Suppression of Crystal Nucleation in Amorphous Si Thin Films by High Energy Ion Irradiation at Intermediate Temperatures
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SUPPRESSION OF CRYSTAL NUCLEATION IN AMORPHOUS Si THIN FILMS BY HIGH ENERGY ION IRRADIATION AT INTERMEDIATE TEMPERATURES
James S. Im, Jung H. Shin, and Harry A. Atwater California Institute of Technology, Pasadena, CA. 91125
ABSTRACT In situ electron microscopy has been used to observe crystal nucleation and growth in amorphous Si films. Results demonstrate that a repeated intermediate temperature ion irradiation/thermal annealing cycle can lead to suppression of nucleation in amorphous regions without inhibition of crystal growth of existing large crystals. Fundamentally, the experimental results indicate that the population of small crystal clusters near the critical cluster size is affected by intermediate temperature ion irradiation. Potential applications of the intermediate temperature irradiation/thermal anneal cycle to lateral solid epitaxy of Si and thin film device technology are discussed.
INTRODUCTION Microstructural evolution in thin Si films has been and continues to be an area of active investigation. The driving force for such activities ranges from fundamental understanding of phase transformations to thin film device applications. Recently, increased attention has been devoted to the amorphous-to-crystal transformation of amorphous Si films. It is now well established that the transformation occurs via nucleation and growth with an incubation period during the initial stage of transformation if the amorphous Si films have been deposited at lowtemperatures [1 ] or have been amorphized via a low temperature ion irradiation process [2]. We are currently investigating the use of ion irradiation to engineer the amorphous-tocrystal phase transformation process on the nanometer scale. In particular, we are interested in finding alternate kinetic paths and/or final microstructures which cannot otherwise be achieved by thermal annealing alone. It was recently demonstrated that high temperature irradiation of Si films during crystallization can lead to a dramatic enhancement in nucleation rates [3,41. Despite considerable technological interest in achieving suppression of random crystal nucleation in amorphous Si (for polycrystalline and single crystalline Si based thin film devices), there currently is no known method to achieve selective suppression of random crystal nucleation. Here we present an in situ experimental study on the effect of periodic ion irradiation at intermediate temperatures on the subsequent isothermal crystallization of an amorphous Si film at high temperatures. The specific purpose of this investigation is to determine whether or not high-energy ion irradiation can be applied to suppress the nucleation of new crystals while allowing the growth of existing crystals.
Mat. Res. Soc. Symp. Proc. Vol. 201.
1991 Materials Research Society
358
EXPERIMENTAL The first step in sample preparation is the deposition of 1000A thick amorphous Si film (via low-pressure chemical vapor deposition) on top of thermally oxidized Si wafers. In order to ensure that the deposited Si films are amorph
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