Island Evolution During Early Stages of Ion-Assisted Film Growth: Ge ON SiO 2
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ISLAND EVOLUTION DURING EARLY STAGES OF ION-ASSISTED FILM GROWTH: Ge ON SiO 2
Shouleh Nikzad and Harry A. Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125 ABSTRACT Microstructure evolution has been studied in the early stages of growth of polycrystalline Ge on SiO 2 in ultrahigh vacuum by ion beam-assisted deposition, and has been systematically compared to conventional thermal film growth. Experimental results indicate that, at equivalent substrate coverages, films grown by ion beam-assisted deposition exhibit increased island density and decreased island size with respect to thermally-deposited films. The increased island density is not a result of an increased island nucleation rate associated with defects produced in the SiO 2 substrate during ion beam-assisted deposition. Instead, a new model for ion beam-induced adatom desorption from islands is proposed to account for the increased island density and decreased island size. INTRODUCTION Ion beam assisted deposition (IBAD) is a deposition technique used to prepare thin films for optical[I], tribological, and corrosion resistant coating applications, as well as reactive compound synthesis[2]. Despite its widespread use, the mechanisms for ion beam-induced modification of microstructure and microstructural evolution remain open questions[3,4]. Many efforts have been made to understand the fundamentals of IBAD[5], however, the results of previous experimental studies do not provide consistent evidence for a single mechanism for ion bombardment-induced modification of island size and density in polycrystalline films[6-10]. Among the many ways that ion bombardment can conceivably effect the microstructure formation, several have been speculated to be the most important: sputtering, adatom diffusion enhancement, ion beaminduced production of nucleation sites, and dissociation of small clusters. In some previous studies, lack of a consistently clean deposition environment left open the possibility of microstructural modification by contamination. A more important problem with IBAD experiments is the difficulty associated with independently controlling different aspects of ion-surface interactions as well as film/substrate interactions. In this paper, we describe results of a systematic study of a model system for IBAD: early stages (before complete island coalescence) of polycrystalline Ge growth on Si0 2 in ultrahigh vacuum (UHV). Simultaneous growth of IBAD and thermally-deposited films allowed ion bombardment-induced microstructural evolution to be distinguished from sample-to-sample variations. In addition, independent experiments were performed to examine the effects of (i) thermallyinduced island coarsening, (ii) sputtering and (iii) bombardment-induced creation of preferential nucleation sites. In order to make reasonable inferences about the nucleation and growth processes, while minimizing effects due to onset of island coalescence, comparisons were made for thermally-deposited and IBAD films at
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