Study of two-subband population in Fe-doped AlxGa1-xN/GaN   heterostructures by persistent photoconductivity effect
  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. 
Ikai Lo, J.K. Tsai, M.H. Gau, Y.L. Chen, Z.J. Chang, W.T. Wang, J.C.   Chiang, K.R. Wang, Chun-Nan Chen, T. Aggerstam