• PDF / 260,376 Bytes
• 6 Pages / 432 x 648 pts Page_size
• 30 Downloads / 282 Views

DOWNLOAD

REPORT

---

Temperature Induced Evolution of Bond-Centered Hydrogen (BCH) Defects in Crystalline Silicon: Dynamical, Electronic, Vibrational and Optical Signatures Zahraa A. Ibrahim, Anatoli I. Shkrebtii *, Frederic Zimmer-De Iuliis and Franco Gaspari Faculty of Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, ON, L1H 7K4 Canada. ABSTRACT The thermal stability, evolution and structure of the bond-centered-hydrogen (BCH) defect in crystalline silicon, its temperature induced dissociation, and the new H complexes formed have been studied in the temperature range from 50 K to 650 K by first-principles molecular dynamics (MD). We demonstrate that BCH is stable at 60 K, but decays at and above 310 K in agreement with experimental results. The dissolved BCH forms new complexes: transitional interstitials, stable monohydride-like and monohydride/dihydride-like complexes. The calculated asymmetric vibrational frequency of H in the BCH complex is 2000 cm-1, very close to the experimental values. Calculated vibrational frequencies, electron charge densities, electron densities of states (DOS), and optical spectra demonstrate noticeable differences for the different geometries with the BCH, interstitial and monohydride-like complexes, especially in the vicinity of the energy gap. The BCH complex is found to induce characteristic donor states below the conduction band, and raises the Fermi level to above the donor state energies. INTRODUCTION Hydrogen and its complexes are among the most important and intriguing defects in crystalline, amorphous and nano-materials, which attract much attention both experimentally and theoretically. H implanted into crystalline silicon (c-Si) at low temperatures resides primarily in the metastable configuration, the so called bond-centered- hydrogen (BCH) complex [1]. The BCH complex was shown experimentally to be only stable below temperatures of 200 K and even at 80 K is unstable if illuminated by above bandgap light [1,2]. At 280 K the BCH complex dissociated irreversibly and new H complexes formed. Different final H configurations were obtained in ion irradiation experiments, where again irreversible decay of BCH was observed [2]. In amorphous Si (a-Si) as well, BCH complexes have drawn special interest, as they have been proposed to exist as metastable intermediate states that facilitate the process of light induced degradation of hydrogenated amorphous Si (a-Si:H) [3]. Molecular Dynamics (MD) simulations can provide a depth of information on the microscopic level on the structure of the BCH complex, its stability and evolution, H migration paths, different Si-H bond types formed after BCH dissociation and energy considerations. We have shown that extensive MD simulations of a-Si:H, in which the structure was melted, cooled and annealed, exhibited the different Si-H and H-H complexes observed in experiments, including the BCH structure [4]. Our preliminary temperature dependent MD on c-Si showed that BCH is stable at 60 K, but dissolves at higher temperatures [5