Critical Laser Fluence Observed in (111) Texture, Grain Size and Mobility of Laser Crystallized Amorphous Silicon
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CRITICAL LASER FLUENCE OBSERVED IN (111) TEXTURE, GRAIN SIZE AND MOBILITY OF LASER CRYSTALLIZED AMORPHOUS SILICON
R. I. JOHNSON, G. B. ANDERSON, J. B. BOYCE, D. K. FORK, P. MEI, S. E. READY, and S. CHEN* Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 *XMR Corporation, 5403 Betsy Ross Drive, Santa Clara, CA 95054 ABSTRACT
This paper describes new results on the relationship between the grain size, mobility, and Si (111) x-ray peak intensity of laser crystallized amorphous silicon as a function of the laser fluence, shot density, substrate temperature, and film thickness. These observations include an unexpected narrow peak found in the silicon (111) xray peak intensity, which occurs at a specific laser fluence for a given film thickness and substrate temperature. Amorphous silicon materials processed at laser energy densities defined by this peak exhibit exceptionally large grain sizes and electron mobilities that cannot be obtained at any other energy and shot density combination above or below the energy at which the Si (111) x-ray peak intensity maximum occurs. INTRODUCTION Laser crystallized a-Si thin films have been studied for a number of years with the hope of improving the properties of polycrystalline material for thin film device applications [1-8]. We have tried to understand the complex relationships that exist between thin film crystal growth, laser fluence, shot density (laser beam overlap), and the substrate temperature [6,8]. We recently observed that a narrow peak exists in the Si (111) x-ray peak intensity at a particular laser fluence for a given substrate temperature and film thickness. Similar peaks, at the same laser energies, were also found to exist in the average grain size and electron mobilities with respect to the laser energy density. The magnitude of these peaks appear to be determined primarily by the shot density. The energy density at which these peaks occur is found to be sensitive to a number of variables, including film thickness, substrate temperature, sample structure, and the laser beam profile. Increasing laser fluence may explain the growth of larger crystals with improved electron mobilities, but it fails to explain the sudden drop in the x-ray intensity, grain size and mobility as the energy continues to increase above the peak energy density position. A model involving the creation of nucleation sites at the silicon-substrate interface at the higher laser fluences is proposed. EXPERIMENTAL CONSIDERATIONS
100 mm quartz wafers coated with 50-100 nm of intrinsic or phosphorous-doped LPCVD a-Si were laser crystallized. As discussed in earlier papers, little difference has been observed between laser crystallized doped and undoped films; the data reported in this paper is based on measurements taken on films containing one atomic % phosphorous. The samples were laser crystallized with an XMR 5100 excimer laser operating at 308 nm with a 50 ns pulse length. The beam was homogenized and focussed down to a beam spot ranging in size from 4.7X4.7 to 5
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