Design for Reliability and Common Failure Mechanisms in Vertical Cavity Surface Emitting Lasers
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Design for Reliability and Common Failure Mechanisms in Vertical Cavity Surface Emitting Lasers Robert W. Herrick1 1 JDSU Inc, 80 Rose Orchard Way, San Jose, CA 95134, U.S.A. ABSTRACT Vertical-Cavity Surface-Emitting Lasers are making up a large and growing share of the world’s production of semiconductor lasers. But the 850 nm GaAs quantum well VCSELs that make up most of present product are highly vulnerable to dislocation networks. In this paper, we discuss how materials selection affects the reliability of semiconductor lasers generally. We then describe the most common failure mechanisms observed in VCSELs, and what precautions are used to prevent them. We finish with a brief discussion of reliability testing and failure analysis. INTRODUCTION Vertical Cavity Surface Emitting Lasers (VCSELs) are one of the most popular types of semiconductor lasers, and command a large and increasing market share over the past several years. It is estimated that approximately 700 million VCSELs have been sold [1] in the 15 years since they have been commercialized, with some estimates ranging as high as one billion VCSELs sold. The three primary applications are as a light source for data communications in short-reach fiber optic transceivers, as an illumination source for optical mice, or as an illumination source for thumb trackpads on cell phones. For these latter two applications, VCSELs are the least expensive semiconductor laser made, with costs in volume that are said to be roughly U.S. $0.12 per VCSEL. The traditional semiconductor laser dates back to the 1970’s for serious commercialization, and emits out cleaved facets, as shown in Figure 1 below. By contrast, the modern VCSEL is a more recent development, dating to the 1990’s for commercialization; it has the light travel perpendicular to the plane of the active region. In addition to low cost, other advantages of VCSELs include lower pumped area that in turn means lower required drive current or power consumption; the VCSEL also has a round, stigmation-free beam that makes for simpler collimation optics. Disadvantages of VCSELs include high thermal resistance, a more limited range of available wavelengths (only 650-1070nm commercially available as of this writing), and limited power per unit area.
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Figure 1: Edge-emitting lasers (left) have light that travels in the plane of the active region, and bounces between the front and back facet before being emitted. In VCSELs (right) the light goes up and down, perpendicular to the plane of the active region. CONSIDERATIONS FOR MATERIALS SELECTION FOR SEMICONDUCTOR LASERS With any semiconductor laser, the choice of “materials system” is key. By “materials system, we primarily mean the choice of the substrate to grow the laser on, and the alloys used to make the laser. Especially important is the material of the quantum wells used to create light, and the barriers that contact the wells. Discussion of materials selection is usually retrospective: after a material is widely adopted commercially, or rejected by researcher
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