Broadband high-reflective distributed Bragg reflectors based on amorphous silicon films for semiconductor laser facet coatings.

We fabricated amorphous silicon (a-Si)-based distributed Bragg reflectors (DBRs) consisting of alternating dense/porous films (i.e., pair) for a center wavelength (λ(c)) of 0.96 μm by oblique angle deposition (OAD) technique using an electron-beam evaporation system. The dense (high refractive index, i.e., high-n) and porous (low-n) a-Si films were deposited at two incident vapor flux angles of 0° and 80° in the OAD, respectively. Their optical reflectance characteristics were investigated in the wavelength range of 0.6-1.5 μm, including theoretical comparison using a rigorous coupled-wave analysis method. Above three pairs, the reflectivity (R) of a-Si DBRs was almost saturated at wavelengths around 0.96 μm, exhibiting R values of >97%. For the a-Si DBR with only three pairs, a broad normalized stop bandwidth (Δλ/λ(c)) of ∼22.5% was obtained at wavelengths of ∼0.87-1.085  μm, keeping high R values of >95%. To simply demonstrate the feasibility of device applications, the a-Si DBR with three pairs was coated as a high-reflection layer at the rear facet of GaAs/InGaAs quantum-well laser diodes (LDs) operating at λ=0.96  μm. For the LDs coated with three-pair a-Si DBR, external differential quantum efficiency (η(d)) was nearly doubled compared to the uncoated LDs, indicating the η(d) value of ∼50.6% (i.e., η(d)∼25.5% for the uncoated LDs).