Deep Centers Research Articles

Гальванические и емкостные эффекты при компенсации проводимости n-SiC радиационными дефектами

AbstractJBS diodes based on 4 H -SiC irradiated with electrons and protons are studied. This is carried out by using the methods of capacitance–voltage and current–voltage characteristics. It is concluded that, for both kinds of irradiation, deep centers are formed in the upper half of the band gap of silicon carbide. This leads to a sharp decrease in the concentration of ionized carriers in the conduction band and to an exponential increase in the resistance of the base regions of the structures under study.

XXII Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 12-15 марта 2018 г. Расчет состояний многозарядных примесно-дефектных центров в эпитаксиальных слоях Hg-=SUB=-1-x-=/SUB=-Cd-=SUB=-x-=/SUB=-Te

AbstractA method for calculating the states of multivalent donors and acceptors in Hg_1 –_ x Cd_ x Te materials is developed. The ionization energies of deep acceptor and donor centers in epitaxial Hg_1 –_ x Cd_ x Te films are calculated. The calculation method takes into account the influence of both the valence band and the conduction band on the states of impurity-defect centers. The calculations of energies for the levels of tetravalent acceptors and donors associated with crystalline structure defects indicate the intercenter nature of lines observed previously in the photoluminescence spectra of Hg_1 –_ x Cd_ x Te films.

A hybrid functional study of native point defects in Cu2SnS3: implications for reducing carrier recombination.

The native point defects in the earth-abundant solar material Cu2SnS3 are studied using the hybrid functional. To generate more accurate formation energies of defects, the extended Freysoldt, Neugebauer, and Van de Walle (FNV) method is used for finite-size corrections in the charged supercell calculations. According to the calculated defect energetics, it is found that the usual experimental conditions can lead to abundant deep centers that deteriorate solar cell performance. To reduce the carrier recombination caused by the deep centers, Sn-rich and S-poor conditions should be attempted. The present calculations also give satisfactory explanations for a recent experimental work on the defect levels in Cu2SnS3.

Влияние глубоких центров на статистическую задержку микроплазменного пробоя в арсенид-галлиевых светодиодах

AbstractA statistical study of the microplasma-breakdown delay in gallium-arsenide light-emitting diodes is performed. The significant effect of deep centers on the microplasma breakdown of gallium arsenide p–n junctions is detected. It is shown that the statistical delay of the microplasma breakdown makes it possible to estimate the energy spectrum of deep levels in the microplasma channel when varying the charge state of deep centers by decreasing the reverse voltage applied to the p–n junction. In the temperature range of 250–350 K, the effect of three deep levels is detected and their parameters are determined.

Влияние глубоких центров на конфайнмент носителей в квантовых ямах InGaN/GaN и эффективность светодиодов

AbstractThe deep-center-assisted tunneling of carriers in p–n structures of light-emitting diodes (LEDs) with InGaN/GaN quantum wells (QWs) makes smaller the effective height of the injection barrier, but leads to a dependence of the radiation efficiency on the density and energy spectrum of defects in GaN. In the case of hopping conduction across the space charge region, the forward voltage mainly drops near the QW boundary, where the density of deep states at the quasi Fermi-level is the lowest. As a result, band bending at the boundary decreases, and, with increasing current, the direction of the electric field also changes, which leads to a weaker confinement of holes, to their non-radiative recombination in the n barrier, and to an efficiency droop. The low efficiency of green GaN LEDs is associated with the dominance of deep centers and insufficient density of shallow centers in the energy spectrum of defects in barrier layers near the boundaries with the QW. The proposed model is confirmed by the stepwise experimental dependences of the current, capacitance and efficiency of green and blue LEDs in the case of forward bias, which reflect the contribution of color centers responsible for the defect photoluminescence bands in GaN.

The TRAMOS pixel as a photo-detection device: Design, architecture and building blocks

The deep trapping gate device concept for charged particle detection was recently introduced in Saclay/IRFU. It is based on an n-MOS structure in which a buried gate, located below the n-channel, collects carriers which are generated by ionizing particles. They deposit their energy in a volume which extends in the bulk, below the buried gate. The n-channel device is based on holes in-buried gate localization. Source–drain current modulation occurs, measurable during readout. The buried gate (Deep Trapping Gate or DTG) contains deep level centers which can be introduced during process or may be made with a Quantum Well. The device can be scaled down providing a micron range resolution. The proof of principle for such a device was verified using 2D device and process simulations. Work under way focusses on the study of building blocks. In this contribution, the pixel proof of design, using existing fabrication techniques will be discussed first. The use of this pixel for photon imaging will be discussed.

Investigation of deep defects in nanocrystalline-Si/Si interfaces using acoustic spectroscopy

Abstract A set of structures with nanocrystalline-Si/Si interfaces formed on p-type Si substrate appropriated for photovoltaic application was prepared. The Acoustic DLTS technique based on the acoustoelectric response signal produced by the structure when a longitudinal acoustic wave propagates through the structure was used together with electric characterization to determine deep defects and the role of both individual layers. Several kinds of interface deep centers with activation energies typical for dangling bonds, oxygen participated Si or point defects were observed as well as a particular influence of individual layers on the interface states. The obtained results are analyzed, discussed and subsequently compared.

Delayed avalanche breakdown of high-voltage silicon diodes: Various structures exhibit different picosecond-range switching behavior

We present a comparative study of silicon high-voltage diodes exhibiting the effect of delayed superfast impact-ionization breakdown. The effect manifests itself in a sustainable picosecond-range transient from the blocking to the conducting state and occurs when a steep voltage ramp is applied to the p+-n-n+ diode in the reverse direction. Nine groups of diodes with graded and abrupt pn-junctions have been specially fabricated for this study by different techniques from different Si substrates. Additionally, in two groups of these structures, the lifetime of nonequilibrium carriers was intentionally reduced by electron irradiation. All diodes have identical geometrical parameters and similar stationary breakdown voltages. Our experimental setup allows measuring both device voltage and current during the kilovolt switching with time resolution better than 50 ps. Although all devices are capable of forming a front with kilovolt amplitude and 100 ps risetime in the in-series load, the structures with graded pn-junctions have anomalously large residual voltage. The Deep Level Transient Spectroscopy study of all diode structures has been performed in order to evaluate the effect of deep centers on device performance. It was found that the presence of deep-level electron traps negatively correlates with parameters of superfast switching, whereas a large concentration of recombination centers created by electron irradiation has virtually no influence on switching characteristics.

Deep Center Supervised Hash for Fast Image Retrieval

With the rapid growth of image data, the current mainstream image retrieval method uses the handcraft visual feature coding, which lack of learning ability, resulting in its image expression is not strong, and the large visual feature dimension which seriously restricting its image retrieval performance. The basic idea of this paper is to learn the hash center of each class, so that the distance of intra-class is closer in Hamming space, while maintaining the Hamming distance between the interclass. Firstly, the deep implicit relationship of the training image is extracted by using the robust learning ability of the CNN, and enhance the distinguishing and expressive ability of the image hash feature. In particular, we have designed a CNN architecture in which a hash layer is added to encourage the input image to approximate discrete value (e.g. +1/-1). At the same time, we have carefully designed a hash center loss function, so that the distance between the intra-class to disperse, closer within the intra-class distance, and simultaneously imposing regularization on the real-valued outputs to approximate desired discrete values. We have conducted evolution on CIFAR-10 and NUS-WIDE images benchmarks, demonstrating that our approach can provide superior image search accuracy than other state-of-the-art hashing methods.

Identification of the spatial location of deep trap states in AlGaN/GaN heterostructures by surface photovoltage spectroscopy

Spatial and spectral origin of deep level defects in molecular beam epitaxy grown AlGaN/GaN heterostructures are investigated by using surface photovoltage spectroscopy (SPS) and pump-probe SPS techniques. A deep trap center ∼1 eV above the valence band is observed in SPS measurements which is correlated with the yellow luminescence feature in GaN. Capture of electrons and holes is resolved by performing temperature dependent SPS and pump-probe SPS measurements. It is found that the deep trap states are distributed throughout the sample while their dominance in SPS spectra depends on the density, occupation probability of deep trap states and the background electron density of GaN channel layer. Dynamics of deep trap states associated with GaN channel layer is investigated by performing frequency dependent photoluminescence (PL) and SPS measurements. A time constant of few millisecond is estimated for the deep defects which might limit the dynamic performance of AlGaN/GaN based devices.

Influences of spray angle and bowl center depth on power and exhaust emissions in a dual fuel direct injection engine

Injection strategy and piston bowl geometry play a very effective role in engine design. Hydrogen fuel in liquid or gaseous form is high in energy, and engine that burns pure hydrogen produces almost no exhaust emissions. In this article, influences of spray angle and bowl center depth on engine performance (indicated power and efficiency), fuel economy, and exhaust emissions (CO, CO2, NO, soot, and hydrocarbon) are investigated for a hydrogen (6%) + diesel dual fuel engine under 1500, 2000, and 3000 r/min speeds. Numerical simulation was performed using computational fluid dynamics code. To ensure validity of simulation, calculated in-cylinder mean pressure and rate of heat release were compared to experimental data, and the results specified a good accordance (for 1200 r/min and 70% load). The results of studies show that 160° spray angle is the best to achieve balanced emissions and more power and efficiency in comparison to other spray angles (120°, 130°, 140°, and 180° spray angles). It is found that taking into account the 160° spray angle in comparison to 120° spray angle, indicated power and efficiency increase by 25%, and the amount of exhaust CO, soot, and hydrocarbon reduces approximately by 76%, 71%, and 95%, respectively. It is determined that a deep bowl center depth leads to higher temperature (further NO emissions) and lower brake-specific fuel consumption. Three bowl center depths (5, 10, and 20 mm) were chosen for further investigation, and the 10-mm depth showed a suitable balance between exhaust emissions and power generated.

General procedure for the calculation of accurate defect excitation energies from DFT-1/2 band structures: The case of the NV− center in diamond

A major challenge in creating a quantum computer is to find a quantum system that can be used to implement the qubits. For this purpose, deep centers are prominent candidates, and ab initio calculations are one of the most important tools to theoretically study their properties. However, these calculations are highly involved, due to the large supercell needed, and the computational cost can be even larger when one goes beyond the Kohn-Sham scheme to correct the band gap problem and achieve good accuracy. In this work, we present a method that overcomes these problems and provides the optical transition energies as a difference of Kohn-Sham eigenvalues; and even more, provides a complete and accurate band structure of the defect in the semiconductor. Despite the original motivations, the presented methodology is a general procedure, which can be used to systematically study the optical transitions between localized levels within the band gap of any system. The method is an extension of the low-cost and parameter-free DFT-1/2 approximate quasi-particle correction, and allows it to be applied in the study of complex defects. As a benchmark, we apply the method to the NV$^-$ center in diamond. The agreement with experiments is remarkable, with an accuracy of 0.1 eV. The band structure agrees with the expected qualitative features of this system, and thus provides a good intuitive physical picture by itself.

Sublinear dose dependence of thermoluminescence as a result of competition between electron and hole trapping centers

This paper presents a model of sublinearity of thermoluminescence (TL) dose response, based on the competitive interaction between active electron trapping centers and deep hole centers during excitation and heating of a phosphor. It is shown by analytical and numerical calculations that the sublinear response is observed at low doses, when the trap occupancy is far from saturation. It is found that the linearity index of TL dose response depends on the concentration of the deep hole traps, and on the coefficient of non-radiative electron recombination. The TL dose response is derived in parametric form. A new analytical expression is derived for the threshold doses, at which the TL response changes from a sublinear growth to an almost linear behavior. This phenomenon of a sublinear TL response changing into a linear behavior, is explained by the deep traps filling up to saturation. The dependence of the linearity index on the model parameters is found by numerical methods.

Deep donor state of the copper acceptor as a source of green luminescence in ZnO

Copper impurities have long been linked with green luminescence (GL) in ZnO. Copper is known to introduce an acceptor level close to the conduction band of ZnO, and the GL has conventionally been attributed to transitions involving an excited state which localizes holes on neighboring oxygen atoms. To date, a theoretical description of the optical properties of such deep centers has been difficult to achieve due to the limitations of functionals in the density functional theory. Here, we employ a screened hybrid density functional to calculate the properties of Cu in ZnO. In agreement with the experiment, we find that CuZn features an acceptor level near the conduction band of ZnO. However, we find that CuZn also gives rise to a deep donor level 0.46 eV above the valence band of ZnO; the calculated optical transitions involving this state agree well with the GL observed in ZnO:Cu.

Capacitance spectroscopy on n-type GaNAs/GaAs embedded quantum structure solar cells

In this study, both deep level transient spectroscopy (DLTS) and admittance spectroscopy (AS) have been used to study the properties of electrically active deep level centers present in GaNAs/GaAs quantum wells (QWs) embedded in p-i-n solar cells. The structures were grown by molecular beam epitaxy (MBE). In particular, the electrical properties of samples with Si (n-type) doping of the QWs were investigated. DLTS revealed four deep level centers in the material, whereas only three were detected by AS. NextNano++ simulation software was used to model the sample band-diagrams to provide reasoning for the origin of the signals produced by both techniques.

Study of the photoluminescence kinetics of heterogeneous nanostructured Pb0.30Cd0.70I2 solid solutions

The study of photoluminescence (PL) kinetics for strongly heterogeneous materials such as nanostructured scintillator Pb0.30Cd0.70I2 solid solutions was carried out. Fitting the obtained results by Kohlrausch-Williams-Watts exponential function allows us to obtain the effective lifetime and the lifetime distribution for the PL decay. At the same time, fitting by a sum (about several hundreds) of single exponential functions enables us to determine the lifetime distribution associated with different radiative recombination processes characteristic of the investigated novel semiconductor materials. It was found that the radiative recombination of free electrons and holes associated with the deep surface acceptor centers is a fast process. The obtained results open a way to optimize defect structure of the investigated semiconductor materials in order to obtain high-sensitive fast non-cooled radiation detector material for X- and γ-rays suitable for biomedical and industrial imaging applications.

Improvement of minority carrier lifetime and conversion efficiency by Na incorporation in Cu2ZnSnSe4 solar cells

The effect of Na incorporation in Cu2ZnSnSe4 (CZTSe) solar cells grown by the coevaporation method was investigated via photoluminescence (PL) and time-resolved PL (TRPL) measurements as well as photovoltaic properties. The TRPL decay curves showed a monotonic increase in CZTSe lifetime from 2 to 15 ns with increasing Na incorporation, which corresponds to the increase in the correction length estimated by quantum efficiency measurements. The TRPL decay curves included two decay components, fast and slow, which were discussed and concluded as originating from the recombination at the surface and bulk of CZTSe, respectively, which is also supported by TPRL measurements with various excitation wavelengths. The lifetime of CZTSe is limited by the surface-related nonradiative recombination compared to Cu(In,Ga)Se2 devices which are fabricated with the same device structure except for the absorber, and at present, it is concluded that the surface recombination of the CZTSe limits the cell performance. In addition to the above investigations, the relationship between the CZTSe bulk lifetime and carrier concentration is discussed; deep nonradiative recombination centers in the CZTSe bulk were found to decrease by one order of magnitude with Na incorporation. The Na incorporation primarily resulted in improvement in the short circuit current density and fill factor and not in the open circuit voltage, and the results are discussed. The best performing CZTSe solar cell with Na incorporation showed a conversion efficiency of 9.57%.

Synthesis and characterization of (Cd,Zn)S buffer layer for Cu2ZnSnSe4 solar cells

In order to improve the electrical performances of Cu2ZnSnSe4 (CZTSe) based solar cells, the standard CdS buffer layer was replaced by (Cd,Zn)S processed by chemical bath deposition. The morphology and composition of the (Cd,Zn)S thin films were studied as a function of [Zn]/([Zn] + [Cd]) ratio in the chemical bath (80, 85 and 90%). The CZTSe/(Cd,Zn)S solar cells with and without Cd partial electrolyte (Cd PE) treatment were compared to CZTSe/CdS reference devices using current–voltage and external quantum efficiency measurements. The (Cd,Zn)S thin films show a non-homogeneity of Zn distribution and phase formation, with a shift from Zn(O,OH)x to ZnS phase when increasing the deposition time and a decrease of the layers thicknesses when increasing the Zn concentration in chemical bath. A model for the growth of (Cd,Zn)S thin films is proposed. The resulting CZTSe/(Cd,Zn)S devices show an important reduction of the barrier at the hetero-interface, which is attributed to the lower density of O contamination in (Cd,Zn)S compared to CdS, inducing a lower density of deep p-type recombination centers. Despite the reduced compensation of the buffer layer, CZTSe/(Cd,Zn)S devices show a deterioration of the open circuit voltage and the fill factor with the increase of Zn content in (Cd,Zn)S. These electrical losses were avoided by Cd PE treatment prior to the deposition of (Cd,Zn)S.

Integrative-existential approach to understanding the personality

У статті висвітлюється проблема особистості з позицій інтегративно-екзитенційного підходу. Окреслено провідні характеристики особистості та cфери її функціонування. Понятійне уявлення про особистість розглянуто в контексті понять “екзистенція” та “культура”. Окреслено провідні лінії особистішого становлення людини. Проаналізовано внутрішню структуру зрілої особистості, котра має інтегроване осереддя, до основних складових якого належать центр на межі внутрішнього і зовнішнього (Его) та глибинний центр (Самість). Его - осередок суб’єктності, що здійснює функцію узгодження імпульсів внутрішньої реальності та вимог зовнішнього світу, постає інструментом саморегуляції, а Самість - осередком єдності особи з усім сущим. Сильне Его зосереджує в собі засадничі важелі та засоби управління, при цьому віддане Самості, яка є джерелом смислу і натхнення. Охарактеризовано варіанти становлення особистості, умови, стадії та можливі її деформації. In article the problem of the personality is considered from the position of integrative-existential approach. The category of personality in the context of categories “culture” and “existence” is analysed. Personality reflects the dynamics and statics of actualization of existence in the individual life. We consider the personality primarily as a spiritual and existential origin (beginning) with which person comes into the world. Leading characteristics of the personality and sphere of its functioning are marked. The basic traits of the personality formation are considered. The internal structure of the mature personality is analysed. A mature personality has an integrated focus, the main components of which are 1) center on the verge of internal and external (the ego) and 2) the deep center (the core-being). Ego is the center of subjectivity, an instrument of self-regulation. Core-being is center of unity with all things, the center of love and conscience. Individual unconscious of such a person, “transparent”, it worked spiritually, not burdened with devastating complexes penetrated for Corebeing signals, which makes it a fruitful dialogue with the Ego. A strong Ego concentrates in itself the main levels and controls, at the same time committed to the Core-being, which is the source of meaning and inspiration.

Dy gradient and coercivity in grain boundary diffusion processed Nd-Fe-B magnet

By controlling Dy vapor deposition process, the amount of Dy that diffused into the magnet was increased gradually from 0.1 wt.% to 0.3 wt.%. Compared with the original status, the coercivity increment was not proportional to the Dy diffusion amount. Subsequent Hcj and Dy content gradient data showed that slope of the 0.3 wt.% sample gradient was bigger than that of 0.1 wt.% one, and the gaps between outer flakes and inner flakes enlarged with the increasement of Dy diffusion amount. Although Dy mostly enriched in triple-junction regions in electron-probe microscope analysis (EPMA) images, the following Auger depth graph showed that Dy content was as high as 3.0 at.% in 1.5 mm deep center. It proved that Dy tended to get into the main phase rather than stayed in the grain boundary during the diffusion process, and over-diffusion of Dy in the main phase was unhelpful for the coercivity enhancement.