Effects of Bragg mirror interface grading and layer thickness variations on VCSEL performance at 1.55 μm

Abstract

A selectively oxidized Vertical Cavity Surface Emitting Laser (VCSEL) has been designed and fabricated for operation at a wavelength of l.546im. The lattice matched device structure was grown on an InP substrate using 111-V quaternary semiconductor alloys for Bragg mirrors and GaInAsP-based unstrained Multi-Quantum Wells (MQW) for the active layer. The mirror reflectivities are 97% for the top Distributed Bragg Reflector (DBR) consisting of 16 pairs of AlGaInAs/InP layers, and 99.9% for the bottom DBR consisting of22 pairs. A threshold current as low as 2.2mA has been achieved. The threshold voltage was typically lower than 2.0 V and the power output exceeded 1mW. The laser spectrum from a 7jtm confmed diode shows a single mode ofoperation at 1.54 m. The single fundamental mode was present at all current levels. The influence ofthe intentional and growth-related compositional grading at the heterointerfaces as well as random and fixed thickness variations of layer thickness on the mirror reflectivity and laser characteristics has been investigated, and key sensitivities to laser performance have been determined through computational simulations. It is shown that the degree of surface roughness and random thickness variation have the strongest impact on the device performance.