STM investigation of energetic insertion during direct ion deposition
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STM investigation of energetic insertion during direct ion deposition Joshua M. Pomeroy, Aaron Couture, Joachim Jacobsen†, Barbara H. Cooper‡, J.P. Sethna, and Joel D. Brock Cornell Center for Materials Research, Clark Hall, Cornell University, Ithaca, NY 14853 USA † currently with Haldor Topsøe A/S, Denmark ‡ deceased August 1999 ABSTRACT Thin copper films have been deposited on single crystal copper substrates and characterized using a UHV Scanning Tunneling Microscope to probe the effect of atomic insertions during hyperthermal ion deposition. At low temperatures, atomic insertions are predicted to provide a net downhill current that offsets the roughening effect due to uphill "Schwoebel" currents leading to a net smoothing of the surface. Films have been grown at several different energies targeted to observe a crossover from insertion driven smoothing to adatom-vacancy dominated roughening. Copper thin films are deposited near 20 eV using a mass selected ion deposition system that allows precise control (+/- 2 eV) over the energy of constituent atoms. Experimental observations are compared with a sophisticated Kinetic Monte Carlo and Molecular Dynamics hybrid (KMC-MD) simulation. INTRODUCTION Techniques for growing thin films tend to fall in two general categories1. Either the film is grown from constituents that arrive at the substrate with a thermal kinetic energy, or they have been accelerated in some fashion to achieve hyperthermal energies. While a community effort has led to the development and refinement of theory and techniques for thermal deposition2, understanding hyperthermal energy effects requires additional attention. During thermal deposition, atoms generally land gently on the surface, often sticking to the first point of contact (Ballistic Deposition)3. Depending on the temperature, the atom may then diffuse on the surface until it achieves a stable enough configuration to remain essentially stationary. Accelerating atoms to hyperthermal energies provides enough momentum that the collision can modify the pre-existing atomic configuration of the surface. Examples of experimental efforts to decipher the role of incidence energy during the evolution of thin film growth include supersonic jets, Ion Beam Assisted Deposition (IBAD), and direct ion deposition1,4. While some success has been enjoyed in controlling morphological transitions5, fundamental understanding has been limited to sub-monolayer deposition6. Three mechanisms are found to be important below 100 eV: atomic insertion, adatom/vacancy formation, and re-sputtering. Insertions occur when an atom strikes the surface just above a step edge and forces itself into the surface by shifting the registration of its new neighbors. Adatom/vacancy pair formation occurs when the incident atom dislodges a member of the underlying crystal lattice. This occurs most often when the atom strikes flat terraces where the atomic configuration is very stable. Re-sputtering is the result of the adatom in adatom/vacancy pair formation being ejected from the sys
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