Peculiar Doping Behavior of Si:Be.
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PECULIAR DOPING BEHAVIOR OF Si:Be. EUGEN TARNOW*, S.B. ZHANG*, K.J. CHANG** and D.J. CHADI* * Electronic Materials Laboratory, Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304 ** Department of Physics, Korea Advanced Institute of Science & Technology, Seoul, South Korea ABSTRACT The total energies and structures of a number of Be-induced defects in Si are investigated using ab-initio local density calculations. Our primary results are: 1) The geometry of the isoelectronic center is found to correspond to a [111] substitutionalinterstitial pair (SIP); 2) The low energy defect spectrum includes large Be complexes containing at least one substitutional atom; and 3) Simple bonding rules exist for the stability of the different types of bonds in the material. Thus the Si-Be bond is found to be stable for all defect configurations while the Be-Be bond is metastable. INTRODUCTION If one attempts to dope Si with the group II element Be one finds that only 10% of the Be atoms become electrically and optically active [1]. The purpose of this article is to explain this intriguing behavior and, in particular, to find out where the remaining Be atoms are. To accomplish this we use ab-initio density functional calculations to investigate the formation energies of a multitude of Be defect structures, ranging from the simplest monatomic defects to more complex defect clusters. Experimentally it is known that the electrically active sites arise from substitutional double acceptor Be dopants. Furthermore it is believed that the inactive defects are neutral Be pairs [2] that behave as "isoelectronic centers". The isoelectronic Be center in Si has been studied experimentally with infrared absorption [3], photoluminescence [4,5], Zeeman measurements [6], and light absorption while under uniaxial stress [7]. From these studies the microscopic structure of the isoelectronic center was indirectly concluded to be either a substitutional-interstitial Be pair [2] or a Be pair occupying a single substitutional site (an "interstitialcy" or split interstitial) [4]. Both configurations were proposed to be axially symmetric along the [100] axis [4.6]. In unpublished work, the experimental results from uniaxial stress [7] and Zeeman measurements [6] were reinterpreted and found to be consistent with a [1111 symmetry axis [8]. A second isoelectronic center has also been seen in photoluminescence [5]. Electrical and optical studies of the Be acceptor centers in Si have been performed 12,9,10]. The total energies associated with some Be defects have been calculated previously [11]. The present work gives qualitatively different results compared to the latter as discussed in a more extensive publication [12]. In this paper we investigated the properties of dopant complexes. First we identify the microscopic geometry of the isoelectronic center. We calculate properties of this center such as local mode frequencies and parameters of the 2state system associated sith the lowest structurally excited state. We then extend
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