Top Dielectric Distributed Bragg Reflector Research Articles

InGaN Resonant-Cavity Light-Emitting Diodes with Porous and Dielectric Reflectors

InGaN based resonant-cavity light-emitting diode (RC-LED) structures with an embedded porous-GaN/n-GaN distributed Bragg reflector (DBR) and a top dielectric Ta2O5/SiO2 DBR were demonstrated. GaN:Si epitaxial layers with high Si-doping concentration (n+-GaN:Si) in the 20-period n+-GaN/n-GaN stacked structure were transformed into a porous-GaN/n-GaN DBR structure through the doping-selective electrochemical wet etching process. The central wavelength and reflectivity were measured to be 434.3 nm and 98.5% for the porous DBR and to be 421.3 nm and 98.1% for the dielectric DBR. The effective 1λ cavity length at 432nm in the InGaN resonant-cavity consisted of a 30 nm-thick Ta2O5 spacer and a 148 nm-thick InGaN active layer that was analyzed from the angle-resolved photoluminescence (PL) spectra. In the optical pumping PL spectra, non-linear emission intensity and linewidths reducing effect, from 6.5 nm to 0.7 nm, were observed by varying the laser pumping power. Directional emission pattern and narrow linewidth were observed in the InGaN active layer with bottom porous DBR, top dielectric DBR, and the optimum spacer layer to match the short cavity structure.

Electrically pumped WSe2-based light-emitting van der Waals heterostructures embedded in monolithic dielectric microcavities

Vertical stacking of atomically thin layered materials opens new possibilities for the fabrication of heterostructures with favorable optoelectronic properties. The combination of graphene, hexagonal boron nitride and semiconducting transition metal dichalcogenides allows fabrication of electroluminescence (EL) devices, compatible with a wide range of substrates. Here, we demonstrate a full integration of an electroluminescent van der Waals heterostructure in a monolithic optical microcavity made of two high reflectivity dielectric distributed Bragg reflectors (DBRs). Owing to the presence of graphene and hexagonal boron nitride protecting the WSe2 during the top mirror deposition, we fully preserve the optoelectronic behaviour of the device. Two bright cavity modes appear in the EL spectrum featuring Q-factors of 250 and 580 respectively: the first is attributed directly to the monolayer area, while the second is ascribed to the portion of emission guided outside the WSe2 island. By embedding the EL device inside the microcavity structure, a significant modification of the directionality of the emitted light is achieved, with the peak intensity increasing by nearly two orders of magnitude at the angle of the maximum emission compared with the same EL device without the top DBR. Furthermore, the coupling of the WSe2 EL to the cavity mode with a dispersion allows a tuning of the peak emission wavelength exceeding 35 nm (80 meV) by varying the angle at which the EL is observed from the microcavity. This work provides a route for the development of compact vertical-cavity surface-emitting devices based on van der Waals heterostructures.

Polarization-pinned emission of a continuous-wave optically pumped nonpolar GaN-based VCSEL using nanoporous distributed Bragg reflectors.

A nonpolar GaN-based vertical-cavity surface-emitting laser (VCSEL) using nanoporous bottom epitaxial distributed Bragg reflector (DBR) is demonstrated at room temperature (RT) under continuous-wave (CW) optical pumping. The porous layers enable the epitaxial growth of lattice-matched high-reflectance DBRs without sacrificing the conductive properties needed for high-performance VCSELs. The 2-λ cavity VCSEL reported here employs a hybrid design with top dielectric DBR and bottom nanoporous DBR. Single longitudinal mode lasing is observed at 462 nm with a threshold power density of ~5 kW/cm2 and a FWHM of ~0.12 nm. The emission polarization was pinned in the a-direction at all measured locations.

Thermal properties of GaN-based semiconductor-metal subwavelength grating VCSELs and novel current injection scheme

This paper provides a thermal and electrical analysis of semiconductor-metal subwavelength-grating (SMSG) vertical-cavity surface-emitting lasers (VCSELs). In the proposed design, the top distributed Bragg reflector (DBR) of a GaN-based VCSEL is replaced with a GaN–Ag SMSG, which serves as both an optical reflector and an electrical contact. Using a numerical 3D self-consistent thermo-electric model combined with a 3D optical model, we show that GaN-based SMSG VCSELs with aperture sizes of over 50 µm are capable of laser operation. We also propose spatial tuning of the injected current density to eliminate the current crowding effect in large area VCSELs. The use of GaN–Ag SMSGs in GaN-based VCSELs eliminates the need for a top dielectric DBR and intracavity injection scheme, making the fabrication of monolithic VCSELs possible. It also eliminates the need to use a tunnel junction with optical confinement built into the cavity. Although the GaN–Ag SMSG introduces higher optical losses than a DBR, its advantage is that it permits vertical current injection into the active region, enabling laser emission from broader-area active regions in comparison to conventional VCSELs.

Fabrication and Characterization of GaN-Based Resonant-Cavity Light-Emitting Diodes with Dielectric and Metal Mirrors

We demonstrated GaN-based resonant-cavity light-emitting diodes (RCLEDs) fabricated on a copper substrate by electroplating and laser lift-off techniques. A TiO2/SiO2 dielectric distributed Bragg reflector (DBR) and an aluminum metal reflector were employed as top and bottom mirrors, respectively. As compared to the conventional LED without a top mirror, improved performances were achieved in RCLED devices due to the resonant cavity effect. We also investigated the effects of the top TiO2/SiO2 dielectric DBR on device characteristics. By tuning the reflectivity of the top dielectric DBR, the optimum RCLED device with the top DBR reflectivity of 55% shows a high output power of 62 mW, an external quantum efficiency of 14.8%, a full width at half maximum (FWHM) of 12 nm for emission spectrum, a 50% viewing angle of 122° and a -3 dB modulation bandwidth of 48 MHz. The results show that the RCLED device with the optimum reflectors has considerable potential for future high-performance application.

GaN-based vertical-cavity surface emitting lasers with sub-milliamp threshold and small divergence angle

A GaN-based vertical-cavity surface emitting laser (VCSEL) structure featuring a silicon-diffusion-defined current blocking layer for lateral confinement is described. Sub-milliamp threshold currents were achieved for both 3- and 5-μm-aperture VCSELs under continuous-wave operation at room temperature. The vertical cavity was defined by a top dielectric distributed Bragg reflector (DBR) and a bottom epitaxial DBR. The emission spectrum exhibited a single peak at 411.2 nm with a linewidth of 0.4 nm and a side mode suppression ratio of more than 10 dB before device packaging. The full-width-at-half-maximum divergence angle of the 3-μm-aperture VCSEL was as small as approximately 5° which is the lowest number reported. These results implied the 3-μm-aperture VCSEL was in near single-mode operation.

GaN-Based Vertical Cavities with All Dielectric Reflectors by Epitaxial Lateral Overgrowth

GaN-based vertical cavities with all dielectric top and bottom distributed Bragg reflectors (DBRs) on c-plane sapphire were investigated under optical injection and compared with those having AlN/GaN bottom and dielectric top DBRs on freestanding GaN. A novel fabrication method employing two epitaxial lateral overgrowth steps is introduced to produce a cavity on bottom dielectric DBRs without the need to remove the sapphire substrate. Under high optical excitation, the cavity with all dielectric DBRs exhibited quality factors up to 1400 and an order of magnitude lower stimulated emission threshold density (5 µJ/cm2) than those employing top dielectric DBRs but semiconductor AlN/GaN bottom DBRs on freestanding GaN. This novel approach is expected to lead to injection vertical cavity lasers with naturally formed nearly defect-free active regions and current confinement without any oxidation steps.

Effects of Inhomogeneous Gain and Loss on Nitride-Based Vertical-Cavity Surface Emitting Lasers

Nitride-based vertical cavity surface emitting lasers (VCSEL) with hybrid mirror has been investigated. We further measured the as-grown samples (without top dielectric distributed Bragg reflector) by µ-photoluminescence, scanning near-field optical microscopy, and cathodoluminescence (CL). By different excitation power density, the experimental results indicated that the VCSEL devices have different lasing modes and spot sizes, represented inhomogeneous gain and loss distribution in the whole structure. The non-uniform emission intensity distribution including several bright spots of about 1 to 2 µm was observed for both VCSEL devices and as-grown samples, which was obtained from CL measurement, due to the effect of indium clusters and non-uniform micro-cavity resonant in InGaN multi-quantum wells (MQWs). The results show the significant influence on commercial applications such as light emitting diodes, lasers devices and so on.

Blue-Violet Lasing of Optically Pumped GaN-Based Vertical Cavity Surface-Emitting Laser With Dielectric Distributed Bragg Reflectors

Optically pumped GaN-based vertical cavity surface-emitting laser (VCSEL) with two Ta2O5/SiO2 dielectric distributed Bragg reflectors (DBRs) was fabricated via a simplified procedure: direct deposition of the top DBR onto the GaN surface exposed after substrate removal and no use of etching and polishing processes. Blue-violet lasing action was observed at a wavelength of 397.3 nm under optical pumping at room temperature with a threshold pumping energy density of about 71.5 mJ/cm2. The laser action was further confirmed by a narrow emission linewidth of 0.13 nm and a degree of polarization of about 65%. The result suggests that practical blue-violet GaN-based VCSEL can be realized by optimizing the laser lift-off technique for substrate removal.

Blue lasing at room temperature in an optically pumped lattice-matched AlInN/GaN VCSEL structure

Laser action with low threshold average pump power density (∼50 W · cm−2) at room temperature is reported for a crack-free planar vertical cavity surface emitting laser (VCSEL) structure based on a bottom lattice-matched AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n- and p-type GaN layers surrounding only three InGaN/GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL. In addition to low threshold, a spontaneous emission coupling factor β∼2×10−3 is derived for this ready-to-be-processed laser structure.

Single-mode 1.3-/spl mu/m photonic crystal vertical-cavity surface-emitting laser

We report the use of a photonic crystal in a 1.3-mum vertical-cavity surface-emitting laser to achieve single fundamental-mode operation. A focused ion beam was used to mill the photonic crystal into a top dielectric distributed Bragg reflector. The laser operated continuous-wave at room temperature with greater than 1 mW of single-mode output power observed with a sidemode suppression ratio of more than 37 dB and little change to the electrical properties of the device

GaN-Based Green Resonant Cavity Light-Emitting Diodes

GaN-based resonant cavity light-emitting diodes (RCLEDs) have been successfully fabricated on Si substrate by laser lift-off and wafer bonding techniques. A five-pair TiO2/SiO2 dielectric distributed Bragg reflector (DBR) (with 85% reflectivity) and an Ag layer (with 99% reflectivity) were employed as top and bottom mirrors, respectively, for front emission RCLEDs. The room temperature light output power of the RCLED was 1.5 times that of similar LED structures without a top DBR mirror under 20 mA injection current. The cavity modes exhibit a linewidth of 5.5 nm at 525 nm wavelength, which corresponds to a quality factor about 100. Moreover, the full width at half maximum of the emission can be reduced to 35 nm, as a result of the effect of the resonant cavity.