Solute Binding at Void Surfaces in Silicon and Germanium

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SOLUTE BINDING AT VOID SURFACES IN SILICON AND GERMANIUM S.M.MYERS, D.M.BISHOP, D.M.FOLLSTAEDT, HJ.STEIN AND W.R.WAMPLER Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT The strongly exothermic reactions of H and Cu with internal surfaces in Si and Ge were examined in experiments employing ion implantation, ion-beam analysis, transmission electron microscopy, and infrared spectroscopy. The dissociation energy of the Si-H surface bond was determined to be 2.6±0.1 eV, so that the monohydride is more stable than molecular H 2, whose dissociation energy per atom is 2.26 eV. Initial experiments indicate a dissociation energy for the Ge- Hsurface bond of =1.9 eV. Copper is bound to the Si surface with an energy of 2.2±0.2 eV relative to solid solution, as compared to a reported binding energy of 1.5 eV for Cu in the precipitated Cu 3Si phase. INTRODUCTION AND APPROACH

Internal surfaces within Si and Ge are highly reactive because of the dangling bonds which protrude unobstructed into the overlying void. Solute reactions at these sites are important for passivation and modification of electronic states and for the potential use of internal surfaces to "getter" metallic impurities from solution. We are conducting experiments to quantify the surface binding energies for technologically important adsorbates, and in the present paper we report results for H in Si, H in Ge, and Cu in Si. The Si-H surface bond energy is not known from the numerous studies of external-surface desorption because the measured desorption activation energy involves an incompletely understood interplay of Si-H dissociation and H-H recombination. We circumvent this limitation by observing release from internal surfaces, where the promotion of atoms from surface states into lattice solution is rate-determinin& [1]. In the present experiments, microscopic cavities were formed within Si and Ge by ion implanting He at room temperature and then vacuum annealing. The annealing, at 973 or 1073 K, enlarged the He bubbles, annealed ion damage, and induced He permeation from the cavities [2,3] leaving closed voids. Resulting microstructures were characterized by cross-section transmission electron microscopy (TEM). Hydrogen was introduced into the cavity-containing layer by ion implantation or by heating in H 2 gas, and detrapping and release were subsequently induced by vacuum annealing. The uptake, redistribution, and release were observed by nuclear-reaction analysis (NRA). The bonding of the H was examined by infrared (IR) spectroscopy of the local stretching vibrations of Si-H and Ge-H bonds. In the experiments on Cu in Si, Cu was introduced by ion implantation, and during subsequent vacuum annealing the migration of Cu to and from cavity-containing layers was observed by Rutherford backscattering spectrometry (RBS). Surface binding energies for the H and Cu were extracted by numerically solving the mathematical formalism for solute diffusion in a field of traps [4] and adjusting the trap strengths to produce agreement with experiment. We prev