Photoionization cross-section studies of the platinum-donor center in silicon

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Abstract
The relative photoionization cross section of the platinum donor center in silicon was measured over the wavelength range of 2.4 to 3.9 μm by electrical deep-level optical spectroscopy on an n+p junction at 80 K. The data were analyzed in terms of the lattice-coupling model proposed by Ridley and Amato which was modified for valence-band nonparabolicity. Good agreement was obtained between the experimental results and the model calculations of the cross section with the energy level of the donor at 0.320±0.005 eV above the valence-band edge and a Huang–Rhys factor S of approximately 1.4. This S value corresponds to a Franck–Condon energy shift of 70 meV with a phonon energy of 50 meV. Previously reported photoionization data of the gold donor were also fit by the same model, yielding S≂0.4, a surprisingly small value. Estimates were made of the majority-carrier capture cross section for these two levels and for the platinum acceptor center in silicon, which was measured previously. These estimates, based on Ridley’s quantum-defect model and our measured S values, are several orders of magnitude smaller than the corresponding measured values, indicative of the complex nature of these 5d-transition elements in silicon. More detailed models, perhaps including anharmonicity of the defect vibrations, are required to understand these large capture cross sections.