Nonuniform Carrier Distribution Research Articles

Multiple-carrier-lifetime model for carrier dynamics in InGaN/GaN LEDs with a non-uniform carrier distribution

We introduce a multiple-carrier-lifetime model (MCLM) for light-emitting diodes (LEDs) with non-uniform carrier distribution, such as in multiple-quantum-well (MQW) structures. By employing the MCLM, we successfully explain the modulation response of V-pit engineered MQW LEDs, which exhibit an S21 roll-off slower than −20 dB/decade. Using the proposed model and employing a gradient descent method, we extract effective recombination and escape lifetimes by averaging the carrier behavior across the quantum wells. Our results reveal slower effective carrier recombination and escape in MQW LEDs compared with LEDs emitting from a single QW, indicating the advantages of lower carrier density achieved through V-pit engineering. Notably, the effective carrier recombination time is more than one order of magnitude lower than the effective escape lifetime, suggesting that most carriers in the quantum wells recombine, while the escape process remains weak. To ensure the reliability and robustness of the MCLM, we subject it to a comprehensive threefold validation process. This work confirms the positive impact of spreading carriers into several quantum wells through V-pit engineering. In addition, the MCLM is applicable to other LEDs with a non-uniform carrier distribution, such as micro-LEDs with significant surface recombination and non-uniform lateral carrier profiles.

Room-temperature high-speed electrical modulation of excitonic distribution in a monolayer semiconductor

Excitons in monolayer semiconductors, benefitting from their large binding energies, hold great potential towards excitonic circuits bridging nano-electronics and photonics. However, achieving room-temperature ultrafast on-chip electrical modulation of excitonic distribution and flow in monolayer semiconductors is nontrivial. Here, utilizing lateral bias, we report high-speed electrical modulation of the excitonic distribution in a monolayer semiconductor junction at room temperature. The alternating charge trapping/detrapping at the two monolayer/electrode interfaces induces a non-uniform carrier distribution, leading to controlled in-plane spatial variations of excitonic populations, and mimicking a bias-driven excitonic flow. This modulation increases with the bias amplitude and eventually saturates, relating to the energetic distribution of trap density of states. The switching time of the modulation is down to 5 ns, enabling high-speed excitonic devices. Our findings reveal the trap-assisted exciton engineering in monolayer semiconductors and offer great opportunities for future two-dimensional excitonic devices and circuits.

Underwater Optical Wireless Communications With InGaN LEDs Grown With an Asymmetric Multiple Quantum Well for Light Emission or Detection

InGaN light-emitting diodes (LEDs) grown with an asymmetric multiple quantum well (MQW) are proposed for use in an optical link with an avalanche photodiode (APD) based receiver. In contrast to the high photoresponse of red AlGaInP LEDs in APDs, the proposed blue LEDs provide improved light output and enhanced system bandwidth for directed line-of-sight optical links passing through a 100-cm-long water tank. This improvement is due to the nonuniform carrier distribution within the InGaN MQWs being mitigated by using a thin GaN barrier near the n-GaN to facilitate hole transport capacity. In addition, bandwidth degradation resulting from APD module saturation can also be avoided by using these blue LEDs, successfully establishing a 300 Mbit/s LED-based underwater data link. The proposed InGaN LEDs (zero bias) under illumination exhibit a peak responsivity of 0.133 at λ = 370 nm, an ultraviolet (UV)-to-visible rejection ratio of 4849 and a 3-dB cut-off frequency of 33.3 MHz. Using violet UV laser diodes and the proposed LEDs respectively as the optical transmitter and receiver, an underwater optical link (L = 100 cm) with a data transmission rate of up to 130 Mbit/s and a bit error rate of 4.2 × 10^−9 is also demonstrated.

Electron Blocking Layer Free Full-Color InGaN/GaN White Light-Emitting Diodes

Indium gallium nitride (InGaN) compound semiconductor alloys have been intensively studied and utilized for white light-emitting diodes (LEDs) [1, 2]. Compared to conventional planar structures, III-nitride nanowires can exhibit significant advantages including greatly reduced dislocation densities and polarization fields, due to the effective lateral stress relaxation. Additionally, the nanowire LEDs with the incorporation of quantum dots/disks in the active region may lead to superior carrier confinement, promising for high efficiency LEDs with tunable emission [2]. However, nanowire LEDs still contain several challenges to further improve the quantum efficiency and light output power, which includes inefficient carrier confinement in the active region, nonuniform carrier distribution, and electron overflow. The integration of an electron blocking layer (EBL) in the LED mitigates electron leakage problem to an extent but it also lowers the hole injection into the active region if not designed properly causing poor hole transport problem. Hence, the LED without EBL, with efficient carrier confinement capability, negligible carrier leakage from active region is the optimal choice [3].In this work, we have demonstrated EBL free high-performance phosphor-free InGaN/GaN nanowire white LEDs. The proposed structure is grown on Si (111) substrate by RF plasma-assisted MBE (Veeco Gen II MBE) under nitrogen-rich condition, then fabricated devices for characterization. The proposed LED structure (LED3) consists of a 200 nm thick n-GaN nanowire template, 10 multiple quantum wells (MQWs) of 3 nm GaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) in the active region and a 100 nm thick p-GaN. In addition, we have incorporated 30 nm n-In0.2Ga0.8N QW between the n-GaN and the active region to control electron overflow. Moreover, to utilize the electrons escaped from the active region, a second 10 nm p-In0.2Ga0.9N QW is introduced between the active region and p-GaN. This second QW reduces electron loss to the p-GaN and contributes blue light emission to relatively control the white light emission from the LED device. The schematic diagram of the proposed structure is shown in Fig. 1(a). A comparison has been made with the conventional LED structure (LED1: 200 nm n-GaN, 10 MQWs (3 nm)/MQBs (3 nm), 100 nm p-GaN) and EBL based structure (LED2: 200 nm n-GaN, 10 MQWs (3 nm)/MQBs (3 nm),10 nm p-Al0.1Ga0.9N EBL, 100 nm p-GaN) to understand the enhanced performance of the proposed LED structure. As a result, we observed that proposed LED has better performance in terms of less electron leakage, high hole injection, improved internal quantum efficiency (IQE), external quantum efficiency (EQE) and output optical power as compared to other conventional and EBL based InGaN/GaN LEDs. The resulting device exhibits high IQE of 58.5% with highly stable emission characteristics. The relative EQE and output optical power of the LEDs are shown in Figs. 1(b) and 1(c). Such unique EBL free nanowire LED structure is a promising approach to achieve high-power phosphor-free LEDs for general illumination and display applications.

Numerical investigation on the device performance of electron blocking layer free AlInN nanowire deep ultraviolet light-emitting diodes

We report on the illustration of the first electron blocking layer (EBL) free AlInN nanowire light-emitting diodes (LEDs) operating in the deep ultraviolet (DUV) wavelength region (sub-250 nm). We have systematically analyzed the results using APSYS software and compared with simulated AlGaN nanowire DUV LEDs. From the simulation results, significant efficiency droop was observed in AlGaN based devices, attributed to the significant electron leakage. However, compared to AlGaN nanowire DUV LEDs at similar emission wavelength, the proposed single quantum well (SQW) AlInN based light-emitters offer higher internal quantum efficiency without droop up to current density of 1500 A/cm2 and high output optical power. Moreover, we find that transverse magnetic polarized emission is ∼ 5 orders stronger than transverse electric polarized emission at 238 nm wavelength. Further research shows that the performance of the AlInN DUV nanowire LEDs decreases with multiple QWs in the active region due to the presence of the non-uniform carrier distribution in the active region. This study provides important insights on the design of new type of high performance AlInN nanowire DUV LEDs, by replacing currently used AlGaN semiconductors.

Full-Color III-Nitride Nanowire Light-Emitting Diodes

III-nitride nanowire-based light-emitting diodes (LEDs) have been intensively studied as promising candidates for future lighting technologies. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-conned Stark effect due to the effective lateral stress relaxation, promising high-efficiency full-color LEDs. Beside these advantages, however, several issues have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, electron overflow, and the nonradiative recombination along the nanowire lateral surfaces. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. Consequently, reported nanowire LEDs show relatively low output power. Recently, III-nitride core-shell nanowire LED structures have been reported as the most efficient nanowire white LEDs with a record-high output power which is more than 500 times stronger than that of nanowire white LEDs without using core-shell structure. In this context, we will review the current status, challenges, and approaches for the high-performance IIInitride nanowire LEDs. More specifically, we will describe the current methods for the fabrication of nanowire structures including top-down and bottom-up approaches, followed by characteristics of III-nitride nanowire LEDs. We will then discuss the carrier dynamics and loss mechanism in nanowire LEDs. The typical designs for the enhanced performance of III-nitride nanowire LEDs will be presented next. The color-tunable nanowire LEDs with emission wavelengths in the visible spectrum and phosphor-free nanowire white LEDs will be finally discussed.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited.

Plasmon-induced efficient hot carrier generation in graphene on gold ultrathin film with periodic array of holes: Ultrafast pump-probe spectroscopy.

Using ultrafast pump-probe reflectivity with a 3.1 eV pump and coherent white light probe (1.1-2.6 eV), we show that graphene on gold nanostructures exhibits a strong coupling to the plasmonic resonances of the ordered lattice hole array, thus injecting a high density of hot carriers in graphene through plasmons. The system being studied is single-layer graphene on an ultrathin film of gold with periodic arrangements of holes showing anomalous transmission. A comparison is made with gold film with and without hole array. By selectively probing transient carrier dynamics in the spectral regions corresponding to plasmonic resonances, we show efficient plasmon induced hot carrier generation in graphene. We also show that due to high electromagnetic field intensities at the edge of the submicron holes, fast decay time (10-100 fs), and short decay length (1 nm) of plasmons, a highly confined density of hot carriers (very close to the edge of the holes) is generated by Landau damping of plasmons within the holey gold film. A contribution to transient decay dynamics due to the diffusion of the initial nonuniform distribution of hot carriers away from the hole edges is observed. Our results are important for future applications of novel hot carrier device concepts where hot carriers with tunable energy can be generated in different graphene regions connected seamlessly.

Possibilities of Applying Geometric Optics for Calculations of Nano- and Microstructures in Photovoltaic Devices

The optical characteristics used in the photovoltaics of nano- and microstructure, evaluated using electromagnetic radiation transfer computation software, mismatch with data obtained at high estimated times. The ability to apply the geometric optics approximations is shown for this purpose. Relevant computations using a 3D ROKS-RG programming tool (the Monte Carlo method) are performed for various experimental results on the optical parameters of nano- and microstructures available in the scientific literature. Given that surface roughness exists as a byproduct of etching during the preparation of structures, the theory is found to satisfactorily agree with experiment. Moreover, most of the radiation is absorbed on account of the surface roughness of substrates, which is expected to induce a nonuniform charge carrier distribution over the structure height and may be among the reasons for the insufficiently high efficiency of solar elements with radial p–n geometry.

Short cavity InGaN-based laser diodes with cavity length below 300 μm

We report on a practical method for developing InGaN-based edge emitting laser diodes of cavity length down to 45 μm. Samples consisting of one uncoated cleaved facet and one etched facet coated with a high-reflectivity (HR) dielectric distributed Bragg reflector (DBR) exhibit lasing in the continuous wave (cw) regime for cavity lengths down to 250 μm and lasing under pulsed injection for lengths as short as 100 μm. For samples having a second HR dielectric DBR, we could demonstrate cw lasing for a cavity length as short as 45 μm with a threshold current below 10 mA being reported for a 75 μm long device. Through a systematic study of the threshold current (Ith) and the slope efficiency (ηs) as a function of cavity length, it is proposed that the parameters underpinning the evolution of Ith and ηs with decreasing cavity length and their overall degradation in the short cavity regime are free carrier absorption, Auger processes and the decrease in the recombination losses due to nonuniform carrier distribution across the multiple quantum well active region.

Interwell carrier transport in InGaN/(In)GaN multiple quantum wells

Uniform carrier distribution between quantum wells (QWs) of multiple QW light emitting diodes (LEDs) and laser diodes is important for the efficiency of device operation. In lasers, the uniform distribution ensures that all the QWs contribute to lasing; in LEDs, it enables high power operation with minimal Auger losses and a maximal efficiency. The carrier distribution between the QWs takes place via interwell (IW) transport. In polar GaN-based structures, the transport might be hindered by the strong carrier confinement and the internal electric fields. In this work, we study the IW transport in InGaN/(In)GaN multiple QW structures typical for ultraviolet-emitting devices with different well and barrier parameters. Experiments have been performed by means of time-resolved photoluminescence. We find that the IW transport rate is limited by the hole thermionic emission, which for InGaN/GaN QWs produces long transport times, ∼1 ns per well, and a nonuniform IW carrier distribution. However, adding 5% In to the barriers completely changes the situation with the transport time decreasing by a factor of four and the hole thermionic emission energy from 200 meV to 70 meV. This study shows that using InGaN barriers is a promising pathway toward efficient high power InGaN LEDs.

:Hydrogen Doping Oxide Transistors: Analysis of Ultrahigh Apparent Mobility in Oxide Field‐Effect Transistors (Adv. Sci. 7/2019)

In article number 1801189, Chuan Liu and co-workers present hydrogen doped IGZO thin-film transistors induced by simple SINX encapsulation which exhibits substantially enhanced current and stability. Combing photoelectron spectroscopy, potential mapping and simulation, the nonuniform carrier distribution is revealed as the cause for the ultra-high current and apparent device mobility, which is 40 times higher than the carrier mobility.

Theoretical modeling of the gas humidification effect on the characteristics of proton ceramic fuel cells

Proton ceramic fuel cells are promising electrochemical converters that produce electrical energy by oxidizing hydrogen. Understanding the impact of gas humidification on the characteristics of such fuel cells is pivotal for the development of high-performance devices. This study develops a theory elucidating the effect of hydrogen and air humidity on the output characteristics of fuel cells based on proton-conducting oxide membrane with mixed (proton and hole) conductivity. The theory is based on a self-consistent description of the strong non-uniform distribution of charge carriers inside the membrane and the variation of the gas phase composition along the fuel and air channels. It is shown that the total current density significantly and nonmonotonously depends on humidity of the inlet hydrogen fuel, while humidification of the inlet air has minor effect on the fuel cell characteristics. The influence of the anode and cathode gas humidity on the distribution of the various fuel cell parameters (current densities, resistivity, etc.) along the membrane is also determined. The obtained results reveal the possibility of enhancing the performance of the proton ceramic fuel cells with low polarization losses by optimizing the composition of the inlet hydrogen fuel.

Возможности применения геометрической оптики для расчетов нано- и микроструктур в фотовольтаике

Numerical simulation of nano and microstructured photovoltaic materials be means radiation transport software may become quite resource-intensive due to poor convergence of the applied optical -properties estimation method. In this paper, applicability of the geometrical optics approximation to microstructure-property modeling was illustrated by mutual comparison of the results of Monte-Carlo simulation in 3D POKS-RG program with a number of available experimental data about optical properties of nano- and microstructures.Numerical simulation accounting peculiar roughness of the surfase as a side effect of etching in microfabrication were in acceptable agreement with the experimental results. The furher analysis revealed essential influence of substrate surfase roughness onto the absorption of the incident light. The aforementioned peculiarity results in nonuniform structure-height distribution of change carriers and might partially cause the modest efficiency of radial p-n junction solar gells.

Design of GaN-Based Multicolor Tunnel-Junction Light-Emitting Diodes

The design of monolithic multicolor tunnel-junction (TJ) light-emitting diodes (LEDs) are investigated numerically. This paper primarily aims to design and study monolithic TJ LED devices that possess high luminous efficiency and wide color gamut using a simple fabrication process. First, the characteristics of green and blue single LEDs are explored. Nonuniform carrier distribution inside quantum wells (QWs), which is due to the deep QWs and severe polarization effect, is observed for both single LEDs under study. Based on structural analysis of the single LEDs, streamline dual-color and broadband TJ LED structures, in which the blue unit LED has two pairs of 3-nm-thick QWs and the green unit LED has 3-nm-thick single QW, are designed and investigated. Specifically, monolithic dual-color blue/green (459/530 nm) TJ LED is proposed and analyzed using effective n+-GaN/i-In0.2 Ga0.8N/p+-GaN TJ. Furthermore, broadband TJ LED with simulated full width at half maximum of around 100 nm is achieved.

Electrical current at micro-/macro-scale of undoped and nitrogen-doped MWPECVD diamond films

Chemical, structural, morphological and micro-/macro-electrical properties of undoped and nitrogen-(N-)doped diamond films are determined by X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, field emission scanning electron microscopy, atomic force microscopy, scanning capacitance microscopy (SCM) and two points technique for I–V characteristics, respectively. The characterization results are very useful to examine and understand the relationship among these properties.The effect of the nitrogen incorporation in diamond films is investigated through the evolution of the chemical, structural, morphological and topographical features and of the electrical behavior. The distribution of the electrical current is first assessed at millimeter scale on the surface of diamond films and then at micrometer scale on small regions in order to establish the sites where the carriers preferentially move. Specifically, the SCM images indicate a non-uniform distribution of carriers on the morphological structures mainly located along the grain boundaries.A good agreement is found by comparing the electrical currents at the micro- and macro-scale. This work aims to highlight phenomena such as photo- and thermionic emission from N-doped diamond useful for microelectronic engineering.

Electron and proton transfer in chloroplasts in silico. 2: The effect of diffusion limitations on the process of photosynthesis in spatially inhomogeneous thylakoids

The lateral mobility of protons and mobile electron carriers (plastoquinone and plastocyanin) is subjected to diffusion limitations; the effect of these limitations on the kinetics of photoinduced pH i changes has been investigated in the present work for metabolic states 3 (conditions of intensive ATP synthesis) and 4 (the state of photosynthetic control). Computer simulations were based on a mathematical model of electron and proton transport in chloroplasts developed earlier by the authors. Non-uniform distribution of electron carriers and ATP synthase complexes in the membranes of grana and intergranal thylakoids was taken into account in the model. The kinetics of intrathylakoid pH i changes and the lateral profiles of distribution of the mobile electron transporters in granal and intergranal thylakoids were studied. The formation of non-uniform pH i profiles (with lumen acidification in the central parts of the grana being substantially slower than in the stromal thylakoids) was shown to occur under the conditions of ATP synthesis. Variation of the diffusion coefficients of intrathylakoid hydrogen ions and mobile electron carriers (plastoquinone and plastocyanin) can have substantial effects on the lateral pH i profiles and the redox state of the mobile electron carriers.

A recovery principle provides insight into auxin pattern control in the Arabidopsis root

Regulated auxin patterning provides a key mechanism for controlling root growth and development. We have developed a data-driven mechanistic model using realistic root geometry and formulated a principle to theoretically investigate quantitative auxin pattern recovery following auxin transport perturbation. This principle reveals that auxin patterning is potentially controlled by multiple combinations of interlinked levels and localisation of influx and efflux carriers. We demonstrate that (1) when efflux carriers maintain polarity but change levels, maintaining the same auxin pattern requires non-uniform and polar distribution of influx carriers; (2) the emergence of the same auxin pattern, from different levels of influx carriers with the same nonpolar localisation, requires simultaneous modulation of efflux carrier level and polarity; and (3) multiple patterns of influx and efflux carriers for maintaining an auxin pattern do not have spatially proportional correlation. This reveals that auxin pattern formation requires coordination between influx and efflux carriers. We further show that the model makes various predictions that can be experimentally validated.

Theoretical Investigation of Efficient Green Tunnel-Junction Light-Emitting Diodes

Green tunnel-junction (TJ) light-emitting diodes (LEDs) are investigated theoretically in this letter. The influence of Auger recombination on the optical performance of green single LED and TJ LED is explored. Simulation results show that non-uniform carrier distribution and Auger recombination are critical issues influencing the performance of conventional green LED. The TJ LED can be operated at relatively low current density that possesses suppressed Auger recombination and high quantum efficiency when compared with its single LED counterpart at the same level of output power. Simulated wall-plug efficiency of the TJ LEDs with two and three unit LEDs is increased from 5.67% to 10.51% and 12.54%, respectively, in comparison with the single LED when the output power is 21 mW.

Self-Consistent Analysis of GaInNAsSb/GaSb Quantum Well Lasers Emitting at 2.3–3.3-μm -Long Wavelength

Double-quantum-well GaInNAsSb laser structures grown on GaSb substrate emitting between 2.3 and 3.3 μm are proposed, and modeled self-consistently using a commercial software that combines gain calculation with 2-D simulations of carrier transport and waveguiding. The model is calibrated using experimental results obtained from conventional GaInAsSb lasers emitting at 2.3 μm. The simulated results show that the incorporation of a small concentration of nitrogen in the conventional InGaAsSb can extend the wavelength to 3.3 μm with a reasonable threshold current density increase. This is obtained as a result of preserving good confinement for holes as compared to conventional InGaAsSb 2.3-μm laser. The threshold current density increase with wavelength is attributed mainly to nonuniform distribution of carriers due to the high-conduction band offset. This drawback of the structure can be overcome by incorporating AlGaInAsSb material as a barrier and waveguide instead of AlGaAsSb. The simulation shows that the bandgap of the AlGaInAsSb quinternary material should be reduced linearly when increasing the wavelength to maintain a good laser performance in terms of emitted optical power.

Electronic properties of the n-type PDI8-CN2 organic semiconductor at the interface with SiO2: addressing the role of adsorbed water molecules by means of optical second-harmonic generation

We investigate the interfacial electronic properties of N,N’-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bisdicarboximide (PDI8-CN2) organic semiconductor films grown on silicon dioxide (SiO2) by polarization-resolved second harmonic generation optical spectroscopy. The analysis shows a non-uniform distribution of charge carriers in PDI8-CN2, whose spatial profile is affected by hydrophobic passivation of SiO2 surfaces by hexamethyldisilazane. An interpretation model strengthened by photoluminescence analysis is developed, based on the presence of the net charge localized at the SiO2 surface and on consequent charge redistribution in the organic semiconductor. Considerations are expounded suggesting a common and ‘universal’ mechanism for the bias stress effect in p-channel and n-channel organic field-effect transistors, related to proton migration toward SiO2 gate dielectrics.