PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY OF THE FLUCTUATING HYPERFINE INTERACTION AT CD-DONOR PAIRS IN SILICON - AN APPROACH TO ELECTRONIC-TRANSITIONS AT IMPURITIES IN SEMICONDUCTORS

Cadmium-donor pairs in n-type silicon (donors: P, As, Sb) are investigated by perturbed-angular-correlation (PAC) spectroscopy at the probe atom In-111/Cd-111. The pairs are formed between In-111 and the donors; the hyperfine interaction is measured by PAC directly after the radioactive decay of In-111 to Cd-111 and thus provides information about the electronic charge distribution around the Cd atom. For each type of donor the PAC spectra reveal two specific electric-field gradients (EFG's). It is demonstrated that these EFG's correspond to two different states of the same atomic configuration (one probe atom and one donor atom). PAC spectra are measured as a function of temperature (280-1100 K) and donor concentration (greater-than-or-equal-to 10(17) CM-3) and under the influence of a Schottky contact. Direct evidence for a fluctuating hyperfine interaction is found. The experimental results imply that the Cd-donor pairs form electrically active centers with one energy level close to the center of the band gap. Both relaxation effects initiated by the In-111 --> Cd-111 decay as well as equilibrium fluctuations between the two charge states of the pairs give rise to a time-dependent hyperfine interaction. The ionization probability and the transition rates between different charge states are extracted and are discussed in terms of Schottky-Read statistics. Both energy level and capture cross sections are derived. Within experimental errors, the electronic properties do not depend on the type of donor. The quantitative determination of dynamic properties from PAC spectra is enabled by the calculation of the perturbation function G(t) for a two-state model allowing fluctuations between axially symmetric EFG's of different strength. Some general properties of this function are discussed.