Concentrations of nitrogen shallow donors, boron shallow acceptors, charge carriers, and electron traps were measured as a function of position along the growth axis in a series of undoped 6H-SiC boules grown by sublimation method with and without addition of hydrogen to the growth atmosphere. Elemental analysis by secondary ion mass spectrometry and measurements of electrical properties indicate that the addition of hydrogen suppresses nitrogen incorporation and formation of all electron traps. Concentration of boron is not affected by hydrogen presence. The addition of hydrogen to the growth ambient improves the uniformity of nitrogen incorporation and deep trap distribution along the growth axis. The results are interpreted as due to increased carbon transport and corresponding shift of crystal stoichiometry toward carbon-rich side of the SiC existence range. (c) 2007 Elsevier B.V. All rights reserved.