Enhancement Of Internal Quantum Efficiency Research Articles

Broadband near infrared emission in Cr3+,Yb3+, Li+ tri-doped In2TeO6

Cr3+ near infrared (NIR) emission is witnessed in tellurite compound for the first time. Broadband emission that peaks at 840 nm and has 157 nm of full width at half maxima (FWHM) appears in the Cr3+ doped In2TeO6 phosphor when excited at 455 nm. Further introduction of Li+ ions into the In2TeO6 can distort the octahedral environment of the doped Cr3+ ions, resulting in 4 times enhancement in luminescence intensity and a 1.75-fold enhancement in internal quantum efficiency (IQE). Moreover, the FWHM increases to 282 nm in the In2TeO6: Cr3+,Li+,Yb3+ phosphor induced by the Cr3+ to Yb3+ energy transfer. LED device utilizing this NIR phosphor has been constructed. At 150 mA drive current, the optoelectrical efficiency is 11.189 % and the NIR output power is 50.72 mW. The In2TeO6: Cr3+,Li+,Yb3+phosphor exhibits enormous potential in NIR phosphor-converted LED (pc-LED).

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

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

Mid-infrared photoluminescence revealing internal quantum efficiency enhancement of type-I and type-II InAs core/shell nanowires: publisher's note.

This publisher's note contains corrections to Opt. Lett.47, 5208 (2022)10.1364/OL.473154.

Mid-infrared photoluminescence revealing internal quantum efficiency enhancement of type-I and type-II InAs core/shell nanowires.

Internal quantum efficiency (IQE) is an important figure of merit for photoelectric applications. While the InAs core/shell (c/s) nanowire (NW) is a promising solution for efficient quantum emission, the relationship between the IQE and shell coating remains unclear. This Letter reports mid-infrared PL measurements on InAs/InGaAs, InAs/AlSb, and InAs/GaSb c/s NWs, together with bare InAs NWs as a reference. Analyses show that the IQE is depressed by a shell coating at 9 K but gets improved by up to approximately 50% for the InGaAs shell coating at 40-140 K and up to approximately 20% beyond 110 K for the AlSb shell. The effect is ascribed not only to the crystal quality but more importantly to the radial band alignment. The result indicates the high-temperature IQE improvement of the type-I and type-II c/s NWs and the appropriateness of the mid-infrared PL analyses for narrow-gap NW evaluation.

Highly Transparent p‐AlGaN‐Based (326–341 nm)‐Band Ultraviolet‐A Light‐Emitting Diodes on AlN Templates: Recent Advances and Perspectives

The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed.

Polarization Modulation at Last Quantum Barrier for High Efficiency AlGaN-Based UV LED

The performance of AlGaN-based light-emitting diodes (LEDs) emitting at UVA–UVC regions can be severely compromised due to the polarization difference (ΔP) between the last quantum barrier (LQB) and the electron blocking layer (EBL). In this work, the different situations of the bandgap difference (ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) and ΔP of InAlN/AlGaN and AlGaN/AlGaN heterojunctions fully strained on GaN and AlN substrates are discussed. It shows that the InAlN/AlGaN heterojunctions could produce positive or negative sheet charges at the heterointerface under ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> >0, which could not be realized by the conventional AlGaN/AlGaN heterojunctions. To demonstrate and utilize the feature, the polarization-modulated InAlN LQBs with 0.14–0.16 indium compositions of 320 nm UVB LEDs are designed and investigated. It is observed that the InAlN LQBs could replace the conventional AlGaN LQB to improve electron confinement and hole injection by affecting effective barrier heights. By modulating the LQB/EBL polarization using InAlN, the proposed UV LED has a 32% enhancement in internal quantum efficiency and lower efficiency droop (from 16.9% to 0.7%) compared with the conventional one without modulation. The operation voltage at the same current also significantly decreases. The improvement of optical output power and wall plug efficiency at 60 mA in proposed structures are near 90% and 100%, respectively. This study provides a novel and highly effective methodology for development of high efficiency UV LEDs.

One-Step Fabrication Method of GaN Films for Internal Quantum Efficiency Enhancement and Their Ultrafast Mechanism Investigation.

The third-generation semiconductors are the cornerstone of the power semiconductor leap forward and have attracted much attention because of their excellent properties and wide applications. Meanwhile, femtosecond laser processing as a convenient method further improves the performance of the related devices and expands the application prospect. In this work, an approximate 3 times improvement of the internal quantum efficiency (IQE) and a 5.5 times enhancement of the photoluminescence (PL) intensity were achieved in the GaN film prepared using a one-step femtosecond laser fabrication method. Three types of final micro/nanostructures were found with different femtosecond laser fluences, which could be attributed to the decomposition, melting, bubble nucleation, and phase explosion of GaN. The mechanisms of the microbump structure formation and enhancement of IQE were studied experimentally by the time-resolved reflection pump-probe technique, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Simulations for the laser-GaN interaction have also been performed to ascertain the micro/nanostructure formation principle. These results promote the potential applications of femtosecond lasers on GaN and other wide band gap semiconductors, such as UV-light-emitting diodes (LEDs), photodetectors, and random lasers for use in sensing and full-field imaging.

(Keynote) Dynamics of Electron Transfer in Enhanced Water Splitting Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) nanoparticles shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and semiconductor.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[7],[8] However, in general Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limits its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film (ATA) photoelectrode.[9],[10] Furthermore, the dynamics of the hot electron transfer under the modal strong coupling conditions are further studied by transient reflection spectroscopy.Modal strong coupling between the Fabry–Pérot nanocavity mode of the TiO2thin-film/Au-film and the LSPR mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this modal strong coupling regime, a key feature of the ATA photoelectrode is partially inlaying of Au-NPs into the TiO2 nanocavity by several nanometers. Under a three-electrode system measurement with a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and an electrolyte of KOH (0.1 mol/dm3), we demonstrated that the action spectrum of incident photon to current conversion efficiency (IPCE) exhibited two bands, which almost corresponds to the absorption spectrum of ATA. The IPCE of ATA is extraordinarily enhanced as compared to that of Au-NPs/TiO2photoanode. Most importantly, under the modal strong coupling conditions, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the wavelengths of strong coupling induced hybrid branches. The increase in IQE implies the possibility of increasing the generation of hot electrons in the modal strong coupling regime. The plasmon-induced water splitting using a two-electrode system was also quantitatively measured. To understand the IQE enhancement under the strong coupling conditions, the dynamics of the hot electrons transfer from Au-NPs to TiO2 were further explored by transient measurement using a visible pump/infrared probe (at 3500 nm) femtosecond transient reflection spectrometers. An ultrafast hot electrons injection from Au-NPs to TiO2 was observed. Interestingly, under the strong coupling conditions, the hot electrons excited in the ATA photoelectrode that fabricated on Al film instead of Au film exhibited a much longer lifetime than that on Au film. The contributions for the long lifetime of the injected electrons to TiO2 are also discussed. References Nishijima, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 1, 2031 (2010). Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H. Inoue, H. Misawa, Angew. Chem. Int. Ed., 53,10350(2014).Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 53, 9802 (2014).Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 55,3942 (2016).Nakamura, T. Oshikiri, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett., 7, 1004 (2016).V. Hoang, K. Hayashi, K. Ueno, H. Misawa et al. Nat. Commun., 8, 771 (2017).Ueno, T. Oshikiri, H. Misawa, ChemPhysChem, 17, 199 (2016).Ueno, T. Oshikiri, Q. Sun, X. Shi, H. Misawa, Chem. Rev., 118, 2955 (2018). Shi, K. Ueno, T. Oshikiri, Q. Sun, K. Sasaki, H. Misawa, Nat Nanotechnol., 13, 953 (2018).Y. Cao, T. Oshikiri, X. Shi, K. Ueno, J. Li, H. Misawa, ChemNanoMat, 5, 1008 (2019).

GaN/AlN multiple quantum wells grown by molecular beam epitaxy: effect of growth kinetics on radiative recombination efficiency

Ultraviolet (UV) optoelectronic devices based on binary GaN quantum wells have been widely reported in the literature. The internal quantum efficiency (IQE) of such structures is relatively low due to the large dislocation densities generated during heteroepitaxial deposition on to non lattice-matched substrates. Enhancement of IQE is possible through the use of expensive lattice-matched substrates or by using complex dislocation density reducing mechanisms. In this paper we have investigated growth mechanisms of GaN/AlN multiple quantum wells (MQWs) using Plasma Assisted Molecular Beam Epitaxy. Specifically the modulation of the surface diffusivity of adatoms has been carried out through choice of appropriate growth parameters, such as the group-III to group-V flux ratio. Our results indicate that this leads to modification of not only the surface morphology, but also the abruptness of the well-barrier interface. Under conditions of growth where surface morphology was atomically flat, the interfaces are relatively diffuse. The IQE for such structures, as measured by the ratio of room temperature photoluminescence intensity to that measured at 4 K, is rather low typically ~10%. Use of near stoichiometric growth conditions however lead to a reduction of the surface diffusivity of adatoms, and the formation of spontaneous nanostructures in the form of nano-dots of about 20 nm in diameter and high levels of uniformity. The IQE for GaN/AlN MQWs grown under such conditions is increased to as high as 28% even for samples with large dislocation densities. Thus, growth under such conditions can mitigate the detrimental effects of non-radiative recombination centers associated with dislocations by spatial localization of electron-hole pairs. These results are important to many applications, including UV light emitting diodes.

Internal quantum efficiency enhancement employing composition-graded last quantum barrier and electron blocking layer

We reduce the problem of asymmetrical distribution of carriers (electrons and holes) by engineering the last barrier and electron blocking layer (EBL) of green indium gallium nitride (InGaN)-based multiquantum well light-emitting diodes. We employ stair-engineered EBL with a graded InGaN last quantum barrier to enhance the device performance. The efficiency droop ratio of the proposed device is ∼15 % at 100 A / cm2. Similarly, the light output power is also enhanced by about three times, as compared to the reference structure. In addition, a carrier transport issue across the active region is also mitigated in our design.

Controlled bunching approach for achieving high efficiency active region in AlGaN-based deep ultraviolet light-emitting devices with dual-band emission

Developing efficient active region structures with low sensitivity to threading dislocation density (TDD) is of great technical significance for AlGaN-based deep ultraviolet (DUV) light-emitting devices. Here, we propose an active region strategy by introducing bunching effect to form self-assembled sidewall quantum wells (SQWs) with much stronger carrier confinement, resulting in a significant enhancement of internal quantum efficiency (from 46% to 59%) compared to the commonly adopted multiple quantum wells (MQWs) due to the lower sensitivity to TDD. As a demo, an AlGaN-based DUV light-emitting diode (LED) with the proposed active region involving both SQWs and MQWs presents dual-band emission and a consequent 68% enhancement in light output power compared to the DUV-LED with only MQWs as the active region, suggesting that the proposed architecture is fully suitable for the development of high performance DUV light-emitting devices even based on poor or medium quality materials.

(Keynote) Dynamics of Electron Transfer in Enhanced Water Splitting Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) nanoparticles shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and semiconductor.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[7],[8] However, in general Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limits its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film (ATA) photoelectrode.[9],[10] Furthermore, the dynamics of the hot electron transfer under the modal strong coupling conditions are further studied by transient reflection spectroscopy.Modal strong coupling between the Fabry–Pérot nanocavity mode of the TiO2thin-film/Au-film and the LSPR mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this modal strong coupling regime, a key feature of the ATA photoelectrode is partially inlaying of Au-NPs into the TiO2nanocavity by several nanometers. Under a three-electrode system measurement with a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and an electrolyte of KOH (0.1 mol/dm3), we demonstrated that the action spectrum of incident photon to current conversion efficiency (IPCE) exhibited two bands, which almost corresponds to the absorption spectrum of ATA. The IPCE of ATA is extraordinarily enhanced as compared to that of Au-NPs/TiO2photoanode. Most importantly, under the modal strong coupling conditions, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the wavelengths of strong coupling induced hybrid branches. The increase in IQE implies the possibility of increasing the generation of hot electrons in the modal strong coupling regime. The plasmon-induced water splitting using a two-electrode system was also quantitatively measured. To understand the IQE enhancement under the strong coupling conditions, the dynamics of the hot electrons transfer from Au-NPs to TiO2were further explored by transient measurement using a visible pump/infrared probe (at 3500 nm) femtosecond transient reflection spectrometers. An ultrafast hot electrons injection from Au-NPs to TiO2was observed. Interestingly, under the strong coupling conditions, the hot electrons excited in the ATA photoelectrode that fabricated on Al film instead of Au film exhibited a much longer lifetime than that on Au film. The contributions for the long lifetime of the injected electrons to TiO2are also discussed.

Giant enhancement of silicon plasmonic shortwave infrared photodetection using nanoscale self-organized metallic films

Many consumer technologies rely on photodetection of infrared light, such as lidar, low visibility imaging, proximity sensors/range finders, etc. However, silicon, the standard material of the semiconductor industry, becomes transparent for wavelengths above 1.1 µm, as the photons no longer have sufficient energy to stimulate direct band-to-band absorption. We report here a Schottky photodetector design that extends silicon’s optical detection range beyond this 1.1 µm limit, by utilizing internal photoemission of hot carriers. Our design relies on an ultra-thin fractally nanostructured aluminum optical absorber and yet remarkably achieves over 50% absorption of incident light. We demonstrate 2 orders of magnitude improvements of responsivity, noise-equivalent power, and specific detectivity as compared to a reference Schottky photodetector made of bulk metal films. We attribute this to the combination of superior transport and momentum relaxation processes from the nanoscale fractal geometries. Specifically, we show a direct link between internal quantum efficiency enhancement and structural parameters such as perimeter-to-surface ratio. Finally, our devices also function as bulk refractive index sensors. Our approach uses an exceedingly simple complementary metal-oxide-semiconductor (CMOS)-compatible “bottom up” fabrication that is cheap and scalable and is a promising candidate for future cost-effective and robust shortwave infrared photodetection and sensing applications.

Pyrophosphate Phosphor Solid Solution with High Quantum Efficiency and Thermal Stability for Efficient LED Lighting.

SummaryPhosphors with high quantum efficiency and thermal stability are greatly desired for lighting industries. Based on the design strategy of solid solution, a series of deep-blue-emitting phosphors (Sr0.99-xBax)2P2O7:0.02Eu2+ (SBxPE x = 0–0.5) are developed. Upon excitation at 350 nm, the optimized SB0.3PE phosphor shows a relatively narrow full width at half maximum (FWHM = 32.7 nm) peaking at 420 nm, which matches well with the plant absorption in blue region. Moreover, this phosphor exhibits obvious enhancement of internal quantum efficiency (IQE) (from 74% to 100%) and thermal stability (from 88% to 108% of peak intensity and from 99% to 124% of integrated area intensity at 150°C) compared with the pristine one. The white LED devices using SB0.3PE as deep-blue-emitting component show good electronic properties, indicating that SB0.3PE is promising to be used in plant growth lighting, white LEDs, and other photoelectric applications.

Photoluminescence enhancement in non-polar ZnO films through metallodielectric mediated Al surface plasmons.

Non-polar ZnO thin films are grown on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. Emission enhancement from non-polar ZnO thin films coated with Al/AlOx has been studied by photoluminescence spectroscopy. AlOx has been used to mediate the surface plasmon (SP) energy of Al nanoparticles. Taking advantage of the resonant coupling between the UV emission of non-polar ZnO film and Al nanoparticle SPs, an 84-fold enhancement of the UV emission and an 8.3-fold enhancement of internal quantum efficiency (ηint) have been achieved under the optimized sputtering time and energy of SPs.

Numerical Investigation of Purcell Enhancement of the Internal Quantum Efficiency of GaN-based Green LED Structures

GaN-based green light-emitting diode (LED) structures suffer from low internal quantum efficiency (IQE), known as the “green gap” problem. The IQE of LED structures is expected to be improved to some extent by exploiting the Purcell effect. In this study, the Purcell effect on the IQE of green LED structures is investigated numerically using a finite-difference time-domain simulation. The Purcell factor of flip-chip LED structures is found to be more than three times as high as that of epi-up LED structures, which is attributed to the high-reflectance mirror near the active region in the flip-chip LED structures. When the unmodified IQE is 20%, the relative enhancement of IQE can be greater than 50%, without utilizing the surface-plasmon coupling effect. Based on the simulation results, the “green gap” problem of GaN-based green LEDs is expected to be mitigated significantly by optimizing flip-chip LED structures to maximize the Purcell effect.

Efficiency Improvement of Deep‐Ultraviolet Light Emitting Diodes with Gradient Electron Blocking Layers

In this paper, a gradient electron blocking layer (GEBL) is introduced to improve efficiency of deep‐ultraviolet light‐emitting diodes (DUV‐LEDs). Various structures of DUV‐LEDs are simulated to determine the energy band diagram variation and carrier injection mechanism resulting from the insertion of the GEBL. The simulation results show the improved electron and hole transport behavior in AlGaN multi quantum wells (MQWs). Especially, the injection efficiency of holes is improved with the increasing number of GEBL steps, which lead to the enhancement of internal quantum efficiency (IQE). The DUV‐LED structures with GEBL are grown in a high‐temperature metal‐organic chemical vapor deposition (HT‐MOCVD) system. Electroluminescence (EL) spectra show that the emission intensity at a wavelength of 280 nm from a DUV‐LED with a 12‐steps EBL is about 2.3 times that from a DUV‐LED with a single EBL.

Efficiency enhancement of InGaN amber MQWs using nanopillar structures

Abstract We have investigated the use of nanopillar structures on high indium content InGaN amber multiple quantum well (MQW) samples to enhance the emission efficiency. A significant emission enhancement was observed which can be attributed to the enhancement of internal quantum efficiency and light extraction efficiency. The size-dependent strain relaxation effect was characterized by photoluminescence, Raman spectroscopy and time-resolved photoluminescence measurements. In addition, the light extraction efficiency of different MQW samples was studied by finite-different time-domain simulations. Compared to the as-grown sample, the nanopillar amber MQW sample with a diameter of 300 nm has demonstrated an emission enhancement by a factor of 23.8.

Dependencies of surface plasmon coupling effects on the p-GaN thickness of a thin-p-type light-emitting diode.

The high performance of a light-emitting diode (LED) with the total p-type thickness as small as 38 nm is demonstrated. By increasing the Mg doping concentration in the p-AlGaN electron blocking layer through an Mg pre-flow process, the hole injection efficiency can be significantly enhanced. Based on this technique, the high LED performance can be maintained when the p-type layer thickness is significantly reduced. Then, the surface plasmon coupling effects, including the enhancement of internal quantum efficiency, increase in output intensity, reduction of efficiency droop, and increase of modulation bandwidth, among the thin p-type LED samples of different p-type thicknesses that are compared. These advantageous effects are stronger as the p-type layer becomes thinner. However, the dependencies of these effects on p-type layer thickness are different. With a circular mesa size of 10 μm in radius, through surface plasmon coupling, we achieve the record-high modulation bandwidth of 625.6 MHz among c-plane GaN-based LEDs.

Significant internal quantum efficiency enhancement of GaN/AlGaN multiple quantum wells emitting at ~350 nm via step quantum well structure design

Significant internal quantum efficiency (IQE) enhancement of GaN/AlGaN multiple quantum wells (MQWs) emitting at ~350 nm was achieved via a step quantum well (QW) structure design. The MQW structures were grown on AlGaN/AlN/sapphire templates by metal-organic chemical vapor deposition (MOCVD). High resolution x-ray diffraction (HR-XRD) and scanning transmission electron microscopy (STEM) were performed, showing sharp interface of the MQWs. Weak beam dark field imaging was conducted, indicating a similar dislocation density of the investigated MQWs samples. The IQE of GaN/AlGaN MQWs was estimated by temperature dependent photoluminescence (TDPL). An IQE enhancement of about two times was observed for the GaN/AlGaN step QW structure, compared with conventional QW structure. Based on the theoretical calculation, this IQE enhancement was attributed to the suppressed polarization-induced field, and thus the improved electron–hole wave-function overlap in the step QW.