Vibrational Spectra of Nitrogen-Oxygen Defects in Nitrogen Doped Silicon using Density Functional Theory
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Vibrational Spectra of Nitrogen-Oxygen Defects in Nitrogen Doped Silicon using Density Functional Theory F. Sahtout Karoui, A. Karoui, N. Inoue*, G. A. Rozgonyi Materials Science and Engineering Dept. North Carolina State University, Raleigh, NC 27695-7916, U.S.A. * Japan Electronics & Information Technology Association, RIAST, Osaka Prefecture University, Japan. ABSTRACT The vibrational spectra of N-pairs and nitrogen-vacancy-oxygen defects in nitrogen doped Czochralski silicon have been investigated using density functional theory calculations. We found that 771 cm-1 and 967 cm-1 lines measured by FTIR are fingerprints for N-pairs in interstitial position. These confirm that nitrogen atoms are paired and bonded to Si atoms. Calculated local vibration modes of N2On complexes provide the best matching with observed FTIR frequency of N-O complexes. Nonetheless, VmN2On (m,n =1,2) can develop during crystal cooling or wafer processing, as revealed by local vibrational modes falling around, 806 and 815 cm-1 FTIR frequencies. INTRODUCTION Nitrogen is an important dopant in silicon even though at concentration levels as low as 1015 cm-3, because of its strong locking effect of dislocations. As a result, large diameter Czochralski (CZ) or float zone (FZ) silicon wafers toughness is enhanced [1]. In addition, nitrogen interacts with point defects as well as impurities, thus can be used to control the formation of micro-defects. It significantly reduces swirl defects and vacancy related defects, improving the gate oxide integrity (GOI) [2, 3]. Nitrogen also dramatically enhances oxygen precipitation which is valuable for gettering of metallic impurities in the bulk [4, 5]. As much as 80% of nitrogen are paired and bonded to silicon at concentration much larger than the solid solubility limit [6]. Nitrogen-pairs might exist either in interstitial positions forming N2 complexes, or in substitutional sites capturing a vacancy (V) or divacancy (V2) forming nitrogenvacancy complexes [6]. The absorption bands 771 cm-1, 967 cm-1 (at T < 15 K) [6, 7] have been associated with localized modes for N-pairs and are commonly used to measure nitrogen concentration in N-FZ/N-CZ Si wafers. The FTIR absorption lines of the N-O complexes occur at 806, 815, 1000, 1021, and 1031 cm-1 [7]. In this paper, we present results from Density Functional Theory (DFT) calculations of N2, VN2, V2N2 and VmN2On vibrational properties (i.e. N2On, VN2On, V2N2On, n=1, 2). The calculated local vibrational modes (LVMs) are compared to FTIR measured data. COMPUTATIONAL METHOD Defect Atomic Structure In order to simulate the crystal real structure and to preserve all symmetries of the diamond structure we have used a periodic cubic system consisting of a supercell of 64 silicon atoms containing the defect in its center. All defects have been considered in their neutral state. To avoid defect-defect interaction from one supercell to another we maintain immobile the atoms
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close to the boundaries while relaxing the system. The neutral N2 has a C2h
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