The Emergence of an Amorphous-Silicon Based Photonic Technology; Optical Memories to 3-D Photonic Crystals
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The Emergence of an Amorphous-Silicon Based Photonic Technology; Optical Memories to 3-D Photonic Crystals N. Hata and C. M. Fortmann1 Materials Science Division, Electrotechnical Laboratory, Tsukuba, Ibaraki 305-8568, JAPAN. 1 Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8502, JAPAN. ABSTRACT Amorphous silicon is an ideal system for optical engineering since its optical properties can drastically be changed through its ability to absorb such impurities as hydrogen whose contents range from zero to beyond twenty percent. In this work we report light-induced changes in its optical properties which may add potential in active optical engineering. Considerable changes in the phases in reflected polarized lights from transparent-substrate side of amorphous silicon films are observed both after prolonged illumination of intense light at the light-soaking temperature range from 40 to 250 °C. Most of the changes are localized to amorphous silicon region near substrate-film interfaces. Illumination time dependencies and annealing characteristics are examined, and physics behind these observed changes are discussed. INTRODUCTION The long investigative search to find improved hydrogenated amorphous silicon (a-Si:H) materials for electronic and photovoltaic device applications has produced a wealth of knowledge concerning the optical properties of this material. For example, Manfredotti et al. [1] reported on the change in the a-Si:H refractive index as a function of hydrogen and void contents. Elsewhere, Kessels et al. [2] reported on the effect of deposition conditions on the refractive index and on the hydrogen content. These reports and the many others provide us with much insight into the optical properties of a-Si:H and lead us to the conclusion that these optical properties can be altered through alloying and hydrogen contents. The ability of a-Si:H to absorb large quantities of hydrogen or other impurities combined with its tunable optical parameters make it an ideal base for photonic device patterning. Fortmann and Jaen [3] described how the refractive index of a-Si:H could be varied by over 50% in some cases by changing the hydrogen content and how in-situ patterning of the a-Si:H hydrogen content can be used to define photonic devices. Photonic devices include three-dimensional (3-D) photonic crystals with features scaled to interact with visible light. The optical band gap depends on a sub-set of the total hydrogen content which is bonded into Si-H2 sites [4]. Together, the tunable a-Si:H band gap and tunable refractive index make it an appealing basis for the engineering of passive optical devices.
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Recently, the light-induced changes are found to be exploitable for the engineering of active optical devices: Hata et al, [5] described reversible, light induced above-gap changes observed at a-Si:H region near its interface with transparent substrate. By taking advantage of this effect a write-read erasable optical memory is proposed in which the m
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