Bottom Distributed Bragg Reflector Research Articles

Characteristics of Multileaf Holey Light-Emitting Diodes for Fiber-Optic Communications

An oxide-confined multileaf holey light-emitting diode (LED) in the 780 nm range is reported. The device is consisted of bottom distributed Bragg reflector (DBR), quantum wells (QWs), and top DBR, with a multileaf holey structure within the p-ohmic contact ring for light extraction. The spontaneous emission from under the etched holes and internally reflected lights can be extracted and collimated out of the smaller etched leaf holes. High-resolution imaging studies indicate that the device emits with smaller beams mainly through the multileaf etched holes made it suitable for fiber-optic communications.

Optical loss and interface morphology in AlGaAs∕GaAs distributed Bragg reflectors

It is shown that n-type doping of AlGaAs∕GaAs distributed Bragg reflectors (DBRs) grown by metal-organic vapor-phase epitaxy has a profound negative impact on the performance of vertical-cavity surface-emitting lasers (VCSELs) based on such mirrors. Using an intracavity contact scheme, 1.3-μm-range InGaAs VCSELs with and without doping in the bottom DBR are directly compared. Doped mirrors lead to lower slope efficiency, lower output power, and higher threshold current. From x-ray diffraction, high-accuracy reflectance measurements, and atomic force microscopy studies, it is suggested that this performance degradation is due to the doping-enhanced Al–Ga interdiffusion, leading to interface roughening and increased scattering loss.

Characteristics of P-Substrate Small-Aperture Holey Light-Emitting Diodes for Fiber-Optic Applications

A small-aperture oxide-confined holey light-emitting diode (LED) on p-type GaAs substrate in the 830 nm range is reported. The device is consisted of bottom distributed Bragg reflector (DBR), quantum wells (QWs), and top DBR, with a small-aperture holey structure at the center for light extraction. The internally reflected spontaneous emission can be extracted and collimated out of the etched hole. High-resolution imaging studies indicate that the device emits with a narrower beam mainly through the central etched hole region made it suitable for fiber-optic applications.

Fabrication and Characteristics of GaN-Based Microcavity Light-Emitting Diodes with High Reflectivity AlN/GaN Distributed Bragg Reflectors

In this paper, we report a GaN-based microcavity light-emitting diode (MCLED) which is composed of 25 pairs of high-reflectivity GaN/AlN distributed Bragg reflector (DBR) and 6 pairs of ex-situ deposited SiO2/TiO2 dielectric mirrors. The electroluminescence peak of this structure matched well with the high reflectance area of the top and bottom DBRs, and shows a narrow emission of approximately 6.7 nm. The fabricated device also shows a more excellent performance on the stability of the emission peak wavelength while varying injection current density and operating temperature than a regular LED.

Monolithic polymer microcavity lasers with on-top evaporated dielectric mirrors

We report on a monolithic polymeric microcavity laser with all dielectric mirrors realized by low-temperature electron-beam evaporation. The vertical heterostructure was realized by 9.5 TiOx∕SiOx pairs evaporated onto an active conjugated polymer, that was previously spincast onto the bottom distributed Bragg reflector (DBR). The cavity supports single-mode lasing at 509nm, with a linewidth of 1.8nm, and a lasing threshold of 84μJ∕cm2. We also report on the emission properties of the polymer we used, investigated by a pump-probe technique. These results show that low-temperature electron-beam evaporation is a powerful and straightforward fabrication technique for molecular-based fully integrable microcavity resonators.

OPTIMAL DESIGN OF SINGLE-PHOTON SOURCE EMISSION FROM A QUANTUM-DOT IN MICRO-PILLAR MICROCAVITY

We have modelled wavelength scale micro-pillar microcavities of group III-V semiconductor materials using the 3-D finite difference time domain (FDTD) method. A broad band dipole source within the microcavity probes the microcavity mode structure and spectrum. We then investigated the modifications to spontaneous emission of photons form narrowband emitters (e.g. quantum dots) at the centre of the resonance. We find strongly enhanced emission due to small modal volumes and high quality factor (Q-factor). A large fraction of the quantum-dot spontaneous emission is coupled into the fundamental cavity mode. Increasing the number of mirror pairs in the bottom distributed Bragg reflector (DBR) obviously reduces the bottom light leakage, leading to light collection efficiency up to 90%. Moreover, we are now looking at more sophisticated structures with both lateral and perpendicular confinements based on annular and photonic crystal defect cavities in order to suppress the remaining sidewall scattering.

Polymer microcavities by room temperature electron-beam evaporation of TiO x and SiO x

We report on monolithic symmetric vertical microcavities exploiting two TiC x/SiO x distributed Bragg reflectors (DBRs), realized by means of electron-beam evaporation. The whole nanofabrication process is carried out without heating the chamber atmosphere, thus allowing the direct evaporation of the top dielectric mirror onto the organic layer previously spin-cast on the bottom DBR stack. The high adhesion to the substrate, the possibility of employing active media of low softness temperature, and the absence of any degradation of the emission properties of the organic compound opens the way for novel vertical emitting resonators and monolithic photo-and electroluminescent microcavities. Our mirrors exhibit a stop-band reflectivity higher than 95% and a spectral width of more than 170 nm.

An approach to the design of highly selective resonant-cavity-enhanced photodetectors

The information capacity of a communication system employing a wavelength-division multiplexing scheme can be increased by reducing the free spectral range between different signals, reducing the full width at half maximum (FWHM), and detecting these closely spaced channels. After critically analyzing the design parameters of existing resonant-cavity-enhanced (RCE) p–i–n photodetectors, we have determined that more closely spaced channels can be detected either by increasing the length of a RCE p–i–n detector or by reducing the stop-band width of the bottom mirror. A masking procedure is described to determine the maximum cavity length attainable for any bottom distributed Bragg reflector (DBR) materials so that photodetector filters and detects only one wavelength, being insensitive to all other wavelengths. The optimized cavity length of single wavelength selective RCE detector operating at 1.3μm using 42 12 pairs of InAlGaAs∕InP as a bottom DBR is 7.5μm and its theoretical FWHM is 2.5nm.

Continuously Tunable 1.55-<tex>$mu$</tex>m VCSEL Implemented by Precisely Curved Dielectric Top DBR Involving Tailored Stress

We present an optically pumped and continuously tunable 1.55-μm vertical-cavity surface-emitting laser (VCSEL). The device shows 26-nm spectral tuning range, 400-μW maximum output power, and 57-dBm side-mode suppression ratio. The VCSEL is implemented using a two-chip concept. The movable top mirror membrane is precisely designed to obtain a tailored air-gap length (L'=16 μm) and a radius of curvature (ROC=4.5mm) in order to efficiently support the fundamental optical mode of the plane-concave resonator. It consists of a distributed Bragg reflector (DBR) with periodic, differently stressed silicon nitride and silicon dioxide multilayers implemented by plasma-enhanced chemical vapor deposition. The lower InP-based part, comprising the InP-InGaAsP bottom DBR and the active region, is grown monolithically using metal-organic vapor phase epitaxy.

Design and technology of vertical-cavity surface-emitting lasers with nonconducting epitaxial mirrors

Design and technology problems in the fabrication of vertical-cavity surface-emitting lasers (VCSELs) equipped with nonconducting distributed Bragg reflectors (DBRs) and fabricated using molecular-beam epitaxy are considered. VCSELs with an active region were formed on the basis of InGaAs quantum wells and incorporated an AlGaAs/GaAs bottom DBR and an oxidized AlGaO/GaAs top DBR; the diameter of the oxidized aperture was equal to 7–12 µm. The devices exhibit a continuous-wave lasing at room temperature with threshold currents of 0.5–1.5 mA, a differential efficiency as high as 0.5 mW/mA, and a highest output power of 3 mW.

Long wavelength MEMS tunable VCSEL with InP-InAlGaAs bottom DBR

In this letter, we present the first demonstration of a long wavelength-tunable vertical-cavity surface-emitting laser (VCSEL) using an InP-InAlGaAs distributed Bragg reflector (DBR). The use of such a DBR improves the thermal resistance of the VCSEL while keeping the growth process simple. The devices show operation to temperatures greater than 75/spl deg/C, and single-mode devices emit powers as high as 0.9 mW at room temperature. The tuning range is as high as 17 nm.

Wafer-Bonded 850-nm Vertical-Cavity Surface-Emitting Lasers on Si Substrate with Metal Mirror

An 850-nm vertical-cavity surface-emitting laser (VCSEL) with a Au/AuBe/TaN/Ta/Si mirror substrate has been realized by low-temperature wafer bonding. It is found that the mirror substrate can be used as the bottom reflector to enhance the reflectivity of a bottom distributed Bragg reflector. The metal mirrors also served as the adhesive layers and ohmic contact layers to bond the Si substrate and the VCSEL epilayers. When the mirror-substrate-bonded VCSELs are excited by continuous-wave current at room temperature, they exhibit lower threshold current density and differential resistance (22 A/cm2, 35 Ω) as compared with the original VCSELs on GaAs substrates (77 A/cm2, 60 Ω). This feature is attributed to the finding that the Si substrate provides a good heat sink.

Low temperature wafer bonding by spin on glass

We report the development of a low temperature (∼400 °C) and low pressure (∼0.5 kg/cm2) spin-on-glass (SOG) wafer bonding technique that can bond compound semiconductors and silicon without using chemical-mechanical polishing, surface etching or other intermediate materials in the bonding process. The relation of bonding quality and applied bonding pressure was studied. Cross sectional transmission electron microscopy analysis shows that the bonding interface is smooth, uniform and did not generate dislocations. Using this SOG bonding method, simulated vertical-cavity surface-emitting laser (VCSEL) structures that consist of GaInAs and InP cavities sandwiched by Al–oxide/Si distributed Bragg reflectors (DBRs) were successfully bonded to a silicon substrate with the bonding interface located outside the bottom DBR away from the VCSEL cavity.

Quantum dots for VCSEL applications at [formula omitted

GaAs-based vertical cavity surface-emitting lasers (VCSELs) using self-organized quantum dots (QDs) emitting at 1.3 μm demonstrate device-acceptable parameters. Threshold currents below 2 mA , operation voltage below 2 V and differential efficiency in excess of 60% are demonstrated. Maximum CW output power of 0.8 mW is realized for 8 μm oxide-confined aperture device. Using fully oxidized top and bottom distributed Bragg reflectors allows reducing the total thickness of the structure to only 5– 6 μm . Lifetime and temperature cycling tests confirm high reliability of the device. Confinement of nonequilibrium carriers in the QDs facilitates applications in VCSEL arrays with ultrasmall apertures and microcavities. Low homogeneous line width in single QDs makes potentially possible realization of single QD VCSELs.

Design of a 364 nm Electrically Pumped Multi-Quantum Well Continuous Wave Nitride Vertical Cavity Surface Emitting Laser

ABSTRACTShort-wavelength vertical cavity surface emitting lasers (VCSELS) are typically important for high-density optical memory and optical imaging systems. An AlGaN/GaN multiquantum well (MQW) VCSEL was designed for operation at wavelength of 364 nm. The design process consisted of careful selection of materials and optimization of the parameters for distributed Bragg reflector (DBR) stacks, active region, and calculation of the threshold current density, external quantum efficiency, and threshold carrier concentration. InGaN/AlGaN material system was selected for constructing DBR stacks in order to minimize power dissipation and to achieve high reflectivity. A reflectivity of 99% was calculated for 33 pairs in the bottom DBR stack and 97% was calculated for 30 pairs in the top DBR stack. An external quantum efficiency of 85% has been achieved through parametric optimization. Results were comparable to that of the similar structures based on the review of recent literatures.

High Gain, Low Threshold Current GaInAsP Based Vcsels for Operation at 1.24UM

ABSTRACTAn electrically pumped 1.24um GaxIn1-xAsyP1-y Vertical Cavity Surface Emitting Laser (VCSEL) has been designed and simulated. Threshold gain of 153cm-1 and threshold current of 1.00mA was obtained. The external efficiency predicted is 0.31. Also the optimized design of VCSEL structure is presented in this article. GaInAsP and AlInAs constitute the Distributed Bragg Reflectors (DBRs) multilayer stack. Reflectivity of the top DBRs is 0.97 and reflectivity of the bottom DBRs is 0.9978 and is shown to provide a good resonant cavity and sufficient lasing intensity. Compared with other reported structures, the present VCSEL has a lower threshold current and higher threshold gain.

Lasing in the vertical direction in InGaN/GaN/AlGaN structures with InGaN quantum dots

InGaN/GaN structures with dense arrays of InGaN nanodomains were grown by metallorganic chemical vapor deposition. Lasing in vertical direction occurs at low temperatures, indicating ultrahigh gains (∼ 105 cm−1) in the active region. Fabrication of an effective AlGaN/GaN distributed Bragg reflector with reflectivity exceeding 90% enables vertical lasing at room temperature in structures with a bottom distributed Bragg reflector, despite the absence of a well-reflecting upper mirror. The lasing wavelength is 401 nm, and the threshold excitation density is 400 kW/cm2.

Control of wet-etching thickness in the vertical cavity surface emitting laser structure by in situ laser reflectometry

We have investigated the use of in situ laser reflectometry for etch depth control in vertical cavity surface emitting laser (VCSEL) structures. We can obtain the precise etch depth by simply counting the number of oscillations during the wet-etching process. After calibration on an AlGaAs/GaAs 20-period distributed Bragg reflector (DBR) structure, this technique was applied to the monolithic 1.55 μm VCSEL device process that is required to etch epilayers of several μm thickness. In the 1.55 μm VCSEL structure, the mesa was generated by monitoring the number of peaks in the bottom DBR that corresponded to an etch depth of about 11 μm. The overall spatial uniformity of the etched sample was within a layer (1000 Å) by the distinction between layers. This process offers the possibility of precise control of etch depth for any multilayer structure.

Design of vertical cavity lasers with intracavityphotodetectors

The magnitude of photocurrent generated in a resonant cavity enhanced pin photodetector embedded within the top or bottom distributed Bragg reflector of a vertical cavity laser is predicted using a characteristic matrix formalism. Modelling results for an example device emitting at 650 nm are presented and discussed.

Monolithic 1.55μm Surface Emitting Laser Structure With In0.53Al0.14Ga0.33As/In0.52Al0.48As Distributed Bragg Reflector and Single Cavity Active Layer Grown by Meatalorganic Chemical Vapor Deposition Method

AbstractVertical cavity surface emitting laser (VCSEL) structure designed at 1.55 μm was grown by low pressure metalorganic chemical vapor deposition method. The VCSEL structure contains top and bottom distributed Bragg reflector (DBR) and single cavity active layer. The DBR was grown with In0.53Al0.14Ga0.33As and In0.52Al0.48As quarter lambda wavelength layer, alternatively. The growth temperature was as high as 750°C to prevent ordering and phase separation of the In0 52Al0.48As layer. The In0.52Al0.48As buffer layer was subsequently grown on the InP buffer layer in order to make an abrupt uniform interface. Unity reflectance was achieved at the center of 1.55 μm with 35.5 pairs undoped DBR layer. The reflectance spectrum of undoped DBR showed a wide flat-band region (greater than 50 nm) where the reflectivity was more than 99.5 %. The center wavelength of DBR was previously determined by an in-situ laser reflectometry technique during the growth of the whole structure. An infrared laser operating at 1.53 μm, which was the design wavelength of DBR layer was used as an in-situ measuring tool. The In0.53Ga0.47As multiple quantum well was used as a cavity layer. The reflectance spectrum of VCSEL structure, which included a single cavity active layer, showed excellent square shaped stop band and also showed an absorption region at the center of the flat band.