Quantum Confined Stark Effect Research Articles

Bifunctional ultraviolet light-emitting/detecting device based on a SnO2 microwire/p-GaN heterojunction

SnO 2 has attracted considerable attention due to its wide bandgap, large exciton binding energy, and outstanding electrical and optoelectronic features. Owing to the lack of reliable and reproducible p-type SnO 2 , many challenges on developing SnO 2 -based optoelectronic devices and their practical applications still remain. Herein, single-crystal SnO 2 microwires (MWs) are acquired via the self-catalyzed approach. As a strategic alternative, n - SnO 2 MW/p-GaN heterojunction was constructed, which exhibited selectable dual-functionalities of light-emitting and photodetection when operated by applying an appropriate voltage. The device illustrated a distinct near-ultraviolet light-emission peaking at ∼ 395.0 nm and a linewidth ∼ 50 nm . Significantly, the device characteristics, in terms of the main peak positions and linewidth, are nearly invariant as functions of various injection current, suggesting that quantum-confined Stark effect is essentially absent. Meanwhile, the identical n - SnO 2 MW/p-GaN heterojunction can also achieve photovoltaic-type light detection. The device can steadily feature ultraviolet photodetecting ability, including the ultraviolet/visible rejection ratio ( R 360 nm / R 400 nm ) ∼ 1.5 × 10 3 , high photodark current ratio of 10 5 , fast response speed of 9.2/51 ms, maximum responsivity of 1.5 A/W, and detectivity of 1.3 × 10 13 Jones under 360 nm light at − 3 V bias. Therefore, the bifunctional device not only displays distinct near-ultraviolet light emission, but also has the ability of high-sensitive ultraviolet photodetection. The novel design of n - SnO 2 MW/p-GaN heterojunction bifunctional systems is expected to open doors to practical application of SnO 2 microstructures/nanostructures for large-scale device miniaturization, integration and multifunction in next-generation high-performance photoelectronic devices.

Luminescence Properties of InGaN/GaN Green Light-Emitting Diodes with Si-Doped Graded Short-Period Superlattice.

The optical properties of InGaN/GaN green light-emitting diodes (LEDs) with an undoped graded short-period superlattice (GSL) and a Si-doped GSL (SiGSL) were investigated using photoluminescence (PL) and time-resolved PL spectroscopies. For comparison, an InGaN/GaN conventional LED (CLED) without the GSL structure was also grown. The SiGSL sample showed the strongest PL intensity and the largest PL peak energy because of band-filling effect and weakened quantum- confined stark effect (QCSE). PL decay time of SiGSL sample at 10 K was shorter than those of the CLED and GSL samples. This finding was attributed to the oscillator strength enhancement by the reduced QCSE due to the Coulomb screening by Si donors. In addition, the SiGSL sample exhibited the longest decay time at 300 K, which was ascribed to the reduced defect and dislocation density. These results indicate that insertion of the Si-doped GSL structure is an effective strategy for improving the optical properties in InGaN/GaN green LEDs.

Highly stable full-color display device with VLC application potential using semipolar μLEDs and all-inorganic encapsulated perovskite nanocrystal

A promising approach for the development of effective full-color displays is to combine blue microLEDs (μLEDs) with color conversion layers. Perovskite nanocrystals (PNCs) are notable for their tolerance to defects and provide excellent photoluminescence quantum yields and high color purity compared to metal chalcogenide quantum dots. The stability of PNCs in ambient conditions and under exposure to blue light can be improved using a SiO 2 coating. This study proposes a device that could be used for both display and visible light communication (VLC) applications. The semipolar blue μLED array fabricated in this study shows a negligible wavelength shift, indicating a significant reduction in the quantum confined Stark effect. Owing to its shorter carrier lifetime, the semipolar μLED array exhibits an impressive peak 3 dB bandwidth of 655 MHz and a data transmission rate of 1.2 Gb/s corresponding to an injection current of 200 mA. The PNC–μLED device assembled from a semipolar μLED array with PNCs demonstrates high color stability and wide color-gamut features, achieving 127.23% and 95.00% of the National Television Standards Committee standard and Rec. 2020 on the CIE 1931 color diagram, respectively. These results suggest that the proposed PNC–μLED device is suitable for both display-related and VLC applications.

Electric field induced tuning of electronic correlation in weakly confining quantum dots

We conduct a combined experimental and theoretical study of the quantum-confined Stark effect in GaAs/AlGaAs quantum dots obtained with the local droplet etching method. In the experiment, we probe the permanent electric dipole and polarizability of neutral and positively charged excitons weakly confined in GaAs quantum dots by measuring their light emission under the influence of a variable electric field applied along the growth direction. Calculations based on the configuration-interaction method show excellent quantitative agreement with the experiment and allow us to elucidate the role of Coulomb interactions among the confined particles and -- even more importantly -- of electronic correlation effects on the Stark shifts. Moreover, we show how the electric field alters properties such as built-in dipole, binding energy, and heavy-light hole mixing of multiparticle complexes in weakly confining systems, underlining the deficiencies of commonly used models for the quantum-confined Stark effect.

Ultrafast 2 × 2 green micro-LED array for optical wireless communication beyond 5 Gbit/s

A green 2 × 2 micro light-emitting diode ( μ - LED ) array with nanostructured grating patterns grown on a semipolar (20-21)-oriented gallium nitride (GaN) buffered layer on (22-43)-oriented sapphire substrate is specially transistor-outline can (TO-can) packaged with a sub-miniature-A (SMA) connector for high-speed data communication beyond 5 Gbit/s. Through a specific design for suppressing the quantum-confined Stark effect (QCSE) in the green 2 × 2 μ - LED array with a low polarization-related electric field and flat quantum well band diagram, the green 2 × 2 μ - LED array exhibits a turn-on voltage of 2.5 V and output power of 0.3 mW at 1 A / cm 2 . The green 2 × 2 μ - LED array also reveals a wavelength shift from 543 nm to 537 nm smaller than that of conventional devices grown on c -plane buffered GaN substrate due to the inhibited QCSE. The 50 µm emission aperture of the green 2 × 2 μ - LED array ensures a lower capacitance for a larger − 3 dB modulation bandwidth, which exhibits − 1 dB power compression at a larger bias under high-speed operation, as it is less affected by the high resistance of the single μ - LED element. With a specific TO-can+SMA package, the green 2 × 2 μ - LED array exhibits maximal data rates exceeding 1.5 Gbit/s for the non-return-to-zero on–off keying format and beyond 5.02 Gbit/s for the bit-loaded discrete multitone (BL-DMT) format, which is very promising for optical wireless communication. As the sampling rate increases from 4 GSa/s to 16 GSa/s, the μ - LED array’s received signal-to-noise ratio (SNR) improves dramatically from 15.4 dB to 12.2 dB. The SNR remains about 15.4 dB, with a matching bit-error ratio (BER) of 2.7 × 10 − 3 , whereas the 10-fold oversampling of the eight-ray quadrature amplitude modulation orthogonal frequency-division multiplexing (8-QAM OFDM) data stream with 16 GSa/s appears to reduce the SNR by − 3 dB , resulting in a decoded BER of 3.3 × 10 − 3 . The green 2 × 2 μ - LED array has demonstrated greater potential in data transmission beyond 5 Gbit/s using the BL-DMT algorithm for future applications in domains of visible light communication or optical wireless communication when packaged with handed mobile devices.

Monolayer-Scale GaN/AlN Multiple Quantum Wells for High Power e-Beam Pumped UV-Emitters in the 240-270 nm Spectral Range.

Monolayer (ML)-scale GaN/AlN multiple quantum well (MQW) structures for electron-beam-pumped ultraviolet (UV) emitters are grown on c-sapphire substrates by using plasma-assisted molecular beam epitaxy under controllable metal-rich conditions, which provides the spiral growth of densely packed atomically smooth hillocks without metal droplets. These structures have ML-stepped terrace-like surface topology in the entire QW thickness range from 0.75–7 ML and absence of stress at the well thickness below 2 ML. Satisfactory quantum confinement and mitigating the quantum-confined Stark effect in the stress-free MQW structures enable one to achieve the relatively bright UV cathodoluminescence with a narrow-line (~15 nm) in the sub-250-nm spectral range. The structures with many QWs (up to 400) exhibit the output optical power of ~1 W at 240 nm, when pumped by a standard thermionic-cathode (LaB6) electron gun at an electron energy of 20 keV and a current of 65 mA. This power is increased up to 11.8 W at an average excitation energy of 5 µJ per pulse, generated by the electron gun with a ferroelectric plasma cathode at an electron-beam energy of 12.5 keV and a current of 450 mA.

Effect of indium content and carrier distribution on the efficiency and reliability of InGaN/GaN-based multi quantum well light emitting diode

This paper investigates the mechanisms that limit the efficiency and the reliability of InGaN-based LEDs, by presenting an analysis carried out on devices having two quantum wells that emit at different wavelengths (405 nm and 495 nm). A color-coded test structure was designed in order to easily quantify the optical characteristics of both quantum wells, and to describe in detail the stress dynamics as a function of In-content and of the position of the quantum wells.The attention was focused on: (i) electrical degradation induced by constant current stress at high temperature, that was found to be related to a diffusion process; (ii) degradation of the optical parameters, which was dominantly ascribed to an increase in SRH recombination and to a decreased carrier injection efficiency; (iii) effect of indium content and carrier distribution on the efficiency and reliability of the devices. The results strongly suggest that the internal quantum efficiency is limited by a low injection efficiency and by the quantum confinement Stark effect. In addition, the results on optical degradation highlight the presence of different mechanisms that contribute to the degradation: the role of Shockley-Read-Hall recombination, carrier escape and Auger processes are discussed.

Visible Light Communication System Technology Review: Devices, Architectures, and Applications

Visible light communication (VLC) is an advanced, highly developed optical wireless communication (OWC) technology that can simultaneously provide lighting and high-speed wireless data transmission. A VLC system has several key advantages: ultra-high data rate, secure communication channels, and a lack of interference from electromagnetic (EM) waves, which enable a wide range of applications. Light-emitting diodes (LEDs) have been considered the optimal choice for VLC systems since they can provide excellent illumination performance. However, the quantum confinement Stark effect (QCSE), crystal orientation, carrier lifetime, and recombination factor will influence the modulation bandwidth, and the transmission performance is severely limited. To solve the insufficient modulation bandwidth, micro-LEDs (μ-LEDs) and laser diodes (LDs) are considered as new ideal light sources. Additionally, the development of modulation technology has dramatically increased the transmission capacity of the system. The performance of the VLC system is briefly discussed in this review article, as well as some of its prospective applications in the realms of the industrial Internet of Things (IoT), vehicle communications, and underwater wireless network applications.

Multiple-quantum-well-induced unipolar carrier transport multiplication in AlGaN solar-blind ultraviolet photodiode

AlGaN solar-blind ultraviolet (SBUV) detectors have potential application in fire monitoring, corona discharge monitoring, or biological imaging. With the promotion of application requirements, there is an urgent demand for developing a high-performance vertical detector that can work at low bias or even zero bias. In this work, we have introduced a photoconductive gain mechanism into a vertical AlGaN SBUV detector and successfully realized it in a p - i - n photodiode via inserting a multiple-quantum-well (MQW) into the depletion region. The MQW plays the role of trapping holes and increasing carrier lifetime due to its strong hole confinement effect and quantum confinement Stark effect. Hence, the electrons can go through the detector multiple times, inducing unipolar carrier transport multiplication. Experimentally, an AlGaN SBUV detector with a zero-bias peak responsivity of about 0.425 A/W at 233 nm is achieved, corresponding to an external quantum efficiency of 226%, indicating the existence of internal current gain. When compared with the device without MQW structure, the gain is estimated to be about 10 3 in magnitude. The investigation provides an alternative and effective approach to obtain high current gain in vertical AlGaN SBUV detectors at zero bias.

Improvement of Radiative Recombination Rate and Efficiency Droop of InGaN Light Emitting Diodes with In‐Component‐Graded InGaN Barrier

The influences of In‐component‐graded InGaN barrier on the performances of InGaN light emitting diodes (LEDs) are studied. The results show that LEDs with In‐component linearly graded InGaN barrier exhibit better performances of radiative recombination rate and efficiency droop than reference LED with constant‐In‐component InGaN barrier. At 150 mA, the external quantum efficiencies for LED A with constant‐In‐component InGaN barrier, LED B with the In‐component linearly increasing in the InGaN barrier, and LED C with the In‐component linearly decreasing in the InGaN barrier are 30.30%, 36.66%, and 34.03%, respectively. Among the three sets of LED samples, LED B, with the In‐component linearly increasing in the InGaN barrier, has the highest radiative recombination rate due to suppression of quantum confinement stark effect and enhancement of carrier confinement effect in the active layer. The efficiency droop for LED A, LED B, and LED C is 34.83%, 25.67%, and 16.67%, respectively. Analysis indicates that the efficiency droop is mainly determined by the hole injection efficiency and electron leakage from the active layer. LED C with the In‐component linearly decreasing in the InGaN barrier has the lowest efficiency droop due to improved hole injection efficiency and reduced electron leakage.

Ultrafast electric control of cavity mediated single-photon and photon-pair generation with semiconductor quantum dots

Employing the ultrafast control of electronic states of a semiconductor quantum dot in a cavity, we introduce a novel approach to achieve on-demand emission of single photons with almost perfect indistinguishability and photon pairs with near ideal entanglement. Our scheme is based on optical excitation off-resonant to a cavity mode followed by ultrafast control of the electronic states using the time-dependent quantum-confined Stark effect, which then allows for cavity-resonant emission. Our theoretical analysis takes into account cavity-loss mechanisms, the Stark effect, and phonon-induced dephasing allowing realistic predictions for finite temperatures.

18.2: Invited Paper: Semipolar Micro‐LED for Full‐color Display and Visible Light Communication

Micro‐LEDs (μ‐LEDs) are well known to have a thriving future in the next‐generation display field. However, conventional μ‐LEDs grown on c‐plane GaN have a severe wavelength shift due to the quantum confined Stark effect (QCSE), which seriously affects the color quality of μ‐LEDs display. In order to fundamentally solve this problem, we grown blue μ‐LED with a 30 µm diameter on the (20–21) semipolar GaN combined with perovskite quantum dots (PQDs) color conversion layer, to achieve high‐quality full‐color μ‐LED, realizing a wide color gamut with 99.2% National Television Standards Committee (NTSC) space and 93.0% Rec 2020 in the CIE 1931, and the peak wavelength hardly shifts, indicating an excellent inhibitory on QCSE. Meanwhile, the semipolar blue μ‐LED excitation light can achieve a wide 3‐dB bandwidth of 652 MHz due to the faster carrier recombination rate, making semipolar LEDs more attractive in the high‐speed visible light communication (VLC).

18.5: Size‐dependence Characterization of GaN‐based Green Micro‐LED

GaN‐based light‐emitting diode (LED) arrays are a promising technology in a wide range of applications. Research on size dependence performance is important in GaN‐based LED. In this paper, we study the electrical properties of 100µm, 150µm, and 200µm devices. The ideal factor is calculated by measuring the current density‐voltage characteristics to study the carrier transport mechanism. It is found that the forward low bias voltage part is mainly composed of recombination current and diffusion current. The current density in the high bias voltage part depends on the current crowding effect. The temperature variation study proves that the ideal factor has no size dependence. Besides, the electroluminescence (EL) spectrum is studied to understand size dependence in GaN internal structure. As the size decreases, the quantum confinement Stark effect (QCSE) will be suppressed, which is proved by the relationship between optical power and turn‐on voltage. These studies indicate that the application of Micro‐LED and even smaller‐sized Micro‐LEDs has unlimited possibilities and broad prospects.

Investigation of Ultrafast Carrier Dynamics in InGaN/GaN‐Based Nanostructures Using Femtosecond Pump–Probe Absorption Spectroscopy

GaN‐based optoelectronic devices including light‐emitting diodes and lasers realized with quantum‐confined nanostructures, revolutionized the solid‐state lighting. Excited‐state ultrafast nonlinear dynamics of these nanostructures are crucial in determining the device performance in terms of luminescence efficacy, emission spectra, wall‐plug efficiency, turn‐on delay, lasing threshold current, and modulation bandwidth. Therefore, understanding the carrier and photon dynamics of these nanostructures is of utmost importance. A comparative investigation of ultrafast nonlinear carrier‐photon dynamics of 2D, 1D, and 0D InGaN/GaN nanostructures is carried out. Femtosecond pump–probe absorption spectroscopy is used uniformly herein to obtain the wavelength‐dependent ultrafast kinetics of these nanostructures, with a temporal resolution of ≈50 fs. Carrier thermalization and relaxation processes of all the samples are studied both experimentally and theoretically. Distinguished phenomena, such as fast carrier trapping and subsequent detrapping of the carriers from sub‐bandgap defect states are observed. Real‐time monitoring of quantum confined Stark effect, enhanced radiative recombination, and diffusion‐limited carrier kinetics are manifested. Carrier capture and recombination time constants are calculated theoretically, considering quantum properties. Theoretical values are in good agreement with the experimental observations. In addition, transient absorption spectroscopy is used to investigate and decouple the surface and the bulk properties of GaN surfaces.

Crystal Growth and Characterization of n-GaN in a Multiple Quantum Shell Nanowire-Based Light Emitter with a Tunnel Junction.

Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n++/ p++-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n++-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p++-GaN and 3-nm-thick n++-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p++-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 1017 cm-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p++-GaN region was calculated to be 1.1 × 1020 cm-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.

Optical properties of quasi-ordered [formula omitted] AlGaN multi-quantum-well nanorod arrays

Quasi-ordered semi-polar 112̅2 plane AlGaN multiple-quantum-well (MQW) nanorod (NR) arrays with a period of 400 nm were successfully fabricated by a self-assembly technique using polystyrene spheres. The diameter and height of the NRs were about 360 and 600 nm, respectively. The photoluminescence (PL) peak wavelengths of both NR and planar MQWs were located at 266 nm at room temperature, demonstrating that the quantum-confined Stark effect was negligible in the semi-polar AlGaN MQWs. The PL integral intensity of the MQW NR arrays was enhanced by 1.5 times compared to that of planar MQWs. Moreover, the degree of polarization was increased from 15.9% to 26.2% when fabricating the NR arrays, demonstrating enhanced polarized emission. The finite-difference time-domain method was used to explain the enhancement of PL intensity, which was mainly due to the modified light extraction by the quasi-ordered NR arrays.

Time-resolved cathodoluminescence investigations of AlN:Ge/GaN nanowire structures

Light emitting diodes represent a key technology that can be found in many areas of everydays life. Therefore, the improvement of the efficiency of such structures offers a high economic and ecological potential. One approach is electrostatic screening of the quantum-confined Stark effect (QCSE) in polar III-V heterostructures by n-type doping in order to increase the oscillator strength of electronic transitions in quantum structures. In this study, we analyzed the cathodoluminescene (CL) spectra of different functional parts of individual AlN/GaN nanowire superlattices and studied their decay characteristics with sub-nanosecond time resolution. This allows us to extract information about strain and electric fields in such heterostructures with an overall spatial resolution <100 nm. The samples, which were investigated in a temperature range from 10 to 300 K by using time-integrated cathodoluminescence spectroscopy (TICL) and time-resolved cathodoluminescence spectroscopy (TRCL) consist of GaN bottom and top layer and a 40-fold stack of GaN nanodiscs, embedded in AlN barriers that were doped with Ge. We show, that the QCSE is reduced with increasing doping concentration due to a screening of the internal electric fields inside GaN nanodiscs, resulting in a reduction of the carrier lifetimes and a blue shift of the emitted light. Due to the small diameter of the electron excitation beam CL offers the possibility to individually analyze the different functional parts of the nanowires.

Nanohole array structured GaN-based white LEDs with improved modulation bandwidth via plasmon resonance and non-radiative energy transfer

Commercial white LEDs (WLEDs) are generally limited in modulation bandwidth due to a slow Stokes process, long lifetime of phosphors, and the quantum-confined Stark effect. Here we report what we believe is a novel plasmonic WLED by infiltrating a nanohole LED (H-LED) with quantum dots (QDs) and Ag nanoparticles (NPs) together (M-LED). This decreased distance between quantum wells and QDs would open an extra non-radiative energy transfer channel and thus enhance Stokes transfer efficiency. The presence of Ag NPs enhances the spontaneous emission rate significantly. Compared to an H-LED filled with QDs (QD-LED), the optimized M-LED demonstrates a maximum color rendering index of 91.2, a 43% increase in optical power at 60 mA, and a lowered correlated color temperature. Simultaneously, the M-LED exhibits a data rate of 2.21 Gb/s at low current density of 96 A / cm 2 (60 mA), which is 77% higher than that of a QD-LED. This is mainly due to the higher optical power and modulation bandwidth of the M-LED under the influence of plasmon, resulting in a higher data rate and higher signal-to-noise ratio under the forward error correction. We believe the approach reported in this work should contribute to a WLED light source with increased modulation bandwidth for a higher speed visible light communication application.

Quantum Confined Stark Effect on the Linear and Nonlinear Optical Properties of SiGe/Si Semi Oblate and Prolate Quantum Dots Grown in Si Wetting Layer.

We have studied the parallel and perpendicular electric field effects on the system of SiGe prolate and oblate quantum dots numerically, taking into account the wetting layer and quantum dot size effects. Using the effective-mass approximation in the two bands model, we computationally calculated the extensive variation of dipole matrix (DM) elements, bandgap and non-linear optical properties, including absorption coefficients, refractive index changes, second harmonic generation and third harmonic generation as a function of the electric field, wetting layer size and the size of the quantum dot. The redshift is observed for the non-linear optical properties with the increasing electric field and an increase in wetting layer thickness. The sensitivity to the electric field toward the shape of the quantum dot is also observed. This study is resourceful for all the researchers as it provides a pragmatic model by considering oblate and prolate shaped quantum dots by explaining the optical and electronic properties precisely, as a consequence of the confined stark shift and wetting layer.

Insights into Growth-Oriented Interfacial Modulation within Semiconductor Multilayers.

Interfacial engineering plays a crucial role in regulating the quality and property of heterogeneous structures, especially for nanometer-scaled devices. However, traditional methods for interfacial modulation (IFM) generally treat all the interfaces uniformly, neglecting the inherent disparities of interfaces like their growth sequence. Herein, it is found that the growth-oriented characteristic of IFM strongly determines the main regions where the modulation takes effect. Specifically, in a semiconductor quantum well structure, the arsenic atoms modulated at the well-on-barrier (WoB) interface tend to diffuse into and thus affect the next-grown well layer. In contrast, the arsenic atoms introduced at the barrier-on-well (BoW) interface mainly take effect within the next-grown barrier layer. According to theoretical simulations and electron holography (EH) experiments, the depth of quantum wells and the height of potential barriers are extended by introducing arsenic atoms at WoB and BoW interfaces, respectively. Resultantly, while modulating at the BoW interface has little impact on the photoluminescence (PL) spectrum, applying IFM at the WoB interface could dramatically improve the luminescent intensity (about 30%), which demonstrates the impact of the growth-oriented characteristic. Furthermore, in situ bias EH results indicate that IFM at the WoB interface helps to suppress the quantum-confined Stark effect.