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 /spl mu/A for the 6/spl times/6 /spl mu/m/sup 2/ VCSEL's bonded on GaP substrates. VCSEL's with 8.6/spl times/8.6 /spl mu/m/sup 2/ aperture bonded on sapphire substrates also exhibit threshold currents of 800 /spl mu/A 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.