Background Impurities Research Articles

Scattering mechanisms in state-of-the-art GaAs/AlGaAs quantum wells

Motivated by recent breakthrough in molecular beam epitaxy of GaAs/AlGaAs quantum wells [Y. J. Chung \textit{et al.}, Nature Materials \textbf{20}, 632 (2021)], we examine contributions to mobility and quantum mobility from various scattering mechanisms and their dependencies on the electron density. We find that at lower electron densities, $n_e \lesssim 1 \times 10^{11}$ cm$^{-2}$, both transport and quantum mobility are limited by unintentional background impurities and follow a power law dependence, $\propto n_e^{\alpha}$, with $\alpha \approx 0.85$. Our predictions for quantum mobility are in reasonable agreement with an estimate obtained from the resistivity at filling factor $\nu= 1/2$ in a sample of Y. J. Chung \textit{et al.} with $n_e = 1 \times 10^{11}$ cm$^{-2}$. Consideration of other scattering mechanisms indicates that interface roughness (remote donors) is a likely limiting factor of transport (quantum) mobility at higher electron densities. Future measurements of quantum mobility should yield information on the distribution of background impurities in GaAs and AlGaAs.

Mg-doping and free-hole properties of hot-wall MOCVD GaN

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range (2.45×1018 cm−3 up to 1.10×1020 cm−3) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance–voltage (C–V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 Ω cm and 8.4×1017 cm−3, respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

Embedded U-Net: combines multiple feature fusion encode and subpixel reconstruction for microcracks salient object detection

The efficient identification of microcracks is of great significance to the early fault diagnosis of structures. The edge pixels of the microcracks have obvious changes that can be regarded as a salient object in the image for detection. Aiming at the differences in microcracks features at different scales of the encoder network, we proposed an embedded U-Net model that combines a cross-stage feature fusion module (CSF) and a layerwise feature fusion module (LWF). The embedded U-Net model can be used to enhance the spatial and edge information of high-resolution images and refine the extraction of low-resolution and high-dimensional channel information. In the low-scale saliency map produced by the decoder network, the bilinear interpolation method easily blurs and distorts the edges of the microcracks. To solve this problem, we send the low-quality saliency map into the subpixel super-resolution reconstruction module (SPR) to reconstruct high-quality pixel information. The SPR module is used to learn the low-resolution pixel information through the convolution operation, and the high-resolution pixels information can be obtained by the reorganization of learned pixels. A large number of experiments indicate that the 2_CSF+4_LWF+SPR has the best model performance. Its F1 score is 0.709, MAE is 0.0074, and the model parameter is only 55.6 MB. It can be seen from the qualitative analysis that LWF can improve the detection performance of microcrack ends and fuzzy cracks, and the SPR can refine the microcrack edges in the saliency map and reduce some interference from background impurities.

An Improved Cervical Cell Segmentation Method Based on Deep Convolutional Network

The cervical cytology smear test is an effective method for cervical cancer early screening, and segmentation accuracy is essential for computer-aided diagnosis. In this study, an improved cervical nucleus and cytoplasm segmentation method based on a deep convolutional network was proposed. This method consisted of a cellular region proposal and pixel-level segmentation network (CRP-PSN). Data were obtained from the 2014 International Symposium on Biomedical Imaging cervical cell segmentation competition open dataset. In CRP networks, online hard example mining and soft-nonmaximum suppression algorithms were incorporated to solve problems, including background noise, impurities, and other interferences in smear images. For PSN networks, a generative adversarial network-generated adversarial network algorithm and least-squares loss function were used to generate cell region segmentation, thereby improving the cytoplasm segmentation results. Finally, the improved CRP-PSN model was analyzed and compared with other typical cervical cell segmentation methods. The experimental results showed that the proposed model could effectively improve the segmentation accuracy of cytoplasm and nuclei in cervical cytology smear images by 92% and 98.6%, respectively. These findings provided strong support for the application of this method for automated interpretation of cervical cytology smear images and improved diagnostic reliability.

Solid solubility and site preference of Ti in 3C- and 6H-SiC

Ti is a background impurity in SiC due to its presence in the graphite parts of the SiC production furnace, and it has large impacts on the electrical and magnetic performance of SiC. Herein, the solid solubilities of Ti in 3C- and 6H-SiC were measured at the SiC–TiSi2 thermodynamic equilibrium, and the values of the enthalpy and entropy of dissolution for Ti in 3C- and 6H-SiC were estimated through thermodynamic analysis, which were also confirmed using the formation energy calculation of defects. The soft X-ray emission (SXE) lines of Ti-Lℓ for Ti-doped 3C- and 6H-SiC were explained by the calculated hole transition energies (HTEs) between the L1 and M3 levels in different possible defects, experimentally indicating that Ti prefers the substitution of the Si-site in both 3C- and 6H-SiC. Additionally, the zero-phonon line (ZPL) calculation provided a possible explanation for the origin of the ZPLs in the photoluminescence (PL) spectra of the Ti in 6H-SiC.

Effects of biased and unbiased illuminations on two-dimensional electron gases in dopant-free GaAs/AlGaAs

Illumination is performed at low temperature on dopant-free two-dimensional electron gases (2DEGs) of varying depths, under unbiased (gates grounded) and biased (gates at a positive or negative voltage) conditions. Unbiased illuminations in 2DEGs located more than 70 nm away from the surface result in a gain in mobility at a given electron density, primarily driven by the reduction of background impurities. In 2DEGs closer to the surface, unbiased illuminations result in a mobility loss, driven by an increase in surface charge density. Biased illuminations performed with positive applied gate voltages result in a mobility gain, whereas those performed with negative applied voltages result in a mobility loss. The magnitude of the mobility gain (loss) weakens with 2DEG depth, and is likely driven by a reduction (increase) in surface charge density. Remarkably, this mobility gain/loss is fully reversible by performing another biased illumination with the appropriate gate voltage, provided both $n$-type and $p$-type Ohmic contacts are present. Experimental results are modeled with Boltzmann transport theory, and possible mechanisms are discussed.

Influence of impurities and structural defects on the properties of CdTe- and CdZnTe-based detectors

The most researched materials for uncooled semiconductor detectors of ionizing radiation are CdTe:Cl and Cd0.9Zn0.1Te, which allow to obtain detectors with high values of resistivity ρ and electron mobility. In the process of producing detector materials, the background impurities and defects can be introduced into their matrix, and as a result the deep levels appear in the bandgap, acting as centers of capture and recombination of nonequilibrium charge carriers and reducing the registration ability of detectors. The aim of this study was to determine by computer simulation method the nature of the effect of background impurities and structural defects on the electrophysical and detector properties of CdTe and CdZnTe. Quantitative studies were conducted using reliability-tested models. The authors used the examples of Cl, Fe, Pb, Cr, Co, Ti, V, Ni, Ge, Sn to study the effect of doping and background impurities on the resistivity ρ, lifetime of nonequilibrium electrons and holes, the charge collection efficiency η of detectors based on CdTe and Cd0.9Zn0.1Te. The influence of cadmium vacancies on the degradation of the ρ and η of the detectors based on the materials under study was clarified. Impurities were found that reduce ρ and η in detectors based on CdTe:Cl and Cd0.9Zn0.1Te:Al. The ultimate concentration of donor impurities and defects with their uniform distribution over the crystal volume without the formation of clusters was determined. The effect of the Fermi level and defect levels on the change and degradation of the properties of the materials under study was found. The ratios of the concentrations of background impurities and defects were established, making it possible to obtain semiconductors CdTe:Cl and Cd0.9Zn0.1Te of an acceptable detector quality.

Effect of neutron irradiation on the spectrum of deep-level defects in GaAs grown by liquid-phase epitaxy in hydrogen or argon ambient

We present results of experimental studies of capacitance--voltage characteristics, spectra of deep-level transient spectroscopy of graded high-voltage GaAs p+-p0-i-n0 diodes fabricated by liquid-phase epitaxy at a crystallization temperature of 900oC from one solution--melt due to autodoping with background impurities, in a hydrogen or argon ambient, before and after irradiation with neutrons are presented. After neutron irradiation, deep-level transient spectroscopy spectra revealed wide zones of defect clusters with acceptor-like negatively charged traps in the n0-layer, which arise as a result of electron emission from states located above the middle of the band gap. It was found that the differences in capacitance--voltage characteristics of the structures grown in hydrogen or argon ambient after irradiation are due to different doses of irradiation of GaAs p+-p0-i-n0 structures and different degrees of compensation of shallow donor impurities by deep traps in the layers. Keywords: GaAs, neutron irradiation, capacitance spectroscopy, p0-i-n0-junction, liquid-phase epitaxy, hydrogen, argon.

Влияние нейтронного облучения на спектр дефектов с глубокими уровнями в GaAs, изготовленном методом жидкофазной эпитаксии в атмосфере водорода и аргона

The results of experimental studies of capacitance– voltage characteristics, spectra of deep-level transient spectroscopy of graded high-voltage GaAs p+−p0−i−n0 diodes fabricated by liquid-phase epitaxy at a crystallization temperature of 900C from one solution–melt due to autodoping with background impurities, in a hydrogen or argon ambient, before and after irradiation with neutrons. After neutron irradiation, deep-level transient spectroscopy spectra revealed wide zones of defect clusters with acceptor-like negatively charged traps in the n0-layer, which arise as a result of electron emission from states located above the middle of the band gap. It was found that the differences in capacitance–voltage characteristics of the structures grown in hydrogen or argon ambient after irradiation are due to different doses of irradiation of GaAs p+−p0−i−n0 structures and different degrees of compensation of shallow donor impurities by deep traps in the layers.

Значення DX–ЦЕНТРІВ для АКУСТОІНДУКОВАНИХ процесів перебудови дефектів В GaN/AlGaN

The experimental results of amplitude effects are compared (from an ultrasonic wave deformation amplitude – a tension τUS) for electron concentration and changes of the lattice parameter on the same sample GaN/Al0.2Ga0.8N/GaN/AlN. It has been experimentally established that at ultrasonic loading (frequency 5–10 MHz, amplitude – towards 2·104 W/m2) there is a nonlinear increase in the effective electron concentration and an increase in the lattice parameter; at the same time, the mobility of electrons decreases and μН(τUS) ~ |τUS|. The energy parameters of the acoustic activation charge carriers process are calculated from the approximation of experimental amplitude changes – Еа ≈ 50 meV and γn(300 K) ≈ 2,5·10-27 m3. The amplitude dependences (increase) of the relative lattice parameter change (ΔС/С) from the tension τUS have been investigated experimentally at different frequencies. The energy of DX-center transition UDX ≈ 108 meV and the activation volume of this transition γDX ≈ 6,6·10-27 m3 are calculated from the approximation of the experimental amplitude changes. The revealed correlation of the magnitude of acoustic induced effects in different experiments allows to build a quantitative energy model of the acoustic action process based on the properties of metastable DX centers. It is shown that the acoustic induced process occurs due to the dimensional displacement of the DX-center atom (a background impurity of silicon atoms) from the non-central position to the centrally symmetric one; herewith DX-center is ionized, one goes into the d0-state. It is believed that the changes are most likely to occur near penetrating dislocations in the barrier layer Al0.2Ga0.8N – acoustic modulated oscillations of the distance between the possible positions of the donor atom lead to a decrease in the barrier to the displacement of the defect.

Research on the Region-Growing and Segmentation Technology of Micro-Particle Microscopic Images Based on Color Features

Silkworm microparticle disease is a legal quarantine standard in the detection of silkworm disease all over the world. The current common detection method, the Pasteur manual microscopy method, has a low detection efficiency all over the world. The low efficiency of the current Pasteur manual microscopy detection method makes the application of machine vision technology to detect microparticle spores an important technology to advance silkworm disease research. For the problems of the low contrast, different illumination conditions and complex image background of microscopic images of the ellipsoidal symmetrical shape of silkworm microparticle spores collected in the detection solution, a region growth segmentation method based on microparticle color and grayscale information is proposed. In this method, the fuzzy contrast enhancement algorithm is used to enhance the color information of micro-particles and improve the discrimination between the micro-particles and background. In the HSV color space with stable color, the color information of micro-particles is extracted as seed points to eliminate the influence of light and reduce the interference of impurities to locate the distribution area of micro-particles accurately. Combined with the neighborhood gamma transformation, the highlight feature of the micro-particle target in the grayscale image is enhanced for region growing. Mea6nwhile, the accurate and complete micro-particle target is segmented from the complex background, which reduces the background impurity segmentation caused by a single feature in the complex background. In order to evaluate the segmentation performance, we calculate the IOU of the microparticle sample image segmented by this method with its corresponding true value image, and the experiments show that the combination of color and grayscale features using the region growth technique can accurately and completely segment the microparticle target in complex backgrounds with a segmentation accuracy IOU as high as 83.1%.

The improved inverted AlGaAs/GaAs interface: its relevance for high-mobility quantum wells and hybrid systems

Two dimensional electron gases (2DEGs) realized at GaAs/AlGaAs single interfaces by molecular-beam epitaxy (MBE) reach mobilities of about 15 million cm2 V s−1 if the AlGaAs alloy is grown after the GaAs. Surprisingly, the mobilities may drop to a few millions for the identical but inverted AlGaAs/GaAs interface, i.e. reversed layering. Here we report on a series of inverted heterostructures with varying growth parameters including temperature, doping, and composition. Minimizing the segregation of both dopants and background impurities leads to mobilities of 13 million cm2 V s−1 for inverted structures. The dependence of the mobility on electron density tuned by a gate or by illumination is found to be the identical if no doping layers exist between the 2DEG and the respective gate. Otherwise, it differs significantly compared to normal interface structures. Reducing the distance of the 2DEG to the surface down to 50 nm requires an additional doping layer between 2DEG and surface in order to compensate for the surface-Schottky barrier. The suitability of such shallow inverted structures for future semiconductor-superconductor hybrid systems is discussed. Lastly, our understanding of the improved inverted interface enables us to produce optimized double-sided doped quantum wells exhibiting an electron mobility of 40 million cm2 V s−1 at 1 K.

Activation ability of Gd dopant in the ZnSe single crystals

Rare-earth elements are widely used as doping materials as they considerably change the semiconductor optical, electrical, magnetic and radiative properties. This work explores the influence of Gadolinium (Gd) as a dopant on the radiative, optical and magnetic properties of the Zinc Selenide (ZnSe). ZnSe single crystals were grown by the physical transport method and doped during the growth process using a GdSe source. A wide range of characterization equipment was employed to analyze the obtained ZnSe:Gd single crystals. Intracenter radiative transition of the Gd has not been detected. Gd ions activate the background impurities, causing radiative transitions from the conduction band to the Cu2+ level and intracenter transitions within V3+, V2+ and Cr2+ ions. At the same time, the edge band intensity is dependent on the dopant concentration. Optical transmittance decreases, but the position of the fundamental absorption band is unchanged. Single crystals have a zinc blende crystal structure, and Gd ions do not form complexes with native defects or background impurities.

Fault detection for rotating machinery using translation-invariant higher-density wavelet packet sliding window block thresholding

The weak incipient fault feature information of mechanical equipment is often submerged in strong background noise, which increases the difficulty of feature extraction. Herein, a novel method called the translation-invariant higher-density wavelet packet sliding window block thresholding is proposed. A higher-density wavelet transform processes one scale function and two wavelet functions, which has an advantage over single wavelet transform in matching various signal feature information. The translation-invariant higher-density wavelet packet transform effectively eliminates the Gibbs phenomenon, and the fine division of frequency bands breaks through the limitations of the higher-density wavelet transform to obtain more comprehensive feature information. At the same time, to accommodate the periodicity of the impact feature signal and the correlation between the decomposition coefficients, a sliding window block thresholding method is proposed. The sliding window is set to independently retain the weak fault features and block thresholding is applied to select the optimal block length and threshold by minimizing the Stein unbiased risk estimator. The results of the simulation and experiment indicate that the proposed method can more effectively reduce the interference of background noise and impurity frequencies, and successfully extract the characteristic frequencies of incipient fault diagnosis.

The theoretical study of the mobility of a two-dimensional electron gas in ALGaN/GaN/ALGaN double heterostructures

We investigate the drift and Hall mobility of a quasi-two-dimensional electron gas (Q2DEG) confined in wurtzite AlGaN/GaN/AlGaN square quantum wells (QWs). We use the variational-subband-wave-function model for carrier confinement and assume that the electrons only occupy the lowest subband. We take into consideration the most important scattering mechanisms such as the edge and charged dislocation, remote and background impurity, interface roughness, acoustic phonon via deformation potential and piezoelectric field, and polar LO phonon. Assuming the scattering by acoustic phonons to be quasi-elastic and using the iterative method for inelastic regime, where the LO phonon scattering becomes important, we study the dependence of the mobility on the temperature T, carrier density $$N_{\mathrm {s}}$$ , and QW width L.

Solid solubility and site preference of Ti in 3C-SiC

Ti is one of characteristic background impurities in silicon carbide (SiC) crystals, and it has a significant effect on the electronic properties of SiC. Herein, the calculated C-Si-Ti phase diagrams reveal that TiC, Ti3SiC2, and TiSi2 are the most stable phases depending on the chemical potential of Si, and the solid solubilities of Ti in 3C-SiC were determined at the SiC-TiSi2 thermodynamic equilibrium. The calculated hole transition energy between L1 and M3 for different possible defects explains the observed peak shift for the experimental Ti-Lℓ soft X-ray emissions of the Ti-saturated 3C-SiC, which experimentally confirmed that the Si-site substitution by Ti is the preferred type of defects for Ti in narrower-bandgap 3C-SiC. Furthermore, theoretical calculations reveal the different effects of Ti on the Si-L2,3 and C-K emission spectra.

Background impurities in a delta-doped QW. Part II: Edge doping

This is the second part of our study of the background impurity influence on the intersubband energy structure of a single SiGe/Si/SiGe quantum well with the impurity delta layer within the well. By the background impurity we mean sparse shallow donor doping throughout the infinitely wide barriers. In this part we consider a situation where the delta layer is positioned near the edge of the well and the structure symmetry is broken. We explain in detail the necessary modifications of our self-consistent method that includes calculation of impurity binding energy. The results particularly show that the mentioned asymmetry combined with the background impurity in the barriers provides new features to the effect of tuning the intersubband optical transitions by the ionization grade of the impurity in delta-layer that provides new technological possibilities.

MBE growth and donor doping of coherent ultrawide bandgap AlGaN alloy layers on single-crystal AlN substrates

Single-crystal Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lower extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlxGa1−xN epitaxial layers with x ∼0.6–1.0 Al contents at a growth rate of ∼0.3 μm/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ∼4.5 to 5.9 eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN heterojunction with significant implications for the formation and transport of 2D electron or hole gases. Well-controlled intentional Si doping ranging from ∼2 × 1017 to 3 × 1019 atoms/cm3 is obtained, with sharp dopant density transition profiles. In Si-doped Al0.6Ga0.4N epilayers, a room-temperature free electron concentration of ∼3 × 1019/cm3, an electron mobility of ∼27 cm2/V s, and an n-type resistivity of ∼7.5 m Ω cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.

Analysis of carrier species in arsenic-implanted p- and n-type Hg0.7Cd0.3Te

Carrier species in arsenic-implanted p– and n–type Hg0.7Cd0.3Te films grown by molecular-beam epitaxy were investigated with the use of the Hall-effect studies and mobility spectrum analysis. The implantation was performed with ion energy 190 or 350 keV and ion fluence ranging from 1012 to 1015 cm−2. A substantial difference between carrier species in Hg0.7Cd0.3Te and Hg0.8Cd0.2Te films subjected to arsenic implantation and post-implantation activation annealing was established. In particular, arsenic implantation in p–type Hg0.7Cd0.3Te in most cases lead to the formation of n+–p– (not n+–n–p–) structures, and in n–type Hg0.7Cd0.3Te films post-implantation activation annealing lead to modification of the electrical parameters of the n–type ‘base’, in contrast to Hg0.8Cd0.2Te material studied earlier. The difference in carrier species formed in arsenic-implanted Hg0.7Cd0.3Te and Hg0.8Cd0.2Te films was tentatively explained by different background impurity concentrations in the films with different chemical composition.

Determination of scattering mechanisms in AlInGaN/GaN heterostructures grown on sapphire substrate

The electron mobility limited by different scattering mechanisms in the quaternary AlInGaN alloy grown on a GaN layer is investigated with the classical Hall measurement, which is performed at a temperature range of 12 and 350 K and a magnetic field of B = 0.51 T. The effect of the thickness and alloy composition of the quaternary AlInGaN layer on the mobility is also determined. The experimentally determined temperature-dependent Hall mobility was compared with mobility calculated by using Matthiessen's rule. The main scattering mechanisms, including acoustic phonon scattering (piezoelectric and deformation potential), polar optical phonon scattering, alloy disorder scattering, interface roughness scattering, ionized impurity scattering, dislocation scattering, background impurity scattering, were used in the calculations for all temperatures. The results show that the dominant scattering mechanisms, depending on the investigated sample, are the interface roughness scattering and alloy disorder scattering at almost all temperatures. At a low-temperature, mobility is limited by ionized impurity scattering. High-temperature mobility is limited by polar optical phonon scattering. Furthermore, our results suggest that the thickness and alloy composition of the quaternary AlInGaN layer should be optimized for better transport properties.