Carrier Decay Research Articles

Time resolved study of carrier relaxation dynamics in α-Al2O3

The relaxation of excited carriers in α–Al2O3 is complex, depending for instance on the type of ionizing radiation. Using femtosecond time-resolved absorption spectroscopy, we can induce a controllable excitation density on a wide range, and follow the relaxation dynamics from 30 fs to 7 ns. We show that the excited carrier decay is non-exponential: it is dependent on the pump intensity, i.e. on the initial carrier concentration. We describe the relaxation as a two-steps process, involving the trapping of initially free electron–hole pairs, followed by recombination. A numerical model taking into account the initial electronic excitation by multiphoton absorption and the subsequent relaxation allows to quantitatively reproduce the amplitude of the measured absorption and its temporal evolution.

Carrier recombination in SrTiO3 single crystals: impacts of crystal faces and Nb doping

We observed the carrier recombination in SrTiO3 single crystals with several crystal faces and Nd doping concentrations using time-resolved photoluminescence and microwave photoconductivity decay methods. The shapes of the decay curves were independent of the excited carrier concentrations, and thus the carriers recombined through the Shockley–Read–Hall and surface recombination processes. From the measurements for different crystal faces, we found that, although the surface recombination is significant for the (100), (110), and (111) faces, they have similar surface recombination velocities of ∼106 cm s−1. Therefore, as photocatalysts, the shape of the materials has a minor effect on the energy conversion efficiency. Based on the dependence of the Nb doping concentration, a high doping concentration significantly enhances the carrier recombination, whereas a moderate Nb doping induces only a slight reduction in the time constants of the carrier decay. In addition, the time constant increases with temperature, and thus moderate Nb doping will have a positive effect on the energy conversion efficiency of SrTiO3 photocatalysts at high temperatures.

Synchronization of polarization chaos in mutually coupled free-running VCSELs.

We numerically demonstrate and analyze polarization chaos synchronization between two free-running vertical cavity surface emitting semiconductor lasers (VCSELs) in the mutual coupling configuration under two scenarios: parallel injection and orthogonal injection. Specifically, we investigate the effect of external parameters (the bias current, frequency detuning and coupling coefficient) and internal parameters (the linewidth enhancement factor, spin-flip relaxation rate, field decay rate, carrier decay rate, birefringence and dichroism) on the synchronization quality. Finally simulation results confirm that in the parallel injection, chaotic synchronization can reach a cross-correlation coefficient of 0.99 within a range of parameter mismatch ±12%. On the other hand, the chaos synchronization for orthogonal injection only reaches a cross-correlation coefficient of 0.95 within a range of parameter mismatch ±3%.

Experimental evidence for a metastable state in FeTe1−xSex following coherent-phonon excitation

Changes of the electronic structure of FeTe1−xSex (x = 0, 0.3) upon ultrafast excitation with near-infrared laser pulses are investigated using time- and angle-resolved extreme ultraviolet photoemission spectroscopy. At different temperatures and for different doping levels we observe a global oscillation of energy bands near the Fermi energy driven by the coherent excitation of the A1g(Te) phonon mode in FeTe1−xSex. Prominently, photo-induced band shifts and a spectral weight reduction persist for at least 300 ps, long after the decay of the photo-excited carriers and the damping of the coherent phonons. We argue that the system escapes from the original equilibrium state and is transiently trapped in a possibly metastable state envisioning a purely optical means of manipulating the electronic structure in an iron-based superconductor.

Nanosecond dynamics in intrinsic topological insulator Bi2−xSbxSe3 revealed by time-resolved optical reflectivity

$\text{Bi}_2\text{Se}_3$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets. We report a substantial slowing of the bulk carrier relaxation time in bulk-insulating $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets as compared to $n$-type bulk-metallic $\text{Bi}_2\text{Se}_3$ at low temperatures, which approaches $3.3 \text{ ns}$ in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in $n$-type $\text{Bi}_2\text{Se}_3$, namely the slow bimolecular recombination of bulk carriers.

Investigation of charge carrier mobility and recombination in PBDTTPD thin layer structures

Abstract In this paper we present investigation of hole transport properties in sandwich and OFET structures with single active layer of PBDTTPD (Poly[(5,6-dihydro-5-octyl-4,6-dioxo-4H-thieno[3,4-c]pyrrole-1,3-diyl)[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]]). Sandwich structures were investigated by photo-CELIV and TOF techniques, obtained results showed strongly time dependent hole mobility and non-Gaussian DOS tail. Photogenerated charge carrier decay experiment demonstrated that bimolecular recombination coefficient is smaller than calculated Langevin recombination coefficient and this was explained by faster holes escaping recombination area and not participating in Langevin recombination process. Organic field-effect transistor structure was investigated by current transients technique to find hole mobility near the dielectric layer and to study OTS treatment influence on hole transport. The study of hole mobility dependence on temperature was performed in order to evaluate energetic disorder of interface DOS in the channel of OFET structures.

Thickness-dependent ultrafast hot carrier and phonon dynamics of PtSe2 films measured with femtosecond transient optical spectroscopy

We systematically investigate the ultrafast hot carrier and phonon dynamics of PtSe2 with various van der Waals (vdW) layer numbers using femtosecond transient optical spectroscopy. The hot carrier dynamics of PtSe2 is found to be strongly dependent on its thickness. For semiconducting PtSe2 films (≤4 vdW layers), electron–phonon coupling, interlayer charge transfer and nonradiative recombination of electrons and holes contribute to the hot carrier decay, while electron–phonon coupling dominates the carrier decay in metallic PtSe2 films (with vdW layers). The PtSe2 films with 2 vdW layers show the most pronounced interlayer charge transfer decay process and are thus most suitable for photoelectric sensors and energy harvesting devices. The phonon dynamics of PtSe2 is also thickness dependent; its LB mode phonon frequency decreases as the number of vdW layers increases, as a result of the thickness-dependent dielectric screening of the long-range Coulomb interaction.

Time-Resolved Changes in Dielectric Constant of Metal Halide Perovskites under Illumination.

Despite their impressive performance as a solar absorber, much remains unknown on the fundamental properties of metal halide perovskites (MHPs). Their polar nature in particular is an intense area of study, and the relative permittivity (εr) is a parameter widely used to quantify polarization over a range of different time scales. In this report, we have exploited frequency-dependent time-resolved microwave conductivity (TRMC) to study how εr values of a range of MHPs change as a function of time, upon optical illumination. Further characterization of charge carriers and polarizability are conducted by femtosecond transient absorption and stimulated Raman spectroscopy. We find that changes in εr are roughly proportional to photogenerated carrier density but decay with a shorter time constant than conductance, suggesting that the presence of charge carriers alone does not determine polarization.

Detection and analysis of structural changes in MoO3/Ag heterostructure by ultrafast transient absorption spectroscopy

Ultrafast transient absorption spectroscopy (UFTS) is a widely used technique to determine the change in optical parameters on laser irradiation and can highlight the pathways and the lifetime of decay of the charge carriers. Here, UFTS was used to detect the structural changes occurring in a MoO3/Ag heterostructure because of dewetting and dealloying. A comparative study of transient absorption behavior of samples exposed to different temperatures indicated a change in phase and crystallinity of the film. Later through GI-XRD, it was concluded that the MoO3 film has become crystalline at 350 °C along α-(110) plane. A wide plasmonic band in UFTS data of dealloyed sample followed by FESEM showed randomly distributed Ag-Mo nanoporous clusters (315 nm - 6 μm). Tuning of surface plasmon resonance (~19 nm) was seen with etching time. This study proves UFTS's suitability for correlating the structural change in a film or heterostructure of films with spectroscopic results.

Anomalous Hot Carrier Decay in Ferromagnetic Cr2Ge2Te6 via Spin-Phonon Coupling.

The link between spin-phonon coupling (SPC) and coherent phonon excitation as well as hot carrier decay dynamics is investigated with femtosecond transient optical spectroscopy. Coherent phonon excitation via SPC is directly observed in a van der Waals ferromagnet Cr2Ge2Te6 (CGT). Such coherent phonon excitation is strongly dependent on spin ordering of CGT and facilitates considerably hot carrier decay. While hot carriers decay normally via direct electron-phonon coupling, hot carrier decay in ferromagnetic CGT is also achieved via indirect electron-phonon coupling, with spin ordering acting as the intermediate between hot carriers and coherent phonons.

Anomalous Charge Carrier Decay Spotted by Clustering of a Time-Resolved Microscopic Phase Image Sequence

We have developed a new type of time-resolved phase microscope through which the dynamics of photo-excited charge carriers at local positions are visualized by a refractive index change. To categor...

Origin of carrier localization in AlGaN-based quantum well structures and implications for efficiency droop

We investigate carrier localization in Al-rich AlGaN/AlN quantum well (QW) structures. Low temperature time-resolved photoluminescence (PL) experiments reveal a strong variation of the carrier decay times with detection photon energy, suggesting a strong impact of carrier localization, which is found to depend primarily on the QW width. In combination with time-integrated PL measurements and numerical band structure calculations, we are able to provide conclusive evidence that the localization strength in AlGaN-based QW structures is directly coupled to the oscillator strength, providing an explanation for its strong dependence on the QW width. This is further supported by the observation of a strong polarization field dependency of the carrier localization, which excludes excitons and may be explained by the accumulation of electrons close to the QW interface, while holes are independently localized across the QW. We complete our discussion by proposing a model to explain the well-known phenomenon of efficiency droop in accordance with our findings, suggesting delocalization-induced Auger recombination as the responsible loss channel.

Femto-Second Carrier and Photon Dynamics in Site Controlled Hexagonal InGaN/GaN Isolated Quantum Dots: Natural Radial Potential Well and Its Dynamic Modulation

Quantum dot (QD) is slated to play a significant role in quantum technologies through its usage as a single-photon source. It also has promising applications as efficient light emitters through strain-relaxed structures. This work demonstrates the fabrication of high-quality site controlled InGaN/GaN QDs through a large contribution of chemical etching in an otherwise reactive ion etching process. Uniform sized QDs with a hexagonal base and radii of 15 and 6 nm are fabricated and characterized by photoluminescence and femtosecond transient absorption spectroscopy. The natural exposition of the inherent hexagonal base is a signature of the reduced defects in these dots. The QDs show the intuitive additional quantum confinement due to size reduction, and the photoluminescence peaks manifest a blue shift. The absorption spectra indicate that the QDs are heavily strain-relaxed at smaller radii, and their characteristics as a function of size and pump power are investigated. The degree of strain relaxation and change in energy-band diagram as a function of position along the radius are also determined. The changes are prominent near the periphery, which creates a natural potential well for both electrons and holes, restricting the carriers from reaching the sidewalls. Femtosecond carrier dynamics indicate a lower capture rate for the smaller size of the dots, indicating excessive electrical or optical pumping will not necessarily lead to increased luminescence. The radiative decay of carriers is faster for strain-relaxed QDs due to higher oscillator strength originating from increased electron–hole wave function overlap. Higher pump power is found to decrease the radiative rate due to the increased effective size of the QDs and thereby reducing carrier injection density. This is in stark contrast to the frequently observed increased decay rate due to Auger recombination. Dynamic modification of the size of the QDs by pump power may find potential use in optoelectronic applications.

Ultrafast photoinduced band splitting and carrier dynamics in chiral tellurium nanosheets

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.

Carrier Transport Limited by Trap State in Cs2AgBiBr6 Double Perovskites.

Understanding the photoinduced carrier dynamics in Cs2AgBiBr6 double perovskites is essential for their application in optoelectronic devices. Herein, we report an investigation on the temperature-dependent carrier dynamics in a Cs2AgBiBr6 single crystal (SC). The time-resolved photoluminescence (TRPL) measurement indicates that the majority of carriers (>99%) decay through a fast trapping process at room temperature, and as the temperature decreases to 123 K, the population of carriers with a slow fundamental decay kinetics rises to ∼50%. We show that the carrier diffusion coefficient (theoretical diffusion length) varies from 0.020 ± 0.003 cm2 s-1 (0.70 μm) at 298 K to 0.11 ± 0.010 cm2 s-1 (2.44 μm) at 123 K. However, in spite of the long diffusion length, the population of carriers that can perform long-distance transport is restricted by the trap state, which is likely a key reason limiting the performance of Cs2AgBiBr6 optoelectronic devices.

Slow carrier cooling in methylammonium lead bromide quantum dots: Evidence of hot phonon bottleneck

Photoinduced charge carrier dynamics studies in lead halide perovskites helps in explaining key processes like carrier trapping, diffusion and multiexciton dynamics etc. which are of importance to understand the fate of excitons. In the present study, we investigate the dynamics in methylammonium lead bromide perovskite nanocrystals under low and high pump intensity to study hot carrier cooling mechanisms. The single exciton studies conducted under low pump fluences (<N> = 0.3) revealed slow carrier decay with a significant fraction of >1 ns decay times as expected due to long radiative carrier recombination times. This also indicated that fast carrier trapping does not occur as evidenced by absence of any fast decay components. The hot carrier (HC) cooling mechanisms were verified by studying carrier density dependence of carrier temperature decays as extracted from the higher energy part of exciton bleach. These studies revealed a very fast cooling of ∼150 fs under low carrier density 1.5 × 1017/ cm3. However additional experiments under higher carrier densities of ∼1018-1019/cm3 showed a very slow carrier temperature decay with decay time ∼100 ps for the highest intensity investigated. The analysis of hot carrier lifetimes revealed Auger heating and acoustic phonon up-conversion as reason for hot carrier relaxations at higher intensities. Additionally, we carried out experiments under hole quencher, we found lower initial carrier temperatures and faster decay indicating highly efficient hole transfer to the quencher in a time scale faster than cooling of carrier at higher intensities. These results hint at possibility of molecular design for enabling efficient hot carrier extraction from these Quantum Dots (QDs).

Time-Frequency Spectroscopy of GaAs Transient Dispersion Using Few-Cycle Pump-Probe Reflectometry

The rapid development of new photonic materials calls for simple yet effective approaches to characterize ultrafast dynamics in materials and devices. Here we demonstrate time-frequency transient reflectometry based on a compact, few-cycle pump-probe reflectometer. The system simultaneously features a broad spectral range (300 nm) and fine temporal resolution (10 fs), and it requires only a femtosecond oscillator. With this capability, we report a time-frequency spectroscopic measurement of the transient dispersion of $\mathrm{Ga}\mathrm{As}$ near its band gap. Our results map important carrier dynamics such as carrier cooling, carrier decay, band filling, and band-gap renormalization on a single two-dimensional graph, offering a comprehensive picture of these processes and revealing details unattainable with conventional methods.

Tunable optical loss and multi-band photodetection based on tin doped CdS nanowire

Abstract Inorganic semiconductor nanowires is the fundamental connector in on-chip nano-photonic circuitry systems. The optical loss during light propagation as the key parameter influence its performance as waveguide. So accurate determination of optical loss is imperative for the optical information communication between different active and passive optical components. In this work, pumping power dependent optical loss along the tin doped CdS nanowires were investigated. By calculating the out-coupling photoluminescence intensity from one end of a nanowire with respect to the propagation length, the attenuation coefficient of the near band edge emission changed from 0.097 μm−1 to 0.061 μm−1 with the laser power increases. Similarly, with the increasing of excitation power, the attenuation coefficient of the defects related emission can be tuned from 0.098 μm−1 to 0.067 μm−1. In addition, the activation energy is determined to be 34.29 meV by the low temperature photoluminescence experiment, demonstrating the main carriers decay channel is exciton recombination at room temperature (T = 300 K). Therefore, the individual tin doped CdS nanowire photodetector is fabricated and systematically testing. Under a low illumination intensity (142 μW/cm2@405 nm), the detector exhibit a high responsivity of 51.2 A/W, together with the gain of 313.3. Furthermore, the nanowire detector shows stable photos-witching and multi-band response characteristic. Based on the tunable optical loss and broad spectrum response property, the tin doped CdS nanowires used as waveguide or active material can be electively applied in integrated nanophotonic and optoelectronic systems.

The effects of interstitial iodine in hybrid perovskite hot carrier cooling: A non-adiabatic molecular dynamics study.

Understanding the defect chemistry of lead-halide perovskites and its effects on the hot-carrier lifetime is of significance for both fundamental understanding and applications as solar cell light absorbing materials. In this study, the mechanistic details of hot carrier decay in hybrid perovskites are investigated using a newly developed non-adiabatic molecular dynamics method. In this approach, the nuclear trajectory is based on Born-Oppenheimer ground state molecular dynamics, which is then followed by the evolution of carrier wave function including the detailed balance and decoherence effects. We found the longer decay time for hot electrons due to the incorporation of interstitial iodine in the hybrid lead-halide perovskites (MAPbI3), while the hot hole decay time is not affected significantly by the interstitial iodine. The underlying mechanism for such modulation of hot carrier dynamics is attributed to the changes of carrier density of states and the electron-phonon coupling strength. Hence, iodine interstitial is the necessary condition to create long-lived hot electrons in perovskites, which is further demonstrated by the comparative analysis with the pure MAPbI3.

Hot carrier dynamics and phonon anharmonicity of ZrTe5 revealed with femtosecond transient optical spectroscopy

We present a time-resolved ultrafast optical spectroscopy study on thermoelectric material ${\mathrm{ZrTe}}_{5}$, regarding its carrier decay dynamics and anharmonic decay of coherent optical phonons at 5--280 K. A reflectivity time series consists of a single exponential decay due to the relaxation of carriers via electron-phonon coupling and decaying oscillations due to coherent phonon vibration of the ${A}_{g}$ mode. The carrier decay characteristics follow the temperature-induced electronic transitions of ${\mathrm{ZrTe}}_{5}$. The coherent optical phonon vibrations can be well described with the anharmonicity model including lattice thermal expansion and phonon-phonon coupling. The phonon vibration frequency and dephasing time are obtained as a function of temperature, and decrease with increasing temperature. The effects of pump fluence are also explored.