  We report on beating appearance in Shubnikov-de Haas oscillations in
conduction band of 18-22nm HgTe quantum wells under applied top-gate voltage.
Analysis of the beatings reveals two electron concentrations at the Fermi level
arising due to Rashba-like spin splitting of the first conduction subband H1.
The difference dN_s in two concentrations as a function of the gate voltage is
qualitatively explained by a proposed toy electrostatic model involving the
surface states localized at quantum well interfaces. Experimental values of
dN_s are also in a good quantitative agreement with self-consistent
calculations of Poisson and Schrodinger equations with eight-band kp
Hamiltonian. Our results clearly demonstrate that the large spin splitting of
the first conduction subband is caused by surface nature of $H1$ states
hybridized with the heavy-hole band.
