2.1.3 Segregation

At a material interface a jump of the dopant concentration can often be established, even in thermodynamic equilibrium. This phenomenon is called segregation effect and can be observed with all species which are used in silicon techniques (boron, arsenic, etc.).

In general the dopant concentration within the two materials is not in the equilibrium thus a resulting flux over the interface tries to change the concentration towards equilibrium (Fig. 2.1).

Figure 2.1: Model of a segregation effect at a material interface between silicon and silicon dioxide

In case of equilibrium it is usual to write

C_si^eq = m ^. C_ox^eq (2.21)

where C_si^eq denotes the doping concentration in silicon, C_ox^eq the doping concentration in silicon dioxide and m the so called segregation coefficient.

In case of non equilibrium a diffusion flux over the interface results trying to balance the dislocation of the dopants. The equation for the flux can be written as

$${\frac{C_{si}^{eq}}{m}}$$ (2.22)

where h stands for a transport coefficient which can be interpreted as a velocity term of particles. The flux at the interface forces a diffusion within the two materials to reach homogeneity and finally to fulfill (2.21).