High temperature electrical transport properties of MBE-grown Mg-doped GaN and AlGaN materials

This paper discusses the results of high temperature resistivity and Hall effect studies of Mg-doped GaN and Al_xGa_1−xN epilayers (0.05 < x < 0.23). The studied samples were grown by molecular beam epitaxy on low temperature buffers of GaN and AlN deposited on a sapphire substrate. The experiments were carried out at temperatures ranging from 300 up to 1000 K. Up to a certain critical temperature T_C (around 800 K), a typical increase of the conduction processes due to the excitation of impurity states has been observed with an activation energy of about E_A = 200 meV. However, at this critical temperature T_C, an annealing effect was observed in all the investigated samples. At this critical temperature, the increase in the free carrier concentration as a function of time leads to an irreversible decrease in the samples’ resistivity of more than 60%. The observed temperature dependences of the electrical transport properties are analyzed in the frame of an impurity model including shallow donors and Mg-related acceptors (with E_A ∼ 200 meV). In some cases, an additional conduction channel not related to free carriers in the valence band must be taken into account. This can lead to an incorrect determination of hole concentration in the valence band, an important parameter in the process of radiative recombination.