Progress and challenges in electrically pumped GaN-based VCSELs

Abstract

The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, medical applications, and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs is challenging, but during recent years several groups have managed to demonstrate electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm 2 . The performance is limited by challenges such as achieving high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control and lateral optical mode confinement. This paper summarizes different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span.

Effects of Lateral Optical Confinement In GaN VCSELs With Double Dielectric DBRs

“…In recent years, several academic and industrial research groups have demonstrated electrically pumped GaN-based VCSELs [3]- [11]. However, the performance of GaN-based VCSELs is still not good enough and challenges are necessary in such as achieving high-reflectivity mirrors, efficient lateral current spreading and lateral optical confinement (LOC) [12]. Further improvements in GaNbased VCSELs are required toward applications.…”

Improved Output Power of GaN-based VCSEL with Band-Engineered Electron Blocking Layer

“…Among those methods, Al x Ga 1−x N electron blocking layer (EBL) is usually placed between the active region and p-GaN layer, since it provides a higher energy barrier in the conduction band between quantum wells (QWs) and the p-layers. The higher energy barrier in the conduction band contributes to the confinement of the electrons in the active region, so stronger output power can be achieved [15]. However, the very commonly used Al x Ga 1−x N EBL sometimes cannot efficiently reduce electron leakage due to the polarization charge in that layer, which decreases the barrier height for electron transport [16,17].…”

Vertical Electrical Conductivity of ZnO/GaN Multilayers for Application in Distributed Bragg Reflectors

“…sensing and optical communication. However, III-nitride based VCSELs with emission wavelengths in the UV and visible, which could find applications in pico projectors, head-up and near-eye displays, biomedicine, and visible-light communication [1], [2], are still not commercially available. Several research groups have reported lasing of electrically injected III-nitride VCSELs [3]- [12] but there is still a need to reach higher output powers, lower threshold currents and improved thermal stability.…”