Behavior of background impurities in 4H-SiC layers is studied in terms of several growth process parameters. The layers were produced by sublimation epitaxy in Ta and Hf, as well as in graphite growth cell environment. Cathodoluminescence imaging and spectroscopy of cleaved samples demonstrate the impurity-thickness uniformity along thick (40-260 mum) layers. The effect of the Ta and Hf environment on the levels of residual impurities is considered through calculations of cohesive energies of Ta-X and Hf-X diatomic molecules and comparing them with those obtained for N-, Al- and B-containing vapor molecules.