Formation Mechanism of Plateau, Rapid Fall and Tail in Phosphorus Diffusion Profile in Silicon Based on the Pair Diffusion Models of Vacancy Mechanism and Interstitial Mechanism

Experimental P diffusion profile in Si with a constant P surface concentration of 3 ×10 cm −3 at 900°C under an inert atmosphere is shown in Fig. 1. The abscissa is x / t = λ , where x is the distance from a specimen surface and t is the diffusion time. The profile has the plateau, rapid fall and tail. Yoshida et al. studied P diffusion in Si based on the pair diffusion models of the vacancy mechanism and the interstitial mechanism. They obtained the effective P diffusion coefficient from the P diffusion equation, then proposed the limiting process of P diffusion. Based on these, the formation mechanism of the plateau, rapid fall and tail is studied. 3,4,5) P diffuses predominantly by the interstitial mechanism. 6) Therefore the basic process of P diffusion is the diffusion of (PI), where I and (PI) denote self-interstitials and P-I pairs. In the high P concentration region, excess I is generated by the dissociation of (PI) and the limiting process of P diffusion depends on whether or not excess I is controlled. That is, [1] if the concentration of excess I decreases relatively due to the effect of the decrease in quasi I formation energy, or [2] if excess I is removed by the recombination with vacancies, P diffuses fast and the plateau is formed; if not, P diffuses slowly and the rapid fall is formed. In the tail region, the P concentration is low and the limiting process of P diffusion is the basic process of P diffusion or the diffusion of (PI). Excess I generated in the high P concentration region diffuses into the tail region and I is supersaturated there. Therefore the concentration of (PI) increases, resulting in the fast diffusion of P and the formation of the tail.Two methods, [1] and [2], were described above for the control of excess I. To investigate which of them actually occurs is a problem in future.