Formation process of interface states at grain boundaries in sputtered polycrystalline Si films
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Formation process of interface states at grain boundaries in sputtered polycrystalline Si films Yoshitaka Nakano,a) Jiro Sakata, and Yasunori Taga Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-1192 Japan (Received 20 February 1998; accepted 22 April 1998)
A systematic investigation has been made on surface defect states of crystallites in the crystallization process of sputtered amorphous silicon films by isothermal annealing. Transmission electron microscopic observations indicate a pronounced vertical columnar structure in the upper part of the films, where the crystallization is delayed. Admittance spectroscopy reveals that two newly generated energy levels with the crystallization are attributed to the crystallites in the lower and upper parts of the films in view of the anisotropic crystallization. These thermally induced changes can be well explained by Si–Si shearing modes at the interfaces of crystallites through the process of crystallization.
I. INTRODUCTION
Polycrystalline silicon (poly-Si) films have become increasingly important as a material for thin-film transistors and solar cells. It is well known that grain boundaries (GB’s) of poly-Si strongly affect the electrical properties, because its electronic behavior is dominated by their inherent GB planes where electrostatic potential barriers with two depletion layers are generated from charges in interface states. So, many researchers have been focusing their attention on a quantitative determination of the energy distribution of the interface states at GB’s using various methods such as photocapacitance, optical absorption, and admittance spectroscopy techniques.1–3 Their results clarified the nature of band tails and dangling bond states at GB’s. The observed band tails and dangling bond states are attributed to the structural disorder such as bond distortions and coordinated defects, respectively. Considering that GB’s are composed of disordered local structures, both electronic states may be organically connected to each other. In general, the electrical conduction in poly-Si films is explained by a model of monoenergetic trapping states at GB’s, i.e., the dangling bond states superimposed on the exponential band tails. Thus, since the dangling bond states at GB’s affect the device performance, it is necessary to understand the formation process of the interface states at GB’s for reducing their influence. In this study, we have systematically investigated thermally induced changes of surface defect states (SDS) of crystallites in the crystallization process of sputtered amorphous silicon (a-Si) films by isothermal annealing. By using transmission electron microscopy (TEM), a pronounced vertical columnar structure is observed in a)
e-mail: [email protected] J. Mater. Res., Vol. 14, No. 2, Feb 1999
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the upper part of the films, where the crystallization is delayed. In view of the anisotropic crystalliz
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