Nonequilibrium Carriers Research Articles

Improvement of the timing properties of Ce-doped oxyorthosilicate LYSO scintillating crystals

The aim of this work has been to improve the time resolution of radiation detectors for future high-energy physics experiments and medical imaging applications. Ce-doped oxyorthosilicate Lu2SiO5:Ce (LSO) and mixed oxyorthosilicate Lu1.6Y0.4SiO5:Ce (LYSO) have been investigated as prospective scintillators for such high-time-resolution applications. A differential optical absorption technique with sub-picosecond time resolution upon selective excitation of Ce3+ ions to different excited states has been adopted to study carrier dynamics in these scintillators, and coincidence time resolution measured using 511 keV γ-quanta has been exploited to test their timing properties. A delay in population of the emitting level of Ce3+ has been observed, and is interpreted in terms of electron trapping, which is more pronounced in mixed yttrium-containing LYSO crystals due to composition fluctuations. It is shown that the delay, which affects the luminescence response time, can be eliminated by co-doping of LYSO:Ce with calcium at concentrations as low as 5 ppm. The faster kinetics of electron transfer correlates with a better coincidence time resolution. Thermalization and spatial distribution of non-equilibrium carriers has been studied theoretically to link the results obtained by the time-resolved differential optical absorption technique with the behavior of the non-equilibrium carriers generated by irradiation.

Impact of the dangling bond defects and grain boundaries on trapping recombination process in polycrystalline 3C SiC

The bulk polycrystalline (pc) 3C SiC of n- and p-type obtained by thermal decomposition of methyltrichlorosilane vapor have been studied by applying the electron paramagnetic resonance (EPR), direct current (DC) conductivity, photoconductivity (PC) and PC time-resolved decay methods. The energy level of the two donor-like minority carrier traps at ε1 = 77 meV and ε2 = 92.6 meV located at the grain boundaries (GB) of pc-3C SiC has been obtained using the measurements of the temperature dependence of DC conductivity and PC in the range of 80–600 K. The minority carrier traps assigned to carbon and silicon dangling bonds with the carbon back bonds were observed in the EPR spectra of pc-3C SiC of n- and p-type at g = 2.0029(3), g = 20042(3), respectively. The PC time decay after the termination of the photo-excitation was studied in monocrystalline and pc-3C SiC of n- and p-type at 80 K. The persistent relaxation of PC has been described by kinetic equations accounting the trapping, ionization, and recombination processes of non-equilibrium charge carriers bound dynamically to shallow donors and acceptors. We have concluded that the main process responsible for the long-lived relaxation of the PC is trap-assisted electron-hole recombination in n-type pc-3C SiC and ionization of boron acceptors, as well as the hole escape/capture at the boron level in p-type pc-3C SiC. The differences in the relaxation process of the PC in n- and p-type pc-3C SiC were explained by the presence of the potential barrier height of about 8.6 meV at GB for the capture of the majority carriers in p-type pc-3C SiC.

Degradation Under Influence of Radiation Defects of Detector Properties of CdTe and Cd0.9Zn0.1Te Irradiated by Neutrons

This work is devoted to the study by computer simulation of the mechanisms of the influence of radiation defects, arising under the influence of neutron irradiation, on the changes in electrical properties: resistivity ρ, electron mobility μn, lifetime of nonequilibrium electrons τn and holes τp in Cd0.9Zn0.1Te and charge collection efficiency η of uncooled ionizing radiation detectors based on this material. Radiation defects, which are corresponded by deep energy levels in the band gap, act as trapping centers of nonequilibrium charge carriers, noticeably affect the degree of compensation by changing ρ of the detector material, the recombination processes, decreasing τn and τp, and also the scattering of conduction electrons, decreasing μn, that ultimately can cause degradation of the charges collection efficiency η. The specific reasons for the deterioration of the electrophysical and detector properties of this semiconductor under the influence of neutron irradiation were identified, and the main factors affecting the increase in the resistivity of Cd0.9Zn0.1Te during its bombardment by low-energy and high-energy neutrons, leading to complete degradation of the recording ability of detectors based on this materials, were found. The recombination of nonequilibrium charge carriers is noticeably stronger than the decrease in μn affects the degradation of detector properties, therefore, the effect of recombination processes at deep levels of radiation defects on the degradation of τn, τp, and η of detectors based on Cd0.9Zn0.1Te was studied. A comparative analysis of the properties of Cd0.9Zn0.1Te with the previously studied CdTe:Cl was made. An attempt was made to explain the higher radiation resistance of Cd0.9Zn0.1Te compared to CdTe:Cl under neutron irradiation by the influence of the radiation self-compensation mechanism with participation of deep donor energy levels: interstitial tellurium and tellurium at the site of cadmium. In addition, the rate of recombination at defect levels in Cd0.9Zn0.1Te is, ceteris paribus, lower than in CdTe:Cl due to the smaller difference between the Fermi level and the levels of radiation defects in cadmium telluride. The relationship between the band gaps of Cd0.9Zn0.1Te and CdTe:Cl, the concentration of radiation defects, the Fermi level drift during irradiation, and the radiation resistance of the detectors were also noted. The important role of purity and dopant shallow donor concentration in initial state of the detector material is indicated.

Effect of photonic processing of rutile thin films with cadmium sulfide quantum dots on the formation of conditions for the separation of nonequilibrium charge carriers

Effect of photonic processing of rutile thin films with cadmium sulfide quantum dots on the formation of conditions for the separation of nonequilibrium charge carriers

Влияние радиационных дефектов на электрофизические и детекторные свойства CdTe:Cl, облученного нейтронами

A promising material for semiconductor detectors of ionizing radiation is CdTe:Cl which allows obtaining detectors with high resistivity ρ and electron mobility μn. During operation, the detector materials may be exposed to neutron irradiation, which causes radiation defects to form in crystal lattice and deep levels to appear in the band gap, acting as centers of capture and recombination of nonequilibrium charge carriers, thus reducing the detection capability. The aim of this study was to use computer simulation to investigate the mechanisms of the influence of such radiation defects on the electrophysical properties (ρ, μn) of CdTe:Cl and the charge collection efficiency η of radiation detectors based on this material. The simulations were based on the models tested for reliability. It was found that the increase of the CdTe:Cl resistivity ρ during low-energy neutrons bombardment and at the initial stages of high-energy neutrons bombardment is caused by an increase in the concentration of radiation donor defect Z (with an energy level EC – 0.47 eV), presumably interstitial tellurium, which shifts the Fermi level into the middle of the band gap. The sharp rise of ρ observed at high-energy neutron bombardment is probably caused by the restructuring of the crystalline structure of the detector material with a change in the lattice constant and with an increase of the band gap, accompanied by a change in the conductivity properties. The degradation of the detector properties of CdTe:Cl during neutron irradiation is due to the capture and recombination of nonequilibrium electrons at radiation defects: Te interstitial, Te substitutional at the cadmium site, on tellurium vacancies and cadmium vacancies. The degradation of electron mobility μn can be caused by the scattering of electrons at microscopic areas of radiation defect clusters. The increase in concentration of the defects over the volume of the crystal at their uniform distribution of up to 1016 cm–3 does not significantly affect the electron mobility at room temperature.

Разогрев и релаксация энергии электронно-дырочного газа в треке первичного атома отдачи

Based on the Monte Carlo algorithm, a method has been developed for calculating the energy spectrum of hot nonequilibrium electrons and holes in the track of the primary recoil atom when exposed to single fast neutrons. The calculations of the heating and subsequent relaxation of nonequilibrium charge carriers in silicon in the track of a charged particle with initial energies in the range of 50...200 keV are carried out. The characteristic temperatures of the electron and hole plasma were obtained, which amounted to 5400 K and 2700 K, respectively. The effect of radiation-induced heating of charge carriers on the failure stability of static memory elements is discussed.

Оптические и транспортные свойства эпитаксиальных пленок Pb-=SUB=-0.74-=/SUB=-Sn-=SUB=-0.26-=/SUB=-Te(In) с модифицируемой поверхностью

Abstract The results of studies of the optical and transport properties of epitaxial Pb_0.74Sn_0.26Te(In) films with a differently treated surface are presented. A comparative analysis of the results makes it possible to establish that the characteristic features of the photoconductivity spectra are defined by processes in the bulk while the surface states have practically no effect on the spectra. At the same time, the photoresponse kinetics and the rate of relaxation processes are heavily dependent on the state of the surface. The characteristic relaxation times of nonequilibrium charge carriers (about several milliseconds) are such that it is possible to use the methods of standard Fourier-transform spectroscopy to characterize optical transitions in the bulk.

Влияние условий формирования геттера в высокоомном кремнии на характеристики PIN-фотодиодов

The influence of gettering conditions in high resistivity silicon during the PIN photodiode fabrication process on the reverse dark currents has been studied. It was demonstrated that the getter formation of backside substrate by a combination of phosphorus ion implantation and deposition of polysilicon film followed by phosphorus doping at the temperatures below 900 0C results in reduction of reverse dark current value and increasing of nonequilibrium carrier lifetime.

Effect of Hole Extraction from the Base Region of a Silicon <i>p–n–p</i> Transistor on its Reactive Impedance

Transistor structures are the basic elements of integrated circuitry and are often used to create not only transistors themselves, but also diodes, resistors, and capacitors. Determining the mechanism of the occurrence of inductive type impedance in semiconductor structures is an urgent task, the solution of which will create the prerequisites for the development of solid-state analogs of inductors. The purpose of the work is to establish the effect of extraction of non-equilibrium charge carriers from the base region on the reactive impedance of a bipolar p–n–p transistor.Using impedance spectroscopy in the frequency range 20 Hz–30 MHz, the structures based on p–n–p transistors KT814G manufactured by JSC “INTEGRAL” were studied. It is shown that in the transistor structures it is possible to observe the “effect of negative capacitance” (inductive type impedance). It is established that the most probable cause of the inductive type impedance is the accumulation of uncompensated charge of holes in the base, the value of inductive impedance is influenced by both the injection efficiency in the base–emitter junction and the extraction efficiency in the base–collector junction.The results can be applied in the elaboration of technologies for the formation of elements of silicon based integrated circuits with an impedance of inductive type.

EFFECT OF SUNLIGHT ON THE ELECTRICAL CHARACTERISTICS OF THE HETEROSTRUCTURE TITANIUM DIOXIDE/SILICON

Electrical characteristics of the heterostructure titanium dioxide/silicon illuminated by the sun light were theoretically modeled. The modeling process includes consideration of generation of the charge carriers and their transport through the practically important heterostructure n-TiO2/p-Si. The current through the structure under small external bias up to 0.6 V was found to depend nonlinearly on the light wavelength. It is controlled by the movement of the electrons from silicon to the titanium dioxide. The highest current corresponds to the wavelengths of about 600 nm. The results obtained are explained by the difference in the absorption coefficients and reflectivity of titanium dioxide and silicon which determine generation of nonequilibrium charge carriers in the heterostructure n-TiO2/p-Si. It was demonstrated that under illumination of the unbiased heterostructure with the light of 500–600 nm the generated electrons freely move from the titanium dioxide to silicon while the movement of holes is blocked. It helps to concentrate electrons in the relatively thin nearsurface layer of titanium dioxide and use them for catalytic purification of water and air by oxidation of organic pollutants at its surface. The regularities observed are important in the detailed analysis of electronic processes at the surface of wide band gap semiconducting metal oxides and their practical application in photocatalytic processes.

Effects of morphology on the visible-light-driven photocatalytic and bactericidal properties of BiVO4/CdS heterojunctions: A discussion on photocatalysis mechanism

BiVO4/CdS composites with different morphologies and mass ratios were successfully prepared via a facile deposition-precipitation method by adding different dispersants. Compared with spherical BiVO4 (BS), sheet-like BiVO4 nanoarrays (BA) showed higher (020) exposed areas, as confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) revealed further details on the microstructure of the composites namely, CdS was deposited on the surface of BiVO4 forming an S-(111) interface layer. The photocatalytic properties of the samples were tested by studying the degradation of Rhodamine B (RhB) and the sterilization of Escherichia coli (E. coli) under visible-light irradiation. The BAC-60 sample showed the best photocatalytic performance among all the catalysts owing to its: (i) improved high separation efficiency of the photo-induced charge carriers and (ii) high exposure activity of the (020) plane. Electrochemical tests with non-equilibrium carrier injection and free radical trapping experiments are introduced to analyze the mechanism, such as the effects of different crystal planes on the photocatalytic reaction and the flow direction of carriers at the CdS–BiVO4 interface.

Long-range ballistic propagation of carriers in methylammonium lead iodide perovskite thin films

The performance of semiconductor devices is fundamentally governed by charge-carrier dynamics within the active materials1–6. Although advances have been made towards understanding these dynamics under steady-state conditions, the importance of non-equilibrium phenomena and their effect on device performances remains elusive7,8. In fact, the ballistic propagation of carriers is generally considered to not contribute to the mechanism of photovoltaics (PVs) and light-emitting diodes, as scattering rapidly disrupts such processes after carrier generation via photon absorption or electric injection9. Here we characterize the spatiotemporal dynamics of carriers immediately after photon absorption in methylammonium lead iodide perovskite films using femtosecond transient absorption microscopy (fs-TAM) with a 10 fs temporal resolution and 10 nm spatial precision. We found that non-equilibrium carriers propagate ballistically over 150 nm within 20 fs of photon absorption. Our results suggest that in a typical perovskite PV device operating under standard conditions, a large fraction of carriers can reach the charge collection layers ballistically. The ballistic transport distance appears to be limited by energetic disorder within the materials, probably due to disorder-induced scattering. This provides a direct route towards optimization of the ballistic transport distance via improvements in materials and by minimizing the energetic disorder. Our observations reveal an unexplored regime of carrier transport in perovskites, which could have important consequences for device performance. Charge-carrier dynamics are fundamental to the operation and performance of semiconductor devices. In methylammonium lead iodide perovskites, carriers in the non-equilibrium regime after excitation propagate ballistically over 150 nm within 20 fs.

Influence of Holes Capture Efficiency on Photoluminescence Temperature Dependence of n-AlGaAs/GaAs Quantum-Well Structures

The process of excess current carriers capture from wide-gap barrier layers into a quantum well plays a decisive role in the operation of devices based on semiconductor structures. Among these devices are lasers, light-emitting diodes, and photodetectors. The oscillating dependence of the capture rate on the quantum-well width is poorly experimentally studied. The purpose of this study is to clarify the influence of oscillations of holes capture efficiency on the temperature behavior of photoluminescence in modulation-doped n-AlGaAs/GaAs heterostructures. Temperature measurements of the integrated photoluminescence intensity of structures with different quantum-well widths corresponding to different capture rates of holes (under conditions of resonant and nonresonant capture) are carried out. It is shown that the efficiency of hole capture influences not only the integrated photoluminescence intensity, but its temperature dependence as well. In resonant structures at low temperatures (77–140 K), the photoluminescence intensity decreases with increasing temperature more sharply than that in structures with inefficient capture. Such a difference is attributed to the fact that, under resonance conditions, the capture rate of holes is so high that its change with temperature does not influence the photoluminescence intensity, and the temperature quenching of photoluminescence is defined only by a decrease in the efficiency of radiative recombination β in the quantum well. In nonresonant structures, the temperature behavior of photoluminescence depends not only on the quantum efficiency β, but also on the local hole capture rate which increases, as temperature is elevated. The results of this study are of interest for optimizing the parameters of heterostructures in designing of devices, for which the efficiency of the capture of nonequilibrium charge carriers into a quantum well plays a decisive role.

Fundamental limits of hot carrier injection from metal in nanoplasmonics

Abstract The evolution of non-equilibrium carriers excited in the process of decay of surface plasmon polaritons (SPPs) in metal is described for each step – from the generation of carriers to their extraction from the metal. The relative importance of various carrier-generating mechanisms is discussed. It is shown that both the generation of carriers and their decay are inherently quantum processes as, for realistic illumination conditions, no more than a single SPP per nanoparticle exists at a given time. As a result, the distribution of non-equilibrium carriers cannot be described by a single temperature. It is also shown that the originally excited carriers that have not undergone a single electron-electron scattering event are practically the only ones that contribute to the injection. The role of momentum conservation in carrier extraction is discussed, and it is shown that, if all the momentum conservation rules are relaxed, it is the density of states in the semiconductor/dielectric that determines the ultimate injection efficiency. A set of recommendations aimed at improving the efficiency of plasmonic-assisted photodetection and (to a lesser degree) photocatalysis is made in the end.

Multifield‐Inspired Tunable Carrier Effects Based on Ferroelectric‐Silicon PN Heterojunction

AbstractTuning the carrier behavior in a heterostructure is important for designing well‐performing electro‐optical devices. Scientific research has mainly focused on 2D and layered materials due to their high carrier mobility. Ferroelectrics exhibit a strong response to external fields and are widely employed in microwave and terahertz (THz) tunable devices. By constructing a ferroelectric‐silicon PN heterojunction, a new strategy is proposed to construct a nonequilibrium carrier layer and achieve tunable carrier effects under multifield coupling. Theoretically, the formation of a carrier layer based on the interaction between free carriers and bound surface charges is analyzed, and a design method for PN heterojunction engineering is presented. Experimentally, a PN heterojunction of Ba0.7Sr0.3TiO3 thin film on silicon substrate is constructed, and three tunable carrier effects are achieved. A broadband tunable THz modulator with an amplitude modulation depth of 99.5% is realized, which is superior to other works. The volt–ampere effect of the PN heterojunction can be dynamically adjusted. The shape of the hysteresis loops can be dynamically controlled in real time. This proposed strategy for carrier modulation is a giant step forward in ferroelectrics applications and a path for the development of electro‐optical applications with new materials.

Structural and optical characteristics investigations in oxygen ion implanted GaN epitaxial layers

This paper presents the lattice disorder-induced effect by oxygen ion implantation in GaN epitaxial layers. Oxygen ion implantation in GaN epitaxial layers is done with fluencies 5 × 1014 cm−2 and 5 × 1015 cm−2 at 120 keV energy on two different pieces of a GaN sample. Unimplanted and implanted GaN layers are investigated by High-resolution X-ray diffraction (HRXRD), Optical absorption and Raman spectroscopy. The in-plane and out of plane lattice parameters (i.e. a and c) of the distorted and undistorted region in the GaN samples are determined from the respective XRD peak positions. Due to the incorporation of oxygen ions in GaN layers up to implantation depth, lattice parameters are extended, as a result, the samples are under hydrostatic strain. Raman spectroscopic studies on the unimplanted and oxygen ion-implanted GaN samples are done by green and ultra-violet (UV) lasers. Raman spectroscopy results indicate that the broadening of the E2 (High) peak and intensity of A1 (LO) peak increases with increasing fluence. A doublet in A1 (LO) peak is found in UV excited Raman spectra in unimplanted and implanted GaN samples. The occurrence of the A1 (LO) doublet is due to surface defects and phonon-plasmon coupled-mode generated due to nonequilibrium photoexcited carriers. In this article, the origin of doublet has been studied and reported.

Impurity-assisted terahertz photoluminescence in compensated quantum wells

The terahertz radiation in GaAs/AlGaAs quantum wells with different types of doping was experimentally studied under interband optical excitation of nonequilibrium charge carriers. The structures doped only with donors, and the structures with a high degree of compensation of donors by acceptors were studied. The spectra of terahertz radiation associated with charge carrier transitions with the participation of shallow impurity states were obtained and analyzed. It was shown that the compensation of donors by acceptors leads to a significant increase in the intensity of terahertz photoluminescence. The spectra of near-infrared photoluminescence with the participation of donor and acceptor states were also studied and analyzed.

The Experimental Proof of Non - Thermal Nature of the Mechanism of Pulse-Photon Annealing in Semiconductor Materials

Nanosecond laser annealing of GaAs amorphized with B+ ions implantation was investigated. The recrystallization process observed in the experiment does not depend on the initial temperature of the samples (77K or 300K) and can be additive; the efficiency of laser annealing (LA) is determined by the light generated nonequilibrium charge carriers (NCC) rather than by crystal heating; the results of the experiments cannot be explained by the purely thermal mechanism of LA. The hypotheses for the low-temperature LA of semiconductors based on the concept of the change in the quantum state of valence electrons affecting a chemical bond are proposed. If the power of LA ensures light generation of antibonding quasiparticles with a concentration of ncr in the thickness equal to or greater than the amorphous (defective) layer, melting occurs with further epitaxial growth on the substrate, which agrees well with the results of the experiment where the concentration of light generated NCC was Δn≈ 4·5 1019cm-3.

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.

Feasibility of surface plasmon lasing in HgTe quantum wells with population inversion

Surface plasmon lasing in semiconductor gain media at far-infrared frequencies requires simultaneously long non-radiative recombination times and large plasmon propagation length. In this paper, we show that these conditions are realized in mercury-telluride quantum wells (HgTe QWs) near the topological transition. We derive the conditions of surface plasmon amplification in HgTe QWs with interband population inversion. To this end, we calculate the spatially-dispersive high-frequency conductivity of pumped HgTe QWs taking into account their realistic band structure, and compare the interband gain with Drude absorption and collisionless Landau damping. An extra necessary condition of plasmon lasing is revealed, namely, the non-equilibrium carrier density should be high enough to make the plasmon spectrum overlap with the frequency domain of interband excitations. The latter condition limits the processes of both stimulated and spontaneous plasmon emission at low temperatures, and should have a strong impact on the recombination kinetics of HgTe QWs at low temperatures.