Nitrogen in Crystalline Si
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NITROGEN IN CRYSTALLINE Si
HERMAN J. STEIN Sandia National Laboratories,
PO Box 5800, Albuquerque,
New Mexico 87185
ABSTRACT Use of N doping, N gas ambients and N implantation in Si processing has stimulated study of the incorporation and behaviour of N in Si. Studies of N-implanted Si and comparisons with melt-doped Si have shown that N pairing is the dominant mode for bonding of N into crystalline Si for a wide range of processing conditions. Studies have also shown that laser annealing quenches a small percentage of implanted N into a substitutional site. Nitrogen diffusion and aggregation have been shown to occur in N-implanted Si upon furnace annealing at temperatures >700 0 C and there is a simultaneous formation of shallow donors. Interactions between N and other impurities occur in both as-grown and ion-implanted Si. Additional studies are needed to describe more fully the electrical and structural consequences of N-N, Nimpurity and N-defect interactions in Si. I.
INTRODUCTION
Potential use of processing steps which introduce nitrogen into Si has provided motivation for studies of the chemical bonding and behaviour of N in Si. Nitrogen is known to substitute for the Group V constituent in GaP [1] and GaAs [2] and to form N-N pairs [3] in GaP. Since N is a Group V impurity, donor activity might be expected for substitutional N in Si. Although little attention was given to N in Si for a number of years because an early study [4] showed that the equilibrium solid solubility is low and donor activation is small(< 1%), an academic interest remained in why N is different from other Group V elements [5]. In addition, silicon nitride is widely used as a diffusion barrier and as a dielectric so that N introduction and interactions with other impurities in Si are technological concerns [5,6]. In recent exploratory processing N was intentionally introduced into Si during growth [7] to strengthen low-oxygen-content wafers, and N-ion implantation [5,6] has been used to form diffusion barriers and buried silicon nitride dielectric layers [8]. Solubility [9], diffusivity [5], equilibrium coefficient [9] and predominant bonding [10] for N in Si are compared in Table I to those for 0 [9,11], C [9,12] and H [13,14]. There are significant differences among the characteristics for these impurities in Si. Carbon is the only one of these light impurities which is predominantly bonded on substitutional sites. The activation energy for diffusion is >2ev except for H where a small activation energy precludes a meaningful determination of equilibrium solubility or distribution coefficient. Two expressions are listed in Table I for diffusivity of H and N illustrating the uncertainty in describing diffusivity for these elements. Hydrogen bonds at defect sites so that diffusivity and solubility are probably defect limited [15]. Oxygen has the highest solubility, a distribution coefficient near unity and is bridge bonded into Si. Among these light impurities, N has the lowest measured equilibrium solid solubility and the smallest
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