Thermodynamics and Microstructure Development in the Thin Film Reaction of Aluminum on Silicon Carbide
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INTRODUCTION Aluminum is currently used for p-type ohmic contacts to SiC, but exhibits unacceptable interfacial resistivity [1]. Dopant drive-in requires annealing above the melting point of Al, resulting in a chemical reaction displacing Si to the reel and forming Al4C3 at the interface. Although this thin film reaction has not been studied, the interaction of aluminum with silicon carbide has been investigated extensively in the area of Al-SiC structural composites [2-6]. The main emphasis of these studies was to control the formation of A14C3 at the interface between Al and SiC, which has been shown to degrade mechanical properties [4]. Similarly, interfacial A4C 3 may degrade the electrical properties of the Al/SiC interface, although no studies directly linking interface microstructure to contact resistivity have been reported. A refined thermodynamic analysis of the Al-SiC reaction is presented in this paper, and its implications for controlling interface microstructure developnmnt are discussed. THERMODYNAMIC ANALYSIS The reaction between rrolten Al and SiC to produce A14C3 is given by reaction equilibrium (3). The standard free energy of reaction (3) was obtained by combining the standard free energy of formation of the carbides, reactions (1) and (2), from Kubaschewski and Alcock [7].
4Al(l) + 3C(s) = AJ4C3(s) 3SiC(s) = 3Si(l) + 3C(s) 4Al(l) + 3SiC(s) =A14C3(s) + 3 Si(l)
(1) (2) (3)
AG 0(1) = -63,700 + 23.0 T AGO(2) = (27,100 + 2.73 T log(T) - 18.1 T) x 3 AGO(3) = 17,600 + 8.19 T log(T) - 31.3 T
(4) (5) (6)
At equilibrium the standard free energy can be expressed as,
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Mat. Res. Soc. Symp. Proc. Vol. 403 @1996 Materials Research Society
AGO =-RTl f 4~l
31 35
(7) Assuming the activities of SiC and A14C3 are unity, equation (7) can be evaluated for the ratio of aluminum activity to silicon activity as a function of temperature. If the activity coefficients of Al and Si in the Al-Si alloy are known, the equilibrium concentration of Si can be calculated. Activity coefficients for Al and Si were calculated from activity-composition data at 1200'C assessed by Murray and McAlister [8]. The variation of activity coefficient with temperature and composition was determined through the Gibbs-Helmholtz relation, d in yi
dnl
AH"
1
R
(8)
using the partial molar enthalpy of solution from Kubaschewski and Alcock [7]. The calculated variation of activity for Al and Si in the Al-Si system is shown in figure 1. The equilibrium Si concentration for reaction (3) is determined by finding the pair of activities for Al and Si which satisfy the free energy expression of equation (7) at the temperature of interest. The calculated Si concentration for reaction (3) as a function of temperature is shown in figure 2, along with other calculated and measured values reported in the literature. The present analysis predicts significantly higher equilibrium Si concentrations than the previous studies, as discussed later. 1
0.8 0.6 °(-
0.41
0.21
0.4
0.6
1.0
Mole fraction Si Figure 1. Calculated variation of activit
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