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Chapter 30:
Silicon Photonics - Recent Advances in Device Development
In 1958 while working at Texas Instruments, Jack Kilby demonstrated that it was possible to fabricate a resistor, capacitor, and transistor using single-crystal silicon. This technological landmark led directly to the first truly integrated circuit and its importance was recognized by the award of a Nobel Prize to Kilby in 2000. In the subsequent almost five decades, the microelectronics industry witnessed a miraculous reduction in individual device size, and hence increases in chip functionality. This trend has seen the doubling of device density approximately every 24 months, roughly in line with the prediction of Gordon Moore in 1962. Moore's law has, more or less, remained relevant to the present day, forming the motivation for the International Technology Roadmap for Semiconductors (ITRS) in 1993. The roadmap is a needs-driven document that assumes that the industry will be dominated by complementary metal oxide semiconductor (CMOS) silicon technology. In fact, the MOSFET transistor forms the basic element of many standard products such as high-speed MPU, DRAM, and SRAM. Of some significance is the fact that no single material possesses the optimum properties for each individual device found in an integrated electronic circuit; however, silicon provides a base material from which all the required devices can be fabricated to an acceptable performance specification. Borrowing many of the design and manufacturing principles from the microelectronics industry, several researchers began projects in the 1980s on the adoption of silicon as the base material for the fabrication of photonic circuits, i.e., those circuits that have light as the carrier of information as opposed to electrical charge. Of note at that time was the work of Richard Soref at the Rome Air Development Center in Maine and Graham Reed at the University of Surrey, UK. The Surrey work was of particular importance for the future commercialization of silicon photonic technology because Reed's group showed that very low-loss propagation was possible in silicon-on-insulator (SOI) rib waveguides, a structure in which light could be confined and manipulated. Many of the optical properties of silicon would suggest it to be an ideal material for planar lightwave circuit (PLC) fabrication (not least the availability of waveguide structures in the form of SOI as shown by Reed). Silicon is virtually transparent to wavelengths < 1100 nm, while silicon dioxide (SiO2) shares its chemical composition with glass fiber, providing a degree of compatibility with long-haul, fiber-optic technology. Silicon has a relatively high refractive index around 3.5 (compared to that for glass fiber, for example, which is around 1.5), which allows the fabrication of waveguides on the nanometer scale. However, there remains an outstanding limitation of silicon in the photonics arena, in the form of the size and nature of its indirect bandgap, which prevents the straightforward formation of efficient optical sources (maybe the greatest challenge to silicon photonics researchers), and detectors compatible with subbandgap wavelengths.
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