In Situ IR Absorption Study of H Bonding in a-Si:H Thin Films
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IN SITU IR ABSORPTION STUDY OF H BONDING IN a-Si:H THIN FILMS M. KATIYAR, G.F. FENG, J.R. ABELSON AND N. MALEY Coordinated Science Laboratory and Department of Materials Science and Engineering, University of Illinois, 1101 W. Springfield Ave., Urbana, IL 61801. ABSTRACT We have used real time, in situ infrared reflectance spectroscopy to study the evolution of Si-H bonding during the growth of hydrogenated amorphous silicon (a-Si:H) by dc magnetron reactive sputtering. The surface Si-H stretching mode (2100 cm- 1 ) is observed in films 5 A thick and the 2000 cm- 1 mode, which has been attributed to isolated monohydrides in the bulk, appears for thickness (d) > 10-15 A. For larger thicknesses the intensity of both modes increases approximately linearly. These trends are interpreted in terms of the nucleation and coalescence of islands for d< 15 A, followed by bulk-like growth. INTRODUCTION Understanding the mechanisms of the growth of a-Si:H and its alloys is of considerable interest for fundamental reasons and for improving material properties and device performance. Hydrogen plays a key role during growth and in determining material properties. Yet very little is known about the mechanisms of hydrogen incorporation in a-Si:H. Infrared spectroscopy is ideally suited to study the evolution of Si-H bonding during film growth since i) it distinguishes between isolated and clustered Si-H bonds and ii) it can be used for real time, in situ probing of film growth in a plasma environment. As a-Si:H is only weakly absorbing in IR, transmission measurements on c-Si substrates are not sensitive enough to observe Si-H bonds in very thin films (d
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