  Al-$\delta$-GaAs structures are studied where tunneling to one or more
subbands of the 2D electron system of a near-surface delta-doped layer is
observed. Reflection of electrons at the threshold of the emission of LO-phonon
is observed. This phenomenon occurs when a new subband is involved in the
tunneling process and when intersubband transitions of the electrons in a
2D-system with the emission of LO-phonon are added to the inelastic tunneling
within a single subband. It is shown that, under the conditions of an
intersubband polaron resonance, the reflection processes dominate during
tunneling into the delta-layer. If, however, tunneling from the delta-layer
occurs, the reflection processes are observed when two subbands are occupied.
