Wafer-bonded bottom-emitting 850 nm VCSELs for short distance free-space optical interconnections

Free-space optical interconnects (FSOI) have been the subject of intense research for application to parallel computing and communication systems. Meeting the demands of higher speed computer systems require alternatives to bandwidth limited and power-dissipative electrical interconnects. Implementing FSOI at chip-to-chip or board-to-board level by hybrid integration of optoelectronic devices onto Si-based systems is a promising solution to achieve higher performance computer systems.; Vertical-cavity surface-emitting lasers (VCSELs) are of great interest for FSOI, because they can be fabricated into 2-D arrays, their output beams are circularly symmetric with controllable divergence angle, and their ultra-low threshold current laser operation eliminates the need for array element pre-bias circuits. Bottom-emitting VCSELs are particularly suitable for hybrid integration with Si circuitry using a flip-chip bonding technology. A large VCSEL array can be transferred in one step and the resultant device capacitance is largely reduced. Bottom-emitting VCSEL arrays to date have predominately operated at 980 nm to make use of the transparency of GaAs substrates at this wavelength.; To enable 850 nm VCSELs to emit light through the substrates, a wafer bonding technology was employed in this study to replace the absorbing GaAs substrates with transparent substrates. Various substrates---p-GaP, i-GaP, and sapphire---and different device configurations were used to optimize the device performance. Carrier and optical confinement for the devices were achieved by using thin (200 A) AlAs oxide apertures placing at the standing wave node position in the first period of p-DBR to improve the manufacturability of oxide-confined VCSELs. The elements of the developed uniform 5 x 5 VCSELs array bonded on sapphire substrate exhibit an average threshold current of 346 muA and an average external quantum efficiency of 57% with maximum variation of 4%. The devices operate in single mode up to 3 mA of current excitation and the output power is more than 2 mW under this condition. A large, 8 x 8, VCSEL array was also demonstrated after improving the uniformity of wafer bonding and device fabrication processes. Short (visible) wavelength bottom-emitting VCSELs are possible by using our approach.