High-performance wafer-bonded bottom-emitting 850-nm VCSEL's on undoped GaP and sapphire substrates

We have demonstrated high performance wafer-bonded bottom-emitting 850-nm vertical-cavity surface-emitting lasers (VCSEL's) on transparent substrates. The free-carrier absorption of the substrate was avoided by using undoped GaP or sapphire substrates. The maximum external quantum efficiency approaches 48% while the threshold current remains as low as 550 μA for the 6×6 μm2VCSEL's bonded on GaP substrates. VCSEL's with 8.6×8.6 μm2aperture bonded on sapphire substrates also exhibit threshold currents of 800 μA and external quantum efficiencies of 33.2%. The difference in efficiency between these two devices results from the change of the refractive index of the exit medium.