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

Abstract

Abstract NiO and Ni-cluster-embedded NiO epitaxial films grown on c-sapphire substrates using pulsed laser deposition technique are annealed at different temperatures in an oxygen environment. The structural, morphological, and electrical properties of these samples are investigated as functions of the annealing temperature. It is observed that, in all cases, the sample surface becomes smoother with the increase in annealing temperature. The resistivity of the Ni-cluster-free NiO epilayers is found to decrease with the increase in annealing temperature beyond ∼300 °C. Annealing at 450 °C results in about a two orders of magnitude decrease of resistivity from the highly insulating state of the as-grown. The resistivity shows a gradual increase when annealed at temperatures higher than 450 °C. A reduction of resistivity with annealing can be attributed to the enhancement of the density of Ni vacancies, which are known to act as shallow acceptors in NiO. The formation energy of the Ni vacancies, which has been estimated from the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>ln</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:msup> <mml:mi>ρ</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> versus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>A</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow/> </mml:math> plot, comes out to be 623 meV. The density of 60° dislocations in these films increases sharply when annealed above 500 °C. A reduction of conductivity of films annealed above 500 °C can thus be explained in terms of the enhancement of the rate of scattering of carriers by these defects. The resistivity of the Ni-cluster-embedded highly conducting NiO epilayers is found to increase rapidly by more than several orders of magnitude beyond an annealing temperature of ∼200 °C. This has been ascribed to percolation-driven metal-to-insulator transition arising as a result of the reduction of the cluster density with annealing.