ON THE CARRIER LIFETIME IN PERIODICALLY DELTA-DOPED GAAS

Redistribution of dopants (Si and Be) in periodically delta-doped (or nipi delta-doped) GaAs during molecular beam epitaxial growth has been investigated. A redistribution would cause a reduction of the space-c harge-induced potential modulation amplitude with a consequent decreas e in the excess carrier lifetime, consistent with recent experimental observations. The spatial localization of the dopants was assessed usi ng secondary-ion mass spectroscopy. Samples with only one type of dopa nt (Si or Be) as well as a sample with alternating Si and Be planes, g rown under identical conditions with a substrate temperature of 500 de grees C, were used in order to investigate any possible electric-field -induced migration of the dopants during growth. In all samples the di stance between neighbouring planes was 30 nm and the sheet densities w ere 3 x 10(12) cm(-2) No evidence for Si redistribution was found. The Be planes, on the other hand, showed an increased FWHM in the alterna tely doped sample, suggesting that electric-field-induced migration co ntributes to the Be redistribution. When corrected for the instrument resolution we find a maximum Be redistribution of 8 nm under the prese nt conditions. The estimated redistribution is, however, smaller than that needed for a substantial reduction of the carrier lifetime.