Ion Beam Enhanced Grain Growth in Thin Films
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ION BEAM ENHANCED GRAIN GROWTH IN THIN FILMS
HARRY A. ATWATER(a), CARL V. TIIOMPSON(b), AND HENRY I. SMITH(W) (a)Department of Electrical Engineering and Computer Science (b)Department of Materials Science and Engineering Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT Ion beam enhanced grain growth has been investigated in thin films of Ge. Grain boundary mobilities are greatly enhanced over their thermal equilibrium values and exhibit a very weak temperature dependence. We propose that defects which are generated by the ion beam at or near the grain boundary are responsible for the boundary mobility enhancement. Films of Ge deposited under different conditions, either unsupported or on thermally oxidized Si, exhibit similar normal grain growth enhancement when implanted with 50 keV Ge+. Beam-enhanced grain growth in Ge was also demonstrated using Xe+, Kr+, and Ar+ ions. The variation in growth enhancement with projectile ion mass is in good agreement with the enhanced Frenkel defect population calculated using a modified Kinchin-Pease formula and Monte Carlo simulation of ion transport in thin films. Calculations based on experiments suggest that there is approximately one atomic jump across the grain boundary per defect generated. Also, the grain growth rate for a given beam-generated defect concentration near the boundary is approximately equal to the expected growth rate for the same defect concentration if thermally generated. INTRODUCTION Ion beams have been used extensively to enhance the kinetics of processes in solids such as crystallization [1] and impurity diffusion 12], however their use in enhancing grain growth is relatively new[3]. Ion bombardment is attractive as a potential lowtemperature method for enhancing grain growth in thin films. More generally, study of grain growth during ion bombardment may provide insight into the detailed mechanism of grain boundary motion. Recently, we reported ion beam enhanced grain growth(IBEGG) in thin Ge films, during implantation at T = 600 °C with a 50 keV Ge+ ion beam. In this paper we discuss the mechanism for grain boundary mobility enhancement during IBEGG, and present new experimental results for Ge. EXPERIMENTAL In order to study ion beam enhanced grain growth, we chose ion bombardment energies so that the ion-beam-generated damage distribution is peaked at approximately one half of the film thickness. We also chose the film thickness to be no greater than a few times the standard deviation of the damage distribution, as shown in Fig 1. Under these conditions, a large fraction of the volume of the film is damaged by ions and recoil atoms. For 500 A Ge films, ion energies ranged from 50 keV for Ar+, Ge+, and Kr+ to 100 keV for Xe+. The projectile ions were chosen to be either native species (i.e., Mat. Res. Soc. Syrup. Proc. Vol. 74. '1987 Materials Research Society
500
Ge+ in Ge) or noble gases in order to avoid confusion between physical and chemical kinetic enhancement, such as dopant enhanced grain boundary migration[4]. Ion dos
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