I Studies of Chemical Splittings, Triplet Levels and Tunneling States at Electronic Defects in Silicon.

Infrared spectroscopy, combined with temperature dependence, magnetic field and uniaxial stress, has been used to study a variety of modifications to the hydrogen model for donors and acceptors in semiconductors. These are spin-triplet levels for double donors, tunneling or rotational motion for molecular acceptors, a novel chemical -splitting scheme for a hydrogen-related donor, and a dependence of level lifetimes on chemical-splitting magnitude. The significance of many-body interactions for double donors has been demonstrated by the observation of weak absorption lines, not predicted by the hydrogen model, in the spectra of selenium and telurium in silicon. Zeeman splitting, interpreted by the theory of Lande g-factors, confirms their identification as transitions to spin-triplet terms. In addition, as a test of our analysis, Zeeman spectroscopy of the singly-ionized double-donors is studied. To investigate the influence of the nuclear degrees of freedom on the electronic spectrum, the temperature - and stress-dependences of the electronic 2p^ ' line for the acceptors (Li,Be), (D,Be) and (H,Be) in silicon have been measured. The results demonstrate that the (Li,Be) center has a fixed < 111>-orientation with a ground level split by local distortion and that the (D,Be) and (H,Be) centers undergo either tunneling or hindered -rotor motion between equivalent < 111 > equilibrium orientations. The temperature-dependent infrared spectra of a new hydrogen-activated donor in silicon reveals three low-lying energy levels with degeneracies in the ratio 1:2:2. No isotope effect is observed when hydrogen is replaced by deuterium, so any model must be based on electronic degrees of freedom. Our observations are consistent with a D_{rm 3d} interstitial site for monatomic hydrogen whose level structure arises from an "inverted" valley-orbit interaction. Laser saturation has been used to prove a transition of the deep-donor Se in AlSb. The intensity dependence of the absorptivity is explained by a two-photon process. We find an excited-state lifetime which is about three orders of magnitude smaller than that for shallow donors in III -V compounds.