The Electronic Structure and Atomic Symmetry of The Oxygen Donor in Silicon
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THE ELECTRONIC STRUCTURE AND ATOMIC SYMMETRY OF THE OXYGEN DONOR IN SILICON MICHAEL STAVOLA AND KEON M. LEE AT&T Bell Laboratories, Murray Hill, New Jersey 07974 ABSTRACT The infrared spectrum of oxygen donor complexes in silicon under uniaxial stress has been examined for the neutral and singly ionized charge states. Our results are consistent with an effective mass-like ground state wave function that is constructed from a single pair of conduction band valleys for both charge states. A thermal ionization experiment in which the stress split components of the ground state are monitored by the absorption of polarized light confirm this interpretation and provide correlation with DLTS and EPR results. Additional small splittings, due to deviations from effective mass theory, show that the electronic wave function of the oxygen donor is distorted by an extended "central cell" with C2 v symmetry. Previously observed splittings of 1s -" np± transitions for the singly ionized charge state at zero stress are interpreted in terms of the effect of the anisotropic oxygen donor structure upon excited state wave functions constructed from the single pair of conduction band valleys. INTRODUCTION Prolonged heat treatment near 450"C of oxygen rich silicon leads to the formation of cm- 3 shallow, oxygen related, donor complexes. Since the discovery of the oxygen donor effect 30 years ago [I], many workers have attempted to elucidate the microscopic structure and growth mechanism for the defect. Early work focussed on the formation kinetics of the oxygen donor that were determined through resistivity measurements. The initial growth rate is proportional to [014 and the maximum concentration of oxygen donors is proportional to [013. These findings led to the proposal by Kaiser, Frisch, and Reiss [21 that the oxygen donor is a Si,0 4 complex that is built up in successive reaction steps, _1016
Si,,O_ + 0 -' Sin O"+jA specific structure and a mechanism to explain the electrical activity were not proposed. In subsequent work the oxygen donor has been studied extensively by several techniques that include Hall effect [31, IR absorption [3-71, EPR [8,91, DLTS [10-121, and ENDOR [131. Several recent reviews summarize the expanding body of experimental work and the many structural models suggested for the oxygen donor complex [14-16]. In spite of tremendous effort there is not a generally accepted structural model or an accepted model for the formation kinetics of oxygen donors. In IR absorption, two groups of effective mass-like features have been observed (Fig. 1). The compensation behavior and temperature dependences of these groups of features demonstrate that the oxygen donor is a double donor [4]. We denote the neutral charge state by TD * and the singly ionized charge state by TD+. Further, the oxygen donor is not a unique defect but is a family of complexes with closely spaced ground state energies that give rise to at least nine distinct series of helium atom-like absorption features [5-71. We adopt the assignments and notation
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