Investigation on many-body effects in micro-LEDs under ultra-high injection levels.

Micro-LEDs can work under an extremely high injection level and are widely used in high-brightness micro-displays and visible light communication. With the increase of carrier concentration, many-body effects gradually become important factors affecting devices' characteristics. Considering the effects of carrier scattering, bandgap renormalization, and Coulomb enhancement (CE), changes in the electroluminescence spectra of micro-LEDs are analyzed as the current density increases from 49.2 to 358.2 kA/cm2, the latter representing an ultra-high injection level. Affected by plasma screening, CE decreases below about 150 kA/cm2. After that, polarization screening dominates and effectively alleviates the spatial separation of electrons and holes, which results in CE increases to the maximum injection level of 358.2 kA/cm2. It is established that CE promotes radiative recombination processes. Different from the traditional phenomenon of "efficiency droop", the enhanced attraction between carriers leads to an abnormal increase of external quantum efficiency at high current density.

Investigation on strain relaxation distribution in GaN-based µLEDs by Kelvin probe force microscopy and micro-photoluminescence.

GaN/InGaN multi-quantum-wells (MQWs) micron light emitting diodes (µLEDs) with the size ranging from 10 to 300 µm are fabricated. Effects of strain relaxation on the performance of µLEDs have been investigated both experimentally and numerically. Kelvin probe force microscopy (KPFM) and micro-photoluminescence (µPL) are used to characterize the strained area on micron pillars. Strain relaxation and reducing polarization field in MQWs almost affects the whole mesa for 10 µm LEDs and about 4% area around the lateral for 300 µm LEDs. It makes a great contribution to high performance for smaller size µLEDs. Moreover, an indirect nanoscale strain measurement for µLEDs are provided.

Angular distribution of polarized light and its effect on light extraction efficiency in AlGaN deep-ultraviolet light-emitting diodes.

Angular distribution of polarized light and its effect on light extraction efficiency (LEE) in AlGaN deep-ultraviolet (DUV) light-emitting diodes (LEDs) are investigated in this paper. A united picture is presented to describe polarized light's emission and propagation processes. It is found that the electron-hole recombinations in AlGaN multiple quantum wells produce three kinds of angularly distributed polarized emissions and propagation process can change their intensity distributions. By investigation the change of angular distributions in 277nm and 215nm LEDs, this work reveals that LEE can be significantly enhanced by modulating the angular distributions of polarized light of DUV LEDs.

Enhancement of optical polarization degree of AlGaN quantum wells by using staggered structure.

Staggered AlGaN quantum wells (QWs) are designed to enhance the transverse-electric (TE) polarized optical emission in deep ultraviolet (DUV) light- emitting diodes (LED). The optical polarization properties of the conventional and staggered AlGaN QWs are investigated by a theoretical model based on the k·p method as well as polarized photoluminescence (PL) measurements. Based on an analysis of the valence subbands and momentum matrix elements, it is found that AlGaN QWs with step-function-like Al content in QWs offers much stronger TE polarized emission in comparison to that from conventional AlGaN QWs. Experimental results show that the degree of the PL polarization at room temperature can be enhanced from 20.8% of conventional AlGaN QWs to 40.2% of staggered AlGaN QWs grown by MOCVD, which is in good agreement with the theoretical simulation. It suggests that polarization band engineering via staggered AlGaN QWs can be well applied in high efficiency AlGaN-based DUV LEDs.

[The electroluminescence spectra of InGan/GaN blue LEDs during aging time].

The luminescence spectra of InGaN/GaN multiple quantum wells light-emitting diodes under low level injection current (<4 mA) during aging process was investigated for the first time. Comparing the electroluminescence (EL) spectra of LEDs before and after aging time it was found that the peak wavelength and the full width at half maximum (FWHM) decreased with stress time and the changes of EL spectrum had two different stages-drastic decrease at the early stress stage and slow decrease later showing the same trend with the output optical power of LEDs, which indicates that the effective polarization electric field of LEDs becomes weak during the aging process and the change has a clear correlation with the increase of the defects in the multiple quantum wells of LEDs. Electrical measurement revealed that junction capacitance (C(j)) under the same junction voltage (V(j) = 1.8 V) and the junction voltage (V(j)) with the same injection current 1 mA calculated by ac small-signal IV method increased along with aging time, which explicates that the carrier density under the same low injection increases as the aging time increases. Analyses indicate that the polarization field in the quantum well is more seriously screened by the increased carriers captured by defects activated during stress time, the weaker effective polarization electric field makes the tilt of the energy band smaller, the energy radiated through the band edge and the density of energy states of the band edge increase which leads to the behaviors of peak wavelength and the FWHM of InGaN/GaN multiple quantum wells LEDs under low level injection current.

Performance improvement of GaN-based LEDs with step stage InGaN/GaN strain relief layers in GaN-based blue LEDs.

The performance of nitride-based LEDs was improved by inserting dual stage and step stage InGaN/GaN strain relief layer (SRL) between the active layer and n-GaN template. The influences of step stage InGaN/GaN SRL on the structure, electrical and optical characteristics of GaN-based LEDs were investigated. The analysis of strain effect on recombination rate based k·p method indicated 12.5% reduction of strain in InGaN/GaN MQWs by inserting SRL with step stage InGaN/GaN structures. The surface morphology was improved and a smaller blue shift in the electroluminescence (EL) spectral with increasing injection current was observed for LEDs with step stage SRL compared with conventional LEDs. The output power of LEDs operating at 20 mA was about 15.3 mW, increased by more than 108% by using step stage InGaN/GaN SRL, which shows great potential of such InGaN/GaN SRL in modulating InGaN/GaN MQWs optical properties based on its strain relief function.

Resonant absorption and scattering suppression of localized surface plasmons in Ag particles on green LED.

The metallic-structure dependent localized surface plasmons (LSPs) coupling behaviors with InGaN QWs in a green LED epitaxial wafer are investigated by optical transmission, scanning electron microscopy (SEM) and photoluminescence (PL) measurements. Ag nanoparticles (NPs) are formed by thermal annealing Ag layer on the green LED wafer. SEM images show that for higher annealing temperature and/or thicker deposited Ag layer, larger Ag NPs can be produced, leading to the redshift of absorption peaks in the transmission spectra. Time resolved PL (TRPL) measurements indicate when LSP-MQW coupling occurs, PL decay rate is greatly enhanced especially at the resonant wavelength 560 nm. However, the PL intensity is suppressed by 3.5 folds compared to the bare LED. The resonant absorption and PL suppression are simulated by three dimension finite-difference-time-domain (FDTD), which suggests that Ag particle with smaller size and lower height lead to the larger dissipation of LSP.

Noise-insensitive discriminative subspace fuzzy clustering.

Discriminative subspace clustering (DSC) can make full use of linear discriminant analysis (LDA) to reduce the dimension of data and achieve effective clustering high-dimension data by clustering low-dimension data in discriminant subspace. However, most existing DSC algorithms do not consider the noise and outliers that may be contained in data sets, and when they are applied to the data sets with noise or outliers, and they often obtain poor performance due to the influence of noise and outliers. In this paper, we address the problem of the sensitivity of DSC to noise and outlier. Replacing the Euclidean distance in the objective function of LDA by an exponential non-Euclidean distance, we first develop a noise-insensitive LDA (NILDA) algorithm. Then, combining the proposed NILDA and a noise-insensitive fuzzy clustering algorithm: AFKM, we propose a noise-insensitive discriminative subspace fuzzy clustering (NIDSFC) algorithm. Experiments on some benchmark data sets show the effectiveness of the proposed NIDSFC algorithm.

Valence subband coupling effect on polarization of spontaneous emissions from Al-rich AlGaN/AlN quantum wells.

The optical polarization properties of Al-rich AlGaN/AlN quantum wells (QWs) were investigated using the theoretical model based on the k·p method. Numerical results show that there is valence subband coupling which can influence the peak emission wavelength and emission intensity for TE and TM polarization components from Al-rich AlGaN/AlN QWs. Especially the valence subband coupling could be strong enough when CH1 is close to HH1 and LH1 subbands to modulate the critical Al content switching dominant emissions from TE to TM polarization. It is believed that the valence subband coupling may give important influence on polarization properties of spontaneous emissions and should be considered in designing high efficiency AlGaN-based ultraviolet (UV) LEDs.

Enhancement of surface emission in deep ultraviolet AlGaN-based light emitting diodes with staggered quantum wells.

The optical polarization properties of staggered AlGaN-AlGaN/AlN quantum wells (QWs) are investigated using the theoretical model based on the k·p method. The numerical results show that the energy level order and coupling relation of the valence subband structure change in the staggered QWs and the trend is beneficial to TE polarized transition compared to that of conventional AlGaN/AlN QWs. As a result, the staggered QWs have much stronger TE-polarized emission than conventional AlGaN-based QWs, which can enhance the surface emission of deep ultraviolet (DUV) light-emitting diodes (LEDs). The polarization control by using staggered QWs can be applied in high efficiency DUV AlGaN-based LEDs.

[Optical characteristics of InGaN/GaN light emitting diodes on patterned sapphire substrate].

In the present paper, optical characteristics of InGaN/GaN multiple quantum wells light emitting diodes fabricated on the conventional (C-LEDs) and patterned sapphire substrates (PSSLEDs) were investigated by electroluminescence (EL) and photoluminescence (PL). PSSLEDs show higher light output power and smaller full width at half maximum (FWHM) of EL than those of C-LEDs for their better crystalline quality. The FWHM of EL for PSSLEDs exhibits faster broadening as current increases than C-LEDs, while the same broadening of FWHM of PL as excitation power increases is shown in PSSLEDs and C-LEDs, which indicate that there is stronger band filling effect in PSSLEDs caused by relatively higher carrier density in their QWs at the same current injection level. Besides, smaller blue-shift of peak wavelength of EL as injection current increases in PSSLEDs suggests that piezoelectric field in PSSLEDs is weaker than that in C-LEDs. Therefore, comparing the changes in external quantum efficiency as current increases for PSSLEDs and C-LEDs, more serious efficiency droop for PSSLEDs could not be mainly caused by piezoelectric field in QWs.

GaN-based light-emitting diodes with photonic crystals structures fabricated by porous anodic alumina template.

In this paper, we propose and demonstrate a convenient and flexible approach for preparation large-area of photonic crystals (PhCs) structures on the GaN-based LED chip. The highly-ordered porous anodic alumina (AAO) with pitch of wavelength scale was adopted as a selective dry etching mask for PhCs-pattern transfer. The PhCs with different pore depths were simultaneously formed on the entire surfaces of GaN-based LED chip including ITO, GaN surrounding contacts and the sidewall of the mesa by one-step reactive ion etching (RIE). The light output power improvement of PhCs-based GaN LED was achieved as high as 94% compared to that of the conventional GaN-based LED.

[Case control study on anatomical locking plate and ordinary steel plate for the treatment of closed calcaneal fracture with Sandersâ ¡and â ¢].

OBJECTIVE: To compare clinical efficacy between anatomical locking plate (ALP) and ordinary steel plate (OSP) in treating closed calcaneal fractures with SandersⅡ and Ⅲ. METHODS: From May 2016 to May 2018, 68 patients with closed Sanders typeⅡ and Ⅲ calcaneal fractures were retrospectively analyzed, and were divided into anatomical locking plate group (ALP group) and ordinary steel plate group (OSP group) according to two kinds of plate fixation, and 34 patients in each group. In ALP group, there were 21 males and 13 females aged from 20 to 63 years old with average of (35.16±8.45) years old; 14 patients were typeⅡand 20 patients were type Ⅲaccording to Sanders classification;treated with ALP. In OSP group, there were 20 males and 14 females aged from 19 to 63 years old with average of (35.05±8.39) years old;19 patients were typeⅡand 15 patients were type Ⅲ according to Sanders classification;treated with OSP. Operative time, intraoperative blood loss and complications between two groups were observed and compared;preoperative and postoperative Böhler angle and gissane angle were also compared;American Orthopaedic Foot & Ankle Society (AOFAS) ankle and hind foot scores, foot and ankle disability index (FADI) scores were applied to evaluate clinical effect. RESULTS: All patients were followed up from 11 to 14 months with an average of (12.06±0.81) months. There were no statistical differences in opertive time, intraoperative blood loss, incision infection and refracture rate in complications between two groups (P>0.05);while there was significant difference in the number of screw loosening (P<0.05). Böhler angle and Gissane angle in ALP group at 6 and 12 months after opertaion were higher than that of OSP group (P<0.05), and the degree of improvement of Böhler angle and Gissane angle in ALP group were also higher than that of OSP group (P<0.05). Postopertaive AOFAS score and FADI score at 6 and 12 months in ALP group were higher than that of OSP group (P<0.05), while no statistical difference in AOFAS grading between two groups(P>0.05). CONCLUSION: Compared with OSP, ALP in treating SandersⅡ and Ⅲ calcaneal fractures could achieve better therapeutic effect, avoid screw loosening, reduce complications, and improve limb function in further.

Single-Photon Emission from Point Defects in Aluminum Nitride Films.

Quantum technologies require robust and photostable single-photon emitters. Here, room temperature operated single-photon emissions from isolated defects in aluminum nitride (AlN) films are reported. AlN films were grown on nanopatterned sapphire substrates by metal organic chemical vapor deposition. The observed emission lines range from visible to near-infrared, with highly linear polarization characteristics. The temperature-dependent line width increase shows T3 or single-exponential behavior. First-principle calculations based on density functional theory show that point defect species, such as antisite nitrogen vacancy complex (NAlVN) and divacancy (VAlVN) complexes, are considered to be an important physical origin of observed emission lines ranging from approximately 550 to 1000 nm. The results provide a new platform for on-chip quantum sources.

Coordinated stress management and dislocation control in GaN growth on Si (111) substrates by using a carbon nanotube mask.

A novel method is proposed to realize coordinated stress management and dislocation control in GaN growth on Si (111) substrates. This method is facilely implemented by using a unique carbon nanotube (CNT) mask with a nanoscale diameter and a high fill factor. It is found that most dislocations are directly blocked by high-density nanovoids formed around CNTs within 150 nm from the GaN/AlN interface. In this way, without dislocation inclination, the threading dislocation density (TDD) of the GaN epilayer is dramatically reduced. Thus the compressive stress relaxation due to dislocation inclination during GaN growth is significantly decreased and more compressive stress can be preserved. In addition, the high-density nanovoids also contribute to decreasing the thermal tensile stress during cooling down. As a result, coordinated stress management and dislocation control are realized and a 3.5 µm-thick crack-free GaN epilayer with a homogeneously low TDD of 2.51 × 108 cm-2 is obtained. With the facile fabrication of the CNT mask and a simple epitaxy procedure, the quality of the GaN epilayer grown on Si substrates is greatly improved, which extends the application of the nanomaterial in the GaN epitaxy on the Si substrate and demonstrates great application potential for high-performance electronic devices.

Anomalous Pressure Characteristics of Defects in Hexagonal Boron Nitride Flakes.

Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressure-dependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.

Study on the Coupling Mechanism of the Orthogonal Dipoles with Surface Plasmon in Green LED by Cathodoluminescence.

We analyzed the coupling behavior between the localized surface plasmon (LSP) and quantum wells (QWs) using cathodoluminescence (CL) in a green light-emitting diodes (LED) with Ag nanoparticles (NPs) filled in photonic crystal (PhC) holes. Photoluminescence (PL) suppression and CL enhancement were obtained for the same green LED sample with the Ag NP array. Time-resolved PL (TRPL) results indicate strong coupling between the LSP and the QWs. Three-dimensional (3D) finite difference time domain (FDTD) simulation was performed using a three-body model consisting of two orthogonal dipoles and a single Ag NP. The LSP–QWs coupling effect was separated from the electron-beam (e-beam)–LSP–QW system by linear approximation. The energy dissipation was significantly reduced by the z-dipole introduction under the e-beam excitation. In this paper, the coupling mechanism is discussed and a novel emission structure is proposed.

Effect of dipole polarization orientation on surface plasmon coupling with green emitting quantum wells by cathodoluminescence.

Ag nanoparticles (NPs) are fabricated on the cross-section of green emitting quantum wells (QWs). The effect of the dipole polarization orientation on the localized surface plasmon (LSP)-QW coupling can be studied by setting the incident direction of the electron beam parallel to the plane of the QWs. Cathodoluminescence (CL) measurements on the QWs show that the intensity with the Ag NPs is enhanced 6.1 times compared with that without the Ag NPs. Total energy loss profiles for an electron beam in the GaN and Ag NP are accurately simulated using a Monte Carlo program (CASINO). The orientations of the in-plane dipoles in the QWs can vary from 0° to 360°. Through a two-step simulation process using the three-dimensional (3D) finite difference time domain (FDTD) method, the weighted average of CL intensities are simulated for QWs with the Ag NPs. The simulation results agree well with the experimental results. Lastly, the dipole orientation dependent LSP-QW coupling process is discussed.

Enhanced Ferromagnetism in Nanoscale GaN:Mn Wires Grown on GaN Ridges.

The problem of weak magnetism has hindered the application of magnetic semiconductors since their invention, and on the other hand, the magnetic mechanism of GaN-based magnetic semiconductors has been the focus of long-standing debate. In this work, nanoscale GaN:Mn wires were grown on the top of GaN ridges by metalorganic chemical vapor deposition (MOCVD), and the superconducting quantum interference device (SQUID) magnetometer shows that its ferromagnetism is greatly enhanced. Secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy (EDS) reveal an obvious increase of Mn composition in the nanowire part, and transmission electron microscopy (TEM) and EDS mapping results further indicate the correlation between the abundant stacking faults (SFs) and high Mn doping. When further combined with the micro-Raman results, the magnetism in GaN:Mn might be related not only to Mn concentration, but also to some kinds of built-in defects introduced together with the Mn doping or the SFs.

The effects of nanocavity and photonic crystal in InGaN/GaN nanorod LED arrays.

InGaN/GaN nanorod light-emitting diode (LED) arrays were fabricated using nanoimprint and reactive ion etching. The diameters of the nanorods range from 120 to 300 nm. The integral photoluminescence (PL) intensity for 120 nm nanorod LED array is enhanced as 13 times compared to that of the planar one. In angular-resolved PL (ARPL) measurements, there are some strong lobes as resonant regime appeared in the far-field radiation patterns of small size nanorod array, in which the PL spectra are sharp and intense. The PL lifetime for resonant regime is 0.088 ns, which is 40 % lower than that of non-resonant regime for 120 nm nanorod LED array. At last, three dimension finite difference time domain (FDTD) simulation is performed. The effects of guided modes coupling in nanocavity and extraction by photonic crystals are explored.