High hole concentration in Mg-doped AlN/AlGaN superlattices with high Al content

We studied hole generation in Mg-doped AlN/Al 0.75 Ga 0.25 N superlattices (SLs) with an average Al content of 0.8. High hole concentrations on the order of 10 18 cm −3 were obtained in the SLs. The temperature dependence of the hole concentration indicated an effective acceptor ionization energy of 40–67 meV, which is much lower than that of Mg-doped Al 0.8 Ga 0.2 N alloy (>400 meV). The hole concentration increased with increasing SL period thickness and became almost constant at about 30 nm. These results indicate that the band bending caused by strong spontaneous and piezoelectric polarization fields enhances the ionization of the Mg acceptors.