The Role of Low-Energy Ion/Surface Interactions During Crystal Growth From the Vapor Phase
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THE ROLE OF LOW-ENERGY ION/SURFACE INTERACTIONS DURING CRYSTAL GROWTH FROM THE VAPOR PHASE J.E. GREENE, A. ROcKETr, AND J.-E. SUNDGREN Materials Science Department, the Coordinated Science Laboratory, and the Materials Research Laboratory University of Illinois, Urbana, Illinois, 61801 ABSTRACT Low-energy (often < 100 eV) ion bombardment during thin film deposition is commonly used in such diverse application areas as microelectronics, optical coatings, magnetic recording layers, and hard wear resistant coatings to modify the microstructure and microchemistry of films deposited by a variety of techniques (e.g. sputtering, primary ion deposition, plasma-assisted CVD. and accelerated-beam MBE). Ion irradiation has been shown to affect every phase of deposition including nucleation and growth kinetics, crystal structure and phase stability, the average grain size and degree of preferred orientation of polycrystalline films, the epitaxial temperature of single-crystal films, defect concentrations, elemental incorporation probabilities, surface segregation, and, hence, film properties. As discussed in this brief review, a detailed understanding of many of these processes is beginning to emerge. Effects such as trapping, preferential sputtering, enhanced diffusion, and collisional mixing have been used to interpret and, in some cases, model experimental results. Nevertheless, there are still large gaps in our knowledge of the role of ion bombardment on fundamental processes such as nucleation kinetics. I. INTRODUCTION The requirements of increasingly sophisticated thin-film device and processing technologies have resulted in a continuing drive to obtain better control over the microchemistry and microstructure of as-deposited layers and to devise lower-temperature growth techniques. Unfortunately, these objectives are often mutually exclusive. Nevertheless, a general strategy has emerged involving the evolution of growth techniques away from near-equilibrium deposition toward kinetically-limited processes. With good experimental design and an understanding of the rate-controlling steps and reaction paths, the crystal grower can manipulate the growth kinetics to tailor the resulting film properties. (Clear evidence that this is possible is provided by recent results demonstrating the growth of new single-crystal thermodynamically metastable semiconductor alloys such as (GaAs)(l_) 0 (Ge 2 )x and (GaSb) 1 0- (Ge 2 )x which exhibit non-linear optical properties [1.2].) General approaches which have been explored include the use of lowenergy ion bombardment of the film and photo-stimulated gas-phase and surface reactions during deposition. The first approach is discussed in this article while photo-stimulated reactions are reviewed in reference 3. Low-energy (often < 100 eV) particle bombardment during vapor-phase deposition has been shown to play an important and sometimes dominant role in controlling film growth kinetics and physical properties. Ion-irradiation-induced effects have been observed in films deposited by glow d
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