W. Conard Holton
Low-cost vertical-cavity surface-emitting lasers (VCSELs) have proved successful in 850 and 980 nm applications. For metropolitan and 10 Gbit/s Ethernet applications, development of a low-cost 1550-nm VCSEL would be a boon (see a related feature, page 35). The lack of a mature technology for making distributed Bragg reflectors (DFB) on an indium phosphide (InP) substrate has been a major hindrance to manufacturability and reliability.
At the University of California at Santa Barbara, researchers have developed an all-epitaxial, lattice-matched approach using aluminum gallium arsenide antimony/aluminum arsenide antimony (AlGaAsSb/AlAsSb) on InP. The design takes advantage of the materials` high refractive index contrast to create highly reflective DBRs. The research team, led by Larry Coldren, developed a double-intracavity design and an air aperture to overcome the poor thermal and electrical properties of the DBRs. The VCSELs have a threshold current of 800 µA, a differential efficiency of 23%, and a maximum output power greater than 1 mW at 20C.
The double-intracavity contacted structure clads the AlInGaAs-based active region with two thick, n-type InP layers that provide for efficient current injection and heat dissipation. The injected current can bypass the AsSb-based DBRs with little resistance or optical loss. The 8-µm undercut air aperture in the active region funnels carriers from the edges of the device to the region of the quantum wells beneath the etched-pillar DBRs. In effect, the aperture confines the current precisely where it is most useful in the active region. An overall improvement in performance can be achieved by reducing the size of the aperture, which further reduces scattering loss at the sidewalls of the etched-pillar (see figure).
By growing the VCSELs on InP wafers using molecular beam epitaxy, the researchers are relying on a well-understood growth process that produces nearly defect-free, single-crystal material by a technique that promises advantages compared to multistep VCSEL manufacturing. Coldren says, "It has the best chance of being a repeatable, manufacturable, and reliable process." For more information, contact Larry Coldren at [email protected].The intracavity design for a VCSEL includes a 16-µm etched pillar above an air aperture, which acts as a strong index guide directing the current to the active region. Reducing the aperture size results in larger differential efficiency, lower threshold current, higher maximum output power, and higher maximum operating temperature.