Background Impurities Research Articles

Polarization Effects in Graded AlGaN Nanolayers Revealed by Current-Sensing and Kelvin Probe Microscopy.

We experimentally demonstrate that the conductivity of graded AlxGa1-xN increases as a function of the magnitude of the Al concentration gradient (%Al/nm) due to polarization doping effects, without the use of impurity dopants. Using three up/down-graded AlxGa1-xN nanolayers with Al gradients ranging from ∼0.16 to ∼0.28%Al/nm combined in one structure, the effects of polarization engineering for localized electric fields and current transport were investigated. Cross-sectional Kelvin probe force microscopy and conductive atomic force microscopy were used to directly probe the electrical properties of the films with spatial resolution along the thickness of the growth. The experimental profiles of the built-in electric fields and the spreading current found in the graded layers are shown to be consistent with simulations of the field distribution as well as of the electron and hole densities. Finally, it was directly observed that for gradients less than 0.28%Al/nm the native n-type donors still limit polarization-induced hole doping, making p-type conductivity still a challenge due to background impurities and defects.

RADIATION SENSITIVITY OF IONIC CRYSTALS. ONE-DIMENSIONAL MODEL. І. IONIC CRYSTALS DOPED WITH ISOVALENT IMPURITIES

Виникнення радіаційного забарвлення в іонних кристалах є результатом локалізації створених радіацією вільних носіїв заряду на дорадіаційних точкових дефектах кристалічної ґратки. Із накопиченням у кристалах центрів забарвлення вступають в дію зворотні процеси. Вільні носії заряду рекомбінують на центрах забарвлення, дорадіаційні дефекти відновлюються. Під час довготривалого опромінення кристалів встановлюється динамічна рівновага між процесами генерації центрів забарвлення та висвітлювальною дією рентгенівських променів. Запропоновано одновимірну модель іонних кристалів, в якій розраховано параметри радіаційної чутливості кристалів флюоритів і лужно-галоїдних кристалів, легованих ізовалентними домішками. Встановлено, що незалежно від концентрації активатора в кристалі імовірність утворення центрів забарвлення при розпаді електронно-діркової пари завжди менша за імовірність їх руйнування. Це пояснюють тим, що в першому випадку носії заряду взаємодіють із електронейтральними цетрами, а в другому – із зарядженими. Імовірність генерації центрів забарвлення під час розпаду електронно-діркової пари різко зменшується зі зменшенням концентрації активатора, а імовірність висвітлювальної дії практично не залежить від вмісту домішки у кристалі. Зі зменшенням концентрації активатора зростає величина енергії, що витрачається на створення однієї комплементарної пари центрів забарвлення, а відповідно радіаційна чутливість кристала знижується. Неконтрольовані фонові домішки, якими є передусім ізовалентні чужорідні іони, якщо їх концентрація менша за величину с<0,01 мол. %, не впливають на радіаційну чутливість кристала.

Direct measurement of the to lifetime ratio in a single trapped

We present for the first time a direct measurement of the lifetime ratio between the and metastable states in a single trapped 40Ca+. A high-efficiency quantum state detection technique is adopted to monitor the quantum jumps, and a high precision and synchronized measurement sequence is used for laser control to study the rule of spontaneous decay. Our method shows that the lifetime ratio is a constant and is irrelevant to the dwell time; it is only determined by the spontaneous decay probabilities of the two metastable states at one random decay time, independent of the lifetimes of the two metastable states. Systematic errors such as collisions with background gases, heating effects, impurity components, the shelving and pumping rates and the photon counts are analyzed, and the lifetime ratio between the and states is directly measured to be 1.0257(43) with an uncertainty of 0.42%. Our result is in good agreement with the most precise many-body atomic structure calculations. Our method can be used to obtain the lifetime of a state which is usually difficult to measure and can be used to determine the magnetic dipole transition matrix elements in heavy ions such as Ba+ and Ra+, and can also be universally applied to lifetime ratio measurements of other single atoms and ions with a similar structure.

Influence of impurities on the high temperature conductivity of SrTiO3

In studies of high temperature electrical conductivity (HiTEC) of dielectrics, the impurity in the highest concentration is assumed to form a single defect that controls HiTEC. However, carrier concentrations are typically at or below the level of background impurities, and all impurities may complex with native defects. Canonical defect models ignore complex formation and lump defects from multiple impurities into a single effective defect to reduce the number of associated reactions. To evaluate the importance of background impurities and defect complexes on HiTEC, a grand canonical defect model was developed with input from density functional theory calculations using hybrid exchange correlation functionals. The influence of common background impurities and first nearest neighbor complexes with oxygen vacancies (vO) was studied for three doping cases: nominally undoped, donor doped, and acceptor doped SrTiO3. In each case, conductivity depended on the ensemble of impurity defects simulated with the extent of the dependence governed by the character of the dominant impurity and its tendency to complex with vO. Agreement between simulated and measured conductivity profiles as a function of temperature and oxygen partial pressure improved significantly when background impurities were included in the nominally undoped case. Effects of the impurities simulated were reduced in the Nb and Al doped cases as both elements did not form complexes and were present in concentrations well exceeding all other active impurities. The influence of individual impurities on HiTEC in SrTiO3 was isolated and discussed and motivates further experiments on singly doped SrTiO3.

Synthesis and characterization of dense Gd2Ti2O7 pyrochlore thin films deposited using RF magnetron sputtering

Thin films of phase-pure pyrochlore Gd2Ti2O7 have been synthesized by RF magnetron sputtering. The films were prepared from oxide targets in 50%O2/Ar atmosphere and deposited on 111 yttria-stabilized zirconia (YSZ) substrates at a temperature of 800°C. The pyrochlore structure was confirmed via grazing angle X-ray diffraction and selected area electron diffraction (SAED). Transmission electron microscopy (TEM) analysis also showed that the films were dense and of uniform thickness with surface roughness of approximately 8nm. The total conductivity measured with AC impedance spectroscopy was found to be independent of thickness and comparable in magnitude to that of bulk Gd2Ti2O7. Differences were observed in the Arrhenius behavior between the bulk and thin film samples and are attributed to varying states of order in the crystalline lattice compounded by different levels of background impurities.

Relaxation Rate and Mobility of a Two-dimensional Electron Gas in MgZnO/ZnO Heterostructures Including Exchange and Correlation Effects

We investigate the relaxation rate and mobility of a two-dimensional electron gas (2DEG) confined in MgZnO/ZnO heterostructures (HSs) for temperatures , taking into account exchange and correlation effects. We use the variational-subband-wave-function model for carrier confinement and assume that the electrons are confined to the lowest subband and scattered by acoustic phonons via deformation potential (DP) and piezoelectric (PE) fields, polar LO phonons, interface roughness (IRS), interface charges (IFCs) and the background impurities (BIs). The calculations are based on the linearized Boltzmann equation (BE) and the relaxation time approximation, assuming the scattering by acoustic phonons to be quasi-elastic. We consider three physically distinct temperature ranges with respect to phonon scattering: the Bloch-Grüneisen (BG), equipartition (EP), and inelastic regimes. In the inelastic regime at high temperatures, where the scattering from polar LO phonons becomes important, we solve directly the linearized BE by an iterative method and compare the obtained results with those of the low-temperature and high-energy relaxation-time approximation. Our calculated low-temperature mobility is in good agreement with the recent experiment.

Effects of surface tunneling of two-dimensional hole gases in undoped Ge/GeSi heterostructures

We investigate the effect of surface tunneling on charge distributions of two-dimensional hole gases (2DHGs) in undoped Ge/GeSi heterostructures. As in the electron channel case, the 2DHG density saturates at a high gate voltage. As the channel depth of 2DHGs increases, a crossover of charge distributions in the system from equilibrium to nonequilibrium is observed at a depth of $\ensuremath{\sim}50$ nm. A surface tunneling model is proposed to explain the density crossover. Magnetotransport analysis is performed to investigate the limiting scattering mechanisms. The power law dependence of mobility on density suggests that the dominant scattering mechanisms for the shallow- and deep-channel 2DHGs are remote impurity and background impurity scattering, respectively. Clear quantum Hall plateaus and vanishing longitudinal magnetoresistance are observed in the 2DHG device of channels as shallow as 9 nm.

Universality of electron mobility in LaAlO3/SrTiO3 and bulk SrTiO3

Metallic LaAlO3/SrTiO3 (LAO/STO) interfaces attract enormous attention, but the relationship between the electron mobility and the sheet electron density, ns, is poorly understood. Here, we derive a simple expression for the three-dimensional electron density near the interface, n3D, as a function of ns and find that the mobility for LAO/STO-based interfaces depends on n3D in the same way as it does for bulk doped STO. It is known that undoped bulk STO is strongly compensated with N≃5×1018 cm−3 background donors and acceptors. In intentionally doped bulk STO with a concentration of electrons n3D&amp;lt;N, background impurities determine the electron scattering. Thus, when n3D&amp;lt;N, it is natural to see in LAO/STO the same mobility as in the bulk. On the other hand, in the bulk samples with n3D&amp;gt;N, the mobility collapses because scattering happens on n3D intentionally introduced donors. For LAO/STO, the polar catastrophe which provides electrons is not supposed to provide an equal number of random donors and thus the mobility should be larger. The fact that the mobility is still the same implies that for the LAO/STO, the polar catastrophe model should be revisited.

Electron transport lifetimes in InSb/Al1-xInxSb quantum well 2DEGs

We report magnetotransport measurements of InSb/Al1-xInxSb modulation doped quantum well (QW) structures and the extracted transport and quantum lifetime of carriers at low temperature We consider conventional transport lifetimes over a range of samples with different doping levels and carrier densities, and deduce different transport regimes dependent on QW state filling calculated from self-consistent Schrödinger–Poisson modelling. For samples where only the lowest QW subband is occupied at electron densities of cm−2 and cm−2 quantum lifetimes of ps, and ps are extracted from Shubnikov–de Haas oscillations below a magnetic field of T. The extracted ratios of transport to quantum lifetimes, and are similar to values reported in other binary QW two-dimensional electron gas systems, but are inconsistent with predictions from transport modelling which assumes that remote ionized donors are the dominant scattering mechanism. We find the low ratio and the variation in transport mobility with carrier density cannot be explained by reasonable levels of background impurities or well width fluctuations. Thus, there is at least one additional scattering mechanism unaccounted for, most likely arising from structural defects.

Investigation of the Effect of the Electron-Beam Crucible Zone Melting of Metallurgical Silicon on Refining and Structure Formation of Ingots

The paper presents the results of studies obtained with the development of the technology of electron-beam crucible impregnation of metallurgical silicon. It is shown that refining of silicon from background and dopant impurities in electron-beam crucible-free zone melting occurs by zone purification during melting and as a result of evaporation of impurities from the sample surface. The mathematical model and computational experiment were been performed to determine the temperature gradient at different rates of zone melting. It was found the diapason of temperature gradients, which provides the columnar structure of crystallites and the purification of the samplesdue the melting with the zone recrystallization procedure. The level of the resistivity of the ingots increases at the end.

Defect mechanisms of coloration in Fe-doped SrTiO3 from first principles

To understand the underlying defect mechanisms governing the coloration of Fe-doped SrTiO3 (Fe:STO), density functional theory calculations were used to determine defect formation energies and to interpret optical absorption spectra. A grand canonical defect equilibrium model was developed using the calculated formation energies, which enabled connection to annealing experiments. It was found that FeTi0 is stable in oxidizing conditions and leads to the optical absorption signatures in oxidized Fe:STO, consistent with experiment. Fe:STO was found to transition from brown to transparent as PO2 was reduced during annealing. The defect equilibrium model reproduces a consistent PO2 of this coloration transition. Most critical to reproducing the PO2 of the coloration transition was inclusion of a FeTi-VO first nearest neighbor complex, which was found to be strongly interacting. The coloration transition PO2 was found to be insensitive to the presence of minority background impurities, slightly sensitive to Fe content, and more sensitive to annealing temperature.

Scattering mechanisms of highest-mobilityInAs/AlxGa1−xSbquantum wells

We study molecular beam epitaxially grown, undoped Al$_{x}$Ga$_{1-x}$Sb/InAs/AlSb quantum wells with different buffer and barrier designs and varying quantum well width. The highest mobilities were achieved with Al$_{0.33}$Ga$_{0.67}$Sb buffers and lower barriers and a quantum well width of 24 nm. These quasi-single-interface InAs/AlSb quantum well devices reached a gate-tuned mobility of 2.4$~\times~10^{6}~$cm$^2$/Vs at a density of 1$\times 10^{12}$ cm$^{-2}$ and 1.3 K. In Hall bar devices boundary scattering is found to strongly influence the mobility determination in this mobility regime. Ionized background impurity scattering at low electron densities, device boundary scattering at intermediate electron densities, and intersubband scattering at high electron densities were identified as most likely dominant scattering processes. Ringlike structures in the Landau fan can be explained using a single-particle model of crossing Landau levels.

Upper limit of a tunneling reaction rate for D−+H2 →HD+H−

The exothermic proton transfer reaction ${\mathrm{D}}^{\ensuremath{-}}+{\mathrm{H}}_{2}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\text{HD}+{\mathrm{H}}^{\ensuremath{-}}$ is known to proceed over a barrier of about 0.33 eV. Here we investigate whether this reaction may occur at low temperatures via tunneling through this barrier. The experiments were carried out in a cryogenic 22-pole ion trap, which provides a high sensitivity for slow ion-molecule reactions. Our experiments show no sign of the tunneling reaction with an upper limit to the rate coefficient of $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{3}/\mathrm{s}$ obtained from the decrease of the ${\mathrm{D}}^{\ensuremath{-}}$ signal and $9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{3}/\mathrm{s}$ from the absence of an increase of ${\mathrm{H}}^{\ensuremath{-}}$. Background impurities were identified to be the main limitation of the sensitivity.

Detection of glycoprotein through fluorescent boronic acid-based molecularly imprinted polymer

A novel strategy based on the fluorescent molecularly imprinted polymers (FMIP) for specific recognition and sensitive sensing glycoprotein from biological samples was developed. The FMIP prepared by introducing a fluorescent boronic acid quinoline-based on the PGMA/EDMA bead surface and assembled a glycoprotein molecularly imprinted polymer using surface imprinting technology. The resultant material showed specific recognition to model glycoprotein. At the same time, the change of fluorescence caused by the amount of model glycoprotein could achieve quantitative determination. The method provided a good linear relationship of the concentrations of HRP in the range of 0.05–1 μM, and the detection limit was 0.02 μM. It is 10 times lower than the previous fluorescence nanosensor for glycoprotein. The established method combined the desired selectivity of molecularly imprinted polymers and high sensitivity of fluorescence spectroscopy. The influence of background impurities could be effectively eliminated. The outstanding features guarantee that it can be successfully applied to detect glycoprotein from biological samples under physiological conditions.

Doping marker layers for ex situ growth characterisation of HVPE gallium nitride

With this work we investigate several different ways to create marker layers for the ex situ characterisation of hydride vapour phase epitaxial (HVPE) gallium nitride (GaN) growth. Significant variations of the charge carrier concentration of GaN crystals become visible as marker lines in cross section investigations using optical microscopy and secondary electron microscopy (SEM). Pulsed intentional silicon and germanium doping leads to a good brightness contrast in cross section SEM micrographs and optical microscopy images. They appear as a black contrast in SEM. Pulsed manganese doping results in bright contrast marker layers in SEM, which is a result of the variation of the charge carrier concentration in the opposite direction, with respect to the background impurity level of the material. Marker layers are employed to determine the temperature transition point between 3D- and step flow GaN growth mode for the used set of growth parameters. A trend for the development of the growth rate of the c-facet as a function of the temperature is observed.

Electronic structure and spatial distribution of the spin density of shallow nitrogen donors in the SiC lattice

The discovery of unique magnetooptical properties of paramagnetic centers in silicon carbide, which make it possible to control spins of small arrays of centers of atomic sizes to single centers at room temperatures, using the techniques of optical detection of the magnetic resonance, posed a number of problems, among which one of the main ones is the creation of conditions under which spin relaxation effects are minimized. As studies of properties of spin nitrogen-vacancy centers in diamond showed, the main contribution to spin relaxation is made by the interaction with nitrogen donors, being a major impurity in diamond. A similar problem exists for silicon carbide, since nitrogen donors are also basic background impurities. The objective of this work is to study the spatial distribution of the spin density of nitrogen donors in two basic silicon carbide polytypes, i.e., 4H-SiC and 6H-SiC, to use this information for minimizing the interaction of nitrogen donors with paramagnetic centers in silicon carbide. The results of the study are analyzed by magnetic resonance methods; the spin density distribution on the nearest coordination spheres of nitrogen donors occupying carbon sites in silicon carbide is determined. It is concluded that paramagnetic centers in the 4H-SiC polytype, including silicon vacancies, can be more stable to the interactions with unpaired donor electrons, since electrons are not localized on the coordination sphere closest to the paramagnetic center in this case.

Precision measurement of the3dD3/22–state lifetime in a single trappedCa+40

We present a high-precision measurement of the $3d\phantom{\rule{4pt}{0ex}}^{2}D_{3/2}$--state lifetime in a single trapped $^{40}\mathrm{Ca}^{+}$. The measurement was performed using a high-efficiency quantum-state detection technique to monitor quantum jumps and a high-precision and highly synchronous measurement sequence for laser control. A feature in our measurement is the pumping rate of the 729-nm laser that was corrected in a real-time way. The $3d\phantom{\rule{4pt}{0ex}}^{2}D_{3/2}$--state lifetime was obtained through the measurement of the spontaneous decay rate after incoherent shelving of the ion to the $3d\phantom{\rule{4pt}{0ex}}^{2}D_{3/2}$ state with a wait time. Systematic errors, such as collisions with background gases, heating effects, impurity components, the shelving and pumping rates, and state detection, were carefully analyzed and estimated. We determined an improved value of the $3d\phantom{\rule{4pt}{0ex}}^{2}D_{3/2}$--state lifetime to be ${\ensuremath{\tau}}_{3/2}=1.195(8)$ s. Furthermore, the $3d\phantom{\rule{4pt}{0ex}}^{2}D_{3/2}\ensuremath{\rightarrow}4s\phantom{\rule{4pt}{0ex}}^{2}S_{1/2}$ quadrupole transition matrix element was measured to be ${S}_{ki}=7.936(26)e{a}_{0}^{2}$, and the ratio between the lifetimes of $3d{\phantom{\rule{4pt}{0ex}}}^{2}{D}_{3/2}$ and $3d{\phantom{\rule{4pt}{0ex}}}^{2}{D}_{5/2}$ was determined to be 1.018(11). Our method can be universally applied to lifetime measurements of other single ions and atoms with a similar structure.

Real-time monitoring and control of the load phase of a protein A capture step.

ABSTRACTThe load phase in preparative Protein A capture steps is commonly not controlled in real‐time. The load volume is generally based on an offline quantification of the monoclonal antibody (mAb) prior to loading and on a conservative column capacity determined by resin‐life time studies. While this results in a reduced productivity in batch mode, the bottleneck of suitable real‐time analytics has to be overcome in order to enable continuous mAb purification. In this study, Partial Least Squares Regression (PLS) modeling on UV/Vis absorption spectra was applied to quantify mAb in the effluent of a Protein A capture step during the load phase. A PLS model based on several breakthrough curves with variable mAb titers in the HCCF was successfully calibrated. The PLS model predicted the mAb concentrations in the effluent of a validation experiment with a root mean square error (RMSE) of 0.06 mg/mL. The information was applied to automatically terminate the load phase, when a product breakthrough of 1.5 mg/mL was reached. In a second part of the study, the sensitivity of the method was further increased by only considering small mAb concentrations in the calibration and by subtracting an impurity background signal. The resulting PLS model exhibited a RMSE of prediction of 0.01 mg/mL and was successfully applied to terminate the load phase, when a product breakthrough of 0.15 mg/mL was achieved. The proposed method has hence potential for the real‐time monitoring and control of capture steps at large scale production. This might enhance the resin capacity utilization, eliminate time‐consuming offline analytics, and contribute to the realization of continuous processing. Biotechnol. Bioeng. 2017;114: 368–373. © 2016 The Authors. Biotechnology and Bioengineering published by Wiley Periodicals, Inc.

Extended Numerical Modeling of Impurity Neoclassical Transport in Tokamak Edge Plasmas

AbstractUnderstanding of impurity transport in tokamaks is an important issue in order to reduce the impurity contamination in fusion core plasmas. Recently, a new kinetic numerical scheme of impurity classical/neoclassical transport has been developed. This numerical scheme makes it possible to include classical self‐diffusion (CL SD), classical inward pinch (CL IWP), and classical temperature screening effect (CL TSE) of impurity ions. However, impurity neoclassical transport has been modeled only in the case where background plasmas are in the Pfirsch‐Schluter (PS) regime. The purpose of this study is to extend our previous model to wider range of collisionality regimes, i.e., not only the PS regime, but also the plateau regime. As in the previous study, a kinetic model with Binary Collision Monte‐Carlo Model (BMC) has been adopted. We focus on the modeling of the neoclassical self‐diffusion (NC SD) and the neoclassical inward pinch (NC IWP). In order to simulate the neoclassical transport with the BCM, velocity distribution of background plasma ions has been modeled as a deformed Maxwell distribution which includes plasma density gradient. Some test simulations have been done. As for NC SD of impurity ions, our scheme reproduces the dependence on the collisionality parameter in wide range of collisionality regime. As for NC IWP, in cases where test impurity ions and background ions are in the PS and plateau regimes, parameter dependences have been reproduced. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magneto-transport analysis of an ultra-low-density two-dimensional hole gas in an undoped strained Ge/SiGe heterostructure

We report the magneto-transport, scattering mechanisms, and effective mass analysis of an ultra-low density two-dimensional hole gas capacitively induced in an undoped strained Ge/Si0.2Ge0.8 heterostructure. This fabrication technique allows hole densities as low as p ∼ 1.1 × 1010 cm−2 to be achieved, more than one order of magnitude lower than previously reported in doped Ge/SiGe heterostructures. The power-law exponent of the electron mobility versus density curve, μ ∝ nα, is found to be α ∼ 0.29 over most of the density range, implying that background impurity scattering is the dominant scattering mechanism at intermediate densities in such devices. A charge migration model is used to explain the mobility decrease at the highest achievable densities. The hole effective mass is deduced from the temperature dependence of Shubnikov-de Haas oscillations. At p ∼ 1.0 × 1011 cm−2, the effective mass m* is ∼0.105 m0, which is significantly larger than masses obtained from modulation-doped Ge/SiGe two-dimensional hole gases.