Fast light-induced change in ellipsometry spectra of hydrogenated amorphous silicon measured through a transparent subst
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Fast light-induced change in ellipsometry spectra of hydrogenated amorphous silicon measured through a transparent substrate upon bias light illumination N. Hata 1, C. M. Fortmann 2, and A. Matsuda 1 1 National Institute of Advanced Industrial Science and Technology, AIST Central 2, Tsukuba, Ibaraki 305-8568, Japan 2 Dept. Applied Mathematics, State University of New York at Stony Brook, Stony Brook, NY 11794-3600 USA ABSTRACT Previous ellipsometric studies of the stability of amorphous silicon (a-Si:H) found reversible changes in the pseudo-dielectric functions. These changes were slow to generate and slow to anneal away. These slow changes are associated with a dangling bond related structural change. Since any light-induced change in the dielectric function is useful for photonic engineering, we undertook the present more detailed study of light induced optical effects in a-Si:H. The optical pseudo-dielectric functions of hydrogenated amorphous silicon (a-Si:H) were measured using spectroscopic ellipsometry (SE) and the “through-the-substrate” measurement technique as a function of measurement temperature and bias light illumination. For the first time we report a light-induced change in a-Si:H materials that is fast, bias-light-dependent, reversible, and temperature dependent. This effect, while not completely understood, offers exciting new prospects for photonic engineering. INTRODUCTION A monolithic means of fabricating optical (photonic) elements for the manipulation of light energy and light-conveyed information, i.e., the analog of the electronic integrated circuit, is of vast technological and commercial importance. The photonic industry is expected to soar in the next ten years. For example a recent optical fibers newsletter [1] reported that the global photonic switch and switch matrix consumption is predicted to be greater than USD 16 billion in 2010, up from USD 308 million in 2000. One of the challenges of photonic engineering is that a set of material properties completely different from those relevant for electronic applications must be patterned at the sub-micron level. In particular refractive index patterns are used to guide the flow of light whereas electronic circuits are patterned using conductors and carrier type in semiconductor elements to guide the flow of current. Several aspects of a-Si:H make it an appealing matrix for photonic engineering. The amorphous state has a well-documented ability to absorb large quantities of impurities such as hydrogen, and such impurities can markedly change the refractive index. Also, a-Si:H has a band gap which is controllable over a very wide range (~ 1.1 to almost 2.4 eV) through variation of the hydrogen content (e.g., see [2], [3] and the references therein) and/or through alloying with germanium, carbon, nitrogen, or oxygen. Furthermore any optically induced change in refractive index offers the possibility of using illumination to trigger a refractive index change. This in turn is used to switch the direction of a second light beam. Therefore it is
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