Spatial localisation of Be in δ-doped material, within a few lattice constants (less than 2nm), was achieved at low growth temperatures for Be concentrations of less than 1014/cm2; as revealed by capacitance-voltage profiles and secondary ion mass spectroscopy. At high growth temperatures, and at higher Be concentrations, significant spreading of the dopants occurred and was explained in terms of Fermi-level pinning-induced segregation, repulsive Coulomb interactions of dopants, and diffusion. The highest Be concentration which was obtained at low growth temperatures exceeded 2 x 1020/cm3, and was limited by repulsive dopant interactions. It was shown that the repulsive Coulomb interaction resulted in a correlated non-random dopant distribution. It was deduced that the diffusivity of Be (table 12) could be described by:

D (cm2/s) = 0.00002 exp[-1.95(eV)/kT]

The present values were much lower than those previously reported.

E.F.Schubert, J.M.Kuo, R.F.Kopf, H.S.Luftman, L.C.Hopkins, N.J.Sauer: Journal of Applied Physics, 1990, 67[4], 1969-72

Table 12

Diffusivity of Be in GaAs