Si wafer bonded of a-Si∕a-SiNx distributed Bragg reflectors for 1.55-μm-wavelength vertical cavity surface emitting lasers

Abstract

Amorphous silicon (a-Si) and amorphous silicon nitride (a-SiNx) layers deposited by magnetron sputtering have been analyzed in order to determine their optical and surface properties. A large value of ∼1.9 of index difference is found between these materials. Distributed Bragg reflectors (DBRs) based on these dielectric material quarter wave layers have been studied by optical measurements and confronted to theoretical calculations based on the transfer matrix method. A good agreement has been obtained between the experimental and expected reflectivities. A maximum reflectivity of 99.5% at 1.55μm and a large spectral bandwidth of 800nm are reached with only four and a half periods of a-Si∕a-SiNx. No variation of the DBR reflectivity has been observed with the time nor when annealed above 240°C and stored during few months. This result allows us to use this DBR in a metallic bonding process to realize a vertical cavity surface emitting laser (VCSEL) with two dielectric a-Si∕a-SiNx DBRs. This bonding method using AuIn2 as the bonding medium and Si substrate can be performed at a low temperature of 240°C without damaging the optical properties of the microcavity. The active region used for this VCSEL is based on lattice-matched InGaAs∕InGaAsP quantum wells and a laser emission has been obtained at room temperature on an optically pumped device.