Dielectric DBRs Research Articles

Photonic engineering of InP towards homoepitaxial short-wavelength infrared VCSELs

Many emerging opportunities, such as three-dimensional (3D) sensing, biophotonics, and optical data links, call for vertical cavity surface-emitting lasers (VCSELs) that operate in the short-wavelength infrared (SWIR) range. In this paper, we report the use of InP distributed Bragg reflector (DBR) mirrors to overcome an impasse in wafer-level mass production of SWIR VCSELs. The DBRs were based on homoepitaxial InP structures and selectively converted through electrochemistry into quarter-wavelength stack structures of alternating nanoporous (NP) and nonporous InP layers with a record index contrast (Δn∼1.0) and near-unity reflectivity. We demonstrated VCSEL operation at both 1380 and 1550 nm from two separate structures prepared on InP substrates using NP–InP DBRs as the bottom mirror and dielectric DBRs as the top mirror. Room temperature continuous-wave (CW) operation of SWIR VCSELs was successfully achieved at both wavelengths with a threshold current density below 2kA/cm2, greater than milliwatt optical output, and a peak power conversion efficiency of 17%. Our work provides strong evidence that the decades-old challenge, in preparing an InP-compatible, high-performance DBR to support the SWIR-emitting vertical cavity, has been addressed and is poised to enable new applications.

Improvement of Thermal Dissipation of GaN-Based Micro Cavity Light-Emitting Devices

Double dielectric DBR based GaN-based micro cavity light emitting devices with two different structures were fabricated, and their thermal characteristics were investigated. To improve thermal dissipation, an AlN current confinement layer with a much higher thermal conductivity than SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and an electroplated copper heat sink were utilized. The thermal resistance of the device decreased from 923 K/W to 457 K/W, half of that obtained with the typically used SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> current confinement layer and bonded substrate. This is the lowest reported value in GaN-based micro cavity light-emitting devices with double dielectric DBRs. Temperature distribution and heat flux inside of the device was simulated based on a steady state quasi three-dimensional cylindrical model. The results show that heat transport in vertical direction is efficiently bypassing the bottom DBR to the copper plate. This work provides an effective method to improve thermal characteristics of GaN-based micro cavity light-emitting devices with double dielectric DBR structure.

Fabrication and spectroscopy of GaN microcavities made by epitaxial lift-off

A novel epitaxial lift-off process for III-nitrides, involving selective removal of a sacrificial (Al, In)N layer in a hot nitric acid etchant, is reported. This was applied to the fabrication of 1−λ GaN planar microcavities bounded by two dielectric DBRs, starting from epitaxial GaN–(Al, In)N–GaN trilayers grown on free-standing GaN or high-quality GaN template material. An optically smooth surface was retained on the GaN (0001̄) surface exposed to the nitric acid etch, with root mean square roughness values as low as 2 nm over 8 μm×8 μm areas. Photoluminescence and reflectivity spectra were recorded from completed microcavities, and the latter showed clear dips in the region of 3.5 eV.