Nonequilibrium Charge Carriers Research Articles

Influence of the content of impurities and structural defects on the properties of the Cd0.9Mn0.1Te:V-based detector

The paper highlights the results of quantitative studies of the influence of the content of impurities and structural defects on the electrophysical and detector properties of Cd0.9Mn0.1Te:V — resistivity and concentrations of free charge carriers, life time of nonequilibrium charge carriers τ, charge collection efficiency η. The optimal ranges of energy change and deep donor concentration, which ensure a high-resistive state and acceptable values of τ and η, are established. The authors study the compensation of cadmium vacancies with vanadium admixture.

Influence of the content of impurities and structural defects on the properties of the Cd0.9Mn0.1Te:V-based detector

The paper highlights the results of quantitative studies of the influence of the content of impurities and structural defects on the electrophysical and detector properties of Cd0.9Mn0.1Te:V — resistivity and concentrations of free charge carriers, life time of nonequilibrium charge carriers τ, charge collection efficiency η. The optimal ranges of energy change and deep donor concentration, which ensure a high-resistive state and acceptable values of τ and η, are established. The authors study the compensation of cadmium vacancies with vanadium admixture.

Excitation of terahertz radiation in p-n-heterostructures based on a-Si:H/c-Si

Studies on the generation of terahertz (THz) radiation in p-n-heterostructures based on a-Si:H/c-Si upon their photoexcitation by a femtosecond titanium-sapphire laser with a wavelength of 800 nm are presented. The properties of observed THz radiation allow to explain its nature by excitation of fast photocurrent of nonequilibrium charge carriers created in the region of the potential barrier under femtosecond interband photoexcitation of the structure. The fast photocurrent, in turn, emits THz electromagnetic waves. The waveforms and amplitude spectra of the observed THz radiation reflect the dynamics of photoexcited charge carriers in the structures. The intensity of THz radiation observed in the studied p-n-heterostructures based on a-Si:H/c-Si is comparable to that generated in n-InAs crystals, which are widely used as emitters in systems of THz time-domain spectroscopy. Therefore, a-Si:H/c-Si p-n-heterostructures can be used as THz emitters for need of THz spectroscopy. Keywords: femtosecond laser photoexcitation, heterostructures, fast photocurrent, terahertz electromagnetic radiation.

Возбуждение терагерцевого излучения в p-n-гетероструктурах на основе a-Si:H/c-Si

The results of studying the generation of terahertz (THz) radiation in p-n-heterostructures based on a-Si:H//c-Si upon their photoexcitation by radiation from a femtosecond titanium-sapphire laser with a wavelength of 800 nm are presented. The properties of the observed THz radiation can be explained by excitation of fast photocurrent of nonequilibrium charge carriers created in the region of the potential barrier under femtosecond interband photoexcitation of the structure. The fast photocurrent, in turn, emits a THz electromagnetic wave. The waveforms and amplitude spectra of the observed THz radiation reflect the dynamics of photoexcited charge carriers in structures. In terms of intensity, the THz radiation observed from the studied p–n heterostructures based on a-Si:H//c-Si is comparable to the THz radiation generated in n-InAs crystals, which are widely used as emitters in systems of THz time-domain spectroscopy. Therefore, a-Si:H//c-Si p-n-heterostructures can be used as THz emitters for solving tasks of THz spectroscopy.

Диполярные биэкситоны в латеральных ловушках в гетероструктурах Si/SiGe/Si

Si/SiGe/Si heterostructures with long-range lateral potential fluctuations that appear in the capping Si layer near the SiGe/Si heterointerface due to the presence of relaxed areas in the SiGe layer have been studied. Analysis of the low-temperature photoluminescence spectra indicates that, upon the photoexcitation of the structure, the accumulation of nonequilibrium charge carriers, formation of dipolar excitons, and their recombination take place in large-scale lateral traps formed by these fluctuations. It is found that, at temperatures T < 10 K, a new narrow line appears with an increase in the excitation level at the blue side of the broad photoluminescence band of dipolar excitons localized by short-range potential fluctuations. At temperatures T ~ 2 K, this line is dominant in the spectra even at the lowest excitation levels. It is shown that, at moderate excitation levels, this line is caused by the recombination of free dipolar biexitons in large-scale traps. At high excitation levels, the width of the new line increases by more than a factor of 2 compared to that at lower excitation levels, and under these conditions this line is associated with the recombination of dipolar electron–hole plasma in large-scale traps.

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.

Photoconductivity and electronic processes in PCDTBT polymer composite with embedded CdSe nanoplatelets

The photoconductivity in thin films of a new composite based on CdSe nanoplatelets (NPLs) and PCDTBT polymer has been studied using photoconductivity temperature dependences, current-voltage and lux-ampere measurements at various temperatures and applied bias. The incorporation of 7 vol% of NPLs into the polymer blend led to an increase in photoconductivity by an order of magnitude compared to the pristine PCDTBT film in the temperature range of 310–360 K and the electric field strength above 200 V cm−1. An analysis of current-voltage characteristics measured at various temperatures demonstrated that the observed increase in the photoconductivity of the composite is due to a change in the transport mechanism of nonequilibrium charge carriers with an increase in their concentration. The observed saturation of photoconductivity with increasing temperature is supposed to occur due to the increase in exciton diffusion length with temperature increasing and limited effective exciton generation area.

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.

Nonlinear Inertial Diode Model Considering the Dependence of Nonequilibrium Charge Carrier Lifetime on Direct Current to Improve Simulation of Radioelectronic Equipment

Introduction. Adequate modeling of semiconductor devices with a p–n-junction in reverse bias represents a relevant research problem. The existing quasistatic and non-quasistatic models fail to provide a satisfactory description for the dependence of nonequilibrium charge carrier lifetime on current density. This leads to significant simulation errors (tens of percent) at pulsed broadband signals. Simulation errors arise, because the existing models regard the lifetime as a constant value.Aim. To propose and investigate an equivalent circuit of a p–n-junction considering the dependence of the lifetime of nonequilibrium charge carriers on direct current, with the possibility of its simple integration into CAD.Materials and methods. The study was carried out on the example of a fast recovery silicon diode BAS16J with a p–n-junction manufactured by Nexperia. A modified diode model is proposed in the form of an equivalent circuit that considers the dependence of the lifetime of nonequilibrium charge carriers on the direct current of the p–njunction at high injection levels.Results. The discrepancy between the experimental and simulated curves did not exceed ±9 % under pulsed diode operation. The extraction of parameters in the proposed model is carried out conventionally, from the current-voltage and capacitance-voltage characteristics of the diode.Conclusion. The proposed non-quasistatic equivalent diode circuit can be used when designing radio electronic devices operated at short-pulse broadband signals. The proposed diode model can be easily implemented in modern CAD systems at the user level.

Generation of Terahertz Radiation under the Femtosecond Laser Excitation of a Multilayer Structure Based on a-Si:H/a-SiC:H/c-Si

Coherent terahertz radiation has been generated in p–n heterostructures based on a-Si:H/a-SiC:H/c-Si excited by 800-nm femtosecond laser pulses at room temperature. Terahertz radiation is generated when a reverse bias voltage is applied to heterostructures. The properties of the generated terahertz radiation strongly depend on the bias voltage, which reflects the dynamics of nonequilibrium charge carriers produced by femtosecond laser pump in the heterostructure.

О корректности математических моделей диффузии и катодолюминесценции

Mathematical models of diffusion and cathodoluminescence of nonequilibrium minority charge carriers generated by a wide electron beam in homogeneous and multilayer semiconductor materials are considered. The use of wide electron beams makes it possible to reduce these problems to one–dimensional ones and to describe these mathematical models by ordinary differential equations.

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.

Excitation of intracenter terahertz radiation by plasma oscillations in electron–hole liquid

A new mechanism for the excitation of impurity related terahertz radiation in semiconductors under the conditions of exciton condensation into an electron–hole liquid is reported. The interaction of impurity centers with plasmons localized on droplets of an electron–hole liquid produces ionization of the centers. The subsequent capture of nonequilibrium charge carriers by ionized impurities is followed by terahertz intracenter radiative transitions. In these processes, impurity centers play the role of antennas that convert the near electromagnetic field of plasmons on droplets of an electron–hole liquid into detected radiation. The main experiments were carried out on lithium-doped silicon crystals at helium temperatures under conditions of interband photoexcitation. A theoretical model of the excitation of impurity centers by localized plasmons is developed, which explains the main regularities observed in the experiment.

Effective lifetime of non-equilibrium carriers in semiconductors from non-adiabatic molecular dynamics simulations.

The lifetimes of non-equilibrium charge carriers in semiconductors calculated using non-adiabatic molecular dynamics often differ from experimental results by orders of magnitude. By revisiting the definition of carrier lifetime, we report a systematic procedure for calculating the effective carrier lifetime in semiconductor crystals under realistic conditions. The consideration of all recombination mechanisms and the use of appropriate carrier and defect densities are crucial to bridging the gap between modeling and measurements. Our calculated effective carrier lifetime of CH3NH3PbI3 agrees with experiments, and is limited by band-to-band radiative recombination and Shockley-Read-Hall defect-assisted non-radiative recombination, whereas the band-to-band non-radiative recombination is found to be negligible. The procedure is further validated by application to the compound semiconductors CdTe and GaAs, and thus can be applied in carrier lifetime simulations in other material systems.

Revealing charge carrier dynamics and transport in Te-doped GaAsSb and GaAsSbN nanowires by correlating ultrafast terahertz spectroscopy and optoelectronic characterization

Recent advances in the growth of III-V semiconductor nanowires (NWs) hold great promise for nanoscale optoelectronic device applications. It is established that a small amount of nitrogen (N) incorporation in III-V semiconductor NWs can effectively red-shift their wavelength of operation and tailor their electronic properties for specific applications. However, understanding the impact of N incorporation on non-equilibrium charge carrier dynamics and transport in semiconducting NWs is critical in achieving efficient semiconducting NW devices. In this work, ultrafast optical pump-terahertz probe spectroscopy has been used to study non-equilibrium carrier dynamics and transport in Te-doped GaAsSb and dilute nitride GaAsSbN NWs, with the goal of correlating these results with electrical characterization of their equilibrium photo-response under bias and low-frequency noise characteristics. Nitrogen incorporation in GaAsSb NWs led to a significant increase in the carrier scattering rate, resulting in a severe reduction in carrier mobility. Carrier recombination lifetimes of 33 ± 1 picoseconds (ps) and 147 ± 3 ps in GaAsSbN and GaAsSb NWs, respectively, were measured. The reduction in the carrier lifetime and photoinduced optical conductivities are due to the presence of N-induced defects, leading to deterioration in the electrical and optical characteristics of dilute nitride NWs relative to the non-nitride NWs. Finally, we observed a very fast rise time of ∼2 ps for both NW materials, directly impacting their potential use as high-speed photodetectors.

Effect of irradiation on pyroelectric and electrocaloric parameters in lead-free relaxor ferroelectric ceramic

Ferroelectric materials have a renaissance due to various photosensitive characteristics. In this study, temperature-dependent dielectric, impedance, and ferroelectric measurements were carried out on lead-free Ba0.6(Bi0.5Li0.5)0.4TiO3 (BBLT) ferroelectric ceramic under the dark and exposure of monochromatic 405 nm light conditions. Profound analysis of complex impedance spectroscopy shows a noticeable reduction in bulk capacitance, relaxation time, and constant phase element parameters under illuminated conditions. Relevant phenomenological approaches towards modified Cole−Cole models confirm that the light induces photogenerated nonequilibrium charge carriers. Thus, the trapped charges alter the internal electric field distribution energy. Under illumination, the enhanced polarization and corresponding pyroelectric coefficient enhanced by 40 % and ~260 % at 5 Hz and 10 Hz, respectively, at 20 ℃ were observed. The cross-coupled Electrocaloric (EC) parameters ∆T and ∆S enhanced 600 % and 571 % at 10 Hz, respectively, and 219 % and 218 %, at 5 Hz respectively at 20 ℃ under illumination. This study provides a comprehensive understanding and new insights into the light-dependent dielectric, ferroelectric, pyroelectric and electrocaloric mechanisms in BBLT electroceramics, which encourages the design of modern thermal sensor devices.

Photovoltaic properties of cd-based ionic liquid crystals with semiconductor nanoparticles

The photovoltaic effect in pure cadmium-alkanoate matrices and in their nanocomposites with semiconductor CdS nanoparticles (NPs) was revealed and investigated. The main mechanism of the phenomenon is the Dember effect, in which the internal field of non-equilibrium charge carriers is formed due to a significant difference in the mobility of positive and negative charges. NPs synthesized in the matrix lead to a significant increase in the concentration of the photo-generated electrons and thus to an increase in the photo-electro-driving force in the material. The magnitude and kinetics of the photocurrent for nanocomposites substantially depend on the content of only a small amount (2 − 4 mol. %) of NPs.

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.

In2O3 Based Hybrid Materials: Interplay between Microstructure, Photoelectrical and Light Activated NO2 Sensor Properties

In this work, organic–inorganic hybrids based on nanocrystalline indium oxide and ruthenium (II) heteroleptic complexes were used as sensitive materials for room temperature light-activated NO2 detection. In2O3 was obtained by chemical precipitation method and then annealed at three different temperatures (T = 300, 500, 700 °C) in order to investigate the influence of the microstructure of indium oxide on sensor characteristics of hybrid materials and on kinetics of the rise and fall of photoconductivity. The results of the X-ray phase analysis demonstrated that the obtained materials are single-phase with a cubic bixbyite structure. The Ru (II) heteroleptic complex, which was used as a photosensitizer, made it possible to shift the optical sensitivity range of the hybrids to the low energy region of the spectrum and to use a low-power LED (λmax = 470 nm) source for the photoactivation process. The sensor properties were investigated toward NO2 at sub-ppm range at room temperature. It was found that for pure oxides, the sensor signal correlates with a specific surface area, while for hybrid materials, both the sensor signal and photoresponse increase with increasing the matrix crystallinity. In this case, the main role is played by traps of nonequilibrium charge carriers, which are structural defects in the matrix.

Impurity-impurity interaction during the growth of UMG-Si-based mc-Si

This article investigates the relationship between the chemical composition and electrophysical properties of p- and n-type multicrystalline silicon ingots based on metallurgical silicon with a purity of 99.99 at.%. In particular, the role of impurity-impurity interactions in the production of multisilicon by the Bridgman vertical method is evaluated in order to identify approaches to controlling this process effectively. The phase equilibrium calculations in the “silicon–all impurities” and “silicon-impurity-oxygen” systems were carried out based on the Gibbs energy minimization in the Selector software package. The study investigates the rank correlations of the concentrations of various impurities with each other, as well as with the specified electrical resistivity (SER) and the lifetime of nonequilibrium charge carriers (NCC) in the direction of crystal growth. Pair correlations of the element distribution profiles were considered based on the role of the main factor represented by the ratio of individual impurity solubilities in solid or liquid silicon (k0), as well as from the standpoint of direct interaction between two elements. It was found that the k0 value for two individual impurities in silicon does not automatically lead to the pair correlation of their distribution profiles in the ingot. A significant effect on the distribution profiles of impurities in multisilicon with k0→0 has the factor of binding some part of the impurity into such a form that this impurity can be incorporated easily into a growing crystal. Binding may be induced by the interaction of the impurity in the melt with the oxygen background, its segregation at the grain boundaries, and its capture by the crystallization front in the composition of the liquid inclusion. Significant correlations of impurity distribution profiles in the ingot were demonstrated by the pairs whose elements interact without the formation of chemical compounds in the 25–1413 °C temperature range. The conducted phase equilibrium calculations for the “silicon–all impurities” system revealed the possibility of forming the VB2, TiB2, ZrB2, and MgTiO4 solid phases in the melt.