Nonradiative Recombination Time Research Articles

A-Site Cations Impact on Nonradiative Recombination, Mobility, and Defect Dynamics in Sn-Based Perovskites.

Sn-based perovskites with different cations in the A-site exhibit distinct electronic structures and dynamic properties. By utilizing time-dependent density functional theory and nonadiabatic molecular dynamics, we demonstrate that larger FA cations decrease wave function overlap between initial and final states and slow down nuclear motion. In the case of FASnI3, this alteration decreases the nonadiabatic coupling and increases the nonradiative electron-hole recombination time by 130% and 76%, respectively, compared to CsSnI3 and MASnI3 (CH3NH3SnI3). Furthermore, A-site modification significantly improves electron mobility and changes the properties of defects in FASnI3 (HC(NH2)2SnI3), which achieves higher electron mobility through a polar optical phonon-dominated scattering mechanism and exhibits higher defect formation energy and migration barriers of A-site cations due to increased steric hindrance, relative to CsSnI3 and MASnI3. These results emphasize the critical function of A-site cation substitution in controlling nonradiative recombination dynamics, electron mobility, and defect characteristics in Sn-based perovskites and provide theoretical insights for the advancement of novel lead-free perovskite materials.

Ab Initio Time-Domain Study of Charge Relaxation and Recombination in N-Doped Cu2O.

Cu2O is a good photoelectric material with excellent performance, and its crystal structure, electronic structure, and optical properties have been extensively studied. To further illustrate the charge distribution and the carrier transport in this system, the e-h recombination dynamics was studied. It is found that N doping induced a shallower impurity band above the VBM, leading to significant charge localization around the impurity atom. NAMD simulation reveals that the N doping system possesses a longer e-h nonradiative recombination time scale. Therefore, we demonstrate that the formation of the impurity band and charge localization play an essential role in suppressing e-h recombination in N doping systems. This work is conducive for understanding the carrier transport mechanism in N-doped Cu2O.

Charge Recombination Dynamics in a Metal Halide Perovskite Simulated by Nonadiabatic Molecular Dynamics Combined with Machine Learning.

Nonadiabatic coupling (NAC) plays a central role in driving nonadiabatic dynamics in various photophysical and photochemical processes. However, the high computational cost of NAC limits the time scale and system size of quantum dynamics simulation. By developing a machine learning (ML) framework and applying it to a traditional CH3N3PbI3 perovskite, we demonstrate that the various ML algorithms (XGBoost, LightGBM, and random forest) combined with three descriptors (sine matrix, MBTR, and SOAP) can predict accurate NACs that all agree well with the direct calculations, particularly for the combination of LightGBM and sine matrix descriptor showing the best performance with a high correlation coefficient of ≤0.87. The simulated nonradiative electron-hole recombination time scales agree well with each other between the NACs obtained from direct calculations and ML prediction. The study shows the advantage in accelerating quantum dynamics simulations using ML algorithms.

Optical properties of InGaN-based red multiple quantum wells

In this work, we present the characterization of red InGaN/GaN multiple-quantum-well (MQW) light-emitting diode structures. The optical properties of two MQW structures with different n-GaN underlayer thicknesses (4 and 8 μm) are studied and compared. The results of photoluminescence studies show that a thicker n-GaN layer is beneficial for obtaining higher In content for red MQWs. However, the sample with a thicker n-GaN layer has a poorer internal quantum efficiency, a larger full width at half maximum, and a shorter nonradiative recombination time, implying that there are stronger In-content fluctuations and more defects. Furthermore, red MQWs with higher In content are shown to exhibit more deep localized states. Our findings imply that in order to achieve high-efficiency InGaN MQWs for red emission, enhancing the uniformity of In-content distribution in the active region and decreasing nonradiative recombination centers are critical challenges.

Phonon-Assisted Nonradiative Recombination Tuned by Organic Cations in Ruddlesden-Popper Hybrid Perovskites

Significant efforts have been devoted to further increasing the photoconversion efficiency of two-dimensional (2D) organic-inorganic halide perovskites, which are promising photovoltaic materials with superior stability but lower efficiency compared to their three-dimensional counterparts. One of the main factors that limit their photoconversion efficiency is the nonradiative recombination of photoexcited carriers. A widely used strategy for tuning the photoconversion efficiency of 2D perovskites is exchanging various organic cations. However, due to mixed effects contributed to by extrinsic factors, such as defect concentration and sample morphology, experiments alone are insufficient to explain the role of organic cations in phonon-assisted carrier recombination. With time-domain simulations based on first-principles nonadiabatic molecular dynamics, we investigate six prototypical 2D Ruddlesden-Popper perovskites to reveal the impact of organic cations on tuning the nonradiative recombination time through displacing inorganic ions and affecting electron-phonon coupling. The phonon-assisted band-to-band nonradiative recombination time is on the order of a few hundred nanoseconds and can be tuned up to 5 times by modifying organic cations. A distinct correlation between the pure-dephasing time and inorganic atom distortions induced by the motion of organic parts is revealed. Compared to the pure-dephasing time, nonadiabatic coupling (NAC) associated with the strength of electron-phonon coupling plays a more important role in determining the nonradiative carrier recombination time. Notably, fluorinated organic spacers increase the NAC between frontier electronic states and speed up the nonradiative recombination process.

Substitutional alkaline earth metals delay nonradiative charge recombination in CH3NH3PbI3 perovskite: A time-domain study.

Experiments reported that alkaline earth metal dopants greatly prolong carrier lifetime and improve the performance of perovskite solar cells. Using state-of-the-art ab initio time-domain nonadiabatic molecular dynamics (NAMD), we demonstrate that incorporation of alkaline earth metals, such as Sr and Ba, into MAPbI3 (MA = CH3NH3 +) lattice at the lead site is energetically favorable due to the largely negative formation energies about -7 eV. The replacement widens the bandgap and increases the open-circuit voltage by creating no trap states. More importantly, the substitution reduces the mixing of electron and hole wave functions by pushing the hole charge density away from the dopant together with no contribution of Sr and Ba to the conduction band edge state, thus decreasing the NA coupling. The high frequency phonons generated by enhanced atomic motions and symmetry breaking accelerate phonon-induced loss of coherence. The synergy of the three factors reduces the nonradiative recombination time by a factor of about 2 in the Sr- and Ba-doped systems with respect to pristine MAPbI3, which occurs over 1 ns and agrees well with the experiment. The study highlights the importance of various factors affecting charge carrier lifetime, establishes the mechanism of reduction of nonradiative electron-hole recombination in perovskites upon alkaline earth metal doping, and provides meaningful insights into the design of high performance of perovskite solar cells and optoelectronics.

Localized-state ensemble model analysis of InGaN/GaN quantum well structures with different dislocation densities

The influence of dislocations on luminescence of InGaN/GaN multiple quantum wells was investigated by temperature-dependent and time-resolved room-temperature photoluminescence measurements and analyzed via localized-state ensemble model. The results show that dislocations decrease non-radiative recombination time and do not affect either radiative recombination time or non-radiative recombination mechanism. Moreover, dislocation-related broadening, increasing linearly with increased dislocation density, was found to take place. However, a significant part of spectral width (∼55 meV) is not defined by either dislocation-induced or alloy- and thermally-induced broadening, revealing the existence of other broadening mechanisms (e.g. carrier–carrier scattering-induced broadening).

Investigation of Micromorphology and Carrier Recombination Dynamics for InGaN/GaN Multi-Quantum Dots Grown by Molecular Beam Epitaxy

InGaN quantum dots (QDs) are promising candidates for GaN-based all-visible optoelectronic devices such as micro light-emitting diode and laser. In this study, self-assembled InGaN/GaN multi-quantum dots (MQDs) have been grown by plasma-assisted molecular beam epitaxy on c-plane GaN-on-sapphire template. A high density of over 3.8 × 1010 cm−2 is achieved and InGaN QDs exhibit a relatively uniform size distribution and good dispersity. Strong localization effect in as-grown InGaN QDs has been evidenced by temperature-dependent photoluminescence (PL). The variation of peak energy is as small as 35 meV with increasing temperature from 10 K to 300 K, implying excellent temperature stability of emission wavelength for InGaN MQDs. Moreover, the radiative and nonradiative recombination times were calculated by time-resolved PL (TRPL) measurements, and the temperature dependence of PL decay times reveal that radiative recombination dominates the recombination process due to the low dislocation density of QDs structure.

Exciton recombination and spin relaxation in strong magnetic fields in ultrathin (In,Al)As/AlAs quantum wells with indirect band gap and type-I band alignment

The exciton recombination and spin dynamics are studied in monolayer-thick (In,Al)As/AlAs quantum wells characterized by an indirect band gap and a type-I band alignment. The exciton recombination time and the photoluminescence intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to no effect of an in-plane field, at a temperature of 1.8 K a magnetic field applied parallel to the growth axis drastically slows down the recombination and reduces the intensity of photoluminescence. The magnetic-field-induced circular polarization of photoluminescence is studied as a function of the magnetic field strength and direction, as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. Taking into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states, we evaluate the heavy-hole longitudinal $g$ factor of 3.6, the radiative recombination time for the bright excitons of 0.13 ms, and the nonradiative recombination time of the bright and dark excitons of 0.43 ms, as well as the spin relaxation times of electron of $25\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ and heavy hole of $16\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$, bound in the exciton.

Variations of light emission and carrier dynamics around V-defects in InGaN quantum wells

Time- and spectrally-resolved scanning near-field optical microscopy was applied to study spatial variations of photoluminescence (PL) spectra and carrier dynamics in polar InGaN/GaN single quantum wells (QWs) emitting from 410 nm to 570 nm. The main attention was devoted to variations of PL properties and carrier dynamics around V-defects. The PL intensity, peak wavelength, and linewidth, as well as the radiative and nonradiative recombination times, were found to be different in V-defect-rich and defect-free regions. The radiative lifetime close to the defects was longer up to several times, which is attributed to an increased electron and hole wave function separation in the QW plane. PL decay times, measured using excitation and collection through the near-field probe, were one to two orders of magnitude shorter than PL decay times measured in the far field. This shows that the near-field PL decay and the integrated PL intensity are primarily determined by the carrier out-diffusion from under the probe. Only in the immediate vicinity of the V-defects, the near-field PL decays due to the nonradiative recombination at dislocations. The area of such enhanced nonradiative recombination is limited to just a few percent of the total QW area. This shows that recombination via dislocations and V-defects does not play a decisive role in the overall nonradiative recombination and internal quantum efficiency of polar InGaN/GaN QWs.

Feasibility of surface plasmon lasing in HgTe quantum wells with population inversion

Surface plasmon lasing in semiconductor gain media at far-infrared frequencies requires simultaneously long non-radiative recombination times and large plasmon propagation length. In this paper, we show that these conditions are realized in mercury-telluride quantum wells (HgTe QWs) near the topological transition. We derive the conditions of surface plasmon amplification in HgTe QWs with interband population inversion. To this end, we calculate the spatially-dispersive high-frequency conductivity of pumped HgTe QWs taking into account their realistic band structure, and compare the interband gain with Drude absorption and collisionless Landau damping. An extra necessary condition of plasmon lasing is revealed, namely, the non-equilibrium carrier density should be high enough to make the plasmon spectrum overlap with the frequency domain of interband excitations. The latter condition limits the processes of both stimulated and spontaneous plasmon emission at low temperatures, and should have a strong impact on the recombination kinetics of HgTe QWs at low temperatures.

Impact of localization phenomenon and temperature on the photoluminescence spectra of GaSbBi alloys and GaSbBi/GaAs quantum dots

In this work, the low temperature PL spectrum of GaSbBi ternary alloys and GaSbBi/GaAs Quantum Dots (QDs) are investigated using two different mathematical models. The first model based on the Gaussian distribution of the localized states takes into consideration the carrier dynamics associated with bulk and QD systems through the inclusion of different carrier dependent parameters such as carrier generation rate, recapture, thermal escape, radiative and non-radiative recombination times and localization depth to compute and reproduce the temperature dependent of the luminescence keys. The k dot p method realized using a 10 band Hamiltonian is used to calculate the carrier effective masses from the temperature dependent band diagram which serves as an input to reproduce the PL spectra. Two separate expressions of intrinsic carrier concentration for bulk and QD used in this model help to achieve a close agreement between the theoretically calculated results and experimental data. This work imparts a deep level understanding of the behaviour of carriers and optoelectronic parameters in III-V-Bi structures.

Unravelling the Effects of A-Site Cations on Nonradiative Electron-Hole Recombination in Lead Bromide Perovskites: Time-Domain ab Initio Analysis.

Lead bromide perovskites APbBr3 (A = Cs, MA, FA) hold great promise in optoelectronics and photovoltaics. Because the band gaps of the three materials are similar, and also because the A-site cation does not contribute to band edges, one would expect a minor influence of A-site cation on the excited-state lifetime of the perovskites. Experiments defy that expectation. By performing ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we demonstrate that the nonradiative electron-hole recombination times are in the order FAPbBr3 > MAPbBr3 > CsPbBr3, which are determined by the NA electron-phonon coupling because decoherence times are similar. The simulations show that the larger A-site cation and the smaller NA coupling because larger A-site cation suppresses the Pb-Br cages' motion. The electron-hole recombination is slow, ranging from subnanosecond to nanoseconds, because the NA coupling is small, less than 3 meV, and because decoherence time is slow, less than 7 fs. Both the trend of recombination and the time scales show excellent agreement with experiments. The time-domain atomistic simulations rationalize the experimental observations and advance our understanding of the cations' influence on perovskite excited-state lifetimes.

Correlation of Bandgap Reduction with Inversion Response in (Si)GeSn/High-k/Metal Stacks.

The bandgap tunability of (Si)GeSn group IV semiconductors opens a new era in Si-technology. Depending on the Si/Sn contents, direct and indirect bandgaps in the range of 0.4-0.8 eV can be obtained, offering a broad spectrum of both photonic and low power electronic applications. In this work, we systematically studied capacitance-voltage characteristics of high-k/metal gate stacks formed on GeSn and SiGeSn alloys with Sn-contents ranging from 0 to 14 at. % and Si-contents from 0 to 10 at. % particularly focusing on the minority carrier inversion response. A clear correlation between the Sn-induced shrinkage of the bandgap energy and enhanced minority carrier response was confirmed using temperature and frequency dependent capacitance voltage-measurements, in good agreement with k.p theory predictions and photoluminescence measurements of the analyzed epilayers as reported earlier. The enhanced minority generation rate for higher Sn-contents can be firmly linked to the bandgap reduction in the GeSn epilayer without significant influence of substrate/interface effects. It thus offers a unique possibility to analyze intrinsic defects in (Si)GeSn epilayers. The extracted dominant defect level for minority carrier inversion lies approximately 0.4 eV above the valence band edge in the studied Sn-content range (0-12.5 at. %). This finding is of critical importance since it shows that the presence of Sn by itself does not impair the minority carrier lifetime. Therefore, the continuous improvement of (Si)GeSn material quality should yield longer nonradiative recombination times which are required for the fabrication of efficient light detectors and to obtain room temperature lasing action.

Dynamics of exciton recombination in strong magnetic fields in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

The exciton recombination dynamics is studied experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells characterized by an indirect band gap and a type-II band alignment. At cryogenic temperatures, the lifetimes of the excitons that are indirect both in real and $\mathbit{k}$ space are in the millisecond range. The exciton recombination time and the photoluminescence (PL) intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to the very weak influence of an in-plane field, at 2 K temperature a field applied parallel to the growth axis drastically slows down the recombination and reduces the PL intensity. With increasing temperature the magnetic field effects on PL intensity and decay time are vanishing. The experimental data are well described by a model for the exciton dynamics that takes into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states. It allows us to evaluate the lower bound of the heavy-hole longitudinal $g$ factor of 2.5, the radiative recombination time for the bright excitons of 0.34 ms, and the nonradiative recombination time of the bright and dark excitons of 8.5 ms.

Recombination kinetics of photogenerated electrons in InGaAs/InP quantum wells

The electron transport and recombination processes of photoexcited electron-hole pairs were studied in InGaAs/InP single quantum wells. Comprehensive transport data analysis reveals a asymmetric shape of the quantum well potential where the electron mobility was found to be dominated by interface-roughness scattering. The low-temperature time-resolved photoluminescence was employed to investigate recombination kinetics of photogenerated electrons. Remarkable modification of Auger recombination was observed with variation of the electron mobility. In high mobility quantum wells, the increasing pump power resulted in a new and unexpected phenomenon: a considerably enhanced Auger non-radiative recombination time. We propose that the distribution of the photoexcited electrons over different conduction band valleys might account for this effect. In low mobility quantum wells, disorder-induced relaxation of the momentum conservation rule causes inter-valley transitions to be insignificant; as a consequence, the non-radiative recombination time is reduced with the increase in pump power. Thus, interface-roughness scattering was found responsible for both transport properties and dynamic optical response in InGaAs/InP quantum wells.

Carrier dynamics analysis for efficiency droop in GaN-based light-emitting diodes with different defect densities using time-resolved electroluminescence

We developed a direct experimental approach for investigating the correlation between efficiency droop and recombination rate variation under current injection conditions by using time-resolved electroluminescence (EL) technique. We applied this approach to understand the droop phenomenon of GaN-based light-emitting diodes grown on patterned sapphire substrates (LED-on-PAT) and planar sapphire substrates (LED-on-PLA). Because of lower dislocation density and current leakage in LED-on-PAT compared to LED-on-PLA, it was found that the effective carrier density injected into quantum wells (QWs) in LED-on-PAT was higher than that of the LED-on-PLA under the same current injection conditions, based on the analysis of spectral broadening of EL spectra with varying current injection and photoluminescence experiments under resonant and non-resonant excitation conditions. The efficiency droop in LED-on-PAT was found to be much more severe than that of LED-on-PLA, despite the higher overall quantum efficiency of LED-on-PAT. From the time-resolved EL analysis, we could separate radiative and non-radiative recombination contributions and directly observe (i) the decrease and saturation of radiative recombination time and (ii) the increase and following decrease in behavior of non-radiative recombination time with increasing current injection level, showing a strong correlation between efficiency droop and recombination rate variation.

Radiative and non-radiative recombinations in tensile strained Ge microstrips: Photoluminescence experiments and modeling

Tensile germanium microstrips are candidate as gain material in Si-based light emitting devices due to the beneficial effect of the strain field on the radiative recombination rate. In this work, we thoroughly investigate their radiative recombination spectra by means of micro-photoluminescence experiments at different temperatures and excitation powers carried out on samples featuring different tensile strain values. For sake of comparison, bulk Ge(001) photoluminescence is also discussed. The experimental findings are interpreted in light of a numerical modeling based on a multi-valley effective mass approach, taking in to account the depth dependence of the photo-induced carrier density and of the self-absorption effect. The theoretical modeling allowed us to quantitatively describe the observed increase of the photoluminescence intensity for increasing values of strain, excitation power, and temperature. The temperature dependence of the non-radiative recombination time in this material has been inferred thanks to the model calibration procedure.

Carrier recombination dynamics of MBE grown InGaAsP layers with 1 eV bandgap for quadruple-junction solar cells

The carrier recombination dynamics of InGaAsP material with a bandgap energy of 1eV for quadruple-junction solar cells grown by solid-source molecular beam epitaxy have been investigated by the employment of time-resolved photoluminescence (PL) measurement. For the nominally undoped material, the PL decay time increases with increasing temperature, which indicates that radiative recombination dominates the recombination process. The radiative and the nonradiative recombination time constants were calculated on the basis of the temperature-dependent PL decay time and the integrated PL intensity. With the incorporation of Be (as the p-type dopant) into the material, the PL decay time decreases with increasing temperature, and a double-exponential PL decay curve is observed in the case of the material with a higher doping density. An InGaAsP-based single-junction photovoltaic device with a bandgap of 1eV was fabricated, and an efficiency of 16.4% was obtained under the AM1.5G solar spectra.

Time‐resolved free carrier lifetime microscopy in bulk GaN

AbstractA novel setup for lifetime microscopy measurements was designed and applied for carrier lifetime mapping in a bulk GaN. Photoexcitation by a picosecond UV pump and detection of time‐resolved free carrier absorption (FCA) images on a CCD camera enabled the mapping of carrier lifetime distribution with a spatial resolution of 5 μm. The spatial variation of lifetime in the bulk HVPE‐grown GaN revealed the presence of different‐size crystalline grains, with lifetime peaking up to 70 ns in the centers of the largest grains (∼20 μm in diameter) and dropping to 10 ns in the small ones, while the spatially averaged lifetime was 40 ns. The inhomogeneity was ascribed to the interplay of nonradiative diffusion‐limited recombination at grain boundaries and a bulk lifetime in the crystallite centers. The numerical solution of spatially‐resolved carrier decay rate in the crystallite centers at high injection levels and comparison with experimental data provided a bulk nonradiative recombination time of ∼70 ns. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)