Concentration Of Nonequilibrium Carriers Research Articles

Subnanosecond switching of GaAs diode due to impact ionization in collapsing bipolar Gunn domains

Subnanosecond switching of high-voltage GaAs diodes initiated by a steep reverse voltage ramp (≥1 kV/ns) is investigated experimentally and numerically. The triggering occurs at the voltage Um which exceeds stationary breakdown voltage Ub, similar to delayed impact ionization breakdown in Si picosecond-range high-voltage diodes known as silicon avalanche sharpening (SAS) diodes. Despite this similarity, we argue that the triggering mechanism of GaAs diodes is qualitatively different and is based on negative differential electron mobility in GaAs. Spatial patterns of recombination emission indicate formation of narrow conducting channels. Numerical simulations reveal the spontaneous appearance of high-field (∼400 kV/cm) bipolar Gunn domains in these channels. The agreement of experiments and simulations demonstrates that impact ionization within collapsing Gunn domains is responsible for a rapid increase of nonequilibrium carrier concentration and subnanosecond switching of the reversely biased GaAs diode to the conducting state with low (<100 V) residual voltage. In particular, the proposed mechanism explains why GaAs diodes exhibit subnanosecond switching at moderate overvoltage ratio Um/Ub ≈ 1.5 smaller than typical overvoltage Um/Ub ≈ 2 required for switching Si diodes, and the origin of anomalous delay of several nanoseconds that is experimentally observed in such regimes.

Ultrafast broadband carrier and exciton dynamics of Fe-related centers in GaN

The effect of Fe defects on carrier recombination and two-photon induced ultrafast exciton dynamics in GaN crystals were investigated using femtosecond transient absorption spectroscopy. The absorption kinetics exhibited completely different characteristics under different nonequilibrium carrier concentrations and distributions. The carrier recombination mechanisms under different excitation conditions can be interpreted by the model of Fe defect-related energy level. The absorption under one-photon excitation originates from the intraband transition of free holes, and the capture of holes by Fe2+ leads to the decay of the absorption. The ultrafast excitonic state [Fe2+,hVB] formation time under two-photon excitation results in a rapid absorption decay (&amp;lt;20 ps), while the long tail of the absorption response could be attributed to long-lived excitonic state (&amp;gt;100 ns). These findings are crucial to the applications of GaN for ultrafast optoelectronics and integrated nonlinear optics.

Влияние условий оптического возбуждения на спектральные и временные характеристики излучения двумерных фотонных кристаллов с Ge(Si)-наноостровками

The effect of optical excitation conditions on the spectral and temporal characteristics of the radiation from two-dimensional photonic crystals based on the structures with self-assembled Ge(Si) nanoislands has been studied. It was shown that one of the main factors effecting the spectral position and shape of photoluminescence lines, as well as the photoluminescence kinetics of photonic crystals with Ge(Si) nanoislands, along with the mode structure of the photonic crystal, are the local heating of the samples and the concentration of nonequilibrium charge carriers created by the absorption of the pumping radiation.

Diffusion length of non-equilibrium current carriers in nanowire radial p-n junctions: Effect of the curvature

In core-shell nanowire radial p-n junction, spatial (along the radius) distribution of the injected carriers is determined not only by recombination falling of the non-equilibrium carrier concentration but also by specific falling due to cylindrical symmetry of the structure. This forces us to consider an effective diffusion length of non-equilibrium carriers in nanoscale radial structures. This effective diffusion length proves to be larger (up to 25%) than the diffusion length in usual planar p-n junction (made of the same material) under injection from the shell to the core and smaller than it (up to 60%) under injection from the core to the shell.

OPTICAL PROPERTIES OF FeGaInS4 SINGLE CRYSTALS UNDER LASER EXCITATION

We experimentally investigated photoluminescence, photocurrent relaxation, and the lux-ampere characteristic of photoconductivity in FeGaInS4 single crystals upon second-harmonic excitation by a pulsed YAG:Nd3+ laser. It is shown that the photoluminescence band can be associated with the recombination of bands on impurity or the recombination of donor-acceptor pairs. The decrease in the lifetime upon excitation is due to the high concentration of nonequilibrium carriers generated by high-power laser radiation. An explanation of the lux-ampere characteristic of photoconductivity is given.

Optical gain in CdS/ZnSe/ZnSSe heterostructures with type II band offsets

A resonance-periodic gain heterostructure containing 8 ZnSe/CdS/ZnSe quantum wells (QWs) with type II band offsets was grown by metalorganic vapor phase epitaxy (MOVPE). Based on this structure, a semiconductor laser with a microcavity and longitudinal pumping by laser diode radiation with a wavelength of 440 nm was implemented. We obtained a peak power of 0.32 W at a wavelength of 525 nm with a slope efficiency of 10 %. The threshold gain was estimated at ∼ 104 cm−1. The calculation of the gain spectra at different pump levels is provided, which showed that at a concentration of nonequilibrium carriers of 1.5∙1013 cm−2, the gain can exceed 5000 cm−1.

Recombination with plasmon emission in HgTe/CdHgTe multiple quantum well heterostructures

The work is devoted to the theoretical study of the process of recombination of nonequilibrium charge carriers with the emission of plasmons in structures with two and three quantum wells, and also in an infinite number quantum-well structure under conditions of inverse band population. The plasmon spectra in these structures have been calculated. The dependence of the average probability of electron recombination with plasmon emission on the concentration of nonequilibrium carriers for three temperatures has been found. It has been shown that an increase in the number of quantum wells leads to a slight increase in the average recombination probability.

Simulation of far-infrared HgTe/HgCdTe quantum-well vertical-cavity surface-emitting laser

We propose an original design for a far-infrared vertical-cavity surface-emitting laser based on a heterostructure with ten 4.7-nm HgTe / Hg0.3Cd0.7Te quantum wells grown on a [013] GaAs substrate. Its characteristics were calculated, and it was shown that the quality factor of the proposed resonator design can reach 248 at a wavelength of 23.25 μm, which is sufficient for the occurrence of laser generation at this wavelength at a temperature of 20 K and a concentration of nonequilibrium carriers of 1.4 × 1010 cm − 2. The estimate of the threshold pump intensity using CO2 laser radiation at a wavelength of 10.6 μm is 1 W / cm2, which makes it possible to expect continuous wave mode lasing. Such a low-threshold pump intensity should also lead to lasing at substantially higher temperatures.

Plasmon gain in HgTe/CdHgTe multi-quantum-well heterostructures

The work is devoted to the theoretical study of plasmon gain in HgTe/CdHgTe multi-quantum-well heterostructures. The spectra of plasmons and plasmon gain are found in structures with 2–8 quantum wells (QWs) under the condition of inverse band population. A nonmonotonic increase in the plasmon gain with an increase in the number of QWs is shown. The dependence of the threshold concentration of nonequilibrium carriers for stimulated plasmon emission on the number of QWs in structures with 1–8 QWs has been studied.

Коллапсирующие домены Ганна как механизм самоподдержания проводящего состояния в обратносмещенных высоковольтных GaAs-диодах

Switching of a high-voltage GaAs diode to the conducting state in the delayed impact-ionization mode is simulated and the results are compared with experimental data. It is shown that the effect of long-term (up to 100 ns) sustaining of the conducting state of the diode after switching is due to the appearance of narrow (of the order of a micrometer) ionizing Gunn domains, the so-called collapsing domains, in the electron-hole plasma. Impact ionization in collapsing domains and in the edge (cathode and anode) domains of a strong electric field (~300 kV/cm) maintains a high concentration of nonequilibrium carriers (≥1017 cm-3) during the entire duration of the applied reverse polarity voltage pulse.

Recombination in gapless HgTe/CdHgTe quantum well heterostructure

Three recombination mechanisms of nonequilibrium carriers in gapless undoped quantum well in HgTe/CdHgTe heterostructure are considered. Dependencies of ensemble-average probability of recombination (inverse of recombination time) on concentration of nonequilibrium carriers for recombination with emission of optical phonons, recombination with emission of two-dimensional plasmons, and radiative recombination have been calculated. Key words: gapless HgTe quantum well, recombination mechanisms of nonequilibrium carriers.

Recombination in gapless HgTe/CdHgTe quantum well heterostructure

Three recombination mechanisms of nonequilibrium carriers in gapless undoped quantum well in HgTe/CdHgTe heterostructure are considered. Dependencies of ensemble-average probability of recombination (inverse of recombination time) on concentration of nonequilibrium carriers for recombination with emission of optical phonons, recombination with emission of two-dimensional plasmons, and radiative recombination have been calculated. Key words: gapless HgTe quantum well, recombination mechanisms of nonequilibrium carriers.

Collapsing Gunn Domains as a Mechanism of Self-Supporting Conducting State in Reversely Biased High-Voltage GaAs Diodes

Switching of a high-voltage GaAs diode to the conducting state in the delayed impact-ionization mode is simulated and the results are compared with experimental data. It is shown that the effect of long-term (up to 100 ns) sustaining of the conducting state of the diode after switching is due to the appearance of narrow (of the order of a micrometer) ionizing Gunn domains, the so-called collapsing domains, in the electron-hole plasma. Impact ionization in collapsing domains and in the edge (cathode and anode) domains of a strong electric field (~ 300 kV/cm) maintains a high concentration of nonequilibrium carriers (≥ 1017 cm-3) during the entire duration of the applied reverse polarity voltage pulse. Keywords: high-voltage GaAs diodes, impact ionization, subnanosecond switches, Gunn effect, lock-on effect.

СРАВНИТЕЛЬНЫЙ АНАЛИЗ ДЕТЕКТОРОВ ЛИЦ HAAR, HOG, CNN

This work presented in this publication is a continuation of works [1] and [2]. We have shown that in solar cells of the HIT structure, when the thickness of the embedded amorphous layer increases, the efficiency of the solar cell increases, but to a certain point of maximum. The increase in characteristics is due to an increase in the lifetime of non-basic charge carriers in a crystalline silicon substrate. This behaviour of the system can be explained by better passivation of surface States on the silicon substrate. The decrease in the efficiency of the solar cell after reaching the maximum is due to the increase of the resistance structure, which with a point contributes more to the characteristics of the solar cell than the contribution from the passivation of surface states. Using the PECVD method, amorphous silicon films with a thickness of 7, 30 and 50 nm were grown on a single-crystal commercial silicon substrate on both sides. Then, the lifetime of non-basic charge carriers was measured using the contactless Sinton method on these structures. Further, the samples were examined by photoluminescence at room temperature. Using this method, it was possible to obtain the dependence of the concentration of non-equilibrium charge carriers on the intensity of photoluminescence radiation. These dependences showed that the intensity is lower on samples with a lower thickness of amorphous silicon than in samples with a higher thickness of the amorphous layer. The detailed results of this work were first presented by the corresponding author in his dissertation [3], the theses of the work were presented at the conference of young scientists of KazNU [4] and only now the results are published as a full-fledged article in the journal.

Effect of microstructure and contact interfaces of cobalt MOFs-derived carbon matrix composite electrode materials on lithium storage performance

Transition metal oxide materials have attracted the attention of researchers due to their advantages, but their development is hindered by structural instability, a small specific surface area and low electrical conductivity [1,2]. Porous carbon materials are often used in combination with transition metal oxide materials since they can produce a synergistic effect [3]. However, the interfacial properties of electrode materials have an important influence on the energy storage characteristics. In this paper, two types of composites were prepared by using three-dimensional graphene foam (3DGF) and three-dimensional porous carbon (3DPC) as the carbon matrix and Co-MOFs as the precursor. The effect of microstructure and contact interfaces on electrochemical properties was analyzed. Besides, non-equilibrium energy band models were established according to the metal semiconductor contact theory and molecular orbital theory, and the carbon-semiconductor and electrode-electrolyte contact interfaces were studied from the energy storage perspective based on the non-equilibrium carrier concentration distribution during battery cycling. Compared with the precursor conversion type (Pcver) 3DGF/Co3O4, the precursor continuation type (Pctin) 3DPC/Co/CoO had better interface compatibility since the energy band at the semiconductor side of the electrode-electrolyte interface bent more slightly. What’s more, the porous channel structure of the Pctin type 3DPC/Co/CoO electrode material endowed it with better cycle stability and more diversified lithium insertion modes. In summary, the Pctin type composite could improve the cycle stability of the electrode and the lithium intercalation model. This study provided insights into the structure design and interface analysis of electrode materials under the non-equilibrium state.

Fabricating Femtosecond Laser-Induced Periodic Surface Structures with Electrophysical Anisotropy on Amorphous Silicon.

One-dimensional periodic surface structures were formed by femtosecond laser irradiation of amorphous hydrogenated silicon (a-Si:H) films. The a-Si:H laser processing conditions influence on the periodic relief formation as well as correlation of irradiated surfaces structural properties with their electrophysical properties were investigated. The surface structures with the period of 0.88 and 1.12 μm were fabricated at the laser wavelength of 1.25 μm and laser pulse number of 30 and 750, respectively. The orientation of the surface structure is defined by the laser polarization and depends on the concentration of nonequilibrium carriers excited by the femtosecond laser pulses in the near-surface region of the film, which affects a mode of the excited surface electromagnetic wave which is responsible for the periodic relief formation. Femtosecond laser irradiation increases the a-Si:H films conductivity by 3 to 4 orders of magnitude, up to 1.2 × 10−5 S∙cm, due to formation of Si nanocrystalline phase with the volume fraction from 17 to 28%. Dark conductivity and photoconductivity anisotropy, observed in the irradiated a-Si:H films is explained by a depolarizing effect inside periodic microscale relief, nonuniform crystalline Si phase distribution, as well as different carrier mobility and lifetime in plane of the studied samples along and perpendicular to the laser-induced periodic surface structures orientation, that was confirmed by the measured photoconductivity and absorption coefficient spectra.

Role of Dislocations in the Process of Degradation of Semiconductor Lasers with Electronic Energy Pumping. Experimental Research

Light self-destruction-degradation of the second type has been observed in samples of semiconductor lasers with electronic energy pumping with high optical homogeneity and good quality of surface treatment. In these samples, damage appeared in the form of cords perpendicular to the ends of the resonator. According to the current understanding of the passage of powerful light streams through various media, the emergence of narrow light channels is due to the phenomenon of self-focusing. It refers to the fundamental physical mechanisms of propagation of laser radiation and is caused by nonlinear phenomena arising in a medium under the influence of high-power laser radiation. The physical reason for self-focusing is an increase in the refractive index n in a strong light field. Thermal self-focusing is the most probable cause of radiation redistribution in the active region of the crystal. However, it is possible that in the initial stage of the appearance of light channels a certain role is played by the growth of the intensity of radiation in certain sections of the crystal because of the instability of generation or small fluctuations in the pump current density. Then the process acquires an avalanche character, since the localization of the ray in the channel increases the density of light radiation which can lead to overheating of the substance and the activation of the thermal self-focusing mechanism. The experiments performed in this paper have shown that optically homogeneous crystals possess maximum resistance to degradation processes. In them, the critical power of light destruction is determined by the self-focusing threshold of radiation in a material. Since the nonlinear addition to the refractive index Δn = n2E2 at the self-focusing threshold is determined by the change in the concentration of non-equilibrium carriers ΔN(E2), the value of the maximum fluctuation DΔNmax itself is proportional to the value of the non-equilibrium carrier concentration at the generation threshold ΔNpores and the relative excess of the generation threshold J = (j – jn)/jn. Thus, a low threshold concentration of non-equilibrium carriers is one of the conditions for increasing material resistance to degradation processes. In doped crystals ΔNpores is less than in pure materials. This, perhaps, explains the rather higher value of Pcritial in the optimally doped homogeneous n-GaAs. Smaller values of Pcritial in p-type samples doped with zinc can be associated not only with the inhomogeneity of these crystals, but also with large generation thresholds. In addition, the cross section for absorption of radiation by holes is about 3–4 times larger than by electrons, which can also reduce the self-destruction threshold of lasers. At Т = 300 K, the lasing thresholds are higher that naturally reduces the value of the self-focusing threshold.

Photoinduced carrier dynamics in bulk ZnTe using optical-pump terahertz-probe spectroscopy

We have utilized optical-pump terahertz-probe spectroscopy to analyze the relaxation process of photoinduced nonequilibrium carriers in bulk ZnTe with a crystal orientation of ⟨ 110 ⟩ . The experimental results showed that the concentration of nonequilibrium carriers increases with the increase in the energy of pump laser. When the pump energy calculated by a single exponential fitting function reached 63 μJ, the concentration of nonequilibrium carriers attained saturation. In addition, for low pump energy, the relaxation time did not change with increasing pump energy. On the contrary, when the pump energy was high, the relaxation time increased in proportion to the pump laser energy.

Comprehensive Analysis of Electric Properties of Oxide-Confined Vertical-Cavity Surface-Emitting Lasers

We present a comprehensive description of electric properties of vertical–cavity surface–emitting lasers (VCSELs). A complete drift–diffusion model is developed and applied for carrier transport in a multilayer semiconductor laser heterostructure with a p–n junction. We address the impact of interface grading in distributed Bragg reflectors (DBRs), modulation doping of the DBRs and surrounding layers of the quantum well as well as drastically material–dependent carrier mobilities and recombination constants. Solution of the drift–diffusion model yields spatial profiles of the nonequilibrium carrier concentrations and current. The focus is made on oxide–confined GaAs/AlGaAs VCSELs. We evaluate both depletion and diffusion capacitance of the device and show that both contributions to the capacitance as well as the differential series resistance critically depend on the injection current and VCSEL chip design such that, in general, VCSEL cannot be properly modeled by an equivalent circuit approximation. Solution for the current profile demonstrates significant enhancement of the current density at the edges of the oxide–confined aperture (current crowding). We show that, as long as the mesa diameter is small, the effective RC–product can be kept low at small aperture diameters.

The influence various sources of fluctuation on the statistical parameters of the vertical cavity surface emitting lasers output radiation

Based on statistical modeling, a numerical analysis of the effects exerted by different factors (fluctuations of the spontaneous emission intensity, nonequilibrium carrier concentration, injection current density) on the statistical characteristics of radiation at the output of surface emitting semiconductor lasers in the region of polarization instability has been performed. In this region the effect of fluctuations is maximal, offering the possibility for substantiated conclusions about relative effects of the parameters. In a theory of semiconductor lasers it is thought that the intensity fluctuations of spontaneous emission represent the dominant source of fluctuations, whereas all other sources may be neglected. As demonstra ted by the results of conducted statistical modeling, this statement is too rigorous; moreover, such a source is not dominant. Taking into consideration fluctuations of the carrier concentration, which result in fluctuations of the amplification factor, we can derive a complete set of the relationships observed experimentally. This result cannot be associated with features our model because in our theory spontaneous emission is a significant factor. If the influence of spontaneous emission would be the dominant factor, it would affect the simulation results. The obtained data make it possible to doubt the key role of the spontaneous emission intensity fluctuations in the process of statistical characteristics formation for the output radiation and to take into account fluctuations of the nonequilibrium carrier concentration.