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ABSTRACT:We have studied the structure and geometry of neutral and charged atomic clusters consisting of Ga and As atoms via ab initio Hartree-Fock (HF) and second-order Møller-Plesset methods. The Ga m As n cluster with m = n composition prefers a nontetrahedral geometry in the charge neutral (q = 0) state. These clusters tend to be stable in tetrahedral geometry when appropriately charged. The Ga m As n cluster with m = n composition (1:1 ratio of Ga to As atoms) tends to be stable in a tetrahedral geometry in the charge neutral (q = 0) state. With increasing size of the cluster, the geometry of Ga n As n cluster approaches the zinc-belende-type crystalline structure.

Ab initio Hartree-Fock and second-order Möller-Plesset theory calculations have been performed to investigate the stability of triply-coordinated 0+ centers in the Si-O-Si network of amorphous SiO2. The calculations reveal that the H+ ion binds with a bridging O center to form a very stable (De > 6 eV) trivalent O complex. Capture of an electron by the positively charged protonated complex, however, is predicted to immediately lead to the dissociation of the O-H bond. A relatively weaker, but stable bond is also formed between the bridging O atom and a +SiH3 ion.

It is shown on the basis of first-principles quantum mechanical calculations that the observed 420G hyperfine signal attributed to the ET' center in amorphous SiO, could result either from a negatively charged symmetric or from a positively charged asymmetric oxygen-deficient center (ODC). Energetic considerations slightly favor a positively charged ODC at preexisting asymmetric =Si-Si= sites as the local microscopic structure of E?' center. SUMMARYBuried oxide (BOX) in silicon-on-insulator (SOI) materials is known to contain an enhanced density of oxygen-vacancy (V,) related defect precursors. These precursors are known to act as charge trapping centers. One such defect is the E,,' center which has been identified as a paramagnetic center related to V,. The characteristic feature of this defect is a 420 G doublet splitting o f the Electron Spin Resonance (ESR) hyperfine spectrum [ 1,2]. Due to its role as a major source of performance degradation of SO1 devices, the microscopic structure and characteristics of this defect has been a subject of intense research in the recent years. While it is established by now that the E, ' defect is related to V, in BOX, its microscopic structure, electrical characteristics, and generation mechanism are uncertain.The prevailing model of E. , ' center is a trapped hole on an asymmetrically relaxed Si atom with the spin of an unpaired electron localized in a dangling bond on the undisturbed pyramidal (sp3) Si atom around V, [ 3-51. The positive charge on this defect is indirectly supported by a correlation between the density of E, , ' and the hole density in C-V measurements on irradiated thin SiOz films [ 6 ] . However, a negative g-shift in the ESR spectrum suggests a negative charge due to a trapped electron [l]. Also, quantum mechanical calculations [5] suggest that asymmetrical relaxation produces additional strained configurations which are unstable with respect to the symmetrically relaxed 0-vacancy.Due to the fundamental importance of E,,' center in SO1 materials and the uncertainties surrounding its atomic structure and electrical features, we have performed first-principles quantum mechanical calculations within cluster approach to examine existing models and to obtain more accurate information. Model structures for a regular S O , cluster (P) and a neutral ODC (V,") are shown in Fig. 1. Positively and negatively charged oxygen vacancies (Vo+, Vo-) were generated from V," (Fig. lb) by placing appropriate charge. Our calculations on the structure and 29Si hyperfine coupling (Table 1) of symmetric charged species strongly suggest that a negatively charged oxygen vacancy (Vo-) best represents the spin property, such as the hyperfine coupling (U) of the E, ' center. However, a V, ' structure is calculated to be energetically unstable with respect to a neutral oxygen vacancy (Voo) ( Table 1). Results on the asymmetric V, ' structures, which are generated by displacing one of the Si atoms (Si, in Fig. lb) from its position in P (d=O) toward its back-bonded oxygen plane, s...

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