  The electronic properties of Fe-doped Al0.31Ga0.69N/GaN heterostructures have
been studied by Shubnikov-de Haas measurement. Two subbands of the
two-dimensional electron gas in the hetero-interface were populated. After the
low temperature illumination, the electron density increases from 11.99 x 1012
cm-2 to 13.40 x 1012 cm-2 for the first subband and from 0.66 x 1012 cm-2 to
0.94 x 1012 cm-2 for the second subband. The persistent photoconductivity
effect (~13% increase) is mostly attributed to the Fe-related deep-donor level
in GaN layer. The second subband starts to populate when the first subband is
filled at a density n1 = 9.40 x 1012 cm-2. We calculate the energy separation
between the first and second subbands to be 105 meV.
