Dynamics of Microstructure in the Early Stages of Ion Beam Assisted Film Growth
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DYNAMICS OF MICROSTRUCTURE IN THE EARLY STAGES OF ION BEAM ASSISTED FILM GROWTH
Harry A. Atwater, C.J. Tsai, S. Nikzad, and M.V.R. Murty Thomas J. Watson Laboratory of Applied Physics California Institute of Technology, Pasadena, CA 91125
ABSTRACT Recent progress in low energy ion-surface interactions, and the early stages of ion-assisted epitaxy of semiconductor thin films is described. Advances in three areas are discussed: dynamics of displacements and defect incorporation, nucleation mechanisms, and the use of ion bombardment to modify epitaxial growth kinetics in a truly surface-selective manner.
I. INTRODUCTION The study of low energy beam-surface interactions is motivated by the need for more sophisticated understanding and control of film growth and etching processes. In order to produce precisely controlled structures with atomically abrupt interfaces at the nanometer scale, thin film growth techniques have evolved away from processes which occur near thermodynamic equilibrium to those limited by surface kinetic phenomena, such as adsorption, diffusion and surface chemical reactions. Atomic and molecular beams composed of energetic ions and neutrals have the potential to provide additional control over non-equilibrium, kinetically-limited surface and thin film processes. Although energetic ion beams have been employed to a limited extent in epitaxial growth processes, the full potential of low energy ion and neutral beams as sophisticated engineering tools in advanced epitaxial and heteroepitaxial film growth processes is only beginning to be realized at present. Understanding and control of defects created by low energy ion bombardment are crucial to the growth of high-quality electronic materials. Concern about damage has been a major factor limiting more extensive use of low energy ion beams as tools used in conjunction with molecular beam epitaxy. We describe here a systematic study of defect incorporation during low energy (50-300 eV) ion bombardment of Ge and Si (001) surfaces. Recent gains in understanding of surface kinetic mechanisms in conventional epitaxy with thermal beams have in turn stimulated new insights about low energy beam-surface interactions. We discuss here changes in the mechanisms for nucleation during ion bombardment, in three contexts: (i) Volmer-Weber nucleation of cap-shaped islands, (ii) nucleation of two-dimensional islands during growth and sputter erosion, and (iii) suppression of island nucleation during Stranski-Krastanov growth. Finally, we note the prospects for controlling film growth via truly surface-selective atomic displacments using ion bombardment energies near the atomic displacement threshold.
II. DEFECT GENERATION AND DAMAGE Possibly the most critical issues to be addressed in beam-assisted epitaxial growth are those related to the generation and interaction of defects in the near-surface region. In addition to generation of bulk point defects and complexes, low energy beams create "surface" defects, i.e., atomic displacements in the first or second layers
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