Fourier transform spectroscopy of localized electronic excitations in semiconductors: I. Ultrashallow acceptors in neutron transmutation doped silicon. II. Transition metal ions in II-VI semiconductors

A new series of excitation spectrum occurs in Si(B) subjected to a slow neutron irradiation and a partial high temperature anneal. The neutron irradiation produces P by the transmutation of $\sp{30}$Si and radiation damage due to the transmutation process itself as well as the unavoidable fast neutron component of the neutron flux. The spacings and the relative intensities of the lines in the new spectrum are strikingly similar to the excitation spectrum of boron acceptors in Si but with lower energies. The new series corresponds to an acceptor (referred to as B$\sb{\rm NTD})$ with a binding energy E$\sb{\rm I}$(B$\sb{\rm NTD})$ = 28.24 meV, significantly lower than E$\sb{\rm I}$(B) = 45.70 meV. The concentration of these ultrashallow acceptor centers increases with annealing in the temperature range 550 to 675$\sp\circ$C; at higher temperatures these centers are annealed out. On the basis of piezospectroscopy, a tetrahedral$(T\sb{d})$ site symmetry is deduced for these centers and the deformation potential constants of the ground state are determined. It appears that the new acceptor centers are boron impurities in substitutional sites in association with defects. The analogous ultrashallow acceptors in which Al rather than B are involved also occur in Si(Al) subjected to a similar neutron irradiation and thermal anneal. The optical transitions from initial states in the $\sp5\Gamma\sb3$ orbital multiplet to final states in the $\sp5\Gamma\sb5$ orbital multiplet of Fe$\sp{2+}$ at a tetrahedral site in CdTe have been observed with a Fourier Transform Spectrometer. Their dramatic temperature dependence in the range 1.8-10 K and the concentration dependence are interpreted in terms of the energy level structure calculated by Villeret et al. (Villeret90). Similar experiments on CdSe:Fe permit the observation of effects associated with the trigonal symmetry of Fe$\sp{2+}$ in the wurtzite structure. Both systems exhibit phonon assisted electronic transitions as well as features associated with Fe$\sp{2+}$ complexes