High Injection Density Research Articles

Exciton-Exciton Interactions in Van der Waals Heterobilayers

Exciton-exciton interactions are key to understanding nonlinear optical and transport phenomena in van der Waals heterobilayers, which emerged as versatile platforms to study correlated electronic states. We present a combined theory-experiment study of excitonic many-body effects based on first-principle band structures and Coulomb interaction matrix elements. Key to our approach is the explicit treatment of the fermionic substructure of excitons and dynamical screening effects for density-induced energy renormalization and dissipation. We demonstrate that dipolar blueshifts are almost perfectly compensated by many-body effects, mainly by screening-induced self-energy corrections. Moreover, we identify a crossover between attractive and repulsive behavior at elevated exciton densities. Theoretical findings are supported by experimental studies of spectrally narrow, mobile interlayer excitons in atomically reconstructed, h-BN-encapsulated MoSe2/WSe2 heterobilayers. Both theory and experiment show energy renormalization on a scale of a few meV even for high injection densities in the vicinity of the Mott transition. Our results revise the established picture of dipolar repulsion dominating exciton-exciton interactions in van der Waals heterostructures and open up opportunities for their external design. Published by the American Physical Society 2024

Contactless quasi-steady-state photoconductance (QSSPC) characterization of metal halide perovskite thin films

We apply the contactless quasi-steady-state photoconductance (QSSPC) method to co-evaporated methyl ammonium lead iodide (MAPbI3) perovskite thin-films. Using an adapted calibration for ultralow photoconductances, we extract the injection-dependent carrier lifetime of the MAPbI3 layer. The lifetime is found to be limited by radiative recombination at the high injection densities applied during the QSSPC measurement, enabling the extraction of the electron and hole mobility sum in the MAPbI3 using the known coefficient of radiative recombination of MAPbI3. Combining the QSSPC measurement with transient photoluminescence measurements, performed at much lower injection densities, we obtain the injection-dependent lifetime curve over several orders of magnitude. From the resulting lifetime curve, we determine the achievable open-circuit voltage of the examined MAPbI3 layer.

Influences of point defects on electrical and optical properties of InGaN light-emitting diodes at cryogenic temperature

We investigate the cryogenic external quantum efficiency (EQE) for some InGaN light-emitting diodes with different indium contents. We observe a monotonic decrease in EQE with the increasing forward current before the “U-turn” point, beyond which the thermal effect increases the EQE. We discover positive dependences among the droop rate (χ), differential electrical resistance (Rd), and indium content. Also, χ and Rd of individual green samples shift correspondingly during the aging test, when the Mg ions are activated at high injection density and diffuse into the active region. Considering the fact that both In and Mg ions would introduce point defects (PDs), we proposed a model that reveals the mechanism of interplay between PDs and carriers. PDs serve as both energy traps and non-radiative recombination centers. They attract and confine carriers, leading to an increase in Rd and a decrease in EQE.

Carrier-density-dependent recombination dynamics of excitons and electron-hole plasma in m -plane InGaN/GaN quantum wells

We study the carrier-density-dependent recombination dynamics in $m$-plane InGaN/GaN multiple quantum wells in the presence of $n$-type background doping by time-resolved photoluminescence. Based on Fermi's golden rule and Saha's equation, we decompose the radiative recombination channel into an excitonic and an electron-hole pair contribution, and extract the injected carrier-density-dependent bimolecular recombination coefficients. Contrary to the standard electron-hole picture, our results confirm the strong influence of excitons even at room temperature. Indeed, at 300 K, excitons represent up to 63 \ifmmode\pm\else\textpm\fi{} 6% of the photoexcited carriers. In addition, following the Shockley-Read-Hall model, we extract the electron and hole capture rates by deep levels and demonstrate that the increase in the effective lifetime with injected carrier density is due to asymmetric capture rates in presence of an $n$-type background doping. Thanks to the proper determination of the density-dependent recombination coefficients up to high injection densities, our method provides a way to evaluate the importance of Auger recombination.

High-speed flow of interacting organic polaritons

The strong coupling of an excitonic transition with an electromagnetic mode results in composite quasi-particles called exciton polaritons, which have been shown to combine the best properties of their individual components in semiconductor microcavities. However, the physics and applications of polariton flows in organic materials and at room temperature are still unexplored because of the poor photon confinement in such structures. Here, we demonstrate that polaritons formed by the hybridization of organic excitons with a Bloch surface wave are able to propagate for hundreds of microns showing remarkable third-order nonlinear interactions upon high injection density. These findings pave the way for the study of organic nonlinear light–matter fluxes and for a technologically promising route of the realization of dissipation-less on-chip polariton devices operating at room temperature.

Microscopic charge carrier lifetime in silicon from a transient approach

We present an experimental approach to determine the charge carrier lifetime in silicon based on the measured transient decay of the emitted photoluminescence intensity, requiring only a crystal volume of 50 μm in diameter. This becomes feasible by a combination of the time correlated single photon counting technique and confocal microscopy. Using combined pulsed and pulse train laser excitation, we obtain a self-consistent charge carrier lifetime in a high dynamic range from 100 ns to ms and an injection range from 1010 cm−3 to high injection densities. An iterative data evaluation routine incorporates all effects induced by the spatially non-homogeneous charge carrier generation.

Defects dynamics during ageing cycles of InGaN blue light-emitting diodes revealed by evolution of external quantum efficiency--current dependence.

We report in detail the defect dynamics in the active region by monitoring the external quantum efficiency (EQE) - injection current curves, I-V curves, and electroluminescence spectra during the ageing test, under a forward current of 850 mA (85 A/cm2), room temperature. We apply a two-level model to analyze the EQE curves and the electroluminescence spectra. The results suggest that high injection density during the ageing may reduce the density of the Shockley-Reed-Hall nonradiative recombination centers and enhance the carrier mobility and diffusion length. The former effect would directly lead to initial surge of EQE, whereas the latter would enhance the effect of extended defects which leads to reduction in peak EQE and increase in EQE droop rate.

Carrier-diffusion corrected J0-analysis of charge carrier lifetime measurements for increased consistency

We investigate the origin of the apparently reduced recombination parameter J0 of highly doped regions obtained from e.g. QSSPC lifetime measurements at high injection densities as well as the influence of recombination at bulk defects on the recently updated J0-analysis of QSSPC measurements in the Sinton lifetime tester software. Using the example of crystalline silicon, we show that the frequently observed reduction and underestimation of J0, which is strongly pronounced at high injection levels, originate from neglecting the finite carrier diffusion coefficient. This work presents an analysis taking into account the finite carrier diffusion coefficient and thus enabling to evaluate charge carrier lifetime data at high injection levels leading to strongly increased consistency of the determined J0. Furthermore, we give a simple correction term for the recently published analysis leading to a total independence of a reasonably injection-independent Shockley–Read–Hall (SRH)-lifetime in the substrate. Combining the proposed methods, the determination of J0 from numerically simulated lifetime-data is accurate within 3% of the input J0=100fA/cm2 over an injection range of 1014–3·1016cm−3. The application of the proposed method to measured data of crystalline silicon devices is in excellent agreement with the simulations and shows a strongly improved linearity and thus constant extracted J0 compared to the former and recently published methods, which underestimate J0 at high injection levels, nearly independently from doping type and -level of the substrate. The new and simple method is implemented in a spreadsheet calculator enabling fast and robust data evaluation similar to the Sinton lifetime tester software.

Interaction of a galactic wind with halo gas and the origin of multiphase extraplanar material

We study the interaction of galactic wind with hot halo gas using hydrodynamical simulations. We find that the outcome of this interaction depends crucially on the wind injection density and velocity. Various phases of the extraplanar media such as high velocity clouds (HVCs), outflowing clouds, and OVI regions can originate in the interaction zones of wind with the halo gas, depending on the injection velocity and density. In our simulations the size of the HVCs is of the order of 100 pc. The total mass contained in the clouds is 10^5 -- 10^7 M_sun and they have a normal distribution of velocities in the galactic standard of rest frame, similar to HVCs. For high injection density and velocity, a significant number of clouds move outwards and resemble the case of cold neutral outflows. Furthermore a 10^5 -- 10^6 K phase is formed in our simulations which has a column density ~ 10^{18} cm^{-2}, and resembles the observed OVI regions. The injection velocity and density are linked with the mass loading factor of the outflow, efficiency of energy injection due to supernovae and the SFR. Comparison of the predicted morphology of extraplanar gas with observations can serve as a useful diagnostic for constraining feedback efficiency of outflows.

Numerical simulation of effect of vessel bifurcation on heat transfer in the magnetic fluid hyperthermia

This research aimed to analyze the mass and heat transfer mechanisms in magnetic fluid hyperthermia (MFH) treatment, revealing the effect of blood flow in a blood vessel bifurcation on the accurate spatial control of the thermal dose. A three-dimensional multiphysical model was developed to obtain the blood flow velocity distribution, concentration distribution of magnetic fluid, and temperature distribution of the treated tumor tissues. The calculated results demonstrate that the structure, size, and position of a bifurcation vessel greatly affect the selection of injection parameters for MFH treatment. The injection parameters considered in this study are the concentration of magnetic fluid, injection volume, arrangement of injections within the targeted tissues, and distance between the injection site and bifurcation. Diffuse injection patterns, large volumes, and low concentrations generally decrease the temperature differences within the tissues. To achieve uniform heating, high injection density and high-concentration magnetic fluids may be applied to the area near the vessel in order to reduce the cooling effect of blood flow. However, a more diffuse injection pattern is advantageous if the distance between the injection site and blood vessel is relatively short for the purpose of eliminating the heating effect of magnetic fluids.

Efficiency of GaN/InGaN double-heterojunction photovoltaic cells under concentrated illumination

In this study, GaN/InGaN double-heterojunction (DH) photovoltaic (PV) devices were fabricated and their theoretical PV properties were simulated. When the characteristics of PV devices were compared between illumination conditions of one sun and 120 suns, the PV properties – open-circuit voltage, short-circuit current density, and filling factor – measured in fabricated GaN/InGaN PV devices improved more than those calculated from the simulation. This result could be tentatively attributed to the trap-filling effect under a high injection density of photo-generated carriers, which would relatively reduce the series resistance and increase the shunt resistance. The improvements in the conversion efficiencies of the GaN/InGaN PV devices between illumination conditions of one sun and 120 suns were 4.6% and 70.8% for simulated and measured conversion efficiencies, respectively. The differences between the simulated results and measurements of actual GaN/InGaN PV devices are consistent.

Coulomb-enhanced Auger recombination in crystalline silicon at intermediate and high injection densities

Band-to-band Auger recombination in crystalline silicon is investigated by means of quasi-steady-state photoconductance measurements under intermediate- and high-injection conditions. The measurements reveal that the correlation of the charge carriers via Coulombic interaction has to be taken into account to accurately model the observed injection level and doping concentration dependence of the Auger recombination lifetime. A semi-empirical Coulomb-enhanced Auger recombination model is applied in order to simulate the experimental results theoretically.

Optical response of conjugated polymers excited at high intensity

The realisation of optically pumped lasing devices using poly( p-phenylenevinylene) has intensified interest in the optical response of conjugated polymers on ultra-short time-scales and at high optical injection density. We describe an investigation of these properties using time-resolved and time-integrated techniques. We demonstrate the importance of exciton-exciton annihilation at high injection densities and present evidence that this process produces photo-induced absorption (PA) through the generation of charges. We present a model which accounts for PA from both photoexcited singlet excitons and the charge pairs generated by annihilation. This model describes the time-evolution of the PA well and indicates that, in the presence of exciton-exciton annihilation, dissimilar stimulated emission and PA dynamics are consistent with a high quantum yield of excitons as deduced from PL lifetime and efficiency studies.

Comments on lasing and many-exciton physics in strongly confined systems

The stimulated emission of strongly confined heterostructures, namely highly polar II-VI quantum wells, is discussed in terms of a phase transition between the exciton gas and the free-carrier gas. The extent to which the transition has proceeded under the operation condition of the device (high temperature and high injection density) depends on the many-body renormalization of the exciton binding energy. The general applications of the model are finally discussed.

Avalanche-injected electron currents in SiO 2 at high injection densities : J. F. Verwey and B. J. de Maagt. Solid State Electron.17, 963 (1974)

Avalanche-injected electron currents in SiO 2 at high injection densities : J. F. Verwey and B. J. de Maagt. Solid State Electron.17, 963 (1974)

Avalanche-injected electron currents in SiO 2 at high injection densities

Gated P +- N diodes in silicon were used to investigate the avalanche-injection of electrons into SiO 2 at high current densities. The acceptor concentration in the P +-region of the diodes was high in order to minimize surface depletion there by the relatively high positive gate voltage during the injection. Injection current densities up to 10 A/cm 2 were obtained. The oxide current vs. gate voltage characteristics are described quantitatively by a simple model in which the current is injection-limited. Hot electrons from the avalanche plasma in the junction lose energy by inelastic collisions and gain energy by drift in the surface potential. Both processes are assumed to occur in a layer of thickness equal to the surface depletion layer thickness in the P +-region. The temperature dependence of the injection current is ascribed to the temperature dependence of the energy loss term.

Small signal equivalent circuit of unsymmetrical junction diodes at high current densities

Finite difference equation methods and lumped models are applied to the solution of the differential equations for the carrier flow through a quasi-neutral semiconductor bulk region at low, moderate and high injection densities. An unsymmetrically doped diode is approximated by a single section lumped model. The static I-V characteristics for this model at low and high current densities are derived. The small signal equivalent circuit of an unsymmetrically doped diode at high injection densities is presented. Approximations for the elements ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R, L, C</tex> ) of this equivalent circuit are calculated from the single section lumped model. The static characteristic and the small signal equivalent circuit elements are in satisfactory agreements with experimental results for diodes in which the base length is not too long compared to the diffusion length.