Electrically Active and Electrically Inactive 3 d Transition Metal Centers in Si
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1268-EE02-01
Electrically active and electrically inactive 3d transition metal centers in Si S.K. Estreicher and D.J. Backlund Physics Department, Texas Tech University Lubbock TX 79409-1051, USA ABSTRACT Systematic first-principles calculations on transition metal (TM) impurities of the 3d series in Si have been performed. The equilibrium sites, migration energies, electrically-active gap levels, charge and spin states are predicted. While the properties of the isolated interstitials are experimentally well-known, much less experimental information is available about the consequences of their interactions with vacancy-like defects. We discuss here the properties of isolated interstitial Ti, Fe, and Ni, their interactions with vacancies and divacancies, the properties of the resulting substitutional impurities, and of the TM-divacancy {V-TM-V} complexes. In equilibrium, interstitial Ti, Fe, and Ni do not become substitutional, but a number of processing steps commonly used in PV manufacturing introduce highly mobile vacancies into the bulk. These vacancies strongly interact with interstitial TMs. At the substitutional site, Ti, Fe, and Ni have very different electrical properties than at the tetrahedral interstitial site. In particular, the electrical activity (and stable spin state) of Ti and Fe are greatly reduced, suggesting that the passivation by vacancies plays an unrecognized role during a variety of hightemperature processes. INTRODUCTION Transition metals (TMs) from the 3d series are ubiquitous and unwanted interstitial contaminants in Si [1-4], especially for photovoltaics (PV) applications [5-8]. The first elements of the series (such as Ti) diffuse very slowly and are powerful recombination centers, especially in p-Si. The middle element (Fe) is mobile at room temperature and forms pairs with shallow acceptors. Both the isolated interstitial and the Fe-acceptor pair have a donor level in the gap and reduce minority carrier lifetimes in p-Si. The heaviest elements in the series (such as Ni) are very fast diffusers and tend to precipitate at defects (grain boundaries, dislocations, interfaces) and these precipitates are electrically active. Over the decades, numerous studies have been published, and many properties of isolated interstitial and substitutional TMs are now known. The details of these studies are discussed in several reviews [1-4]. The early theoretical studies [9-11] of interstitial and substitutional TM impurities have focused on establishing trends to explain electron paramagnetic resonance data [12] and the electrical activity of the impurities. These studies did not include potential energy surfaces and other energetics of the defects. An ab-initio Hartree-Fock study [13] in small hydrogen-saturated clusters predicted a large barrier for diffusion for Ti and the formation of a strong Ti-H bond forms in the 0 charge state. Attempts at passivating TM impurities with hydrogen have been largely unsuccessful. Many TM-H complexes have been observed [14] by deep-level transient spectroscopy (DL
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