Carrier Recombination Mechanisms Research Articles

Carrier Dynamics and Recombination Pathways in Ag-In-Zn-S Quantum Dots.

Strong tolerance to off-stoichiometry of group I-III-VI semiconductors in their nanocrystal form allows fabrication of multinary, alloyed structures of desired properties. In particular, alloyed Cu-In-Zn-S and Ag-In-Zn-S quantum dots (QDs) have recently emerged as efficient fluorophors, in which tailoring the composition allows tuning the optical properties, and achieving photoluminescence (PL) quantum yields approaching unity. However, poor understanding of the carrier recombination mechanism in these materials limits their further development. In this work, by studying transient absorption and temperature dependent PL on bare QDs and QDs conjugated with electron scavenger molecules, we obtain a detailed picture of carrier dynamics. Our results challenge the prevailing assumption that the PL is due to a donor-acceptor-pair transition. We show that the PL occurs as a result of a recombination of a delocalized electron with a localized hole.

Investigation of the mechanism of carrier recombination in GaN-based blue laser diodes before lasing

Abstract The carrier recombination behavior of GaN-based blue laser diodes (LDs) is studied and analyzed by experiments and simulation calculations before lasing, with a particular focus on the role of Auger recombination. It is found that Auger recombination plays a crucial role in the decrease in differential efficiency and threshold current of GaN-based blue LDs. The theoretical calculation results show that a large Auger recombination rate may lead to a dominant recombination channel before lasing, which could exceed the radiation recombination and result in an obvious decrease in the differential efficiency. Such a high Auger recombination will dissipate a large number of carriers in the quantum well, resulting in deterioration of device performance, a higher threshold current and a lower efficiency. This work presents a method to evaluate Auger recombination through differential efficiency and also provides evidence that suppressing the Auger recombination rate is beneficial to improve the performance of blue LDs.

(Invited) Plasmon Induced Hot Electron Transfer from Doped Semiconductor Nanocrystals

In recent years, a new class of plasmonic materials based on doped semiconductor NCs has been developed. Similar to noble metals, localized surface plasmon resonances in doped semiconductor NCs arises from the coherent collective oscillation of free carriers within the NCs. The wide tunability of free carrier concentration results in tunable plasmon bands spanning from the visible to mid-infrared regime and these plasmonic semiconductor NCs have been reported to drive plasmonic hot carrier photocatalysis under near IR light illumination. These reports suggest that plasmonic semiconductors is an interesting material platform for efficient near IR driven hot carrier photocatalysis. So far the reported efficiencies are low and the mechanisms of hot carrier generation, transfer and recombination are unclear. There are interesting similarities and differences between metal and semiconductor plasmonics. First, plasmons in semiconductors can decay by inter- (band gap) and intra- band plasmon decay. Furthermore, charge transfer transition in semiconductor heterostructures have been extensively reported; can plasmon decay in plasmonic-semiconductor/semiconductor junctions proceed by exciting interfacial ET across the junction, similar to the PICTT pathway reported for metal/semiconductor junctions? Second, in plasmonic semiconductor materials, where there is a lack of states within the p-semiconductor bandgap, the transferred electrons in the accepting semiconductor CB can recombine only with the VB holes in the plasmonic semiconductor. This process requires dissipation of ~ >1 eV energy, which may slow down the recombination rate. In this talk, I will discuss our recent progresses in studying plasmon induced hot electron transfer and recombination in plasmonic semiconductor/semiconductor interfaces.

Numerical analysis of the influence of sidewall defects on AlGaN-based deep ultraviolet micro-light emitting diodes

The optoelectronic performance of AlGaN-based deep ultraviolet micro-light emitting diodes are strongly affected by surface recombination at the mesa sidewall. Herein, the influence of sidewall defect density and location changes on the photoelectric properties and carrier recombination mechanisms were theoretically investigated. The results indicate a significant deterioration in the internal quantum efficiency and optical power with an increase in the sidewall defect density at the edge of the LED mesa. This deterioration is attributed to the Shockley-Read-Hall nonradiative recombination caused by sidewall defects. The sidewall defects also act as traps for electrons and holes, significantly affecting the carrier injection capability. Furthermore, the position dependence of carrier concentration and recombination rate along the lateral dimension of the LED mesa were studied. The results show that etching process not only causes sidewall damage but can damage the quality of internal epitaxial materials. These effects should be minimized by optimizing the dry-etching process.

Defect Engineering and Emission Tuning of Wide-Bandgap MAPbCl3 Perovskite.

Lead-chloride perovskites are promising candidates for optoelectronic applications, such as visible-blind UV photodetection. It remains unclear how the deep defects in this wide-bandgap material impact the carrier recombination dynamics. In this work, we study the defect properties of MAPbCl3 (MA = CH3NH3) based on photoluminescence (PL) measurements. Our investigations show that apart from the intrinsic emission, four sub-bandgap emissions emerge, which are very likely to originate from the radiative recombination of excitons bound to several intrinsic vacancy and interstitial defects. The intensity of various emission features can be tuned by adjusting the type and ratio of precursors used during synthesis. Our study not only provides important insights into the defect property and carrier recombination mechanism in this class of material but also demonstrates efficient strategies for defect passivation and engineering, paving the way for further development of lead-chloride perovskite-based optoelectronic devices.

Compact Model and Detailed Balance Limit for a Dual n-Type Direct Z-Scheme Heterojunction.

This paper presents a comprehensive study on a compact model and the detailed balance limit for a dual n-type direct Z-scheme heterojunction. The compact model developed in this work describes the current-voltage (IV) characteristics of the staggered heterojunction under one-sided illumination. The model incorporates charge neutrality, surface recombination, thermionic emission over the barrier, and surface potentials. By considering these factors, the IV curve of the staggered heterojunction is captured, shedding light on the charge transfer and separation processes within the device. The heterojunction device consists of two photosystems: photosystem one (PSI) with a wide band gap and photosystem two (PSII) with a narrow band gap. Furthermore, the paper establishes the detailed balance limit for the efficiency of the dual n-type direct Z-scheme heterojunction. The maximum achievable efficiency, estimated to be 11.4%, is determined by the interplay between the band gap of PSII and the empirical relation for the maximum barrier for electrons leaving PSII. This detailed balance limit represents the highest efficiency that can be attained, accounting for carrier generation, recombination, and charge transfer mechanisms. The compact model and the derived detailed balance limit provide insights for designing and improving the performance of direct Z-scheme heterojunctions.

Ultrafast photocarrier dynamics in InAs/GaAs self-assembled quantum dots investigated via optical pump-terahertz probe spectroscopy

Optical pump-terahertz probe (OPTP) spectroscopy was performed to measure the lifetime of photogenerated carriers in the barrier and the wetting layer (WL) regions of an indium arsenide on gallium arsenide (InAs/GaAs) single-layer self-assembled quantum dot (QD) sample. A modified rate equation model of carrier dynamics was proposed where possible state-filling in both QD and WL is considered. Drude model fitting was also performed to extract the time-dependent plasma frequency and phenomenological scattering time from the terahertz transmission spectra. The results of the OPTP experiment show two prominent recombination processes that occur at different timescales after photoexcitation. These two processes were attributed to carrier recombination in the GaAs barrier and the quantum well-like states of the WL based on the fitted lifetimes. Calculations using the coupled differential rate equations were also able to replicate the experimental trend at low fluence. The lack of agreement between experimental data and numerical calculations at high optical fluence was mainly attributed to the possible saturation of the GaAs density of states. Lastly, the results of the parameter fitting for the plasma frequency and scattering time indicate a transition from the barrier to the WL recombination as the dominant carrier recombination mechanism within the time scale of the OPTP scan. This further lends credence to the proposed model for carrier dynamics in SAQD systems under state-filling conditions.

Post-annealing-free BaOxFy/LiF-based stack electron-selective contacts for high efficiency crystalline silicon solar cells featuring ultra-low contact resistivity

In silicon heterojunction and tunnel oxide passivating contact crystalline silicon (c-Si) solar cells based on successful passivating contacts technologies, an annealing activation step is indispensable to achieve a suitably low contact resistivity (ρc), which is a costly and non-ideal process. Here, combined with a calcium aluminium (Ca:Al) alloy electrode, a low work function barium oxide and lithium fluoride (BaOxFy/LiF) stack electron-selective contact (ESC) without an annealing activation step on c-Si solar cells clinches an ultra-low ρc of 1.4 mΩ·cm2 delivering a power conversion efficiency (PCE) of 20.5 %. The BaOxFy/LiF/Ca:Al stack structure is built by BaOx/LiF/Ca/Al hybrid structure via a facile continuous thermal evaporation process. As a proof of concept, the stacked BaOxFy/LiF/Ca:Al/Al contact is applied to the rear side of c-Si solar cells, achieving a PCE of 20.5 % with an open-circuit voltage of 625.7 mV, an impressive fill factor of 83.5 % and a short-circuit current density of 39.2 mA/cm2. With the aid of a transient photovoltage setup, the carrier recombination mechanism of the stacked BaOxFy/LiF/Ca:Al structure is explored. This work demonstrates an alternative idea for undoped ESCs, showing potential for next-generation cost-effective ultrathin-slice c-Si solar cells (∼80 μm) with high efficiency, and low-parasitic light absorption losses.

A strategy for boosting photovoltaic performance based on a two-dimensional ZrSSe/HfSSe van der Waals heterostructure.

Identifying high-efficiency solar photovoltaic systems with two-dimensional (2D) materials is still an urgent challenge to meet modern energy requirements. Very recently, a 2D heterostructure with type-II band alignment has been confirmed to be more favorable for application in photoelectric conversion. However, the staggered band offset of 2D type-II heterostructures cannot always be guaranteed, nor the intrinsic hindrance mechanism of carrier recombination being clear. In this study, taking the emerging ZrSSe/HfSSe van der Waals heterostructure (vdWH) as a generic example, a boosting strategy for improving the photoelectric performances of 2D vdWHs is proposed. Through a series of in-depth systematic research studies based on first-principles, we demonstrate that via applying a vertical strain, an anticipated band alignment transition from type-I to favorable type-II of this ZrSSe/HfSSe vdWH can be induced due to the interfacial charge redistribution, during which a corresponding enlarged photocurrent can be detected from the latter based device compared to the former. Essentially, such enhanced photocurrent at the incident photon energy (Eph) around the band gap is attributed to the suppressed recombination rate of photoexcited carriers. Moreover, when Eph is increased into the visible light region, the photoelectric conversion performances can be further controlled by vertical strain. These generalized findings not only provide an effective manipulation strategy for enhancing the performances of 2D solar photovoltaic systems, but the intrinsic physical mechanism can also be extended to the next practical design and regulation of other 2D photovoltaic devices.

Carrier recombination in highly Al doped 4H-SiC: dependence on the injection conditions

We investigate carrier recombination mechanisms in heavily aluminum (Al) doped p-type 4H-SiC, a material crucial for power devices. The recombination mechanisms in Al-doped p-type 4H-SiC have remained unclear, with reports suggesting various possibilities. To gain insights, we employ photoluminescence (PL) measurements, particularly time-resolved PL (TR-PL), as they are well-suited for studying carrier lifetimes in heavily Al-doped p-type 4H-SiC. We examine the temperature and excitation intensity dependencies of TR-PL and PL spectra and discuss the underlying recombination mechanisms. We observe that the dominant recombination mechanism varies with injection conditions for the samples with Al concentration less than 1019 cm−3. Under low injection conditions, recombination via the Al acceptor level appears dominant, exhibiting weak temperature dependence. However, under high injection conditions, Shockley–Read–Hall recombination takes precedence, leading to shorter carrier lifetimes with increasing temperature. This temperature dependence implies that presences of the deep recombination centers with the small capture barrier for holes.

The effective prolongation of the excited-state carrier lifetime of CsPbI2Br with applying strain.

In recent years, all-inorganic perovskites CsPbX3 (X = Cl, Br, I) have emerged as excellent candidates for solar cells due to their remarkable thermal stability and suitable bandgaps. Among them, CsPbI2Br is a hotspot in perovskite material research currently. Non-radiative electron-hole recombination often leads to significant energy losses, impacting the efficiency of solar cells, so a thorough understanding of carrier recombination mechanisms is crucial. Our work investigated the carrier recombination dynamics in detail and proved that strains can effectively reduce nonradiative recombination. In this study, using first-principles calculations combined with nonadiabatic (NA) molecular dynamics (MD), we demonstrate that applying 2% tensile and 2% compressive strains to CsPbI2Br can modify the bandgap, induce moderate disorder, reduce the overlap of electron-hole wavefunctions, decrease NA coupling, and shorten decoherence time, thereby minimizing non-radiative recombination and extending the carrier lifetime. Especially the 2% tensile strain exhibits more effective control performance, significantly reducing non-radiative electron-hole recombination and extending the charge carrier lifetime to 14.59 ns, nearly five times that of the pristine CsPbI2Br system (3.12 ns). This study reveals the impact mechanism of strain on carrier behavior in perovskite solar cells, providing a new non-chemical strategy for modulating the lifetime of photo-generated carriers and enhancing the efficiency of all-inorganic perovskite solar cells.

Influence of period number of superlattice electron barrier layer on the performance of AlGaN-based deep ultraviolet LED

The development of AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs) is currently limited by poor external quantum efficiency (EQE) and wall-plug efficiency (WPE). Internal quantum efficiency (IQE), as an important component of EQE, plays a crucial role in improving the performance of DUV-LEDs. The IQE is related to the carrier injection efficiency and the radiation recombination rate in the active region. In order to improve the IQE of AlGaN-based DUV-LEDs, this work proposes a scheme to optimize the period number of superlattice electron barrier layer (SL-EBL) to achieve better carrier injection efficiency and confinement capability. The effect of the period number of SL-EBL on the luminous efficiency, reliability and carrier recombination mechanism of AlGaN-based DUV-LEDs with an emission wavelength of 273 nm are investigated. The experimental results show that the light output power (LOP), external quantum efficiency (EQE) and wall-plug efficiency (WPE) of the DUV-LEDs tend to first increase and then decrease with the period number of SL-EBL increasing, while the leakage current decreases and the reliability is enhanced. The maximum EQE and WPE of the DUV-LED are 3.5% and 3.2%, respectively, at an injection current of 7.5 mA when the period number of SL-EBL is fixed at 7 (the thickness is 28 nm). Meanwhile, the numerical simulation results show that the electron potential barrier height is enhanced with the period number of SL-EBL increasing, and the variation of the hole potential barrier height is negligible. Therefore, increasing the period number of SL-EBL is beneficial to shielding the dislocations and suppressing the leakage of electrons into the p-type layer, which improves the luminous efficiency and reliability of DUV-LEDs. However, when the period number of SL-EBL exceeds 7, the excessively thick hole potential barrier prevents the holes from entering into the activation region and reduces the radiative recombination efficiency. Therefore, EQE and WPE will show an inflection point with the variation of the period number of SL-EBL. In addition, to investigate the carrier recombination mechanism of the active region, the experimental EQE curves are fitted by the ABC model as well as the different slopes in logarithmic light output power-current (<i>L-I</i> ) curves are calculated after aging. It can be found that increasing the period number of SL-EBL can effectively suppress the non-radiative combination of carriers in the active region. This investigation can provide an alternative way to enhance the photoelectric performance of DUV-LEDs.

Negative Photoresponse Switching via Electron-Hole Recombination at The Type III Junction of MoTe2 Channel/SnS2 Top Layer.

Extensive study on 2Dvan der Waals (vdW) heterojunctions has primarily focused on PN diodes for fast-switching photodetection, while achieving the same from 2D channel phototransistors is rare despite their other advantages. Here, a high-speed phototransistor featuring a type III junction between p-MoTe2 channel and n-SnS2 top layer is designed. The photodetecting device operates with a basis of negative photoresponse (NPR), which originates from the recombination of photoexcited electrons in n-SnS2 and accumulated holes in the p-MoTe2 channel. For the NPR to occur, high-energy photons capable of exciting SnS2 (band gap ≈2.2eV) are found to be effective because lower-energy photons simply penetrate the SnS2 top layer only to excite MoTe2 , leading to normal positive photoresponse (PPR) which is known to be slow due to the photogating effects. The NPR transistor showcases 0.5ms fast photoresponses and a high responsivity over 5000 A W-1 . More essentially, such carrier recombination mechanism is clarified with three experimental evidences. The phototransistor is finally modified with Au contact on n-SnS2 , to be a more practical device displaying voltage output. Three different photo-logic states under blue, near infrared (NIR), and blue-NIR mixed photons are demonstrated using the voltage signals.

Ultrafast broadband carrier and exciton dynamics of Fe-related centers in GaN

The effect of Fe defects on carrier recombination and two-photon induced ultrafast exciton dynamics in GaN crystals were investigated using femtosecond transient absorption spectroscopy. The absorption kinetics exhibited completely different characteristics under different nonequilibrium carrier concentrations and distributions. The carrier recombination mechanisms under different excitation conditions can be interpreted by the model of Fe defect-related energy level. The absorption under one-photon excitation originates from the intraband transition of free holes, and the capture of holes by Fe2+ leads to the decay of the absorption. The ultrafast excitonic state [Fe2+,hVB] formation time under two-photon excitation results in a rapid absorption decay (<20 ps), while the long tail of the absorption response could be attributed to long-lived excitonic state (>100 ns). These findings are crucial to the applications of GaN for ultrafast optoelectronics and integrated nonlinear optics.

Estimation of activation energy using skewness of TL data as symmetry parameter in peak shape method

The retrapping and recombination mechanism of charge carriers in a thermoluminescence (TL) process governs the shape of a glow curve. In peak shape method the shape of a glow curve is exploited through various symmetry parameters to estimate activation energy. In statistical sense, skewness is a measure of distribution asymmetry of a dataset and hence may be useful to reflect the shape of a TL curve through its asymmetry. In this work, we investigate the applicability of skewness as a symmetry parameter and propose a new set of peak shape relations in terms of skewness of the data to extract activation energy from an isolated TL peak. The applicability of present method is verified by employing it to the TL peaks simulated in General Order Kinetics (GOK) and One Trap One Recombination centre (OTOR) equations. The present method yields consistent results for all the glow curves except for highly saturated cases with heavy retrapping. This limitation is investigated by studying the dependence of skewness on the ratio of retrapping to recombination probabilities. Finally, the present peak shape formulae are tried on experimental TL curves reported in literature and the results are quite satisfactory. The present peak shape coefficients are listed explicitly so that they can be used to estimate activation energy using skewness of isolated peaks deconvoluted from experimental TL data.

Interband carrier recombination mechanism in Al-doped ZnO

Due to strong nonlinear optical effects and ultrafast response properties, transparent conductive oxides are promising candidate materials for next-generation THz modulation devices and other photonic applications. However, the mechanisms of photon-induced ultrafast carrier relaxations in those materials have yet to be fully understood. Here we investigated the interband carrier dynamics in a prototype of this family, Al-doped ZnO, with different excitation photon energies through femtosecond optical transmittance measurements. We found that the interband relaxation of the photon-induced carriers is mainly through second-order recombination, which is essentially the direct band-to-band recombination with the assistance of intraband electron relaxation. The high density of the doped electrons provides superior intraband relaxation channels through electron-phonon interaction and electron-impurity scattering, which contribute to the overall ultrafast response, especially under indirect and band tail transitions. This study provides a new insight into the ultrafast response mechanism of Al-doped ZnO and the analyzing method may also be generalized to other heavily doped semiconductors.

Insight into Charge Carrier Recombination Mechanisms in Lead Based Mixed Cation and Halide Perovskite

Insight into Charge Carrier Recombination Mechanisms in Lead Based Mixed Cation and Halide Perovskite

Influence of polarities on optical properties of Mg-doped GaN films grown on GaN free-standing substrates by MOCVD

Time-resolved photoluminescence measurements were performed to investigate the minority carrier recombination mechanism of p-GaN films grown on c-plane, m-plane and (10–11)-plane GaN freestanding substrates by MOCVD. The fast and slow decay lifetimes of the three samples were fitted by the bi-exponential function, in which the slow decay lifetime of the m-plane Mg-doped GaN reached a record long value of 493.7 ps? The effects of stress-induced non-radiative recombination centers and edge dislocations on the slow decay lifetime were ruled out using Raman and XRD. The reason for the longer slow decay lifetime of the m-plane sample is the reduction of the non-radiative recombination centers due to the decrease of the VGa concentration, ensuring by the magnitude of the IYL/INBE ratio in the PL spectra. The low-temperature PL spectra of the m-plane sample were investigated. According to Haynes' rule, the position of acceptor level was calibrated through combining with the position of the bound excitons at 5 K.

Transformation of carrier recombination mechanism as increasing the Germanium content in Cu2ZnGexSn1-xS4 single crystal prepared by molten salt method

Return to the physics origin of Cu2ZnSnS4 type (CZTS-type, include CZGS, CZGTS, CZTSSe, etc.) material system, studying the influence of replacement elements on the optoelectronic properties by preparing high-quality single crystal materials to clarify the relationship between the key defect states of materials and carrier dynamics is the only way which must be passed to find efficient paths to improve the existing performance of CZTS-type optoelectronic devices. Herein, the optical behavior of high-quality Cu2ZnGexSn1-xS4 (CZGTS) single crystal prepared by the molten salt method and its relationship with Ge content were systematically studied using a photoluminescence (PL) and absorption tests at room temperature. Combined with the simple electrical tests, the transformation of carrier recombination mechanism as increasing the Germanium content in CZGTS single crystal before and after phase transition were reported for the first time. The transmission electron microscopy images, X-ray diffraction spectra and Raman scattering spectra of CZGTS were adopted to reveal the evolution mechanism of single crystal growth and phase transition caused by the change of Ge composition. The results show that the tunable blue-shift of PL peak emerges with the increase of Ge content and the excitation power dependence has a saturation phenomenon in the CZGTS single crystal particles. In consideration of the optical bandgap, PL peak position, PL intensity, carrier lifetime and the macroscopic resistivity, it is confirmed that the intrinsic PL behavior could be effectively tailored by different Ge amounts. Compared with the recombination PL of interband carrier (Ep > Eg) in Cu2ZnSnS4, the band tailed carriers (Ep < Eg) dominated by defect states would play a powerful role in CZGTS single crystal. Our work provides a strategy for tailoring the bandgap of CZGTS materials and a new perspective for understanding the influence of defect states on carrier dynamics.

Exciton recombination in few‐layer MoS2 nanosheets: Role of free carriers and defects

A current obstacle to the development of extremely flexible and high‐performance optoelectronic devices using transition metal dichalcogenide nanosheets is the incomplete understanding of the exciton and free carrier recombination in the presence of defects. Here, taking liquid‐phase exfoliated few‐layer MoS2 nanosheets as a model system, we demonstrate the exciton and free carrier recombination mechanism by employing pump energy and fluence‐dependent ultrafast transient absorption spectroscopy. We demonstrate that 3.10 eV pump excitation, much above the lowest energy A exciton of four to six layers of MoS2 nanosheets (~1.84 eV), generates excitons and free carriers. The excitons decay quickly within ~3 ps due to the defect capture via the Shockley–Read–Hall mechanism. In contrast, free carriers show slower recombination (~1000 ps), which is an order of magnitude larger than the exciton recombination time. We verified this idea by exciting the sample with 1.94–2.22 eV pump excitations that predominantly generate excitons decaying within ~3 ps. Our systematic studies in few‐layer MoS2 nanosheets reveal crucial information on the unexplored domain of excitons and free carriers recombination in the presence of defects for several optoelectronic applications.