Superlattice Reflectors Boost VCSEL Performance
A superlattice design for distributed Bragg reflectors in vertical-cavity surface-emitting lasers (VCSELs) simultaneously offers high reflectivity and low series resistance. The low cost and high yield associated with the semiconductor superlattice distributed Bragg reflector VCSEL and the improved confinement offered with proton bombardment make the device an attractive candidate for use in optical communications and in other optoelectronic applications.
VCSELs have an active layer that is very thin relative to the overall device thickness. To achieve lasing, the mirrors must be of high enough reflectivity to not contribute significantly to cavity loss. VCSEL mirrors are typically constructed from distributed Bragg reflectors, quarter-wave layers of alternating materials with different indices of refraction. To get high reflectivity, the difference in refractive index between adjacent layers should be large, but those materials with a large index difference also have high resistance, which reduces the laser efficiency.
Researchers at National Cheng Kung University in Tainan, Taiwan, created the bottom reflector in 19 periods, each comprising 18.5 pairs of alternating thin GaAs/AlAs layers and one thick layer of AlAs. They built a three-quantum-well active region on top of that and deposited a top-distributed Bragg reflector constructed from a 16-period superlattice whose structure was similar to that of the bottom mirror. The reflectivity was as high as 99.7 percent in the lasing region of interest.
In addition to building the distributed Bragg reflectors using traditional molecular beam epitaxy, the researchers added a proton-bombardment step to the process to create areas of high electrical resistance, confining the injection current and electrically isolating each laser from its neighbors. They positioned tungsten wires against the wafer surface to shield selected areas during the bombardment. In a four-step process, they introduced the high-resistance areas around a 10 x 10-µm region of the wafer.
The finished devices exhibited single-mode operation near 840 nm and good uniformity of resistance among 20 x 20-µm regions.
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