InGaN Multiple Quantum Research Articles

Selective area grown photonic integrated chips for completely suppressing the Stokes shift

In this work, we report on the selective area growth (SAG) of InGaN multiple quantum well (MQW) structures to completely suppress the phenomenon of the Stokes shift in monolithically integrated photonic chips. The original green MQW region is designed as the integrated photodetector (PD), while the SAG blue MQW region acts as the integrated light-emitting diode (LED). The detection spectra of the PD can completely cover the emission spectra of the LED, greatly improving the on-chip optical connection by the complete suppression of the Stokes shift. Thus, the bottleneck of on-chip optical connection based on spectra-tail overlap in integrated photonic chips has been broken. Under the same operating current, the photocurrent of the SAG integrated PD reaches 11.8 μA, while the conventional chip achieves only 0.6 μA. By SAG method, the photo-to-dark current ratio of integrated PD exhibits about two orders of magnitude increase under 0 V bias. Undoubtedly, the SAG technology provides a strategy to further improve the on-chip optical signal transmission efficiency of the MQW structure integrated photonic chips.

InGaN multiple quantum well based light-emitting diodes with indium composition gradient InGaN quantum barriers

InGaN multiple quantum well based light-emitting diodes with indium composition gradient InGaN quantum barriers

Design and Simulation of InGaN-Based Red Vertical-Cavity Surface-Emitting Lasers.

We propose a highly polarized vertical-cavity surface-emitting laser (VCSEL) consisting of staggered InGaN multiple quantum wells (MQWs), with the resonance cavity and polarization enabled by a bottom nanoporous (NP) n-GaN distributed Bragg reflectors (DBRs), and top TiO2 high-index contrast gratings (HCGs). Optoelectronic simulations of the 612 nm VCSEL were systematically and numerically investigated. First, we investigated the influences of the NP DBR and HCG geometries on the optical reflectivity. Our results indicate that when there are more than 17 pairs of NP GaN DBRs with 60% air voids, the reflectance can be higher than 99.7%. Furthermore, the zeroth-order reflectivity decreases rapidly when the HCG's period exceeds 518 nm. The optimal ratios of width-to-period (52.86 ± 1.5%) and height-to-period (35.35 ± 0.14%) were identified. The staggered MQW design also resulted in a relatively small blue shift of 5.44 nm in the emission wavelength under a high driving current. Lastly, we investigated the cavity mode wavelength and optical threshold gain of the VCSEL with a finite size of HCG. A large threshold gain difference of approximately 67.4-74% between the 0th and 1st order transverse modes can be obtained. The simulation results in this work provide a guideline for designing red VCSELs with high brightness and efficiency.

Porous pseudo-substrates for InGaN quantum well growth: Morphology, structure, and strain relaxation

Strain-related piezoelectric polarization is detrimental to the radiative recombination efficiency for InGaN-based long wavelength micro-LEDs. In this paper, partial strain relaxation of InGaN multiple quantum wells (MQWs) on the wafer scale has been demonstrated by adopting a partially relaxed InGaN superlattice (SL) as the pseudo-substrate. Such a pseudo-substrate was obtained through an electro-chemical etching method, in which a sub-surface InGaN/InGaN superlattice was etched via threading dislocations acting as etching channels. The degree of strain relaxation in MQWs was studied by x-ray reciprocal space mapping, which shows an increase of the in-plane lattice constant with the increase of etching voltage used in fabricating the pseudo-substrate. The reduced strain in the InGaN SL pseudo-substrate was demonstrated to be transferable to InGaN MQWs grown on top of it, and the engineering of the degree of strain relaxation via porosification was achieved. The highest relaxation degree of 44.7% was achieved in the sample with the porous InGaN SL template etched under the highest etching voltage. Morphological and structural properties of partially relaxed InGaN MQWs samples were investigated with the combination of atomic force and transmission electron microscopy. The increased porosity of the InGaN SL template and the newly formed small V-pits during QW growth are suggested as possible origins for the increased strain relaxation of InGaN MQWs.

Surface lattice resonances for beaming and outcoupling green μ LEDs emission

Abstract Light-Emitting Diodes (LEDs) exhibit a typical Lambertian emission, raising the need for secondary optics to tailor their emission depending on specific applications. Here, we introduce plasmonic metasurfaces to InGaN green emitting quantum wells for LEDs to control their far-field emission directionality and enhance the collection efficiency. The proposed mechanism is based on surface lattice resonances (SLRs) and relies on the near-field coupling between the InGaN multiple quantum wells (MQWs) and periodic arrays of aluminum (Al) nanodisks. Fourier microscopy measurements reveal that the angular photoluminescence emission pattern depends on the lattice constant of the metasurfaces. We demonstrate that integrating Al metasurfaces in LED wafers can enhance the collected outcoupled light intensity by a factor of 5 compared to the same sample without metasurfaces. We have also performed numerical calculations of the far-field emission based on the reciprocity principle and obtained a very good agreement with the experimental data. The proposed approach controls the emission directionality without the need for secondary optics and it does not require post-etching of the GaN, which makes it a potential candidate to control and enhance the generated light from micro-LEDs.

Growth of (101¯1) Semipolar GaN‐Based Light‐Emitting Diode Structures on Silicon‐on‐Insulator

The growth and characterization of semipolar GaN buffer, InGaN multiple quantum wells (MQWs), and light‐emitting diode (LED) structure on patterned silicon‐on‐insulator (SOI) substrates, implementing the aspect ratio technique (ART), are reported. The early growth stages of GaN result in continuous and uniform stripes with small height variations that cause the formation of chevrons. Three coalescence strategies are tested to improve surface morphology and optical quality. Scanning electron microscopy identifies no crack formation but undulations of the surface. A roughness of ≈10 nm is measured by atomic force microscopy on large areas. The impact of MQW growth temperature shows similar surface morphology in terms of undulations and roughness. Room temperature photoluminescence spectra show wavelength emission redshifting when decreasing the MQW growth temperature. Room‐temperature cathodoluminescence (CL) highlights first the presence of threading dislocations (TDs) in between the coalescence boundary despite the use of the ART technique. Second, CL shows a spatially homogeneous emission wavelength of around 485 nm only perturbed by lower‐wavelength emission (455 nm) arising from the chevrons. Blue LED structures exhibit uniform emission wavelength at 450 nm, having a crack‐free surface, and roughness of ≈5 nm. These results pave the way for the fabrication of semipolar micro‐LEDs on SOI substrates.

Effects of GaN cap layer thickness on photoexcited carrier density in green luminescent InGaN multiple quantum wells

The effects of GaN cap layers on the optical properties of green luminescent InGaN-based multiple quantum wells were studied by photoluminescence (PL) spectroscopy. The PL peak energy under the selective excitation of the InGaN well layers was lower than that under the band-to-band excitation of the GaN barrier layers. The difference in the PL peak energies between the selective and band-to-band excitations decreased as the cap layer thickness increased, indicating an increase in the nonradiative recombination of photogenerated carriers in the barrier layers. Moreover, the internal quantum efficiency under selective excitation decreased as the cap layer thickness increased because of the increase in the internal electric field strength.

Luminescence and scintillation properties of the Si doped InGaN/GaN multiple quantum wells

The InGaN multiple quantum wells (MQW) samples with the undoped and Si doped GaN barriers were grown by Metal Organic Vapour Phase Epitaxy (MOVPE). By comparing defects-related emission bands in the undoped GaN and InGaN layers, one may conclude that the band is complex in the InGaN layer, composed of at least two contributions peaking at 2.17 and 2.39 eV, respectively. In and Si affect the intensity of the defects-related band – the larger the In and/or Si concentration the stronger the band. The detailed investigation of the observed phenomena was conducted, and the observed peculiarities were explained.

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.

Role of p-GaN layer thickness in the degradation of InGaN-GaN MQW solar cells under 405 nm laser excitation

GaN-based solar cells with InGaN multiple quantum wells (MQWs) are promising devices for application in space environment, concentrator solar systems, wireless power transmission and multi-junction solar cells. It is therefore important to understand their degradation kinetics when submitted to high-temperature and high-intensity stress. We submitted three samples of GaN-InGaN MQW solar cells with p-AlGaN electron-blocking-layer with different thickness of the p-GaN layer to constant power stress at 310 W/cm2, 175 °C for several hundred hours. The main degradation modes are a reduction of open-circuit voltage, short-circuit current, external quantum efficiency, power conversion efficiency and electroluminescence. In particular, we observed that a thinner p-GaN layer results in a stronger degradation observed on the cell operating parameters. The analysis of the dark I-V characteristics showed an increase in low-forward bias current and the analysis of electroluminescence showed a decrease in the electroluminescene emitted by the (forward biased) cell, as a consequence of stress. This work highlights that the cause of degradation is possibly related to a diffusion mechanism, which results in an increase of defect density in the active region. The impurities involved in the diffusion processes possibly originate from the p-side of the devices, therefore a thicker p-GaN layer reduces the amount of defects reaching the active region.

Effect of Hydrogen Treatment on Photoluminescence and Morphology of InGaN Multiple Quantum Wells.

In this paper, the photoluminescence (PL) properties and surface morphology of InGaN/GaN multiple quantum well (MQW) structures with the hydrogen (H2) heat treatment of InGaN are investigated to elucidate the effect of hydrogen on the structure and surface of the MQWs. The experimental results show that the H2 heat treatment on the as-grown MQWs may lead to the decomposition of InGaN and the formation of inhomogeneous In clusters. The atomic force microscope (AFM) study indicates that although the surface roughness of the uncapped samples increases after H2 treatment, the V-defects are suppressed. Moreover, the luminescence efficiency of the MQWs can be effectively improved by growing a GaN cap layer with an appropriate thickness on the top of the MQWs, which can reduce the effects of the H2 atmosphere and high temperature on the MQWs. In addition, a morphologic transformation from step bunching to shallow steps occurs and a much smoother surface can be obtained when a thicker cap layer is adopted.

Enhanced Resonant Energy Transfer by Decorating Au Nanoparticles on the Sidewalls of InGaN Multiple‐Quantum‐Well Nanorods

An enhanced resonant energy transfer (RET) process between green InGaN multiple quantum wells (MQWs) and CdSe/ZnS quantum dots (QDs) is investigated by decorating the sidewalls of InGaN MQW nanorods with Au nanoparticles. The InGaN MQW nanorods are shallow‐etched to the region of InGaN MQWs or deep‐etched through InGaN MQWs completely. The Au nanoparticles can provide localized surface plasmon coupling to both InGaN MQWs and QDs, leading to accelerated decay rates. The localized surface plasmons also enhance the RET process between InGaN MQWs and QDs. Compared to the bare nanorods without Au nanoparticles, the enhancement factors of the RET process are 1.8 and 5.6 for shallow‐ and deep‐etched MQW nanorods, respectively. The larger enhancement factor for deep‐etched nanorods is mainly attributed to the weaker localized surface plasmon coupling with InGaN MQWs, which is proposed to compete with the RET process. The RET efficiency in the deep‐etched InGaN MQW nanorods is calculated as 62%, demonstrating high potential for usage in energy conversion devices.

Effect of High Temperature Treatment on the Photoluminescence of InGaN Multiple Quantum Wells

In this work, the photoluminescence (PL) properties of three as-grown InGaN/GaN multiple quantum well (MQW) structures which are heat-treated under different temperatures with nitrogen (N2) atmosphere are investigated. Temperature-dependent photoluminescence (PL) analysis was used to characterize the depth of localized states and defect density formed in MQWs. By fitting the positions of luminescence peaks with an LSE model, we find that deeper localized states are formed in the MQWs after high-temperature treatment. The experimental results show that the luminescence intensity of the sample heat-treated at 880 °C is significantly improved, which may be due to the shielding effect of In clusters on defects. While the luminescence efficiency decreases because of the higher defect density caused by the decomposition of the InGaN QW layer when the sample is heat-treated at 1020 °C. Moreover, the atomic force microscope results show that the increase in heat-treatment temperature leads to an increase in the width of surface steps due to the rearrangement of surface atoms in a high-temperature environment.

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of GaN/InGaN asymmetric lower waveguide (LWG) layers on photoelectrical properties of InGaN multiple quantum well laser diodes (LDs) with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of InGaN insertion layer (InGaN-IL) between the GaN lower waveguide layer and the quantum wells, which is achieved with the Crosslight Device Simulation Software (PIC3D, Crosslight Software Inc.). The optimal thickness and the indium content of the InGaN-IL in lower waveguide layers are found to be 300 nm and 4%, respectively. The thickness of InGaN-IL predominantly affects the output power and the optical field distribution in comparison with the indium content, and the highest output power is achieved to be 1.25 times that of the reference structure (symmetric GaN waveguide), which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells. Furthermore, when the thickness and indium content of InGaN-IL both reach a higher level, the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor (OCF) related to the concentrated optical field in the lower waveguide.

Emission color modulation of InGaN/GaN multiple quantum wells by selective area metalorganic vapor phase epitaxy on hexagonal windows

We investigated influences of mask pattern on the emission from InGaN multiple quantum wells through the differences in plane orientation appearing on the multifaceted islands in selective area metalorganic vapor phase epitaxy. Cathodoluminescence mapping confirmed that emission colors changed depending on the crystal plane. Photoluminescence spectroscopy showed that the emission wavelength red-shifted by increasing the mask width. By combining the difference of indium incorporation efficiency depending on the crystal plane and the lateral vapor phase diffusion effect, multiple quantum wells with different emission wavelengths of up to 106 nm were grown simultaneously in the microscale region.

Simultaneous transmission, detection, and energy harvesting.

Due to the electro-optic property of InGaN multiple quantum wells, a III-nitride diode can provide light transmission, photo detection, and energy harvesting under different bias conditions. Made of III-nitride diodes arrayed in a single chip, the combination allows the diodes to transmit, detect, and harvest visible light at the same time. Here, we monolithically integrate a III-nitride transmitter, receiver, and energy harvester using a compatible foundry process. By adopting a bottom SiO2/TiO2 distributed Bragg reflector, we present a III-nitride diode with a peak external quantum efficiency of 50.65% at a forward voltage of 2.6 V for light emission, a power conversion efficiency of 6.68% for energy harvesting, and a peak external quantum efficiency of 50.9% at a wavelength of 388 nm for photon detection. The energy harvester generates electricity from ambient light to directly turn the transmitter on. By integrating a circuit, the electrical signals generated by the receiver pulse the emitted light to relay information. The multifunctioning system can continuously operate without an external power supply. Our work opens up a promising approach to develop multicomponent systems with new interactive functions and multitasking devices, due to III-nitride diode arrays that can simultaneously transmit, detect, and harvest light.

Effect of High Temperature Treatment on the Photoluminescence of InGaN Multiple Quantum Wells

Effect of High Temperature Treatment on the Photoluminescence of InGaN Multiple Quantum Wells

Comparison of growth interruption and temperature variation impact on emission efficiency in blue InGaN/GaN MQWs

Pulsed growth, i.e. the MOCVD growth method, when the metal precursors are delivered with interruptions, allows increasing the internal quantum efficiency (IQE) of InGaN multiple quantum well (MQW) structures. On the other hand, similar results are achieved by increasing the growth temperature. In this work, we analyze the physical mechanisms behind the IQE improvement in differently grown QWs at low and high carrier densities. At low excitations, the In0.1Ga0.9N/GaN MQW structures grown at 800 and 815 °C exhibited the PL efficiency of 4% and 6%, respectively. At high excitations, the benefit of higher growth temperature disappeared: peak IQE at the droop onset was ~34% in all structures. Moreover, the structure grown at 815 °C had lower than intended indium content (9% instead of 10%) and QW width (3.5 nm instead of 4.0 nm). The pulse-grown structures, while very sensitive to interruption duration, provided peak IQE up to 40% without decline in structural quality. From the provided photoluminescence, carrier lifetime and diffusion coefficient data we speculate that higher growth temperature leads to lower point defect density, which increases IQE at low excitations, but becomes saturated at high excitations. Meanwhile, the pulsed growth mainly affects the localizing potential, which governs carrier recombination pathways.

Modeling and analysis of the effects of inhomogeneous carrier distributions in InGaN multiple quantum wells

It is well known that carrier distribution in InGaN multiple quantum wells (MQWs) can be significantly inhomogeneous. However, the conventional ABC recombination model assumes that carriers are uniformly distributed throughout the MQW. In this paper, a modified ABC model that considers the unequal carrier density in the QWs was developed. From the analysis of the developed ABC model, the effective recombination coefficients and modified internal quantum efficiency (IQE) were obtained for an arbitrary carrier distribution in MQWs. The efficiency droop was found to be aggravated as the carrier distribution was increasingly inhomogeneous. However, it was also found that the effect of inhomogeneous carrier distribution alone was not sufficient to explain the IQE droop with the theoretical Auger recombination coefficient based on indirect Auger processes. The developed ABC model is expected to provide insight into the influence of inhomogeneous carrier distributions in MQWs on the efficiency droop in GaN-based light-emitting diodes.

Design and growth of GaN-based blue and green laser diodes

GaN-based laser diodes (LDs) extend the wavelength of semiconductor LDs into the visible and ultraviolet spectrum ranges, and are therefore expected to be widely used in quantum technology, bio & medical instruments, laser displays, lighting and materials processing. The development of blue and green LDs is still challenging, even though they are based on the same III-nitride materials as GaN-based light-emitting diodes. The challenges and progress of GaN-based blue and green LDs are reviewed from the aspects of epitaxial growth and layer structure design. Due to large differences in lattice constants and growth conditions for InN, GaN, and AlN, considerable effort is required to improve the quality of InGaN multiple quantum well (MQW) gain medium for blue and especially green LDs. p-type doping profiles, conditions and layer structures are critical to reduce the internal losses and to mitigate the degradation of InGaN MQWs. Hole injection is also a key issue for GaN-based LDs.