VCSEL TECHNOLOGY NOW INVADING HOMES!
Advanced Semiconductors Vital for FiOs & Verizon
VCSEL is an acronym for Vertical Cavity Surface Emitting Laser. A VCSEL is an advanced semiconductor device that is formed in special, epitaxial layers grown on n-type GaAs or InP substrates. Unlike conventional laser diodes that emit light from the cleaved sidewalls of the chip, VCSELs emit laser light normal to the face of the chip. Vertical light emission is a major practical advantage in packaging VCSELs. The emitted laser beam can be pointed into the desired direction, and can be easily coupled to a fiber optic. VCSELs can also be tested over the area of a wafer prior to dicing. A schematic of a generic VCSEL is shown in Figure 1 - Schematic of a Simplified VCSEL.
FABRICATION & STRUCTURE OF VCSELs
The starting material for a VCSEL is a single crystal wafer of n-type GaAs or InP. The VCSCEL structure is created by depositing multiple, monocrystalline semiconductor layers atop the wafer. These epitaxial layers are grown by molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD). The epitaxial layers are all monocrystalline, but their chemical composition, doping level, refractive index and thickness are carefully controlled.
A VCSEL is comprised of three major sections: the bottom n-type Bragg reflector, the quantum well, and the p-type Bragg reflector. A Bragg reflector is a stack of semiconductor layers that reflect a particular range of light wavelengths. This “mirror” does not include metal, but is instead a stack of semiconductor layers of different compositions and refractive indices which will reflect light over a band of wavelengths. On GaAs substrates, the epitaxial layers are typically AlxGa(1-x)As. The GaAs-AlGaAs system is favored for constructing VCSELs because the lattice constant of the material does not strongly vary as the composition “x” is changed, thus permitting multiple "lattice-matched" epitaxial layers to be grown on a GaAs substrate. Fortunately, the refractive index of AlxGa(1-x)As is very sensitive to composition “x”.
The bottom n-type Bragg reflector is designed to be highly reflective. The top p-type reflector is designed to reflect some light back down to the quantum well, while some light is permitted to pass up out of the device. The quantum well is where the electrons and holes are injected, combine, and emit the recombination energy as light. Essentially, the quantum well is where lasing occurs. Electrical contacts are at the top and bottom of a VCSEL. The electrical contacts typically are titanium + gold, which are deposited by evaporation. Electrical current in a VCSEL flows vertically.
EPI WAFER CHALLENGE
The VCSEL epi structure is one of the most difficult epi stacks among compound semiconductor devices due to the large number of layers and the demanding requirements for each layer. If only one epi layer is out of specification, then the entire wafer may be rejected. VCSEL epi wafers are therefore more costly than epi wafers for edge-emitting laser diodes, pHEMTs and HBTs. For many years, VCSELs were fabricated on 2” or 3” diameter wafers. However, in response to increased market demand for VCSEL devices, epi wafer merchant supplier IQE announced in March 2014 that they would produce 150mm diameter VCSEL epi wafers.
APPLICATIONS OF VCSELs
VCSELs transitioned from R&D to a commercial product in the late 1990s in order to support the roll-out of fiber optic communications. Since VCSELs can be easily coupled to a lens and fiber optic, they were used to send the light signal down the fiber. At that time, the larger market was for transcontinental and intercontinental (submarine) fiber optic communications. More recently, VCSELs have become vital to fiber optics used for distributing cable TV, as well as fiber-to-the home services (e.g., Verizon FiOS.) Fiber communications are also commonly used to link motherboard and memory devices in large server farms, since photons in a fiber optic transit signals faster than electrons in copper wire.
GESTURE RECOGNITION & OTHER APPLICATIONS
An exciting new application of VCSELs is for gesture recognition. Gesture recognition enables computers to be controlled by the motion of a hand or finger waved in the air (instead of tactile contact on a screen, or the movement of a mouse.) One or more light sources illuminate a hand with the movement of the hand being detected by several photodiodes and optical filters. Currently, low cost, low resolution gesture recognition systems use light emitting diodes (LEDs) to illuminate the hand. However, the coherent laser light of two or more VCSELs enable much finer resolution of a hand motion.
There are multiple applications for VCSELs. High power VCSELs can be used for material processing. With the advent of ultra low-loss glass fiber optics, the output aperture of a VCSEL can be coupled to an optical fiber to deliver intense light to a workpiece. High power VCSELs are used in dermatology for hair and wrinkle removal and for industrial processes such as laser cutting, drilling, ablation and engraving.
VCSELs AND MATERION
Materion is involved in the innovative applications of VCSELs. The Advanced Material Group supplies PVD metals used to form metal contacts to VCSELs, including Ti and Au evaporation slugs. Materion Precision Optics is a leading manufacturer of optical filters, which can be used with VCSELs for gesture recognition systems. For more information, contact Richard Koba at Richard.Koba@materion.com.