The diffusion of P from a POCl3 source into (111)-oriented n-type single crystals was studied using sectioning by anodic oxidation, and 4-point resistivity techniques. The concentration profiles were fitted to a solution of Fick’s equation involving a moving boundary which separated into two distinct phases. It was concluded that the surface region constituted a different phase to the remainder of the diffused layer. For the short diffusion times used, the phase boundary reaction was the rate-limiting process and the phase boundary moved at an almost constant rate. In the region beyond the phase boundary, the transport of P was controlled by two diffusing species, characterized by two appreciably different diffusion coefficients. At 900C, the coefficient for fast diffusion was 5.7 x 1014cm2/s, and that for slow diffusion was 2.7 x 10-15cm2/s. The slow-diffusing component was present mainly in the transition region between the phase boundary and the fast-diffusion dominated region. The fast-diffusing component exhibited a maximum concentration at the phase boundary. The diffusivity between 820 and 1100C in the two cases could be described by:

fast: D(cm2/s) = 2.49 x 10-5exp[-2.0(eV)/kT]

slow: D(cm2/s) = 4.93 x 101exp[-3.77(eV)/kT]

As the diffusion temperature was increased, the diffusion constants for the slow and fast diffusion approached each other. At 1100C, the diffusion profile could be represented by a single diffusion constant.

J.C.C.Tsai: Proceedings of the IEEE, 1969, 57[9], 1499-506

Table 81

Diffusion of P

(Phosphine in nitrogen source)