Early electron-spin-resonance (ESR) studies by Woodbury and Ludwig on the Pt acceptor in Si have been refined and extended. The strain dependence of the spectroscopic splitting tensor $\overleftrightarrow{\mathrm{g}}$ has been measured using the strain-modulated electron-spin-resonance technique. The symmetry of the center proves to be monoclinic. The 20 independent elements of the magnetoelastic tensor $\overleftrightarrow{\mathrm{F}}$ allowed in $C_s$ symmetry are determined at $T=4.2\mathrm{}$ K. New details in the ESR spectra are also reported. A superhyperfine structure due to a second Pt atom indicates that Pt enters the lattice as pairs. Experiments on samples with varying Pt and P concentrations show that in the concentration range ${10}^{16}$ < [Pt] < ${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$, Pt is exclusively present as ${\mathrm{Pt}}^{-}$${-\mathrm{P}\mathrm{t}}^0$ pairs. Single ${\mathrm{Pt}}^{-}$ centers are not observed. The doubly charged pair state ${\mathrm{Pt}}^{-}$${-\mathrm{P}\mathrm{t}}^{-}$ is also absent, even in samples with high [P]/[Pt] ratio. This implies that both members of the pair must be located at distinct types of sites. The following model accounts for all observed features: One of the platinums (${\mathrm{Pt}}^{-}$) is at a near-substitutional site. It forms chemical bonds with two neighboring silicon atoms and is shifted in their direction along a [001] axis. The remaining two silicon neighbors form a reconstructed bond. The second ${\mathrm{Pt}}^0$ atom is at an interstitial site; its presence is responsible for the monoclinic ($C_s$) center symmetry.

• Received 10 May 1982

DOI:https://doi.org/10.1103/PhysRevB.27.4002