Epitaxial 4H-SiC structures with heavily boron or aluminium doped layers have been prepared by vapour phase epitaxy. The samples have been annealed in Ar atmosphere in an RF-heated furnace between 1700 and 2000 degreesC for 45 min to 64 h. Secondary ion mass spectrometry has been employed to obtain depth distributions as well as lateral distributions (ion imaging) for boron and aluminium. Transmission electron microscopy has been used to study the crystallinity and determine phase composition. Solubility limits of similar to 1 x 10(20) Al/cm(3) (1700 degreesC) and < 1 x 10(20) B/cm(3) (1900 degreesC) have been deduced. (C) 2002 Elsevier Science B.V. All rights reserved.