Cavity Light-emitting Diodes Research Articles

GeSn/Ge Multiquantum-Well Vertical-Cavity Surface-Emitting p-i-n Structures and Diode Emitters on a 200 mm Ge-on-Insulator Platform

An efficient monolithically integrated light source with complementary metal-oxide-semiconductor (CMOS) compatibility remains the missing component to enable Si photonics for various applications. In particular, vertical-cavity-surface-emitting (VCSE) light sources, such as resonant cavity light-emitting diodes (RCLEDs) and vertical-cavity surface-emitting lasers (VCSELs), are strong contenders due to their compact size, circular emission profile with low beam divergence, wafer-scale fabrication compatibility, high bandwidth, and high coupling efficiency to fiber optic cables. We report the first demonstration of 8-in. wafer-scale GeSn/Ge multiple-quantum-well VCSE p-i-n structures and diodes for laser or light-emitting diode (LED) applications in Si photonics by wafer bonding and layer transfer techniques, which are challenging for all-epitaxy routes. The wafer could be 12 in. (300 mm) since the MQW gain medium is grown on a 300 mm Si wafer, and the bonding is wafer-scale. Alternative dielectric layers (SiO2/SiN/SiO2), introduced by wafer bonding, under the emitting structure, serve as the bottom mirror for the vertical cavity. The Ge0.92Sn0.08/Ge MQW layer is utilized to improve the material quality and to confine injected carriers. As a result, more than 8× enhancement of light emission due to the vertical cavity resonance was demonstrated by photoluminescence spectroscopy. Besides, the spectral purity is enhanced by the single-longitudinal-mode cavity. The intensity of light emission is insensitive to the temperature range from 4 to 300 K and even becomes stronger at higher temperatures since the stronger resonant cavity effect at higher temperatures compensates for the drop in light emission efficiency caused by nonradiative centers. The vertical cavity effect on the light emission is further verified by reflectivity spectroscopy and optical simulations. A positive gain can be achieved as indicated by optical gain calculations. Room-temperature electroluminescence and an excellent carrier injection efficiency of the MQW VCSE diode are observed, showing its potential for electrically injected RCLEDs and VCSELs.

Optoelectronic Simulations of InGaN-Based Green Micro-Resonant Cavity Light-Emitting Diodes with Staggered Multiple Quantum Wells

In this research, we compared the performance of commercial μ-LEDs and three-layered staggered QW μ-LED arrays. We also investigated the self-heating effect. We proposed a green micro-resonant cavity light-emitting diode (µ-RCLED) that consists of a three-layer staggered InGaN with multiple quantum wells (MQWs), a bottom layer of nanoporous n-GaN distributed Bragg reflectors (DBRs), and a top layer of Ta2O5/SiO2 DBRs. We systematically performed simulations of the proposed µ-RCLEDs. For the InGaN MQWs with an input current of 300 mA, the calculated wavefunction overlaps are 8.8% and 18.1% for the regular and staggered structures, respectively. Furthermore, the staggered MQWs can reduce the blue-shift of electroluminescence from 10.25 nm, obtained with regular MQWs, to 2.25 nm. Due to less blue-shift, the output power can be maintained even at a high input current. Conversely, by employing 6.5 pairs of Ta2O5/SiO2 DBRs stacks, the full width at half maximum (FWHM) can be significantly reduced from 40 nm, obtained with ordinary µ-LEDs, to 0.3 nm, and a divergence angle smaller than 60° can be obtained. Our simulation results suggest that the µ-RCLEDs can effectively resolve the wavelength instability and color purity issues of conventional µ-LEDs.

Gallium nitride-based resonant cavity light-emitting diode with single-longitudinal-mode emission.

A novel, to the best of our knowledge, gallium nitride (GaN)-based resonant cavity light-emitting diode (RCLED) with single-longitudinal-mode light emission was demonstrated. A Ta2O5/SiO2 dielectric distributed Bragg reflector (DBR) with a filter-film structure was adopted as the top mirror. In contrast to the flat-topped reflectivity spectrum of the conventional high-reflective-structure DBR, for this filter-film-structure DBR, there is a light-transmitting concave band on the reflectivity spectrum. Owing to the modulation effect of this band on the output light, a single-longitudinal-mode light emission with a full width at half maximum as low as 0.63 nm was realized. Furthermore, the novel RCLED exhibited better wavelength stability. With an increase in the injection current from 50 to 500 mA, the redshift of the emission peak was only 0.2 nm. This novel RCLED with ultra-narrowband emission has a high potential for application in optical communication systems and optical fiber sensing applications.

InGaN-Based Orange-Red Resonant Cavity Light-Emitting Diodes

InGaN-based orange-red resonant cavity light-emitting diodes (RCLEDs), were fabricated by using an AlN current-confinement aperture and double dielectric distributed Bragg reflectors (DBRs). For realizing the structure of device, a substrate transfer technique was employed in process of fabrication. The device exhibited optical resonant effect, a high Q factor (∼3010), and a narrow emission linewidth (FMHW ∼0.2 nm), indicating low optical loss in the resonant cavity. Additionally, due to the three-dimensional (3D) optical confinement effect, the discrete modes of red emission were clearly observed in the far field via an angular resolved measurement system. And then, the energies of the photon states of a red RCLED consists generally with the simulation results based on a circular waveguide model. The saturated emission intensity of the orange-red RCLED was proportional to the area of current injection. This work demonstrated the feasibility of InGaN-based electrically injected orange-red RCLEDs that are useful for the development of displays and communication systems.

Influence of top mirror on performance of GaN-based resonant cavity light-emitting diode

In this paper, two kinds of distributed Bragg reflectors (DBRs) with high-reflective-film structure and filter structure are designed and evaporated on the top of GaN-based resonant cavity light emitting diode (RCLED), respectively. Firstly, the reflectivity spectra of the two kinds of DBRs are simulated. Then, the differences in performance including optical longitudinal modes, spectral linewidth, and output light intensity between the two kinds of RCLED devices with different top mirrors, are compared and analyzed. Finally, the influence of the top mirror reflection characteristics on the output spectrum of the RCLED is studied in detail. The results show that the top mirror is an important part of RCLED, and its reflection characteristics determine the optical performance of the device. For the conventional DBR with high-reflective-film structure, its reflectivity spectrum has a wide high-reflection band. Accordingly, the spectral linewidth of the RCLED can be effectively narrowed by using the conventional DBR as the top mirror. However, the spectrum still consists of multi-longitudinal modes. For the DBR with filter structure, its reflectivity spectrum has a narrow high-transmittance band at the central wavelength. Depending on the modulation effect of the high-transmittance band to the output light, single longitudinal mode light emission is realized for the RCLED with the specially designed DBR as the top mirror, which shows a broad application prospect in optical communication and optical fiber sensing. Moreover, the spectral characteristics of the RCLED can be further optimized to meet its application requirements in much more fields, by designing the top mirror structure and changing its reflectivity spectrum characteristics.

Study on the effect of top mirror on the performance of GaN-based resonant cavity light-emitting diode

本文在GaN基共振腔发光二极管（RCLED）顶部设计制备了高反膜结构分布式布拉格反射镜（DBR）和滤波器结构DBR两种反射镜，对比分析了两种反射镜的反射率曲线特征以及对应的RCLED器件的光输出纵模模式、光谱线宽和输出光强等性能差异，详细研究了顶部反射镜的光反射特性对RCLED器件输出光谱性能的影响机理。研究结果表明，顶部反射镜是RCLED的重要组成部分，其反射率曲线特征决定器件的光输出性能。常规高反膜结构DBR顶部反射镜的反射率曲线具有较宽的高反射带，将其作为顶部反射镜可有效压窄RCLED发光纵模线宽，但是发光光谱仍呈现多纵模光输出特征。滤波器结构DBR顶部反射镜的反射率曲线在中心波长处具有较窄的透光凹带，利用透光凹带对输出光的调制作用，器件可实现单纵模光输出，在光通信、光纤传感等领域展示了广阔的应用前景。通过进一步设计RCLED顶部反射镜结构，可以改变其反射率曲线特性，进而优化RCLED器件的输出光谱特性，以满足器件在多个领域的应用需求。

GaN-on-Si resonant-cavity light-emitting diode incorporating top and bottom dielectric distributed Bragg reflectors

We report here a GaN-based resonant cavity light-emitting diode (RCLED) with top and bottom dielectric TiO2/SiO2 distributed Bragg reflector (DBR) mirrors on a silicon substrate. High data transmission in free space at 200 Mbps with an opening in the eye diagram was achieved. The results show that the combination of GaN-based LED on silicon and double sided dielectric DBR mirror deposition enables a manufacturable process which provides a unique opportunity for commercialization of RCLED in future solid-state lighting and visible light communication applications.

Design and fabrication of double AlGaN/GaN distributed Bragg reflector stack mirror for the application of GaN-based optoelectronic devices

In this work, a near-ultraviolet (380 nm) double Al0.2Ga0.8N/GaN distributed Bragg reflectors (DBRs) stack mirror was designed and fabricated. The double DBRs stack mirror consists of a 30-pair Al0.2Ga0.8N/GaN DBRs centered at 375 nm and a 20-pair Al0.2Ga0.8N/GaN DBRs centered at 385 nm. Our simulation results show that the method of double DBRs stack mirror design can broaden the stopband width greatly and increase the reflected angle efficiently, compared with the single Al0.2Ga0.8N/GaN DBRs mirror. In experiment, the double Al0.2Ga0.8N/GaN DBRs stack mirror and the reference Al0.2Ga0.8N/GaN DBRs mirror were grown on sapphire substrate by metalorganic chemical vapor deposition. The measured stopband width of the double DBRs stack mirror (~ 25 nm) is more than two times that of the reference DBRs mirror (~ 11 nm), which consists well with our simulation results. It is reasonable to believe that this work could provide a valuable information to obtain AlGaN/GaN DBRs with wide stopband width that can be used in the fabrication of GaN-based resonant cavity light-emitting diodes and vertical cavity surface emitting lasers.

A novel modulation scheme for short range polymer optical fiber communications

To make use of the spectrum of a step-index polymer optical fiber (SI-POF) channel, a hybrid modulation scheme is proposed combining discrete multi-tone modulation and M-ary position phase shift keying. The discrete line spectrum interference and sideband interference components of the new modulation scheme are analyzed. The waveform optimization factor and a new numerical symbol generating approach are introduced to strengthen the anti-interference capability of the system. Offline processing with an arbitrary waveform generator and digital storage oscilloscope is used to investigate the transmission performance. A transmission rate of 510.5 Mbps is achieved for a 50 m SI-POF in an intensity modulated direct detection system using eye-safe resonant cavity light-emitting diodes and a PIN diode at red 650 nm. The experimental verification demonstrates that, using the new methodology, an additional bit rate of 19.5 Mbps in the low signal-to-noise ratio region is obtained. Furthermore there 6 dB power is saved at a bit error rate (BER) of 1×10−4, so better spectral efficiency can be achieved. It is demonstrated that the proposed scheme can offer better BER performance and make a better compromise between transmission rate, power consumption, spectral efficiency and BER.

Modeling of Vertical GaN Based Resonant Cavity Light-Emitting Diode

Vertical structure of resonant cavity light-emitting diode (RCLED) is presented in this paper to further improve the photoelectric performance of the device. Modeling and 2D simulation have been proceeded with Crosslight APSYS[, the analysis and modeling software for semiconductor devices. Main results of the simulation are listed and discussed. The peak wavelength locates at about 550 nm with half-wave width of about 10 nm and its output power has been enhanced by 25% than the conventional one. The results prove that vertical RCLED is a considerable source for POF-based fiber communication.

Size-Dependent Resonant Cavity Light-Emitting Diodes for Collimating Concerns

This paper presented the GaN-Based size-dependent ultrathin-film resonant cavity light-emitting diode (uT-RCLED) for highly collimation and light-collective efficiency. The cavity was bound by bottom silver mirror (∼97%) and top distributed Bragg reflector (DBR) with five pairs of SiO2/TiO2 (∼95%). The viewing angle of 50 µm (100 µm) uT-RCLED was near ±51° (±47°) through the beam profiler and presented the best light-collective efficiency of 8.18% (9.73%) within ±15° light extraction cone contrasted with regular non-DBR ultrathin-film light-emitting diodes (uT-LEDs) (6.57%). In addition, the absolute light output power of the 50 µm (100 µm) uT-RCLED was 95.26% (127.36%) much higher than the uT-LED. Besides, the micromation of RCLED influenced neither the resonant effect nor the collimating properties which opens a potential application for the LED microprojectors to meet the etendue limitation for maximum luminance output.

Enhanced Output Power of GaN-Based Resonance Cavity Light-Emitting Diodes With Optimized ITO Design

We fabricated and measured GaN-based resonant cavity light-emitting diodes with a 30 nm thick Indium tin oxide (ITO) thin film as a transparent contact layer. Four different ITO structures on p-type GaN samples were deposited by sputter and e-gun, and the corresponding device performance was compared. Each of these four samples has been annealed by its optimal parameters. The ITO thin film deposited by sputter demonstrated better electrical characteristics, surface morphology, specific contact resistance, and the overall device light output compared to those of the e-gun samples. Between the two sputtered ITO types, the hybrid type shows higher roll-over current density of 14 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and the output power is increased from 15 to 39 μ W. From statistical data of the 2-D light intensity under the same current, we saw the lateral current spreading of the pure crystalline ITO by sputter is worst. The hybrid type, which combines the crystalline and amorphous ITO, has the best overall performance when we consider all the electrical, optical, and metrology measurements. From these results, we believe the 30 nm thick hybrid ITO thin film has the best potential to be applied in light emitting devices such as light-emitting diodes, laser diodes, etc.

Comparison of Different Modulation Formats for 1-Gb/s SI-POF Transmission Systems

In the framework of the currently ongoing effort to find the best transmission solution for Gigabit Ethernet over step-index polymer optical fiber (SI-POF), we give a novel contribution through the comparison of three possible modulation formats: pulse amplitude modulation (PAM)-2, PAM-4, and duobinary, all coupled with electronic equalization at the receiver. We show that on a 50-m SI-POF link using resonant cavity light-emitting diode (RC-LED), duobinary gives the best performance, followed by PAM-2 and then by PAM-4. All evaluations were performed by offline processing experiments.

DFE Versus MLSE Electronic Equalization for Gigabit/s SI-POF Transmission Systems

In this letter, we compare three different types of electronic equalization techniques to compensate the limited bandwidth of gigabit/s step-index polymer optical fiber (SI-POF) transmission systems. In particular, we experimentally analyze the performance of a feed-forward equalizer (FFE), a feed-forward plus decision feedback equalizer (FFE+DFE), and a maximum likelihood sequence estimator (MLSE) equalizer in an optical link composed of a resonant cavity light-emitting diode (RC-LED), 50 m of SI-POF, and a photodiode. The investigation is carried out on a real hardware experiment based on available POF optoelectronics, while the equalization is implemented in software using an off-line processing approach. We show that the FFE+DFE solution pays only a limited penalty versus the much more complex MLSE algorithm. Our result shows that the FFE+DFE solution is a very good compromise between performance and complexity for a gigabit/s SI-POF system.

CW substrate-free metal-cavity surface microemitters at 300 K

In this paper substrate-free metal-cavity surface microemitters are demonstrated. The optical cavity is formed by a metal reflector, metal-surrounded sidewall and n-doped distributed-Bragg reflector, which provides optical feedback and carrier injection. We describe a simple design principle with the modal properties modified by geometry and metal-insulator cladding. Both resonant cavity light-emitting diodes (1.85 µm diameter and 0.6 µm height) and lasers (2.0 µm diameter and 2.5 µm height) are successfully fabricated and characterized. These two types of devices operate at room temperature under continuous-wave (CW) operation. Since the devices are substrate-free, they can be bonded to any substrates. From the threshold currents of the lasers, we obtain a high characteristic temperature of 425 K in the range of 10–27 °C. We also discuss a general approach to improve the diffraction from small-aperture devices.

Effect of top distributed Bragg reflectors on the performance of 650 nm AlGaInP resonant cavity light-emitting diodes

Three kinds of 650 nm AlGaInP resonant cavity light-emitting diodes (RCLEDs) are fabricated by metal organic chemical vapor deposition (MOCVD) with different numbers of pairs of top distributed Bragg reflectors (DBRs), which are 15, 10 and 5, respectively. By comparing the full width at half maximum (FWHM), light power and the angular far-field emission of the devices, the device with 15 pairs of top DBRs shows the best performance. Its FWHM is 13.4 nm and the light power is 0.63 mW at a driving current of 30 mA.

1.3 μm emitting GaInNAs/GaAs quantum well resonant cavity LEDs

The electrical and optical characteristics of a set of MBE-grown 1.3μm emitting GaInNAs/GaAs quantum well (QW) resonant cavity light-emitting diodes (RCLEDs) have been analyzed as a function of the growth temperature of the distributed Bragg reflector (DBR) and the contact alloying. The RCLEDs consist of a λ-long microcavity delimited on one side by an 8-pair AlAs/GaAs DBR and on the opposite side by a metallic mirror, with the QW located at one of the antinodes of the standing wave. The structures are designed for extraction of the light through the substrate. Room-temperature electroluminescence at 1.3μm is demonstrated with 38% In and 1.3% N in the quantum well. It is found that the RCLEDs with the DBR grown at the lowest temperature of 670°C show a lower differential slope efficiency and degraded current–voltage characteristics, i.e. higher ideality factor, lower shunt resistance and lower threshold voltage. This degradation can be explained to arise from the higher roughness observed in the DBRs grown at 670°C, which likely enhance the formation of point defects and compositional inhomogeneities in the quantum well. Alloying of the ohmic contacts also results in a widening of the electroluminescence emission, probably due to the degradation of the top metallic mirror sharpness caused by the diffusion of the metal into the GaAs. The best results in terms of differential slope efficiency and electrical characteristics are obtained when the DBRs are grown at 750°C and the contacts are left non-alloyed.

Enhancement of Light Extraction From Resonant Cavity Light-Emitting Diodes Using a 2-D Grating Embossed in TiO$_{2}$ Sol-Gel

To enhance the light extraction efficiency of a resonant cavity light-emitting diode (RCLED), a soft embossing technique was developed to fabricate a 2-D grating on the RCLED output surface. The optimal 2-D grating parameter was designed by a rigorous coupled wave analysis code. By embossing the 2-D grating on the output of a commercial RCLED, a 49% light extraction enhancement was achieved using a novel high refractive index ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> = 2) TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sol-gel and a 24% enhancement using a UV curable polymer with lower refractive index ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> = 1.56). The overall emission pattern was also modified making the patterned RCLED potentially easier to couple into a fiber.

Chromatic dispersion penalty by angular coupling between resonant cavity light-emitting diode and plastic optical fibre

Chromatic dispersion penalty by angular coupling between a resonant cavity light-emitting diode (RCLED) and a plastic optical fibre is presented. To evaluate the influence of the angular-dependent spectral characteristics of the RCLED on the chromatic dispersion penalty, the effective spectral linewidth of the RCLED was defined and the bit error rate performances for 500 Mbit/s by 50 m transmission were calculated. The calculated results were in good agreement with the experimental ones that showed the chromatic dispersion penalty of 1.2 dB at BER 10 -12 in the angle of 26deg.

Directional light extraction from thin-film resonant cavity light-emitting diodes with a photonic crystal

We report directional light extraction from AlGaInP thin-film resonant cavity light emitting diodes (RCLEDs) with shallow photonic crystals (PhCs). Diffraction of guided modes into the light extraction cone enhances the light extraction by a factor of 2.6 compared to unstructured RCLEDs, where the farfields still show higher directionality than Lambertian emitters. The external quantum efficiency is 15.5% to air and 26% with encapsulation, respectively. The PhC-RCLEDs are also more stable to a temperature induced wavelength shift than unstructured RCLEDs.