Introductory Invited PaperRecent developments in silicon optoelectronic devices
Introduction
Although the aspiration of “computation at the speed of light” may still be several light-years beyond our present technological frontier, the optoelectronic system integration has already come to light [1], [2], [3], [4], [5], [6], [7], [8], [9]. There are urgent needs and tremendous commercial values for the development of optoelectronics integrated circuits (OEICs) for the globally rapid growth of the internet and multi-media communication networks in recent years. Several attempts have already been made. Silica-based planar lightwave circuits (PLC) for fiber-matching have been developed [1], [2], [3]. These integrated circuits provide various components for fiber-to-the-home (FTTH) [4], [5] optical networks and wavelength-division-multiplexing (WDM) transmission networks [6], [7], [8]. Integrating the semiconductor laser with the microcavity for wavelength tuning has also been attempted [9]. However, these OEICs are still very primitive and the costs are still very high. Unlike the microelectronic systems where all the components are built on a single material––silicon, the photonic components often require some incompatible materials. For example, light sources are often built on compound semiconductors and waveguiding parts are built on LiNBO3 film.
Using silicon-based materials, e.g. oxide or silica, nitride, oxynitride, and porous silicon as well as the microfabrication technology will be a promising technology for integrated optics. There are many merits for Si-OEICs. Since the mainstream electronic devices, microprocessors, and memory are made of silicon, fabricating optoelectronic devices on silicon is the most cost effective way for electro-optic system integration. In addition, silicon technology has the following attributes. It is the greatest and most mature technology available to date. It could be as fine as nanometer in structure and as great as a giga scale in complexity. With silicon technology, we can make any kind of geometries, e.g. optical cavity, 3D and motion structure, nano structures. With nano geometry, even the quantum effect is possible in Si. On the other hand, silicon is the largest (as large as 30 cm in diameter) substrate available. It is particularly suited for large-scale integration and mass production with excellent uniformity and reproducibility.
Silicon material is now also possible for use in light generation. Silicon was not used for light-emitting devices (LEDs) as it is a non-direct band-gap material. However, quantum confinement of electronics and holes are found in quantum dots and nanocluster silicon structures [10], [11]. On the other hand, strong luminescence can also be generated from the radiative centers in silicon oxide and nitride [12], [13], [14], [15]. A lot of investigations on the luminescence properties of silicon-based structures, including porous silicon (PS) [10], [16], [17], silicon nanoclusters in amorphous SiO2 [11], [18], [19], [20], [21], have been conducted. A wide range of luminescence, including red-orange band (luminescence peak with energy 1.6–2.0 eV) and blue band (2.6–2.8 eV) in these structures were observed. A green line has also been found by making use of the silicon nitride (Si3N4) structure. This provides the possibility for fabricating full-color devices based on the silicon technology [11].
In guided wave applications, the silicon-based devices have features of low insertion loss, polarization-independent behavior and efficient fiber pigtails. Since silicon is a low-contrast material, a high-efficiency fiber-chip coupling can be obtained [3], [4]. The successful development of microfabrication technology for nanoscale structures will also have a great impact on optoelectronics. Optical systems in the past have been bulky with expensive optical components like mirrors, filters, beam splitters. These components require precise alignment in the micron scale. By employing the microfabrication technology, optical microelectro-mechanical systems (OMEMS) are possible and the size of the present optical system can be reduced greatly [9]. Thus the Si-based optoelectronics have features of low cost, high robustness and multifunctional. This leads to a bright future for low-cost mass production of OMEMS and OEICs.
Section snippets
Luminescent properties
Applications of silicon oxynitride in various integrated optical devices have been attempted [12], [22], [23]. The luminescence of the silicon nitride and SiO2 can be due to the radiative centers. Different centers will produce different luminescence bands [12]. Table 1 shows the various defect centers in oxide and nitride [12]. The luminescence bands, ranging from red band to UV, depend on the defect types. The applications of these radiation centers in LEDs need to be explored. Particular
Porous silicon
Both light emission and guided wave applications are possible with the porous silicon. PS has been widely studied recently [10], [31], [32], [33], [34], [35]. Many applications of this material, e.g. LEDs, optical waveguide and photodetectors, have been widely explored [36], [37], [38]. It was soon found that porous silicon fabricated on polycrystalline silicon films (so called PPS or porous poly-Si) offers further advantages over conventional PS [36], [39]. PPS films can be formed on silicon,
Some optoelectronic devices
In the past few years, several attempts for making optical devices using microelectronics fabrication technologies have been made. Although some of them are still very primitive, we believe that their characteristics can be improved with modified designs, deposition process and other technological processing steps like lithography and etching. Here we describe briefly some of the designs. More details can be found in the references.
Concluding remarks
The rapid growth of the internet and multi-media communication networks have called for urgent needs of the development of optoelectronic integrated circuits (OEICs). Silicon-based will have high potential because of many technological merits. This work reviews the recent development and prospect of silicon-based integrated optoelectronic systems. The technological aspects of porous silicon and oxynitride and silicon quantum devices for integrated optoelectronic applications are discussed. Some
Acknowledgements
The authors would like to thank his collaborators Dr V.A. Gritsenko of Institute of Semiconductor Physics, Novosibirsk, Russia, Dr M.C. Poon, Hong Kong University of Science and Technology, and his former student Dr P.G. Han for their contributions to the previous published papers which had formed a solid ground of this review. This work is partially supported by research project no. 7001134 of City U.
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