Nonradiative Auger Recombination Research Articles

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.

Colloidal quantum dot lasers

Semiconductor nanocrystals represent a promising class of solution-processable optical-gain media that can be manipulated via inexpensive, easily scalable colloidal techniques. Due to their extremely small sizes (typically <10 nm), their properties can be directly controlled via effects of quantum confinement; therefore, they are often termed colloidal quantum dots (CQDs). In addition to size-tunable emission wavelengths, CQDs offer other benefits for lasing applications, including low optical-gain thresholds and high temperature stability of lasing characteristics. Recent progress in understanding and practical control of processes impeding light amplification in CQDs has resulted in several breakthroughs, including the demonstration of optically pumped continuous-wave lasing, the realization of optical gain with direct current electrical injection and the development of dual-function electroluminescent devices that also operate as optically pumped lasers. The purpose of this Review is to assess the status of the field of CQD lasing and discuss the existing challenges and opportunities. A particular focus is on approaches for suppressing nonradiative Auger recombination, novel optical-gain concepts enabled by strong exciton–exciton interactions and controlled CQD charging, effects of nanocrystal form factors on light amplification and practical architectures for realizing electrically pumped CQD lasers. This overview suggests that the accumulated knowledge, along with the approaches developed for manipulating the optical-gain properties of colloidal nanostructures, perfectly position the CQD field for successfully addressing a long-standing challenge: the realization of CQD-based laser diodes.

Suppressing Auger Recombination in Multiply Excited Colloidal Silicon Nanocrystals with Ligand-Induced Hole Traps

Nonradiative Auger recombination in multiply excited nanocrystals is a dominant efficiency loss pathway for nanocrystal-containing optoelectronic devices that rely on high-rate emission and absorpt...

Auger Recombination and Multiple Exciton Generation in Colloidal Two-Dimensional Perovskite Nanoplatelets: Implications for Light-Emitting Devices

Improving the understanding of multiple exciton interactions and dynamics in semiconductor nanostructures is mandatory for their successful use as photoactive materials in light convertors such as electroluminescent diodes, lasers, or single-photon sources. Here high-fluence and high-energy excitation effects are investigated in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs) using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Nonradiative Auger recombination (AR) is the dominant pathway for multiexciton recombination. Its dynamics are found to be subquadratic with the exciton density. Indeed, because of the limited exciton wave-function delocalization length, AR is limited by exciton diffusion in the 2D plane at moderate excitation fluence and takes place in several hundreds of picoseconds, with typical recombination rates on the order of 10–2 cm2/s. At high excitation fluence leading to an average interexciton distance comparable with the exciton delocalization length, the measured “intrinsic” AR time is faster than 10 ps and independent of the NPL composition. The strong dependence of the AR rate on the interexciton distance allows us to identify the recombination resulting from multiple exciton generation, involving the reaction of “geminate biexcitons”, upon excitation at low fluence with high-energy photons.

Biexciton Dynamics in Single Colloidal CdSe Quantum Dots.

The investigation of biexciton dynamics in single colloidal quantum dots (QDs) is critical to biexciton-based applications. Generally, a biexciton exhibits an extremely low photoluminescence (PL) quantum yield as well as very fast PL decay due to strong nonradiative Auger recombination, making it difficult to investigate the biexciton dynamics. Here, we develop a quantitative method based on intensity- and time-resolved photon statistics to investigate the biexciton dynamics in single colloidal QDs. This robust method can be used under high-excitation conditions to determine the absolute radiative and Auger recombination rates of both neutral and charged biexciton states in a single QD level, and the corresponding ratios between the two states agree with the theoretical predictions of the asymmetric band structures of CdSe-based QDs. Furthermore, the surface traps are found to provide additional nonradiative recombination pathways for the biexcitons, and their contributions are quantified by the method.

Quantitative Electrochemical Control over Optical Gain in Quantum-Dot Solids.

Solution-processed quantum dot (QD) lasers are one of the holy grails of nanoscience. They are not yet commercialized because the lasing threshold is too high: one needs >1 exciton per QD, which is difficult to achieve because of fast nonradiative Auger recombination. The threshold can, however, be reduced by electronic doping of the QDs, which decreases the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete absorption. Here, we show that by electrochemically doping films of CdSe/CdS/ZnS QDs, we achieve quantitative control over the gain threshold. We obtain stable and reversible doping of more than two electrons per QD. We quantify the gain threshold and the charge carrier dynamics using ultrafast spectroelectrochemistry and achieve quantitative agreement between experiments and theory, including a vanishingly low gain threshold for doubly doped QDs. Over a range of wavelengths with appreciable gain coefficients, the gain thresholds reach record-low values of ∼1 × 10–5 excitons per QD. These results demonstrate a high level of control over the gain threshold in doped QD solids, opening a new route for the creation of cheap, solution-processable, low-threshold QD lasers.

Suppressing Auger Recombination in Cesium Lead Bromide Perovskite Nanocrystal Film for Bright Light-Emitting Diodes.

All-inorganic cesium lead halide perovskite colloidal nanocrystals are attractive for next-generation light-emitting diodes because of their high color purity, but the nonradiative Auger recombination in perovskite nanocrystal film limits the efficiency and brightness of the fabricated devices. Here, we introduce a surface-engineering process to exchange the original long-chain oleic acid/oleylamine ligands by the cerium-tributylphosphine oxide hybrid ligands to suppress nonradiative Auger recombination in CsPbBr3 NC film for bright and low-efficiency roll-off light-emitting diodes. Using ultrafast transient absorption and time-resolved photoluminescence spectroscopy, we demonstrate that the hybrid ligand passivation can efficiently remove surface trap states to enhance radiative recombination and homogenize the exciton concentration to suppress nonradiative Auger recombination in the CsPbBr3 nanocrystal thin film. Consequently, we fabricate a light-emitting diode with efficient charge injection into the CsPbBr3 nanocrystal emitting layer, achieving a pronounced improvement of electroluminescence with an external quantum efficiency from 5.5% to 9.1%. More importantly, the efficiency roll-off characteristics of high-brightness light-emitting diodes is effectively mitigated. Our reported hybrid ligand passivation suppressed Auger recombination strategy shows a great potential for fabricating high-brightness cesium lead halide perovskite light-emitting diodes.

Trap Density Assessment on Multilayer WS2 using Power-Dependent Indirect Photoluminescence

Material or interface defectivity assessment of 2D materials remains a challenge, specifically in terms of simple techniques which can be integrated in a CMOS process line. Here we demonstrate an optical technique that assesses interface trap densities, based on the indirect photoluminescence emission. We achieved that by demonstrating the modulation of the indirect/direct photoluminescence peak intensity ratio by the exciton concentration and then linking the modulation to the trap-sensitive non-radiative Auger recombination. Calibration is achieved through theoretical modeling of the recombination mechanisms and, as an example of the methodology, a trap density between 1.6 × 1010 cm−2 and 1.2 × 1011 cm−2 is extracted from (hBN/)WS2/SiO2 structures.

Strain dependence of Auger recombination in 3 μm GaInAsSb/GaSb type-I active regions

We differentiate the effect of strain induced by lattice-mismatched growth from strain induced by mechanical deformation on cubic nonradiative Auger recombination in narrow-gap GaInAsSb/GaSb quantum well (QW) heterostructures. The typical reduction in the Auger coefficient observed with lattice-mismatched growth appears to be due to the concomitant compositional change rather than the addition of strain, with implications for mid-IR semiconductor laser design. We induced a range of internal compressive strain in five samples from −0.90% to −2.07% by varying the composition during the growth and mechanically induced a similar range of internal strain in analogous quantum well membrane samples. We performed time-resolved photoluminescence and differential reflectivity measurements to extract the carrier recombination dynamics, taken at 300 K with carrier densities from 2.7×1018 cm−3 to 1.4×1019 cm−3. We observed no change with strain in the cubic Auger coefficient of samples that were strained mechanically, but we did observe a trend with strain in samples that were strained by the QW alloy composition. Measured Auger coefficients ranged from 3.0×10−29 cm6 s−1 to 3.0×10−28 cm6 s−1.

Trion Emission Dominates the Low-Temperature Photoluminescence of CdSe Nanoplatelets.

Colloidal nanoplatelets (NPLs) are atomically flat, quasi-two-dimensional particles of a semiconductor. Despite intense interest in their optical properties, several observations concerning the emission of CdSe NPLs remain puzzling. While their ensemble photoluminescence spectrum consists of a single narrow peak at room temperature, two distinct emission features appear at temperatures below ∼160 K. Several competing explanations for the origin of this two-color emission have been proposed. Here, we present temperature- and time-dependent experiments demonstrating that the two emission colors are due to two subpopulations of uncharged and charged NPLs. We study dilute films of isolated NPLs, thus excluding any explanation relying on collective effects due to NPL stacking. Temperature-dependent measurements explain that trion emission from charged NPLs is bright at cryogenic temperatures, while temperature activation of nonradiative Auger recombination quenches the trion emission above 160 K. Our findings clarify many of the questions surrounding the photoluminescence of CdSe NPLs.

Bright trion emission from semiconductor nanoplatelets

The trion, a quasiparticle comprising one exciton and an additional charge carrier, offers unique opportunities for generating spin-photon interfaces that can be used in developing quantum networks. Trions are also actively sought after for integrated optoelectronic devices including photovoltaics, photodetectors, and spintronics. However, formation of trions in strongly confined low-dimensional materials is often deemed detrimental. This is because trion emission in such materials is typically prohibited due to the predominant nonradiative Auger recombination processes. Semiconductor nanoplatelets with their strong confinement in the thickness direction and extended lateral geometries exhibit large exciton coherence sizes and reduced carrier-carrier interactions that may enable unprecedented trion properties. Here, we perform optical spectroscopic studies of individual CdSe nanoplatelets at cryogenic temperatures and observe bright trion emission with intensities comparable to that of neutral exciton emission. We perform carrier dynamics studies of the nanoplatelets and find that due to their extended lateral geometry, the fast radiative decay rate of the nanoplatelets at cryogenic temperatures is comparable to the inhibited Auger recombination rate, leading to the bright trion emission. Our tight-binding theory further reveals distinct size-tunable trion emission in the nanoplatelets that is advantageous for efficient trion emission. These properties make semiconductor nanoplatelets potential candidates as photon sources for optoelectronic and quantum logic devices.

(Invited) Colloidal Semiconductor Nanocrystals Under Intense Optical Excitations Towards Efficient Lasing and Hot-Carrier Harvesting

Colloidal semiconductor nanocrystals appeal to broad range of applications including bio-imaging and displays where essential optical properties such as photoluminescence quantum yields are typically characterized under weak optical excitation conditions producing less than one exciton per particle on average. On the other hand, applications such as lasing and high-brightness lighting require intense excitation conditions producing more than one exciton per nanocrystal on average. Unfortunately, prior reports showed that multiple excitons in these nanocrystals are annihilated at extremely high rates due to nonradiative Auger recombination. Therefore, energy of the multiple excitons is lost to heating before it can be used and converted into electrical or light energy.In the first of the talk, I will present a colloidal nanocrystal system, which provides remarkable performance in lasing applications. Recently, we showed that atomically-flat colloidal CdSe nanoplatelets exhibit record-high optical gain coefficients under intense optical excitation conditions [1]. In contrast, optical gain coefficients in conventional CdSe nanocrystals (e.g., core/shell quantum dots) saturate at very low excitation fluences; typically at a fluence that is two times the gain threshold. In the case of the CdSe nanoplatelets, optical gain saturates at an exceptionally high level which is an order of magnitude larger than those of prototypical nanocrystals. As a result, CdSe nanoplatelets attain giant modal gain coefficients which we directly measure to be on the order of 6000 cm-1 with the intrinsic material gain coefficients exceeding 15,000 cm-1. The measured modal gain in the nanoplatelets is the highest among any lasing media measured at room temperature to date. This indicates that nanoplatelets sustain optically excited multiple excitons and enable their strong contribution to the gain before the nonradiative Auger recombination kicks in. We hypothesize that multiexcitons in the nanoplatelets form highly stable biexcitonic species as claimed in epitaxial quantum wells at low temperatures before [2].In the second part of the talk, I will present a new technique to monitor how colloidal nanocrystals heat up at the nanoscale following an intense optical excitation, which is important for applications requiring intense excitation conditions. Previously, a phonon bottleneck effect between excited state electrons and optical phonons have been predicted but has never been observed experimentally [3]. This implied that nanocrystals should heat up rather quickly (sub-picosecond to picosecond timescale) by transferring the energy stored in the electronic system into the lattice via electron – phonon coupling. However, timescales of the heating up nanocrystals, which involve both electron – phonon and phonon – phonon couplings, could not be straightforwardly characterized with the existing time-resolved techniques. Here, we utilize ultrafast electron diffraction measurements, where we probe the lattice of the nanocrystals through following an optical pump. With this method, we resolve the intrinsic timescales of the heat transfer within the colloidal nanocrystals [4]. Distinct from previous optical studies, we observe highly slow heating up of the nanocrystals which strikingly depend on the excitations whether generated in the core or the shell of the nanocrystals.[1] B. Guzelturk et al. Nano Letters 19, 277 (2019)[2] D. A. B. Miller, “Optical physics of Quantum Wells”[3] R. Schaller et al. Phys. Rev. Lett. 95, 196401 (2005)[4] B. Guzelturk et al. (in-preparation)

Suppressing Efficiency Roll-Off at High Current Densities for Ultra-Bright Green Perovskite Light-Emitting Diodes.

Perovskite light-emitting diodes (PeLEDs) have undergone rapid development in the last several years with external quantum efficiencies (EQEs) reaching over 21%. However, most PeLEDs still suffer from severe efficiency roll-off (droop) at high injection current densities, thus limiting their achievable brightness and presenting a challenge to their use in laser diode applications. In this work, we show that the roll-off characteristics of PeLEDs are affected by a combination of charge injection imbalance, nonradiative Auger recombination, and Joule heating. To realize ultrabright and efficient PeLEDs, several strategies have been applied. First, we designed an energy ladder to balance the electron and hole transport. Second, we optimized perovskite materials to possess reduced Auger recombination rates and improved carrier mobility. Third, we replaced glass substrates with sapphire substrates to better dissipate joule heat. Finally, by applying a current-focusing architecture, we achieved PeLEDs with a record luminance of 7.6 Mcd/m2. The devices can be operated at very high current densities (J) up to ∼1 kA/cm2. Our work suggests a broad application prospect of perovskite materials for high-brightness LEDs and ultimately a potential for solution-processed electrically pumped laser diodes.

Gate-Voltage Control of Quantum Yield in Monolayer Transition-Metal Dichalcogenide

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 photo-induced charge carriers relaxation in 2D-TMDs, which allows obtaining an analytical dependence of the quantum yield on the voltage applied to the FET gate. We consider a standard experimental situation, when the 2D-TMD monolayer and the metal gate are plates of a flat capacitor, and the capacitor charge is proportional to the gate voltage. The dependences of the TMD monolayers quantum yield on the gate voltage and the carrier generation rate have been calculated for the cases of the prevailing recombination of free electrons and holes (radiative and non-radiative Auger recombination) and recombination of excitons (radiative and Auger recombination). In both cases analytical expressions were derived for the dependence of quantum yield on the gate voltage and photo-induced carriers generation rate at a fixed gate voltage. Quantitative agreement with experiment allows concluding about the relevance of the proposed theoretical model for the description of carriers photo-generation and recombination in 2D-TMD monolayers. Obtained results demonstrate the possibilities of 2D-TMD quantum yield control by the gate voltage and indicate that 2D-TMDs are promising candidates for modern optoelectronics devices.

Effects of interface-potential smoothness and wavefunction delocalization on Auger recombination in colloidal CdSe-based core/shell quantum dots.

Auger nonradiative recombination dominates decay of multicarrier states in high quality colloidal quantum dots (QDs) and thus is critical for many of their optical and optoelectronic applications. Controlling interface-potential smoothness and wavefunction delocalization are proposed as two main strategies for Auger engineering in core/shell QDs. Here, a series of CdSe-based core/shell QDs with nearly ideal optical quality of their single-exciton states are developed and applied for studying biexciton quantum yields and Auger nonradiative recombination rates. Comparative experiments find that the interface-potential smoothness has little influence on biexciton quantum yield and Auger rates of these core/shell QDs with the same CdS outer shells. In contrast, with a fixed total size of the series of QDs, the decreasing hole wavefunction delocalization can increase the Auger rates of positive trions by ∼400%. A mild decrease in electron wavefunction delocalization among the series of QDs results in a small increase in the Auger rates of negative trions (∼50%). Smoothing the core/shell interface can indeed affect the Auger rates, but this is by the way of altering wavefunction delocalization. These findings highlight the importance of control of wavefunction delocalization among the strategies of Auger engineering and provide guidelines for rational design QDs for applications.

Nitride dot-in-nanowire light emitters with suppressed auger process

In wurtzite III-Nitride nano-devices, the non-radiative Auger recombination is the primary mechanism responsible for the degradation of internal quantum efficiency (IQE), especially under high current density. In this paper, by employing an atomistic tight-binding framework, we theoretically study the effects of Auger recombination in recently reported InGaN/GaN dot-in-nanowire light emitters. The effects of strain and polarization, which can be strong in realistically-sized structures, have been considered. We demonstrate that the use of graded interfacial confinement leads to a weaker Auger recombination as compared to the abrupt counterpart, especially for thinner nanowires. The atomistically simulated Auger recombination coefficient for the core quantum dot buried in nanowire with different diameter is then incorporated into a TCAD simulator to obtain the device terminal (efficiency vs. current) characteristics. Overall, the simulation results indicate that increasing the diameter of the host nanowire (that is, the volume of the active region) remains the most efficient way to suppress Auger recombination.

Fundamental Limits to Far-Infrared Lasing in Auger-Suppressed HgCdTe Quantum Wells

The challenge of bridging the terahertz gap with semiconductor lasers faces an inevitable problem of enhanced nonradiative Auger recombination with reduction of photon energy. We show that this pro...

A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites.

Mixed-halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution-processed triple-cation mixed-halide (Cs0.06 MA0.15 FA0.79 )Pb(Br0.4 I0.6 )3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar-equivalent illumination. It is found that the illumination leads to localized surface sites of iodide-rich perovskite intermixed with passivating PbI2 material. Time- and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide-rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed-halide mixed-cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.

Ionization and Neutralization Dynamics of CsPbBr3 Perovskite Nanocrystals Revealed by Double-Pump Transient Absorption Spectroscopy.

Ionization of nanocrystals (NCs) causes both photoluminescence intermittency and a reduction in luminescence quantum efficiency and thus plays a critical role in the optoelectronic performance of NC-based devices. Here, we study the ionization and neutralization processes of CsPbBr3 perovskite NCs under strong photoexcitation by means of double-pump transient absorption spectroscopy. A strong initial pulse is used to generate ionized NCs, and their optical responses are investigated by varying the excitation intensity and delay time of the second pump pulse. We find that charging can occur either via nonradiative Auger recombination of biexcitons or via any possible recombination of trions. The presence of the extra charge inside of an ionized perovskite NC significantly reduces its absorption cross section. The experiments reveal that ionized NCs exhibit two types of neutralization processes with time constants on the order of nanoseconds and microseconds. These results are useful for the optimal design of NC-based photonic devices.

Single Halide Perovskite/Semiconductor Core/Shell Quantum Dots with Ultrastability and Nonblinking Properties.

The further practical applications of halide perovskite quantum dots (QDs) are blocked by problems of instability and nonradiative Auger recombination manifested as photoluminescence blinking. Here, single core/shell structured perovskite semiconductor QDs are successfully fabricated by capping CsPbBr3 QD core with CdS shell. It is demonstrated that CsPbBr3/CdS core/shell QDs exhibit ultrahigh chemical stability and nonblinking photoluminescence with high quantum yield due to the reduced electronic traps within the core/shell structure. Efficiency of amplified spontaneous emission exhibits obvious enhancement compared to that of pure CsPbBr3 QDs, originating from the mitigated competition between stimulated emission and suppressed nonradiative biexciton Auger recombination. Furthermore, low‐threshold whispering‐gallery‐mode lasing with a high‐quality factor is achieved by incorporating CsPbBr3/CdS QDs into microtubule resonators. Density functional theory (DFT)‐based first‐principles calculations are also performed to reveal the atomic interface structure, which supports the existence of CsPbBr3/CdS structure. An interesting feature of spatially separated charge density at CsPbBr3/CdS interface is found, which may greatly contribute to the suppressed Auger recombination. The results provide a practical approach to improve the stability and suppress the blinking of halide perovskite QDs, which may pave the way for future applications for various optoelectronic devices.