Electron-hole Recombination Energy Research Articles

Direct Assessment of Auger Recombination Rates of Charged Excitons via Opto-Electrical Measurements

Auger recombination (AR), whereby the electron–hole recombination energy is transferred to a third charge carrier, prevails in nanocrystal quantum dots (QDs) and governs the performance of QD-based devices including light-emitting diodes and lasers. Thus, precise AR evaluation of QDs is essential for understanding and improving the characteristics of such applications. So far, conventional charging approaches, such as the stir-versus-static method, photochemistry, or electrochemistry, have been able to assess the AR decay rate of either positively (two holes and one electron, X+) or negatively (one hole and two electrons, X–) charged excitons, and the decay dynamics of the other type of charged exciton is presumably estimated by the superposition principle of the biexciton Auger process. Herein, we demonstrate an opto-electrical method that enables us to precisely assess AR rates of X+ and X– in core/shell heterostructured QDs. Specifically, we devise electron-only devices and hole-only devices to inject extra charge carriers into QDs without unwanted side reactions or degradation of QDs and probe AR characteristics of these charged QDs via time-resolved photoluminescence measurements. We find that AR rates of charged excitons, both X+ and X–, gained from the present method agree well with those attained from conventional approaches and the superposition principle, corroborating the validity of the present approach. This present method permits us to comprehend multicarrier dynamics in QDs, prompting the use of QDs in light-emitting diodes and laser devices based on QDs.

Visible-light responsive PVDF/carbon sphere@TiO2 membrane for dye scavenging and bacteria inactivation

Titanium dioxide (TiO2) is wildly used for dye scavenging and bacteria inactivation in wastewater treatment. However, its photocatalytic efficiency is significantly limited by two inherent limitations including quick electron-hole pair recombination and wide bandgap energy (∼3.2 eV). In this work, we present a composite membrane that shows superior photocatalytic properties under visible light in dye scavenging and bacteria inactivation. The carbon sphere@TiO2 (CST) was manufactured via a two-step hydrothermal progress as a visible-light activated nano-photocatalyst and then electrospun with poly(vinylidene fluoride) (PVDF) to fabricate PVDF/CST composite membrane. When exposed to visible light, the PVDF/CST composite membrane performs well in terms of organic dye degradation. Furthermore, antibacterial activity assays show that the composite membrane has excellent sterilizing characteristics. Overall, this composite membrane appears to be a potential material for dye scavenging and bacterial inactivation in wastewater treatment.

Trap creation, trap conversion and thermoluminescence process in barite – effect of flux

ABSTRACT Among the undoped metal sulfates, on sintering at high temperatures, only BaSO4 was found to exhibit TL glow peaks in the region of radiation dosimetric interest i.e. ∼200°C (peak I) and 250°C (peak II). The TL intensity of peak I is very weak in purified BaSO4 but increased 4 times on 500°C, 1 h sintering. The intensity of peak II and the peak II/I height ratio increased with sintering temperature between 500°C and 1000°C. NaCl flux enhances the TL sensitivity of BaSO4 unlike that of Na2SO4. TL response of BaSO4 is in general linear below 1 kGy but unlike other phosphors, the TL response in the high dose region (> 1 kGy) does not saturate but increases sublinearly with dose (1–15 kGy) as a result of slow creation of traps by radiolysis. XRD shows that BaSO4 made with NaCl flux corresponding to a single phase of orthorhombic Barite crystal. The oxygen interstitial ions, are envisioned to be the hole traps and the radicals/anion vacancies formed by thermal dissociation of ions constitute the electron traps for peaks I and II, respectively. During TL, in the absence of dopants, the electron–hole recombination energy is observed under relaxed anion exciton emission in the 380–410 nm region. The phosphor is reusable after irradiation and TL readout. High dose irradiations, however, causes sensitization of TL after 400°C, 1 h post-irradiation anneal but a 700°C, 1 h anneal reduced the TL sensitivity. The creation of TL traps causing peak I precede that of peak II and a trap conversion between them occurs during high-temperature sintering. The study thus reveals for the first time that the defects causing TL peaks I and II are both thermally induced. Further studies to extend the saturation dose limit are proposed.

Effect of Interfacial Alloying versus "Volume Scaling" on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots.

Auger recombination is a nonradiative three-particle process wherein the electron-hole recombination energy dissipates as a kinetic energy of a third carrier. Auger decay is enhanced in quantum-dot (QD) forms of semiconductor materials compared to their bulk counterparts. Because this process is detrimental to many prospective applications of the QDs, the development of effective approaches for suppressing Auger recombination has been an important goal in the QD field. One such approach involves "smoothing" of the confinement potential, which suppresses the intraband transition involved in the dissipation of the electron-hole recombination energy. The present study evaluates the effect of increasing "smoothness" of the confinement potential on Auger decay employing a series of CdSe/CdS-based QDs wherein the core and the shell are separated by an intermediate layer of a CdSexS1-x alloy comprised of 1-5 sublayers with a radially tuned composition. As inferred from single-dot measurements, use of the five-step grading scheme allows for strong suppression of Auger decay for both biexcitons and charged excitons. Further, due to nearly identical emissivities of neutral and charged excitons, these QDs exhibit an interesting phenomenon of lifetime blinking for which random fluctuations of a photoluminescence lifetime occur for a nearly constant emission intensity.

Color Control of Pr3+ Luminescence by Electron-Hole Recombination Energy Transfer in CaTiO3 and CaZrO3.

Controlling luminescence in phosphors able to produce several emissions from different stable excited states determines their use in optical devices. We investigate the color control mechanism that quenches the greenish-blue emission in favor of the red one in the archetype phosphor CaTiO3:Pr3+. State-of-the-art ab initio calculations indicate that direct host-to-dopant energy transfer (released by electron-hole recombination following the interband excitation and structural reorganization) selectively populates the 1D2 red luminescent state of Pr3+ and bypasses the 3P0 greenish-blue emitter. Local defects can modulate the electron-hole recombination energy and therefore increase the red emission efficiency, as experimentally observed. The selection of red emission does not happen in CaZrO3:Pr3+ because the electron-hole recombination energy is much higher. The calculations could not support the widely accepted color control mechanism based on metal-to-metal charge transfer states. The conclusion sets new points of view for the color control of lanthanide activated inorganic phosphors.

Influence of Indium-Percentage Variation on Dynamical Characteristics of InxGa1-xAs/GaAs(001) Quantum Dot Lasers

The influence of indium percentage on dynamical characteristics of InxGa1-xAs/GaAs(001) quantum dot lasers (QDLs) is investigated. Energy levels of self-organized truncated-cone-shape QDs are calculated by means of the eight-band k.p model and their dependence to indium percentage is surveyed. Then, by presenting a four-level model and numerical solution of the resulting rate equations, laser properties are determined. Our results show that inclusion of more indium gives rise in the reduced bandgap and electron–hole recombination energy. Moreover, lasing for both Ground State (GS) and Excited States (ES) sounds to be sensitive to indium percentage. It is shown that rise of indium percentage at fixed injected current results in the increased ES turn-on delay and GS photon number and 3 dB modulation bandwidth, and decreased ES photon number, GS turn-on delay, amplitude of relaxation oscillations, output power, and ES 3 dB modulation bandwidth; but has no effect on threshold current and laser gain. At last, we find an optimized cavity length which was likely to be independent from indium percentage.

Auger Recombination in Self-Assembled Quantum Dots: Quenching and Broadening of the Charged Exciton Transition.

In quantum dots (QDs), the Auger recombination is a nonradiative process in which the electron-hole recombination energy is transferred to an additional carrier. It has been studied mostly in colloidal QDs, where the Auger recombination time is in the picosecond range and efficiently quenches the light emission. In self-assembled QDs, on the other hand, the influence of Auger recombination on the optical properties is in general neglected, assuming that it is masked by other processes such as spin and charge fluctuations. Here, we use time-resolved resonance fluorescence to analyze the Auger recombination and its influence on the optical properties of a single self-assembled QD. From excitation-power-dependent measurements, we find a long Auger recombination time of about 500 ns and a quenching of the trion transition by about 80%. Furthermore, we observe a broadening of the trion transition line width by up to a factor of 2. With a model based on rate equations, we are able to identify the interplay between tunneling and Auger rate as the underlying mechanism for the reduced intensity and the broadening of the line width. This demonstrates that self-assembled QDs can serve as an ideal model system to study how the charge recapture process, given by the band-structure surrounding the confined carriers, influences the Auger process. Our findings are not only relevant for improving the emission properties of colloidal QD-based emitters and dyes, which have recently entered the consumer market, but also of interest for more visionary applications, such as quantum information technologies, based on self-assembled quantum dots.

Two-dimensional energy transfer between self-assembled quantum dots modulated by an external electric field

Energy transfer processes between InAs/GaAs self-assembled quantum dots within an intrinsic region of a p-i-n device have been studied. Three emission peaks associated with quantum dots (QDs) energy states could be identified in the photoluminescence (PL) spectrum in such a way that the energy transfer between lateral QDs could be individualized for each state. Using the spatially resolved micro-PL technique, several scans have been performed in the whole range of QDs emissions at different values of direct applied bias Vb. Curves were fitted according to the diffusion theory, and the parameters associated with the energy transfer (L) were plotted as a function of Vb and the QD recombination energy. We found that the energy transfer between QDs is faster for lower energy states and increases, with different rates, for each QD energy state by increasing the direct applied bias. The energy transfer process has been associated with the carrier diffusion within the barriers. We also found that the energy transfer between QDs decreases linearly with the electron-hole recombination energy. From this result, we conclude that the energy transfer phenomenon occurs following a Gaussian distribution of diffusing carriers, in which the concentrations of electron-hole pairs far away from the origin are just enough to fill the lower energy states of QDs.

Radiative properties of carriers in CdSe-CdS core-shell heterostructured nanocrystals of various geometries

We report a model on core-shell heterostructured nanocrystals with CdSe as the core and CdS as the shell. The model is based on one-band Schrödinger equation. Three different geometries, nanodot, nanorod, and nanobone, are implemented. The carrier localization regimes with these structures are simulated, compared, and analyzed. Based on the electron and hole wave functions, the carrier overlap integral that has a great impact on stimulated emission is further investigated numerically by a novel approach. Furthermore, the relation between the nanocrystal size and electron-hole recombination energy is also examined.

Electro-optical characterization of Ti/Au–ZnTe Schottky diodes with CdTe quantum dots

In this study the electric and optical spectroscopy techniques have been applied to investigate ZnTe (p-type)–Ti/Au Schottky diodes containing a layer of CdTe self-assembled quantum dots (SAQDs). The reference sample was the ZnTe–Ti/Au diode without dots. Both samples were grown by molecular beam epitaxy technique. Raman measurements confirmed the presence of the CdTe layer while the photoluminescence proved that CdTe quantum dots were formed in the investigated structure. The PL spectra reveal the CdTe QD electron-hole recombination energy equal to 2.1eV at 10K. Based on the temperature PL measurements the activation energy of PL quenching has been determined to be equal to 22meV. Further confirmation for the QD formation has been obtained from the C–V characteristics which exhibited a step related to the charge accumulation at the QD states. DLTS spectra for the sample with QDs yield three hole-related signals with apparent activation energies equal to EH1=0.16eV, EH2=0.2eV and EH3=0.4eV. For the reference ZnTe–Ti/Au diode solely single signal was observed of signature close to the level H3 in the QD sample. Detailed characterization of the traps as well as the PL studies lead to the conclusion that the level H2 is related to the defects located close to the QDs created during the growth while the other traps are associated with defects present in the ZnTe bulk material.

Suppressed Auger Recombination in “Giant” Nanocrystals Boosts Optical Gain Performance

Many potential applications of semiconductor nanocrystals are hindered by nonradiative Auger recombination wherein the electron-hole (exciton) recombination energy is transferred to a third charge carrier. This process severely limits the lifetime and bandwidth of optical gain, leads to large nonradiative losses in light-emitting diodes and photovoltaic cells, and is believed to be responsible for intermittency ("blinking") of emission from single nanocrystals. The development of nanostructures in which Auger recombination is suppressed has recently been the subject of much research in the colloidal nanocrystal field. Here, we provide direct experimental evidence that so-called "giant" nanocrystals consisting of a small CdSe core and a thick CdS shell exhibit a significant (orders of magnitude) suppression of Auger decay rates. As a consequence, even multiexcitons of a very high order exhibit significant emission efficiencies, which allows us to demonstrate optical amplification with an extraordinarily large bandwidth (>500 meV) and record low excitation thresholds. This demonstration represents an important milestone toward practical lasing technologies utilizing solution-processable colloidal nanoparticles.

Recombination-Induced Stacking Faults: Evidence for a General Mechanism in Hexagonal SiC

We report on optically induced nucleation and expansion of stacking faults in hexagonal SiC structures. The activation energy for partial dislocation glide under optical excitation is found to reduce to 0.25 +/- 0.05 eV, which is about 2 eV lower than for pure thermal activation. From the measurements of thermal activation and below-gap excitation spectroscopy of dislocation glide, we conclude that the elementary process controlling expansion of stacking faults is kink pair nucleation aided by the phonon-kick mechanism. We propose that solitons on 30 degrees Si(g) partials with a silicon core act as deep 2.4 eV + Ev trap sites, readily providing electron-hole recombination energy to enhance the motion of dislocations. Our results suggest that this is a general mechanism of structural degradation in hexagonal SiC.

Degradation of hexagonal silicon-carbide-based bipolar devices

Only a few years ago, an account of degradation of silicon carbide high-voltage p-i-n diodes was presented at the European Conference on Silicon Carbide and Related Compounds (Kloster Banz, Germany, 2000). This report was followed by the intense effort of multiple groups utilizing varied approaches and subsequent progress in both fundamental understanding of this phenomenon and its elimination. The degradation of SiC p-i-n junctions is now well documented to be due to the expansion of Shockley-type stacking faults in the part of the devices reached by the electron-hole plasma. The faults can gradually cover most of the junction area, impeding current flow and, as a result, increasing the on-state resistance. While in most semiconductors stacking faults are electrically inactive, in hexagonal silicon carbide polytypes (4H- and 6H-SiC) they form quantum-well-like electron states observed in luminescence and confirmed by first-principles calculations. The stacking-fault expansion occurs via motion of 30° silicon-core partial dislocations. The Si–Si bond along the dislocation line induces a deep level in the SiC band gap. This state serves as both a radiative and a nonradiative recombination center and converts the electron-hole recombination energy into activation energy for the dislocation motion. Dislocation motion is typically caused by shear stress, but in the case of SiC diodes, the driving force appears to be intrinsic to the material or to the fault itself, i.e., the fault expansion appears to lower the energy of the system. Stable devices can be fabricated by eliminating stacking-fault nucleation sites. The dominant type of such preexisting defects is the segment of basal plane dislocations dissociated into partials. The density of such defects can be reduced to below 1cm−2 by conversion of all basal plane dislocations propagating from the substrate into threading ones in the epitaxial layer. Remarkable progress in fabrication of low basal plane dislocation density material offers hope of bipolar SiC devices being available commercially in the near future.

Detection of directed electron-hole recombination energy transfer from an ionic crystal matrix to self-assembled nanocrystals

It is found that the energy released in the spin-dependent tunneling recombination of electron-hole pairs and self-trapped excitons in an ionic crystal matrix is directionally transferred to low-dimensional semiconductor structures embedded in the matrix as a result of self-assembled growth. The EPR spectra of electron and hole centers in the matrix crystal are detected by tunneling afterglow and photostimulated luminescence that are excited in the low-dimensional structure.

Self-consistent calculations of exciton, biexciton and charged exciton energies in InGaN/GaN quantum dots

We present theoretical calculations of the variation of exciton energies in truncated conical InGaN quantum dots (QDs) in a GaN matrix with dot size and indium composition. We compute the built-in strain-induced and spontaneous piezoelectric fields using a surface integral method that we have recently derived, and confirm that the built-in fields can be of the order of a few MV/cm, resulting in a spatial separation of the electrons and holes. The ground state wavefunctions of the exciton ( X 0 ) , biexciton ( 2 X 0 ) and the two charged excitons ( X − and X + ) are then calculated in the Hartree approximation, using a self-consistent finite difference method. We find that the electron–hole recombination energy is always blue-shifted for the charged excitons X − and X + , with a further blue-shift for the biexciton, and this blue-shift increases with increasing indium content. We describe the trends in interband transition energy and the scale of the blue-shift with dot size, shape and composition. We conclude that spectroscopic studies of the exciton, charged excitons and biexciton should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.

Role of Mn as a co-dopant in [formula omitted]:Mn,Pr TL phosphor

CaSO 4 :Pr, CaSO 4 :Mn and CaSO 4 :Mn,Pr thermoluminescence (TL) phosphors have been prepared and their TL glow curves and TL emission spectra are recorded. TL emission bands at 360, 440, 490 and 610 nm were observed for CaSO 4 :Pr in the scanned range. Co-doping of Mn in CaSO 4 :Pr enhances its TL sensitivity. TL sensitivity of CaSO 4 :Mn,Pr (0.5, 0.1 mol%) is about 47 times that of CaSO 4 :Pr (0.1 mol%). TL emission band at 500 nm was observed for CaSO 4 doped with Mn. TL emission of 220 °C TL peak in CaSO 4 :Mn,Pr consists predominantly of 500 nm band. The presence of Mn 2 + emission in 220 °C peak in CaSO 4 :Mn,Pr is because of transfer of electron-hole recombination energy to Mn 2 + ions during TL readout. The presence of weak Pr 3 + emission, along with Mn 2 + emission, indicates that Pr 3 + ions compete in absorbing the electron–hole recombination energy.

Surface integral determination of built-in electric fields and analysis of exciton binding energies in nitride-based quantum dots

We present a simple analytical approach to calculate the built-in strain-induced and spontaneous piezoelectric fields in nitride-based quantum dots (QDs) and then apply the method to describe the variation of exciton, biexciton and charged exciton energy with dot size in GaN/AlN QDs. We first present the piezoelectric potential in terms of a surface integral over the QD surface, and confirm that, due to the strong built-in electric field, the electrons are localised near the QD top and the holes are localised in the wetting layer just below the dot. The strong localisation and smaller dielectric constant results in much larger Coulomb interactions in GaN/AlN QDs than in typical InAs/GaAs QDs, with the interaction between two electrons, J ee, or two holes, J hh, being about a factor of three larger. The electron–hole recombination energy is always blue shifted in the charged excitons, X − and X +, and the biexciton, and the blue shift increases with increasing dot height. We conclude that spectroscopic studies of the excitonic complexes should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.

A Novel Technique for Probe Intensity Profile Characterisation in the Environmental Scanning Electron Microscope

The environmental scanning electron microscope (ESEM) allows imaging and analysis in a gaseous environment at pressures up to 20 torr. Gas phase scattering of primary electrons leads to a loss of useable probe current and produces an electron skirt around the beam that can degrade the spatial resolution in microanalytical studies. Progress in ESEM microanalysis requires accurate modeling and measurement of the beam intensity profile. Previously, x-ray techniques and beam sensitive materials have been used to measure the extent of the skirt. These investigations have yielded inconsistent skirt distances possibly due to dose dependence of the measurement techniques. We present a new, high gain method to measure the useable probe current and skirt profile using the electron beam induced current (EBIC) from a P-N junction.The interface of an N- and a P-type semiconductor contains an electrostatic donor-acceptor dipole (“depletion layer”) created by an electron-hole recombination energy minimization mechanism.

Dynamic process of anti-Stokes photoluminescence at a long-range-orderedGa0.5In0.5P/GaAsheterointerface

Efficient anti-Stokes photoluminescence (PL) has been observed in ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ and GaAs single heterostructure. PL of ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ was observed when photoexciting the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ interface. The anti-Stokes PL intensity from the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ layer is about 1% of the GaAs PL. The observed anti-Stokes PL exhibits a characteristic intensity dependence in the double-logarithmic plot, which reveals two straight lines having slopes of about 2 and 4. Time-resolved measurements show that the anti-Stokes PL is described by two components: the rapid decay caused by the two-step two-photon absorption of the excitation laser light and the slower decay by energy transfer of electron-hole recombination energy in the GaAs. We found that many-particle effects in the GaAs under high excitations cause the characteristic intensity dependence of the anti-Stokes PL.

Effect of interfacial states on the binding energies of electrons and holes in InAs/GaAs quantum dots

The interface between an InAs quantum dot and its GaAs cap in ``self-assembled'' nanostructures is nonhomogeneously strained. We show that this strain can lead to localization of a GaAs-derived ${X}_{1c}$-type interfacial electron state. As hydrostatic pressure is applied, this state in the GaAs barrier turns into the conduction-band minimum of the InAs/GaAs dot system. Strain splits the degeneracy of this ${X}_{1c}$ state and is predicted to cause electrons to localize in the GaAs barrier above the pyramidal tip. Calculation (present work) or measurement (Itskevich et al.) of the emission energy from this state to the hole state can provide the hole binding energy, ${\ensuremath{\Delta}}_{\mathrm{dot}}^{(h)}.$ Combining this with the zero-pressure electron-hole recombination energy gives the electron binding energy, ${\ensuremath{\Delta}}_{\mathrm{dot}}^{(e)}.$ Our calculations show ${\ensuremath{\Delta}}_{\mathrm{dot}}^{(h)}\ensuremath{\sim}270\mathrm{meV}$ (weakly pressure dependent) and ${\ensuremath{\Delta}}_{\mathrm{dot}}^{(e)}\ensuremath{\sim}100\mathrm{meV}$ at $P=0.$ The measured values are ${\ensuremath{\Delta}}_{\mathrm{dot}}^{(h)}\ensuremath{\sim}235\mathrm{meV}$ (weakly pressure dependent) and ${\ensuremath{\Delta}}_{\mathrm{dot}}^{(e)}\ensuremath{\sim}50\mathrm{meV}$ at $P=0.$ We examine the discrepancy between these values in the light of wave-function localization and the pressure dependence of the hole binding energy.