Annealing-dependent changes in the structural and electrical properties of NiO epitaxial films

Abstract

NiO and Ni cluster embedded NiO epitaxial films grown on c-sapphire substrates by PLD technique are annealed at different temperatures in oxygen environment. Structural, morphological and electrical properties of these samples are investigated as functions of the annealing temperature. It has been observed that in all cases, sample surface becomes smoother with the increase in annealing temperature. Resistivity of the Ni-cluster free NiO epilayers is found to decrease with the increase in annealing temperature beyond ~300℃. Annealing at 450℃, results in about two orders of magnitude decrease of resistivity from the highly insulating state of the as grown. Resistivity shows a gradual increase when annealed at temperatures higher than 450℃. Reduction of resistivity with annealing can be attributed to the enhancement of the density of Ni-vacancy, which are known to act as shallow acceptors in NiO. Formation energy of the Ni-vacancies, which has been estimated from the ln⁡(ρ^(-1) ) versus 1/T_A plot, comes out to be 623 meV. Density of 60°-dislocations in these films increases sharply when annealed above 500℃. Reduction of conductivity of these films for annealing above 500℃ can thus be explained in terms of the enhancement of the rate of scattering of carriers by these defects. Resistivity of the Ni-cluster embedded highly conducting NiO epilayers are found to increase rapidly by more than several orders of magnitude beyond an annealing temperature of ~200℃. This has been ascribed to percolation driven metal-to-insulator transition arising as a result of the reduction of the cluster density with annealing.

Study of lithium incorporation in (111) NiO epitaxial layers grown on c-sapphire substrates using the pulsed laser deposition technique

“…It should be noted that the stoichiometric NiO is an insulator. The p-type conduction in the film can be ascribed to the nickel vacancies (V Ni ) [17][18][19][20]. It is noteworthy that NiO epitaxial films with high crystalline quality have been reported by different growth techniques such as pulsed laser deposition [8,21,22], mist chemical vapor deposition [23], RF magnetron sputtering [24] and molecular beam epitaxy [5] on various substrates, such as sapphire, cubic yttria-stabilized zirconia and MgO.…”