Stability of Nanometer-Size Si Crystals in Amorphous Si Thin Films under Ion Irradiation
- PDF / 1,499,267 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 75 Downloads / 242 Views
STABILITY OF NANOMETER-SIZE Si CRYSTALS IN AMORPHOUS Si THIN FILMS UNDER ION IRRADIATION Chih M. Yang and Harry A. Atwater California Institute of Technology, Thomas J. Watson Laboratory of Applied Physics, Pasadena, CA 91125 ABSTRACT In this paper we discuss the size-dependent shrinkage rate of nanometer-size Si crystals in amorphous Si thin films during ion irradiation. We obtain shrinkage rate of nanometersize crystals at low temperature under ion irradiation by studying the evolution of the crystal size distribution using transmission electron microscopy. Our results indicate that crystals less than 10 nm in diameter decrease in size faster than larger crystals under ion irradiation. In addition, we fit our data to a capillary model for growth of Si crystals in amorphous Si under ion irradiation. INTRODUCTION Ion irradiation can be used to control the evolution of microstructures during crystallization of amorphous Si (a-Si). At high temperatures (> 500 °C), ion irradiation enhances nucleation rate of Si crystals in a-Si[l, 2]. At intermediate temperatures (200 'C to 450 °C), ion irradiation can induce both growth and shrinkage of crystals[3, 4]. In particular, recent experiments have shown that under ion irradiation only a fraction of the pre-existing crystals in a-Si can grow[3], which suggests that growth rate of small crystals might be affected by capillarity. Consequently, if one can obtain the dependence of crystal growth rate on size, one can use ion irradiation as a method to alter the crystallization kinetics of a-Si. For example, one can use ion irradiation to obtain a more uniform grain size distribution, to increase the average grain size, or to control the grain density. In addition, studying the size-dependence of the growth rate of small crystals under ion irradiation will increase our understanding of the importance of capillarity on the stability of Si crystals in a-Si under ion irradiation. We have performed our experiments at a low temperature (170 *C), so that all crystals decrease in size, and obtained size-dependent amorphization rate for the crystals. By performing the ion irradiation at low temperature, crystals that were smaller than the smallest observable size, d", do not grow; consequently we do not need to make any assumptions about the population of these crystals in order to obtain the amorphization rate of crystals larger than d4 6. Amorphization of Si crystals in a-Si via ion irradiation at low temperature has been reported previously[5], but in those experiments the size of crystals studied were relatively large (about 100 nm), and the amorphization rate was found to be size-independent. For our experiments, we study crystals between 6 nm and 50 nm in size.
BACKGROUND A model for ion beam-induced crystallization and amorphization has been developed which fits existing experimental data very well[6]. This kinetic model assumes a planar crystalline-amorphous (c-a) interface and proposes that the rate of interface movement, Mat. Res. Soc. Symp. Proc. Vol. 311. 01993 Materials Res
Data Loading...
