  We report on the study of magneto-photogalvanic and magnetotransport
phenomena in 200 nm partially strained HgTe films. This thickness is slightly
larger than the estimated critical thickness of lattice relaxation leaving the
film partially relaxed with the value of the energy gap close to zero. We show
that illumination of HgTe films with monochromatic terahertz laser radiation
results in a giant resonant photocurrent caused by the cyclotron resonance in
the surface states. The resonant photocurrent is also detected in the reference
fully strained 80 nm HgTe films previously shown to be fully gapped 3D
topological insulators. We show that the resonance positions in both types of
films almost coincide demonstrating the existence of topologically protected
surface states in thick HgTe films. The conclusion is supported by
magnetotransport experiments.
