Semiconducting p-type MgNiO:Li epitaxial films fabricated by cosputtering method

Li-doped ternary Mg_xNi_1-xO thin films were deposited on (0001) Al₂O₃ substrates by a radio frequency (RF) magnetron cosputtering method with MgO and NiO:Li targets. The Mg mole fraction and Li content were relatively controlled by changing RF power for the MgO target over a range of 0-300 W, while the NiO:Li target was kept at 150 W. As a result, all films were epitaxially grown on (0001) Al₂O₃ substrates with the relationship of [1 ̄ 1 ̄ 0]_NiO | | [11 1 ̄ 0]_Al2O3, [1 ̄ 1 2 ̄]_NiO | | [2 1 ̄ 1 ̄ 0]_Al2O3 (in-plane), and [1 ̄ 11]_NiO | | [000 1 ̄]_Al2O3 (out-of-plane), and showed p-type semiconducting properties. Furthermore, from x-ray diffraction patterns, the authors found that MgO was effectively mixed with NiO:Li without structural deformation due to low lattice mismatch (0.8%) between NiO and MgO. However, the excess Li contents degraded the crystallinity of the MgNiO films. The band-gap of films was continuously shifted from 3.66 eV (339 nm) to 4.15 eV (299 nm) by the RF power of the MgO target. A visible transmittance of more than 80% was exhibited at RF powers higher than 200 W. Ultimately, the electrical resistivity of p-type MgNiO films was improved from 7.5 to 673.5 Ωcm, indicating that the Li-doped MgNiO films are good candidates for transparent p-type semiconductors.