Two Si-doped specimens, which had been grown by using molecular beam epitaxy techniques, were used to study Si diffusion at temperatures ranging from 700 to 950C. Each specimen structure consisted of well-characterized regions which ranged in type from undoped semi-insulating to heavily Si-doped (4 x 1019/cm3). In one structure, the Si doping increased step-wise from the surface to the semi-insulating substrate. The other structure was grown in the reverse fashion, with the maximum Si doping situated near to the surface. Annealing was carried out after encapsulating the samples with a plasma-enhanced chemical vapour deposited nitride or oxide layer. Both structures exhibited almost identical diffusion behaviours which were best modelled by using an electron-dependent diffusion model. A least-squares fit to both sets of data showed that the Si diffusivity could be described by:

D(cm2/s) = 60.1 exp[-3.9(eV)/kT](n/ni)2

This diffusion behavior was independent of the encapsulating material which was used, and of the proximity of the dopant to the surface region. The results indicated the existence of a Fermi level dependent diffusion behavior which was governed by compensating acceptor charged point defects, VGam-. These were a consequence of Si doping during molecular beam epitaxial growth. On the basis of the observed electron concentration dependence of the diffusivity, and assuming the operation of a simple Ga vacancy diffusion mechanism, these compensating vacancies were suggested to be at least doubly negatively charged.

J.J.Murray, M.D.Deal, E.L.Allen, D.A.Stevenson, S.Nozaki: Journal of the Electrochemical Society, 1992, 137[7], 2037-41