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Photoluminescence of composites containing carbon nanostructures is critical for many modern applications. Of particular significance are metamaterials capable of generating white light photoluminescence within a single structure, as the white luminescence usually needs separate red-green-blue emitters. This work provides an insight into the photoluminescent properties of a promising family of hierarchical metamaterials made of carbon nanocones in a silicon oxide matrix in view of their use as efficient single-layer white light emitters. The composites were prepared using a facile plasma-enhanced technology, followed by a thermal treatment. ATR-FTIR, Raman, SEM, and AFM microscopy and profilometry characterization confirmed the presence of silica and carbon nanocones. The advanced comprehensive photoluminescence studies conducted using solid-state Yb:KGW laser revealed a significant difference in photoluminescent properties for the composites containing sharp nanocones of similar parameters. A comprehensive morphological analysis performed using several analytical techniques including the 2D fast Fourier transform spectra, Hough distributions, spectral density function, and Minkowski functionals revealed the Minkowski boundary functional, ordering, and connectivity to be the most important morphological descriptors for the photoluminescent response. This study suggests that these morphological parameters play a critical role in defining the key properties of advanced metamaterials via the overlap of ψ functions and may therefore be targeted for informed material design and intelligent fabrication.

GeSn coatings on commercial Si substrates have gained increased interest for application in infrared detectors and lasers. The characterization of these materials is crucial for their quality assessment and in describing device performance for commercialization. The time-resolved photoluminescence is an efficient method for contactless, time-resolved characterization of GeSn optoelectronic properties. For this purpose, in this work, we developed an infrared streak camera attachment based on the broadband upconversion of infrared photoluminescence by using powerful nanosecond 1064 nm pulses. The attachment achieved picosecond time resolution, being limited by the laser pulse duration, jitter, and temporal resolution of the streak camera. The spectral range for time-resolved photoluminescence in the 1100–2400 nm interval was obtained, outperforming the range of commercial infrared InGaAs streak cameras. The developed setup was applied for excitation-dependent time-resolved photoluminescence decay measurements in a GeSn and compared to the conventional upconversion technique with the optically delayed picosecond gate pulses. The new setup provided 2D spectro-temporal images for analysis. The photoluminescence decay times in the 30–80 ps range were obtained in the GeSn layer depending on the excitation pulse energy and spectral emission wavelength. Carrier thermalization was observed as a redshift of the photoluminescence spectra with time.

Quasi-two dimensional perovskites demonstrate unique excitonic properties due to their multilayer structure making them attractive for various optoelectronic applications. However, the thickness of individual perovskite sheets in wet cast quasi-2D layers tends to randomly fluctuate giving rise to a specific type of disorder, which impacts on the carrier dynamics and is rather complex and remains understudied. Here, we present a study of carrier transport in Ruddlesden–Popper type (PEA)2FAn−1PbnBr3n+1 layers of order n from one to four, and in the bulk FAPbBr3 layer. We use a light induced transient grating technique to measure the carrier diffusion coefficient directly, and the transient absorption via photoluminescence to investigate the energy relaxation pathways. We observe two distinct energy transfer processes on different time scales. Fast energy funnelling in thicker (n ≥ 3) layers is observed up to 10 ps after excitation; we attribute this to short-distance transfer of excitons to neighbouring perovskite sheets of higher order. On the longer timescale of hundreds of picoseconds, carrier in-plane transport is governed by exciton diffusion in n = 1 and 2 layers and by free carrier plasma in thicker ones. Within the carrier density range of (0.5–4) × 1019 cm−3, the exciton diffusion coefficient in n = 1, 2 increases slowly from 1 to 2.8 cm2 s−1, whereas in thicker layers the dependence is much stronger and the diffusivity grows from 0.09 to 1.9 cm2 s−1. We explain these dependencies by a higher structural order in the thinner samples and the stronger localization of carriers in thicker ones. Also, amplified spontaneous emission (ASE) is observed in thicker (n ≥ 3) layers in electron–hole plasma, as evidenced by the typical ASE line redshift upon excitation.

A commercial grade low-cost image intensifier was modified with a fast transistor gating circuit for time-resolved photoluminescence spectra and decay measurements. Gating exposure of 20 ns and spectral sensitivity in the 300–1000 nm range were achieved. Operation frequency in 5 Hz to 0.3 MHz range was verified by a pulsed laser diode. Pulsed repetitive image detection was integrated on a cooled CMOS camera. High dynamic range and wide spectral resolution were achieved. The thermal noise, reduced by three orders of magnitude at -16°C cooling and a high gain of 104, allowed single photon detection. Application of the spectrometer for sensitive photoluminescence decay measurements at pulsed laser excitation was performed on MAPbBr3 perovskite and CsI(Tl) scintillator crystals. The decay lifetimes of 400–600 ns and 100–6000 ns were determined, respectively. CsI(Tl) scintillator showed much stronger x-ray luminescence intensity due to larger defect density.

Applications of lead halide perovskites in solar cells and photo- and ionising radiation detectors are based on effective charge carrier generation and transport. The perovskites exhibit large carrier recombination lifetimes and diffusion coefficients leading to exceptionally long carrier diffusion lengths for solution processed materials. However, carrier density dependences of these parameters are not very well established. Therefore, in this work, we have used non-destructive optical techniques such as time-resolved differential absorption, time-resolved photoluminescence and light induced transient gratings (LITG) for the simultaneous determination of carrier density dependent recombination and diffusion rates as well as diffusion length in MAPbI3 and MAPBr3 crystals in a wide range of excitation densities upon single and two photon injections. At low carrier densities, recombination processes on bulk and surface traps were found to be dominant providing 1–2 μs lifetime, and 6 × 103; ∼30 cm s−1 surface recombination velocities in MAPbI3 and MAPbBr3 crystals, respectively. For higher carrier densities, an enhancement in carrier recombination rate as well as an increase in carrier diffusivity is observed. We related the lifetime reduction with excitation to the bimolecular and Auger recombination processes with B0 = 1.0 (4) × 10−10 cm3 s−1 and C = 1.5 (1.2) × 10−29 cm6 s−1 coefficients in MAPbI3 and MAPbBr3, respectively. The LITG measurements provide a direct increase in the carrier density dependent diffusion coefficient from 1.35 to 3 cm2 s−1 in MAPbI3 and from 0.45 cm2 s−1 to 1.7 cm2 s−1 in MAPbBr3 due to the carrier plasma degeneracy and saturation of localized states. A high carrier density related diffusion length drop from 10 to 0.1 μm in the 1016–1019 cm−3 carrier density range was observed.

Nonlinear nonradiative recombination processes are considered as a main obstacle to efficient lead halide perovskite light emitters as light emitting diodes and lasers. In this work, crystal structure ideality, described by the Goldschmidt tolerance factor, impact on Auger and bimolecular recombination rates in spin-coated mixed-cation CsxMA1–xPbBr3 and CsxFA1–xPbBr3 perovskite layers was investigated. The excitation-dependent carrier lifetime and diffusion coefficient were determined using optical pump–probe techniques: time-resolved photoluminescence and light-induced transient grating. Layers with a Goldschmidt tolerance factor closer to unity revealed Auger and bimolecular recombination rates smaller by up to an order of magnitude. The bimolecular coefficient (0.6–5 × 10–10 cm3/s) increased simultaneously with the Auger coefficient (0.05–2 × 10–27 cm6/s) upon increase of x and simultaneous decrease of tolerance factor, due to variation in the localized carrier population and Rashba splitting. A larger...

Photoexcited carrier dynamics in 1014 – 1018 cm-3 density range was investigated by using complementary optical and microwave techniques. Bulk lifetime decrease from 4 s to 460 ns with excitation and its increase with temperature were observed in bulk n-type 4H-SiC crystal. The latter data and modeling of excitation-dependent hole capture with subsequent electron capture provided the trap position at EV + 0.19 eV and electron (hole) lifetimes of 360 ns (100 ns), correspondingly, at high excitation conditions. Lifetime increase with temperature was observed due to the trap thermal activation and reduction of electron capture cross section. The trap origin was discussed in terms of silicon vacancy related and titanium point defects. Photoluminescence spectra in 24-300 K range revealed bound exciton luminescence and nitrogen-aluminum donor-acceptor pair band, which excitation dependent and time-resolved dynamics was analyzed in wide temperature range.

Tin iodide perovskite CH3NH3SnI3 is often considered as a replacement for toxic lead halide perovskites. Tin iodide is not only suitable for production of solar cells, but also it emits in the near...

Heat management of optoelectronic devices is of critical significance in lead halide perovskites due to the intrinsically low thermal conductivity of this material. Despite its importance, thermal conductivity remains understudied, particularly in polycrystalline perovskite layers with different halides. Here, we employ a novel method for investigation of thermal properties in perovskite layers, which is based on a light-induced transient diffraction grating technique. We demonstrate the applicability of thermal grating technique by determining in an all-optical way the thermo-optic coefficient, speed of sound, and thermal conductivity in vapor-deposited polycrystalline layers of MAPbX3 (X = Cl, Br, I), MAPbBr2I, and MAPbCl2Br perovskites. We reveal the spatial anisotropy of thermal conductivity, which is noticeably lower in the direction along the layer surface (0.2–0.5 W/(m K)) if compared to that across the layer (0.3–1.1 W/(m K)). Finally, we demonstrate that for both directions the thermal conductivi...

We explored a Mg-alloyed ZnO material (or MgZnO alloy) on a lattice-matched scandium aluminum magnesium oxide substrate for obtaining most effective photodetectors with highest (∼10 A/W) and fastest (up to 3 GHz) responses and two orders of magnitude UV/VIS rejection ratio. Device operation was related to its material electronic properties (carrier lifetime, diffusivity, and diffusion length) via photoluminescence and light-induced pump–probe techniques. Defect-related response is found suitable for sensitive logarithmic detectors, while exciton-related response is favorable to ultrafast linear UV solar blind detectors. Two-contact planar detector design allows their application in low-cost environmental detection systems.

Time-resolved photoluminescence and light-induced transient grating techniques were applied for the comparative investigation of the evolution of the internal quantum efficiency and of the carrier diffusion in AlxGa1-xN (x > 0.6) heterostructures in 80 – 550 K temperature range and for excitations from 1 µJcm-2 to 1 mJcm-2. The decrease of the photoluminescence efficiency measured at high excitations and temperatures is quantitatively correlated to the increase of the diffusion coefficients of carriers and to the increase of their non-radiative recombination rate on vacancies. Complex numerical modelling provides a full set of carrier and exciton recombination parameters.

Carrier lifetime characterization in solar cell and light emitting diode materials by time-resolved photoluminescence and differential transmission is of high importance, indicating cost-efficient streak camera suitable for such tests would be highly demanded. In this work an electro-optic deflecting device is extended to full functionality streak camera operating in wide spectral range (200-2000 nm) by using electro-optic DKDP crystals. Setup, consisting of picosecond laser for sample excitation, electro-optic deflector and imaging spectrograph with silicon and InGaAs cameras, allows spectral and temporal imaging of recombination dynamics in semiconductor materials, with variable bandgap and composition (tested on metal-halide perovskite crystal), achieving temporal resolution up to 50 ps. For the case of time-resolved differential transmission, broadband

ZnO materials have been at the centre of many studies for decades. Doping of ZnO by lithium atoms is a prospective approach for compensation of n-type conductivity in the unintentionally doped ZnO aimed at obtaining p-type semiconductor. In this study, we have synthesized ZnO rod-like powders doped with lithium ions (0–0.65 atom%) by the new microwave-assisted solvothermal method in order to obtain greater photoluminescence intensity of ZnO emissions in the UV region. The obtained powders contain nanoparticles from 20 nm up to 250 nm depending on Li content. X-ray diffractometry and Raman spectroscopy were employed to characterise the structure of ZnO powders, scanning electron microscopy was used to determine the morphology, and most importantly the photoluminescence technique was used to investigate the optical properties of the obtained materials. As a result, the photoluminescence intensity has been increased up to an order of magnitude with the Li content in the ZnO crystals. The increase of the excitation laser power has led to an increased carrier lifetime and photoluminescence response intensity that is observed for all the samples.

International audience; Time-resolved photoluminescence and light-induced transient grating techniques were applied for temperature and excitation dependent internal quantum efficiency and carrier diffusion investigation in silicon-doped epilayer and AlxGa1-xN MQWs (x > 0.6). Decrease of photoluminescence efficiency at high excitations correlated with increase of diffusion coefficient as well as nonradiative recombination rate, indicating excitation and temperature dependent localized exciton density reduction. Excitation-dependent lifetime decrease was less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency dominated by thermal dissociation of excitons. At lowest excitations excitons were found captured to vacancy complexes. With increase of excitation exciton ionization appeared, leading to enhanced nonradiative recombination of holes on aluminum vacancies and simultaneous droop of PL efficiency due to saturated bimolecular free carrier plasma recombination. At initial recombination stages and high excitations diffusive recombination on dislocations was also revealed. Modelling provided free exciton binding energies of 104, 140 meV in the MQWs and in the layer; exciton localization energies in 12-35 meV range; localized exciton lifetimes in 2-4 ns range; free exciton and carrier radiative recombination rate coefficients of rex = (0.6 0.2)109(T/300)-1 1/s and Brad = (7 1)10-10(T/300)-3/2 cm3/s, respectively. Vacancy complex capture cross section for excitons was determined to be = (2 1)10-16(300/T)2 cm2. Electron and hole capture cross sections to aluminum vacancy were modeled (1.5 1)10-13 cm2 and (7 1)10-13 cm2, respectively. Dislocation Coulomb radius for free carrier recombination was found to be 12-15 nm.

We report a comprehensive study of the time-resolved photoluminescence (PL), carrier recombination, and carrier diffusion under diverse laser pulse excitation in TlInS2. The 2D-layered crystals were grown by the Bridgman method without or with the presence of a small amount of erbium in the melt. The investigation exposes large differences in two crystal types, although, a linear nonradiative lifetime and carrier diffusivity attain close values under high excitation with no contribution of the Auger recombination and the absence of the band gap narrowing effect. Moreover, at high pulse power, we detect imprinted transient grating fringes which are attributed to a new crystal phase formed by 2D electron-hole charge separation on local layers. The versatile model of the spontaneously polarized 2D-crystal has been developed to explain the observed features and ergodicity of charge dynamic processes. The model embraces the planar stacking fault (PSF) which edge provides a distortion and act as sink of strong recombination. The reduced occurrence of the PSFs in the erbium doped TlInS2 is the main attribute which determines the enhancement of PL by a factor of 50 and improves carrier diffusion along 2D-layers. The simulation permits evaluation of the PSF sizes of about 0.7 μm. The presented results allow improving 2D-crystal growth technology for novel sensor devices with separated excess charges.

Two types of BiOBr thin films with substantially different crystal stacking and photoelectrochemical (PEC) properties were grown on conducting ITO substrates. In the first case, electrophoretically deposited closely-packed layers with parallel arrangement of (001) faceted plate crystallites (BiOBr||) exhibit p-type PEC behavior in Br2/Br− aqueous solution generating cathodic photocurrent under electrochemical polarization. In the second case, BiOBr electrodes composed of plates with predominant perpendicular orientation to the ITO surface (BiOBr⊥) demonstrate opposite PEC behavior and generate anodic photocurrent. This fundamental difference in PEC properties of BiOBr⊥ and BiOBr|| photoelectrodes, which have close values of BET specific surface area (25 and 18 m2/g), plate thickness (15 and 16 nm) and comparable non-equilibrium charge carrier lifetime (in a sub-ns range), is explained by different stacking of plate crystals in them. Since BiOBr has better electronic conductivity within the (001) plane compared to that along the [001] direction, BiOBr⊥ crystallites have better electrical contact with ITO and are subjected to electrochemical polarization, while BiOBr|| crystallites are predominantly in an open circuit condition and their properties are influenced by the boundaries between plate crystals.

Carrier mobility is one of the crucial parameters determining the electronic device performance. We apply the light-induced transient grating technique to measure independently the carrier diffusion coefficient and lifetime, and to reveal the impact of additives on carrier transport properties in wet-cast CH3NH3PbI3 (MAPbI3) perovskite films. We use the high excitation regime, where diffusion length of carriers is controlled purely by carrier diffusion and not by the lifetime. We demonstrate a four-fold increase in diffusion coefficient due to the reduction of localization center density by additives; however, the density dependence analysis shows the dominance of localization-limited diffusion regime. The presented approach allows us to estimate the limits of technological improvement—carrier diffusion coefficient in wet-cast layers can be expected to be enhanced by up to one order of magnitude.

Metal halide perovskites are attractive materials for the realization of cheap and effective solar cells, thin film transistors, and light emitters. Carrier diffusion at high excitations, however, is poorly addressed in perovskites, even though it governs the diffusion length and determines the efficiency of photonic devices. To fully understand the dependence of diffusion length on carrier density, we performed direct and independent measurements of the carrier diffusion coefficient and recombination rate in several methylammonium lead-halide perovskite layers by applying the light-induced transient grating technique. We demonstrate the existence of two distinct carrier diffusion regimes within the density range of 1018–1020 cm–3. In the perovskite films of high compositional quality, diffusion is governed by a bandlike transport of free carriers. The diffusivity is high (0.28–0.7 cm2/s) in these samples, even at low carrier density, and further increases with excitation due to carrier degeneracy. The op...

Time-resolved induced absorption (IA) and light induced transient grating (LITG) techniques were applied for the investigation of nonequilibrium exciton-carrier diffusion and recombination processes in a high-purity CVD diamond. Injection range from 10 15 to 10 20 cm −3 carrier density was achieved by combining one photon and two photon excitations. The measurements were performed in the 10–750 K temperature range. The LITG diffusion coefficient peaked at 44 cm 2 /s value at room temperature under low injection conditions. At lower temperatures it transferred to much lower exciton diffusion coefficient. A strong decrease of diffusion coefficient under higher injection conditions was explained by exciton formation with a low diffusion coefficient and many body effects, as polyexciton and electron-hole droplet formation. High temperature phonon-limited diffusion coefficient was weakly injection dependent. Low excitation carrier lifetime was about 700 ns above 200 K. At lower temperatures, the decay time reduced by two orders of magnitude, which was explained by the formation of biexcitons. At lowest temperatures, an increase of the carrier recombination rate at high injection was attributed to Auger recombinations of polyexcitons and electron-hole droplets. While at high temperatures, the increase of the recombination rate with 490 meV activation energy was observed. The combination of IA and LITG measurements provided effective diffusion lengths in a 0.3–50 µm range, being strongly dependent on the excess carrier density and temperature.

Charge carrier trapping in diamond crystals containing well-defined concentrations of dislocations was investigated by several complementary techniques. Samples with dislocation densities ndis between

The research on halide perovskites is in its peak activity at the moment due to the material's potential application in photovoltaics. It is well known that slow processes, from seconds to minutes, are very significant in perovskite films and devices as compared to conventional photovoltaic materials. The kinetics of photoluminescence (PL) is informative data for studying such processes. In particular, tracing the change in PL intensity under continuous laser excitation provides information on charge carriers’ recombination and the efficiency of their extraction from the absorber layer. Although widely available, this method was applied mainly for the research on thin films. However, monitoring PL kinetics on complete solar cells as well as on incomplete device stacks at each fabrication and lifespan stage can provide an important information on the device optimization and determination of the layers and interfaces that limit device efficiency and stability. Here, we demonstrate the advantages of the method by tracing the evolution of PL kinetics at seconds-to-minutes timescale upon: (1) varying perovskite composition (MAPbI3, Cs0.15FA0.85PbI2.7Br0.3, Cs0.05(MA0.15FA0.85)0.95PbI2.55Br0.45); (2) layer-by-layer stack construction of Cs0.15FA0.85PbI2.7Br0.3-based solar cell and (3) the device on-shelf degradation. In this way, information on charge carrier recombination, diffusion and extraction is revealed purely by optical, contactless and non-destructive means.

Carrier dynamics in n-type 4H-SiC epilayers of varying thicknesses and low Z1/2 defect concentrations are investigated here in wide ranges of excess carrier density and temperature. Several techniques are employed to monitor carrier diffusion and recombination processes, including light induced transient grating (LITG), microwave photoconductance decay (MPCD) and free carrier absorption (FCA) using ps-laser pulses at 355 nm. The observed increase of the diffusion coefficient with the increasing excitation level is explained by the transition from the minority to the bipolar transport regime. Its subsequent decrease with even higher excitations is found to be governed by band-gap renormalization and degeneracy effects. The bulk lifetime, limited by hole traps at 0.19–0.24 eV above the valence band at lower excitations, was found to decrease from few microseconds to hundreds of nanoseconds during the transition regime from the minority to the bipolar transport. Our temperature-dependent measurements confirmed the trap activation energy and provided the approximate functional form of electron and hole lifetimes as τe = 340 × (T/300 K)3/2 ns and τh = 100 × (T/300 K)–1/2 ns, for the temperature T range 80–800 K. It was found to hold for 65 and 120 μm sample thicknesses, while the lifetimes were found to be twice shorter for the sample 35 μm thick.

In this work we present a study of spatial and temperature dependencies of carrier lifetime and photoluminescence in pseudomorphic Ge0.95Sn0.05 layer on silicon by applying contactless time-resolved differential absorption and photoluminescence techniques. The observed small lifetime and photoluminescence intensity spatial variations (~10%) indicate for good material homogeneity. Differential transmission kinetics exhibit fast bleaching and slow absorption components related to thermalization (few hundreds of picoseconds) and surface recombination (few tens of nanoseconds) processes. Temperature dependent photo-luminescence measurements indicate activation of carriers from L to Γ valley, governing dominant direct photoluminescence emission band at 1850 nm at 300 K. These direct transitions explain more efficient and blue-shifted by 150 nm photoluminescence emission at room temperature. At low temperatures direct emission is related to non-thermalized carriers, while indirect one is caused by depopulation of light hole band with temperature.

Photo-detection in the near-infrared is commonly performed by Ge or InGaAs-based photodetectors. Further extension of the detection range to the mid-infrared region can be performed by germanium-tin (GeSn) material which shows promising characteristics and is fully compatible with silicon electronics as can be directly grown on silicon substrates. In this study, we focused on the optoelectronic properties of the photodiodes prepared by using 200 nm thick Ge0.95Sn0.05 epitaxial layers on Ge/n-Si substrate with aluminium contacts. Photodiodes were formed on non-irradiated and Nd:YAG laser irradiated Ge0.95Sn0.05 layers. The samples were irradiated by pulsed Nd:YAG laser with 61.5–259.2 MW/cm2 intensity. The photodiodes were characterized by using short laser pulses with the wavelength in 2.0–2.6 μm range. The laser-irradiated diode was found more sensitive in the long wavelength range due to laser induced Sn atoms redistribution providing formation of graded bandgap structure. Sub-millisecond photocurrent relaxation in the diodes revealed their suitability for image sensors. Our findings open the perspective for improving the photo-sensitivity of GeSn alloys in the mid-infrared by pulsed laser processing.

Carrier-transport mechanisms are studied in high-purity diamond irradiated with 6 MeV electrons in the dose range of 1012–1016 cm−2 and annealed at different temperatures up to 1450 °C. Lifetimes and diffusion coefficients are extracted using two pump–probe techniques based on free-carrier absorption and transient-grating principles and then correlated with the corresponding defect evolution from spectroscopic measurements. The neutral monovacancy is revealed as the main carrier recombination center in the as-irradiated diamond, providing bipolar carrier lifetimes of a few nanoseconds at the highest irradiation dose. Carrier-capture cross sections are reduced during annealing as vacancies aggregate into divacancies at ≤1000 °C and extended vacancy clusters at 1450 °C.

The development of new technology, which would be able to shift photosensitivity of Si devices to the mid-infrared range, preserving the benefits of cheap silicon readout circuits, is of high priority for short-wave infrared photo-detection in defense, medical, night vision, and material production applications. Group IV GeSn-based materials have recently shown promising optoelectronic characteristics, allowing extension of the detection range to the mid-infrared region. However, the electronic properties of the material are not well understood and need further research. In this work, we provide temperature dependent studies of carrier lifetime, diffusion coefficient, and diffusion length in Ge0.95Sn0.05 epilayer on silicon by applying contactless light induced transient grating technique. The observed temperature dependence of lifetime was explained by the recombination of carriers on vacancy-related defects. The electron and hole capture cross sections were calculated. The temperature dependence of the diffusion coefficient indicated hole mobility limited by phonon and defect scattering. Weakly temperature dependent diffusion length of ∼0.5 μm verified material suitability for efficient submicrometer-thick optoelectronic devices.

Low equilibrium solid solubility of Sn atoms in Ge (less than 1%) leads to limitations in application of this material for IR detectors and emitters. Providing of non-equilibrium conditions by powerful pulsed laser radiation can be successfully applied for enhancement of solubility of impurity atoms in the host material. Here we present laser-induced monotonous redistribution of Sn atoms in Ge, based on the thermogradient effect aiming overcoming equilibrium limitations in the solubility. We applied pulsed nanosecond laser radiation to epitaxial Ge0.96Sn0.04 layer grown on Si substrate to increase Sn atomic concentration up to 14% at the surface layer. As a result, indirect-direct graded bandgap GeSn structure was formed. The TEM/EDS cross-section analysis, X-ray photoelectron spectroscopy, Raman and UV reflection spectra confirmed the increase of Sn atomic content at the surface by order of magnitude. SEM and AFM imaging provided evident microstructure changes, while carrier lifetime changes, determined by differential transmittivity, were not observed, indicating that laser irradiation does not generate defects which reduce electronic quality of the material.

We report an investigation of a planar GeSn p–i–n diode for a high-detectivity photodetector based on an undoped GeSn film. By fabricating n- and p-type regions on the plane of the GeSn film using the complementary metal–oxide–semiconductor technology of ion implantation, a low dark current density is revealed and attributed to the low defect density of the film and current flow suppression around the diode periphery. This yields a specific 1550-nm detectivity of ∼1010 cm Hz1/2 W−1, an order of magnitude higher than that of conventional vertical GeSn-based diodes and comparable to that of commercially available Ge-based diodes. This work provides an alternative approach for achieving a high-detectivity GeSn photodetector that may facilitate its potential applications.

An optical pump–probe technique was used to detect spatial distribution of carrier lifetimes across the thickness of a high-quality diamond device structure. Two samples with as-received and boron implanted surfaces were compared to assess the role of implantation and related processing on carrier recombination mechanisms. It was found that the two implanted surfaces show very different behaviors despite undergoing the same treatment. At one of the surfaces, carrier lifetimes remained relatively unchanged, indicating surface recombination rates in the 102–103 cm/s range. At the other surface, carrier lifetimes were almost a magnitude lower and correlated with the locally detected nitrogen vacancy defect that was attributed to the elevated concentration of residual nitrogen.

We applied time-resolved free carrier absorption and light-induced transient grating techniques for monitoring carrier dynamics in lightly Fe-doped GaN bulk crystals. Comparison of measured decay times at single- and two-photon carrier photoexcitation with those reported for higher Fe doping revealed two different branches of lifetime dependence on iron density. A deeper insight was reached by using different wavelengths for probing carrier dynamics, which enabled discrimination of electron and hole capture processes by Fe ions. The capture cross section for holes, σh = (4 ± 1) × 10−15 cm2 for the Fe2+* state (5E → 4T2 transition) was determined to be ten times larger than that for electrons, σe = (4 ± 1) × 10−16 cm2 for the Fe3+* state (4T1 → 3T1 transition). Thermal activation of the [Fe2+(5T2),hVB] complex, where hVB is a free hole, required an energy of 315 meV. Light-induced transient grating decay revealed decreasing diffusivity of electrons with Fe doping due to initial carrier capture to Fe3+ states. Simultaneous measurements of carrier diffusion coefficient and lifetime at low excitation conditions (3 × 1017 cm−3) revealed a decrease of bipolar diffusion length from 2 μm in the undoped GaN down to 0.08 μm in the Fe-doped samples at 300 K.

We report a comprehensive study of the time-resolved photoluminescence (PL), carrier recombination, and carrier diffusion under diverse laser pulse excitation in TlInS2. The 2D-layered crystals were grown by the Bridgman method without or with the presence of a small amount of erbium in the melt. The investigation exposes large differences in two crystal types, although, a linear nonradiative lifetime and carrier diffusivity attain close values under high excitation with no contribution of the Auger recombination and the absence of the band gap narrowing effect. Moreover, at high pulse power, we detect imprinted transient grating fringes which are attributed to a new crystal phase formed by 2D electron-hole charge separation on local layers. The versatile model of the spontaneously polarized 2D-crystal has been developed to explain the observed features and ergodicity of charge dynamic processes. The model embraces the planar stacking fault (PSF) which edge provides a distortion and act as sink of strong recombination. The reduced occurrence of the PSFs in the erbium doped TlInS2 is the main attribute which determines the enhancement of PL by a factor of 50 and improves carrier diffusion along 2D-layers. The simulation permits evaluation of the PSF sizes of about 0.7 μm. The presented results allow improving 2D-crystal growth technology for novel sensor devices with separated excess charges.

It was demonstrated in our previous work that the photoelectrochemical (PEC) reduction processes occur with a giant incident photon-to-current conversion efficiency (IPCE ≫ 100%) at bismuth oxysulfide (BOS) semiconductor films in aqueous solutions containing acceptors of photoelectrons ([Fe(CN)6]3−). The anomalously high IPCE was related to the photoconductivity of the semiconductor. In this work, we analyze the dynamics of the chemical and phase composition of BOS films with variation of their deposition time, as well as the dependence of photocurrent on the film thickness and wavelength of the incident light. We demonstrate that in the case of illumination with a short-wavelength light (λ = 465 nm), the photocurrent is reduced down to a complete disappearance with an increase in the film thickness in the range of 0.3–1.3 μm, and for a fixed thickness of the bismuth oxysulfide film, the photocurrent decreases with the reduction of the wavelength indicating that photogeneration of the charge carriers over the entire thickness of the film is necessary for the giant IPCE effect. Using the light induced transient grating (LITG) method, the lifetime of the charge carriers (τ) was determined in the range of 25–80 ps depending on the film thickness, whereas the diffusion coefficient (D) does not exceed 1 cm2 s−1 meaning that the charge transport across the films is determined only by drift.

Carrier mobility is one of the crucial parameters determining the electronic device performance. We apply the light-induced transient grating technique to measure independently the carrier diffusion coefficient and lifetime, and to reveal the impact of additives on carrier transport properties in wet-cast CH

The change in reflectivity of black-Si (b-Si) upon optical excitation was measured by the pump-probe technique using picosecond laser pulses at 532 (pump) and 1064 nm (probe) wavelengths. The specular reflection from the random pattern of plasma-etched b-Si nano-needles was dominated by the photo-excited free-carrier contribution to the reflectivity. The kinetics of the reflectivity were found to be consistent with surface structural and chemical analysis, performed by scanning and transmission electron microscopy, and spectroscopic ellipsometry. The surface recombination velocity on the b-Si needles was estimated to be ~ 10 2 cm / s . Metalization of b-Si led to much faster recombination and alteration of reflectivity. The reflectivity spectra of random b-Si surfaces with different needle lengths was modeled by a multi-step refractive index profile in the Drude formalism. The dip in the reflectivity spectra and the sign reversal in the differential reflectivity signal at certain b-Si needle sizes is explained by the model.

We investigated carrier dynamics and photoluminescence in undoped and n-type phosphorus-doped diamond epilayers under interband picosecond-pulse photoexcitation at 213 nm. Carrier/exciton lifetime and diffusion coefficient D were determined in 4 × 10 17 to 4 × 10 19 cm − 3 carrier density range. The low D = 7 ÷ 11 cm 2 /s value in the undoped epilayer was ascribed to exciton formation and contribution of many-body effects. At low excitations, photoluminescence spectra in the undoped layer exhibited exciton emission, while at the higher ones (above 2 × 10 18 cm − 3 ) free carrier emission became dominant. The calculated radiative lifetime of excitons (860 ns) was found much higher than the experimentally measured nonradiative lifetime of 11 ns, governed by excess carrier diffusive flow to the surface, with S = (3 ± 2) × 10 5 cm/s surface recombination velocity. In the n-type layer, donor-related emission dominated and the phosphorus-related defect complexes diminished the carrier lifetime to 30–50 ps. Optically recharged phosphorus states resulted in millisecond long decay tails of trapped carriers.

The combined temporally, spatially and spectrally-resolved optical techniques, namely the photoluminescence, light induced transient grating, and differential reflectivity were used for investigation of excitation-dependent PL efficiency, exciton lifetime, and diffusion coefficient in Si-doped Al-rich multiple quantum wells and epilayers at various temperatures. Novel features of carrier recombination and in-plane diffusion were observed. Low-excitation radiative lifetime of 1–2 ns was found temperature-independent in 80–150 K interval, while it sublinearly decreased with excitation at excess carrier densities above 1018 cm−3. The lifetime decrease correlated with the increase of diffusion coefficient, indicating excitation-enhanced delocalization of localized excitons and therefore enhanced capture to nonradiative centers. The droop of photoluminescence efficiency with excitation was the strongest at 80–150 K due to strong delocalisation at low-temperatures, while at higher temperatures the thermal activation prevailed in photoluminescence excitation dependence. The photoluminescence efficiency quenching at T > 200 K provided rather high activation energies of ∼100 and 160 meV for Al-rich multiple quantum wells and epilayers, correspondingly.

To mymargin evaluate nitrogen-doped diamond as a candidate for Photoconductive Semiconductor Switches (PCSS) triggered in the sub-bandgap visible range, we have fabricated and tested diamond PCSS from a set of diamond grades of varying nitrogen concentration. The nitrogen-doped diamond PCSS have higher responsivity than other diamond optoelectronic devices in the visible range. We present a novel demonstration of sub-bandgap triggered diamond PCSS, which when scaled to higher voltage, can compete with and even surpass performance of existing ultraviolet-triggered diamond PCSS. This work suggests that nitrogen-doped diamond is a promising candidate for high-power, fast optoelectronic devices.

Non-equilibrium carrier dynamics was studied in a free-standing 160 μm thick 3C-SiC wafer using time-resolved differential transmittivity (DT), differential reflectivity (DR), and light induced transient grating (LITG) techniques. To enable investigation of carrier recombination and diffusion features in the middle part of the wafer, a wedge-shaped strip was prepared by polishing out the most-defective part of the wafer on the substrate side. Measurements of DT and DR decay kinetics on both sides of the wedged sample and their numerical fitting provided bulk lifetimes of 180 ns in the middle part and 60 ns in the edge part (∼50 μm thick subsurface region). On the as-grown side, the surface recombination velocity was equal to 3 × 10 3 cm/s. Carrier diffusivity, determined by LITG technique, and recombination rate were found injection-dependent. The low-injection value of diffusion length, L D = 8 μm, was found the longest in the middle part of the wafer, while at high injections it decreased to 1 μm due to an impact of Auger recombination with a coefficient, C = (4 ± 1) × 10 −32 cm 6 /s. The 2.7 times lower diffusion length value in the edge part was explained by an impact of point defects to the carrier lifetime and diffusivity.

Perovskite light converting layers optimization for cost-efficient white light emitting diodes (LED) was demonstrated. High excitation independent internal quantum efficiency (IQE) of 80% and weakly excitation dependent PL spectra suitable for white light generation were obtained in the mixed cation CsxMA1-xPbBr3 perovskite nanocrystal layers with optimal x = 0.3 being determined by effective surface passivation and phase mixing as revealed by x-ray diffraction. Enhancement of the PL homogeneity and the external quantum efficiency (EQE) were secured when using 2,2',2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi) additive in the layer preparation process. Excitation dependent PL intensity, decay time, and IQE revealed that the high emission efficiency of the layers originates from a dominant radiative localized exciton recombination (130 ns) weakly influenced by the nonradiative free carrier recombination (750 ns). Warm and cool white LEDs with correlated color temperature 3000 K and 5600 K, and color rendering index 82 and 74, respectively, were realized by using the optimized perovskite layers, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) red emitter and a blue LED.

We applied time-resolved nonlinear optical techniques for investigation of carrier recombination and diffusion processes in a 420 μm thick CVD diamond, relatively free from structural and point defects. Injection of 1015–1017 cm−3 carrier density by two-photon excitation and monitoring free carrier absorption decay in 80–800 K range provided lifetime of 720 ns at T ≥ 300 K. At low temperatures, a linear increase of the carrier recombination rate with the injection level was fitted by an effective bimolecular recombination coefficient B = 10−11–10−9 cm3 s−1 and attributed to the impact of excitons and biexcitons, contributing to phonon-assisted indirect Auger recombination. At high temperatures, increase of recombination rate was governed by Coulomb enhanced direct Auger process with a 520 meV threshold. An ambipolar carrier mobility with its peak value of ∼1500 cm2 V−1 s−1 at room temperature was determined by transient grating technique at ∼2 × 1015 cm−3 excess carrier density. A strong decrease of mobility at higher injections revealed the role of carrier-carrier scattering, exciton formation and bandgap renormalization. The simultaneous measurements of carrier lifetime and diffusivity provided carrier diffusion length, which varied in a 1–50 μm range, depending on injection and temperature.

Carrier dynamics under interband carrier injection conditions (213 nm) was studied in undoped and boron-doped microcrystalline diamond layers with different grain size. The grain size, determined by scanning electron microscopy and electron backscattered diffraction, varied from 130 μm on the growth side to ∼1–2 μm on the nucleation side of a 1.0 mm-thick undoped layer. Carrier lifetimes measured by differential transmittivity (DT) technique varied from 1 to 4 ns on the growth side to ∼220 ps on the nucleation side. Also the carrier diffusivity was found higher on the growth side. The B-doped layer with 40 μm grain size at the growth side exhibited 380 ps carrier lifetime, which decreased to 130 ps on the nucleation side. Even shorter lifetimes (∼100–200 ps and ∼10 ps, correspondingly) were revealed in this layer by differential reflectivity decay due to impact of subsurface defects. Therefore we conclude that the recombination rate in presence of large grains is dominated by bulk non-radiative traps, as diffusion time of carriers to reach grain boundaries is much longer (few μs). An impact of grain boundaries to recombination is expected in case of smaller grains. Thermal grating decay provided values of thermal diffusivity Dth in the range of 12–6 cm2 s−1, respectively, on the growth and nucleation side of the layers, and its decrease with reduction of grain size was attributed to phonon scattering on grain boundaries.