Evidence for a shallow Cu acceptor in Si from infrared spectroscopy and photoconductivity

Copper impurities affect optical and electronic properties in a diverse range of materials. Using IR spectroscopy, we provide evidence for a shallow Cu acceptor in Si. The observation of ${P}_{1/2}$ excitation lines indicates a Cu acceptor with a hole ionization energy of only 27 meV, which is shallower than any of the group-III acceptors. The observed center has an IR spectrum that is characteristic of a hydrogenic acceptor. Variable-temperature Hall-effect measurements confirm this result. The observation of shallow, hydrogenic behavior is consistent with a universal level model, in which the Cu acceptor level is fixed relative to vacuum, irrespective of the host.