Recombination Of Nonequilibrium Charge Carriers Research Articles

The Effect оf Laser Radiation оn Functional Properties of MOS Structures

The electrophysical properties of instrument MOSFET structures (capacitor, field-effect transistor with an isolated gate and an induced channel, CMOS integrated circuit) when exposed to unmodulated laser radiation are studied. Static and dynamic characteristics were measured. The theoretical study was carried out using the developed SPICE models and numerical experiments. An expression is obtained for the volt-ampere characteristic of a field-effect transistor operating in a mode with constant optical illumination. It is shown that the characteristics of the structures are determined by the generation and recombination of nonequilibrium charge carriers, the field effect, the photovoltaic effect in p—n junctions, the photo-Dember effect and tunneling of charge carriers through a gate dielectric. The results of the work are of interest from the point of view of creating high-speed transistors and integrated circuits of a new type.

InSb фотодіоди (огляд. Частина V)

The results of research on the recombination of non-equilibrium charge carriers in n-InSb single crystals are analyzed. It is shown that the Shockley-Reed-Hall recombination models described in the literature are unable to explain the experimental results obtained in single crystals of applied quality, which are used to create infrared photodiodes. The proposed model of recombination with the participation of two independent levels, one of which is an acceptor. The model is based on the results of experimental studies of the lifetime in n-type InSb obtained from measurements of photoconductivity relaxation under pulsed laser excitation in the temperature range of 77-250 K. For the measurements, n-type conductivity single crystals grown by Czochralski method and obtained from several sources were used. The concentration of charge carriers in the samples at a temperature of 77 K varied within 1014-1016 cm-3. The kinetics of photoconductivity was studied in n-InSb samples before and after the formation of diffused p+-n junctions. The diffusion temperature of the cadmium acceptor impurity varied between 380 and 420 0С. It is shown that in the samples after the formation of the p-n junction, the lifetime can be limited by the effect of the trapping of minority carriers to the acceptor level, which is located at a distance of 60 meV from the bottom of the conduction band. The model allows describing the dependence of lifetime on temperature and carrier concentration in the doping range of 1014 - 1016 cm-3. From the comparison of stationary and transient lifetime, conclusions are drawn regarding the existence of the effect of the trapping of minority carriers in the material of n-type conductivity, which is responsible for the excess tunnel current in photodiodes, generation-recombination and low-frequency 1/f noise. It is assumed that acceptor traps can be caused by the generation of dislocations during the formation of a diffused junction.

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.

Optical and Photosensitive Properties of Flexible n (p)-InSe/In2O3 Heterojunctions.

In this work, optical, including photoluminescence and photosensitivity, characteristics of micrometer-sized flexible n (p)–InSe/In2O3 heterojunctions, obtained by heat treatment of single-crystalline InSe plates doped with (0.5 at.%) Cd (Sn), in a water-vapor- and oxygen-enriched atmosphere, are investigated. The Raman spectrum of In2O3 layers on an InSe:Sn substrate, in the wavelength range of 105–700 cm−1, contains the vibration band characteristic of the cubic (bcc-In2O3) phase. As revealed by EDX spectra, the In2O3 layer, ~2 μm thick, formed on InSe:Cd contains an ~18% excess of atomic oxygen. The absorption edge of InSe:Sn (Cd)/In2O3 structures was studied by ultraviolet reflectance spectroscopy and found to be 3.57 eV and ~3.67 eV for InSe:Cd and InSe:Sn substrates, respectively. By photoluminescence analysis, the influence of doping impurities on the emission bands of In2O3:Sn (Cd) was revealed and the energies of dopant-induced and oxygen-induced levels created by diffusion into the InSe layer from the InSe/In2O3 interface were determined. The n (p)–InSe/In2O3 structures display a significantly wide spectral range of photosensitivity (1.2–4.0 eV), from ultraviolet to near infrared. The influence of Cd and Sn concentrations on the photosensitivity and recombination of nonequilibrium charge carriers in n (p)–InSe layers from the heterojunction interface was also studied. The as-obtained nanosized InSe/In2O3 structures are suitable for optoelectronic applications.

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.

Degradation Mechanisms of the Detector Properties of CdTe and CdZnTe Under the Influence of Gamma Irradiation

This work is devoted to the study of the mechanisms of the influence of radiation defects, arising under the influence of gamma irradiation, on the change in resistivity ρ, lifetime of nonequilibrium electrons τn and holes τp, in CdTe:Cl and Cd0.9Zn0.1Te as well as the collection efficiency η of uncooled radiation detectors based on these materials, by computer simulation method. Radiation defects, that 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 by decreasing τn and τp, what ultimately can cause degradation of the charges collection efficiency η. The specific reasons for the deterioration of the detector properties of CdTe:Cl and Cd0.9Zn0.1Te under the influence of gamma irradiation were identified, and the main factors leading to complete degradation of the recording ability of detectors based on these semiconductors during their bombardment by 60Co gamma quanta were determined. The gradual degradation of the spectroscopic performance of CdTe:Cl-based detectors during gamma irradiation occurs due to the continuous formation of cadmium vacancies VCd and acceptor complexes VCd – Cl, which continuously shift Fermi level towards valence band and decrease ρ. The complete performance degradation of CdTe detectors takes place mainly due to the capture of nonequilibrium electrons at energy level of interstitial tellurium Te(I). The invariable spectroscopic properties of CdZnTe-based sensors under gamma irradiation up to 25 kGy occur due to the mechanism of radiative self-compensation by formation of substitutional defect TeZn. At the final stage of irradiation, a sharp deterioration in the detector properties of CdZnTe occurs, mainly due to the capture and recombination of nonequilibrium charge carriers at the level of the Te(I) defect. The different radiation resistances of CdZnTe and CdTe:Cl can be explained by different behavior of Fermi level EF in these semiconductors under gamma irradiation. EF in CdZnTe under radiation exposure remains near the middle of band gap, and in CdTe it drifts to the valence band. The rate of capture and recombination through Te(I) donor level in CdTe:Cl is lower than in CdZnTe due to the larger difference between the Fermi level and the radiation defect Te(I) level in cadmium telluride. Thereby, the complete degradation of the CdTe:Cl detector occurs at a higher concentration of radiation defect Te(I), and hence after a higher irradiation dose of 50 kGy compared with a dose of 30 kGy required for degradation of CdZnTe detector properties.

Near-infrared stimulated emission from indium-rich InGaN layers grown by plasma-assisted MBE

We report on the stimulated emission (SE) in the near-infrared range from the planar InGaN epitaxial layers grown on sapphire substrates. By varying the indium content from 100% to 75%, the emission wavelength is tunable from 1.65 to 1.1 μm and minimum threshold intensities of 20–30 kW/cm2 at T = 77 K were obtained for layers with an 80%–90% indium fraction emitting around 1.3 μm. We suggest that at low temperatures, limitations for observation of SE from heavily doped InN and InGaN layers are mostly imposed by strong losses due to free-carrier absorption approaching material gain at residual electron concentrations in excess of several 1019 cm−3 and not directly by Auger recombination of non-equilibrium charge carriers. At higher temperatures, Shockley–Read–Hall (SRH) recombination sets on as a dominant nonradiative process, which determines temperature quenching of stimulated emission at Tmax ∼ 190–210 K in a way very similar to that observed earlier for InN layers with a much lower background doping level of about 3 × 1017 cm−3. Thus, different defect centers are expected to provide free electrons for InN and InGaN layers under consideration and to contribute to the SRH recombination rate, which affects SE at high temperatures.

Structural Characteristics and Photoluminescence of Thin Films of Cu(In1–xgax)(Syse1–y)2 Solid Solutions

The phase composition and structural and optical characteristics of thin films of Cu(In1–xGax)(SySe1–y)2 solid solutions with the chalcopyrite structure were investigated. The unit-cell constants according to x-ray diffraction analysis were a ~ 5.720 A and c ~ 11.52 A. The elemental compositions were x = Ga/(Ga + In) ~ 0.14 and y = S/(S + Se) ~ 0.11 for Cu(In1–xGax)(S ySe1–y)2 thin films. The optical band gap of Cu(In1–xGax)(S ySe1–y)2 solid solutions determined from measurements of photoluminescence spectra and luminescence excitation spectra at ~4.2 K was Eg ~ 1.122 eV. The mechanism of radiative recombination of nonequilibrium charge carriers in thin films of Cu(In1–xGax) (S ySe1–y)2 solid solutions is discussed.

Излучательная рекомбинация на ионно-индуцированных дефектах в тонких пленках твердых растворов Cu(In, Ga)Se-=SUB=-2-=/SUB=-

In thin films of Cu(In,Ga)Se2 solid solutions radiation-induced effects after irradiation of hydrogen ions with an energy of 2.5, 5 and 10 keV with dose of ~ 3∙10^15 cm-2 were studied. A comparative analysis of the optical characteristics of non-implanted and implanted Cu(In,Ga)Se2 thin films was carried out based on the measurements of photoluminescence spectra and the luminescence excitation spectra at liquid helium temperature of ~ 4.2 K. The bandgap of Cu(In,Ga)Se2 solid solutions determined from the data of mathematical processing of the luminescence excitation spectra was ~ 1.171 eV. An intense band with a maximum of ~ 1.089 eV was found in the photoluminescence spectra of non-implanted and hydrogen-implanted Cu(In,Ga)Se2 thin films caused by the recombination of free electrons with holes localized in the tails of the valence band. It was established that appearance of intense broad bands in the photoluminescence spectra with maxima in the energy range of ~ 0.92 eV and ~ 0.77 eV is due to radiative recombination of nonequilibrium charge carriers at deep energy levels of acceptor type ion-induced defects formed in the bandgap of Cu(In,Ga)Se2 solid solutions. The conditions for the appearance of the ionic passivation effect of dangling electronic bonds on the surface and in the bulk of Cu(In,Ga)Se2 polycrystalline films, possible nature of point defects in the structure and the mechanisms of radiative recombination are discussed.

Study on inertial characteristics of photoresistors in a physical workshop

The article describes a measuring complex based on the 32-bit STM32F103VET6 microcontroller for studying the lux-ampere (light), frequency and inertial characteristics of a photoresistor with different laws of recombination of nonequilibrium charge carriers arising under the influence of light. The developed complex is connected to a personal computer (smartphone) and allows to analyze the rise and fall edges of the photoresistor current, as well as to determine the lifetime of excess charge carriers at different illumination levels. Unlike traditional methods for measuring the parameters of photoresistors operating in a dynamic mode, the proposed measuring complex does not use an oscilloscope and a separate modulator of the luminous flux (generator or light interrupter), this made it possible to significantly reduce the size and cost of the complex, as well as automate the measurement process. To determine the frequency response of the photoresistor, it is proposed to use a frequency synthesizer, which allows, together with a digital-to-analog converter of the microcontroller, to form an amplitude-modulated light flux of the required frequency and modulation depth. The complex described in the work can be connected to the Internet using a Wi-Fi module based on an ESP8266 microcontroller, which allows conducting research in a remote mode. It is also possible to determine the parameters of the photoresistor at different values of the load resistance.

Optical and structural properties of n− and p−InSe/In2O3 heterostructures

In this work crystal structure and surface morphology, photoluminescence and fundamental optical absorption of n− InSe and p−InSe:Cd (0.05 at. %) single crystals and related heterojunctions with native oxide (In2O3) formed by surface oxidation are investigated. Oxidation-induced defects and Cd impurities shield exciton bonds, contribute to the increase of fundamental absorption, and determine photoluminescence features of InSe layer from the n (p)−InSe/In2O3 interface.Doping-induced impurity states and structural defects in the interfacial InSe layer are determining factors for the photon absorption at energies below the band gap.Impurity photoluminescence of interfacial InSe layer originates from donor−acceptor pair recombinations, for inter-pair distance much greater than thickness of elementary Se–In–In–Se packings.The violet−red emission band is determined by nonequilibrium charge carrier recombination in the interfacial In2O3 layer. Its intensity and shape are determined by structural defects at the p−InSe surface.

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.

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.

Влияние примесей и структурных дефектов на электрофизические и детекторные свойства CdTe и CdZnTe

Solid-state ionizing radiation detectors based on high-resistance semiconductors can be used to monitor the safety of nuclear reactors. High-resistance CdTe and CdZnTe have very good electrophysical and detector properties. The objective of this study was to use computer simulation to determine how impurities and structural defects, as well as their clusters, affect electrophysical and detector properties of Cd1-xZnxTe (0≤x≤0.3). The calculations were based on well-tested models, the reliability of which was confirmed when comparing simulation results with well-known experimental data. It has been established that deep donors with energy levels near the middle of the band gap considerably extend the area of the high-resistance state of CdTe and CdZnTe, which is suitable for the creation of radiation detectors. The capture and recombination of non-equilibrium charge carriers occurs at the deep levels of cadmium vacancies owing to the influence of Ti, V, Ge, Ni, and Sn impurities. For this reason, such impurities are considered to be harmful, noticeably reducing the efficiency of charge collection η in CdTe and CdZnTe detectors. The decrease of electron mobility in CdTe and CdZnTe can be caused by the distribution heterogeneity of impurities (impurity clusters).When concentration of harmful impurities Ti, V, Ni, Sn, Ge does not exceed the content of the "background", provided that the impurities are distributed over the crystal uniformly, it is possible to obtain high-resistance CdZnTe of an acceptable detector quality. The obtained results could help determining conditions for producing CdTe and CdZnTe materials of high detector quality.

Identification of the Diffusion Length in Gallium Nitride by Cathodoluminescence in Cases of the Exciton Recombination and Impurity Recombination of Nonequilibrium Charge Carriers

A number of possibilities of a model of the quantitative cathodoluminescence of direct-gap materials is shown. It is based on using the dependence of the monochromatic cathodoluminescence intensity on the electron-beam energy in the case of a constant level of electron—hole pair generation. In accordance with the results of experimental studies of single-crystal n-GaN, estimates of the diffusion length and the lifetime of minority charge carriers for the spectral range corresponding to impurity recombination and also estimates of the diffusion length and the lifetime of bound excitons are obtained by means of this model.

Noncontact determination of the rate of surface recombination of nonequilibrium charge carriers at the p–p+ (n–n+) boundaries of n+–p(n)–p+ silicon structures by means of compensation

Calculations show that the rate of the surface recombination of nonequilibrium charge carriers at the back of a p(n) region with known lifetime values can be determined for n+–p(n)–p+ structures of silicon by non-contact measurement. This allows us to determine the photosensitivity contrast along a surface structure upon its local illumination.

Kinetics of photoconductivity of c-Si with amorphous heterogeneities

The influence of amorphous inclusions of a cylindrical shape on the photoconductivity of crystalline silicon under the action of radiation from the photosensitivity region of the structure under study was theoretically investigated. The photoconductivity of the structure is analyzed depending on the geometric dimensions of the inclusions, their location in the structure, and the rate of surface recombination of nonequilibrium charge carriers. It is established that when the fraction of inclusions in the structure increases, the processes of generation of nonequilibrium charge carriers are determined mainly by an amorphous matrix. The appearance of the effect of negative photoconductivity under certain conditions has been revealed.

Influence of surface potential barriers on the dependence of photocurrent on intensity of exciting light in cadmium sulfide films with a nanostructured surface

The influence of the photoexcitation intensity on the value of the stationary photocurrent in pure and Li-doped pyrolitic CdS films with a nanostructured surface has been studied. The brightness dependence of the photocurrent of the structures under study is approximated by a power function whose power index is a decay function of the excitation intensity. The unusual peculiarity of the photocurrent dependence on illumination intensity is explained by a modulation of the surface potential barriers for the drift and recombination of nonequilibrium charge carriers.

Recombination of charge carriers in heterostructures with Ge nanoislands grown on Si(100)

In this paper, the study of the recombination of non-equilibrium charge carriers and determination of recombination mechanisms in Ge/Si heterostructures with nanoislands have been presented. The effects of long-term photoconductivity decay in Ge/Si heterostructures with Ge nanoislands have been found as caused by variations of the electrostatic potential in the near-surface region of Si(100) substrate and spatial separation of electron-hole pairs between localized states of Ge nanoislands and states of wetting layer and Si. It has been shown that the photoconductivity decay depends on the excitation energy and temperature, while Ge nanoislands are Shockley-Read recombination centers with a higher recombination rate as compared with Si.

THE INFLUENCE OF GROWTH CONDITIONS AND DONOR DOPING ON CONDUCTIVITY MODE AND DEEP TRAPS SPECTRA IN TLBR SINGLE CRYSTALS

Studies of electrical characteristics, deep traps spectra, microcathodoluminescence (MCL) spectra of undoped and donor (Pb, Ca) doped TlBr crystals as influenced by growth conditions (Br pressure, Ar pressure, growth in air) are presented. It is shown that, for the 85−320 K temperature range, the crystal conductivity was determined not by ionic conductance but by the density of electrons and holes supplied by the ionization of deep centers. Centers with activation energies of 1−1.2 eV that pin the Fermi level in donor doped crystals are shown to play a prominent role in the recombination of nonequilibrium charge carriers. In undoped crystals the Fermi level is pinned near Ev+0.8 eV and these centers are also active in the recombination of charge carriers and are responsible for the MCL band peak near 1.85 eV. The temperature dependence of photocurrent in undoped crystals is strongly influenced by electron trapping on relatively shallow centers located 0.1−0.2 eV below the conduction band edge. Deep traps spectra revealed the presence of centers with activation energies 0.36, 0.45, 0.6 eV whose concentration increases with donor doping. Doping with Pb or Ca increases the dark resistivity of the crystals by about an order of magnitude, but Pb doping enhances the density of deep traps, which is not favorable for use of this material in radiation detectors.