Impact of Stress Current on Electro-Optical Properties of the Active Cavity Region in 850 Nm VCSELs

Vertical-cavity surface-emitting lasers ( VCSELs) are of utmost importance as key components for high-speed datacom, sensor and free-space applications. Therefore, for a successful further optimization of their performance understanding their behavior during operation is of crucial importance. A set of 850 nm VCSEL samples employing different doping of the active cavity zone are studied during operation by means of reverse current-voltage (IV) characteristics as well as photocurrent spectroscopy (PCS) under reverse bias. Reverse IV characteristics exhibits avalanche breakdown which enables an estimation of the electric field in the active region as a function of applied bias. Photocurrent spectroscopy is a powerful, nondestructive technique which measures essentially the convolution of the top mirror and intrinsic region absorption spectra and reveals quantum well transitions which redshift with reverse bias due to quantum-confined Stark effect (QCSE). The VCSELs are characterised before and after high current operation. VCSELs with a controlled doping of the active cavity region do not alter neither avalanche breakdown nor the QCSE shift of the quantum well transitions during operation. However, VCSELs without doping of the active cavity region show a systematic shift in breakdown voltage towards lower values, which is accompanied by an operation-induced redshift of quantum well transitions observed by PCS. These results indicate an increase of the built-in electric field in the active cavity zone after high current operation which is discussed in terms of conceivable processes such as dopant diffusion, impurity electromigration, burn-in of contacts and/or the activation of dopants during operation.