  Wightman function, the vacuum expectation values of the field squared and the
energy-momentum tensor are investigated for a scalar field with general
curvature coupling parameter in the geometry of a plate in the de Sitter
spacetime. Robin boundary condition for the field operator is assumed on the
plate. The vacuum expectation values are presented as the sum of two terms. The
first one corresponds to the geometry of de Sitter bulk without boundaries and
the second one is induced by the presence of the plate. We show that for
non-conformal fields the vacuum energy-momentum tensor is non-diagonal with the
off-diagonal component corresponding to the energy flux along the direction
perpendicular to the plate. In dependence of the parameters, this flux can be
either positive or negative. The asymptotic behavior of the field squared,
vacuum energy density and stresses near the plate and at large distances is
investigated.
