Wafer-bonded bottom-emitting 850 nm VCSELs on GaP substrates

We report high performance bottom-emitting 850 nm VCSELs wafer-bonded to transparent GaP substrates. The VCSEL structure is grown on a lattice-matched [100] n/sup +/-GaAs substrate using MOCVD. The structure consists of an AlAs etch stop layer, 40.5 pairs n-type bottom DBR, 1 wavelength cavity, and 28 pairs p-type top DBR. The epi-structure is optimized for top emission at present which has an effect on the results achieved. The epi-structure is then wafer-bonded to a p-type GaP substrate at 750/spl deg/C for 30 minutes in H/sub 2/ ambient under uniaxial pressure applied by a graphite fixture. Prior to the wafer bonding, both epi-structure and GaP substrate were cleaned with solvent and diluted HF solution, and shallow trenches were etched on GaP substrates to help the gas or moisture trapped at the interface to escape before bonding took place. After wafer bonding, the GaAs substrate was lapped down to about 25 /spl mu/m, and then a NH/sub 4/OH:H/sub 2/O/sub 2/ solution was employed to selectively etch off the remaining GaAs substrate. Finally, the VCSEL structure on GaP substrate can be processed using standard fabrication procedure of oxide VCSELs without any special care. The device characteristics of top-emitting and bottom-emitting 850 nm VCSELs are shown. Both devices were fabricated from the same VCSEL wafer. The threshold current of the bonded VCSELs increases about 3 times of that of the top emitting VCSELs. Despite the unoptimized L-I characteristics of the bonded VCSELs, the turn on voltage of the devices still remains 1.5 volts. These are the first low voltage GaP/GaAs VCSEL's reported to date.