Direct growth of crack-free stoichiometric silicon nitride on sapphire for wide-band photonics

Abstract

Low-loss stoichiometric silicon nitride on oxidized silicon has emerged as the major wide-band photonics platform for nonlinear and quantum photonics. These films, deposited by low-pressure chemical vapor deposition (LPCVD), are well known to have around 1000 MPa of residual tensile stress, which often leads to cracking when film thickness exceeds 400 nm. Although various stress-mitigation approaches can isolate or prevent cracking, this stress issue remains a major fabrication challenge. Low-residual-stress LPCVD stoichiometric silicon nitride on sapphire has also been reported and recently explored for photonics at telecommunication wavelengths. Here, we advance its development through systematic characterization, including thin-film stress analysis, ellipsometry and prism-coupling measurements of refractive index and loss, and swept-wavelength spectroscopy of fabricated photonic devices across a broad range of wavelengths. We demonstrate the direct growth of crack-free stoichiometric silicon nitride up to 2 μm thick with only 200 MPa of stress. Our fabricated devices show propagation losses of 0.35 dB/cm around 780 nm, 0.37 dB/cm around 1064 nm, and 0.25 dB/cm around 1630 nm. In addition, prism-coupling measurements indicate film losses below 1 dB/cm at wavelengths down to 500 nm. Our work highlights stoichiometric silicon nitride on sapphire as a promising wide-band photonics platform.