InGaN Quantum Wells Research Articles

Theoretical and Experimental Studies on Material Gain for Wide Polar InGaN Quantum Well‐Mechanism Leading to Electric Field Screening and Lasing

AbstractThe width of polar InGaN quantum wells (QWs) is usually limited to a few nanometers to ensure a sufficient overlap between the wave functions of the ground electron and hole state. This paper presents the results of material gain measurements for thin (2.6 nm), wide (10.4 nm), and extremely wide (25 nm) InGaN QWs. The comparison of experimental data with theoretical predictions allows for correlation of the current density with the carrier concentration, which contributes to the positive material gain, and determination of the coefficients in the standard ABC model. It is shown that the mechanism in wide and narrow polar QWs that leads to electric field screening and lasing is different. For wide QWs, the optical transitions between excited states become dominant in the gain spectrum. The reason for such a gain mechanism is the built‐in electric field, which must first be screened. The ground electron and hole states play a very important role in the screening; however, their contribution to the material gain and lasing in wide QWs is very weak.

Design of InGaN-ZnSnGa2N4 quantum wells for high-efficiency amber light emitting diodes

A novel type-II InGaN-ZnSnGa2N4 quantum well (QW) structure is proposed based on recent experimental achievements for the successful epitaxy of ZnSnN2-GaN alloys and the determination of their band offsets with GaN. The simulation results indicate that this structure is promising as the active region for high-efficiency InGaN-based amber (λ ∼ 590 nm) light-emitting diodes (LEDs). The hole wavefunction in the valence band is better confined with the insertion of a monolayer scale of ZnSnGa2N4 into the InGaN QW while the electron wavefunction in the conduction band is better confined with the incorporation of an AlGaN layer in the GaN quantum barrier. The band structure of the InGaN-ZnSnGa2N4 QW is numerically simulated based on the experimentally measured band offsets between ZnSnGa2N4 and GaN. With the InGaN-ZnSnGa2N4 QW design, a low In content (20%) is required in the InGaN layer to reach a peak emission wavelength of ∼590 nm, yet an In composition of 25% is needed to reach the same emission wavelength for a conventional InGaN QW with the same layer thicknesses. Moreover, the electron-hole wavefunction overlap (Гe1−hh1) for the InGaN-ZnSnGa2N4 QW design reaches 18% for an emission wavelength at ∼590 nm. This result is much improved over the conventional InGaN QW overlap of 5% emitting at the same wavelength. The increase in electron-hole wavefunction overlap results in an approximately 14 times enhancement in the predicted spontaneous emission radiative recombination rate of the InGaN-ZnSnGa2N4 QW as compared to that of the conventional InGaN QW. This InGaN-ZnSnGa2N4 QW structure design can be promising to pave a new way to achieve high efficiency amber LEDs.

Unveiling light collection and pump enhancement from quantum wells with plasmonic metasurfaces using power dependent measurements

Low light extraction efficiency (LEE) is the greatest limiting factor for the brightness of reduced-size light-emitting diodes (LEDs) as it limits their emission intensity. In addition, LEDs have a Lambertian emission, which requires secondary optics to control the emission directionality. Plasmonic metasurfaces can introduce a way of manipulating the generated light from LEDs to enhance their LEE and steer the emitted light by reshaping the far-field emission. Here, we fabricate resonant plasmonic metasurfaces on top of a typical blue emitting wafer consisting of InGaN/gallium nitride quantum wells developed for commercial LED devices. The metasurface is separated from the InGaN quantum wells by p-GaN and indium-tin-oxide (ITO) layers with a cumulative thickness of 110 nm. Since this distance value is close to the emission wavelength in the corresponding medium, enhanced near-fields of localized plasmonic resonances do not reach the active region. Despite this, we observe a strong influence of the metasurfaces on the far-field photoluminescence emission from the quantum wells as demonstrated by Fourier imaging. Power-dependent excitation measurements of the samples allow us to retrieve the pump and light collection enhancement factors provided by the plasmonic metasurfaces. We demonstrate that the plasmonic metasurfaces can provide a pump enhancement factor of up to 4.1 and a collection enhancement factor of up to 3.2. We have also performed simulations based on the reciprocity principle and achieved a good qualitative agreement with the experimental results.

Impact of nanoscale fluctuations and cap-layer thickness in buried InGaN single quantum wells probed by tip-enhanced Raman scattering

Ternary semiconductors such as InGaN thin films, quantum wells, and superlattices commonly exhibit alloy fluctuations that become increasingly pronounced with higher In-content. The thickness fluctuations of quantum wells and their thin cap-layers further introduce nanoscale inhomogeneities that alter the potential landscape. In this work, we present a combined theoretical and experimental study of InGaN single quantum wells with thin GaN cap-layers to unravel the influence of cap-layer thickness, compositional inhomogeneity, and thickness fluctuations on their electronic and optical properties. A pronounced spectral shift of quantum well emission for thin cap-layers between 1 and 10 nm is observed by micro-photoluminescence spectroscopy. The origin of this shift is explained by calculations of electronic band profiles and probability density overlap of carriers in the quantum well. The impact of alloy fluctuations and homogeneity for different cap-layer thicknesses is studied on both the microscale and nanoscale using UV micro-Raman scattering and tip-enhanced Raman spectroscopy (TERS). On the microscale, the alloy composition as determined by micro-Raman mapping appears very homogeneous except for the thinnest 1 nm cap-layer where small fluctuations are visible. On the nanoscale, TERS reveals local fluctuations on a 20–30 nm length scale. The influence of the cap-layer thickness on the TERS spectra is discussed regarding both the nanoscale homogeneity and the depth resolution of the near-field Raman scattering technique. Our results demonstrate the capabilities of TERS to resolve nanoscale thickness fluctuations and compositional inhomogeneities in ultra-thin semiconductor layers, even when they are buried by thin cap-layers with thicknesses below 10 nm.

The effect of ammonia partial pressure on the growth of semipolar (11–22) InGaN/GaN MQWs and LED structures

This paper presents the effect of ammonia partial pressure at a relatively low fixed V/III (∼765) growth atmosphere on the growth of multi-quantum well (MQW) structures and the light emitting diode (LED) emission wavelength. At fixed V/III ratio, ammonia partial pressure together with the consequent group III partial pressures are shown to influence the growth of InGaN/GaN MQWs in term of growth rate and the indium incorporation for the quantum well. It is also observed that the growth of InGaN quantum well (QW) behaves differently as compared to GaN quantum barrier (QB) at low V/III ambient. For the growth of InGaN QW, the data collected shows the existence of threshold point for ammonia partial pressure of ∼300 Pa. In the regime below the threshold value, the decomposition of InGaN due to severe dissociation of In-N bonds caused the growth rate to be extremely low. As the ammonia partial pressure increases above the threshold value, the growth rate of InGaN QW increases sharply together with an improvement in indium incorporation, as proven by the XRD and (11–24) asymmetric reciprocal space mapping (RSM) results. The XRD result is consistent with the atomic force microscopy (AFM) results. As the partial pressures increases, the RMS and peak-to-valley roughness increases which is credited to the increment in indium incorporation. Furthermore, “needle-like” structures can be observed over all samples when the MQWs were grown with limited ammonia partial pressure. During the growth for the LED structure with thin layers of InGaN/GaN, the partial pressure of ammonia became a dominating factor in the formation of well-abrupt layer interfaces. Through the XRD and room-temperature photoluminescence (RTPL) results, it is shown that abrupt layers can be formed at higher ammonia partial pressures, while indium clusters tend to form at a lower partial pressure, which are most probably due to the insufficient N concentration, while the electroluminescence (EL) results were consistent as well. Therefore, it is once again confirmed that the performance of the InGaN/GaN LED is highly governed by the partial pressures of the ammonia.

Fabrication and optical characterization of GaN micro-disk cavities undercut by laser-assisted photo-electrochemical etching

GaN micro-disk cavities undercut by laser-assisted photo-electrochemical (PEC) etching are fabricated and optically characterized. The laser source used in the PEC etching is tuned to be absorbed by the InGaN/GaN superlattice beneath the GaN disk to selectively etch the superlattice. Whispering gallery modes (WGMs) in fabricated GaN micro-disk cavities are evaluated by micro-photoluminescence spectroscopy of light emission from the embedded InGaN quantum wells. Quality factors estimated for the WGMs reach approximately 6700 at blue-violet wavelengths. Detailed analysis suggests that the high-Q WGMs are the fundamental WGMs. These results indicate the high applicability of laser-assisted PEC etching to the fabrication of air-clad GaN micro-cavities.

Factors Affecting Terahertz Emission from InGaN Quantum Wells under Ultrafast Excitation

InGaN quantum wells (QWs) grown on c-plane sapphire substrate experience strain due to the lattice mismatch. The strain generates a strong piezoelectric field in QWs that contributes to THz emission under ultrafast excitation. Physical parameters such as QW width, period number, and Indium concentration can affect the strength of the piezoelectric field and result in THz emission. Experimental parameters such as pump fluence, laser energy, excitation power, pump polarization angle, and incident angle can be tuned to further optimize the THz emission. This review summarizes the effects of physical and experimental parameters of THz emission on InGaN QWs. Comparison and relationship between photoluminescence properties and THz emission in QWs are given, which further explains the origin of THz emission in InGaN QWs.

Highly Uniform Array of Hexagonally Symmetric Micro‐Pyramid Structures for Scalable and Single Quantum Dot Emitters

AbstractControlling the site, size, and shape of group III‐nitride quantum dots (QDs) is critical for the development of mass‐producible single‐photon sources for scalable quantum technologies operable at room temperature. Herein, a methodology is proposed for fabricating high‐purity single QD emitters by controlling site‐controlled GaN micro‐pyramid structures with a high degree of uniformity and symmetry. To achieve a uniformly grown, hexagonally symmetric micro‐pyramid array, the H2/N2 carrier gas ratio, growth temperature, and V/III ratio are controlled to attain self‐limited growth regime and self‐limited width at the GaN pyramid apex. A thin InGaN layer is consecutively grown on a pyramid array under the growth condition for enhancing the growth rate anisotropy to hinder the growth of InGaN quantum wells (QWs) at semi‐polar facets. As a result, single‐photon emission is observed from apex QD with suppressed background side QW emission while maintaining more than 90% high hexagonal QD symmetry over the large area of the wafer.

Evidence for "dark charge" from photoluminescence measurements in wide InGaN quantum wells.

Wide (15-25 nm) InGaN/GaN quantum wells in LED structures were studied by time-resolved photoluminescence (PL) spectroscopy and compared with narrow (2.6 nm) wells in similar LED structures. Using below-barrier pulsed excitation in the microsecond range, we measured increase and decay of PL pulses. These pulses in wide wells at low-intensity excitation show very slow increase and fast decay. Moreover, the shape of the pulses changes when we vary the separation between them. None of these effects occurs for samples with narrow wells. The unusual properties of wide wells are attributed to the presence of "dark charge" i.e., electrons and holes in the ground states. Their wave functions are spatially separated and due to negligible overlap they do not contribute to emission. However, they screen the built-in field in the well very effectively so that excited states appear with significant overlap and give rise to PL. A simple model of recombination kinetics including "dark charge" explains the observations qualitatively.

InGaN-based LEDs on convex lens-shaped GaN arrays toward multiwavelength light emitters

InGaN-based LEDs on convex lens-shaped GaN microstructures are fabricated. The gently sloping microstructures enable us to employ simple device processes similar to those for conventional planar LEDs. The fabricated LEDs show a clear rectifying property and broadband electroluminescence spectra ranging from ∼380 to ∼500 nm due to the multiwavelength emission properties of the InGaN quantum wells. Furthermore, the peak wavelength continuously changes from ∼470 to ∼410 nm within a particular current range.

Efficiency enhancement mechanism of piezoelectric effect in long wavelength InGaN-based LED.

Improving the luminescence efficiency of InGaN-based long wavelength LEDs for use in micro-LED full-colour displays remains a huge challenge. The strain-induced piezoelectric effect is an effective measure for modulating the carrier redistribution at the InGaN/GaN heterointerfaces. Our theoretical results reveal that the hole injection is significantly improved by the diminution of the valence band offset (VBO) of the InGaN/GaN heterointerfaces along the [0001] direction, and inversely, the VBO increases along the [0001] direction. The energy band structures showed that the tensile strain of the GaN film grown on a silicon (Si) substrate could weaken the internal electric field of the InGaN well layer leading to a flattening of the energy band, which increases the overlap of electron and hole wave functions. In addition, the strain-induced piezoelectric polarisation of the InGaN layer on the Si substrate generates opposite sheet-bound charges at the heterointerfaces, which causes a reduction in the depletion region of the InGaN/GaN quantum wells (QWs). A systematic analysis illustrates that the control of the piezoelectric polarisation of the InGaN QW layer is available improve the internal quantum efficiency of the InGaN-based LEDs.

Investigation of InGaN-Based Green Micro-Photonic-Crystal- Light-Emitting-Diodes with Bottom, Nanoporous, Distributed Bragg Reflectors

In this work, an InGaN-based, green micro-photonic crystal-light-emitting-diode (µ-PCLED), which incorporates a nanoporous, GaN-distributed Bragg reflector (DBR) to form a Fabry–Perot (FP) cavity, was fabricated and characterized. Simulations for the µ-PCLED’s optical features were systematically performed and analyzed. Numerical results revealed that the p-GaN photonic crystal (PC) with a filling factor of 0.3 is beneficial for improving the coupling constants of the first- and second-order Bragg diffractions. In addition, based on the product of quantum well (QW) and PC confinement factors, four to six pairs of InGaN QWs should be the preferable design. In order to achieve single-wavelength emission and small full-width at half-maximum (FWHM), the thickness of the n-GaN layer was controlled to be thinner than 920 nm, leading to more than 20 nm wavelength separation between two adjacent FP modes. Experimentally, the fabricated InGaN-based µ-PCLED with a mesa diameter of 30 µm can emit 545 nm green light with FWHM of about 10 nm and negligible blue-shift of about 3 nm in spontaneous emission under the injection current of 1 to 10 mA. Our simulation and experimental results demonstrate that the p-GaN PC design can effectively resolve the wavelength instability issue.

Effect of the AlN strain compensation layer on InGaN quantum well red-light-emitting diodes beyond epitaxy.

An atomically thick AlN layer is typically used as the strain compensation layer (SCL) for InGaN-based-red light-emitting diodes (LEDs). However, its impacts beyond strain control have not been reported, despite its drastically different electronic properties. In this Letter, we describe the fabrication and characterization of InGaN-based red LEDs with a wavelength of 628 nm. A 1-nm AlN layer was inserted between the InGaN quantum well (QW) and the GaN quantum barrier (QB) as the SCL. The output power of the fabricated red LED is greater than 1 mW at 100 mA current, and its peak on-wafer wall plug efficiency (WPE) is approximately 0.3%. Based on the fabricated device, we then used numerical simulation to systematically study the effect of the AlN SCL on the LED emission wavelength and operating voltage. The results show that the AlN SCL enhances the quantum confinement and modulates the polarization charges, modifying the device band bending and the subband energy level in the InGaN QW. Thus, the insertion of the SCL considerably affects the emission wavelength, and the effect on the emission wavelength varies with the SCL thickness and the Ga content introduced into the SCL. In addition, the AlN SCL in this work reduces the LED operating voltage by modulating the polarization electric field and energy band, facilitating carrier transport. This implies that heterojunction polarization and band engineering is an approach that can be extended to optimize the LED operating voltage. We believe our study better identifies the role of the AlN SCL in InGaN-based red LEDs, promoting their development and commercialization.

Strain-engineered N-polar InGaN nanowires: towards high-efficiency red LEDs on the micrometer scale

The absence of efficient red-emitting micrometer-scale light emitting diodes (LEDs), i.e., LEDs with lateral dimensions of 1 μm or less is a major barrier to the adoption of microLEDs in virtual/augmented reality. The underlying challenges include the presence of extensive defects and dislocations for indium-rich InGaN quantum wells, strain-induced quantum-confined Stark effect, and etch-induced surface damage during the fabrication of quantum well microLEDs. Here, we demonstrate a new approach to achieve strong red emission ( > 620 nm ) from dislocation-free N-polar InGaN/GaN nanowires that included an InGaN/GaN short-period superlattice underneath the active region to relax strain and incorporate more indium within the InGaN dot active region. The resulting submicrometer-scale devices show red electroluminescence dominantly from an InGaN dot active region at low-to-moderate injection currents. A peak external quantum efficiency and a wall-plug efficiency of 2.2% and 1.7% were measured, respectively, which, to the best of our knowledge, are the highest values reported for a submicrometer-scale red LED. This study offers a new path to overcome the efficiency bottleneck of red-emitting microLEDs for a broad range of applications including mobile displays, wearable electronics, biomedical sensing, ultrahigh speed optical interconnect, and virtual/augmented reality.

Cubic GaN and InGaN/GaN quantum wells

LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material.

The direct evidence of the composition pulling effect and its role in InGaN multiple quantum wells

Direct evidence of composition pulling effect (CPE) in InGaN` quantum wells was obtained by combining energy dispersive x-ray spectrometry (EDS) and scanning tunneling electron microscope (STEM). The indium content increases along the c-axis in the InGaN quantum wells, forming an In-rich sublayer on the surface. The indium content in the In-rich sublayer will reach saturation earlier. After that, the incorporation of indium atoms is hindered, resulting in a decrease in the average indium composition in the InGaN quantum wells. Moreover, an aggregation behavior of indium atoms has been observed in samples with a stronger CPE. This leads to a lot of dark spot defects, seriously deteriorating the luminescence quality. By restricting the migration ability of indium atoms, dark spot defects could be greatly suppressed, and a higher quality InGaN/GaN MQW with higher indium content was obtained.

InGaN micro-light-emitting diodes monolithically grown on Si: achieving ultra-stable operation through polarization and strain engineering

Micro or submicron scale light-emitting diodes (µLEDs) have been extensively studied recently as the next-generation display technology. It is desired that µLEDs exhibit high stability and efficiency, submicron pixel size, and potential monolithic integration with Si-based complementary metal-oxide-semiconductor (CMOS) electronics. Achieving such µLEDs, however, has remained a daunting challenge. The polar nature of III-nitrides causes severe wavelength/color instability with varying carrier concentrations in the active region. The etching-induced surface damages and poor material quality of high indium composition InGaN quantum wells (QWs) severely deteriorate the performance of µLEDs, particularly those emitting in the green/red wavelength. Here we report, for the first time, µLEDs grown directly on Si with submicron lateral dimensions. The µLEDs feature ultra-stable, bright green emission with negligible quantum-confined Stark effect (QCSE). Detailed elemental mapping and numerical calculations show that the QCSE is screened by introducing polarization doping in the active region, which consists of InGaN/AlGaN QWs surrounded by an AlGaN/GaN shell with a negative Al composition gradient along the c-axis. In comparison with conventional GaN barriers, AlGaN barriers are shown to effectively compensate for the tensile strain within the active region, which significantly reduces the strain distribution and results in enhanced indium incorporation without compromising the material quality. This study provides new insights and a viable path for the design, fabrication, and integration of high-performance µLEDs on Si for a broad range of applications in on-chip optical communication and emerging augmented reality/mixed reality devices, and so on.

Excitons in a disordered medium: A numerical study in InGaN quantum wells

Excitons in InGaN quantum wells are investigated numerically, considering random alloy disorder and Coulomb interaction on equal footing in the Schr\"odinger equation. Their statistical properties are systemically explored as a function of the quantum well thickness and composition, revealing a complex competition between disorder-induced carrier localization, Coulomb attraction, and field-induced wave function separation. This results in a class of semiconductor quasiparticle with hybrid properties in between hydrogenoid excitons and disorder-localized free particles. Exciton screening by free carriers is investigated and shows distinct behavior from the screening of bulk excitons. Finally, a highly accurate approximate solution of the excitonic Schr\"odinger equation, with reduced numerical complexity, is introduced.

Multiwavelength-emitting InGaN quantum wells on convex-lens-shaped GaN microstructures

We fabricated InGaN quantum wells on GaN microlens structures by employing a thermal reflow method. The peak emission wavelengths shift from ∼490 nm on the top of the microstructure to ∼400 nm on the bottom. This variation is attributed to the In composition distribution caused by the continuously changing off-angles from the (0001) plane. The obtained results suggest that not only stable planes exposed in microstructures by selective area growth technique but also unstable planes can be useful for InGaN-based microstructures with multiwavelength emission properties.

Atomic Diffusion of Indium through Threading Dislocations in InGaN Quantum Wells.

The compositional and structural investigations of threading dislocations (TDs) in InGaN/GaN multiple quantum wells were carried out using correlative transmission electron microscopy (TEM) and atom probe tomography (APT). The correlative TEM/APT analysis on the same TD reveals that the indium atoms are diffused along the TD and its concentration decreases with distance from the InGaN layer. On the basis of the results, we directly observed that the indium atoms originating from the InGaN layer diffuse toward the epitaxial GaN surface through the TD, and it is considered to have occurred via the pipe diffusion mechanism induced by strain energy relaxation.