Background Impurities Research Articles

Reducing disorder in Ge quantum wells by using thick SiGe barriers

We investigate the disorder properties of two-dimensional hole gases in Ge/SiGe heterostructures grown on Ge wafers, using thick SiGe barriers to mitigate the influence of the semiconductor–dielectric interface. Across several heterostructure field effect transistors, we measure an average maximum mobility of (4.4±0.2)×106 cm2/Vs at a saturation density of (1.72±0.03)×1011 cm−2, corresponding to a long mean free path of (30±1)μm. The highest measured mobility is 4.68×106 cm2/Vs. We identify uniform background impurities and interface roughness as the dominant scattering mechanisms limiting mobility in a representative device, and we evaluate a percolation-induced critical density of (4.5±0.1)×109 cm−2. This low-disorder heterostructure, according to simulations, may support the electrostatic confinement of holes in gate-defined quantum dots.

Vacuum Polarization in the Point Impurity Background

The vacuum polarization effect of a massive scalar field φ(x) near the point δ-like source is considered. The corresponding interaction is introduced within a technique of self-adjoint extension of the Laplace operator. This method has been widely discussed within the framework of quantum mechanics. We propose to use it to investigate vacuum field effects. This approach allows computing the renormalized Hadamard function and the renormalized vacuum energy density hT00(x)iren for massive real scalar field with minimal curvature coupling. The dependence of the vacuum polarization effect upon the fields’s mass is analyzed.

3D-Mapping and Manipulation of Photocurrent in an Optoelectronic Diamond Device.

Establishing connections between material impurities and charge transport properties in emerging electronic and quantum materials, such as wide-bandgap semiconductors, demands new diagnostic methods tailored to these unique systems. Many such materials host optically-active defect centers which offer a powerful in situ characterization system, but one that typically relies on the weak spin-electric field coupling to measure electronic phenomena. In this work, charge-state sensitive optical microscopy is combined with photoelectric detection of an array of nitrogen-vacancy (NV) centers to directly image the flow of charge carriers inside a diamond optoelectronic device, in 3D and with temporal resolution. Optical control is used to change the charge state of background impurities inside the diamond on-demand, resulting in drastically different current flow such as filamentary channels nucleating from specific, defective regions of the device. Conducting channels that control carrier flow, key steps toward optically reconfigurable, wide-bandgap optoelectronics are then engineered using light. This work might be extended to probe other wide-bandgap semiconductors (SiC, GaN) relevant to present and emerging electronic and quantumtechnologies.

Vacuum Polarization in the Point Impurity Background

Vacuum Polarization in the Point Impurity Background

Effect of Gamma Irradiation on Electrophysical Parameters of Silicon Solar Cells Doped with Nickel

The results of the studies of changes in the electro-physical parameters (Voc – no-load voltage, Isc – short-circuit current density; τ – lifetime of nonequilibrium charge carriers) of photocells made on plates of monocrystalline silicon of the p-type conductivity with the specific resistance ρ 0.5 Ohm cm, doped with nickel, under irradiation with γ-quanta from 60Co source are presented. It is shown that the efficiency of the solar energy conversion in nickel-doped photovoltaic cells remains higher than in standard cells up to irradiation doses of 108 rad. It was established that with increasing diffusion temperature of nickel atoms radiation stability of electrophysical parameters of photovoltaic cells also increases. A decrease in the concentration of recombination-active radiation defects is due to the getterization by nickel atoms of technological (background) impurities and the action of nickel clusters as effluents for radiation-induced vacancies.

Epitaxy and characterization of undoped Si/SiGe heterojunctions

Silicon-based semiconductor quantum computing with spin as the encoding unit is compatible with traditional microelectronic processes, easy to expand, and can improve isotope purification and decoherence time, thus attracting much attention. There are fewer reports on the work related to undoped Si/SiGe heterostructures grown by molecular beam epitaxy than those on chemical vapor deposition. An undoped Si/SiGe heterostructure is grown by molecular beam epitaxy (see the attached figure below). The results from scanning transmission electron microscopy and energy-dispersive spectroscopy mapping show an atomic-scale interface with a characteristic length of 0.53 nm. The surface root-mean-square roughness measured by atomic force microscope is 0.44 nm. The X-ray diffraction data show that the Si quantum well is fully strained and the in-plane strain is 1.03%. In addition, the performance of the two-dimensional electron gas is evaluated by low-temperature Hall measurements, which are conducted in the Hall-bar shaped field-effect transistor. The peak mobility is 20.21×10<sup>4</sup> cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> when the carrier density is about 6.265×10<sup>11</sup> cm<sup>–2</sup> at 250 mK. The percolation density is 1.465×10<sup>11</sup> cm<sup>–2</sup>. The effective mass of the two-dimensional electron gas is approximately 0.19<i>m</i><sub>0</sub>. The power exponential between carrier density and mobility at different gate voltages is 1.026, and the Dingle ratio of the two-dimensional electron gas is in a range of 7–12, indicating that the electrons are scattered by background impurities and semiconductor/oxide interfaces charges. The atomically sharp interface of Si/SiGe heterostructures created by molecular beam epitaxy is beneficial for studying the valley physics properties in silicon. The structural and transport characterizations in this paper lay the foundation for the optimization of Si-based semiconductor quantum dot quantum computing materials.

Melt-grown semi-insulating Mn:β-Ga2O3 single crystals exhibiting unique visible absorptions and luminescence

Several acceptor dopants have been explored in β-Ga2O3 to produce semi-insulating substrates and epitaxial films. Fe and Mg make up the majority of research thus far; however, other transition metals provide potential alternatives for optimized performance. β-Ga2O3 bulk single crystals were grown by the Czochralski and vertical gradient freeze methods with a nominal dopant concentration of 0.25 at. % Mn. Ultraviolet-visible-near infrared spectroscopy and photoluminescence revealed polarization- and orientation-dependent optical absorptions (pleochroism) coupled with an orange luminescence. All samples were electrically insulating, on the order of 109–1011 ohm cm at room temperature, indicative of acceptor doping. Actual dopant concentrations of the intentionally doped transition metal and background impurities were determined via glow discharge mass spectrometry, indicating the macroscale segregation behavior. High-temperature resistivity measurements indicated an experimental acceptor level of 1.7 ± 0.2 eV. Hydrogenation of samples resulted in an increase in the orange luminescence and O–H stretching modes observable in the infrared spectrum. Density functional theory calculations were performed to determine the likely site-occupancy and acceptor level of Mn in the bandgap.

Phytoplankton Image Segmentation and Annotation Method Based on Microscopic Fluorescence.

Microscopic phytoplankton segmentation is an important part of water quality assessment. The segmentation of microscopic phytoplankton still faces challenges for computer vision, such as being affected by background impurities and requiring a large number of manual annotation. In this paper, the characteristics of phytoplankton emitting fluorescence under excitation light were utilized to segment and annotate phytoplankton contours by fusing fluorescence images and bright field images. Morphological operations were used to process microscopic fluorescence images to obtain the initial contours of phytoplankton. Then, microscopic bright field images were processed by Active Contour to fine tune the contours. Seven algae species were selected as the experimental objects. Compared with manually labeling the contour in LabelMe, the recall, precision, FI score and IOU of the proposed segmentation method are 85.3%, 84.5%, 84.7%, and 74.6%, respectively. Mask-RCNN was used to verify the correctness of labels annotated by the proposed method. The average recall, precision, F1 score and IOU are 97.0%, 86.5%, 91.1%, and 84.2%, respectively, when the Mask-RCNN is trained with the proposed automatic labeling method. And the results corresponding to manual labeling are 95.3%, 86.1%, 90.3%, and 82.8% respectively. The experimental results show that the proposed method can segment the phytoplankton microscopic image accurately, and the automatically annotated contour data has the same effect as the manually annotated contour data in Mask-RCNN, which greatly reduces the manual annotation workload.

Effect of spontaneous polarization field on diffusion thermopower in AlGaN/GaN heterostructures

Polarization field on diffusion thermopower Sd, of a two-dimensional electron gas (2DEG) formed at a wurtzite AlGaN/GaN heterojunction is investigated. The investigations of macroscopic polarization effect on diffusion thermopower over the wide range of temperature and barrier fluctuations bring out the role of spontaneous and piezoelectric polarization fields present in the heterostructure. The induced 2DEG charge density formed at the interface of AlGaN/GaN heterojunction due to in-built spontaneous and piezoelectric fields are assumed to be scattered by acoustic phonons via deformation and piezoelectric couplings, background and remote impurities via Coulomb interaction, interface roughness and interface charges. We find, the dominant contribution is from interface roughness scattering mechanism and that from acoustic phonons via deformation potential is negligible. Theoretical investigations of Sd show that, the effect of the spontaneous polarization is to reduce the diffusion thermopower significantly at practically relevant temperatures.

Radiation-balanced lasing in Yb3+:YAG and Yb3+:KYW.

Radiation-balanced lasing and thermal profiling is reported in two Yb-doped laser crystals at room temperature. In 3% Yb3+:YAG a record efficiency of 30.5% was achieved by frequency-locking the laser cavity to the input light. Both the average excursion and axial temperature gradient of the gain medium were maintained within 0.1 K of room temperature at the radiation balance point. By including saturation of background impurity absorption in the analysis, quantitative agreement was obtained between theory and the experimentally measured laser threshold, radiation balance condition, output wavelength, and laser efficiency with only one free parameter. Radiation-balanced lasing was also achieved in 2% Yb3+:KYW with an efficiency of 2.2% despite high background impurity absorption, losses from Brewster end faces that were not parallel, and non-optimal output coupling. Our results confirm that relatively impure gain media can be operated as radiation-balanced lasers, contrary to earlier predictions which ignored background impurity properties.

Holes Outperform Electrons in Group IV Semiconductor Materials

A record‐high mobility of holes, reaching 4.3 × 106 cm2 V−1 s−1 at 300 mK in an epitaxial strained germanium (s‐Ge) semiconductor, grown on a standard silicon wafer, is reported. This major breakthrough is achieved due to the development of state‐of‐the‐art epitaxial growth technology culminating in superior monocrystalline quality of the s‐Ge material platform with a very low density of background impurities and other imperfections. As a consequence, the hole mobility in s‐Ge appears to be ≈2 times higher than the highest electron mobility in strained silicon. In addition to the record mobility, this material platform reveals a unique combination of properties, which are a very large and tuneable effective g*‐factor (>18), a very low percolation density (5 × 109 cm−2) and a small effective mass (0.054 m 0). This long‐sought combination of parameters in one material system is important for the research and development of low‐temperature electronics with reduced Joule heating and for quantum‐electronics circuits based on spin qubits.

Trends in runaway electron plateau partial recombination by massive H2 or D2 injection in DIII-D and JET and first extrapolations to ITER and SPARC

Experimental trends in thermal plasma partial recombination resulting from massive injection into high-Z (Ar) containing runaway electron (RE) plateaus in DIII-D and JET are studied for the purpose of achieving sufficiently low electron density () to increase RE final loss MHD levels. In both DIII-D and JET, thermal electron density is found to drop by ∼100 when the thermal plasma partially recombines, with a minimum at a vacuum vessel-averaged density in the range . RE effective resistivity also drops after partial recombination, indicating expulsion of the Ar content. The level after partial recombination is found to increase as RE current is increased. The amount of initial Ar in the RE plateau is not observed to have a strong effect on partial recombination. Partial recombination timescales of order 5 ms in DIII-D and 15 ms in JET are observed. These basic trends and timescales are matched with a 1D diffusion model, which is then used to extrapolate to ITER and SPARC tokamaks. Within the approximations of this model, it is predicted that ITER will be able to achieve sufficiently low values on time scales faster than expected RE plateau vertical drift timescales (of order 100 ms), provided sufficient or is injected. In SPARC, it is predicted that achieving significant recombination will be challenging, due to the very high RE current density. In both ITER and SPARC, it is predicted that achieving low will be easier with Ar as a background impurity (rather than Ne).

Gettering of epitaxial indium arsenide by the rare earth element holmium

The results of a study of the galvanomagnetic properties of indium arsenide grown by liquid-phase epitaxy are presented. It is shown that the use of the rare earth element holmium in the growth of InAs epitaxial layers makes it possible to reduce the electron concentration by two orders of magnitude to n=2.1·1015 cm-3 at T=77 K. This effect is due to the gettering of shallow background impurities with the formation of their compounds in the melt. With an increase in the holmium content of more than 0.12 mol.% the concentration of current carriers in the material begins to increase, while mobility decreases due to the influence of VAs-Ho donor centers. This method of gettering is promising for obtaining A3B5 materials with a low concentration of current carriers, which are in demand in the optoelectronic industry. Keywords: indium arsenide, rare earth element, Hall coefficient, concentration of current carriers, mobility of current carriers.

Геттерирование эпитаксиального арсенида индия редкоземельным элементом гольмием

The results of a study of the galvanomagnetic properties of indium arsenide grown by liquid-phase epitaxy are presented. It is shown that the use of the rare earth element holmium in the growth of InAs epitaxial layers makes it possible to reduce the electron concentration by two orders of magnitude to n=2.1•1015 cm−3 at T=77 K. This effect is due to the gettering of shallow background impurities with the formation of their compounds in the melt. With an increase in the holmium content of more than 0.12 mol.% the concentration of current carriers in the material begins to increase, while mobility decreases due to the influence of VAs – Ho donor centers. This method of gettering is promising for obtaining AIIIBV materials with a low concentration of current carriers, which are in demand in the optoelectronic industry.

Применение метода Бриджмена для получения термоэлектрического кремния, легированного германием и фосфором

Heavily donor or acceptor doped metallurgical silicon is a promising candidate as a high-temperature thermoelectric energy converter due to the extremely low cost of its fabrication. The problem of silicon-based thermoelectric materials is the high value of the thermal conductivity; however, modern technologies offer several options for solving this problem at once. In the present work, silicon ingots heavily doped using Si:P compound were grown by the Bridgman directional crystallization method with a small (up to 5 at. %) germanium impurity fraction. The main thermoelectric parameters of the material were measured in a wide temperature range (50 – 800 °C). These are Seebeck coefficient, electrical conductivity and thermal conductivity. Based on the measurement results, the value of the thermoelectric figure of merit was calculated. The latter determines the value of the thermoelectric conversion efficiency. The study of electrical properties shows that phosphorus from the SiP compound is introduced into the lattice as a dopant and creates a high concentration of conduction electrons. The chemical analysis of the ingots showed the presence of additional background impurities, the concentration and composition of these impurities vary over the bulk of the sample. Despite the presence of impurities, the material demonstrates relatively high thermoelectric characteristics, and the efficiency is at the level of the best world results. A further potential for optimizing thermoelectric characteristics due to the possibility of a fine-grained polycrystalline structure formation is discussed.

Enhanced Bayesian sparse representation of mechanical fault signals by structural feature-oriented matching composite dictionary construction

The traditional orthogonal matching pursuit algorithm exploits only the overall sparsity of signals without considering the effects caused by structural characteristics. To this end, this paper proposes an enhanced Bayesian sparse representation (EBSR) of mechanical fault signals by structural feature-oriented matching redundant dictionary construction. First, an EBSR model which can improve sparse reconstruction results is proposed. The proposed model improves the recovery accuracy and robustness of the sparse representation by using the structural information as a priori information. Subsequently, a composite dictionary is designed combining a Sin-Chirplet dictionary with an Impulse dictionary, and a multi-group and multi-strategy grey wolf optimizer algorithm is employed to enable the composite dictionary match the structural features of the fault signal and reduce its redundancy degree. Finally, the optimized matching composite dictionary is introduced into the EBSR algorithm, endowing it with an efficient atom selection strategy and reducing the complexity of the sparse representation. The simulation and experimental results demonstrated that the proposed method can effectively reduce the interference from background noise and impurity frequencies, verifying the effectiveness and applicability of the proposed method for the sparse representation of mechanical faults.

A Review of Recent Progress in β‐Ga2O3 Epitaxial Growth: Effect of Substrate Orientation and Precursors in Metal–Organic Chemical Vapor Deposition

Gallium oxide (Ga2O3) is a highly promising ultrawide‐bandgap semiconductor for power electronics that emerged about a decade ago. Epitaxial growth Ga2O3 at the small scale is demonstrated. In order to develop scalable manufacturing of high‐performance epitaxial structures, in‐depth understanding of the fundamental growth processes, control parameters, and mechanism is imperative. This review discusses the recent progress in epitaxial growth of β‐Ga2O3 films and highlights challenges in obtaining high growth rate, low defects, and high carrier mobilities. Compared with the other epitaxy methods, metal–organic chemical vapor deposition (MOCVD) offers a wider growth window and precursor selection option, to minimize the tradeoff between crystal quality and growth rate. Growth rate is inversely proportional to temperature, within a certain temperature window, because of the unavoidable premature gas‐phase reactions and desorption of the highly volatile gallium suboxide (Ga2O) at elevated temperatures. Growth defects, background impurity incorporation, and carrier mobilities can be affected by the choice of MOCVD precursors, nucleation, and adsorption/desorption/diffusion of adatoms on substrate surfaces of different orientations, including the effect of growing on cleavage and noncleavage planes. This review summarizes the current status of the epitaxial growth of β‐Ga2O3 and analyzes the major factors that enhance mobility and reduce background doping concentration. The insights gained help advance the manufacturability of device‐grade epitaxial thin films.

Carbon and silicon background impurity control in undoped GaN layers grown with trimethylgallium and triethylgallium via metalorganic chemical vapor deposition

The unintentional impurity incorporation in GaN epitaxial layers impacts the electrical conductivity and optical properties of the films grown by metalorganic chemical vapor deposition (MOCVD). It is critical to control impurity-related states for device structure development. The aim of this work is to contribute to the understanding of the reasons for the presence of certain background impurities. In this paper, the unintentional impurity incorporation of carbon, hydrogen, oxygen, and silicon in undoped (u-) GaN films grown by low-pressure MOCVD were studied. Background impurity concentrations were evaluated by secondary ion mass spectroscopy (SIMS) to optimize growth parameters including V/III ratio, growth temperature (Tg), growth pressure (Pg), and gallium (Ga) precursors of trimethylgallium (TMG) or triethylgallium (TEG). Unintentional [C] and [Si] incorporations are found to be highly dependent on the growth parameters. For the same growth conditions, u-GaN films grown with a TEG precursor exhibited a lower background concentration of C compared to that of GaN grown with TMG. However, the lowest background [C] achieved was grown with TMG using optimized conditions and exhibited [C] < 4 × 1015 cm−3 which was at the detection limit of the SIMS measurement. These results were measured for a u-GaN layer grown with TMG at 200 Torr, 1030 °C, and a V/III of 3700. The lowest background [Si] of 4 × 1015 cm−3 was achieved by growing with TMG at 200 Torr, 1000 °C, and V/III of 650.

Impact of in situ controlled disorder screening on fractional quantum Hall effects and composite fermion transport

We examine the impact of random potential due to remote impurites (RIs) and its in situ controlled screening on fractional quantum Hall effects (FQHEs) around Landau-level filling factor $\ensuremath{\nu}=1/2$. The experiment is made possible by using a dual-gate GaAs quantum well (QW) that allows for the independent control of the density ${n}_{e}$ of the two-dimensional electron system in the QW and that (${n}_{\mathrm{SL}}$) of excess electrons in the modulation-doping superlattice. As the screening is reduced by decreasing ${n}_{\mathrm{SL}}$ at a fixed ${n}_{e}$, we observe a decrease in the apparent energy gap of the FQHEs deduced from thermal activation, which signifies a corresponding increase in the disorder broadening $\mathrm{\ensuremath{\Gamma}}$ of composite fermions (CFs). Interestingly, the increase in $\mathrm{\ensuremath{\Gamma}}$ is accompanied by a noticeable increase in the longitudinal resistivity at $\ensuremath{\nu}=1/2$ (${\ensuremath{\rho}}_{1/2}$), with a much stronger correlation with $\mathrm{\ensuremath{\Gamma}}$ than electron mobility $\ensuremath{\mu}$ has. The in situ control of RI screening enables us to disentangle the contributions of RIs and background impurities (BIs) to ${\ensuremath{\rho}}_{1/2}$ with the latter in good agreement with the CF theory. We construct a scaling plot that helps in estimating the BI contribution to ${\ensuremath{\rho}}_{1/2}$ for a given set of ${n}_{e}$ and $\ensuremath{\mu}$.

Indium as a surfactant: Effects on growth morphology and background impurity in GaN films grown by ammonia-assisted molecular beam epitaxy

We report on the improvement of the surface morphology of c-plane GaN films grown at high growth rates (∼1 µm/h) using ammonia molecular beam epitaxy through a series of growth optimizations as well as the introduction of indium as a surfactant. The indium surfactant was expected to help with the adatom mobility and, thus, provide smoother growth surfaces. Through a combination of varying V/III ratios, In flux, and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved. At higher Ga fluxes for fast growth rates (∼1 µm/h and beam equivalent pressures of ∼5 × 10−7 Torr), higher ammonia flows were necessary to preserve the surface morphology. In addition, indium was an effective surfactant—reducing the roughness and improving the overall surface morphology. However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant also resulted in a reduction of background Si impurity concentrations in the film. These effects allow for the growth of thick drift layers with low background dopant concentrations for vertical GaN power devices.