  We discuss problem of Rashba field in bulk GaN and in GaN/AlGaN
two-dimensional electron gas, basing on results of X-band microwave resonance
experiments. We point at large difference in spin-orbit coupling between bulk
material and heterostructures. We observe coupled plasmon-cyclotron resonance
from the two-dimensional electron gas, but no spin resonance, being consistent
with large zero-field spin splitting due to the Rashba field reported in
literature. In contrast, small anisotropy of g-factor of GaN effective mass
donors indicates rather weak Rashba spin-orbit coupling in bulk material, not
exceed 400 Gauss, alpha_BIA < 4*10^-13 eVcm. Furthermore, we observe new kind
of electron spin resonance in GaN, which we attribute to surface electron
accumulation layer. We conclude that the sizable Rashba field in GaN/AlGaN
heterostructures originates from properties of the interface.
