Silicon Photonics: Fundamentals and Devices

13.4 Material Modification

There are different methods of modifying the properties of a semiconductor near its surface. The two widely used techniques are diffusion and ion implantation. We briefly describe the principles involved in these processes.

13.4.1 Diffusion

In this process, the substrate may be placed in intimate contact with the dopant material which may be a solid, liquid, or gas. Both the dopant and the substrate are heated to temperatures as high as 800–1000̊C, because at higher temperatures the atoms in both of them are considerably more mobile than at room temperature. However, the substrate atoms are more fixed to lattice sites and the dopant atoms migrate to any of the vacancies existing in the substrate material. The dopant atoms may also occupy interstitial sites.

Diffusion of impurities occurs in all materials, whenever a concentration gradient of impurities and random thermal motion to drive the process exists. The following one-dimensional model describes the flux of impurities as a function of time and depth:

(13.6)

where N is the impurity concentration per unit volume; D is the diffusion coefficient, which may depend on N and temperature; and x is the depth into the substrate, measured from the surface. By knowing the boundary conditions, Eq. (13.6) may be solved to yield the concentration profile as a function of time.

In one common method of diffusion, ...