Quantum Wells Research Articles

Recombination efficiency in c-plane (In,Ga)N/GaN quantum wells: saturation of localisation sites versus Auger–Meitner recombination

Abstract Light emitting diodes based on c-plane (In,Ga)N/GaN quantum wells (QWs) can have >90% emission efficiency at modest current densities but this drops significantly at higher excitation, an effect known as efficiency droop that limits device efficacy at high brightness. Several explanations for this have been proposed including the saturation of carrier localisation sites at high excitation densities, resulting in a greater exposure of carriers to defects and hence a significant increase in the associated non-radiative recombination processes. Here, power- and temperature-dependent photoluminescence spectroscopy of c-plane (In,Ga)N/GaN QWs is used to investigate the relationship between the saturation of localised states and emission efficiency. For the samples studied, we find that the saturation of localised sites broadly coincides with the onset of efficiency droop. However, it is also found that as the localised states saturate with increasing excitation, the relative contribution of defect-associated non-radiative processes to overall recombination decreases rather than increases. Based on these observations and on modelling of recombination processes in the QW, it is concluded that the saturation of localised states does not significantly contribute to the reduction in emission efficiency at high excitation. Our studies rather suggest that defect-related non-radiative recombination is out-competed by radiative and Auger–Meitner recombination at the carrier densities required for saturation.

(In,Ga)N-GaN resonant Bragg structures with single and double quantum wells in the unit supercell

We study the formation of a superradiant optical mode in the room temperature reflection spectra from resonant Bragg structures (RBSs) composed of single and double (In,Ga)N quantum wells (QWs) in the unit cell. The appearance of the mode manifests itself by a significant increase in the resonant optical reflectivity due to the electromagnetic coupling of quasi-two-dimensional excitons in the QWs. The implementation of the supercells with double (In,Ga)N QWs results in an increase in the oscillator strength of the quasi-2D excitons and corresponding rise of the radiative broadening parameter to the value as high as 0.3 ± 0.02 meV. We also show that the supercells with double QWs are preferable for RBS with large number of periods due to better tolerance to deviations from the exact periodicity.

Introducing boron gallium nitride as carriers’ source layer for efficient near-ultraviolet microLED

Abstract This study explored the impact of boron gallium nitride (BGaN) in buffer layer and hole source layer. We employed B0.05Ga0.95N which reduced the lattice mismatch as well as the electric field. BGaN not only minimized the number of electrons leaking out of quantum wells (QWs) but also improved the hole injection. It is evident from our simulations that internal quantum efficiency (IQE) is enhanced significantly as more carriers are available for radiative recombination in multiple quantum wells (MQWs). Along with the increase in IQE, droop is also reduced in BGaN ultraviolet light-emitting diodes. Significantly high luminous power and emission intensity were observed along with slight blueshift because of minimized quantum confinement stark effect (QCSE).

Deep Ultraviolet Excitation Photoluminescence Characteristics and Correlative Investigation of Al-Rich AlGaN Films on Sapphire

AlGaN is attractive for fabricating deep ultraviolet (DUV) optoelectronic and electronic devices of light-emitting diodes (LEDs), photodetectors, high-electron-mobility field-effect transistors (HEMTs), etc. We investigated the quality and optical properties of AlxGa1−xN films with high Al fractions (60–87%) grown on sapphire substrates, including AlN nucleation and buffer layers, by metal–organic chemical vapor deposition (MOCVD). They were initially investigated by high-resolution X-ray diffraction (HR-XRD) and Raman scattering (RS). A set of formulas was deduced to precisely determine x(Al) from HR-XRD data. Screw dislocation densities in AlGaN and AlN layers were deduced. DUV (266 nm) excitation RS clearly exhibits AlGaN Raman features far superior to visible RS. The simulation on the AlGaN longitudinal optical (LO) phonon modes determined the carrier concentrations in the AlGaN layers. The spatial correlation model (SCM) analyses on E2(high) modes examined the AlGaN and AlN layer properties. These high-x(Al) AlxGa1−xN films possess large energy gaps Eg in the range of 5.0–5.6 eV and are excited by a DUV 213 nm (5.8 eV) laser for room temperature (RT) photoluminescence (PL) and temperature-dependent photoluminescence (TDPL) studies. The obtained RTPL bands were deconvoluted with two Gaussian bands, indicating cross-bandgap emission, phonon replicas, and variation with x(Al). TDPL spectra at 20–300 K of Al0.87Ga0.13N exhibit the T-dependences of the band-edge luminescence near 5.6 eV and the phonon replicas. According to the Arrhenius fitting diagram of the TDPL spectra, the activation energy (19.6 meV) associated with the luminescence process is acquired. In addition, the combined PL and time-resolved photoluminescence (TRPL) spectroscopic system with DUV 213 nm pulse excitation was applied to measure a typical AlGaN multiple-quantum well (MQW). The RT TRPL decay spectra were obtained at four wavelengths and fitted by two exponentials with fast and slow decay times of ~0.2 ns and 1–2 ns, respectively. Comprehensive studies on these Al-rich AlGaN epi-films and a typical AlGaN MQW are achieved with unique and significant results, which are useful to researchers in the field.

High performance deep violet InGaN TQW laser diode through multi-layer thickness optimization using particle swarm optimization method

The paper details the optimization of efficiency parameters for a InGaN TQW laser diode using the particle swarm optimization algorithm through SILVACO software. By adjusting layer thickness based on output power, slope efficiency, and threshold current, significant enhancements were achieved. Three optimized laser diodes (LD1, LD2, LD3) were compared with a primary LD, revealing notable improvements in output power and overall performance. Analysis of carrier recombination rates in the active region highlighted increased stimulated recombination and decreased non-radiative recombination, particularly in the n-side well. The study also showed a more uniform radiative recombination in the quantum wells of the optimized LDs, especially in LD3. Investigation of electron and hole concentrations, current densities, and energy levels demonstrated higher electron and hole current density in the optimized LDs, with LD3 exhibiting substantial improvements over the primary LD. Notably, the optimized LDs displayed reduced threshold current and improved slope efficiency compared to the primary LD. The study identified optimal layer thicknesses based on various cost functions, resulting in significant enhancements in output power (0.561 W), slope efficiency (2.515 W/A), and reduced threshold current (less than 0.029 A), indicative of enhanced TQW laser diode performance. Overall, the research showcases the effectiveness of the particle swarm optimization approach in optimizing GaN-based TQW laser diodes, leading to improved efficiency characteristics and demonstrating the potential for enhanced performance in practical applications.

Analysis of Electron Penetration into the Insulator in Finite Quantum Wells for Nanosheet MOSFETs

Analysis of Electron Penetration into the Insulator in Finite Quantum Wells for Nanosheet MOSFETs

Bright dipolar excitons in twisted black phosphorus homostructures.

Bright dipolar excitons, which contain electrical dipoles and have high oscillator strength, are an ideal platform for studying correlated quantum phenomena. They usually rely on carrier tunneling between two quantum wells or two layers to hybridize with nondipolar excitons to gain oscillator strength. In this work, we uncovered a new type of bright infrared dipolar exciton by stacking 90°-twisted black phosphorus (BP) structures. These excitons, inherent to the reconstructed band structure, exhibit high oscillator strength. Most importantly, they inherit the linear polarization from BP, which allows light polarization to be used to select the dipole direction. Moreover, the dipole moment and resonance energy can be widely tuned by the thickness of the BP. Our results demonstrate a useful platform for exploring tunable correlated dipolar excitons.

Electro-optically Modulated Nonlinear Metasurfaces.

Electrically reconfigurable nonlinear metasurfaces provide dynamic control over nonlinear phenomena such as second-harmonic generation (SHG), unlocking novel applications in signal processing, light switching, and sensing. Previous methods, like electric-field-induced SHG in plasmonic metasurfaces and Stark-tuned nonlinearities in quantum well metasurfaces, face limitations due to weak SHG responses from metals and mid-infrared constraints of quantum wells, respectively. Addressing the need for efficient SHG control in the visible and near-infrared ranges, we present a novel approach using the electro-optic (EO) effect to modulate SHG. By leveraging the exceptional EO and SHG properties of lithium niobate (LN), we integrate the EO effect with SHG within a metasurface framework for the first time. Our LN metasurface achieves an 11.3% modulation depth in SHG amplitude under a ±50 V alternating voltage. These results open new avenues for reconfigurable photonic applications. including tunable nonlinear light sources, quantum optics, and nonlinear information processing.

Isolated Red Quantum Wells with Strain Relaxation Induced by V-pits and Trench Structures

Isolated Red Quantum Wells with Strain Relaxation Induced by V-pits and Trench Structures

Time-resolved counterpropagating edge transport in the topological insulating phase

We have investigated the time-resolved low-energy charge dynamics in counterpropagating edge channels formed in the quantum Hall regime of an electron-hole bilayer system in band-inverted InAs/InGaSb composite quantum wells. In the studied magnetic field range, the system remains in the topological phase where an equal number of counterpropagating electronlike and holelike edge channels exist in the bulk energy gap, analogous to the helical edge channels in a quantum spin Hall insulator. When the Fermi level was set in the conduction band, the injected charge pulses propagated unidirectionally as chiral edge magnetoplasmon (EMP) wave packets. In contrast, in the nonchiral regime with the Fermi level set in the band gap, the injected charge pulses propagated identically in both directions with significantly reduced velocity. In addition, as the channel length increased, we observed a sharp crossover from EMP transport to diffusive transport, manifested as pronounced waveform broadening and dissipation. Simulations using a distributed capacitance model including interchannel tunneling well reproduced the observed behavior in both chiral and nonchiral regimes, revealing that interchannel charge transfer between counterpropagating channels plays a crucial role. The slow EMP transport in the nonchiral regime and its crossover to diffusive transport are consequences of the strong interchannel interaction and interchannel tunneling, respectively. Our results will thus provide insights into charge dynamics and scattering mechanism in analogous one-dimensional systems, including a quantum spin Hall insulator with helical edge modes and its expected Tomonaga-Luttinger liquid behavior. Published by the American Physical Society 2024

Manipulating formation of different InGaAs/GaAs nanostructures via tailoring As4 flux

This research provides a flexible approach to manipulate formation of InGaAs nanostructures on the GaAs (100) surface by varying arsenic (As4) beam equivalent pressure (BEP). By selecting the As4/(In+Ga) BEP ratio to be 4, 8, 20, 50 and 100, we were able to obtain different quantum structures from quantum well (QW) to quantum dots (QDs), then to spatially ordered quantum dot chains (QD-chains), and finally to quantum wires (QWRs), respectively. This transformation of nanostructures was explained by anisotropic surface diffusion coupled with the strain relieving Stranski–Krastanov growth mode, while the anisotropy was modulated by increasing As4 flux and subsequently enhanced by multilayer-stacking growth with a suitable spacer thickness. Photoluminescence characteristics show correlation to the nanostructure morphology for each sample. In particular, the formation of QD-chains and QWRs results in anisotropic features that offer potential device applications.

Impact of modal gain and waveguide design on two-state lasing in quantum well-dot lasers.

We study the current-controlled lasing switching from the ground state (GS) to the excited state (ES) transition in broad-area (stripe width 100 µm) InGaAs/GaAs quantum well-dot (QWD) and quantum well (QW) lasers. In the lasers with one QWD layer and a 0.45 µm-thick GaAs waveguide, pure GS lasing takes place up to an injection current as high as 8 A (40 kA/cm2). In contrast, in QW lasers with a similar design, ES lasing emerges already at 3 A (15 kA/cm2). The ES lasing in the QWD lasers is observed only in the devices with a waveguide thickness of 0.78 µm that supports a 2nd order transverse mode at the wavelength of the ES transition. Increasing the modal gain in the lasers with 0.78 µm-thick waveguide by using two QWD layers in the active region suppresses the ES lasing.

Effects of magnetic, electric, and non-resonant intense laser fields on exciton binding energy and exciton absorption in a symmetric Gaussian single quantum well

Effects of magnetic, electric, and non-resonant intense laser fields on exciton binding energy and exciton absorption in a symmetric Gaussian single quantum well

Quasi-bound state in the continuum enhancing background limited infrared detectivity

Quasi-bound states in the continuum (quasi-BIC) have tunable high Q factors and thus have many potential applications in optoelectronics. Here, we propose to construct a quasi-BIC with a perturbated elliptical-cylinder photonic crystal slab, and then develop a new way based on the quasi-BIC to address the background noise problem of infrared detectors. By integrating the photonic crystal slab with a quantum well infrared photodetector (QWIP), and by pushing the quasi-BIC system with absorption to the critical coupling state, an ultra-narrow photoresponse band (Q factor equal to 1150) with a high peak quantum efficiency (over 60%) is achieved in the long-wavelength infrared range. The critical coupling state is realized by matching the radiation Q factor, which is mainly controlled by the quasi-BIC, to the absorption Q factor, which is mainly controlled by the doping of the quantum wells (QWs). This device rejects most background radiation and thus significantly suppresses the background noise, resulting in a background limited specific detectivity (DBLIP∗) 11.85 times that of a conventional ideal photoconductor. A detailed formula is employed for calculation of DBLIP∗, incorporating the wavelength and incident angle dependence of the quantum efficiency. The ultra-narrow photoresponse band with a high peak responsivity can be tuned over the photosensitive wavelength range of the QWs by simply modifying the in-plane structural parameters. This work opens a new avenue to greatly enhance the specific detectivity for infrared detectors based on the joint effect of quasi-BIC and critical coupling.

Saturation Mobility of 100 cm2 V-1 s-1 in ZnO Thin-Film Transistors through Quantum Confinement by a Nanoscale In2O3 Interlayer Using Spray Pyrolysis.

In this study, we present a comprehensive study on the fabrication and characterization of heterojunction In2O3/ZnO thin-film transistors (TFTs) aimed at exploiting the quantum confinement effect to enhance device performance. By systematically optimizing the thickness of the crystalline In2O3 (c-In2O3) layer to create a narrow quantum well, we observed a significant increase in saturation mobility (μSAT) from 12.76 to 97.37 cm2 V-1 s-1. This enhancement, attributed to quantum confinement, was achieved through the deposition of a 3 nm c-In2O3 semiconductor via spray pyrolysis. Various In2O3 layer thicknesses (2-5 nm) were obtained by adjusting precursor solution concentration, flow rate, and number of spray cycles. Post annealing treatments were employed to reduce the defects at the interface and within the oxide film, enhancing device stability and performance. Transmission electron microscopy (TEM) confirmed the uniformity of the c-In2O3 film thickness, while variations in thickness significantly influenced TFT performance, particularly the turn-on voltage (VGS) due to changes in the carrier concentration. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) supported the formation of a potential well with a two-dimensional electron gas (2DEG). The study of single and multiple superlattice structures of consecutive c-In2O3 and c-ZnO layers provided insights into the effects of multiple quantum wells on the TFT performance. This research presents an advanced approach to TFT optimization, highlighting high reliability, and environmental and bias stabilities. These lead to enhanced mobility and performance uniformity through the precise control of c-In2O3 layer thickness for the quantum confinement effect.

Dimensional downscaling and quantum engineering: A path to high-performance micro-LEDs

The investigation of the optoelectronic performance of AlGaN-based ultraviolet-C (UV-C) micro light-emitting diodes (μLEDs) emitting at 273 nm is carried out numerically by reducing the chip area from large LED (300 × 300 μm2) to μLED (25 × 25 μm2). However, due to the high surface to volume ratio of μLED, surface recombination becomes dominant that is generated due to robust sidewall defects. The enhanced current spreading in μLED further affects the carrier injection in the active region as the electrons and holes are captured by sidewall defects. These effects are more dominant at low current density in μLED while at high current density, the sidewall defects get saturated, and the surface recombination weakens. Various optimization strategies, such as quantum wells (QWs) width, quantum barriers (QBs) width, and QW number are carried out to study the effect on the performance of 25 × 25 μm2 UV-C μLED. These optimization strategies at low current density (0.1 A/cm2) further improved the electrical/optical properties of AlGaN-based UV-C μLEDs.

Degradation mechanism during catastrophic optical damage in 385 nm GaN-based ultraviolet laser diodes

The failure mechanism of 385 nm GaN-based ultraviolet (UV) laser diodes (LDs) has been investigated. The degradation factors were studied by analyzing scanning electron microscopy (SEM), cathodoluminescence, and transmission electron microscopy (TEM) after aging. In degraded UV LD, degradation is easily observed on the anterior cavity facet, with holes found in the epitaxial GaN material, primarily concentrated in the quantum wells and waveguide regions of the LDs. This shows that the degradation may be closely related to the higher photon energy of the UV laser beam emitted by UV LDs. In the aging process of samples, emission of UV lasers leads to deterioration at the interface and in the semiconductor. This degradation leads to continuous heat accumulation and may create a positive feedback loop. Ultimately, this results in the failure of the UV LD. This study presents a possible major factor for the shorter lifetime of UV LD compared to blue LD. Therefore, it is very significant to improve the interface quality for extending the lifetime of UV LDs.

Modeling of planar germanium hole qubits in electric and magnetic fields

Hole-based spin qubits in strained planar germanium quantum wells have received considerable attention due to their favorable properties and remarkable experimental progress. The sizeable spin-orbit interaction in this structure allows for efficient qubit operations with electric fields. However, it also couples the qubit to electrical noise. In this work, we perform simulations of a heterostructure hosting these hole spin qubits. We solve the effective mass equations for a realistic heterostructure, provide a set of analytical basis wavefunctions, and compute the effective g-factor of the heavy-hole ground state. Our investigations reveal a strong impact of highly excited light-hole states located outside the quantum well on the g-factor. We find that sweet spots, points of operations that are least susceptible to charge noise, for out-of-plane magnetic fields are shifted to impractically large electric fields. However, for magnetic fields close to in-plane alignment, partial sweet spots at low electric fields are recovered. Furthermore, sweet spots with respect to multiple fluctuating charge traps can be found under certain circumstances for different magnetic field alignments. This work will be helpful in understanding and improving the coherence of germanium hole spin qubits.

Novel 2D/3D Heterojunction for UV Light‐Emitting Diodes Using Hexagonal Boron Nitride as Hole Injection Layer

The AlGaN materials system has been extensively studied in order to improve the efficiency of UV‐B and UV‐C light‐emitting diodes (LEDs). While progress has been made, significant challenges remain at shorter wavelengths. Most notably, increased Al composition for shorter‐wavelength operation results in increased activation energy of Mg dopants, resulting in low p‐doping. Although p‐doped h‐BN, with a bandgap of 5.9 eV, has been proposed as a potential replacement of p‐doped AlGaN, there have not been demonstrations of LEDs fabricated from p‐doped h‐BN/AlGaN heterostructures. Such unique heterostructures combine 2D p‐doped h‐BN materials with 3D AlGaN materials. Herein, fabrication and characterization of p‐doped h‐BN/AlGaN multiple quantum wells (MQWs)/n‐AlGaN LEDs, demonstrating emission of light at 290 nm corresponding to the AlGaN MQWs, with weaker emission at 262 nm corresponding to the AlGaN barrier, are reported. These results conclusively show hole injection through p‐doped h‐BN into AlGaN and provide a proof of concept that p‐doped h‐BN can be an alternative hole injection layer for UV LEDs.

Dirac fermions and spin transport in the SrVO3/SrTiO3 quantum well

The type-II Dirac fermions are characterized by a tilted Dirac cone and sustain exotic quantum transport phenomena. Here, we show the emergence of a type-II Dirac nodal loop in the SrVO3/SrTiO3 quantum well structure by density functional theory calculations and symmetry arguments. When the spin–orbit coupling (SOC) is neglected, the unavoidable crossing between two t2g bands gives rise to a nodal loop in the Brillouin zone, which is protected by the mirror symmetry. Such a nodal loop is fully gapped by including SOC, which results in a large spin Hall effect due to large spin Berry curvatures. Our findings add the unexplored functionality to oxide quantum wells and offer a practical platform to explore the interplay between topological fermions and spin transport phenomena.