The Electronic Structure of Interstitial Iron in Silicon
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THE ELECTRONIC STRUCTURE OF INTERSTITIAL IRON IN SILICON A THILDERKVIST, G GROSSMANN, M KLEVERMAN, AND H G GRIMMEISS Department of Solid State Physics, University of Lund, Box 118, S-221 00 Sweden ABSTRACT A donor-like spectrum in Fe-doped silicon has been studied by means of high-resolution Zeeman spectroscopy. Previous work had unambigouosly identified the center as the interstitial iron impurity and the spectrum was interpreted as due to the transitions Fei + hv -- Fe+ + e", where an electron is excited to shallow effective-masslike donor states. In this paper, we can, by studying the transitions in a magnetic field, verify the effective-mass-like character of the loosely bound electron. Furthermore, we also obtain information on the impurity core whose level structure is reflected in the observed superposition of donor-like Rydeberg series and whose g values determines the Zeeman splitting pattern. INTRODUCTION The electronic properties of the transition metal (TM) impurities in silicon have attracted considerable interest, both experimentally and theoretically, since the pioneering electron- aramagnetic resonance (EPR) work by Ludwig and Woodbury in the early 60"s [1]. The theoretical understanding of TM impurities has increased during the last years and it is no exaggeration to state that a basic understanding has ben achieved (2]. Nevertheless, experiments reveal many finer structures which are still impossible to predict on theoretical grounds, e. g. symmetry-breaking lattice distortions; nor can the energy-level structure be calculated with an accuracy comparable to experimental observations. Transmission spectroscopy has proven to be a valuable too! for gaining deeper insight into the electronic structure of TMs in silicon. No internal d transitions have yet been identified and the optical transitions so far assigned are from a localized ground state to shallow donor or acceptor like states. Recently, the observation of a Fe-related line spectrum in silicon was reported [3]. An isochronal annealing experiment performed on the same sample in both EPR and transmission showed that the lines originate from transitions at the neutral charge state of interstitial Fe, Fe°0. Furthermore, the photoionization cross section for electrons was measured by photo-EPR and photoconductivi ty, and compared with previously published junction space charge results. The excellent agreement between the different results showed that the same center was probed by the three methods. The most intense lines were well accounted for by assuming two superimposed shallow donor-like spectra. Additionalweaker lines, indicated that also a third series may be involved. Considering the delocalization of the electron in one of the shallow-donor states, it is reasonable to assume that the interaction between the core and the excited electron is negligible. Therefore, it should be feasible to consider the final state as built up from two decoupled systems: a shallow-donor electron and the Fei+ core. The origin of the superimposed spectra was te
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