Ultrafast Dynamics Of Photoexcited Carriers Research Articles

Ultrafast dynamics of photoexcited carriers in tellurium in the vicinity of Weyl nodes

Ultrafast dynamics of photoexcited carriers in tellurium in the vicinity of Weyl nodes

Comprehensive study of the ultrafast photoexcited carrier dynamics in Sb2Te3–GeTe superlattices

Abstract Chalcogenide superlattices Sb2Te3-GeTe is a candidate for interfacial phase-change memory (iPCM) data storage devices. By employing terahertz emission spectroscopy and the transient reflectance spectroscopy together, we investigate the ultrafast photoexcited carrier dynamics and current transients in Sb2Te3-GeTe superlattices. Sample orientation and excitation polarization dependences of the THz emission confirm that ultrafast thermo-electric, shift and injection currents contribute to the THz generation in Sb2Te3-GeTe superlattices. By decreasing the thickness and increasing the number of GeTe and Sb2Te3 layer, the interlayer coupling can be enhanced, which significantly reduces the contribution from circular photo-galvanic effect (CPGE). A photo-induced bleaching in the transient reflectance spectroscopy probed in the range of ~1100 nm to ~1400 nm further demonstrates a gapped state resulting from the interlayer coupling. These demonstrates play an important role in the development of iPCM-based high-speed optoelectronic devices.

Investigation of self-trapped excitonic dynamics in hematite nanoforms through non-degenerate pump–probe transmission spectroscopy

Non-degenerate pump–probe transmission spectroscopy is used to examine the ultrafast dynamics of photo-excited carriers in hematite nanoforms at various pump fluences. Using coupled rate equations, the kinetics of self-trapped exciton (STE) formation and its interaction with free excitons resulting in exciton annihilation were studied. It is shown from this model that the majority of the excitons were trapped by polaronic trap states to form self-trapped excitons within ∼3.5 ps. The findings indicate that free excitons and STEs interact non-linearly, similar to trap-assisted bi-molecular Auger recombination to annihilate one another. It is observed that there is substantial dependence of kinetics of STE formation and exciton decay on photo-excited exciton density, and the nature of this dependence indicates the reduced screening of electron–phonon interaction. Using the screening model applied to the rate constants of STE formation and decay, we estimate the saturation exciton density to be ∼3.3 × 1017 cm−3 and the average STE density to be ∼3.8 × 1018 cm−3 in the hematite nanoforms. We also noticed that doping K and Ni to hematite nanoforms up to 5% did not remarkably change the nature of the exciton dynamics.

Laser-combined multiprobe microscopy and its application to the materials with atomic layer thickness

We have developed a method of evaluating the photo-induced carrier dynamics of nanostructures by combining optical technologies with multiprobe microscopy techniques. Using multiple probes, measurement can be carried out even for a small sample without complicated pretreatments, such as attaching electrode structures. Using transition metal dichalcogenides as a sample and a continuous laser or an ultrashort pulse laser as the light source, we demonstrated analyses of the carrier dynamics related to trap levels in a millisecond to second time domain and the ultrafast photoexcited carrier dynamics in the picosecond region.

Excitation-density and excess-energy dependence of ultrafast dynamics of photoexcited carriers in intrinsic bulk CdTe

In this study, ultrafast dynamics of photoexcited carriers in an intrinsic CdTe film are investigated deeply and systematically as a function of carrier density and excess energy using femtosecond-resolved pump–probe transient differential transmission spectroscopy. A dynamic model is developed based on an interband transition absorption model in direct bandgap semiconductors by introducing the dynamics of thermalization, cooling and recombination of photoexcited carriers into the interband transition absorption model. Then, the model is used to best fit the measured ultrafast dynamics to retrieve the initial temperature of thermalized electrons and time constants of the thermalization, cooling and recombination dynamic processes as a function of carrier density and excess energy. The increase with carrier density and the decrease with excess energy of thermalization time constant reveal that thermalization process of photoexcited carriers is dominated by carrier-phonon scatterings. We find for the first time that cooling of thermalized carriers depend strongly on carrier density and excess energy, and cooling time constant varies in a wide range from 0.2 to 5 ps. It is also found that carrier recombination in the intrinsic CdTe film is dominated by the radiative recombination of electron-hole pairs, rather than Auger recombination as reported in doped CdTe films. These new results are very important to understanding of microscopic mechanism of relaxation processes of photoexcited carriers and applications of CdTe-based optoelectronic devices.

Ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi2Te3 thermoelectric films

Ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi2Te3 thermoelectric films

Photoexcited carrier and phonon morphology of InSb observed with an ultrafast pump-probe microscope

The ultrafast pump-probe microscope has been utilized to spatially map the ultrafast dynamics of photoexcited carriers and phonons in InSb with varied carrier concentrations. In this study, the negative and positive components of transient reflectivity-change spectra (ΔR/R) are associated with the dynamics of photoexcited holes and phonons, respectively. The mapping of phonon morphology (i.e. positive ΔR/R on a ps-ns timescale) shows the delocalized electron-phonon coupling in InSb, especially for large carrier concentrations. In contrast, the recombination of electrons and holes is rather localized inside the pumped spot. According to morphological analyses of oscillatory signals in ΔR/R, we found that an electric field applied to InSb can further manipulate the phonon strength and lifetime. For the case of large carrier concentrations, the spatial breadth of phonons has been first observed in InSb.

Ultrafast dynamics of photoexcited carriers in perovskite semiconductor nanocrystals

Abstract Perovskite semiconductor nanocrystals have emerged as a promising family of materials for optoelectronic applications including light-emitting diodes, lasers, light-to-electricity convertors and quantum light emitters. The performances of these devices are fundamentally dependent on different aspects of the excited-state dynamics in nanocrystals. Herein, we summarize the recent progress on the photoinduced carrier dynamics studied by a variety of time-resolved spectroscopic methods in perovskite nanocrystals. We review the dynamics of carrier generation, recombination and transport under different excitation densities and photon energies to show the pathways that underpin the photophysics for light-emitting diodes and solar cells. Then, we highlight the up-to-date spin dynamics and coherent exciton dynamics being manifested with the exciton fine levels in perovskite semiconductor nanocrystals which are essential for potential applications in quantum information technology. We also discuss the controversial results and the possible origins yet to be resolved. In-depth study toward a comprehensive picture of the excited-state dynamics in perovskite nanocrystals may provide the key knowledge of the device operation mechanism, enlighten the direction for device optimization and stimulate the adventure of new conceptual devices.

Ultrafast Charge Transfer Coupled to Quantum Proton Motion at Molecule/Metal Oxide Interface

Understanding the ultrafast dynamics of photoexcited carriers at the semiconductor/molecule interface is essential for improving the solar energy conversion efficiency. At such interfaces, the hydrogen bond (H-bond) network is often formed, and the quantum proton motion is expected. However, how the nuclear quantum effects (NQEs) will influence the photoexcited charge transfer is still unknown. By combining two kinds of emerging molecular dynamics methods at the ab initio level - the path-integral based molecular dynamics and time-dependent nonadiabatic molecular dynamics, and choosing CH3OH/TiO as a prototypical system to study, we find that the quantum proton motion in the H-bond network is strongly coupled with the ultrafast photoexcited charge dynamics at the interface. The hole trapping ability of adsorbed CH3OH molecule is then notably enhanced by the NQEs, and thus, it behaves as a hole scavenger on TiO2. It is concluded that the quantum proton motion in the H-bond network may play a critical role in enhancing the energy conversion efficiency based on photoexcitation.

Transient carrier dynamics of GaAs at room temperature

We use time-resolved optical reflectivity with femtosecond resolution to study the ultrafast dynamics of photoexcited carriers in bulk GaAs. We observe an emergence of short-lived excitons at room temperature right after photoexcitation, in terms of a sharp dip and sub-ps lifetime in the transient reflectivity changes, as long as the pump fluence increases above a critical value Fc. Our findings help understand ultrafast physics in this material.

Modulation of carrier lifetime in MoS2 monolayer by uniaxial strain**Project supported by the Natural Science Foundation of Beijing, China (Grant No. JQ19004), the Excellent Talents Training Support Fund of Beijing, China (Grant No. 2017000026833ZK11), the National Natural Science Foundation of China (Grant Nos. 51991340 and 51991342), the National Key Research and Development Program of China (Grant Nos. 2016YFA0300903 and 2016YFA0300804),

Carrier lifetime is one of the most fundamental physical parameters that characterizes the average time of carrier recombination in any material. The control of carrier lifetime is the key to optimizing the device function by tuning the electro–optical conversion quantum yield, carrier diffusion length, carrier collection process, etc. Till now, the prevailing modulation methods are mainly by defect engineering and temperature control, which have limitations in the modulation direction and amplitude of the carrier lifetime. Here, we report an effective modulation on the ultrafast dynamics of photoexcited carriers in two-dimensional (2D) MoS2 monolayer by uniaxial tensile strain. The combination of optical ultrafast pump–probe technique and time-resolved photoluminescence (PL) spectroscopy reveals that the carrier dynamics through Auger scattering, carrier–phonon scattering, and radiative recombination keep immune to the strain. But strikingly, the uniaxial tensile strain weakens the trapping of photoexcited carriers by defects and therefore prolongs the corresponding carrier lifetime up to 440% per percent applied strain. Our results open a new avenue to enlarge the carrier lifetime of 2D MoS2, which will facilitate its applications in high-efficient optoelectronic and photovoltaic devices.

Relevance of collinear processes to the ultrafast dynamics of photoexcited carriers in graphene

Abstract The importance of interband transitions on the ultrafast relaxation process in photoexcited pristine graphene is evaluated by means of an ensemble Monte Carlo simulator. Impact ionization and Auger recombination in the collinear limit are considered, together with phonon-induced generation and recombination and intraband scattering mechanisms. The results show that collinear impact ionization is dominant in the first 100 femtoseconds, creating an important excess carrier population that is finally eliminated in the picosecond scale, together with the photoexcited population, by Auger and optical phonon-assisted recombination. The hot phonon effect is also important, stimulating phonon absorption and indirectly reducing the net collinear recombination in the hundreds of femtosecond range. The substrate type is an important factor, appeasing collinear impact ionization via screening and creating additional cooling channels that speed up the relaxation process. The results evidence that interband collinear generation processes are critical to explain the fastest stages of the relaxation process in graphene.

Ultrafast dynamics of pure many-body effect and its competition with bandgap widening via electron–phonon coupling in PbTe thin films

Bandgap renormalization (BGR), or bandgap narrowing effect, is one of the many-body effects predicted by many-body theory in semiconductor physics. However, pure BGR effect has never been observed experimentally although the mixed effect of BGR with lattice-heating induced bandgap narrowing was reported in GaAs and CdTe semiconductors. PbTe has a positive temperature coefficient of bandgap. Its lattice-heating induced bandgap narrowing can be eliminated so that the pure BGR effect is possibly observed. However, that has never been reported so far because the BGR effect is weak in PbTe due to its huge dielectric screening. We study the ultrafast dynamics of photo-excited carriers in PbTe films using near-infrared femtosecond-resolved transient differential transmission (TDT) spectroscopy. A complex oscillation-like damped decay dynamics is observed. A negative dip, a fingerprint signature of BGR effect, occurs immediately after the thermalization process of the photo-excited carriers. As such, we have observed a pure BGR effect in PbTe for the first time, strongly supporting the many-body theory. Meanwhile, we have also observed a dynamic transition from a transient absorption enhancement to absorption saturation, revealing the emergence of a slow lattice-heating induced bandgap widening via carrier-phonon couplings. Based on a two-temperature model, a simulation computation is carried out, supporting the lattice-heating induced bandgap widening leads to the dynamic transition. A phenomenological dynamic model is developed, including three dynamic components: the thermalization of the photo-excited carriers, recombination of the thermalized carriers and lattice-heating induced bandgap widening, and used to best fit the excitation-power dependent TDT dynamics. Time constants and strengths of the three dynamic components are obtained for different excitation powers. Time constant variation with the excitation power reveals different roles of the carrier–carrier and carrier-phonon scattering in the thermalizing process, as well as of free carrier and exciton recombination in the decaying process of hot carriers.

Measurement of ultrafast dynamics of photoexcited carriers in β-Ga2O3 by two-color optical pump-probe spectroscopy

We report results from ultrafast two-color optical pump-probe spectroscopy on bulk β-Ga2O3. A two-photon absorption scheme is used to photoexcite carriers with the pump pulse and free-carrier absorption of the probe pulse is used to record the subsequent dynamics of the photoexcited carriers. Our results are consistent with carrier recombination via defect-assisted processes. We also observe transient polarization-selective optical absorption of the probe pulse by defect states under nonequilibrium conditions. A rate equation model for electron and hole capture by defects is proposed and used to explain the data. Whereas the rate constants for electron capture by defects are found to be temperature-independent, they are measured to be strongly temperature-dependent for hole capture and point to a lattice deformation/relaxation process accompanying hole capture. Our results shed light on the mechanisms and rates associated with carrier capture by defects in β-Ga2O3.

Ultrafast selective extraction of hot holes from cesium lead iodide perovskite films

Lead halide perovskites have some unique properties which are very promising for optoelectronic applications such as solar cells, LEDs and lasers. One important and expected application of perovskite halide semiconductors is solar cell operation including hot carriers. This advanced solar cell concept allows overcoming the Shockley–Queisser efficiency limit, thereby achieving energy conversion efficiency as high as 66% by extracting hot carriers. Understanding ultrafast photoexcited carrier dynamics and extraction in lead halide perovskites is crucial for these applications. Here, we clarify the hot carrier cooling and transfer dynamics in all-inorganic cesium lead iodide (CsPbI3) perovskite using transient absorption spectroscopy and Al2O3, poly(3-hexylthiophene-2,5-diyl) (P3HT) and TiO2 as selective contacts. We find that slow hot carrier cooling occurs on a timescale longer than 10 ps in the cases of CsPbI3/Al2O3 and CsPbI3/ TiO2, which is attributed to hot phonon bottleneck for the high photoexcited carrier density. An efficient ultrafast hole transfer from CsPbI3 to the P3HT hole extracting layer is observed. These results suggest that hot holes can be extracted by appropriate selective contacts before energy dissipation into the halide perovskite lattice and that CsPbI3 has a potential for hot carrier solar cell applications.

Hybridized electronic states between CdSe nanoparticles and conjugated organic ligands: A theoretical and ultrafast photo-excited carrier dynamics study

Formation of densely packed thin films of semiconductor nanocrystals is advantageous for the exploitation of their unique optoelectronic properties for real-world applications. Here we investigate the fundamental role of the structure of the bridging ligand on the optoelectronic properties of the resulting hybrid film. In particular, we considered hybrid films formed using the same CdSe nanocrystals and two organic ligands that have the same bidentate dithiocarbamate binding moiety, but differ in their bridging structures, one bridged by ethylene, the other by phenylene that exhibits conjugation. Based on the results of photo-excited carrier dynamics experiments combined with theoretical calculations on the electronic states of bridged CdSe layers, we show that only the phenylene-based ligand presents a strong hybridization of the molecular HOMO state with CdSe layers, that is a marker of formation of an effective bridge. We argue that this hybridization spread favors the hopping of photo-excited carriers between nanocrystals, which may explain the reported larger photo-currents in phenylene-based hybrid films than those observed in ethylene-based ones.

Ultrafast carrier dynamics and the role of grain boundaries in polycrystalline silicon thin films grown by molecular beam epitaxy

We have used time-resolved terahertz spectroscopy to study microscopic photoconductivity and ultrafast photoexcited carrier dynamics in thin, pure, non-hydrogenated silicon films grown by molecular beam epitaxy on quartz substrates at temperatures ranging from 335 °C to 572 °C. By controlling the growth temperature, thin silicon films ranging from completely amorphous to polycrystalline with minimal amorphous phase can be achieved. Film morphology, in turn, determines its photoconductive properties: in the amorphous phase, carriers are trapped in bandtail states on sub-picosecond time scales, while the carriers excited in crystalline grains remain free for tens of picoseconds. We also find that in polycrystalline silicon the photoexcited carrier mobility is carrier-density-dependent, with higher carrier densities mitigating the effects of grain boundaries on inter-grain transport. In a film grown at the highest temperature of 572 °C, the morphology changes along the growth direction from polycrystalline with needles of single crystals in the bulk of the film to small crystallites interspersed with amorphous silicon at the top of the film. Depth profiling using different excitation wavelengths shows corresponding differences in the photoconductivity: the photoexcited carrier lifetime and mobility are higher in the first 100–150 nm from the substrate, suggesting that thinner, low-temperature grown polycrystalline silicon films are preferable for photovoltaic applications.

Terahertz Spectroscopy: Studying Carrier Dynamics in Semiconductor Nanostructures

Understanding the ultrafast dynamics of photoexcited carriers in semiconductor nanostructures and their dependence on sample morphology is crucial for their incorporation into photonic devices. Time-resolved terahertz (THz) spectroscopy (TRTS) is an all-optical, contact-free technique that directly measures the transient mobile carrier dynamics and terahertz conductivity in materials over picosecond time scales, and is uniquely suited as a probe of conductivity in nanomaterials. Using low temperature MBE-grown silicon films as an example, we show how TRTS can be used to probe microscopic photoconductivity as well as obtain crucial insights into sample morphology. The thin silicon films consist of a mixture of amorphous and crystalline phases, and their relative content changes drastically with growth temperature. Photoexcited carrier dynamics in these films are determined by film crystallinity: in the amorphous phase, carriers are trapped in bandtail states on sub-picosecond time scales, while the carriers excited in crystalline grains remain free for tens of picoseconds. The complex THz conductivity spectra obtained from the TRTS measurements show that the long range conductivity is significantly higher in films grown at higher temperatures that contain a larger fraction of crystalline material with larger crystal grain sizes.

Terahertz Spectroscopy: Studying Carrier Dynamics in Semiconductor Nanostructures

Understanding the ultrafast dynamics of photoexcited carriers in semiconductor nanostructures and their dependence on sample morphology is crucial for their incorporation into photonic devices. Time-resolved terahertz (THz) spectroscopy is an all-optical, contact free technique that allows directly measuring the transient mobile carrier dynamics and terahertz conductivity over picosecond time scales, and is uniquely suitable as a probe of conductivity in nanomaterials. We have applied time-resolved THz spectroscopy to investigate ultrafast carrier dynamics to a variety of nanostructured semiconducting systems such as silicon nanocrystal films, nanogranular iron pyrate (FeS2) and vanadium dioxide (VO2). Furthermore, terahertz emission spectroscopy of nanomaterials, where the sample itself emits THz radiation in response to optical excitation, provides important insights into carrier dynamics. We have recently generated THz pulses from optically excited macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs). We propose that top-bottom asymmetry present in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized THz radiation by a photocurrent surge.

Evolution of hot carrier dynamics in graphene with the Fermi level tuned across the Dirac point

The ultrafast dynamics of photoexcited carriers closely depends on the excitation processes pertaining to the energy band of the materials and the relevant relaxation pathway relies on the interactions between hot carriers and lattice phonons. By using ultrafast optical-pump terahertz (THz)-probe spectroscopy with an ion-gel gate to tune the Fermi energy level ${E}_{F}$ in graphene, we are able to reveal the relaxation dynamics of hot carriers at different ${E}_{F}$. It is found that the relaxation time increases while the pump-induced differential transmission decreases as ${E}_{F}$ approaches the Dirac point. Through self-consistent model calculations, we quantitatively interpret that a temperature-dependent scattering rate is responsible for a negative photoinduced conductivity, and the relaxation transient directly manifests the Dirac spectrum dependence of the optical phonon emission and the carrier scattering rate. More interestingly, the scattering rate of hot carriers also exhibits a strong ${E}_{F}$ dependence, which is the most likely to originate from charged impurities inevitably present in graphene. The diminution of photoresponse efficiency across the Dirac point implies that the graphene-based optoelectronic devices may be operable only in the highly doped regime.