Distributed Bragg Reflectors for Vertical-Cavity Surface-Emitting Lasers
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Distributed Bragg
Reflectors for Vertical-Cavity Surface-Emitting Lasers
W.G. Breiland, A.A. Allerman, J.F. Klem, and K.E. Waldrip Abstract Distributed Bragg reflectors (DBRs) not only serve as high-reflectance mirrors to define the laser cavity of a vertical-cavity surface-emitting laser (VCSEL), but they also must conduct electricity, confine currents, and provide a single-crystal template for the gain region of the laser. Basic optical and electrical properties of DBRs are presented in this article. Three examples of DBR structures used in VCSEL applications from the ultraviolet to the infrared are given to illustrate the complexity and range of materials science issues that are encountered in DBR growth. Fabrication issues are also discussed. Keywords: chemical vapor deposition (CVD), compound semiconductors, optoelectronic materials, vertical-cavity surface-emitting lasers (VCSELs).
Introduction The distributed Bragg reflector (DBR) is used primarily in vertical-cavity surfaceemitting laser (VCSEL) applications as a mirror for confining the electric field within the optical cavity and gain region. It thus serves the same function as a metallic or dielectric mirror in a conventional laser. Very high reflectivity is required because the single-pass gain can be less than 1% in the short cavities used for VCSELs. If the reflectance property were the only function served by the DBR, fabrication would be straightforward. However, the DBRs in a VCSEL must also satisfy several additional constraints. The DBR must often carry current to the gain region, requiring that it be a low-resistance electrical conductor. The current and optical fields must be laterally confined to optimize device performance. Additionally, VCSELs exhibit complex temperature effects, and the thermal conductivity of the DBR can be a
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critical property. To preserve single-crystal band-state properties that allow the gain region to function, the bottom DBR of a VCSEL must be grown epitaxially. These electronic and materials constraints pose challenges to the device designer and grower. It is common for a DBR structure to contain over 100 individual layers with complex, graded composition and doping profiles. Such complexity results in a VCSEL whose total thickness is mostly taken up by two DBRs, with only a small fraction of the device devoted to the optical cavity and gain region. This article describes the use of DBRs in VCSEL applications. Basic optical and electronic properties are reviewed. Fabrication issues arising from optical and electronic constraints are discussed. Finally, applications of three representative VCSEL devices are described to illustrate the complexity and range of materials sci-
ence issues that are encountered in DBR growth.
Distributed Bragg Reflectors: Basics Optical Properties of a Distributed Bragg Reflector The distributed Bragg reflector nomenclature traces back to the use of external periodic grating structures as wavelengthselective cavity mirrors in edge-emitting lasers. However, for VCSEL applications,
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