Recombination Time Research Articles

Electric field ionization of boron acceptors in single-crystalline diamond

Vertical hole transport in single-crystalline diamond films with ohmic and Schottky contacts was studied at dc and pulsed electric fields up to ∼ 5⋅105 V/cm. Conductivity mechanisms at different fields were identified. The concentrations of free carriers (holes) and acceptors were determined. The hole recombination time at boron acceptors has been estimated. The mechanisms of electric field ionization of boron acceptors are discussed.

The Basic Category and Application of Graphene-based Hybrid Photodetector

A photodetector is an optoelectronic device that can be divided into inorganic photodetector, which consists of traditional semiconductors, and organic photodetector, the recent direction of researching that made of organic semiconductors with the help of organic dye molecular or other organic compounds. The core of the improvement of photodetector is graphene, a single layer of the two-dimensional (2D) carbon crystalline structure, which provides the property with wide spectral bandwidth and considerably high response speed. This review focuses on the basic knowledge about graphene-based hybrid photodetector, because the hybrid photodetectors have the advantages of breaking the limitation of light absorption and shortening the recombination time of electrons and holes to increase photoresponsivity. Photodetectors can also behave differently when lights are in the infrared, ultraviolet, and terahertz region, so that the selection of materials based on graphene to detect the specific wavelengths of light becomes a challenge. In order to explain the complicated advanced technology briefly, this review can be regarded as two main parts. The first one is the basic properties of inorganic and organic photodetectors with the addition of perovskite, a new material that has been extensively studied recently, and the second part is about how photodetectors can be applied in a different wavelength of lights.

Effect of Surface Recombination on Diffusion Length and Active Cavity Life time

This work presents an analytical study for simulating a Fabry-Perot Bi-stable etalon (F-P cavity) filled with a dispersive optimized nonlinear optical material (Kerr type) such as semiconductors Indium Antimonite (InSb). Depending on the obtained results and because of a trade-off between the optical path length of the sample and active cavity lifetime, an optimization procedure was applied on the InSb etalon/CO laser parameters; critical switching irradiance (Ic) was applied via simulation systems of optimization procedures of optical cavity (Matlap program was used to study the optical Bi-stability of a nonlinear Fabry-Perot cavity). In order to achieve minimum switching power and faster switching time, the optimization surface recombination on the diffusion length and effective cavity lifetime was studied.
 In addition, for a specific absorption value 400 cm-1, the lifetime coefficient values were 0.33 , 0.091 , 0.0172 ns for sample thickness (D) = 500 , 60 , 20 mm, respectively. Also, for a bulk recombination time (Tl) of 200 ns, specific absorption (α) of 50 cm1, and D of 20 mm, the surface recombination speed value was 2.845 x 105 cm/sec, whereas the active lifetime, which is defined as the thickness over the surface recombination speed (sυ) (D/2sυ), was equal to 3.5ns.

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.

Plasmon induced charge transfer mechanism in gold-TiO2 nanoparticle systems: The size effect of gold nanoparticle

The application of surface plasmon in the solar-cell design has become a hot topic in the field of photovoltaic research. The enhancement of the photoelectric conversion efficiency is due to charge transfer caused by photoinduced injection of electrons from the metal to the corresponding acceptors. Revealing the basic physical mechanism further is of very important practical significance. We used the femtosecond time-resolved IR ultrafast spectroscopy technology and chose to excite the plasmon band of gold while changing the size of the gold nanoparticle to regulate the complex nanoprocess of the separation and recombination of photogenerated electrons in gold assembled with TiO2 systems. Behavior of hot holes in gold was also considered. We found that larger gold particles resulted in longer charge recombination times. The mechanism is discussed in detail in terms of restricted carrier diffusion in the nanospace.

A comprehensive study of ultrafast carrier dynamics of LT-GaAs: Above and below bandgap regions

We report the study of ultrafast carrier dynamics of low temperature grown GaAs over wide spectral range (2.75 eV–0.88 eV) with above and below bandgap (Eg) optical excitations using ultrafast transient absorption spectroscopy. The presence of defects and their subsequent effect on the pathway and lifetime of carrier relaxation with probing at different energy levels is presented. The carrier relaxation pathways and their corresponding models are predicted. It is observed that for pumping (2.58 eV) and probing above Eg, the transient behavior is dominated by band filling while renormalization is in prominence when probed near to Eg. For below Eg pumping (0.88 eV) the optical behavior is mostly governed by defects. The calculated trap time is in the range of ~128 fs to ~807 fs while overall recombination time shows a wide variation from ~1.38 ps to ~1.25 ns depending on the position of traps/defects in the forbidden band.

A new synthesizing method of TiO2 with montmorillonite: Effective photoelectron transfer to degrade Rhodamine B

Montmorillonite (MMT), with many excellent features, such as nano-layered structure, ion exchange ability and high hydrophilicity, could be considered as a kind of modified materials, so it is widely applied in many fields. In this study, novel photocatalyst material-TMC, is synthesized via the method of filling cations, accomplishing with prepared by loading TiO2 on the surface of MMT, which is proposed to remove Rhodamine B (Rh-B) via photocatalytic degradation. In the preparing process, the negative potential of MMT plays an important role in uniformly distributing TiO2 as well as limiting the size of distributed particles. Meanwhile, obtaining more oxygen vacancies could be attributed to optimal by screen calcination temperature. In addition, the specific surface area of TMC is measured to be three times larger than that of single TiO2. Furthermore, coupled with UV-DRS, FT-IR, XPS and PL, it is confirmed that Ti-O-Si bonds have been formed, and the narrowest bandgap (2.79 eV) is formed with the sample that calcinated at 450 °C. Meanwhile, the positions of conduction band and valence band with TMC contribute that it reaches higher Fermi level as compared with single TiO2. In addition, the negative potential (on the surface of MMT) acts as effectively extending the recombination time between electrons (e-) and holes (h+), so that the transferred photoelectrons can directly form O2− to degrade Rh-B that adsorbed on the surface of MMT. Finally, the main free radical scavenging experiment is conducted to indicate that O2− and OH are the dominant free radicals that involve in photocatalytic process, and the degradation pathways for Rh-B are speculated with characterization of HPLC-MS.

Biodegradable Fe-Doped Vanadium Disulfide Theranostic Nanosheets for Enhanced Sonodynamic/Chemodynamic Therapy.

Sonodynamic therapy (SDT), a noninvasive and highly penetrating tumor therapy, which employs ultrasound and sonosensitizers, has attracted extensive attention because of its ability to treat deep tumors. However, many current sonosensitizers have drawbacks in phototoxicity and limited sonodynamic effect. Herein, as a novel kind of sonosensitizer, iron-doped vanadium disulfide nanosheets (Fe-VS2 NSs) are constructed by a high-temperature organic-solution method and further modified with polyethylene glycol (PEG). With Fe doping, the sonodynamic effect of Fe-VS2 NSs is greatly enhanced, owing to the prolonged electron-hole recombination time. Simultaneously, such Fe-VS2-PEG NSs as a good Fenton agent can be utilized for chemodynamic therapy (CDT) by using the endogenous H2O2 in the tumor microenvironment (TME). Moreover, the multivalent Fe and V elements in the Fe-VS2 NSs can consume glutathione to amplify the reactive oxygen species-induced oxidative stress by SDT and CDT. Utilizing the strong near-infrared optical absorbance and enhanced magnetic resonance (MR) contrast by Fe-VS2 NSs, photoacoustic/MR biomodal imaging reveals a high accumulation of Fe-VS2-PEG NSs in the tumor. The great tumor suppression effect is then achieved by the in vivo combined CDT&SDT treatment. Importantly, most of the injected Fe-VS2-PEG NSs can be gradually decomposed and excreted from the mice, making them as safe sonosensitizers for cancer treatment. Our work highlights a new type of biodegradable sonosensitizer with the ability of regulating TME for applications in cancer theranostics.

Annealing temperature dependence of the performance of bulk heterojunction polymer: Fullerene solar cells under short and open circuit conditions

The current density-voltage(J-V) characteristics and cole-cole impedance plots (-Im(Z) vs Re(Z)) of ITO/PEDOT:PSS/AnE-PVab:PCBM/Ca/Ag solar cells were measured under thermal annealing range 25–125 °C using AM 1.5 G (80 mW/cm2) solar simulator (white light). The fill factors (FF), the short-circuit current density (JSC) and the open-circuit voltage (VOC) initially increased up to 110 °C and then decayed considerably for higher annealing temperatures. Under short circuit conditions at 0 V DC, The Cole-Cole plots demonstrate an increase in the semicircle radius for devices annealed at 25 °C and 100 °C. Then, the semicircle radii decrease with increasing annealing temperatures from 105 °C and above. However, at VOC conditions, we observe that the semicircle radii increase with increasing the thermal annealing. To reproduce theoretically the observed cole-cole impedance plots at 0 V DC and VOC conditions for different annealing temperatures, we used an equivalent circuit in the framework of the transmission line model, incorporating the chemical capacitance (Cµ), the recombination resistance (Rrec), the transport resistance (Rt) and the contact electrical resistance (Rco). We determined the diffusion time (τdif), the recombination time (τrec), the diffusion length (Ln) at 0 V DC and VOC voltages. At VOC voltage, average mobility of global carriers for the device is around 4 10−3 cm2V−1s−1 , which is in good agreement with that derived using PCBM electron-only devices.

Nonexponential Photoluminescence Dynamics in an Inhomogeneous Ensemble of Excitons in WSe2 Monolayers

The spectral and spatiotemporal dynamics of photoluminescence in monolayers of transition metal dichalcogenide WSe2 obtained by mechanical exfoliation on a Si/SiO2 substrate is studied over a wide range of temperatures and excitation powers. It is shown that the dynamics is nonexponential and, for times t exceeding ∼50 ps after the excitation pulse, is described by a dependence of the form 1/(t + t0). Photoluminescence decay is accelerated with a decrease in the temperature and in the energy of emitting states. It is shown that the observed dynamics cannot be described by a bimolecular recombination process, such as exciton—exciton annihilation. A model that describes the nonexponential photoluminescence dynamics by taking into account the spread of radiative recombination times of localized exciton states in a random potential gives good agreement with experimental data.

Recombination and spin dynamics of excitons in thin (Ga,Al)(Sb,As)/AlAs quantum wells with an indirect band gap and type-I band alignment

The dynamics of exciton recombination and spin relaxation in thin (Ga,Al)(Sb,As)/AlAs quantum wells (QWs) with indirect band gap are studied. The band alignment in these QWs is identified as type I. The exciton recombination time exceeds hundreds of microseconds, while the spin relaxation times of the electron and the heavy hole in an exciton do not exceed hundreds of nanoseconds. The heavy-hole longitudinal $g$ factor is determined to be $+2.5$. Despite the long exciton lifetimes, the photoluminescence circular polarization degree induced by a magnetic field is unexpectedly small and does not exceed $25%$.

Computing the Effect of Metal Substitution in Metal–Organic Frameworks on the Recombination Times of Charge Carriers

Metal–organic frameworks (MOFs) have been attracting increasing attention in the fields of photoactive applications, for which understanding the factors governing their excited-state properties is critical. One of the major challenges includes reducing the energy losses due to the phonon-assisted nonradiative recombination and extending charge-carrier lifetimes. Metal substitution constitutes a way to modify the electronic structure of MOFs. Among different options, Ce-MOFs are often considered; however, from the theoretical perspective, little is known about the charge-carrier dynamics and recombination pathways in such systems. Therefore, in this work, the electron–hole recombination in a prototype Ce-based MOF is investigated. The limit of a radiative charge-carrier lifetime is estimated theoretically. The nonradiative recombination process is simulated using nonadiabatic molecular dynamics within the decoherence-induced surface-hopping approach and the corresponding electron–hole recombination rates are calculated, demonstrating that the charge-carrier lifetimes in Ce-MOFs are in excellent agreement with the experimental data. Importantly, the vibrational modes, which are the main contributors to the nonradiative decay, are analyzed. In particular, the role of the soft phonon modes in the charge-carrier recombination process is highlighted. Based on this data, the routes for modulating electron–hole lifetimes are suggested.

Micro-LED as a Promising Candidate for High-Speed Visible Light Communication

Visible Light Communication (VLC) technology is an emerging technology using visible light modulation that, in the modern world, will mainly facilitate high-speed internet connectivity. VLC provides tremendous advantages compared to conventional radio frequency, such as a higher transmission rate, high bandwidth, low-power consumption, no health hazards, less interference, etc., which make it more prominent in recent days. Due to their outstanding features, including low cost, low power consumption, etc., µ-light-emitting diodes (LEDs) have gained considerable attention for VLC implementation, but mostly for the ability to be used for lighting as well as communications. In this review paper, we will focus mainly on recent developments in VLC applications and various factors affecting the modulation bandwidth of VLC devices. Numerous factors, such as quantum confined stark effect (QCSE), carrier lifetime, carrier recombination time, crystal orientation, etc. affect the modulation bandwidth of LEDs, and more information will be discussed in the following sections. This paper will focus on VLC applications based on LEDs but mainly on semipolar μ-LEDs and μ-LED-based arrays with high bandwidths. Another important application of VLC is underwater optical wireless communication (UOWC), which has drawn a huge interest in marine exploration and underwater connectivity, but still faces some challenges because visible light is being used. In addition, this paper will focus on how the current VLC system modulation bandwidth can be enhanced. Many methods have been introduced, such as decreasing the active layer thickness or effective active area or using doping, but the bandwidth is restricted by the recombination time when the system configuration reaches its limit. Therefore, it is important to find alternative ways such as optimizing the system, using the blue filter or using the equalization technology, which will be addressed later. Overall, this review paper provides a brief overview of the VLC-based system performance and some of its potential prospects.

Simultaneous Observation of Carrier-Specific Redistribution and Coherent Lattice Dynamics in 2H-MoTe2 with Femtosecond Core-Level Spectroscopy.

We employ few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to reveal simultaneously the intra- and interband carrier relaxation and the light-induced structural dynamics in nanoscale thin films of layered 2H-MoTe2 semiconductor. By interrogating the valence electronic structure via localized Te 4d (39-46 eV) and Mo 4p (35-38 eV) core levels, the relaxation of the photoexcited hole distribution is directly observed in real time. We obtain hole thermalization and cooling times of 15 ± 5 fs and 380 ± 90 fs, respectively, and an electron-hole recombination time of 1.5 ± 0.1 ps. Furthermore, excitations of coherent out-of-plane A1g (5.1 THz) and in-plane E1g (3.7 THz) lattice vibrations are visualized through oscillations in the XUV absorption spectra. By comparison to Bethe-Salpeter equation simulations, the spectral changes are mapped to real-space excited-state displacements of the lattice along the dominant A1g coordinate. By directly and simultaneously probing the excited carrier distribution dynamics and accompanying femtosecond lattice displacement in 2H-MoTe2 within a single experiment, our work provides a benchmark for understanding the interplay between electronic and structural dynamics in photoexcited nanomaterials.

Ultrafast control of fractional orbital angular momentum of microlaser emissions

On-chip integrated laser sources of structured light carrying fractional orbital angular momentum (FOAM) are highly desirable for the forefront development of optical communication and quantum information–processing technologies. While integrated vortex beam generators have been previously demonstrated in different optical settings, ultrafast control and sweep of FOAM light with low-power control, suitable for high-speed optical communication and computing, remains challenging. Here we demonstrate fast control of the FOAM from a vortex semiconductor microlaser based on fast transient mixing of integer laser vorticities induced by a control pulse. A continuous FOAM sweep between charge 0 and charge +2 is demonstrated in a 100 ps time window, with the ultimate speed limit being established by the carrier recombination time in the gain medium. Our results provide a new route to generating vortex microlasers carrying FOAM that are switchable at GHz frequencies by an ultrafast control pulse.

Nanosecond pulsed-bias-actuated and exciton-dynamics-induced chirp in InGaN/GaN LEDs towards realizing electrically-tunable broadband light emitters

Here, we show that a nanosecond electrical pulsed-bias actuation induces a chirp in the emission spectra of strained InGaN quantum well light emitting diodes (LEDs) that can be maneuvered to generate a broad tunable emission. The carrier and photon dynamics show that the emission wavelength can be tuned not only by the amplitude, but also by the on-time of the pulse. Whereas, the optical power can be tuned only by the on-time of the pulse. The time duration of the pulse is of the order of the carrier recombination time and the chirp is a consequence of the dominance of quantum confined Stark effect over junction temperature. Effect of polarization charges, applied bias, screening by injected carriers, carrier localization, and Bosonic exciton dynamics are amongst the imperative aspects that are considered for an accurate design of the pulsed-bias.

Interlayer Polarization Explains Slow Charge Recombination in Two-Dimensional Halide Perovskites by Nonadiabatic Molecular Dynamics Simulation.

Two-dimensional (2D) perovskites for applications in photovoltaics and optoelectronics are attracting a great deal of research interest. The nonradiative electron-hole (e-h) recombination is the major efficiency loss channel. Herein, we report a study of the thickness dependence of the e-h recombination dynamics in diamine-based 2D perovskite via ab initio NAMD. For multilayer structures, due to the emergence of spontaneous interlayer electric polarization, which is induced by the collective and correlated reorientation of methylammonium molecules, the electron and hole at the band edges are localized in different inorganic layers, suppressing the e-h recombination. Furthermore, a broad range of phonon excitation also inspired rapid pure dephasing related to the microscopic origin for longer recombination times. The combination of the two effects leads to the observation of a prolonged carrier lifetime in multilayer 2D perovskites, which is essential to understanding the nonradiative e-h recombination mechanism in such materials.

SN 2018fif: The Explosion of a Large Red Supergiant Discovered in Its Infancy by the Zwicky Transient Facility

Abstract High-cadence transient surveys are able to capture supernovae closer to their first light than ever before. Applying analytical models to such early emission, we can constrain the progenitor stars’ properties. In this paper, we present observations of SN 2018fif (ZTF 18abokyfk). The supernova was discovered close to first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil Gehrels Swift Observatory. Early spectroscopic observations suggest that the progenitor of SN 2018fif was surrounded by relatively small amounts of circumstellar material compared to all previous cases. This particularity, coupled with the high-cadence multiple-band coverage, makes it a good candidate to investigate using shock-cooling models. We employ the SOPRANOS code, an implementation of the model by Sapir & Waxman and its extension to early times by Morag et al. Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shock-cooling model as well as allowing usage of the entirety of the early UV data. We find that the progenitor of SN 2018fif was a large red supergiant with a radius of and an ejected mass of . Our model also gives information on the explosion epoch, the progenitor’s inner structure, the shock velocity, and the extinction. The distribution of radii is double-peaked, with smaller radii corresponding to lower values of the extinction, earlier recombination times, and a better match to the early UV data. If these correlations persist in future objects, denser spectroscopic monitoring constraining the time of recombination, as well as accurate UV observations (e.g., with ULTRASAT), will help break the extinction/radius degeneracy and independently determine both.

Charge localization induced by reorientation of FA cations greatly suppresses nonradiative electron-hole recombination in FAPbI3 perovskites: A time-domain Ab Initio study

Recent experiments report the rotation of FA (FA=HC[NH2]2+) cations significantly influence the excited-state lifetime of FAPbI3. However, the underlying mechanism remains unclear. Using ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we have demonstrated that reorientation of partial FA cations significantly inhibits nonradiative electron-hole recombination with respect to the pristine FAPbI3 due to the decreased NA coupling by localizing electron and hole in different positions and the suppressed atomic motions. Slow nuclear motions simultaneously increase the decoherence time, which is overcome by the reduced NA coupling, extending electron-hole recombination time scales to several nanoseconds and being about 3.9 times longer than that in pristine FAPbI3, which occurs within sub-nanosecond and agrees with experiment. Our study established the mechanism for the experimentally reported prolonged excited-state lifetime, providing a rational strategy for design of high performance of perovskite solar cells and optoelectronic devices.

Ultrafast dynamics and short-lived carriers in Cu nitride and oxynitride layers

Ultrafast carrier dynamics in copper nitride (Cu3N) have been investigated using femtosecond pump-probe spectroscopy. Cu3N films were prepared on fused SiO2 substrates coated with thin Cu layers under NH3:O2 flow. The obtained differential transmission data revealed discrete maxima in the spectra at different probing energies. Lower energy maxima are attributed to direct bandgap transitions, while transient transmission peaks at higher energies are due to probing defect state transitions in the material. These states originate from crystalline structure defects and are energetically located close to the conduction and valence bands. The recombination times extracted from the data were of the order of few picoseconds which raises the question of how suitable this material is for energy conversion applications. Ultrafast pump-probe spectra obtained at a low temperature of 78 K revealed almost no shifting of the transition energies compared to room temperature spectra, although small differences in decay times were observed due to the absence of thermal effects. Measurements using longer probing wavelengths in the infrared region indicated state filling of defect states within the bandgap for wavelengths between 800 nm and 1400 nm while probing at longer wavelengths (1400–1600 nm) revealed initial free-carrier absorption and re-excitation of carriers to higher energy defect states above the conduction band. In addition, copper oxynitride layers were produced and studied in a similar fashion to investigate the effects of oxygen on the band structure and recombination times.