Tunneling via X valley related states is investigated in a GaAs/AlAs/GaAs s
ingle-barrier structures. Features in the tunnel current which are observed
experimentally are associated with resonant tunneling through X-valley AlA
s quantum well states derived from conduction band minima both perpendicula
r (X-x and X-y) and parallel (X-z) to the (100) growth direction and Si-don
or states:linked to X valley. Tunnelling through impurity; states associate
d with X-z and X-x,X-y valleys is observed for the first time.