Analysis of defect-related optical degradation of VCSILs for photonic integrated circuits

Abstract

Laser diodes are of paramount importance for on-chip telecommunications applications, and a wide range of sensing devices that require near-infrared sources. In this work, the devices under test are vertical-cavity silicon-integrated lasers (VCSILs) designed for operation at 845 nm in photonic integrated circuits (PICs). We focus on the analysis of the degradation of the optical performance during aging. To investigate the reliability of the devices, we carried out several stress tests at constant current, ranging from 3.5 mA to 4.5 mA representing a highly accelerated stress condition. We observed two different degradation modes. In the first part of the experiments, the samples exhibited a worsening of the threshold current, but the sub-threshold emission was unaffected by degradation. We associated this behavior to the diffusion of impurities that, from the p-contact, were crossing the upper mirror implying a worsening of the DBR optical absorption. In the second stage of the stress test, the devices showed a higher degradation rate of the threshold current, whose variation was found to be linearly correlated to the worsening of the sub-threshold emission. We related this second degradation mode to the migration of the same impurities degrading the top DBR that, when reaching the active region of the laser, induced an increase in the non-radiative recombination rate. In addition to that, we related the two degradation modes to the change in series resistance, which was ascribed to the resistivity increment of the top DBR first and of oxide aperture afterwards.

Lifetime-limiting mechanisms of integrated IR sources for silicon photonics

“…Also the epitaxy of III-As vertical-cavity 850 nm lasers heterogeneously integrated on SiN substrate was found to be affected by an impurity diffusion process 11 . The very same devices described in 12 , featuring an oxide aperture of 4 m, exhibit a remarkable optical degradation process when submitted to prolonged accelerated aging at currents in excess of 20 kA/cm 2 (the typical threshold current density at room temperature (RT) is approximately 1.35 kA/cm 2 ).…”