Design and fabrication of long-wavelength vertical-cavity surface-emitting lasers using wafer bonding technologies.

摘要

Long-wavelength vertical-cavity surface-emitting lasers (VCSELs) emitting at 1.55 μm from GaInAsP/InP heterostructures are important optical sources for high-speed fiber-optic commutations. However, their development has been obstructed by the material disadvantages associated with the GaInAsP/InP distributed Bragg reflector (DBR) and the fabrication complexity of direct wafer bonding. In this work, a novel VCSEL fabrication method facilitated with new wafer bonding techniques has been developed to overcome these limitations. This process scheme provides more options on the DBR and substrate materials for long-wavelength VCSELs. In addition, the new wafer bonding techniques are less stringent on process requirements than the previous arts. Thus, the GaInAsP/InP VCSELs can be easily fabricated on lattice-mismatched substrates with high-contrast composite DBRs for a better performance, and therefore the need to grow thick epitaxial DBRs is eliminated. During the process development, substrate-independent DBRs such as polycrystalline-GaAs/Al-oxide, amorphous Si/Al-oxide, Si/Mg-oxide, and (Ga,As)/(Al,As) were investigated. New wafer bonding techniques using either spin-on glass or AuGeNiCr metal alloys as the bonding medium were developed. Using these wafer bonding techniques, the bonding interface can be formed outside the VCSEL cavity at low temperatures, reducing the mechanical and thermal impacts to the active region. In addition, processes for device formation such as ohmic contacts and chemical etching were studied and integrated. The results lead to the use of Si/Al-oxide DBRs and the metallic wafer bonding for the VCSEL fabrication. The Si/Al-oxide DBR provides a reflectance above 99.5% in six periods with a low thermal resistance and a broad stop bandwidth. The metallic wafer bonding offers a robust bonding interface, excellent thermal stability and optical reflectance. It also has a large process latitude and integration capability. Using the metallic bonding process, we successfully demonstrated 1.55 μm GaInAsP/lnP VCSELs on Si substrates. The fabricated VCSELs lase at 1.545 μm under pulsed operation at room temperature. The threshold current density is 1.2 kA/cm2 and the output power is 5 μW from a 30 μm × 30 μm device, which is limited by the heating of the resistive p-contact materials and the resulting offset of material gain. This process is useful to long-wavelength VCSELs and other applications.