Precipitation of copper in silicon

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Abstract
The structure and morphology of precipitates of copper, decorating dislocations in high‐purity nearly perfect silicon single crystals were determined by high‐voltage (1 MV) transmission electron microscopy and are discussed in light of the covalent bond formation by copper in silicon. The precipitates are observed to form during cooling, in clusters or colonies, on and around dislocations. They are spherical, 50–500 Å in size, and exhibit parallel moiré fringes. Those which are heterogeneously nucleated on dislocations are always found to be in the larger size range. Dislocations are thought to be present prior to precipitation and they climb down, emitting vacancies for precipitating substitutional copper atoms, and thus become heavily jogged. No ``rel‐rod'' streaks are found in diffraction patterns. Only a few extra spots are found and then only when the precipitate density is very high (5×1014/cm3). The crystal structure of the precipitates was determined by combined analyses of moiré spacings and the few extra spots. It is found to be diamond cubic (B3 type, a0=5.72 Å), isomorphous with the structure (A4). The B3 structure is derived from the diamond cubic (A4) silicon lattice by replacing corner (000) and face‐center (1/2 1/2 0, 1/2 0 1/2, 0 1/2 1/2) sites by copper atoms. The precipitate has a chemical composition of Cu–Si and is not stable at room temperature but is thought to be stabilized by strain energy of the precipitate/matrix system. The new interpretation of covalency of copper in the diamond cubic silicon is given on the basis of stereochemical considerations. The proposed tetrahedral coordination of copper atoms implies d10sp3 orbitals which are rationalized in terms of state of ionization (triple acceptor) of copper in silicon. The choice of conductivity type, reduction in conductivity after precipitation, and quick recovery time in depletion‐capacitance measurements are qualitatively explained in terms of the proposed covalency of copper.