Proof of the Acceptor State of the Trigonal Iron-Boron Pair in Silicon by Electron Paramagnetic Resonance Measurements
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PROOF OF THE ACCEPTOR STATE OF THE TRIGONAL IRON-BORON PAIR IN SILICON BY ELECTRON PARAMAGNETIC RESONANCE MEASUREMENTS W.Gehlhoff and U.Rehse* AG EPR im Wissenschaftler-Integrations-Programm, Rudower Chaussee 5, D/O-1 199 Berlin *Institut fu'r Kristallziichtung im Forschungsverbund Berlin e.V., Rudower Chaussee 6, D/O-1199 Berlin, Federal Republic of Germany ABSTRACT For the first time the acceptor state of iron-boron pairs the occurence of which are suggested by electrical measurements and theoretical considerations is directly proved by EPR measurements. The corresponding EPR spectrum is observed upon suitable illumination of samples which have been co-doped with boron and iron. It shows trigonal symmetry, and the finestructure pattern can be described with the spin S=1, the g-values g, = 2.1345±0.0001, g• = 2.1345±0.0004 and the zero-field splitting parameter IDI = (1.418±0.001) cm-1. 1 INTRODUCTION It is a general practical experience that iron is one of the most important elements for an unintentional contamination of silicon wafers since it is a main constituent of many materials and a common impurity in chemicals. Because of its high diffusion coefficient, isolated interstitial iron is unstable even at room temperature and tends to form complexes with other impurities. The simplest ones are iron-shallow acceptor pairs in p-type silicon. Such pairs have been extensively studied during the last years ,2. Detailed information on the identity, symmetry and electronic structure of the neutral pairs were obtained by Electron Paramagnetic Resonance (EPR) studies. These investigations have revealed that Fe-acceptor pairs have two structural configurations with trigonal and orthorhombic symmetry, except for FeB which has only trigonal symmetry. The correspondence between the two donor levels detected by DLTS "and the trigonal and orthorhombic defect configuration observed in EPR was demonstrated by combined EPR and DLTS experiments ". In addition to the well-established (FeB)y+ level at Ev+0. leV an acceptor level at Ec-0.29 eV I (Ec-0.23 eV 7) was noted in p-type silicon with identical annealing characteristic to the Ev+0. leV donor level and was suggested as the acceptor state of the iron-boron pair. But a microscopic identification of the defect has not been established so far. In this paper we report the observation of a new EPR center in silicon co-doped with boron and iron upon suitable illumination of the samples. The results show that the EPR spectrum is indeed due to the negativ charge state of the trigonal iron-boron pair which caused an acceptor level at Ec-(0.25±0.05) eV. 2. EXPERIMENTAL DETAILS The samples were prepared from boron-doped p-type FZ silicon with a resistivity of 2 01cm. The crystal was oriented and pieces of 2.5x2.5x10 mm3 were cut with the long axis parallel to < 110 >. The iron doping was performed by encapsulating a small piece of metallic iron and a carefully etched sample in an evacuated quartz ampule. The ampule was heat treated for 60 to 90 minutes at 1200 *C. After the diffusion pro
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