  Surface states of topological insulators (TIs) have been playing the central
role in the majority of outstanding investigations in low-dimensional electron
systems for more than 10 years. TIs based on high-quality strained HgTe films
demonstrate a variety of subtle physical effects. The strain leads to a bulk
band gap but limits a maximum HgTe strained film thickness, and therefore, the
majority of experiments were performed on films with a thickness of less than
100 nm. Since a spatial separation of topological states is crucial for the
study of a single-surface response, it is essential to increase the HgTe
thickness further. In this work, by combining transport measurements together
with capacitance spectroscopy, we perform an analysis of a 200-nm partially
relaxed HgTe film. The Drude fit of the classical magnetotransport reveals the
ambipolar electron-hole transport with a high electron mobility. A detailed
analysis of Shubnikov-de Haas oscillations in both conductivity and capacitance
allows us to distinguish three groups of electrons, identified as electrons on
top and bottom surfaces and bulk electrons. The indirect bulk energy gap value
is found to be close to zero. It is established that the significant gap
decrease does not affect the surface states, which are found to be well
resolved and spin nondegenerate. The presented techniques allow investigations
of other three-dimensional TIs, regardless of the presence of bulk
conductivity.
