Ion-Surface Interactions During Epitaxy
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ION-SURFACE INTERACTIONS DURING EPITAXY S.A. BARNETT, C.-H. CHOI, and R. KASPI Department of Materials Science and Engineering and the Materials Research Center, Northwestern University, Evanston, IL 60208 ABSTRACT Recent work on low-energy ion-assisted deposition of epitaxial films is reviewed. Much of the interest in this area has centered on the use of very low ion energies (- 25 eV) and high fluxes (> 1 ion per deposited atom) obtained using novel ion-assisted deposition techniques. These methods have been applied in ultra-high vacuum, allowing the preparation of high-purity semiconductor materials. The following ion-surface interaction effects during epitaxy are discussed: improvements in crystalline perfection during low temperature epitaxy, ion damage effects, improved homogeneity and properties in III-V alloys grown within miscibility gaps, and changes in nucleation mechanism from Stranski-Krastanov to layer-by-layer. I. INTRODUCTION Epitaxial growth processes are modified by low-energy ion irradiation of the growth surface. Several reviews on this topic have been published in the past several years [1-4]. Ion-induced changes in growth mechanisms include lowering of epitaxial temperatures [1,2], changes in elemental incorporation rates [1,2], altered dopant site selectivity in compounds [51, improved selected area growth [2], and alloy growth within miscibility gaps [6]. Molecular dynamics [7] and Monte Carlo [4] simulations of ion-surface interactions give considerable insight into the atomic processes that can influence epitaxy: including trapping, sputtering, enhanced desorption, enhanced atomic mobilities, mixing, and defect production. One of the main results of both the experimental and theoretical studies described above is that ion damage occurs in some growth parameter ranges. For example, in semiconducting films grown by ion-assisted molecular beam epitaxy (IAMBE), increasing the ion energy E above critical values led to decreased carrier mobilities [8] and decreased the quality of photoluminescence spectra [9] due to ion-induced defects. For the growth of high quality semiconductors, ion-assisted epitaxy should thus be carried out at ion energies near the threshold for displacing bulk atoms in order to minimize ion damage effects [4]. On the other hand, ion fluxes should be relatively high in order to maximize beneficial ion-surface interaction effects. The purpose of the present review is to describe recent work on ion-assisted epitaxy, in particular for the case of very-low-energy (- 25 eV), high- flux (> 1 ion per deposited atom) ion irradiation. The development of techniques that allow controlled irradiation in this range under very high purity conditions has led to significant new results on semiconductor epitaxy. A brief description of the experimental techniques is given in section II. In section III, the effect of irradiation on nucleation is described. Effects of ion irradiation on defect densities are discussed in section IV. In section V, effects of ion irradiation on alloy formation a
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