Sputtered hydrogenated amorphous silicon thin films for distributed Bragg reflectors and long wavelength vertical cavity surface emitting lasers applications

In this work, we report a study of hydrogenated amorphous silicon (a-SiH) films deposited by radio frequency magnetron sputtering for application in Vertical Cavity Surface Emitting Lasers (VCSEL) elaboration. The influence of the hydrogen dilution in the plasma during the deposition on the optical and surface properties is investigated. After selection of the deposition parameters, a-SiH films have been combined with amorphous silicon nitride (a-SiN_x) films to provide high reflectivity Bragg reflectors. Distributed Bragg reflector (DBR) based on these quarter wavelength thick dielectric layers have been realized and characterized by optical measurements and compared with theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.2% at 1.6 μm and a large spectral bandwidth of 700 nm have been reached with only four and a half periods of a-SiH/a-SiN_x deposited on a glass substrate. Residual absorption at 1.55 μm has been measured to be as low as 60 cm⁻¹ with a-SiH layers, compared with 400 cm⁻¹ loss with amorphous silicon without hydrogenation step. Finally, DBR comprising six a-SiH/a-SiN_x periods have been included in an InP-based VCSEL. Laser emission is demonstrated at room temperature in continuous wave operation with a photopumping experiment.