The role of band alignment in p-type conductivity of Na-doped ZnMgO: Polar versus non-polar

We investigate the electrical properties of polar and non-polar ZnMgO:Na films that have been fabricated on c-plane and r-plane sapphire substrates using intervened ZnO layers (10-30nm thick) by pulsed laser deposition. Hall-effect measurements indicate that the a-plane ZnMgO:Na film exhibits p-type conductivity with a carrier concentration of about 3.5 x 10(16) cm(-3), while the polar film shows a compensatory conductivity. Meanwhile, the dependence of the band alignment on the orientation of the ZnMgO/ZnO heterojunctions has been investigated using photoelectron spectroscopy. The heterojunctions form in the type-I straddling alignment with valence band offsets of 0.07 (0.02) eV for the (non-)polar heterojunction. The difference in valence band offsets is primarily attributed to the spontaneous polarization effect. We propose that the smaller valence band offsets and larger conduction band offsets would reduce the Na-Zn acceptor level and enhance the relative intrinsic donor levels. Such effects consequently lead to p-type conductivity in non-polar ZnMgO:Na films. The band alignment of non-polar ZnMgO/ZnO can be used to facilitate p-type doping with a shallower acceptor state in the ZnO-like alloy. (C) 2014 AIP Publishing LLC.