Observation of novel conductance structure in GaAs/Ga/sub x/Al/sub 1-x/As resonant tunneling heterostructures

A novel pre-resonant conductance structure is observed in single and double well GaAs/Ga/sub x/Al/sub 1-x/As resonant tunneling heterostructures. This structure is attributed to single electron tunneling through donor bound states in the quantum well as they cross the Fermi level. Peaks in conductance are observed in devices as large as (64/spl mu/)/sup 2/. We observe for the first time donor binding energies in quantum wells as large as 35 meV, which is probably due to the formation of a donor complex. An impressive impurity tunneling conductance structure is observed in double quantum well structures with conductance peak heights varying over 3 orders of magnitude. Conductance peaks are observed before and after the main resonant current peak which are attributed to tunneling of electrons through impurities in one quantum well and the quantum state in the other well.< >