Nonequilibrium Carrier Density Research Articles

Light Inorganic Scintillation Materials for Neutron and Charge Particle Detection

The technological aspects of the light inorganic crystalline compounds suitable to create scintillation materials to detect charged particles and neutrons in a wide energy range have been examined. Among them, Li2CaSiO4:Eu was found to be a prospective candidate to control the valent state of the Rare Earth (RE) and to provide a high intensity of luminescence. It was demonstrated that the material has room for future improvement; however, this requires precise engineering of its composition—an experimental search of compositions or additives that will provide the maximum Eu2+/Eu3+ ratio to achieve a high scintillation light yield. The benefits of light inorganic materials are disclosed through the modeling of the linear density of nonequilibrium carriers along secondary particle tracks created in scintillators utilized for neutron detection. It is shown that oxide matrices have a larger linear density in comparison with halide crystalline compounds under alpha-particles and tritons, whereas light oxides can provide smaller numbers under protons.

SCALING MODELS OF ELECTRICAL PROPERTIES OF PHOTOAND BETA-CONVERTERS WITH NANO-HETEROJUNCTIONS

The new methodology is developed and the computer simulation of scaling the electrical properties of nanochips-generators of a semiconductor energy converter based on nanoscale contact heterojunctions to ensure maximum power is considered. The variant of optimization of the scaling solution is represented by the connection of nanoheterojunctions with an increase in the current density of nonequilibrium carriers and the open circuit voltage. A generalized equivalent scheme for variations of internal properties and identification of experimental data is presented. The influence of the type of scaling and model parameters is analyzed.

The Ubiquitous Memristive Response in Solids

Basic ingredients universally available in the carrier transport of conductive solids are proved to be sufficient (not necessary) conditions for the formation of memory while symmetry, or the lack of it, plays a decisive role in the nature of this response. It suffices that the input driving frequency is of the order of the inverse relaxation time and that the nonequilibrium carrier densities are in the order of the quasi-equilibrium ones. Thus, memristive response with ON and OFF states at zero voltage, Type I, and vanishing ON– OFF ratio, Type II, arise naturally according to simple symmetry constraints and are contrasted here in a fully analytical picture. Figures of merit for the conductance and the robustness of the memory response are presented in the form of concise correlations between general intrinsic microscopic parameters such as relaxation times, activation energies, and efficiencies with external drives: voltage pulses, temperature, and illumination. Explicit analytical expressions unveil the mimicry of inductive- and capacitive-like leaning impedances without the need of relaxing the interpretation of flux that has been used for more stringent definitions of memristors.

Thermionic graphene/silicon Schottky infrared photodetectors

Optical communications, imaging, and biomedicine require efficient detection of infrared radiation. Growing demand pushes for the integration of such detectors on chips. It is a challenge for conventional semiconductor devices to meet these specs due to spectral limitations arising from their finite band gap, as well as material incompatibilities. Single layer graphene (SLG) is compatible with complementary metal-oxide-semiconductor (CMOS) Si technology, while its broadband (UV to THz) absorption makes the SLG/Si junction a promising platform for photodetection. Here we model the thermionic operation of SLG/Si Schottky photodetectors, considering SLG's absorption, heat capacity, and carrier cooling dependence on temperature and carrier density. We self-consistently solve coupled rate equations involving electronic and lattice temperatures, and nonequilibrium carrier density under light illumination. We use as an example the infrared photon energy of 0.4 eV, below the threshold for direct photoemission over the Schottky barrier, to study the photothermionic response as a function of voltage bias, input power, pulse width, electronic injection, and relaxation rates. We find that device and operation parameters can be optimized to reach responsivities competitive with the state of the art for any light frequency, unlike conventional semiconductor-based devices. Our results prove that the SLG/Si junction is a broadband photodetection platform.

Auger recombination in narrow gap HgCdTe/CdHgTe quantum well heterostructures

We present detailed theoretical analysis of nonradiative Auger recombination in narrow-gap mercury-cadmium-telluride quantum wells (HgCdTe QWs). We suggest a microscopic model to calculate Auger recombination rates in the QWs with different Cd fraction as functions of non-equilibrium carrier density with account to the complex band dispersions and wavefunctions, degenerate carrier statistics, and screening effects. Our model is validated by the comparison of measured photoconductivity kinetics with the simulated curves. Furthermore, we use the developed calculation technique to evaluate different designs of HgCdTe/CdHgTe QWs for the far-IR emitters. In particular, we consider a series of QWs with the fixed bandgap of 40 meV (lasing wavelength about 30 μm) and find out that lasing may be favored in the QWs with moderate (6%–9%) cadmium content and not in the pure binary HgTe QWs, which is in contrast to intuitive expectations within threshold energy concept for Auger recombination. Though cadmium-free QWs do provide the highest possible Auger threshold energies, Cd-containing QWs feature much more efficient screening of Coulomb potential (and so Auger interaction) by free charge carriers. The latter effect contributes decisively into the suppression of Auger processes at low temperatures and high carrier concentrations.

Effect of recombination process in femtosecond laser-induced modification on Ge crystal* *Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11804227).

The dynamics of produced excited carriers under the irradiation of Ge crystal is investigated theoretically by using femtosecond laser pulse. A two-temperature model combined with the Drude model is also used to study the nonequilibrium carrier density, carrier and lattice temperatures, and optical properties of the crystal. The properties of the surface plasmon wave when excited are also studied. The influences of non-radiation and radiative recombination process on the photoexcitation of the semiconductor during pulse and the relaxation after the pulse are described in detail. The results show that the effects of Auger recombination on the nonequilibrium carrier density and optical properties of the crystal and the properties of the surface plasmon polariton are great, whereas the effect of radiative recombination is extremely small.

Ultra-long carrier lifetime in neutral graphene-hBN van der Waals heterostructures under mid-infrared illumination

Graphene/hBN heterostructures are promising active materials for devices in the THz domain, such as emitters and photodetectors based on interband transitions. Their performance requires long carrier lifetimes. However, carrier recombination processes in graphene possess sub-picosecond characteristic times for large non-equilibrium carrier densities at high energy. An additional channel has been recently demonstrated in graphene/hBN heterostructures by emission of hBN hyperbolic phonon polaritons (HPhP) with picosecond decay time. Here, we report on carrier lifetimes in graphene/hBN Zener-Klein transistors of ~30 ps for photoexcited carriers at low density and energy, using mid-infrared photoconductivity measurements. We further demonstrate the switching of carrier lifetime from ~30 ps (attributed to interband Auger) down to a few picoseconds upon ignition of HPhP relaxation at finite bias and/or with infrared excitation power. Our study opens interesting perspectives to exploit graphene/hBN heterostructures for THz lasing and highly sensitive THz photodetection as well as for phonon polariton optics.

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.

Measurement of lifetime of nonequilibrium charge carriers in single-crystal silicon

An equation of continuity describing decay of nonequilibrium charge carriers after illumination of semiconducting silicon by a long duration pulse of light with wavelength of 1.06 µm is solved by numerical methods in a one-dimensional approximation. It is shown that, when the volume recombination lifetime of minority carriers is 500 µs and higher, the known formula for “thin” samples 1/τeff = 1/τV + 2s/d cannot be used to evaluate the effective lifetime of thin plates (10 µm or above). Estimations of the range of variation of excess density in which the instantaneous lifetime achieves saturation are carried out; this takes place in the second part of the relaxation curve if the wafer thickness does not exceed three diffusion lengths of minority carriers. It is demonstrated that the formula scaled to an infinite recombination rate is applicable for thicknesses of the sample within the range from one to five diffusion lengths. For higher thicknesses, it is necessary to apply corrections taking into account the dependence of instantaneous lifetime on the excess density of nonequilibrium charge carriers.

Dynamics of nonequilibrium carrier decay in AlGaN epitaxial layers with high aluminum content

Carrier dynamics in high-Al-content AlGaN epilayers with different dislocation densities from 5 × 10(8) cm(-2) to 5 × 10(9) cm(-2) is studied by comparing the photoluminescence decay with the decay of carrier density. The carrier density decay was investigated using the light-induced transient grating technique. This comparison shows that the luminescence at the nonequilibrium carrier densities expected in operating light-emitting diodes depends on the recombination of free carriers and the localized exciton-like electron-hole pairs and localization-delocalization processes. In addition, a fraction of the nonequilibrium carriers is captured by the deep capture centers with extremely long lifetimes. These carriers have an insignificant contribution to the band-to-band radiative recombination. This capture is an important factor in decreasing the emission efficiency.

Addendum to “Switching effect and the metal–insulator transition in electric field” by A.L. Pergament et al. [J. Phys. Chem. Solids 71 (2010) 874

In the paper mentioned above we reported on the switching mechanism in vanadium dioxide which was shown to be based on the electronically-induced Mott insulator-to-metal transition occurring in conditions of the non-equilibrium carrier density excess in the applied electric field, and the proposed model involved the dependence of the carrier density n on electric field (the Poole–Frenkel effect), as well as the dependence of the critical electric field on n. The data on the n(T) dependence were obtained on the assumption of a temperature-independent carrier mobility μ, and the problem of n reduction at lower temperatures was not fully understood. In this Letter we revisit this problem in the light of some recent data on the μ(T) dependence for VO2. It is shown that the adjusted values of n, taking into account this μ(T), correspond to the Mott critical density within an order of magnitude.

Growth mechanism and optical properties of InGaAs/GaAsSb Su-perlattice structures

In this paper, we report the growth of GaAsSb and its crystalline property under various Sb2/As2 flux ratios and growth temperatures. We simulated the incorporation difference between Sb2 and As2 by using a non-equilibrium thermodynamic model. Our study of GaAsSb growth has successfully yielded, high quality InGaAs/GaAsSb Type II superlattice for which the optical properties were characterized by photoluminescence at different excitation power and temperature. A blue-shift in luminescence peak energy with excitation power was observed and was described by a non-equilibrium carrier density model. We measured and analyzed the dependences of peak energy and integrated intensity on temperature. Two thermal processes were observed from intensity dependent photoluminescence measurements.

Low threshold for optical damage in AlGaN epilayers and heterostructures

Laser pulses with duration much shorter than the effective carrier lifetime cause permanent photoluminescence (PL) quenching and enhancement of PL decay rate in bare-faceted and capped AlGaN epilayers and multiple quantum wells at pulse energies about an order of magnitude lower than those causing the surface to melt and degrade. In contrast, GaN epilayers exhibit no photomodification in the same excitation intensity range. PL spectra and decay kinetics show that lattice heating is not responsible for the observed changes in AlGaN, which result from the formation of nonradiative recombination centers via recombination-enhanced defect reactions occurring at high nonequilibrium carrier densities.

Temperature- and excitation-dependent carrier diffusivity and recombination rate in 4H-SiC

Time-resolved optical ‘pump-probe’ techniques were applied to monitor carrier dynamics in CVD-grown 4H-SiC for investigation of carrier diffusivity and recombination rate over a wide temperature (80–800 K) and excitation (1016–1019 cm−3) range. The light-induced transient grating technique provided the dependence of ambipolar diffusion coefficient on nonequilibrium carrier density at various temperatures, attributable to bandgap renormalization at low excitation levels and to plasma degeneracy at high ones. The free carrier absorption technique allowed us to monitor carrier lifetimes at various excitation levels and revealed its essential decrease from microseconds to a few nanoseconds. The latter effect was attributed to phonon-assisted Auger recombination with a temperature-independent, unscreened coefficient, C0 = (5 ± 1) × 10−31 cm6 s−1, which was Coulomb-enhanced at low excitations (up to C300 K ∼ 10−30 cm6 s−1 at 1018 cm3) but gradually decreased with increasing excitation due to screening of the carrier–polar optical phonon interaction. The measurements provided an ambipolar diffusion length of ∼30 µm which was almost temperature independent at low excitations (up to ΔN = 1017 cm−3) but decreased to 1 µm at high excitations, mainly due to the impact of nonlinear recombination.

Interface recombination influence on carrier transport

A theory of interface recombination in the semiconductor–semiconductor junction is developed. The interface recombination rate dependence on the nonequilibrium carrier densities is derived on the basis of a model in which the interface recombination occurs through the mechanism of trapping. The general relation between the interface recombination parameters at small carrier density deviation from the equilibrium ones is obtained. The validity of this relation is proved considering the generation of the Hall electric field in the extrinsic semiconductor sample. The anomalous Hall electromotive force in a weak magnetic field was investigated and interpreted by means of a new interface recombination model. The experimental data corroborate the developed theory.

Effect of diffusion and fluctuations in the nonequilibrium charge carrier density on the angular distribution of the output intensity for injection lasers based on an InAsSb/InAsSbP heterostructure

We have used numerical modeling to study the effect of diffusion and fluctuations in the nonequilibrium carrier density in the active layer of injection lasers based on an InAsSb/InAsSbP heterostructure on the angular distribution of the output intensity. We show that diffusion smoothes out the nonequilibrium carrier distribution in the active layer, and the fundamental lasing mode is stable over a much broader range of stripe contact widths. At the same time, diffusional processes can lead to formation of local regions with a jump in the density of nonequilibrium charge carriers, fluctuations in which can act as a source of instability for the fundamental lasing mode. Analysis of the numerical modeling results gives qualitative agreement with experimental data on the dependence of the angular distribution of the output radiation for different stripe contact widths.

Analysis of the theory of the electron avalanche domain (EAD) in GaAs photoconductive semiconductor switches

An electron avalanche domain (EAD) theory for the filamentary current (streamer) formation and propagation in semi-insulating (SI) GaAs photoconductive semiconductor switches (PCSS) has been derived from the analysis of the experiments. The theory perfects the EAD concept and reveals that the steady increase of the non-equilibrium carrier density for the strong electric field and the high carrier density effect can lead to form the streamer in the local area of the high gain GaAs PCSS. The EAD theory is used successfully to explain the following experimental phenomena happened in the high gain GaAs PCSS which include the formation and propagation of the current filament, the occurrence of carrier avalanche in SI GaAs PCSS when the bias field across the device is below the threshold electric field in GaAs for intrinsic impact ionization. The results show that the EAD theory is a generalized theory to analyze the streamer formation in such transferred-electron effect semiconductor devices.

Ultraviolet light emitting diodes

The paper presents a review of the recent development of III-nitride based deep UV light emitting diodes (LEDs). Main applications of the deep UV LEDs are introduced. Review of material issues is focused on the lattice mismatch between the substrate and the active layer and at heterojunctions in multiple quantum well structures forming the active layer, the localization of nonequilibrium carriers, the material properties limiting the internal quantum efficiency, and the effect of efficiency droop at high density of nonequilibrium carriers. AlGaN is currently the semiconductor of choice for development of deep UV LEDs, so this material is the most discussed one in this review, though some information on AlInGaN is also provided. Keywords: light emitting diodes, ultraviolet light sources, III-nitride semiconductors, AlGaN

Switching effect and the metal–insulator transition in electric field

We measured S-shaped I–V characteristics of vanadium dioxide in a wide temperature range (15–340 K). The proposed switching mechanism in materials with metal–insulator transition (MIT) is based on an electronically induced MIT occurring in conditions of the non-equilibrium carrier density excess under the applied electric field. The mechanism is developed on the basis of a phenomenological approach, and this model not only allows the qualitative description of the switching mechanism, but it is in quantitative agreement with the experimental results, in particular, with those concerning the critical concentration and threshold field temperature dependence. The model takes into account the dependence of the carrier density on the electric field strength. The experimental results for vanadium dioxide are examined within the frameworks of the developed approach. Finally, the validity of the proposed model for other systems is discussed.

Luminescence study of ZnSe based scintillators in frequency domain

Photoluminescence of ZnSe-based scintillation crystals is studied. Photoluminescence intensities and spectra of isoelectronically doped ZnSe(Te), ZnSe(O), and ZnSe(O,Al) are compared. Frequency domain luminescence lifetime measurement technique was applied to study carrier dynamics at low density of nonequilibrium carriers, which is typical for operation of high-sensitivity ZnSe-based radiation detectors. Temperature-dependent competition between donor-acceptor-pair-type and exponential luminescence decay components was observed.