Recombination Time Research Articles

N-polar GaN p-n junction diodes with low ideality factors

High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy. The room-temperature current–voltage characteristics reveal a high on/off current ratio of >1011 at ±4 V and an ideality factor of 1.6. As the temperature increases to 200 °C, the apparent ideality factor gradually approaches 2. At such high temperatures, Shockley–Read–Hall recombination times of 0.32–0.46 ns are estimated. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level and acceptor-related luminescence is greatly suppressed.

Tunability of solar cell with double quantum dot structure

This work introduces a double quantum dot (QD) structure as an intermediate band for high-performance solar cells. The density matrix equations, coupled with the continuity-current equation, are solved numerically to address the interaction between all the states, which is impossible with the rate equation modeling. This modeling allowed us to obtain quantum efficiency (QE). Using the orthogonalized plane wave assumption for WL-QD, the momentum matrix elements of QD-QD, QD-wetting layer (WL), WL-barrier transitions are calculated. Results are simulated in both the excitonic and non-excitonic (electron-hole, eh) cases and exhibit the importance of adding the QD layer.The results confirm the importance of adding the intermediate band (QD layer). Both band-to-band recombination rates and relaxation rates in the DQD structure are modulated according to the energy difference between DQD states. Compared to the eh model, the excitonic model reduces the QD-QD relaxation rates and all band-to-band recombination rates. The quantum efficiency (QE) is attaining at longer recombination and relaxation times that are confirmed depending on the wider energy difference by manipulating the transitions of the DQD structure.

Microscopic optical nonlinearities and transient carrier dynamics in indium selenide nanosheet.

This work systematically investigates the third-order nonlinear optical (NLO) properties and ultrafast carrier dynamics of layered indium selenide (InSe) obtained by mechanical exfoliation (ME). The two-photon absorption (TPA) effect of layered InSe was tested using micro-Z/I-scan techniques. The results indicate that InSe flakes undergo the TPA response under the excitation of both 520 nm and 1040 nm fs pulses, and that InSe is more likely to achieve TPA saturation under visible light excitation. Furthermore, ultrafast carrier dynamics revealed that InSe flakes in the visible region undergo a transition from photoinduced absorption to photobleaching and exhibit a fast recombination time of ∼0.4-1ps, suggesting a high optical modulation speed as high as ∼1-2.5 THz.

Assuming Ionization Equilibrium and the Impact on the Lyα Forest Power Spectrum during the End of Reionization at 8 ≥ z ≥ 5

We explore how the assumption of ionization equilibrium modulates the modeled intergalactic medium at the end of the hydrogen epoch of reionization using the cosmological radiation hydrodynamic Technicolor Dawn simulation. In neutral and partially ionized regions where the metagalactic ultraviolet background is weak, the ionization timescale t ion ≡ Γ−1 exceeds the Hubble time. Assuming photoionization equilibrium in such regions artificially boosts the ionization rate, accelerating reionization. By contrast, the recombination time t rec < t ion in photoionized regions, with the result that assuming photoionization equilibrium artificially increases the neutral hydrogen fraction. Using snapshots in the range 8 ≥ z ≥ 5, we compare the predicted Lyα forest (LAF) flux power spectrum with and without the assumption of ionization equilibrium. Small scales (k > 0.1 rad s km−1) exhibit reduced power from 7 ≤ z ≤ 5.5 in the ionization equilibrium case, while larger scales are unaffected. This occurs for the same reasons: ionization equilibrium artificially suppresses the neutral fraction in self-shielded gas and boosts ionizations in voids, suppressing small-scale fluctuations in the ionization field. When the volume-averaged neutral fraction drops below 10−4, the signature of nonequilibrium ionizations on the LAF disappears. Comparing with recent observations indicates that these nonequilibrium effects are not yet observable in the LAF flux power spectrum.

Fabrication of fractal structured soot templated titania-silver nano-surfaces for photocatalysis and SERS sensing

A novel approach of candle soot templated TiO2/Ag nanostructures to fabricate highly active photocatalyst and self-cleaning Surface-Enhanced Raman Scattering (SERS) substrates is demonstrated. A three-stage synthetic approach involves the replication of the fractal structure of candle soot in forming a high surface area porous TiO2 nanostructure using chemical vapour deposition and subsequent electroless deposition of silver nanoparticles on the high surface area fractal network that resulted in the formation of TiO2/Ag nanostructures. The unique morphology of TiO2/Ag renders these materials to exhibit photocatalytic and SERS activity. These surfaces showed an enhanced photocatalytic effect due to the Schottky junction which increases the recombination time of the charge carriers. The size and the coverage of the silver nanoparticles were optimized to obtain high conversion towards photocatalytic degradation of a model dye molecule such as Rhodamine B (RhB). The same substrate was also shown to enhance the Raman scattering spectra of RhB. The SERS activity of the substrate can be regenerated by photocatalytic cleaning the substrates. Overall, this present work demonstrated that fractal structured TiO2/Ag materials can be synthesized using fractal soot structures and this multi-functional material offers good potential in those applications, where photo-catalysis, SERS sensing, and self-cleaning sensing multifunctionality are required.

Temperature variation of photoconductivity in the layered quasi one-dimensional compound TiS3: Semiconducting and unconventional behavior

The layered quasi one-dimensional compound TiS3 is remarkable for its optical properties, especially, photoconductivity. Up to now, photoconductivity in TiS3 was studied only at room temperature. Here, we report photoconductivity, δσ, of the TiS3 whiskers in the temperature range of 5–300 K under irradiation in the near-infrared region. With a temperature decrease from room temperature down to T ∼ 100 K, δσ grows moderately, dominated by the variation of the mobility of the electrons. The estimates based on the values of δσ give the recombination time ∼3–25 μs at 300 K. The results show that above ∼100 K, TiS3 behaves as a semiconductor, where electron–hole pairs are excited across the gap, while the recombination goes through transitions of the excited electrons to a donor level with a long lifetime. Below 60 K, a drop of δσ is likely to reveal a phase transition of electrons into a collective state. Another feature in δσ(T) is seen around 17 K, resembling the temperature variation of the conductivity. This feature indicates a drop in the mobility of the electrons. While the behavior of δσ(T) is consistent with the transition of electrons into a collective state, it is strikingly different from δσ(T) of the known quasi one-dimensional conductors with charge-density waves.

Analyzing the Hubble tension through hidden sector dynamics in the early universe

The recent analysis from the SH0ES collaboration has confirmed the existence of a Hubble tension between measurements at high redshift (z > 1000) and at low redshift (z < 1) at the 5σ level with the low redshift measurement giving a higher value. In this work we propose a particle physics model that can help alleviate the Hubble tension via an out-of-equilibrium hidden sector coupled to the visible sector. The particles that populate the dark sector consist of a dark fermion, which acts as dark matter, a dark photon, a massive scalar and a massless pseudo-scalar. Assuming no initial population of particles in the dark sector, feeble couplings between the visible and the hidden sectors via kinetic mixing populate the dark sector even though the number densities of hidden sector particles never reach their equilibrium distribution and the two sectors remain at different temperatures. A cosmologically consistent analysis is presented where a correlated evolution of the visible and the hidden sectors with coupled Boltzmann equations involving two temperatures, one for the visible sector and the other for the hidden sector, is carried out. The relic density of the dark matter constituted of dark fermions is computed in this two-temperature formalism. As a consequence, BBN predictions are upheld with a minimal contribution to ΔN eff. However, the out-of-equilibrium decay of the massive scalar to the massless pseudo-scalar close to the recombination time causes an increase in ΔN eff that can help weaken the Hubble tension.

Cage-shaped [P(W3O10)4]3- nanoclusters deposited carbon nitride to construct a dual-functional catalyst for photocatalytic mineralization wastewater and dye detector

In this work, [P(W3O10)4]-g-C3N4 composite catalyst is prepared from phosphotungstic acid and thin-layer carbon nitride at high temperature, which can efficiently remove Rhodamine B (RhB) organic wastewater by photocatalysis. The results of UV–vis, TPC, PL and EIS tests show that the introduction of [P(W3O10)4]3- increases the light absorption range of the g-C3N4 and prolongs the recombination time of photogenerated electrons and holes. The degradation efficiency of g-C3N4 and the [P(W3O10)4]-g-C3N4 catalyst are compared and analyzed, and the corresponding active species (O2·-) is proposed. The new catalyst can decolor and mineralize the RhB solution in a short time, and its degradation rate and mineralization rate are 98.0% and 81.0%, respectively. Interestingly, [P(W3O10)4]-g-C3N4 catalyst and RhB solution mixture can be used as a chili powder dye detector. Its special performance can be attributed to the dissociation of the composite catalyst under acid conditions, and the newly formed H3PW12O40 combines with the RhB solution to change color. In short, a dual-function catalyst is synthesized to remove water pollution, and its advantages such as high efficiency, rapidity and strong practicability are worthy of attention.

Azadipyrromethene Dye-Assisted Defect Passivation for Efficient and Stable Perovskite Solar Cells.

Organic-inorganic perovskite solar cells (PSCs) provide one of the most outstanding photovoltaic (PV) technologies, yet their efficiency, stability, and defect passivation engineering still remain challenging. We demonstrate the use of low-cost, eco-friendly, and multi-functional aza-dipyrromethene (Aza-DIPY) dye molecules to promote the power conversion efficiency (PCE) and the operating stability of PSC devices. The Aza-DIPY dye was meticulously synthesized and incorporated into PSC devices via a one-step solution processing approach. The pyrrole, benzene ring, and chlorine functional groups on the dye have intense interactions with perovskite to passivate surface defects and obtain high-quality perovskite absorbers, resulting in the lengthened carrier recombination time and enhanced fill factor of PSCs. Additionally, the hydrophobic phenyl and halogen functional groups on the Aza-DIPY perform as a protecting barrier against moisture and ameliorate the stability of PSCs. As a consequence, the PV performance of PSCs is considerably improved, with the average PCE increased from 16.71% to 19.71%, and the champion device with Aza-DIPY shows a PCE of 20.46%. The unencapsulated PSC devices with multi-functional molecular Aza-DIPY maintains 89.06% of their beginning PCEs after storage in ambient air (25-30 °C, 50-70% relative humidity) under dark conditions for 100 h, exhibiting a significantly enhanced ambient stability compared with the case of the reference cells without the dye. Furthermore, the Aza-DIPY-modified PSC devices exhibit strong and reversible photoresponses, with a high responsivity of 0.739 mA/W to near-infrared (NIR) laser beams. Our results highlight the potential of synthesizing multi-functional Aza-DIPY dyes-incorporated PSC devices with sensitive NIR/visible light responses, high PV efficiency, and stability.

Charge carrier nonadiabatic dynamics in non-metal doped graphitic carbon nitride.

Graphitic carbon nitride (GCN) has attracted significant attention due to its excellent performance in photocatalytic applications. Non-metal doping of GCN has been widely used to improve the efficiency of the material as a photocatalyst. Using a combination of time-domain density functional theory with nonadiabatic molecular dynamics, we study the charge carrier dynamics in oxygen and boron doped GCN systems. The reported simulations provide a detailed time-domain mechanistic description of the charge separation and recombination processes that are of fundamental importance while evaluating the photovoltaic and photocatalytic performance of the material. The appearance of smaller energy gaps due to the presence of dopant states improves the visible light absorption range of the doped systems. At the same time, the nonradiative lifetimes are shortened in the doped systems as compared to the pristine GCN. In the case of boron doped at a carbon (B-C-GCN), the charge recombination time is very long as compared to the other two doped systems owing to the smaller electron-phonon coupling strength between the valence band maximum and the trap state. The results suggest B-C-GCN as the most suitable candidate among three doped systems studied in this work for applications in photocatalysis. This work sheds light into the influence of dopants on quantum dynamics processes that govern GCN performance and, thus, guides toward building high-performance devices in photocatalysis.

Non-equilibrium spin noise spectroscopy of a single quantum dot operating at fiber telecommunication wavelengths

We report on the spin and occupation noise of a single, positively charged (InGa)As quantum dot emitting photons in the telecommunication C-band. The spin noise spectroscopy measurements are carried out at a temperature of 4.2 K in dependence on intensity and detuning in the regime beyond thermal equilibrium. The spin noise spectra yield in combination with an elaborate theoretical model the hole-spin relaxation time of the positively charged quantum dot and the Auger recombination and the electron-spin relaxation time of the trion state. The extracted Auger recombination time of this quantum dot emitting at 1.55μm is comparable to the typical Auger recombination times on the order of a few μs measured in traditionally grown InAs/GaAs quantum dots emitting at around 900 nm.

Linear bounded potential model for semiconductor band bending

We propose a simple but unexplored model for the semiconductor band bending with the aim to obtain a relatively simple expression to calculate the energy spectrum for the confined levels and the analytical expressions for wave-functions. This model consists of a linear potential but it is bounded or trimmed in energy unlike the well known wedge potential (WP) model. We present exact solutions for this potential in the frame of the effective mass approximation and they are valid for electron or hole confinement potential. This model provides a more adequate physical scenario than the WP since it takes into account the charge balance involved in the band bending potential. These results allow to treat confined potential problems as in the case of a two-dimensional electron gas in a simplified way. We discuss the application of this approximation to the recombination time of electrons an holes and for the Franz–Keldysh effect.

Flexible Sb0.405Te0.595 photodetectors with broadband spectral response up to 4.5 µm

As one of the third-generation topological insulators (TIs), antimony telluride (Sb2Te3) is considered as a prospective candidate for extremely sensitive and broadband photodetectors ascribed to its extremely narrow direct band gap and exotic optical properties. However, due to the ultrafast carrier recombination time and small carrier lifetime of Sb2Te3, it continues to be an enormous challenge to achieve broadband detection in the mid-wave infrared (MWIR) region. Here, distinctive broadband photodetectors based on stable and scalable high-quality Sn-catalyzed Sb0.405Te0.595 films grown by physical vapor deposition (PVD) was described, and they were found to be photoconductive and sensitive in the visible light (VL) - MWIR region (405–4500 nm). The optimized responsivity (Ri) and specific detectivity (D*) of photodetectors achieved 588 A/W and 6.435 × 108 Jones, respectively. Additionally, these devices exhibit fantastic mechanical flexibility, durability and stability following 200 bending cycles without obvious degradation of photoelectric performance. These excellent properties provide new strategy and insight into the manufacture of high-performance and wearable photodetectors for energy efficient nanoelectronics.

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.

Charge Transfer in Self-Assembled Fullerene-Tetraphenylporphyrin Non-Covalent Multilayer

Self-assembly of organic molecules is a promising method for generating multilayer systems for fabrication of functional devices. In particular, fullerene (C60) and porphyrin molecules offer a variety of binding modes, including π–π interactions, dipole electrostatic attraction, and hydrogen bonding, to tailor the charge separation and charge recombination limiting device performance. Here, we investigate multilayer systems obtained by the sequential physical vapor deposition of C60 and tetraphenylporphyrin (H2TPP) layers, focusing on the effect of the interfaces on the charge transfer processes. Absorbance spectra indicate noncovalent-like π-stacking, with the increment of fullerene interfaces shifting the porphyrin Soret band toward the blue. Similarly, surface photovoltage measurements in the multilayer systems show that as the number of interfaces increases, so does the photogeneration of charge. Charge separation follows carrier generation given that the recombination time, associated to trap states, decreases. This behavior indicates that the Donor-Acceptor nature of the fullerene-porphyrin bilayer system is conserved, and even enhanced in the multilayer film, and that the number of interfaces aids to the formation of selective paths for charge carrier collection, demonstrating its potential in optoelectronic devices.

The effect of maintaining a high conductivity state in high-voltage GaAs diodes switched-on in the delayed avalanche breakdown mode

It is experimentally established that high-voltage GaAs diodes maintain conducting state with low residual voltage after switching to the conducting state in the delayed impact-ionization mode. The duration of the constant current in the reversely-biased structure is determined by the duration of the applied rectangular voltage pulse (up to 100 ns) and significantly exceeds drift extraction and recombination times. The discovered effect of self-supporting conducting state resembles the &amp;quot;lock-on&amp;quot; effect in optically activated GaAs semiconductor switches and S-diodes with deep centers. The effect can be explained by shock ionization in narrow collapsing Gann domains. Keywords: high-voltage GaAs diodes, &amp;quot;lock-on&amp;quot; effect.

Эффект самоподдержания проводящего состояния в обратносмещенных GaAs-диодах, переключаемых в режиме задержанного лавинного пробоя

It is experimentally established that high-voltage GaAs diodes maintain conducting state with low residual voltage after switching to the conducting state in the delayed impact-ionization mode. The duration of the constant current in the reversely-biased structure is determined by the duration of the applied rectangular voltage pulse (up to 100 ns) and significantly exceeds drift extraction and recombination times. The discovered effect of self-supporting conducting state resembles the lock-on effect in optically activated GaAs semiconductor switches and S-diodes with deep centers. The effect can be explained by shock ionization in narrow collapsing Gann domains.

Exploring the photocatalytic properties and carrier dynamics of 2D Janus XMMX' (X = S, Se; M = Ga, In; and X' = Te) materials.

Recently, two-dimensional (2D) Janus structures have been extensively explored because of their robust electron mobility and unique photocatalytic properties. In spite of the increasing interest, the origin of high photocatalytic activities and the behaviors of photoinduced carriers in this kind of materials have not been well understood. Herein, we present a step-by-step protocol based on the first-principles calculations combined with the ab initio non-adiabatic molecular dynamics (NAMD) simulations to unveil the origin of high photocatalytic activity of highly stable typical 2D Janus XMMX' structures (X = S, Se; M = Ga, In; and X' = Te). Their band structures, optical properties, exciton binding energies, carrier effective masses, solar-to-hydrogen efficiency, hot carrier relaxation and recombination times, etc. have been calculated. We find that the difference between X and X' atoms on the two surfaces of the XMMX' monolayer not only builds an out-of-plane electric field, which significantly affects the charge distributions on the valence band maxima (VBM) and the conduction band minima (CBM) and subsequently decreases the exciton binding energy, but also transforms the indirect band structures of XM into the direct ones with well suitable energy gaps for visible-light absorption as well as endows the XMMX' structures with unequal electron and hole mobility, rapid hot carrier relaxation and slow electron-hole recombination processes on a timescale of tens of nanoseconds. The current work suggests that Janus XMMX' monolayers are good photocatalytic materials for overall water splitting and provides a guide to regulate the materials' properties for efficient energy harvesting and optoelectronic applications.

Recombination in gapless HgTe/CdHgTe quantum well heterostructure

Three recombination mechanisms of nonequilibrium carriers in gapless undoped quantum well in HgTe/CdHgTe heterostructure are considered. Dependencies of ensemble-average probability of recombination (inverse of recombination time) on concentration of nonequilibrium carriers for recombination with emission of optical phonons, recombination with emission of two-dimensional plasmons, and radiative recombination have been calculated. Key words: gapless HgTe quantum well, recombination mechanisms of nonequilibrium carriers.

Out-of-plane dipole-modulated photogenerated carrier separation and recombination at Janus-MoSSe/MoS2 van der Waals heterostructure interfaces: an ab initio time-domain study.

Out-of-plane mirror symmetry-breaking provides a powerful tool for engineering the electronic properties and the exciton behavior of two-dimensional materials. Here, by combining time-domain density functional theory with nonadiabatic dynamics, we investigate the underlying mechanism of how the vertical dipole moment modulates the photoexcited carrier transport and the electron-hole recombination dynamics in polar Janus MoSSe/MoS2 stacked heterostructures. It is shown that the stronger nonadiabatic coupling, interlayer-state delocalization and the built-in electric field caused by charge redistribution facilitate a more rapid photocarrier separation across the interface in the S/S stacked bilayer compared with the S/Se bilayer, explaining the experimentally observed stronger photoluminescence quenching effect in the S/S heterostructure. We also found that the photocarrier recombination of the heterostructure with the S/Se interface has a timescale up to nanoseconds, which is ∼4 times longer than that of the S/S bilayer. Such a prolonged recombination time originates from the dipole-weakened nonadiabatic coupling between occupied and unoccupied states instead of quantum coherence and the band gap effect. Overall, Janus MoSSe/MoS2 heterostructures exhibit superior photocatalytic activity, reflecting the ultrafast photocarrier separation triggered by the built-in electric field, suppressed carrier recombination, high solar-to-hydrogen conversion efficiency and the strong absorption coefficient expanding from visible-light to near-infrared-light. The above atomistic and time-domain findings reveal the intrinsic dipole as an effective freedom to regulate the nonadiabatic photocarrier dynamics in Janus-based 2D heterostructures for efficient energy harvesting and optoelectronic applications.