Distributed Bragg Reflector Structure Research Articles

P‐9.1: QD based color converter with DBR Structure and its application on Micro‐LED

An enhancement of light extraction efficiency of quantum dots (QD) in light‐emitting diodes (LEDs) with Bi‐functional TiO2/Al2O3 distributed Bragg reflector (DBR) nanolaminate structure grown by atomic layer deposition (ALD) has been demonstrated. The DBRs were simulated and optimized with TFCalc, and they exhibited excellent and tunable optical properties, as well as reliable moisture barrier performance. These DBRs were integrated in the QD‐LED, enabling an obvious increase in red emission and a strong decrease in blue light transmittance, which can achieve color conversion greatly. Furthermore, these DBRs can prolong the lifetime of QDs evidently by isolating the QDs from the moisture vapor. These results highlight the potential application of DBRs in the QLEDs and QD‐LEDs.

Photon-Recycling in Ultraviolet GaN-Based Photodiodes with Porous AlGaN Distributed Bragg Reflectors

GaN-based ultraviolet photodiode with porous distributed Bragg reflectors (DBR) structure was demonstrated. The n+-AlGaN:Si epitaxial layers with high refractive index had been etched as porous-AlG...

Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers

A distributed Bragg reflector (DBR) is a key building block in the formation of semiconductor microcavities and vertical cavity surface emitting lasers (VCSELs). The success in epitaxial GaAs DBR mirrors paved the way for the ubiquitous deployment of III-V VCSELs in communication and mobile applications. However, a similar development of GaN-based blue VCSELs has been hindered by challenges in preparing DBRs that are mass producible. In this article, we provide a review of the history and current status of forming DBRs for GaN VCSELs. In general, the preparation of DBRs requires an optimization of epitaxy/fabrication processes, together with trading off parameters in optical, electrical, and thermal properties. The effort of epitaxial DBRs commenced in the 1990s and has evolved from using AlGaN, AlN, to using lattice-matched AlInN with GaN for DBRs. In parallel, dielectric DBRs have been studied since 2000 and have gone through a few design variations including epitaxial lateral overgrowth (ELO) and vertical external cavity surface emitting lasers (VECSEL). A recent trend is the use of selective etching to incorporate airgap or nanoporous GaN as low-index media in an epitaxial GaN DBR structure. The nanoporous GaN DBR represents an offshoot from the traditional epitaxial approach and may provide the needed flexibility in forming manufacturable GaN VCSELs. The trade-offs and limitations of each approach are also presented.

High-Performance White Organic Light-Emitting Diodes Using Distributed Bragg Reflector by Atomic Layer Deposition

White organic light-emitting diodes (WOLEDs) with higher performance, which have enjoyed application in high-quality lighting sources, are here demonstrated with improved optical and electrical properties. The integration of a novel transparent distributed Bragg reflector (DBR), which consists of periodically alternating layers of atomic layer deposition-fabricated ZrO2/Zircone films and sputtered tin-doped indium oxide into OLEDs microcavities were studied to obtain four-peak electroluminescence (EL) spectra. Three types of OLEDs with two-peak, three-peak, and four-peak EL spectra have been developed. The results of the two-peak spectra show that the DBR structures have an outstanding effect on carrier capture; as a result, the device exhibits a stronger stability in color at various applied voltages. The Commission Internationale de L’Eclairage (CIE) coordinates of the two-peak device at 5–13 V shows few displacements and a negligible slight variation of (±0.01, ±0.01). In addition, the four-peak WOLED also yields a high color purity white emission as the luminance changes from 100 cd m−2 to 10,000 cd m−2.

Optical Properties of Alq3/TiO2 BDR Structure Processed by Spin Coating Technique

ABSTRACTOne-dimensional hybrid Distributed Bragg Reflector (DBR) is constructed using Tris (8-hydroxy) quinoline aluminum (Alq3) molecules and Titanium dioxide (TiO2) nanoparticles via spin coating process. Light emission from thin films of low molecular weight organic semiconductor of Alq3 is dominated by excitons. This material has been widely used as a superior emitter for organic light emitting diodes. Titanium dioxide (TiO2) is an inorganic semiconductor with a high band gap. Photoluminescence (PL) of thin films of Alq3 showed a broad PL peak at 530 nm. In DBR structures, PL quenching is observed but there is no shift in the PL peak of the Alq3. The PL quenching is tentatively attributed to energy transfer via sensitization to wide band gap TiO2 layers. A simple excitonic model is suggested to explain the observation. Fabrication process and optical properties of the structure are presented.

Strategy toward the fabrication of ultrahigh-resolution micro-LED displays by bonding-interface-engineered vertical stacking and surface passivation.

In this study, we proposed a strategy to fabricate vertically stacked subpixel (VSS) micro-light-emitting diodes (μ-LEDs) for future ultrahigh-resolution microdisplays. At first, to vertically stack the LED with different colors, we successfully adopted a bonding-interface-engineered monolithic integration method using SiO2/SiNx distributed Bragg reflectors (DBRs). It was found that an intermediate DBR structure could be used as the bonding layer and color filter, which could reflect and transmit desired wavelengths through the bonding interface. Furthermore, the optically pumped μ-LED array with a pitch of 0.4 μm corresponding to the ultrahigh-resolution of 63 500 PPI could be successfully fabricated using a typical semiconductor process, including electron-beam lithography. Compared with the pick-and-place strategy (limited by machine alignment accuracy), the proposed strategy leads to the fabrication of significantly improved high-density μ-LEDs. Finally, we systematically investigated the effects of surface traps using time-resolved photoluminescence (TRPL) and two-dimensional simulations. The obtained results clearly demonstrated that performance improvements could be possible by employing optimal passivation techniques by diminishing the pixel size for fabricating low-power and highly efficient microdisplays.

Hybrid method of plane-wave and cylindrical-wave expansions for distributed Bragg-reflector pillars: formalism and its application to topological photonics

A hybrid computational method of plane-wave and cylindrical-wave expansions for distributed Bragg-reflector (DBR) pillars is proposed. The plane-wave expansion is employed to represent the one-dimensional periodic structure of the DBR. The cylindrical-wave expansion is employed to describe the scattering by circular pillars with the DBR structure inside. This formalism enables us to calculate the radiation fields, $t$-matrices, scattering cross sections, photonic band structures, and quality factors of the DBR pillars. Furthermore, optical properties of arrayed DBR pillars are also investigated with the aid of the multiple-scattering method. Using this formalism, we demonstrate explicitly that high $Q$ photonic band modes including the so-called bound states in continuum are obtained both in isolated and arrayed DBR pillars. We also present a novel formation of gapless Dirac-cone surface states in a three-dimensional photonic crystal composed of a two-dimensional periodic arrangement of core-shell DBR pillars.

Fabrication of HfO2/TiO2–based conductive distributed Bragg reflectors: Its application to GaN-based near-ultraviolet micro-light-emitting diodes

Distributed Bragg reflector (DBR) has been used to enhance the performance of various optoelectronic devices because of its higher reflectance than metal reflector, particularly at a specific wavelength. However, the insulating property of the DBR structure have limited its use where the current injection is required (e.g., below electrodes). Here, we introduce a way to overcome this limit, by creating conductive paths in the DBR-electrode structure using an electrical breakdown process; thereby, achieving an ohmic contact with p-GaN contact layers, and finally apply to ultraviolet micro-light-emitting diodes (μLEDs) to verify the validity of the method. Specifically, by inserting three pairs of TiO2/HfO2-based conductive DBR structures under a Cr/Ni/Au-based p-type electrode, the reflectance of the p-type electrode was increased up to 95%, simultaneously increasing the output power of the μLED by 5% by reducing the light absorption at the p-type electrode by the reflection of light without electrical losses. This approach is expected to offer a great flexibility in the design of conventional devices using DBR structures.

Fabrication of vertically conducting near ultraviolet LEDs on SiC substrates

In this work, GaN-based vertically conducting near ultraviolet light-emitting diodes (LEDs) were grown on n-SiC substrates by metal-organic chemical vapor deposition. 10 pairs or 20 pairs of Si-doped n-Al0.2Ga0.8N/n-GaN distributed Bragg reflectors (DBRs) were used to improve the performance of the LEDs. The central reflected wavelength of the DBRs structures agreed well with the emission wavelength of the LEDs. A 389 nm near ultraviolet electroluminescence (EL) emission peak could be clearly observed in all the LED samples in our study. The EL intensity of LEDs was obviously enhanced because of the application of DBRs. Angle-dependent EL spectra were measured as well. For all the measurement angle region, the near ultraviolet EL emission intensity of the LEDs with DBRs were higher than that of the LED without DBRs.

A New Material and Structures for Light-Emitting Thyristor Applications

We have demonstrated that GaInP materials with GaP and distributed Bragg reflector (DBR) structures can be applied in light-emitting thyristors (LEThs) and depleted optical thyristors (DOTs). The GaP epitaxial film greatly improved the current spreading, so that the brightness of LEThs was also greatly improved. The luminous intensity increased by 70% at an injection current of 250 mA with the GaP epitaxial film, compared to the LEThs without a GaP film. Moreover, the DBR structure played a role to prevent the emission light from being absorbed by the opaque GaAs substrate. The DBR structure can improve the droop efficiency, reduce the operation voltage, and maintain the wavelength at high-current operation. The luminous intensity of LEThs with the 18-pairs DBR structure increased by 71% at an injection current of 900 mA, compared to LETh with only three-pairs DBR and GaP window structure. It also showed nonlinear s-shaped ${I}$ – ${V}$ characteristics as a function of the input light intensity. The turn-on voltages were significantly reduced, from 5.36 to 1.66 V, as the external optical input intensity changed from dark to light with 300 mW. Under certain conditions, it also showed standard diode curve characteristics; as a result, the LETh can also be used as a DOTs.

Resonant light emission from uniaxially tensile-strained Ge microbridges

A highly strained germanium microbridge is a promising platform for realizing monolithically integrated lasers on a silicon substrate. However, it remains challenging to combine it with optical resonators. Here, we have observed resonant light emission peaks with Q-factors of about 180 in room-temperature photoluminescence spectra from uniaxially tensile-strained germanium microbridges. These peaks are found to correspond to the resonance in Fabry–Perot (FP) cavities formed transversely to the uniaxial stress axis. On the basis of this phenomenon, we design a Fabry–Perot cavity by adding distributed Bragg reflectors (DBRs) laterally to the microbridge. With this design, the optical performance can be optimized without disturbing to the mechanical structure. A Q-factor as high as 1400 is obtained from numerical simulation. Moreover, we prove by theoretical analysis deduction and calculation that the lateral structure will not decrease the strain, unlike the on-pad DBR structure. The structure thus provides a promising solution for the realization of highly strained germanium lasers in the future.

Monolithic Red/Green/Blue Micro-LEDs With HBR and DBR Structures

In this letter, monolithic red, green, and blue (RGB) micro light-emitting diodes (LEDs) were fabricated using gallium nitride based blue micro LEDs and quantum dots (QDs). Red and green QDs were sprayed onto individual region surrounded by patterned black matrix photoresist on the blue micro LEDs to form color conversion layers. Owing to its light-blocking capability, the patterned black matrix photoresist improved the contrast ratio of the micro LEDs from 11 to 22. To enhance the color conversion efficiency and the light output intensity, a hybrid Bragg reflector (HBR) was deposited on the bottom side of the monolithic RGB micro LEDs, thus reflecting the RGB light emitted to the substrate. To further improve the color purity of the red and green light, a distributed Bragg reflector (DBR) with high reflection for the blue light was deposited on the top side of the QDs/micro LEDs. The red and green light output intensities of the micro LEDs with HBR and DBR were enhanced by about 27%.

Mirror design for long-wavelength vertical-cavity surface-emitting lasers

A kind of bottom mirror, fabricated using a nano-scale epitaxy method, is proposed for long-wavelength InP-based vertical-cavity surface-emitting lasers (VCSELs). The design of the bottom mirror combines the conventional dielectric distributed Bragg reflector structure with a fabrication procedure using nano-scale epitaxial lateral overgrowth (ELO). Hence, this ELO mirror structure can provide both a high reflectivity, to form an optical resonance cavity, and a virtual InP substrate for the growth of high quality active regions in the VCSEL. The dependence of the reflectivity on the epitaxial lateral overgrowth mirror structure parameters is analyzed in detail using numerical methods. The results demonstrate that a stopband at more than 99% reflectivity is achieved in the wavelength range from 1.49 to 2.09 µm, and the threshold optical gain can be as low as 202, 257, and 311 cm−1 for the internal loss coefficients of 10, 15 and 20 cm−1.

GaN/AlGaN ultraviolet light-emitting diode with an embedded porous-AlGaN distributed Bragg reflector

A GaN/AlGaN-based ultraviolet light-emitting diode (LED) structure with an embedded porous-AlGaN reflector was fabricated by a doping-selective electrochemical (EC) wet-etching process. The n+-AlGaN/undoped-AlGaN (u-AlGaN) stack structures with different Al contents were transformed into porous-AlGaN/u-AlGaN stack structures that acted as the embedded distributed Bragg reflectors (DBRs). The porosity of the EC-treated AlGaN layer was increased by decreasing the Al content in the n+-AlGaN layer. The reflectivity of the porous-AlGaN DBR structure was measured to be 90% at 379.3 nm with a 37.2 nm stopband width. The photoluminescence emission intensity of the DBR-LED was enhanced by forming the embedded porous-AlGaN DBR structure.

Design of ITO/SiO2/TiO2 distributed Bragg reflectors as a p-type electrode in GaN-based flip-chip light emitting diodes

Two different SiO2/TiO2 distributed Bragg reflectors (DBR) with ITO ohmic contacts were investigated as p-type reflective electrodes in InGaN/GaN flip-chip light-emitting diodes (FC-LEDs). The DBR structures were designed to have reflectance over 95% at wavelengths of 400–520nm (DBR 1) and 400–720nm (DBR 2). These ranges are wider than those of the electroluminescence spectrum of blue FC-LEDs. The FC-LEDs with DBR 1 showed an output power of 114mW at 200mA, while those with DBR 2 presented higher output power of 135mW. A ray tracing simulation showed that the anomalously low output power of the FC-LEDs with DBR 1 can be attributed to the low incidence angle to the DBR due to the diffraction of the light at the patterned sapphire substrate. A finite-difference time-domain simulation and angle-dependent reflectance measurement of the DBRs were also carried out. The results showed that DBR 2 yielded high reflectance even at low incidence angles, while DBR 1 presented low reflectance at incidence angles lower than 30°, followed by a reduction of the output power.

Tunable and multichannel terahertz perfect absorber due to Tamm surface plasmons with graphene

In this paper, we have shown that perfect absorption at terahertz frequencies can be achieved by using a composite structure where graphene is coated on one-dimensional photonic crystal (1DPC) separated by a dielectric. Due to the excitation of optical Tamm states (OTSs) at the interface between the graphene and 1DPC, a strong absorption phenomenon occurs induced by the coupling of the incident light and OTSs. Although the perfect absorption produced by a metal–distributed Bragg reflector structure has been researched extensively, it is generally at a fixed frequency and not tunable. Here, we show that the perfect absorption at terahertz frequency not only can be tuned to different frequencies but also exhibits a high absorption over a wide angle range. In addition, the absorption of the proposed structure is insensitive to the polarization, and multichannel absorption can be realized by controlling the thickness of the top layer.

Tunable InGaN quantum dot microcavity light emitters with 129 nm tuning range from yellow-green to violet

An electrically pumped wavelength-tunable InGaN quantum dot (QD) based microcavity (MC) lighter emitter with a large tuning range of 129 nm was demonstrated. The multi-mode emission spectrum was tuned by injected current from 564 nm (yellow-green) to 435 nm (violet). The MC light emitter is featured with a double dielectric distributed Bragg reflector structure and a copper substrate fabricated using substrate transfer and laser lift off techniques. By utilizing an InGaN QD active layer with a tunable broad emission spectrum and a Fabry-Pérot cavity which allows multi-longitudinal mode resonating, the emission spectrum could be tuned among several particular cavity modes, which are decided by the gain enhancement factor. In addition, both the enhancement and suppression of MC emission modes caused by the gain enhancement factor were observed in a single MC device. As the first electrically driven III-V nitride semiconductor based tunable MC light emitter with a tuning range of 129 nm, the device is promising for applications such as in wide-gamut compact displays and projectors.

Enhanced optical absorption of monolayer WS2 using Ag nanograting and distributed Bragg reflector structures

A hybrid novel structure of monolayer WS2 with Ag nanograting and distributed Bragg reflector (DBR) on Si substrate has been proposed to obtain broad-band absorption response for two-dimensional (2D) materials. It is effective to reduce light loss and reflect the incident light efficiently for monolayer WS2 absorption with DBR dielectric layers. Moreover, by combining Ag nanograting with DBR structure, the average absorption achieves as high as 52.9% within broad wavelength ranging from 420 to 700 nm, which is attributed to the plasmonic resonant effect of metal nanostripes. The absorption would be influenced by the period and height of the Ag nanograting, and shows incident angle dependent characteristics, while an average absorption higher than 50% has been obtained at the incident angle around 40°. These results indicate that 2D WS2 in combination with DBR and metal nanograting have a promising potential applications for optical nano-devices.

Size effect caused significant reduction of thermal conductivity of GaAs/AlAs distributed Bragg reflector used in semiconductor disk laser

Thermal properties of the distributed Bragg reflector (DBR) used in the semiconductor gain element are crucial for the performance of a semiconductor disk laser (SDL). For the purpose of more reasonable semiconductor wafer design, so as to improve the thermal management of SDLs, accurate thermal conductivity value of a DBR is under considerable requirement. By the use of equilibrium molecular dynamics method, thermal conductivities of GaAs/AlAs DBRs, which are widely employed in 1μm waveband SDLs, are calculated, and simulated results are compared with the reported experimental data. Influences of the layer thickness on the thermal conductivities of the DBR structure and the effects of Al composition on the AlxGa1−xAs ternary alloy values are focused and analyzed.

An improved design for AlGaN solar-blind avalanche photodiodes with enhanced avalanche ionization**Project supported by the State Key Project of Research and Development Plan, China (Grant No. 2016YFB0400903), the National Natural Science Foundation of China (Grant Nos. 61634002, 61274075, and 61474060), the Key Project of Jiangsu Province, China (Grant No. BE2016174), the Anhui University Natural Science Research Project, China (Grant No.

To enhance the avalanche ionization, we designed a new separate absorption and multiplication AlGaN solar-blind avalanche photodiode (APD) by using a high/low-Al-content AlGaN heterostructure as the multiplication region instead of the conventional AlGaN homogeneous layer. The calculated results show that the designed APD with Al0.3Ga0.7N/Al0.45Ga0.55N heterostructure multiplication region exhibits a 60% higher gain than the conventional APD and a smaller avalanche breakdown voltage due to the use of the low-Al-content Al0.3Ga0.7N which has about a six times higher hole ionization coefficient than the high-Al-content Al0.45Ga0.55N. Meanwhile, the designed APD still remains a good solar-blind characteristic by introducing a quarter-wave AlGaN/AlN distributed Bragg reflectors structure at the bottom of the device.