface features and spatial distribution of dopants to increase the concentration of photoexcited electrons and therefore the water splitting activity. By recognizing the key factors that control the photocatalytic properties of a highly active catalyst, we can then devise proper strategies to design new photocatalyst materials with breakthrough performances.

Direct Observation of the Reduction of a Molecule on Nitrogen Pairs in Doped Graphene.

Incorporating functional atomic sites in graphene is essential for realizing advanced two-dimensional materials. Doping graphene with nitrogen offers the opportunity to tune its chemical activity with significant charge redistribution occurring between molecules and substrate. The necessary atomic scale understanding of how this depends on the spatial distribution of dopants, as well as their positions relative to the molecule, can be provided by scanning tunneling microscopy. Here we show that a noncovalently bonded molecule such as CoPc undergoes a variable charge transfer when placed on N-doped graphene; on a nitrogen pair, it undergoes a redox reaction with an integral charge transfer whereas a lower fractional charge transfer occurs over a single nitrogen. Thus, the charge state of molecules can be tuned by suitably tailoring the conformation of dopant atoms.

Water-Splitting Activity of La-Doped NaTaO3 Photocatalysts Sensitive to Spatial Distribution of Dopants

Understanding the science behind highly active materials is essential for advancement in the field of photocatalytic water splitting for solar energy harvesting. Sodium tantalate (NaTaO3) doped with La cations is one of the best engineered materials for efficient water splitting to evolve hydrogen. In this study, physical insights into the sensitivity of the water-splitting activity to the spatial La distribution are discussed. The spatial distribution of La cations placed at the Na site was found to dictate the energy gradient of the conduction band bottom (CBB), resulting in a tunable electron population and hence water-splitting activity. A less homogeneous sample with a sufficiently large CBB gradient exhibited higher water-splitting activity. The mechanism of gradient tuning of the CBB through controlling the spatial dopant distribution is expected to be applicable to a broad range of metal oxide perovskites for artificial photosynthesis.

Tuning charge transport dynamics via clustering of doping in organic semiconductor thin films

A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient. Conventional wisdom posits that the polymer alone dictates thermoelectric efficiency. Herein, we show that doping — in particular, clustering of dopants within conjugated polymer films — has a profound and predictable influence on their thermoelectric properties. We correlate Seebeck coefficient and electrical conductivity of iodine-doped poly(3-hexylthiophene) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)] films with Kelvin probe force microscopy to highlight the role of the spatial distribution of dopants in determining overall charge transport. We fit the experimental data to a phonon-assisted hopping model and found that the distribution of dopants alters the distribution of the density of states and the Kang–Snyder transport parameter. These results highlight the importance of controlling dopant distribution within conjugated polymer films for thermoelectric and other electronic applications.

Dopant profiling in p-i-n GaN structures using secondary electrons

We show that secondary electrons in a scanning electron microscope can provide important information about spatial dopant distribution in p-i-n GaN structures, with the highest contrast observed for a primary electron beam accelerating voltage in the range of 1 to 2 kV. The current through the specimen is used to determine the total backscattered and secondary electron yield. We establish a correlation between the secondary electron emission intensity and the doping characteristics of the material. The secondary electron emission intensity was found to be highest for p-type GaN, intermediate for n-type GaN, and lowest for undoped GaN. Specimen currents are found to have a strong correlation with the Mg concentration in p-GaN films. The contrast associated with dopants is shown to depend on experimental parameters such as primary electron beam voltage, total electron beam exposure, and specimen surface history. This technique can serve as a powerful tool for the development and characterization of thin films for GaN power electronics.

Effect of acceptor doping on phase transitions of HfO2 thin films for energy-related applications

Fluorite structured HfO2 or ZrO2 thin films have been intensively studied for memory- and energy-related applications since their ferroelectricity was first reported in 2011. The phase transition between the nonpolar tetragonal and the polar orthorhombic phase in these new ferroelectric materials is believed to be promising for energy harvesting, energy storage and solid state cooling. The temperature dependent phase transition and resulting strong pyroelectric and electrocaloric effect have been reported for Si-doped HfO2 and (Hf,Zr)O2 thin films. In this study, the effect of acceptor (Al and Gd) doping into HfO2 thin films on their temperature dependent phase transition was systematically examined. The phase transitions in Al- and Gd-doped HfO2 thin films were much broader compared to Si-doped HfO2 and (Hf,Zr)O2 films. The maximum adiabatic temperature change (ΔT) values of Al- and Gd-doped HfO2 film were 5.7 and 3.1K, respectively. A giant negative electrocaloric effect with ΔT of −7.4K could be observed for Al-doped HfO2. The various factors which can potentially affect the phase transitions of HfO2 films, such as dopant size, grain size distribution, spatial dopant distribution, and oxygen vacancy distribution were carefully examined to understand the different phase transition behavior. From the various factors, the distribution of oxygen vacancies is suggested as the origin of the different phase transitions of HfO2 films doped with trivalent and tetravalent dopants.

Study of doping uniformity of a 200 kV ion implanter by RBS and sheet resistance measurements

The ion implantation uniformity is of vital importance for an ion implanter. In this paper, we report the, uniformity measurement for a large current ion implanter (LC-16 type) by implanting of 190-keV Ar ions into Si to 3 × 1016 atoms/cm2, followed by Rutherford backscattering spectroscopy (RBS) and sheet resistance measurement providing quantitative information on spatial distribution of dopants. The implant doses obtained from RBS at selected points of the sample give a spatial uniformity of <5 %, which are confirmed by the sheet resistance measurement. While sheet resistance is an indirect method for dose evaluation of ion-implanted samples, RBS provides a competent technique for calibration of the ion implantation system. And both measurements show that good uniformity can be achieved for the ion implanter by tuning of the scanning process.

Supramolecular Chemistry Controlled by Conformational Space during Structure Direction of Nanoporous Materials: Self-Assembly of Ephedrine and Pseudoephedrine

In an attempt to produce chiral solids, where chirality arises from an asymmetric ordering of the spatial distribution of dopants, we have studied the supramolecular aggregation of π–π-type interactions between aromatic rings of two closely related diastereoisomers, (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine, during structure direction of the microporous AFI large-pore aluminophosphate framework. Despite their very similar molecular structures, differing only in the stereochemistry of the hydroxyl substituent, fluorescence spectroscopy clearly shows completely different aggregation behaviors of the two isomers, with ephedrine displaying a much stronger aggregation trend than pseudoephedrine in aqueous solution; interestingly, this supramolecular behavior is reproduced when the molecules are occluded within the AFI framework during crystallization. A molecular mechanics study of the conformational behavior of the two chiral flexible molecules, both in a vacuum and in aqueous solution, revealed that the lower trend of pseudoephedrine to form supramolecular aggregates is caused by the occurrence of a particular conformation with a folded molecular structure. This conformation, which is stable for pseudoephedrine but not for ephedrine, provokes steric hindrance that prevents the approaching of a neighboring molecule to form an aggregate, hence providing an explanation for the experimental observations. Our work represents the first example in which the conformational space of a molecule determines its supramolecular aggregation behavior during structure direction and could play a role in molecular-recognition phenomena in host–guest systems.

Supramolecular chemistry of chiral (1R,2S)-ephedrine confined within the AFI framework as a function of the synthesis conditions

In an attempt to promote chirality in AFI-based materials through spatial distributions of dopants, a systematic variation of the synthesis conditions of Mg-doped aluminophosphate-based microporous materials using chiral (1R,2S)-ephedrine as structure-directing agent has been carried out in order to understand the supramolecular chemistry and packing arrangements of these molecules when confined within the AFI channels. The formation of ephedrine dimers is strongly dependent on the synthesis conditions, especially the ephedrine/H2O ratio (higher concentrations of ephedrine favour the incorporation of dimers), crystallisation temperature (higher temperatures enhance the supramolecular aggregation) and Mg content (lower Mg contents increase the formation of dimers). The latter influence is explained by molecular simulations in terms of an electrostatic repulsion between the ammonium groups of closely-located adjacent dimers when a high density of protonated ephedrine molecules is required in order to compensate for the high Mg incorporation. In this situation, dimers are only incorporated if the system is forced by increasing the concentration of ephedrine in the gel and increasing the temperature. Our results show that a similar number of ephedrine molecules and Mg dopants are incorporated in the AFI unit cell because of charge-balancing purposes, suggesting a strong electrostatic interaction between both species with opposite charge.

Determination of the Degree of Homogeneity for n Ge Crystals by Measurements of Magnetoresistance in Classically Weak Magnetic Fields

In a wide range of values of the resistivity 0.0212 £ r £ 35 Ohm×cm for n‑Ge crystals with known crystallographic orientation the ratio , which was predicted by the theory, was experimentally confirmed. The experimentally measured deviations from the relationship , which are associated with the heterogeneity of the crystals, it is proposed to use for qualitative assessment of the degree of heterogeneity in the spatial distribution of dopants in the samples.

Probing Ultrafast Carrier Dynamics by Laser-Combined STM

With the progress of science and technology, we are now facing limitations of current technologies that cannot be overcome by the simple expansion of existing engineering concepts. Thus, the introduction of new concepts based on new ideas, as well as the discovery of new physical properties and their applications, is a key factor in breaking this deadlock. However, for example, the technology node of semiconductor devices has become as small as 20-30 nm, and the fluctuations in the spatial distribution of dopants, which are introduced to control the properties of devices, and in the nanostructured interface between dissimilar materials directly affect the device functions obtained. Charge transfer in composite materials governing, for example, phase transitions, photoelectric conversion and catalysis is generally over the picoseond-to-subpicosecond range. Phonon-related phenomena are much faster. Namely, with the miniaturization of functional devices consisting of composite materials, quantum dynamics, which has been analyzed by techniques providing spatially and/or temporally averaged information, does not provide a sufficiently detailed description for the analysis and design of macroscopic functions.

Towards chiral distributions of dopants in microporous frameworks: helicoidal supramolecular arrangement of (1R,2S)-ephedrine and transfer of chirality.

A molecular-mechanics computational study is performed in order to analyze the arrangement of (1R,2S)-(-)-ephedrine molecules within the 12-MR channels of the AFI aluminophosphate microporous framework and the influence on the spatial distribution of dopants embedded in the tetrahedral network. Results showed that ephedrine molecules arrange exclusively as dimers by π-π stacking of the aromatic rings within the AFI channels. Interestingly, the asymmetric nature of ephedrine and the presence of H-bond-forming groups (NH2 and OH) involve a preferential orientation where consecutive dimers within the channels are rotated by an angle of +30°; this is driven by the establishment of inter-dimer H-bonds. This preferential orientation leads to the development of a supramolecular enantiomerically-pure helicoidal (chiral) arrangement of ephedrine dimers. In addition, the computational results demonstrate that the particular molecular structure of ephedrine imparts a strong trend to attract negative charges to the vicinity of the NH2(+) positively-charged groups. Hence divalent dopants such as Mg, whose replacement by trivalent Al in the aluminophosphate network involves the generation of a negative charge, will tend to locate close to the NH2(+) molecular groups, suggesting that an imprinting of the organic arrangement to the spatial distribution of dopants would be feasible. Combined with the trend of ephedrine to arrange in a helicoidal fashion, an enantiomerically-pure helicoidal distribution of dopants would be expected, thus inducing a new type of chirality in microporous materials.

Two-dimensional vacancy trapping in yttria doped ceria

Abstract Thin film processing methods offer a number of means to investigate and engineer ion conduction in solid electrolytes. In this work, we present fabrication and characterization of Y-doped CeO 2 thin films where the Y-dopants were distributed homogeneously or were condensed into increasingly concentrated layers, to the limit of alternating layers of pure Y 2 O 3 and pure CeO 2 . Both the entire film thickness and net Y-concentration were kept constant such that only the spatial distribution of dopants was altered. Space charge regions formed at interfaces between regions with varying vacancy concentrations, yielding vacancies trapped within two-dimensionally arranged accumulation regions. A Gouy–Chapman model was implemented in order to further investigate the distribution of the accumulated oxygen vacancies in the space charge regions of pure CeO 2 layers. Comparison of the measured activation energy of conduction indicates that in films with intermediate dopant condensation, conduction occurred predominantly by vacancies trapped in the Y-containing layers. Conversely, in the film composed of alternating layers of Y 2 O 3 and CeO 2 , vacancies trapped in the CeO 2 space charge regions became significantly conductive, thus providing a means to determine the properties of vacancies in ceria that are trapped near dopants.

High-Quality White Light Using Core–Shell RE3+:LaPO4 (RE = Eu, Tb, Dy, Ce) Phosphors

Rare-earth (RE = Eu, Dy, Tb, and Ce) ion doped core and core–shell LaPO4 phosphors were synthesized to elucidate the effect of spatial distribution of dopants on the emission spectra. The core–shell architecture was designed as a single particle that can be excited by a single wavelength and yield a balanced white light appearance with long emission lifetimes. Specifically, a multishell architecture was employed to separate the Eu3+ and Tb3+ within the phosphor to circumvent the energy transfer between them, passivate the surface quenching sites, and control Ce3+ doping to sensitize other RE ions. To assess the effectiveness of these core–shell phosphors, the International Commission on Illumination (CIE) coordinates and luminescence lifetimes are quantified as the figures of merit. The Eu3+:LaPO4|Ce3+,Dy3+:LaPO4|Tb3+:LaPO4 layering resulted in CIE coordinates of (0.34, 0.35) using 365 nm excitation, nearly at center of the white light regime at (0.35, 0.35). Finally, the emission lifetimes were measured to be 0.85, 4.34, and 3.26 ms and resulted in a total increase of 31, 36, and 16% over the RE3+:LaPO4 reference phosphors, where RE = Dy, Tb, and Eu, respectively. The synthesized phosphor material has high-quality white light with improved emission lifetimes, suitable for application in white light LED devices.

Atom-probe for FinFET dopant characterization

With the continuous shrinking of transistors and advent of new transistor architectures to keep in pace with Moore's law and ITRS goals, there is a rising interest in multigate 3D-devices like FinFETs where the channel is surrounded by gates on multiple surfaces. The performance of these devices depends on the dimensions and the spatial distribution of dopants in source/drain regions of the device. As a result there is a need for new metrology approach/technique to characterize quantitatively the dopant distribution in these devices with nanometer precision in 3D. In recent years, atom probe tomography (APT) has shown its ability to analyze semiconductor and thin insulator materials effectively with sub-nm resolution in 3D. In this paper we will discuss the methodology used to study FinFET-based structures using APT. Whereas challenges and solutions for sample preparation linked to the limited fin dimensions already have been reported before, we report here an approach to prepare fin structures for APT, which based on their processing history (trenches filled with Si) are in principle invisible in FIB and SEM. Hence alternative solutions in locating and positioning them on the APT-tip are presented. We also report on the use of the atom probe results on FinFETs to understand the role of different dopant implantation angles (10° and 45°) when attempting conformal doping of FinFETs and provide a quantitative comparison with alternative approaches such as 1D secondary ion mass spectrometry (SIMS) and theoretical model values.