Recombination Of Trapped Carriers Research Articles

Sonochemical-assisted preparation of mesoporous ZnS quantum dots: Optical, electrical, and temperature-dependent dielectric characteristics

Mesoporous ZnS quantum dots (QDs) were prepared by the sonochemical technique that indicated bi-structural cubic-hexagonal phases with an average particle size of 2.5 nm, a pore size of 4 nm, and 28.85 m2 g−1 surface area. ZnS QDs exhibited a high bandgap of 3.85 eV with a large Urbach energy of 280 meV. The presence of localized defects was verified by the photoluminescence emission that was ascribed to the recombination of trapped carriers in shallow and deep trapping states with photogenerated holes. ZnS QDs revealed a high DC conductivity of 5.37 × 10−6 Ω−1 m−1 and a small activation energy of 248 meV that facilitated the tunnelling of thermionically excited carriers. The frequency-dependent behavior of AC conductivity (σAC), dielectric constant (εr), and dielectric loss (ε′) was examined at various temperatures. It was observed that both σAC, εr, ε′ were enhanced with increasing temperature up to 333 K followed by a gradual decrease at higher temperatures.

Trapped Carrier Recombination in Sb2Se3 Polycrystalline Film

Sb2Se3 has recently emerged as a promising material for optic-electronic applications. In this work, trapped carrier recombination in Sb2Se3 was investigated by joint use of time-resolved microwave conductivity (TRMC) and photoluminescence (PL) spectroscopy. trapped carrier thermal excitation into the continuous band was observed in TRMC kinetics. Based on the exponential band tail model, the depth of the trap state, where trapped carriers are released into a continuous band, was estimated to range from 33.0 meV to 110.0 meV at room temperature. Temperature-varying TRMC and PL were further employed to study the influence of temperature on the trapped carrier recombination. Negative thermal quenchings of PL intensity and quantity of thermal emission carriers were observed and can be well explained by the thermal excitation of deep trapped carriers into shallow trap states and the continuous band. Two thermal activation energies of 12.5 meV and 304.0 meV were also revealed. This work is helpful for understanding the trapped carrier recombination process in polycrystalline Sb2Se3 film.

Unveiling defect-mediated carrier dynamics in few-layer MoS2 prepared by ion exchange method via ultrafast Vis-NIR-MIR spectroscopy

Defect-mediated processes in two-dimensional transition metal dichalcogenides have a significant influence on their carrier dynamics and transport properties, however, the detailed mechanisms remain poorly understood. Here, we present a comprehensive ultrafast study on defect-mediated carrier dynamics in ion exchange prepared few-layer MoS2 by femtosecond time-resolved Vis-NIR-MIR spectroscopy. The broadband photobleaching feature observed in the near-infrared transient spectrum discloses that the mid-gap defect states are widely distributed in few-layer MoS2 nanosheets. The processes of fast trapping of carriers by defect states and the following nonradiative recombination of trapped carriers are clearly revealed, demonstrating the mid-gap defect states play a significant role in the photoinduced carrier dynamics. The positive to negative crossover of the signal observed in the mid-infrared transient spectrum further uncovers some occupied shallow defect states distributed at less than 0.24 eV below the conduction band minimum. These defect states can act as effective carrier trap centers to assist the nonradiative recombination of photo-induced carriers in few-layer MoS2 on the picosecond time scale.

The electroluminescence mechanism of non-doping PhOLEDs based on CBP/Ir(ppy)3 investigated by delayed EL measurements

In this paper, the non-doped PhOLED based on 4,4′-bis(9-carbazolyl)-1,1′-biphenyl(CBP) and fac-tris(2-phenylpyridine) iridium(Ir(ppy)3) with the emitting layer of [CBP/Ir(ppy)3]n/CBP in which n is equal to 2,3,4 and 5, respectively, are prepared. The electroluminescence and the luminance-voltage-current density characteristics of devices are detected. The wavelength range of these devices is varied from 517 nm to 540 nm. And the delay EL measurements are used to elucidate the carrier recombination and light emission mechanism in non-doped PhOLEDs. In this technique, the devices are driven using a square pulse driving scheme, with a forward bias pulse width of 1 ms, which is sufficiently long for prompt EL to reach its steady-state intensity. The delay EL results show that changing the number of the non-doped EML leads to marked changes in the charge-trapping and host–host TTA patterns, which suggests that the carrier transport and recombination processes depends on the number of non-doped EML. When the number of non-doped EML is less than 3, only the trapped carrier recombination signal are detected in the delay EL measurement. For the devices with more non-doped EML than 3, both the trapped carrier recombination and host–host TTA signal are detected. All these results are discussed and give the evidence for the electroluminescence mechanism of prepared devices.

Measurement of transient photoabsorption and photocurrent of BiFeO3thin films: Evidence for long-lived trapped photocarriers

We have studied the optical response and dynamical behavior of photocarriers in BiFeO${}_{3}$ thin films by means of transient absorption (TA) and photocurrent (PC) measurements. PC and absorption spectroscopy indicate that BiFeO${}_{3}$ thin films have an indirect band gap energy of \ensuremath{\sim}2.4 eV. The TA and PC decay dynamics have fast (\ensuremath{\sim}1 ns) and slow (\ensuremath{\sim}100 ns) components that are attributed to the localization of free carriers to shallow trap states and the recombination of trapped carriers, respectively. The long decay time of the PC is caused by the thermal activation of trapped carriers into the conduction band. Long-lived trapped photocarriers can be linked to the ferroelectricity and give rise to unique photoinduced phenomena in BiFeO${}_{3}$.

Photoluminescence and Photo-Catalytic Activity of Synthesized Nanocrystals

Abtract Intrinsic and extrinsic semiconductor nanostructures have attracted great attention due to their size tunable photo-physical and photo-chemical properties. In the present paper, polyvinyl pyrrolidone (PVP) capped Zn 1-x Eu x S (0.00001≤x≤0.1) nanocrystals have been synthesized by means of a facile chemical synthesis method. Crystallography and morphology of synthesized materials have been deliberated using X-ray diffraction (XRD) and transmission electron microscope (TEM), respectively. Diffraction and electron micrograph studies reveal that the synthesized materials are zinc blende nanocrystals having average particle size ~3nm. Elemental and compositional analyses of the nanocrystals have been done using energy dispersive X-ray fluorescence (EDXRF) technique. Steady state photoluminescence spectra have been recorded for optical characterization of synthesized nanomaterials. Photocatalytic activity potential of synthesized nanomaterials under UV radiation exposure has been investigated using methylene blue (MB) dye as a test contaminant in aqueous media. Photo-physical and photo-chemical behaviour dependence on doping concentration has been described in detail. Moreover, the sophistication of competition between charge carrier recombination and charge carrier trapping followed by the competition between recombination of trapped carriers and interfacial charge transfer processes have been presented in a fantastic and elaborative way by comparative study of photoluminescence and photo-catalytic activity results.

Initial charge carrier dynamics in porous silicon revealed by time‐resolved fluorescence and transient reflectivity

AbstractNonequilibrium charge carrier relaxation dynamics in porous silicon was investigated by means of time‐resolved photoluminescence and ultrafast transient reflectance. Fluorescence band shift to the low energy side and intensity decay were observed during several initial nanoseconds under porous silicon excitation at 3.3 eV, and attributed to the spectral diffusion and trapped carrier recombination. Ultrafast transient reflectance pump–probe technique reveals reflectance dynamics on a several ps time scale present with 3.1 eV excitation, but absent with excitation at 1.77 eV.

Optical studies of carrier and phonon dynamics in Ga1−xMnxAs

We present a time-resolved optical study of the dynamics of carriers and phonons in Ga1−xMnxAs layers for a series of Mn and hole concentrations. While band filling is the dominant effect in transient optical absorption in low-temperature-grown GaAs, induced absorption by trapped electrons become important with increasing Mn concentration in Ga1−xMnxAs, as inferred from the sign of the absorption change. We find that carrier trapping and trapped carrier recombination becomes faster as Mn concentration increases in GaMnAs. We also report direct observation on lattice vibrations in Ga1−xMnxAs layers via the reflective electro-optic sampling technique. The data show increasingly fast dephasing of longitudinal-optical phonon oscillations for samples with increasing Mn and hole concentration, which can be understood in term of phonon scattering by the holes.

Optically Detected Magnetic Resonance Study of CdS/HgS/CdS Quantum Dot Quantum Wells

CdS/HgS/CdS nanoparticles consist of a CdS core, epitaxially covered by one or two monolayers of HgS, and additional cladding layers of CdS. The luminescence spectrum of the studied materials contains a dominant exciton band located at the HgS layer and an additional nonexcitonic band, presumably corresponding to the recombination of trapped carriers. The present paper describes our efforts to identify the influence of CdS/HgS/CdS interfaces on the localization of the photogenerated species. These properties were investigated by the utilization of optically detected magnetic resonance spectroscopy. The results have shown the existence of two kinds of electron−hole recombination, trapped either at a twin packing of a CdS/HgS interface or at an edge dislocation of an epitaxial HgS or a CdS cladding layer.

Optically Detected Magnetic Resonance and Double Beam Photoluminescence of CdS/HgS/CdS Nanoparticles

AbstractCdS/HgS/CdS nanoparticles consist of a CdS core, epitaxially covered by one or two monolayers of HgS and additional cladding layers of CdS. The present paper describes our efforts to identify the influence of CdS/HgS/CdS interfaces on the localization of the photogenerated carriers deduced from the magneto-optical properties of the materials. These were investigated by the utilization of optically detected magnetic resonance (ODMR) and double-beam photoluminescence spectroscopy. A photoluminescence (PL) spectrum of the studied material, consists of a dominant exciton located at the HgS layer, and additional non-excitonic band, presumably corresponding to the recombination of trapped carriers at the interface. The latter band can be attenuated using an additional red excitation. The ODMR measurements show the existence of two kinds of electron-hole recombination. These electron-hole pairs maybe trapped either at a twin packing of a CdS/HgS interface, or at an edge dislocation of an epitaxial HgS or a CdS cladding layer.

Temperature dependence of optical phase conjugation in semiconductor-doped glasses

We measured optical phase conjugation via low-power degenerate four-wave mixing at 532 nm as a function of temperature in semiconductor-microcrystallite-doped glasses. We find that the overall efficiency of the phase conjugation process increases with decreasing temperature and is temperature independent between 77 and 100 K. The results are interpreted as the combined effect of the increased transmission due to the increased band gap and an effective χ(3) which increased due to a decrease of the trapped carrier recombination rate.

Recombination of trapped carriers in a-Si:H probed by subbandgap excitation

IR-excitation of non-equilibrium carriers is used to study recombination in undoped and doped glow discharge deposited a-Si:H. In two-beam experiments quenching and enhancement of photoluminescence (PL) and photoconductivity (PC) are observed in the temperature range 5–300 K. At low temperature after bandgap excitation trapped carriers of concentration near 5·10 16cm −3 persist with long lifetimes. Their reexcitation by IR-light causes transients of PL and PC which in connection with light induced ESR yield information on the recombination process. The results are discussed in a model, which contains besides bandtail states only defect states arising from Si-dangling bonds. The quenching phenomena are explained by excitations of trapped holes from the band tail and of electrons from doubly occupied dangling bonds.

Photoluminescence associated with impurity clusters in phosphorous doped silicon

Photoluminescence measurements as a function of excitation level and temperature are presented for Si(P) containing 7.5×10 17 P atomd/cm 3. The luminescence in Si(P) for intermediate concentrations in the range from 1×10 16 to 2×10 18 cm −3 is interpreted in terms of recombination of trapped carriers at clusters of impurities.

Doppelinjektion und Elektrolumineszenz in dotierten Anthracenkristallen

Abstract The current-voltage dependence and the current dependence of the electroluminescence at different wavelengths have been investigated in pure and in tetracene doped anthracene crystals. The influence of traps is shown to be very significant. The current-voltage dependence is calculated by dividing the crystal in a hole injecting region (density of electrons negligible) and in a space-charge-free region. The latter arises by emptying the traps by recombination, its thickness is proportional to the applied voltage. The temporal decay of the electroluminescence was measured after the contacts were shorted. The decay shows a fast component which had the fluorescence spectrum characteristic of tetracene and a delayed component which had the fluorescence spectrum characteristic of anthracene. The delayed component is explained by assuming that each recombination, including free- trapped carrier recombination, has a high probability of yielding a triplet exciton. This will lead to delayed fluorescence by...