Trapping Of Free Carriers Research Articles

Oxygen vacancy-mediated activation of Zn-O bonds at the S-scheme heterostructure interface for efficient uranium collection and resistance to biological contamination

Efficient reduction of U(VI) to U(IV) during seawater uranium extraction remains a challenge. Here, cerium dioxide quantum dots were bonded to the surface of indium zinc sulfide (ZIS) to form ZIS composite cerium dioxide quantum dots (ZIS/QDs-Ce). The oxygen vacancies are able to activate the Zn-O bonds at the ZIS and QDs-Ce interfaces, thus enabling the constructed S-scheme heterojunction to show great potential in facilitating the separation and transfer of photogenerated carriers. ZIS/QDs-Ce exhibited high anti-bio-pollution activity by generating biotoxic free radical reactive oxygen species (ROS). The photo-induced reduction of uranium by ZIS/QDs-Ce in 10 ppm uranium spiked simulated seawater reached 99.6 %. Oxygen vacancies cause distortions in the crystal field, and the Zn-O bond activation process triggers the movement of ZIS/QDs-Ce from the singlet low-spin state to the stable triplet state, resulting in free-carrier trapping centers and optimized electronic energy band structures. ZIS/QD-Ce has superior uranium extraction capabilities and is highly recommended for uranium extraction from seawater.

Preparation of Hybrid Films Based in Aluminum 8-Hydroxyquinoline as Organic Semiconductor for Photoconductor Applications.

In the present work, we have investigated an organic semiconductor based on tris(8-hydroxyquinoline) aluminum (AlQ3) doped with tetracyanoquinodimethane (TCNQ), which can be used as an organic photoconductor. DFT calculations were carried out to optimize the structure of semiconductor species and to obtain related constants in order to compare experimental and theoretical results. Subsequently, AlQ3-TCNQ films with polypyrrole (Ppy) matrix were fabricated, and they were morphologically and mechanically characterized by Scanning Electron Microscopy, X-ray diffraction and Atomic Force Microscopy techniques. The maximum stress for the film is 8.66 MPa, and the Knoop hardness is 0.0311. The optical behavior of the film was also analyzed, and the optical properties were found to exhibit two indirect transitions at 2.58 and 3.06 eV. Additionally, photoluminescence measurements were carried out and the film showed an intense visible emission in the visible region. Finally, a photoconductor was fabricated and electrically characterized. Applying a cubic spline approximation to fit cubic polynomials to the J-V curves, the ohmic to SCLC transition voltage VON and the trap-filled-limit voltage VTFL for the device were obtained. Then, the free carrier density and trap density for the device were approximated to n0=4.4586×10191m3 and Nt=3.1333×10311m3, respectively.

High-k/InGaAs interface defects at cryogenic temperature

Oxide defects in the high-k/InGaAs MOS system are investigated. The behaviour of these traps is explored from room temperature down to 10 K. This study reveals that the exchange of free carriers between oxide states and either the conduction or the valence band is strongly temperature dependant. The capture and emission of electrons is strongly suppressed at 10 K as demonstrated by the collapse of the capacitance frequency dispersion in accumulation for n-InGaAs MOS devices, though hysteresis in the C-V sweeps is still present at 10 K. Phonon assisted tunnelling processes are considered in the simulation of electrical characteristics. The simulated data match very well the experimental characteristics and provide energy and spatial mapping of oxide defects. The multi phonon theory also help explain the impedance data temperature dependence. This study also reveals an asymmetry in the free carrier trapping between n and p type devices, where hole trapping is more significant at 10 K.

Study of ultrafast photocarrier dynamics in polycrystalline CdTe films under low illumination

The photocarrier dynamics in polycrystalline CdTe films is closely related to device performance. However, the test results obtained at different illumination intensity may be different, and even significantly deviate from the ones in practical application environments. Here, the photocarrier dynamics in polycrystalline CdTe films deposited by thermal evaporation was studied through time-resolved transient absorption (TA) under different pump intensity. It was found that the behavior of photocarriers in CdTe films is strongly dependent on the pump intensity. Under low pump intensity close to operating environment of solar cells, the lifetime of free carriers becomes extremely short compared to high intensity, which is interpreted as a rapid trapping of free carriers by defects. For low pump intensity, numerical fitting by a physical model with one defect level shows that the free carriers in band-edge can be rapidly trapped by defects within tens of picoseconds and then slowly recover to the ground state within several nanoseconds. These results provide a meaningful reference to understand the photocarrier dynamics of CdTe polycrystalline films with a low excess carrier concentration close to that under AM1.5G illumination. • Photocarrier dynamics of CdTe films was characterized under low pump intensity by an ultra-sensitive TA test system. • A physical model was employed to describe the photocarrier dynamics of CdTe films under low illumination. • Experimentally revealed that the photocarrier lifetime of CdTe films under low illumination is tens of picoseconds.

Charge Carrier and Exciton Dynamics in Perovskites Revealed by Time‐Integrated Photoluminescence after Double‐Pulse Excitation

AbstractThe rate constants that describe the decays of excited states (free charge carriers, excitons) are important parameters for perovskite materials. Typically, these rates are determined using detectors with high time‐resolution to record the time‐dependent photoluminescence on the nanosecond scale. Herein, a method is applied that uses two excitation pulses with a variable delay and an inexpensive detector capable of measuring only the quasisteady state photoluminescence. Based on how the time‐integrated photoluminescence varies as a function of the delay time between the two excitation pulses, the rate constants for exciton–exciton annihilation, monomolecular exciton decay, bimolecular free charge carrier recombination, and monomolecular trapping of free charge carriers can be extracted. To demonstrate the method quasi‐2D perovskites are investigated as they are known to exhibit excited‐state populations that can be exciton dominated, free charge carrier dominated, or a mixture of the two. The method leads to unique curves and accurate extracted parameters in all cases. The introduced method of determining excited‐state lifetimes can be used with detectors that only have a low temporal resolution. This opens the path to the spatial mapping of excited‐state dynamics using standard cameras which would be attractive for quality control of photovoltaic layers.

Characterization of ion beam induced polarization in scCVD diamond detectors using a microbeam probe

Diamond detectors are increasingly being used in many multidisciplinary areas due to their good spectroscopy properties. However, in certain cases of diamond employment as a nuclear detector, a significant deterioration of the signal properties due to the trapping of free charge carriers by defects can be observed. This phenomenon is known as polarization. Experimentally, a good understanding of the free carrier transport mechanisms can be obtained from direct measurements of the Charge Collection Efficiency (CCE) using the ion beam induced charge technique (IBIC). In this work IBIC was performed by focused proton and carbon ion beams of different energies in the MeV range, to probe the polarization induced changes of the CCE. The detectors being used in this study include a thin scCVD membrane detector (thickness 6 μm) and a scCVD crystal of 65 μm thickness. Furthermore, the detectors have been damaged with transmitted protons to study the influence of defects on polarization. The results that are presented show that the ion microprobe technique IBIC can be successfully used to fully characterize the polarization development in terms of time evolution in scCVD diamonds. More specifically, we show for the thin membrane detectors at high count rates and longer time irradiation a clear dependence of the pulse height signal decrease, induced by the polarization effect, with levels of radiation damage. A significant deterioration of the signal was observed in regions damaged with fluences between 1012 and 1014 ions/cm2 for both types of charge carriers. The results have shown that the energies used for the probing ion beams allow us to explore the electronic features from shallow to deeply penetration range and that has an effective impact on the temporal evolution of the build-up space charge. The experiments carried out have also shown that light can be used to suppress or amplify the polarization phenomena in damaged regions depending on the dominant type of charge carriers.

Electron Trapping and Detrapping in an Oxide Two-Dimensional Electron Gas: The Role of Ferroelastic Twin Walls

The choice of electrostatic gating over the conventional chemical doping for phase engineering of quantum materials is attributed to the fact that the former can reversibly tune the carrier density without affecting the system's level of disorder. However, this proposition seems to break down in field-effect transistors involving SrTiO$_3$ (STO) based two-dimensional electron gases. Such peculiar behavior is associated with the electron trapping under an external electric field. However, the microscopic nature of trapping centers remains an open question. In this paper, we investigate electric field-induced charge trapping/detrapping phenomena at the conducting interface between band insulators $\gamma$-Al$_2$O$_3$ and STO. Our transport measurements reveal that the charge trapping under +ve back gate voltage ($V_g$) above the tetragonal to cubic structural transition temperature ($T_c$) of STO is contributed by the electric field-assisted thermal escape of electrons from the quantum well, and the clustering of oxygen vacancies (OVs) as well. We observe an additional source of trapping below the $T_c$, which arises from the trapping of free carriers at the ferroelastic twin walls of STO. Application of -ve $V_g$ results in a charge detrapping, which vanishes above $T_c$ also. This feature demonstrates the crucial role of structural domain walls in the electrical transport properties of STO based heterostructures. The number of trapped (detrapped) charges at (from) the twin wall is controlled by the net polarity of the wall and is completely reversible with the sweep of $V_g$.

Extremely bias stress stable enhancement mode sol–gel-processed SnO2 thin-film transistors with Y2O3 passivation layers

Herein, a new combination of high-performance and bias-stress-stable SnO2 thin-film transistors (TFTs) with Y2O3 passivation layers is introduced. The Y2O3 layers on SnO2 semiconductors function suitably as passivation layers to minimize H2O adsorption and to avoid free carrier trapping by creating physical gaps that are larger than the characteristic diffusion length. Yttrium is observed to partially diffuse into SnO2, suppressing the formation of oxygen vacancies in the SnO2 semiconductor because of its low electronegativity and standard electrode potential. The SnO2 TFTs with Y2O3 passivation layers exhibit a mobility of 5.7 ± 0.3 cm2/Vs, subthreshold swing of 1.1 ± 0.2 V/decade, threshold voltage (VTH) of + 6.8 ± 0.4 V, and on/off current ratio of 6.1 (±0.3) × 107. The fabricated sol–gel-processed SnO2 TFTs with Y2O3 passivation layers revealed remarkable negative and positive bias stress stabilities without hysteresis and consistently delivered high performance with enhancement mode operations. Owing to this high performance, the SnO2/Y2O3 combination reported herein has potential as a commercial-product-level TFT for next-generation displays.

Dielectric relaxation and electrical conduction mechanism of Neodymium doped Yttrium Chromite

This research article represents the fabrication and characterization of Neodymium doped YCrO3. Different optical parameters like optical bandgap energy and Urbach energy are evaluated. The gradual decrement of activation energy is observed when Neodymium doping content gradually increases. From the capacitance-voltage characteristics, free carrier concentration and trap carrier concentration are calculated, which increase with Nd doping content. Current-Voltage Characteristics of the different samples are measured at different temperatures. It follows the diode equation for the Schottky Barrier Diode (SBD) following thermionic emission. The graphical representation of current and voltage shows non-linearity, which cites the concept of Schottky diode. Another observation is that the barrier height is reduced, and the ideality factor is increased with the increase of doping concentration of Neodymium. The obtained value of the ideality factor is large, and it may be due to generation-recombination of charges inside the depletion layer and also tunneling through the barrier.

Effects of Temperature on the Free Carrier Traps of Shockley Read Hall Recombination Mechanisms for Gallium Sulfide (GaS) Semiconductor

In this paper, we have studied the effects of temperature on the free carrier trap of Gallium Sulfide (GaS) in Shockley read hall recombination mechanism. We have seen dependencies on the energy level in the trap of electrons and holes in their respective bands and classify these energy levels into five regions based on the interactions of the localized states with the conduction or valence bands in the high-temperature region. Gallium sulfide compound semiconductor has material impurities that introduce some intermediate energy levels in the forbidden gap. These levels act as recombination centers (or traps, from which the process is otherwise depicted as trap assisted recombination) and an intermediate step is introduced in the recombination process. Another is described as a hole that ascends to the intermediate level (equivalently described as a hole capture of the trap), recombining with an electron. It has also investigated that, at high temperatures, it shows only the electron trap for a wider range of localized trap energies and it shows a small region of localized trap energies for low temperatures. It also shows similar variations for trap density. Additionally, we also analyse the effects of injection levels on traps of free carriers. The variation of both excess carriers in the conduction and valence bands and excess electrons on trap level is the same with illumination at low injection level.

Phonon-Assisted Trapping and Re-excitation of Free Carriers and Excitons in Lead Halide Perovskites

Despite the advances in solar cells based on lead halide perovskites, the nature of photogenerated charges and trap states within these materials remains unclear. A model describing recombination in CH3NH3PbI3−xClx has been developed that accounts for phonon-assisted free-exciton and free-carrier trapping. We utilize optical spectroscopies and observe significant co-existence of the tetragonal and orthorhombic structural phases at low temperatures. From these measurements, we evaluate the longitudinal−optical phonon energy, exciton binding energy, and temperature-dependent electronic bandgap. We use these parameters to model the temperature- and fluence-dependent time-resolved photoluminescence decays, enabling us to demonstrate how shallow traps from which carriers can be re-excited can account for the delayed recombination in lead halide perovskites. The trap-state density reaches a maximum at the tetragonal to orthorhombic phase transition at ∼140 K, suggesting the formation of disorder-induced trap states, which are shown to dominate the recombination dynamics in CH3NH3PbI3−xClx.

Cathodoluminescence induced in oxides by high-energy electrons: Effects of beam flux, electron energy, and temperature

The cathodoluminescence (CL) induced in four oxide single crystals (α-Al2O3, ZrO2: Y or YSZ, MgAl2O4, and TiO2) by high-energy electrons from 400 keV to 1250 keV was studied as a function of beam parameters (flux and energy). The main CL bands are related to F center (oxygen vacancy) formation by elastic collisions above the threshold displacement energy of oxygen atoms. The beam-intensity dependence is interpreted on the basis of a kinetic-rate model involving F-center formation and annihilation. The temperature effect was also followed from 110 K to 300 K. A broad maximum is found for all bands at about 200 K for sapphire, whereas a monotonous increase with temperature is observed for YSZ. The plots of CL intensity versus temperature are mainly interpreted by the interplay between the thermal dependence of thermalized free-carrier trapping rates and luminescence efficiency. Finally, the dependence of CL intensity on the primary electron energy for F centers in YSZ showing a maximum at about 600 keV is explained on the basis of the interplay between point-defect formation and secondary-electron energy spectra production.

Defect Manipulation To Control ZnO Micro-/Nanowire-Metal Contacts.

Surface states that induce depletion regions are commonly believed to control the transport of charged carriers through semiconductor nanowires. However, direct, localized optical, and electrical measurements of ZnO nanowires show that native point defects inside the nanowire bulk and created at metal-semiconductor interfaces are electrically active and play a dominant role electronically, altering the semiconductor doping, the carrier density along the wire length, and the injection of charge into the wire. We used depth-resolved cathodoluminescence spectroscopy to measure the densities of multiple point defects inside ZnO nanowires, substitutional Cu on Zn sites, zinc vacancy, and oxygen vacancy defects, showing that their densities varied strongly both radially and lengthwise for tapered wires. These defect profiles and their variation with wire diameter produce trap-assisted tunneling and acceptor trapping of free carriers, the balance of which determines the low contact resistivity (2.6 × 10-3 Ω·cm-2) ohmic, Schottky (Φ ≥ 0.35 eV) or blocking nature of Pt contacts to a single nano/microwire. We show how these defects can now be manipulated by ion beam methods and nanowire design, opening new avenues to control nanowire charge injection and transport.

Dielectric relaxations of confined water in porous silica ceramics

In this study, dielectric properties of water confined in porous silica ceramics were investigated. Two porous ceramics were characterized in the frequency range \(10^{-1}~\hbox {to}~10^{7}\) Hz and temperature interval from \(-100\) to \(200^{\circ }\hbox {C}\). While the first sample was a ceramic with opened lateral pores, the second one was a ceramic with sealed lateral pores. In both ceramics, three dielectric processes were identified. The first, which appeared at lower temperatures, was attributed to the reorientation of water molecules in ice-like water cluster structures. The second is the relaxation observed over an intermediate temperature range, associated with the kinetic transition due to water molecule reorientation near a defect. At higher temperatures, the third was relaxation identified as the Maxwell–Wagner–Sillars polarization process due to the trapping of free charge carriers at the interface of the porous media. The first and second dielectric relaxations were analysed to prove the effect of the lateral surface state of the sample on water–inner surfaces of the porous media interaction. These analyses revealed a great similarity in the ice-like structure for both ceramics. However, the lateral surface state of the sample might enhance the dielectric strength of the first relaxation when lateral pores are sealed. Furthermore, it might improve the water–inner surfaces interaction when lateral pores are opened.

Studies on improved hole injection into N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine hole transport layer in the device by thermal annealing of indium tin oxide anode

We demonstrate the improved hole injection into N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) hole transport layer (HTL) by thermal annealing of ITO anode in hole-only devices (HODs). ITO anode was patterned and annealed at different temperatures (200, 300, and 400 °C) under the normal ambient. ITO film annealed at 400 °C shows the higher sheet resistance of 185 Ω/□ and the Hall measurement also confirms the drastic increase in the resistivity after 300 °C. The drastic decrease of Hall carrier concentration and mobility after 300 °C is attributed to the trapping of free carriers and scattering from the chemisorbed oxygen at grain boundaries. XRD analysis shows the increasing crystallite size as annealing temperature increases. SEM images show that ITO thin film annealed at 400 °C exhibits the wrinkle kind of morphological structures. AFM studies also supported the significant changes with an increase in the surface roughness of ITO film annealed after 300 °C. The fabricated HOD based on ITO anode annealed at 300 °C exhibited the improved hole injection than that of pristine and 200 °C annealed ITO-based HODs, and it is drastically decreased for 400 °C annealed ITO-based device below that of pristine device. It is reported that the annealing of ITO film at 400 °C under the normal ambient changes its surface morphology and/or surface chemical properties which play a role in the hole-injection properties. These results show that annealing of ITO anode upto 300 °C under the normal ambient improves the hole injection into TPD HTL in HODs, which will also be useful to enhance the hole injection and improve the charge balance in organic light-emitting diodes to improve the efficiencies.

Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites.

The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic-inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N(1)-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results are consistent with photogenerated carriers self-trapped in a deformable lattice due to strong electron-phonon coupling, where permanent lattice defects and correlated self-trapped states lend further inhomogeneity to the excited-state potential energy surface.

Disordered small defect clusters in silicon

The ionizing radiation induced disordered defect clusters and their relaxation in silicon were simulated by the density functional method. It was found that a non-relaxed disordered cluster gives rise to a great number of localized states having their energy levels within the semiconductor forbidden band gap. After the relaxation, however, the density of these states significantly decreases leaving only several relatively shallow donor and acceptor state levels that may contribute to trapping of free carriers and shrinkage of an effective band gap.

(Invited) MOS Interface Control Technologies for Advanced III-V/ Ge Devices

Critical issues for high quality Ge/III-V MOS gate stacks have been reviewed and discussed. For Ge gate stacks, a plasma post oxidation method can realize high quality ultrathin EOT Al2O3/GeOx/Ge and HfO2/Al2O3/GeOx/Ge gate stacks. We have evaluated limiting factors of electron and hole mobility in Ge MOSFETs. The Ge MOS channel mobility in a high Ns region is significantly degraded by surface roughness scattering as well as trapping of free carriers into interface states inside the Ge bands. It is shown that reduction in surface roughness through low temperature PPO and reduction in Dit inside the bands due to atomic Deuterium annealing can improve the effective mobility in the high Ns region. For InGaAs gate stacks, ALD La2O3 gate stacks yield lower Dit than the Al2O3 ones. For GaSb gate stacks, high ALD temperature or annealing temperature increases Dit at Al2O3/GaSb interfaces. InAs passivation for GaSb surfaces can effectively significantly improve the MOS interface properties and the thermal stability of the Al2O3/GaSb interfaces.

Profiling the local carrier concentration across a semiconductor quantum dot

We profile the local carrier concentration, n, across epitaxial InAs/GaAs quantum dots (QDs) consisting of 3D islands on top of a 2D alloy layer. We use scanning thermoelectric microscopy to measure a profile of the temperature gradient-induced voltage, which is converted to a profile of the local Seebeck coefficient, S. The S profile is then converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. Our combined computational-experimental approach suggests a reduced carrier concentration in the QD center in comparison to that of the 2D alloy layer. The relative roles of free carrier trapping and/or dopant expulsion are discussed.

The effect of diiodooctane on the charge carrier generation in organic solar cells based on the copolymer PBDTTT-C.

Microstructural changes and the understanding of their effect on photocurrent generation are key aspects for improving the efficiency of organic photovoltaic devices. We analyze the impact of a systematically increased amount of the solvent additive diiodooctane (DIO) on the morphology of PBDTTT-C:PC71BM blends and related changes in free carrier formation and recombination by combining surface imaging, photophysical and charge extraction techniques. We identify agglomerates visible in AFM images of the 0% DIO blend as PC71BM domains embedded in an intermixed matrix phase. With the addition of DIO, a decrease in the size of fullerene domains along with a demixing of the matrix phase appears for 0.6% and 1% DIO. Surprisingly, transient absorption spectroscopy reveals an efficient photogeneration already for the smallest amount of DIO, although the largest efficiency is found for 3% DIO. It is ascribed to a fine-tuning of the blend morphology in terms of the formation of interpenetrating donor and acceptor phases minimizing geminate and nongeminate recombination as indicated by charge extraction experiments. An increase in the DIO content to 10% adversely affects the photovoltaic performance, most probably due to an inefficient free carrier formation and trapping in a less interconnected donor-acceptor network.