Injected Carrier Density Research Articles

Mid-Infrared Emission in Ge/Ge1-xSnx/Ge Quantum Well Modeled Within 14-Band k.p Model

Band structure and gain in a Ge/Ge1-xSnx/Ge quantum well are described theoretically using a 14-band k.p model. It has been shown that the quantum well width and the α-Sn concentration considerably modify the conduction and valence subband structure, and, as a result, the optical gain changes with the insertion of a very small concentration of α-Sn. In particular, we have determined the necessary injection carrier density Nj and the critical α-Sn concentration for elevated high gain lasing. It is found that for Nj = 1.5 × 1018 cm−3, we achieved a maximum peak gain for α-Sn concentration of the order 0.155. We can predict that Ge/Ge1-xSnx/Ge QWs should be manufactured with an α-Sn concentration less than 0.155 in devices for optoelectronics applications such as telecommunication and light emitting laser diodes.

High-Performance All-Inorganic Architecture Perovskite Light-Emitting Diodes Based on Tens-of-Nanometers-Sized CsPbBr3 Emitters in a Carrier-Confined Heterostructure.

Developing green perovskite light-emitting diodes (PeLEDs) with a high external quantum efficiency (EQE) and low efficiency roll-off at high brightness remains a critical challenge. Nanostructured emitter-based devices have shown high efficiency but restricted ascending luminance at high current densities, while devices based on large-sized crystals exhibit low efficiency roll-off but face great challenges to high efficiency. Herein, we develop an all-inorganic device architecture combined with utilizing tens-of-nanometers-sized CsPbBr3 (TNS-CsPbBr3) emitters in a carrier-confined heterostructure to realize green PeLEDs that exhibit high EQEs and low efficiency roll-off. A typical type-I heterojunction containing TNS-CsPbBr3 crystals and wide-bandgap Cs4PbBr6 within a grain is formed by carefully controlling the precursor ratio. These heterostructured TNS-CsPbBr3 emitters simultaneously enhance carrier confinement and retain low Auger recombination under a large injected carrier density. Benefiting from a simple device architecture consisting of an emissive layer and an oxide electron-transporting layer, the PeLEDs exhibit a sub-bandgap turn-on voltage of 2.0 V and steeply rising luminance. In consequence, we achieved green PeLEDs demonstrating a peak EQE of 17.0% at the brightness of 36,000 cd m-2, and the EQE remained at 15.7% and 12.6% at the brightness of 100,000 and 200,000 cd m-2, respectively. In addition, our results underscore the role of interface degradation during device operation as a factor in device failure.

The role of nonequilibrium LO phonons, Pauli exclusion, and intervalley pathways on the relaxation of hot carriers in InGaAs/InGaAsP multi-quantum-wells

Under continuous-wave laser excitation in a lattice-matched In0.53Ga0.47As/In0.8Ga0.2As0.44P0.56 multi-quantum-well (MQW) structure, the carrier temperature extracted from photoluminescence rises faster for 405 nm compared with 980 nm excitation, as the injected carrier density increases. Ensemble Monte Carlo simulation of the carrier dynamics in the MQW system shows that this carrier temperature rise is dominated by nonequilibrium LO phonon effects, with the Pauli exclusion having a significant effect at high carrier densities. Further, we find a significant fraction of carriers reside in the satellite L-valleys for 405 nm excitation due to strong intervalley transfer, leading to a cooler steady-state electron temperature in the central valley compared with the case when intervalley transfer is excluded from the model. Good agreement between experiment and simulation has been shown, and detailed analysis has been presented. This study expands our knowledge of the dynamics of the hot carrier population in semiconductors, which can be applied to further limit energy loss in solar cells.

Revealing Ultrafast Carrier Dynamics of Hybrid Perovskites at Various Stages of Nucleation and Growth Kinetics

AbstractWith hybrid organic–inorganic perovskites expanding their technological reach, from photovoltaic solar cells that operate at low carrier densities (1011 − 1013 cm−3) to light‐emitting diodes and lasers that operate at higher carrier densities (1013 − 1019 cm−3), it is critical to know how microstructure dictates carrier dynamics at varying injected carrier densities. For fabrication at scale, it is equally critical to know how quickly during the growth process do hybrid perovskites develop their characteristic optoelectronic properties. This work reports on a facile fabrication method that “freezes” different stages of nucleation and growth kinetics of prototypical hybrid perovskite on the same substrate—providing a unique strategy to assess differences in optoelectronic properties and carrier dynamics from nascent nucleating microcrystals to large‐grain thin films. The solution‐processed fabrication technique, optimized to control the nucleation density of an intermediate phase, successfully decouples the nucleation and growth steps that lead to large‐grain thin films. Ultrafast broadband absorption microscopy finds that the nucleating microcrystals already possess the optoelectronic properties of hybrid perovskites and share similar femtosecond‐to‐nanosecond dynamics as large‐grain thin films. When compared to the large‐grain thin films, the confining microcrystals exhibit enhanced carrier interactions, marked by an increase in bimolecular and Auger recombination rates.

Competition mechanism of exciton decay channels in the stacked multilayer tungsten sulfide.

The competition mechanism of exciton decay channels in the multilayer TMDs remains poorly understood. Here, the exciton dynamics in the stacked WS2 was studied. The exciton decay processes are divided into the fast and slow decay processes, which are dominated by the exciton-exciton annihilation (EEA) and defect-assisted recombination (DAR), respectively. The lifetime of EEA is on the order of hundreds of femtoseconds (400∼1100 fs). It is decreased initially, followed by an increase with adding layer thickness, which can be attributed to the competition between phonon-assisted effect and defect effect. The lifetime of DAR is on the timescale of hundreds of picoseconds (200∼800 ps), which is determined by the defect density especially in a high injected carrier density.

Comparative study of optical characteristics of SQW and DQWs InGaAs/GaAsSb/AlAsSb based compressively strained heterostructures

InP-based InGaAs/GaAsSb/AlAsSb type-II single quantum well (SQW) and double quantum wells (DQWs) heterostructures are compared on the basis of matrix elements, energy dispersion curves, optical gain, and transition energy. The effect on optical gain due to applied external strain and the influence of In-mole fraction and Sb-mole fraction on momentum matrix element and transition wavelength have also been compared between SQW and DQWs heterostructures. The simulation results show that the transition wavelength is located in the short wavelength infrared region (SWIR). SWIR imaging has various applications such as solar cell inspection, surveillance, electronic board inspection, identification and sorting, and more. For the proposed design, a total optical gain of 7450 cm−1 and 13,681 cm−1 is obtained at 0.75 eV and 0.77 eV transition energies for the SQW and DQWs heterostructures. For 1.5 nm QW thickness and 3x1012 cm−2 injected carrier density at 300 K, the transition wavelength increases from 1.67 μm to 1.82 μm and 1.60 μm–1.77 μm for SQW and DQWs heterostructure, respectively on increasing the mole fraction value of In from 0.5 to 0.8 and when the mole fraction value of Sb increases from 0.5 to 0.8, the transition wavelength increases from 1.69 μm to 1.87 μm and 1.62 μm–1.79 μm for SQW and DQWs heterostructure, respectively.

Excitonic optical properties and lasing mode shifts in square CsPbBr3 nanoplate cavities

Cesium lead bromide (CsPbBr3) perovskite micro/nanostructures have potential applications in miniaturized integrated light sources due to their confined modal volume and high luminous efficiency. In this work, we synthesized CsPbBr3 nanoplates with regular square shapes and smooth surface by the chemical vapor deposition method. The temperature-dependent and excitation power-dependent photoluminescence spectra reveal the excitonic photoluminescence properties. The whispering gallery mode (WGM)-like lasing in CsPbBr3 nanoplates was observed at room temperature. We systematically investigated the pump fluence-dependent multi-mode and single-mode lasing actions in square microcavities. The blueshift of individual lasing modes can be attributed to the variation of refractive index resulting from the formation of the electron-hole plasma (EHP) state under highly injected carrier density. The results reveal the dependence of emission properties on excitation intensity, providing further understanding of the carrier-induced mode shift mechanism in the WGM-like lead halide perovskite microcavities.

The low temperature limit of the excitonic Mott density in GaN: an experimental reassessment

The research on GaN lasers aims for a continuous reduction of the lasing threshold. An approach to achieve it consists in exploiting stimulated polariton scattering. This mechanism, and the associated polariton lasers, requires an in-depth knowledge of the GaN excitonic properties, as polaritons result from the coupling of excitons with photons. Under high excitation intensities, exciton states no longer exist due to the Coulomb screening by free carriers; this phenomenon occurs at the so-called Mott density. The aim of this work is to study the bleaching of excitons under a quasi-continuous optical excitation in a bulk GaN sample of high quality through power dependent micro-photoluminescence and time-resolved experiments at 5 K. Time-resolved photoluminescence allows to measure the carrier lifetime as a function of excitation intensity, which is required for a reliable evaluation of the injected carrier density. The vanishing of excitonic lines together with the red-shift of the main emission evidences the occurrence of the Mott transition for a carrier concentration of (6 ± 3) × 1016 cm−3. This value is more than an order of magnitude smaller than previous determinations published in the literature and is in accordance with many-body calculations.

Superfluorescence of Sub-Band States in C-Plane In0.1Ga0.9N/GaN Multiple-QWs.

Superfluorescence is a collective emission from quantum coherent emitters due to quantum fluctuations. This is characterized by the existence of the delay time ( for the emitters coupling and phase-synchronizing to each other spontaneously. Here we report the observation of superfluorescence in c-plane In0.1Ga0.9N/GaN multiple-quantum wells by time-integrated and time-resolved photoluminescence spectroscopy under higher excitation fluences of the 267 nm laser and at room temperature, showing a characteristic from 79 ps to 62 ps and the ultrafast radiative decay (7.5 ps) after a burst of photons. Time-resolved traces present a small quantum oscillation from coupled In0.1Ga0.9N/GaN multiple-quantum wells. The superfluorescence is attributed to the radiative recombination of coherent emitters distributing on strongly localized subband states, Ee1→Ehh1 or Ee1→Elh1 in 3nm width multiple-quantum wells. Our work paves the way for deepening the understanding of the emission mechanism in the In0.1Ga0.9N/GaN quantum well at a higher injected carrier density.

Progress in Germanium Tin (GeSn) Photonic Crystal Lasers

We report on lasing in two types of Germanium-Tin (GeSn) photonic crystal lasers, with band-edge and H4 hexagonal cavities. A GeSn 16% step-graded structure was used as the optical gain material. The strong out-of-plane emission observed in the band-edge cavity was attributed to the thick optical gain layer, in addition with the presence of leaky band-edge modes above the light cones and a high photonic density of states. The maximum lasing temperature in both types of photonic crystal lasers, 180 K, was lower than in a micro-disk laser fabricated from the same stack, i.e., 230 K. Meanwhile, the lasing thresholds at low temperature (15 K) were very similar (120 kW/cm² for a band-edge cavity and 140 kW/cm² for a H4 cavity, to be compared with 134 kW/cm² for a micro-disk cavity). Since the injected carrier density is governed by the pump power, we suggest the strong variation of optical gain at low temperature, for a small variation of injected carrier density, as a possible cause of such a phenomenon.

Effect of thickness on the electronic structure and optical properties of quasi two-dimensional perovskite CsPbBr3 nanoplatelets

The effect of thickness of cesium lead halide perovskite CsPbBr3 nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an effective-mass envelope function theory based on the 8-band k·p model with the consideration of exciton binding energy. We first reported the CsPbBr3 nanoplatelets’ band structure and optical gain with exciton effect. As the thickness of NPLs decreases, their bandgap increases and the band mixing is more obvious which influences the transition matrix element (TME). The optical gain of CsPbBr3 nanoplatelets is calculated by taking into account of TME, Fermi factor, injected carrier density and thickness. A blue shift of peak position in optical gain can be observed as the thickness of NPLs decreases. As the injected carrier density arises, the maximum optical gain of thicker NPLs reach saturation much faster than the thinner ones. To obtain high differential gain, NPLs with less thickness are better choices. Experimental work is carried out and the results agree with our theoretical results very well.

Slow carrier relaxation in tin-based perovskite nanocrystals

The conversion efficiency of solar energy in semiconductors is fundamentally limited by ultrafast hot-carrier relaxation processes, and slowing down these processes is critical for improved energy harvesting. Here we report formamidinium tin iodide (FASnI3) nanocrystals where quantum confinement effects yield an evolution from a continuous band structure to separate energy states with decreasing nanocrystal size, as observed by transient absorption spectroscopy. The appearance of separate energy levels slows down the relaxation of hot carriers by two orders of magnitude at low injected carrier densities (<1 carrier pair per nanoparticle). The observed build up time of the ground-state bleach at the band edge is two orders of magnitude slower in FASnI3 nanocrystals than in lead halide perovskite bulk and nanocrystals, which we attribute to a phonon bottleneck effect. Our results highlight the promise of lead-free perovskite nanocrystals for high-efficiency photovoltaic applications operating above the Shockley–Queisser limit. Tin-based perovskite nanocrystals with slower than usual relaxation dynamics holds promise for superior lead-free perovskite optoelectronic devices.

Efficient interlayer electron transfer in a MoTe2/WS2/MoS2 trilayer heterostructure

Electron transfer and carrier dynamics in MoTe2/WS2/MoS2 trilayer heterostructures are investigated by transient absorption and photoluminescence measurements. Monolayer flakes of MoTe2, WS2, and MoS2 are obtained by mechanical exfoliation from their bulk crystals and are used to fabricate the heterostructures by a dry-transfer technique. Photoluminescence spectroscopic measurements indicate that the recombination of the MoS2 and WS2 intralayer excitons is significantly suppressed in the heterostructure, illustrating the efficient interlayer charge transfer processes. Layer-selective time-resolved differential reflectance measurements show that the electrons excited in MoTe2 can transfer to MoS2 within 0.3 ps. The transferred electrons show a long lifetime of several hundred picoseconds due to their slow recombination with the spatially separated holes that reside in MoTe2. Furthermore, the charge transfer and recombination processes are weakly dependent on the injected carrier density. These results demonstrate the feasibility of constructing van der Waals multilayer heterostructures involving the infrared-sensitive MoTe2 with emergent properties and provide important information to quantify the performance of MoTe2-based devices.

Advanced method for electrical characterization of carrier-selective passivating contacts using transfer-length-method measurements under variable illumination

Carrier-selective passivating contacts have been demonstrated to be crucial to reach the practical efficiency limit of single junction, crystalline silicon (c-Si) based solar cells. Yet, the electrical transport losses affecting the collection of photogenerated carriers remain to be addressed. To this aim, different methodologies and characterization techniques are currently used. In this contribution, we propose the concept of shell as a new terminology to describe carrier-selective passivating contacts. Then, we present a novel characterization methodology using transfer length method (TLM) measurement under variable illumination to investigate the charge-carrier transport in amorphous/crystalline silicon heterojunction (SHJ) n-type contact stacks. We use technology computer-aided design simulation to model a TLM structure and to identify the physical phenomena and the key parameters affecting the contact resistivity (ρc) and the charge carrier accumulation of such contact stacks. Then, the simulation results are compared with experimental data by performing variable-illumination TLM measurements of actual SHJ n-type contact stacks. Specifically, we demonstrate that illumination has a strong impact on the measured ρc value, highlighting the importance of measuring ρc under maximum power point conditions for a relevant characterization of solar cell transport losses. In addition, we investigate the dependence of ρc to a change in the injected carrier density within the c-Si bulk to compare the illumination responses of different SHJ n-type contact stacks. In the quest for maximal efficiency, this method may insightfully complete other characterization techniques to further understand and study the electrical transport in solar cells.

Time-Resolved Observation of Hole Tunneling in van der Waals Multilayer Heterostructures.

We reported a time-resolved study of quantum-mechanical tunneling of holes between two MoSe2 monolayers that are separated by a monolayer WS2 energy barrier. Four-layer heterostructures of MoSe2/WS2/MoSe2/graphene, as well as control samples, were fabricated by mechanical exfoliation and dry transfer techniques. To time-resolve the hole tunneling process, an ultrashort laser pulse was used to excite electrons and holes in both MoSe2 layers. By utilization of the graphene layer to eliminate carriers in the third MoSe2 layer, the first MoSe2 layer is selectively populated with the holes, which then tunnel to the third MoSe2 layer. By monitoring decay of the hole population with an ultrashort probe pulse, we measure a hole tunneling time of about 20 ps, which is found to slightly increase with the injected carrier density. Besides the fundamental interests of real-time observation of the quantum-mechanical tunneling effect across a nanometer barrier, these results provide quantitative understanding on tunneling mechanisms of charge transfer in van der Waals heterostructures, which is useful for designing sophisticated van der Waals multilayer heterostructures.

Limitation of bulk GeSn alloy in the application of a high-performance laser due to the high threshold

In this work, the gain characteristics of the GeSn alloy are systematically investigated with effective mass approximation theory, and the potential of bulk GeSn in the application of a high-performance laser is discussed. The gain could not be enhanced persistently as the mole fraction of Sn continuously increases and becomes negative as the Sn fraction beyond 18%. An Sn fraction dependent doping scheme is proposed to effectively reduce the threshold injected carrier density to the lowest of 1.10×1018 cm-3. The optimum doping type varies from n-type to p-type as the Sn fraction increases with the conversion fraction of 10%. With doping optimization, the lowest threshold current density of bulk GeSn based laser is predicted to be 1.225 kA/cm2 for a designed n-Si0.157Ge0.643Sn0.200/p-Ge0.84Sn0.16/p-Si0.157Ge0.643Sn0.200 double heterostructure laser, indicating the inadequacy of bulk GeSn alloy of being the gain material for a high-performance laser.

Numerical Investigation into Optoelectronic Performance of InGaN Blue Laser in Polar, Non-Polar and Semipolar Crystal Orientation

Recently, InGaN grown on semipolar and non-polar orientation has caused special attraction due to reduction in the built-in polarization field and increased confinement of high energy states compared to traditional polar c-plane orientation. However, any widespread-accepted report on output power and frequency response of the InGaN blue laser in non-c-plane orientation is readily unavailable. This work strives to address an exhaustive numerical investigation into the optoelectronic performance and frequency response of In0.17Ga0.83N/GaN quantum well laser in polar (0001), non-polar (101¯0) and semipolar (101¯2), (112¯2) and (101¯1) orientations by working out a 6 × 6 k.p Hamiltonian at the Γ-point using the tensor rotation technique. It is noticed that there is a considerable dependency of the piezoelectric field, energy band gap, peak optical gain, differential gain and output power on the modification in crystal orientation. Topmost optical gain of 4367 cm−1 is evaluated in the semipolar (112¯2)-oriented laser system at an emission wavelength of 448 nm when the injection carrier density is 3.7 × 1018 cm−3. Highest lasing power and lowest threshold current are reported to be 4.08 mW and 1.45 mA in semipolar (112¯2) crystal orientation. A state-space model is formed in order to achieve the frequency response which indicates the highest magnitude (dB) response in semipolar (112¯2) crystal orientation.

Femto-Second Carrier and Photon Dynamics in Site Controlled Hexagonal InGaN/GaN Isolated Quantum Dots: Natural Radial Potential Well and Its Dynamic Modulation

Quantum dot (QD) is slated to play a significant role in quantum technologies through its usage as a single-photon source. It also has promising applications as efficient light emitters through strain-relaxed structures. This work demonstrates the fabrication of high-quality site controlled InGaN/GaN QDs through a large contribution of chemical etching in an otherwise reactive ion etching process. Uniform sized QDs with a hexagonal base and radii of 15 and 6 nm are fabricated and characterized by photoluminescence and femtosecond transient absorption spectroscopy. The natural exposition of the inherent hexagonal base is a signature of the reduced defects in these dots. The QDs show the intuitive additional quantum confinement due to size reduction, and the photoluminescence peaks manifest a blue shift. The absorption spectra indicate that the QDs are heavily strain-relaxed at smaller radii, and their characteristics as a function of size and pump power are investigated. The degree of strain relaxation and change in energy-band diagram as a function of position along the radius are also determined. The changes are prominent near the periphery, which creates a natural potential well for both electrons and holes, restricting the carriers from reaching the sidewalls. Femtosecond carrier dynamics indicate a lower capture rate for the smaller size of the dots, indicating excessive electrical or optical pumping will not necessarily lead to increased luminescence. The radiative decay of carriers is faster for strain-relaxed QDs due to higher oscillator strength originating from increased electron–hole wave function overlap. Higher pump power is found to decrease the radiative rate due to the increased effective size of the QDs and thereby reducing carrier injection density. This is in stark contrast to the frequently observed increased decay rate due to Auger recombination. Dynamic modification of the size of the QDs by pump power may find potential use in optoelectronic applications.

Subquantum-Well Influence on Carrier Dynamics in High Efficiency DUV Dislocation-Free AlGaN/AlGaN-Based Multiple Quantum Wells

We explore the effect of the subwell centers and related carrier dynamics mechanisms in dislocation-free DUV AlGaN/AlGaN multiple quantum wells (MQWs) homoepitaxially grown on an AlN substrate. Cross-sectional imaging and energy-dispersive X-ray compositional analyses using scanning transmission electron microscopy (STEM) reveal epitaxial layers of very high crystalline quality, as well as ultrathin Al-rich subquantum barrier and subwell layers at the interface between the wells and the barriers. Carrier dynamic analyses studied by power- and temperature-dependent time-resolved and time-integrated photoluminescence (PL) and PL excitation measurements, as well as numerical simulations, reveal the carrier repopulation mechanisms between the MQWs and subwell sites. This advanced analysis shows that the subwell/sub-barrier structure results in additional exciton localization centers, enhancing the internal quantum efficiency via staggered carrier repopulation into the MQWs to reach a maximum of ∼83% internal quantum efficiency, which remains high at high injected carrier densities in the droop region. Both experimental and numerical simulation results show that the slight efficiency droop can be due to Auger recombination, counteracted by a simultaneous increase in radiative recombination processes at high power density, demonstrating the role of the subwells/sub-barriers in efficiency enhancement.

Drone-Based Daylight Electroluminescence Imaging of PV Modules

Electroluminescence (EL) imaging is a photovoltaic (PV) module characterization technique, which provides high accuracy in detecting defects and faults, such as cracks, broken cells interconnections, shunts, among many others; furthermore, the EL technique is used extensively due to a high level of detail and direct relationship to injected carrier density. However, this technique is commonly practiced only indoors—or outdoors from dusk to dawn—because the crystalline silicon luminescence signal is several orders of magnitude lower than sunlight. This limits the potential of such a powerful technique to be used in utility scale inspections, and therefore, the interest in the development of electrical biasing tools to make outdoor EL imaging truly fast and efficient. With the focus of quickly acquiring EL images in daylight, we present in this article a drone-based system capable of acquiring EL images at a frame rate of 120 frames per second. In a single second during high irradiance conditions, this system can capture enough EL and background image pairs to create an EL PV module image that has sufficient diagnostic information to identify faults associated with power loss. The final EL images shown in this work reached representative quality SNRAVG of 4.6, obtained with algorithms developed in previous works. These drone-based EL images were acquired with global horizontal solar irradiance close to one sun in the plane of the array.