Intentional impurities (dopants) can be introduced during epitaxial crystal growth in terms of either elemental doping sources or chemical compounds. This chapter is devoted to aspects of elemental doping sources and to the characteristics of these dopants. The following chapter discusses non-elemental, gaseous doping precursors. The incorporation of elemental doping impurities is mediated through thermal evaporation of the doping element from an effusion cell. Such elemental solid evaporation sources are commonly used for molecular-beam epitaxy (MBE) and related techniques discussed in Chap. 4.

Doping techniques and calibration

The incorporation of doping atoms originating from solid elemental doping sources is mediated through thermal evaporation within a vacuum environment. The solid source is heated to a temperature T at which the desired amount of the doping element evaporates. The rate of evaporation, the source and substrate geometry, and the sticking probability of impurity atoms on the crystal surface determine the incorporation rate of the impurities into the growing epitaxial crystal. These parameters are discussed below.

Doping impurity flux

The concentration of doping impurities in an epitaxial layer is usually controlled by the impurity-cell temperature. Consider an epitaxial crystal of GaAs that grows at a rate of 1 monolayer per second, i.e. at a rate of 6 × 1014 GaAs molecules per cm2 per second.