Having already achieved near-unity quantum yields, with promising properties for light-emitting diode, lasing, and charge separation applications, colloidal core/shell quantum dots have great technological potential. The shell thickness and band alignment of the shell and core materials are known to influence the efficiency of these devices. In many such applications, a key to improving the efficiency requires a deep understanding of multiexcitonic states. Herein, we elucidate the shell thickness and band alignment dependencies of the biexciton Auger recombination lifetime for quasi-type-II CdSe/CdS and type-I CdSe/ZnS core/shell quantum dots. We find that the biexciton Auger recombination lifetime increases with the total nanocrystal volume for quasi-type-II CdSe/CdS core/shell quantum dots and is independent of the shell thickness for type-I CdSe/ZnS core/shell quantum dots. In order to perform these calculations and compute Auger recombination lifetimes, we developed a low-scaling approach based on the stochastic resolution of identity. The numerical approach provided a framework to study the scaling of the biexciton Auger recombination lifetimes in terms of the shell thickness dependencies of the exciton radii, Coulomb couplings, and density of final states in quasi-type-II CdSe/CdS and type-I CdSe/ZnS core/shell quantum dots.
The Auger recombination in bulk semiconductors can depopulate the charge carriers in a non-radiative way, which, fortunately, only has detrimental impact on optoelectronic device performance under the condition of high carrier density because the restriction arising from concurrent momentum and energy conservation limits the Auger rate. Here, we surprisingly found that the Auger recombination in bulk Fe2O3 films was more efficient than narrow-bandgap high-mobility semiconductors that were supposed to have much higher Auger rate constants than metal oxides. The Auger process in Fe2O3 was ascrib…
Auger recombination (AR) of the ground biexciton state in quantum-confined lead salt nanowires (NWs) with a strong coupling between the conduction and the valence bands is shown to be strongly suppressed, and only excited biexciton states contribute to Auger decay. The AR rate is predicted to be greatly reduced when temperature or the NW radius are decreased, and the effect is explained by decrease in both the population of excited biexciton states and overlap of phonon-broadened single- and biexciton states. Suppression of AR of multiexciton states exhibiting strong radiative decay makes obvi…
The fast nonradiative decay of multiexcitonic states via Auger recombination is a fundamental process affecting a variety of applications based on semiconductor nanostructures. From a theoretical perspective, the description of Auger recombination in confined semiconductor nanostructures is a challenging task due to the large number of valance electrons and exponentially growing number of excited excitonic and biexcitonic states that are coupled by the Coulomb interaction. These challenges have restricted the treatment of Auger recombination to simple, noninteracting electron-hole models. Here…
The principal mechanisms of Auger recombination of nonequilibrium carriers in semiconductor heterostructures with quantum wells are investigated. It is shown for the first time that there exist three fundamentally different Auger recombination mechanisms of (i) thresholdless, (ii) quasi-threshold, and (iii) threshold types. The rate of the thresholdless Auger process depends on temperature only slightly. The rate of the quasi-threshold Auger process depends on temperature exponentially. However, its threshold energy essentially varies with quantum well width and is close to zero for narrow qua…
Auger recombination is an important non-radiative carrier recombination mechanism in many classes of optoelectronic devices. The microscopic Auger processes can be either direct or indirect, mediated by an additional scattering mechanism such as the electron-phonon interaction. Phonon-assisted Auger recombination is particularly strong in nitride materials and affects the efficiency of nitride optoelectronic devices at high powers. Here we present a first-principles computational formalism for the study of direct and indirect Auger recombination in direct-band-gap semiconductors and apply it t…
We report Auger recombination rates for wurtzite InGaN calculated from first principles density-functional and many-body-perturbation theory. Two different mechanisms are examined -- inter- and intra-band recombination -- that affect different parts of the emission spectrum. In the blue to green spectral region and at room temperature the Auger coefficient can be as large as 2x10^-30cm^6s^-1; in the infrared even larger. Since Auger recombination scales with the cubic power of the free-carrier concentration it becomes an important non-radiative loss mechanism at high current densities. Our res…
A growing interest in colloidal quantum dot (QD) based light-emitting diodes (QD-LEDs) has been motivated by the exceptional color purity and spectral tunability of QD emission as well as the amenability of QD materials to highly scalable and inexpensive solution processing. One current challenge in the QD-LED field has been a still incomplete understanding of the role of extrinsic factors (e.g., recombination via QD surface defects) versus intrinsic processes such as multicarrier Auger recombination or electron-hole separation due to applied electric field in defining device efficiency. Here,…
Influence of electron-electron relaxation processes on Auger recombination rate in direct band semiconductors is investigated. Comparison between carrier-carrier and carrier-phonon relaxation processes is provided. It is shown that relaxation processes are essential if the free path length of carriers doesn't exceed a certain critical value, which exponentially increases with temperature. For illustration of obtained results a typical InGaAsP compound is used.
The technologically important problem of predicting Auger recombination lifetimes in semiconductors is addressed by means of a fully first--principles formalism. The calculations employ highly precise energy bands and wave functions provided by the full--potential linearized augmented plane wave (FLAPW) code based on the screened exchange local density approximation. The minority carrier Auger lifetime is determined by two closely related approaches: \emph{i}) a direct evaluation of the Auger rates within Fermi's Golden Rule, and \emph{ii}) an indirect evaluation, based on a detailed balance f…
Auger recombination (AR) being electron-hole annihilation with energy-momentum transfer to another carrier is believed to speed up in materials with small band gap. We theoretically show that this rule is violated in gapless three-dimensional materials with ultra-relativistic electron-hole dispersion, Weyl semimetals (WSM). Namely, AR is prohibited by energy-momentum conservation laws in prototypical WSM with a single Weyl node, even in the presence of anisotropy and tilt. In real multi-node WSM, the geometric dissimilarity of nodal dispersions enables weak inter-node AR, which is further supp…
Over the last two decades, intensive research efforts have been devoted to the suppressions of photoluminescence (PL) blinking and Auger recombination in metal-chalcogenide nanocrystals (NCs), with significant progresses being made only very recently in several specific heterostructures. Here we show that nonblinking PL is readily available in the newly-synthesized perovskite CsPbI3 (cesium lead iodide) NCs, and their Auger recombination of charged excitons is greatly slowed down, as signified by a PL lifetime about twice shorter than that of neutral excitons. Moreover, spectral diffusion is c…
We describe theoretically novel excitons in self-assembled quantum dots interacting with a two-dimensional electron gas in the wetting layer. In the presence of the Fermi sea, the optical lines become strongly voltage-dependent. If the electron spin is nonzero, the width of optical lines is given by k_BT_K, where T_K is Kondo temperature. If the spin is zero, the exciton couples with the continuum due to Auger-like processes. This leads to anticrossings in a magnetic field. Some of the described phenomena are observed in recent experiments.
From a systematic study of the threshold current density as a function of temperature and hydrostatic pressure, in conjunction with theoretical analysis of the gain and threshold carrier density, we have determined the wavelength dependence of the Auger recombination coefficients in InGaAsSb/GaSb quantum well lasers emitting in the 1.7-3.2 $μ$m wavelength range. From hydrostatic pressure measurements, the non-radiative component of threshold currents for individual lasers was determined continuously as a function of wavelength. The results are analysed to determine the Auger coefficients quant…
For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, Auger recombination (AR) and carrier multiplication (CM) lifetimes are computed exactly in a three-dimensional real space grid using empirical pseudopotential wave functions. Our results in support of recent experimental data offer new predictions. We extract simple Auger constants valid for NCs. We show that both Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band gap energy, the electrons gain lion's share from the total excess e…
In a semiconductor heterostructure with type II energy band alignment, the spatial separation between electrons and holes slows down their radiative recombination. With increasing excitation intensity, Auger recombination quickly becomes the dominate recombination channel, and it produces carrier ejection from the quantum well. Here, we show that in Si/SiGe three-dimensional nanostructures, this efficient process facilitates the formation of an electron-hole plasma (EHP) and/or electron-hole droplets (EHDs) in thin Si barriers separating SiGe clusters. In contrast to conventional, strongly tem…
Previous single-particle spectroscopic studies of colloidal quantum dots have indicated a significant spread in biexciton lifetimes across an ensemble of nominally identical nanocrystals. It has been speculated that in addition to dot-to-dot variation in physical dimensions, this spread is contributed to by variations in the structure of the quantum dot interface, which controls the shape of the confinement potential. Here we directly evaluate the effect of the composition of the core-shell interface on single- and multi-exciton dynamics via side-by-side measurements of individual core-shell C…
Fulfilling the potential of the colloidal semiconductor quantum dots (QDs) in electrically driven applications remains a challenge largely since operation of such devices involves charged QDs with drastically different photo-physical properties compared to their well-studied neutral counterparts. In this work, the full picture of excited state dynamics in charged CdSe QDs at various time-scales has been revealed via transient absorption spectroscopy combined with electrochemistry as direct manipulation tool to control the negative charging of CdSe QDs. In trions, excited states of single charg…
We calculate the P-shell--to-S-shell decay lifetime τ(P->S) of electrons in lens-shaped self-assembled (In,Ga)As/GaAs dots due to Auger electron-hole scattering within an atomistic pseudopotential-based approach. We find that this relaxation mechanism leads to fast decay of τ(P->S)~1-7 ps for dots of different sizes. Our calculated Auger-type P-shell--to-S-shell decay lifetimes τ(P->S) compare well to data in (In,Ga)As/GaAs dots, showing that as long as holes are present there is no need for an alternative polaron mechanism.
Fully microscopic many-body models based on inputs from first principle density functional theory are used to calculate the carrier losses due to radiative- and Auger-recombinations in bulk tellurium. It is shown that Auger processes dominate the losses for carrier densities in the range typical for applications as lasers. The Auger loss depends crucially on the energetic position of the $H_6$ valence bands. At cryogenic temperatures of 50$\,$K (100$\,$K) the Auger coefficient, $C$, varies by about six (three) orders of magnitude within the range of published distances between these bands and …
Recent photoluminescence experiments presented by M. Binder et al. [Appl. Phys. Lett. 103, 071108 (2013)] demonstrated the visualization of high-energy carriers generated by Auger recombination in (AlInGa)N multi quantum wells. Two fundamental limitations were deduced which reduce the detection efficiency of Auger processes contributing to the reduction in internal quantum efficiency: the capture probability of these hot electrons and holes in a detection well and the asymmetry in type of Auger recombination. We investigate the transport and capture properties of these high-energy carriers reg…
Auger recombination in semiconductors is a many-body phenomenon in which recombination of electrons and holes is accompanied by excitation of other charge carriers. Being nonradiative, it is detrimental to light emission. The excess energy of the excited carriers is normally rapidly converted to heat, making Auger processes difficult to probe directly. Here, we employ a technique in which the Auger-excited carriers are detected by their ability to tunnel out of the semiconductor through a thin barrier, generating a current. We employ vertical van der Waals (vdW) heterostructures with monolayer…
Unraveling the Roles of Shallow, Deep and Auger Trapping in Charge Carrier Recombination in Triple-Cation Perovskites
Lund University（隆德大学）；Leibniz Institute for Solid State and Materials Research Dresden（德累斯顿莱布尼兹固体与材料研究所）；Technical University of Dresden（德累斯顿工业大学）；Indian Institute of Technology Roorkee（鲁尔基印度理工学院）；Forschungszentrum Jülich（于利希研究中心）；University of Duisburg-Essen（杜伊斯堡-埃森大学）
We examine the effect of carrier localization due to random alloy fluctuations on the radiative and Auger recombination rates in InGaN quantum wells as a function of alloy composition, crystal orientation, carrier density, and temperature. Our results show that alloy fluctuations reduce individual transition matrix elements by the separate localization of electrons and holes, but this effect is overcompensated by the additional transitions enabled by translational symmetry breaking and the resulting lack of momentum conservation. Hence, we find that localization increases both radiative and Au…
A comprehensive study of the exciton relaxation processes in Cu2O has led to some surprises. We find that the ortho-para conversion rate becomes slower at high stress, and that the Auger nonradiative recombination rate increases with stress, with apparently no Auger recombination at zero stress. These results have important consequences for the pursuit of Bose-Einstein condensation of excitons in a harmonic potential.
Carrier lifetime measurements reveal that, contrary to common expectations, the high-current non-radiative recombination (droop) in III-Nitride light emitters is comprised of two contributions which scale with the cube of the carrier density: an intrinsic recombination --most likely standard Auger scattering-- and an extrinsic recombination which is proportional to the density of point defects. This second droop mechanism, which hasn't previously been observed, is likely caused by an impurity-assisted Auger process. Further, it is shown that longer-wavelength emitters suffer from higher point …
We report semi-empirical pseudopotential calculations of emission spectra of charged excitons and biexcitons in CdSe nanocrystals. We find that the main emission peak of charged multiexcitons - originating from the recombination of an electron in an s-like state with a hole in an s-like state - is blue shifted with respect to the neutral mono exciton. In the case of the negatively charged biexciton, we observe additional emission peaks of lower intensity at higher energy, which we attribute to the recombination of an electron in a p state with a hole in a p state.
We have demonstrated surface-plasmon induced change in spontaneous emission rate in the ultraviolet regime at ~ 375-380 nm, using AlN/GaN quantum dots (QD). Using time-resolved and continuous-wave photoluminescence measurements, the recombination rate in AlN/GaN QD is shown to be enhanced when spontaneous emission is resonantly coupled to a metal-surface plasmon mode. The exciton recombination process via Ag-surface plasmon modes is observed to be as much as 3-7 times faster than in normal QD spontaneous emission and depends strongly on the emission wavelength and silver thickness.
Significant efficiency droop is a major concern for light-emitting diodes and laser diodes operating at high current density. Recent study has suggested that heavily Bi-alloyed GaAs can decrease the non-radiative Auger recombination and therefore alleviate the efficiency droop. Using density functional theory, we studied a newly fabricated quaternary alloy, GaAs1-x-yPyBix, which can host significant amounts of Bi, through calculations of its band gap, spin-orbit splitting, and band offsets with GaAs. We found that the band gap changes of GaAs1-x-yPyBix relative to GaAs are determined mainly by…
CdSe quantum dots (QDs) with different organic linker molecules are attached to ZnO nanowires (NWs) to study the luminescence dynamics and the electron tunneling from the QDs to the nanowires in time-resolved photoluminescence (PL) and photoconductivity measurements. The PL transients of the QD luminescence indicate two different recombination channels: the direct recombination inside the QD core and the recombination via QD surface defect states. After linking the QDs to the ZnO NW surface, photo-induced electron tunneling from an excited state of the QD into the conduction band of the nanowi…
We report on photoluminescence in the 3-7 $μ$m mid-wave infrared (MWIR) range from sub-100 nm strained thin films of rocksalt PbSe(001) grown on GaAs(001) substrates by molecular beam epitaxy. These bare films, grown epitaxially at temperatures below 400 °C, luminesce brightly at room temperature and have minority carrier lifetimes as long as 172 ns. The relatively long lifetimes in PbSe thin films are achievable despite threading dislocation densities exceeding $10^9$ $cm^{-2}$ arising from island growth on the nearly 8% lattice- and crystal-structure-mismatched GaAs substrate. Using quasi-co…
Two-dimensional transition metal dichalcogenide (2D-TMD) monolayers, which reveal remarkable semiconductor properties, are the subject of active experimental research.Recently it has been shown experimentally that quantum yield in MoS2 and WSe2 monoatomic layers can reach values close to unity when electrostatic doping makes them intrinsic semiconductors. However, the available theoretical description does not give an understanding of the physical mechanisms underlying in the gate voltage control of quantum yield.This work is an attempt to propose a consistent semi-phenomenological theory of p…
We present results on the charge dependence of the radiative recombination lifetime, Tau, and the emission energy of excitons confined to single self-assembled InGaAs quantum dots. There are significant dot-to-dot fluctuations in the lifetimes for a particular emission energy. To reach general conclusions, we present the statistical behavior by analyzing data recorded on a large number of individual quantum dots. Exciton charge is controlled with extremely high fidelity through an n-type field effect structure, providing access to the neutral exciton (X0), the biexciton (2X0) and the positivel…
We present results on photoexcited carrier lifetimes in few-layer transition metal dichalcogenide MoS2 using nondegenerate ultrafast optical pump-probe technique. Our results show a sharp increase of the carrier lifetimes with the number of layers in the sample. Carrier lifetimes increase from few tens of picoseconds in monolayer samples to more than a nanosecond in 10-layer samples. The inverse carrier lifetime was found to scale according to the probability of the carriers being present at the surface layers, as given by the carrier wavefunction in few layer samples, which can be treated as …
We calculate the height dependence of recombination energies, polarization and radiative lifetimes of the optical transitions of various excitonic complexes: neutral excitons (X^0), negatively- (X^-) and positively-charged (X^+) trions, and biexcitons (XX^0) in lens-shaped, self-assembled In_0.6Ga_0.4As/GaAs quantum dots. By using an atomistic pseudopotential method combined with the configuration-interaction method, we predict the following. (i) The recombination energy of the lowest transition of X^- blue-shifts as height increases, whereas that of X^+ red-shifts. Remarkably, the recombinati…
Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. As already shown, optical control of a spin of a magnetic ion is feasible employing photo-generated carriers confined in a quantum dot. A non-radiative recombination, regarded as a severe problem, limited development of quantum dots with magnetic ions. Our photoluminescence studies on, so far unexplored, individual CdTe dots with single co…
In this work, we provide a detailed analysis of phosphorene performance as n-type and p-type active materials. The study is based on first principles calculation of phosphorene electronic structure, and resulting electron and hole scattering rates and lifetimes. Emphasis is put on extreme regimes commonly found in semiconductor devices, i.e. high electric fields and heavy doping, where impact ionization and Auger recombination can occur. We found that electron-initiated impact ionization is weaker than the hole-initiated process, when compared to carrier-phonon interaction rates, suggesting re…
Charge transfer between photoexcited quantum dots and molecular acceptors is one of the key limiting processes in most applications of colloidal nanostructures, most prominently in photovoltaics. An atomistic detailed description of this process would open new ways to optimize existing and create new structures with targeted properties. We achieve a one-to-one comparison between ab-initio non-adiabatic molecular dynamics calculations and transient absorption spectroscopy experiments, which allows us to draw a comprehensive atomistic picture of the charge transfer process, following the time ev…
Continuous and concerted development of colloidal quantum-dot light-emitting diodes over the past two decades has established them as a bedrock technology for the next generation of displays. However, a fundamental issue that limits the performance of these devices is the quenching of photoluminescence due to excess charges from conductive charge transport layers. Although device designs have leveraged various workarounds, doing so often comes at the cost of limiting efficient charge injection. Here we demonstrate that high-field terahertz (THz) pulses can dramatically brighten quenched QDs on…
We report on a multiband microscopic theory of many-exciton complexes in self-assembled quantum dots. The single particle states are obtained by three methods: single-band effective-mass approximation, the multiband $k\cdot p$ method, and the tight-binding method. The electronic structure calculations are coupled with strain calculations via Bir-Pikus Hamiltonian. The many-body wave functions of $N$ electrons and $N$ valence holes are expanded in the basis of Slater determinants. The Coulomb matrix elements are evaluated using statically screened interaction for the three different sets of sin…
We demonstrate a new method of measuring the exciton spin relaxation time in semiconductor nanostructures by continuous-wave photoluminescence. We find that for self-assembled CdTe quantum dots the degree of circular polarization of emission is larger when exciting polarized excitons into the lower energy spin state than in the case when the excitons are excited into the higher energy spin state. A simple rate equation model gives the exciton spin relaxation time in CdTe quantum dots equal to 4.8+/-0.3 ns, significantly longer than the quantum dot exciton recombination time 300 ps.
Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]. Realistic eight-band k.p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are either formed by the A or the B valence band.
The theoretical analysis of photovoltaic conversion efficiency of highly effective silicon solar cells (SC) is performed for n-type and p-type bases. The case is considered when the Shockley-Read-Hall recombination in the silicon bulk is determined by the deep level of Fe. It is shown that due to the asymmetry of the recombination parameters of this level the photovoltaic conversion efficiency is increasing in the SC with the n-type base and decreasing in the SC with the p-type base with the increase in doping. Two approximations for the band-to-band Auger recombination lifetime dependence on …
The dynamic response of InAsP quantum dots grown on InP(001) substrates by low-pressure Metalorganic Vapor Phase Epitaxy emitting around 1.55 $μ$m, is investigated by means of time-resolved microphotoluminescence as a function of temperature. Exciton lifetime steadily increases from 1 ns at low temperature to reach 4 ns at 300K while the integrated photoluminescence intensity decreases only by a factor of 2/3. These characteristics give evidence that such InAsP/InP quantum dots provide a strong carrier confinement even at room temperature and that their dynamic response is not affected by ther…
We use a many-body, atomistic empirical pseudopotential approach to predict the multi-exciton emission spectrum of a lens shaped InAs/GaAs self-assembled quantum dot. We discuss the effects of (i) The direct Coulomb energies, including the differences of electron and hole wavefunctions, (ii) the exchange Coulomb energies and (iii) correlation energies given by a configuration interaction calculation. Emission from the groundstate of the $N$ exciton system to the $N-1$ exciton system involving $e_0\to h_0$ and $e_1\to h_1$ recombinations are discussed. A comparison with a simpler single-band, e…
