Carrier Localization Research Articles

Effect of Si-doped InGaN underlayers on photoluminescence efficiency and recombination dynamics in InGaN/GaN quantum wells

A series of single InGaN/GaN quantum wells (QWs) with a Si-doped InGaN underlayer were studied to investigate the impact of the underlayer on photoluminescence efficiency and recombination dynamics. The thickness of the GaN capping layer was varied between samples, which changed the electric field across the QW due to band bending near the surface. When directly exciting the wells, thermionic emission of carriers results in a rapid drop in the photoluminesence efficiency with increasing temperature such that no emission is observed above 100 K. However, exciting above the energy of the barriers caused the intensity of the QW emission to drop more slowly, with up to 12% of the 10 K emission intensity remaining at 300 K. This difference is attributed to hole transfer from the underlayer into the QW, which increases in efficiency at higher temperatures, and is enhanced by stronger electric fields present in the GaN barriers of samples with thinner GaN capping layers. Further, the sample with the narrowest cap layer of 2 nm has a different shape and characteristic time for its photoluminescence decay transient and a different emission energy temperature dependence than the other samples. This behaviour was ascribed to a change in carrier localisation for this sample due to a reversal of the net field across the well compared to the other samples.

Temperature Dependence of Dark Spot Diameters in GaN and AlGaN

Threading dislocations in c‐plane (Al,Ga)N layers are surrounded by areas with reduced light generation efficiency, called “dark spots.” These areas are observable in luminescence measurements with spatial resolution in the submicrometer range. Dark spots reduce the internal quantum efficiency in single layers and light‐emitting devices. In cathodoluminescence measurements, the diameter of dark spots (full width at half maximum [FWHM]) is observed to be 200–250 nm for GaN. It decreases by 30–60% for AlxGa1−xN with x ≈ 0.5. Furthermore, the dark spot diameter increases with increasing temperature from 83 to 300 K in AlGaN, whereas it decreases in GaN. Emission energy mappings around dark spots become less smooth and show sharper features on submicrometer scales at low temperature for AlGaN and, on the contrary, at high temperature for GaN. It is concluded that charge carrier localization dominates the temperature dependence of dark spot diameters and of the emission energy distribution around threading dislocations in AlGaN, whereas the temperature‐dependent excitation volume in cathodoluminescence and charge carrier diffusion limited by phonon scattering are the dominant effects in GaN. Consequently, with increasing temperature, nonradiative recombination related to threading dislocations extends to wider regions in AlGaN, whereas it becomes spatially limited in GaN.

Bright and stable light-emitting diodes made with perovskite nanocrystals stabilized in metal–organic frameworks

Perovskite nanocrystals are exceptional candidates for light-emitting diodes (LEDs). However, they are unstable in the solid film and tend to degrade back to the bulk phase, which undermines their potential for LEDs. Here we demonstrate that perovskite nanocrystals stabilized in metal–organic framework (MOF) thin films make bright and stable LEDs. The perovskite nanocrystals in MOF thin films can maintain the photoluminescence and electroluminescence against continuous ultraviolet irradiation, heat and electrical stress. As revealed by optical and X-ray spectroscopy, the strong emission originates from localized carrier recombination. Bright LEDs made from perovskite-MOF nanocrystals are demonstrated with a maximum external quantum efficiency of over 15% and a high brightness of over 105 cd m−2 after the device reaches stabilization. During LED operation, the nanocrystals can be well preserved, free of ion migration or crystal merging through protection by the MOF matrix, leading to a stable performance over 50 hours. The use of metal–organic frameworks helps protect perovskite nanocrystals, resulting in bright, stable light-emitting diodes.

Semiconducting nature and magnetoresistance behaviour of ZnO / La0.3Ca0.7MnO3 / SrTiO3 heterostructures

In this report, ZnO /La0.3Ca0.7MnO3 /SrTiO3 (ZnO /LCMO /STO) heterostructures, with various thicknesses of LCMO, were grown successfully using chemical solution deposition (CSD) method. X–ray diffraction (XRD) measurement confirms the presence of LCMO and ZnO phases only without any other unwanted phases. To understand the charge conduction mechanisms across ZnO /LCMO interface as well as for LCMO manganite thin layer, temperature dependent resistivity measurements have been investigated under different applied magnetic fields. The measured ρ (T) data of all the heterostructures in current perpendicular to plane (CPP; for ZnO /LCMO interface studies) mode and current in plane (CIP; for LCMO thin layer studies) mode show the semiconducting behaviour throughout the temperature and magnetic field range studied. The semiconducting state of all the heterostructures have been studied using Mott type variable range hopping (VRH) mechanism to understand the field dependence of charge carrier localization length. Observed negative magnetoresistance (MR) has been discussed in detail with its dependence on applied magnetic field, temperature and manganite layer thickness.

Limiting factors of GaN-on-GaN LED

GaN-on-GaN light emitting diodes (LEDs) have been considered to present the path towards ultimate LED technology. However, at present, the performance of the LEDs is still lower in comparison to GaN-on-sapphire LEDs. This work attempted to identify the possible factors that limit the performance of a GaN-on-GaN LED. To perceive this clearly, a GaN-on-patterned sapphire substrate (PSS) LED was also included in this study for comparison. It was found that the GaN-on-GaN LED exhibited lower performance (e.g. external quantum efficiency) than the GaN-on-PSS LED, especially at higher currents, despite its dislocation density being much lower than the latter. This is because the GaN-on-GaN LED had a lack of V-pits formation and carriers localization. As a result, the LED showed higher operating voltage and lower radiative recombination in comparison to the GaN-on-PSS LED.

Thermal fluctuations and carrier localization induced by dynamic disorder in MAPbI3 described by first-principles based tight-binding model

Halide perovskites are strongly influenced by large amplitude anharmonic lattice fluctuations at room temperature. We develop a tight-binding model for dynamically disordered ${\mathrm{MAPbI}}_{3}$ based on density functional theory calculations to calculate electronic structure for finite temperature crystal structures at the length scale of thermal disorder and carrier localization. The model predicts individual Hamiltonian matrix elements and band structures with high accuracy, owing to the inclusion of additional matrix elements and descriptors for non-Coulombic interactions. We apply this model to electronic structure at length and timescales inaccessible to first-principles methods, finding an increase in band gap, carrier mass, and the subpicosecond fluctuations in these quantities with increasing temperature as well as the onset of carrier localization in large supercells induced by thermal disorder at 300 K. We identify the length scale ${L}^{*}=5$ nm as the onset of localization in the electronic structure, associated with decreasing band edge fluctuations, increasing carrier mass, and Rashba splitting approaching zero.

Nature of Local Charge Carrier Motions in Porphyrin-based Bulk Heterojunction Films Revealed by Time-resolved Optical Pump-terahertz Probe Spectroscopy

Local charge carrier dynamics in diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP)-based bulk heterojunction (BHJ) films are investigated by optical pump-terahertz probe spectroscopy. Here we focus on the effect of the alkyl chain lengths of DPP on the carrier dynamics. In contrast to the large difference in the power conversion efficiency, similar dynamics are observed between C10-DPP-BP and C4-DPP-BP BHJ films. We discuss the origin of similarity in terms of nature of the high frequency response of photo-generated charge carriers.

Demystifying natural gas distribution grid decommissioning: An open-source approach to local deep decarbonization of urban neighborhoods

In this paper, deep decarbonization in an urban neighborhood in Vienna, Austria is proposed focusing on decommissioning of the gas distribution grid for heat supply rather than trying to feed in “green” gas in the future. The core objective is to demonstrate that alternative network infrastructures and energy technologies ensure not only an adequate but also an even superior provision of local heat energy services. Two different deep decarbonization pathways are studied, namely, electrification of almost all energy services and expansion of the district heating network. In addition, future district cooling service supply is considered. The method applied couples and extends two open-source models offering a complete analysis toolkit covering a high spatial and temporal resolution. The results show that deep decarbonization of local multiple-energy carrier systems is possible, without being dependent on the existing distribution grid of natural gas. Possible stranded assets (also at the gas end-user level) must not play a decisive role, especially since the trade-off analyses in this work show that alternative scenarios of lower/zero-emission energy service provision are even more economical in the longer term since the CO2 price is expected to increase in the next decades. Future work may focus, among others, on the energy generation technology mix feeding into the district heating grid, the local mobility service needs, and a higher granularity to improve the assessment of the on-site (building-integrated) renewable generation potential associated with the emergence of energy community concepts.

Carrier localization and magnetoresistance in DNA-functionalized carbon nanotubes

Helical functionalization of carbon nanotubes using DNA strands can polarize carrier spins through chirality induced spin selectivity (or CISS) effect. Detection of this effect using transport experiments unravels an underlying magnetoresistance effect, origin of which is not well understood. In the present study, we investigate this effect, a fundamental understanding of which is crucial for the potential use of this system in spintronic devices. The conduction mechanism has been found to be in the strongly localized regime due to DNA functionalization, with the observed magnetoresistance originating from the interference effects between the forward and backward hopping paths. CISS-induced spin polarization has been estimated to increase the carrier localization length by an order of magnitude in the low temperature range and it affects the magnetoresistance effect in a non-trivial way that is not observed in conventional systems.

Raising Dielectric Permittivity Mitigates Dopant-Induced Disorder in Conjugated Polymers.

Conjugated polymers need to be doped to increase charge carrier density and reach the electrical conductivity necessary for electronic and energy applications. While doping increases carrier density, Coulomb interactions between the dopant molecules and the localized carriers are poorly screened, causing broadening and a heavy tail in the electronic density‐of‐states (DOS). The authors examine the effects of dopant‐induced disorder on two complimentary charge transport properties of semiconducting polymers, the Seebeck coefficient and electrical conductivity, and demonstrate a way to mitigate them. Their simulations, based on a modified Gaussian disorder model with Miller‐Abrahams hopping rates, show that dopant‐induced broadening of the DOS negatively impacts the Seebeck coefficient versus electrical conductivity trade‐off curve. Increasing the dielectric permittivity of the polymer mitigates dopant‐carrier Coulomb interactions and improves charge transport, evidenced by simultaneous increases in conductivity and the Seebeck coefficient. They verified this increase experimentally in iodine‐doped P3HT and P3HT blended with barium titanate (BaTiO3) nanoparticles. The addition of 2% w/w BaTiO3 nanoparticles increased conductivity and Seebeck across a broad range of doping, resulting in a fourfold increase in power factor. Thus, these results show a promising path forward to reduce the dopant‐charge carrier Coulomb interactions and mitigate their adverse impact on charge transport.

Transport properties, charge conduction mechanism and magnetic behavior of La0.3Ca0.7MnO3:ZnO Nanocomposites

Nanocomposites consist of ZnO and La0.3Ca0.7MnO3 (LCMO) nanoparticles, prepared by cost effective sol–gel method, have been studied for their transport and magnetoresistance (MR) behaviors. Different ZnO nanoparticles contents (weight ratio) were mixed with LCMO nanostructured manganite to understand the possible interactions between ZnO and LCMO nanoparticles. X–ray diffraction (XRD) measurement was performed for all composites and structural quality was verified by performing Rietveld refinements. Variation in an average crystallite size for LCMO and ZnO nanoparticles in different composites has been discussed. Temperature dependent resistivity measurements under different applied magnetic fields suggest the semiconducting nature of all nanocomposites under all applied magnetic fields. Mott type variable range hopping (VRH) mechanism has been employed to understand the dependence of charge carrier localization in LCMO nanostructured manganite lattice on the applied magnetic field and ZnO nanoparticles content within the nanocomposites. Observed MR behavior has been understood under the influence of temperature and ZnO nanoparticles content in the context of spin polarized tunneling (SPT) and magnetic field induced improved conduction across the manganite lattice. Grain and grain boundary contributions to the MR behavior have been explained by using theoretical model fits to obtained MR data at different temperatures for all studied LCMO:ZnO nanocomposites. Magnetic nature of studied LCMO:ZnO nanocomposites has been understood by performing zero field cooled (ZFC) and field cooled (FC) measurement protocols based magnetization and magnetic isotherms measurements that confirm the coexisting magnetic phase in all studied nanocomposites. Magnetic nature has been understood on the basis of weakening of zener double exchange (ZDE) mechanism within the magnetic lattice of LCMO manganite and magnetic contribution from the free charge carriers within the ZnO clusters.

Fine tunability of optical gain characteristics of InGaAs/GaAsSb/InAlAs nano-heterostructure under combined effect of field and temperature

The high optical gain of InGaAs/GaAsSb/InAlAs nano-scale heterostructure has motivated us for its further study under variable temperature and electric field. The presented work is devoted to show the impact of variable temperature and electric field on optical gain characteristics of InGaAs/GaAsSb/InAlAs nano-scale heterostructure which has been assumed to grow on lattice matched GaAs substrate. A six band k.p approach has been utilized to optimize the heterostructure in terms of carrier's wavefunctions, local carriers' density in conduction and valence band and dispersion curves followed by calculating momentum matrix elements and the optical gain spectra. According to the calculation results, the increasing range of temperature (having range of 100–300 K at the interval of 50 K) can control the peak gain significantly (6070 cm−1 to 4470 cm−1) with the red shift in wavelength; while enhanced order of electric field at room temperature can control the gain slightly (4470 cm−1 to ~4150 cm−1) with fine red shift in wavelength. The results achieved in this work will definitely prove the method of controlling the gain characteristics as excellent; and the heterostructure studied will be suitable in design of tunable NIR (near infrared) light sources.

Understanding the Correlation Between Temperature Dependent Performance and Trap Distribution for Nickel Oxide Based Inverted Perovskite Solar Cells

The distribution of trap states in the active layer of perovskite solar cells is a key limiting factor that largely controls the charge carrier mobility, rate of recombination of trapped charges, and, subsequently, the power conversion efficiency. In the present study, the charge carrier dynamics and the influences of the energetic distribution of trap states over device performance of nickel oxide based inverted perovskite solar cells have been investigated in detail by analyzing their temperature dependent transient photoresponses. The reduction in temperature during electrical measurement has induced anomalous behavior in current-voltage characteristics and distribution of trap states. The tail states energy of the valance band decreases from 150 to 50 meV as the temperature of the transient photocurrent measurement increases from 83 to 300 K. The applied bias significantly influences the shape and location of the density of states distribution, indicating their ionic origin. The transient photovoltage measurement revealed that the devices show rapid recombination of localized photogenerated charge carriers around the orthorhombic to the tetragonal phase transition region. The detailed study of trap state analysis and their various impacts on the device performances will prove beneficial for the fundamental understanding of the device physics, material design, and performance stabilization for perovskite solar cells and related devices.

Doping and annealing effects on structural, optical and electrical characteristics of Sn-doped ZnS thin film

This work presents the fabrication of Zn1−xS:Snx thin films using the chemical bath deposition method and investigates the effects of Sn doping and annealing temperature on the structural, optical and electrical properties of ZnS thin films. The XRD patterns show that the relative intensities of the major diffraction peaks increase with Sn dopant incorporation and annealing temperature. Because high-temperature annealing forms several nucleation centers throughout the lattice and dopant incorporation enhances point defects leading to crystallite boundary mobility enhancement, crystallinity is also improved. UV–vis-NIR spectrophotometric studies revealed that the films have good transmittance that is larger than 75% in both visible and near-infrared regions and their optical bandgap ranges from 3.34 eV to 3.90 eV. Both the Sn content and annealing temperature cause the transmittance and the optical bandgap to rise. The absorption edge shifts towards the longer wavelength for higher Sn contents and annealing temperatures. Also, the decreased Urbach energy with increased Sn content or annealing temperature can be attributed to the reduced structural disorders and dislocations of ZnS crystals that are indicated by improved crystallinity. Electrical characterization by the two-point probe method exhibits that at higher annealing temperature grain boundary scattering limits the number of mobile carriers by increasing interatomic binding. On the other hand, due to increased carrier concentration and decreased dislocations resulting from the Sn content or annealing, localized carriers dominate in the bulk crystal state and require higher activation energy to replace an interstitial atom and excite the bulk states.

Investigation of Photoexcitation Energy Impact on Electron Mobility in Single Crystalline CdTe.

The exceptional electronic properties of cadmium telluride (CdTe) allow the material to be used in a wide range of high energy radiation detection applications. Understanding the mechanisms of local carrier scattering is of fundamental importance to understand the charge transport in the material. Here, we investigate the effect of photoexcitation on electron transport properties in chlorine doped single crystalline cadmium telluride (SC-CdTe:Cl). For this purpose time of flight measurements were performed on SC-CdTe:Cl in order to study the electron drift mobility in the low injection regime. Measurements were made at the temperature intervals of 80 to 300 K, for an applied electric field between 270 and 1600 V/cm and for wavelengths of 532, 355 and 213 nm. We have found that the electron drift mobility was affected by the excitation energy for temperatures below 200 K. In addition, the measurements revealed that it is possible to determine impurity and shallow trap concentration by this method. The method proves to be extremely sensitive in measuring very low impurity levels and in identifying dominant scattering mechanisms.

Three-dimensional luminescence microscopy for quantitative plasma characterization in bulk semiconductors

Important challenges remain in the development of ultrafast laser writing inside semiconductor materials because the properties of narrow gap materials cause strong propagation distortions to intense infrared light. Here, we introduce a simple and robust imaging method for high-dynamic-range investigations of the laser–matter interactions in bulk semiconductors. Supported by measurements in gallium arsenide and silicon, we show how z-scan imaging of the band-to-band radiative recombination enables quantitative reconstruction of the three-dimensional distributions of free-carriers generated by nonlinear ionization with ultrashort pulses. The validity is confirmed by comparisons with ultrafast transmission microscopy (shadowgraphy) images. The superior sensitivity of the zero-background luminescence method allows the measurement of local carrier densities as low as ≈1016 cm−3 inside GaAs that is inaccessible by shadowgraphy. It provides the first direct evidence of the low density plasma generated far prior to the focus that causes the previously reported intensity clamping phenomenon. The potential of this non-coherent 3D imaging method to assess complex beam distortion features is also exemplified by real-time pre-compensation of aberration for an intense interacting beam.

The Al Doping Effect on Epitaxial (In,Mn)As Dilute Magnetic Semiconductors Prepared by Ion Implantation and Pulsed Laser Melting.

One of the most attractive characteristics of diluted ferromagnetic semiconductors is the possibility to modulate their electronic and ferromagnetic properties, coupled by itinerant holes through various means. A prominent example is the modification of Curie temperature and magnetic anisotropy by ion implantation and pulsed laser melting in III–V diluted magnetic semiconductors. In this study, to the best of our knowledge, we performed, for the first time, the co-doping of (In,Mn)As diluted magnetic semiconductors by Al by co-implantation subsequently combined with a pulsed laser annealing technique. Additionally, the structural and magnetic properties were systematically investigated by gradually raising the Al implantation fluence. Unexpectedly, under a well-preserved epitaxial structure, all samples presented weaken Curie temperature, magnetization, as well as uniaxial magnetic anisotropies when more aluminum was involved. Such a phenomenon is probably due to enhanced carrier localization introduced by Al or the suppression of substitutional Mn atoms.

Off-axis multilayer zone plate with 16 nm × 28 nm focus for high-resolution X-ray beam induced current imaging.

Using multilayer zone plates (MZPs) as two-dimensional optics, focal spot sizes of less than 10 nm can be achieved, as we show here with a focus of 8.4 nm × 9.6 nm, but the need for order-sorting apertures prohibits practical working distances. To overcome this issue, here an off-axis illumination of a circular MZP is introduced to trade off between working distance and focal spot size. By this, the working distance between order-sorting aperture and sample can be more than doubled. Exploiting a 2D focus of 16 nm × 28 nm, real-space 2D mapping of local electric fields and charge carrier recombination using X-ray beam induced current in a single InP nanowire is demonstrated. Simulations show that a dedicated off-axis MZP can reach sub-10 nm focusing combined with reasonable working distances and low background, which could be used for inoperando imaging of composition, carrier collection and strain in nanostructured devices.

Low field magnetotransport behavior of barium hexaferrite/ferromagnetic manganite bilayer

Adding functionalities to existing ferroelectric/ferromagnetic materials showed promising results with exciting physical mechanisms. Pure and bilayer films of strong ferromagnetic oxides, viz, BaFe12O19 (BaM) and La0.67Sr0.33MnO3 (LSMO), were fabricated by pulsed laser deposition. Polycrystalline samples of dense structure, uniform thickness, and monodispersed grain distributions were used to form capacitor-like stack geometry for dielectric and magneto-dielectric (MD) measurements. High dielectric constants at moderately high frequencies with increased relaxation times were observed for the bilayer film and are attributed to the BaM/LSMO strained interface, while Maxwell–Wagner polarization plays an insignificant role. Modeling of dielectric loss tangents and AC conductivity revealed localized carrier hopping between Fe ions in the bilayer film. Pronounced hysteresis loops with a small coercive field and increased saturation magnetization values of BaM/LSMO bilayers, as compared with BaM/Pt, are demonstrated at 300 K; where the role of mixed valence Mn ions in +3 and +4 states at the bottom LSMO electrode is highlighted. MD measurements with varying magnetic fields showed magnetically tunable, large MD coupling values (∼287%) for BaM/LSMO/Pt. The phenomenally high MD values are discussed based on ionic polarization, colossal magnetoresistance of LSMO, and magnetostriction at the BaM/LSMO interface. Our findings propose significant applications of ferromagnetic oxide bilayers in the emerging field of magneto-dielectric coupling devices.

Lead-Free Halide Light-Emitting Diodes with External Quantum Efficiency Exceeding 7% Using Host–Dopant Strategy

Lead-free halide light-emitting diodes (LEDs) are fabricated using nontoxic and earth-abundant CsCu2I3 with a strong yellow emission at a peak wavelength of 568 nm. CsCu2I3-based host–dopant emitters are formed by vacuum thermal evaporation (VTE) film codeposition process instead of the commonly used solution-based film deposition process. Using the VTE process, extremely thin (30 nm) host–dopant emitters have successfully been formed with the CsCu2I3 dopant and various organic host molecules. A bright yellow emission with a photoluminescence quantum yield value of 84.8% is achieved in the 0.5% CsCu2I3-doped halide emitter film due to the successful spatial localization of charge carriers and excitons using an organic host with appropriate energy levels to CsCu2I3. With the further enhancement in charge balance using the cohost system, a record-breaking lead-free halide LED has been fabricated with an EQE of 7.4%. The lead-free halide LEDs are also highly stable in the device operation with LT70 of 20 h at 100 cd/m2.