We performed an infrared study on the highly n-type and semi-insulating GaN single crystals. We also studied epitaxial layers (homoepitaxy) grown on the (0 0 0 (1) over bar)N and (0 0 0 1)Ga faces of these crystals. For the n-type GaN single crystals, we show that the (0 0 0 (1) over bar)N faces always possess much higher free carrier concentration than the (0 0 0 1)Ga face. Moreover, infrared reflectivity of the layers deposited on both polarity of semi-insulating and highly conducting bulk GaN substrates shows that the incorporation of impurities (most probably oxygen and silicon) is systematically higher in the layers grown on the (0 0 0 (1) over bar)N faces. These results clearly indicate that the growth of epitaxial layers on the bulk GaN substrates should be done on the (0 0 0 1)Ca faces of the GaN crystals.