Quantum Confined Stark Effect Research Articles

Tuning Stark effect by defect engineering on black titanium dioxide mesoporous spheres for enhanced hydrogen evolution

Defects can strongly affect the lattice, strain, and electronic structures of nanomaterials photocatalysts, like a double-edged sword of both positive significance and negative influence on photocatalytic performances. To date, most studies into defects only partially elucidated their beneficial or detrimental roles in photocatalysis. However, a quantitative understanding of the photocatalytic performances modulated by defect concentration still needs to be discovered. Here, a series of TiO2−X mesoporous spheres (MS) with different oxygen vacancy concentrations for photocatalytic applications were prepared by high-temperature chemical reduction. The link between oxygen vacancy concentration and photocatalytic performance was successfully established. The localization of carriers dominated by the Stark effect is first enhanced and then weakened with increasing oxygen vacancy concentration, which is a crucial factor in explaining the double-edged sword role of defect concentration in photocatalysis. As the reduction temperature rises to 300 °C, carrier localization dominated by the quantum-confined Stark effect maximizes the separation ability of photo generated electron hole pairs, thus exhibiting the best catalytic performance for photocatalytic hydrogen production and the degradation of organic pollutants, as demonstrated by a hydrogen evolution rate of 523.7 µmol g-1 h-1 and a ninefold higher RhB photodegradation rate compared to TiO2 MS. The work offers excellent flexibility for precisely constructing high-performance photocatalysts by understanding vacancy engineering.

Wavelength-tunable high-fidelity entangled photon sources enabled by dual Stark effects

The construction of a large-scale quantum internet requires quantum repeaters containing multiple entangled photon sources with identical wavelengths. Semiconductor quantum dots can generate entangled photon pairs deterministically with high fidelity. However, realizing wavelength-matched quantum-dot entangled photon sources faces two difficulties: the non-uniformity of emission wavelength and exciton fine-structure splitting induced fidelity reduction. Typically, these two factors are not independently tunable, making it challenging to achieve simultaneous improvement. In this work, we demonstrate wavelength-tunable entangled photon sources based on droplet-etched GaAs quantum dots through the combined use of AC and quantum-confined Stark effects. The emission wavelength can be tuned by ~1 meV while preserving an entanglement fidelity f exceeding 0.955(1) in the entire tuning range. Based on this hybrid tuning scheme, we finally demonstrate multiple wavelength-matched entangled photon sources with f > 0.919(3), paving the way towards robust and scalable on-demand entangled photon sources for quantum internet and integrated quantum optical circuits.

Efficient and droop-free AlGaN-based UV-C LED using the inverted linearly graded active region and engineered hole source layer

The poor efficiency of the aluminum gallium nitride (AlGaN)-based ultraviolet (UV)-C light emitting diode (LED) remains as the major hindrance towards its commercialization. Thus, to enhance the internal quantum efficiency (IQE) and light output power (LOP) and to mitigate the efficiency droop of AlGaN-based UV-C LED, this simulation work proposes and examines a new engineering strategy, i.e., inverted linearly graded quantum wells along with quantum barriers over a series of samples A, B and C. To further improve the characteristics of AlGaN-based UV-C LED, an engineered linearly graded p-AlGaN layer has been proposed in the device structure. This increases the electron and hole concentrations by ∼2-fold and ∼8-fold, respectively. The 1.2-fold reduction in hole effective potential barrier height improves the hole injection. Also, the 1.5-fold increase in electron effective potential barrier height limits electron spill-off from the active region. The proposed structural-engineering reduces quantum confined stark effect (QCSE) confirmed by the reduction of quantum well slopes. Reducing carrier spill-off and QCSE increases average radiative recombination rate by ∼8-fold. The maximum IQE is improved by ∼44 % and the efficiency droop is also reduced to a promising level of ∼2 % in case of the sample under consideration. Finally, the LOP increases 10-fold, promisingly. Thus, the proposed structure is supposed to be highly potential for improving the UV-C LED performance. The electroluminescence spectra show that despite the gradual differences introduced in the structure, all the samples emit at ∼276 nm with similar full width at half maximum providing a reference for the comparison.

Nonvolatile Control of Valley Polarized Emission in 2D WSe2-AlScN Heterostructures.

Achieving robust and electrically controlled valley polarization in monolayer transition metal dichalcogenides (ML-TMDs) is a frontier challenge for realistic valleytronic applications. Theoretical investigations show that the integration of 2D materials with ferroelectrics is a promising strategy; however, an experimental demonstration has remained elusive. Here, we fabricate ferroelectric field-effect transistors using a ML-WSe2 channel and an Al0.68Sc0.32N (AlScN) ferroelectric dielectric and experimentally demonstrate efficient tuning as well as non-volatile control of valley polarization. We measure a large array of transistors and obtain a maximum valley polarization of ∼27% at 80 K with stable retention up to 5400 s. The enhancement in the valley polarization is ascribed to the efficient exciton-to-trion (X-T) conversion and its coupling with an out-of-plane electric field, viz., the quantum-confined Stark effect. This changes the valley depolarization pathway from strong exchange interactions to slow spin-flip intervalley scattering. Our research demonstrates a promising approach for achieving non-volatile control over valley polarization for practical valleytronic device applications.

Performance Enhancement of Multiple Quantum Shell Nanowire‐Based Micro‐Light Emitting Diodes with Underlying GaInN/GaN Superlattices

GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) are of great interest as high‐efficiency micro‐light emitting diodes (micro‐LEDs), mainly due to their quantum confined Stark effect suppression and dry etching insensitivity features. Herein, morphological and device properties corresponding to NW‐LEDs with different numbers of GaInN/GaN superlattices (SLs) are evaluated. The scanning electron microscopy measurements revealed that the polar‐plane MQS are shrunken, while the semipolar‐plane underwent expansion upon the introduction of the SLs. The current density–voltage–light output analysis at low current density indicates that samples with a greater number of SL pairs exhibit higher light output. Electroluminescence spectra show that NWs lacking SLs exhibit an emission wavelength of 700 nm, which is derived from indium‐rich clusters in polar‐plane MQS, whereas those with SLs emit at a significantly shorter wavelength of 560 nm. The reduction in the polar‐plane MQS coupled with the enhancement in MQS quality resulting from the SLs is identified as the primary contributing factor. Additionally, the external quantum efficiency factor for NW‐LEDs, which remained consistent even as the emission area decreased, is assessed. These findings suggest that NW‐LEDs with SLs possess the potential to mitigate the emission degradation associated with sidewall etching and realize high‐efficiency micro‐LEDs.

Impact of Crystal Orientation on Modulation Bandwidth: Towards GaN LED-Based High-Speed Visible Light Communication

Light-emitting diodes (LEDs) with high modulation bandwidth are required for high-speed visible light communication applications. Crystal orientation in the GaN LED structure plays a key factor in its modulation bandwidth as the recombination lifetime is highly dependent on crystal orientation owing to the Quantum-Confined Stark Effect (QCSE). In this study, six different crystal orientation multi-quantum well (MQW) GaN LEDs are simulated to understand the impact of heterostructure orientation on modulation bandwidth, radiative recombination rates, and emission intensity. The results of this study demonstrate that semi-polar 101¯3¯ MQW LEDs provide the highest bandwidth in the current density range of 9–20 kA/cm2 compared to the other five orientations. For instance, the semi-polar 101¯3¯-based LED offers a modulation bandwidth of 912.7 MHz at 20 kA/cm2 current density. These results suggest that the semi-polar 101¯3¯ orientation-based LED has the potential to support a high-speed visible light communication system.

High Performance Electrically‐Injected InGaN Microdisk Lasers through Simultaneous Enhancement of Optical Confinement and Overlap Factor

AbstractDespite the considerable research interest in InGaN‐based microdisk lasers, owing to their unique circular geometry distinct from vertical cavity surface‐emitting lasers (VCSEL) or edge‐emitting lasers, commercial products remain scant due to deficient performance under electrical injection. This study proposes an innovative method integrating a thin‐film configuration with a metallic undercut, significantly augmenting optical confinement, overlap factor, and thus lasing performance. This approach results in a diminished lasing threshold from 1500 to 670 Wcm−2 and a record high quality (Q) factor of 10100 under electrical injection. Strain relaxation in the metallic undercut, identified via scanning near‐field optical microscopy (SNOM), beneficially mitigates the Quantum Confined Stark Effect (QCSE), boosting the internal quantum efficiency of the laser. Furthermore, a beneficial blue‐shift in the emission spectrum aligns with the lasing peak, strengthening the Q factor. A tactfully implemented recessed top contact enables electrical injection lasing without optical performance compromise. The study provides invaluable insights for future microdisk laser technology advancements, potentially leading to specialized functionalities, including quantum optics, thereby opening new avenues for research and applications in this field.

The Influence Mechanism of Quantum Well Growth and Annealing Temperature on In Migration and Stress Modulation Behavior.

This study explores the effects of growth temperature of InGaN/GaN quantum well (QW) layers on indium migration, structural quality, and luminescence properties. It is found that within a specific range, the growth temperature can control the efficiency of In incorporation into QWs and strain energy accumulated in the QW structure, modulating the luminescence efficiency. Temperature-dependent photoluminescence (TDPL) measurements revealed a more pronounced localized state effect in QW samples grown at higher temperatures. Moreover, a too high annealing temperature will enhance indium migration, leading to an increased density of non-radiative recombination centers and a more pronounced quantum-confined Stark effect (QCSE), thereby reducing luminescence intensity. These findings highlight the critical role of thermal management in optimizing the performance of InGaN/GaN MQWs in LEDs and other photoelectronic devices.

The Effect of AlN Epilayer Growth Rate on the Growth of Semipolar (11–22) InGaN/GaN Light Emitting Diode

This article presents the effect of aluminium nitride (AlN) epilayer (EPL) growth rate on the growth of semipolar (11–22) InGaN/GaN light emitting diode (LED). Three samples are grown with different AlN EPL growth rates, which are low (L1, 1.0 nm s−1), intermediate (L2, 1.3 nm s−1), and high (L3, 1.6 nm s−1) growth rate. The root‐mean‐square (RMS) and peak‐to‐valley roughness show an increasing trend with the increasing AlN EPL growth rate which is attributed to an increment in indium composition as justified by the photoluminescence results. The sample with AlN EPL growth rate of 1.3 nm s−1 (L2) exhibits an outstanding crystal quality in terms of defect densities reduction, with a minimum basal stacking fault density of 1.60 × 104 cm−1 as calculated. Besides that, the obvious fringes of InGaN/GaN peaks observed in 2θ−ω scan proves that a high abruptness multi‐quantum well is attained. Sample L2 also shows a single peak profile in both photoluminescence and electroluminescence spectra with green wavelength emission of 506 and 537 nm, respectively, indicating a homogeneous indium composition. As the injection current is swept from 5 to 40 mA for sample L2, a weak blueshift of ≈6 nm is observed which indicates a reduction in quantum‐confined Stark effect (QCSE).

Enhancing the blue GaN laser diodes by modifying the first barrier structure and adjusting the growth conditions of quantum wells

The structure of the first barrier (FB) of quantum wells was modified from the single-layer GaN to the GaN-InGaN-GaN composite structure. Experiments have shown that the composite first barrier can improve the crystal quality of the quantum wells, which increase the luminous power and reduce the stress accumulation before the InGaN quantum well. Furthermore, the composite first barrier can reduce the polarized electric fields and suppress the impact of quantum confinement Stark effect (QCSE) on the luminescence properties of the quantum wells. The growth conditions of the InGaN/GaN quantum wells were studied. Both the growth rate and the ammonia flow are related to the incorporation of In in InGaN quantum wells and the formation of In clusters. These two parameters have optimal values when the other one is fixed. After optimizing the growth rate and the ammonia flow, the internal quantum efficiency of the quantum wells was finally improved from 24.14 % to a commercial level of 77.2 %.

Electrically Tunable Single Polaritonic Quantum Dot at Room Temperature.

Exciton-polaritons confined in plasmonic cavities are hybridized light-matter quasiparticles, with distinct optical characteristics compared to plasmons and excitons alone. Here, we demonstrate the electric tunability of a single polaritonic quantum dot operating at room temperature in electric-field tip-enhanced strong coupling spectroscopy. For a single quantum dot in the nanoplasmonic tip cavity with variable dc local electric field, we dynamically control the Rabi frequency with the corresponding polariton emission, crossing weak to strong coupling. We model the observed behaviors based on the quantum confined Stark effect in the strong coupling regime.

Spatial and time-resolved properties of emission enhancement in polar/semi-polar InGaN/GaN by surface plasmon resonance

Abstract Light-emitting diodes (LEDs) are widely used as next-generation light sources because of their various advantages. However, their luminous efficiency is remarkably low at the green-emission wavelength. The luminous efficiencies of InGaN/GaN quantum wells (QWs) significantly decrease with increasing indium content in the green wavelength region, mainly owing to the quantum-confined Stark effect (QCSE). This green gap problem can be solved using QWs grown on semi-polar GaN substrates, such as the {11–22} planes, to reduce the QCSE. We propose that the use of surface plasmons (SPs) is a promising way to improve the light emission efficiency of light-emitting materials such as InGaN/GaN QWs. SP resonance increases the spontaneous emission rates of the excited states, causes a relative reduction in non-radiative relaxation, and ultimately increases the internal quantum efficiencies. In this study, the light emissions of InGaN/GaN QWs grown on polar and semi-polar GaN were investigated using micro-photoluminescence (PL). We successfully enhanced the light emission of semi-polar GaN via SP resonance. The PL peak intensities and wavelengths were mapped and compared to determine the underlying mechanisms. We also measured the emission lifetimes by time-resolved PL and interpreted the detailed mechanism of SP-enhanced emissions. It was found that SP resonances can control not only the emission efficiency but also the exciton dynamics, such as exciton localization effects, QCSE screening, and defect level saturation. We conclude that the green gap problem can be solved by SP-enhanced light emission in semipolar InGaN/GaN.

Enhanced Photocurrent and Electrically Pumped Quantum Dot Emission from Single Plasmonic Nanoantennas.

Integrating cavity-enhanced colloidal quantum dots (QDs) into photonic chip devices would be transformative for advancing room-temperature optoelectronic and quantum photonic technologies. However, issues with efficiency, stability, and cost remain formidable challenges to reach the single antenna limit. Here, we present a bottom-up approach that delivers single QD-plasmonic nanoantennas with electrical addressability. These QD nanojunctions exhibit robust photoresponse characteristics, with plasmonically enhanced photocurrent spectra matching the QD solution absorption. We demonstrate electroluminescence from individual plasmonic nanoantennas, extending the device lifetime beyond 40 min by utilizing a 3 nm electron-blocking polymer layer. In addition, we reveal a giant voltage-dependent redshift of up to 62 meV due to the quantum-confined Stark effect and determine the exciton polarizability of the CdSe QD monolayer to be 4 × 10-5 meV/(kV/cm)2. These developments provide a foundation for accessing scalable quantum light sources and high-speed, tunable optoelectronic systems operating under ambient conditions.

Strain modulation effect of superlattice interlayer on InGaN/GaN multiple quantum well

The strong piezoelectric field in InGaN/GaN heterostructure quantum wells severely reduces the light emission efficiency of multiple quantum well (MQW) structures. To address this issue, a strain modulation interlayer is commonly used to mitigate the piezoelectric polarization field and improve the luminescence performance of the devices. To investigate the influence and mechanism of strain modulation in the InGaN/GaN superlattice (SL), epitaxial wafers with an n-type InGaN/GaN SL interlayer sample, and their corresponding control samples are prepared. The measured temperature-dependent photoluminescence (PL) spectra of the epitaxial wafers, show that the introduction of an SL interlayer leads to a shorter-wavelength emission and enhancement of internal quantum efficiency. As the temperature increases, a blue shift of the PL peak is observed. However, for the sample with an SL interlayer, the blue shift of the PL peak with temperature increasing is relatively small. Electroluminescence (EL) experiments indicate that the introduction of an SL interlayer significantly increases the integrated intensity of the EL peak and reduces its full width at half maximum. These phenomena collectively indicate that the incorporation of a superlattice interlayer can partly suppress the quantum-confined Stark effect (QCSE) that affects the light emission efficiency. Theoretical calculations show that the introduction of a superlattice strain layer before growing an active multiple quantum well can weaken the polarization-induced built-in electric field in the active quantum well, reduce the tilt of the energy band in the multiple quantum well active region, increase the overlap of electron and hole wave functions, enhance the emission probability, shorten the radiative recombination lifetime, and promote competition between radiative recombination and non-radiative recombination, thereby achieving higher recombination efficiency and improving light emission intensity. This study provides experimental and theoretical evidence that the strain modulation SL interlayer can effectively improve the device performance and offer guidance for optimizing the structural design of devices.

Tunable Vernier Series-Coupled Microring Resonator Filters Based on InGaAs/InAlAs Multiple Quantum-Well Waveguide

We demonstrate a fourth-order Vernier serious-coupled microring resonator (MRR) filter based on the InGaAs/InAlAs five-layer asymmetric coupled quantum-well (FACQW) waveguide. As the tuning of the main resonance wavelength is driven by the electrorefractive index change in the FACQW layer caused by the quantum-confined Stark effect (QCSE), the MRR filter has lower tuning power consumption and high-speed operation. The free spectral range (FSR) of the MRR filter is extended to 9.4 nm. The main resonant peak is discretely tuned within one FSR, leading to an increase in the wavelength channel.

Polarization-matching and carrier confinement in III-nitride deep-ultraviolet light-emitting diodes

The polarization-induced quantum confined Stark effect has been recognized as a significant factor contributing to the Internal Quantum Efficiency (IQE) droop in light-emitting diodes (LEDs). This study focuses on the design of LEDs by investigating the InAlN/AlGaN interface. By incorporating InAlN quantum wells, a polarization-matched (PM) multi-quantum well (MQW) LED architecture was developed. While the flat conduction and valence bands on PM MQWs indicate an improved recombination rate, it is crucial to examine the impact on IQE, considering carrier confinement and injection efficiency influenced by the band offsets. This paper presents a numerical analysis comparing two LEDs emitting at 245 and 275 nm, respectively. The results demonstrate that the PM LED operating at 275 nm exhibits enhanced performance, benefiting from high probability density overlap. Conversely, the PM LED emitting at 245 nm demonstrates poor confinement, resulting in an overall low performance, regardless of polarization matching.

Onset of Quantum-Confined Stark Effects in Multijunction Photovoltaic Laser Power Converters Designed with Thin Subcells

Photovoltaic multijunction power-converting III–V semiconductor devices generate electrical power from the optical energy of laser beams. They exhibit conversion efficiencies reaching values greater than 60% and 50% for the GaAs and the InP material systems, respectively. The applications of optical wireless power transmission and power-over-fiber greatly benefit from employing such laser power converters constructed with multiple subcells; each is designed with either thin GaAs or InGaAs absorber regions. This study elucidates how the application of electric fields on thin heterostructures can create specific current–voltage characteristics due to modifications of the absorption characteristics from Franz–Keldysh perturbations and the onset of quantum-confined Stark effects. Negative differential photocurrent behavior can be observed as the reverse bias voltage is increased, until the corresponding current-clamping subcell reaches its reverse breakdown condition. The reverse voltage breakdown characteristics of the subcells were also measured to depend on the thickness of the subcell and on the optical intensity. The onset of the reverse breakdown was found to be at ~2.0–2.5 V under illumination and the thinner subcells exhibited higher levels of reverse bias currents. These effects can produce distinctive current–voltage behavior under spectrally detuned operations affecting the thinner subcells’ biases, but have no significant impact on the performance and maximum power point of multijunction power converters.

Fabricating and investigating a beveled mesa with a specific inclination angle to improve electrical and optical performances for GaN-based micro-light-emitting diodes.

In this Letter, beveled mesas for 30 × 30 µm2 GaN-based micro-light-emitting diodes (µLEDs) with different inclination angles are designed, fabricated, and measured. We find that µLED with a mesa inclination angle of 28° has the lowest internal quantum efficiency (IQE) and the highest injection current density at which the peak IQE is obtained. This is due to the increased quantum confined Stark effect (QCSE) at the mesa edge. The increased QCSE results from the strong electric field coupling effect. Instead of radiative recombination, more nonradiative recombination and leakage current will be generated in the sidewall regions. Besides, the smallest angle (28°) also produces the lowest light extraction efficiency (LEE), which arises from the optical loss caused by the sidewall reflection at the beveled surface sides. Therefore, the inclination angle for the beveled mesa has to be increased to 52° and 61° by using Ni and SiO2 as hard masks, respectively. Experimental and numerical results show that the external quantum efficiency (EQE) and the optical power can be enhanced for the fabricated devices. Meanwhile, the reduced surface recombination rate also decreases the leakage current.

Achieving atomically ordered GaN/AlN quantum heterostructures: The role of surface polarity.

Interface engineering in heterostructures at the atomic scale has been a central research focus of nanoscale and quantum material science. Despite its paramount importance, the achievement of atomically ordered heterointerfaces has been severely limited by the strong diffusive feature of interfacial atoms in heterostructures. In this work, we first report a strong dependence of interfacial diffusion on the surface polarity. Near-perfect quantum interfaces can be readily synthesized on the semipolar plane instead of the conventional c-plane of GaN/AlN heterostructures. The chemical bonding configurations on the semipolar plane can significantly suppress the cation substitution process as evidenced by first-principles calculations, which leads to an atomically sharp interface. Moreover, the surface polarity of GaN/AlN can be readily controlled by varying the strain relaxation process in core-shell nanostructures. The obtained extremely confined, interdiffusion-free ultrathin GaN quantum wells exhibit a high internal quantum efficiency of ~75%. Deep ultraviolet light-emitting diodes are fabricated utilizing a scalable and robust method and the electroluminescence emission is nearly free of the quantum-confined Stark effect, which is significant for ultrastable device operation. The presented work shows a vital path for achieving atomically ordered quantum heterostructures for III-nitrides as well as other polar materials such as III-arsenides, perovskites, etc.

Electric-Field Fluctuations as the Cause of Spectral Instabilities in Colloidal Quantum Dots.

Spectral diffusion (SD) represents a substantial obstacle toward implementation of solid-state quantum emitters as a source of indistinguishable photons. By performing high-resolution emission spectroscopy for individual colloidal quantum dots at cryogenic temperatures, we prove the causal link between the quantum-confined Stark effect and SD. Statistically analyzing the wavelength of emitted photons, we show that increasing the sensitivity of the transition energy to an applied electric field results in amplified spectral fluctuations. This relation is quantitatively fit to a straightforward model, indicating the presence of a stochastic electric field on a microscopic scale, whose standard deviation is 9 kV/cm, on average. The current method will enable the study of SD in multiple types of quantum emitters such as solid-state defects or organic lead halide perovskite quantum dots, for which spectral instability is a critical barrier for applications in quantum sensing.