Influence of strain and point defects on the electronic structure and related properties of (111)NiO epitaxial films

Abstract (111)NiO epitaxial films are grown on c-sapphire substrates at various growth temperatures ranging from room temperature to 600 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:msup> <mml:mtext>C</mml:mtext> </mml:mrow> </mml:math> using pulsed laser deposition technique. Two series of samples, where different laser fluences are used to ablate the target, are studied here. Films grown with higher laser fluence, are found to be embedded with Ni-clusters crystallographically aligned with the (111)NiO matrix. While the layers grown with lower laser energy density exhibit p-type conductivity, especially at low growth temperatures. X-ray diffraction study shows the coexistence of biaxial compressive and tensile hydrostatic strains in these samples, which results in an expansion of the lattice primarily along the growth direction. This effective uniaxial expansion <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>ε</mml:mi> <mml:mi mathvariant="normal">⊥</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> increases with the reduction of the growth temperature. Band gap of these samples is found to decrease linearly with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>ε</mml:mi> <mml:mi mathvariant="normal">⊥</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> . This result is validated by density functional theory calculations. Experimental findings and the theoretical study further indicate that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>Ni</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo>+</mml:mo> <mml:mrow> <mml:msub> <mml:mi>O</mml:mi> <mml:mi>I</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mi>O</mml:mi> </mml:msub> </mml:mrow> <mml:mo>+</mml:mo> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mtext>i</mml:mtext> </mml:mrow> <mml:mi>I</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> complexes exist as the dominant native defects in samples grown with Ni-deficient (low laser fluence) and Ni-rich (high laser fluence) conditions, respectively. P-type conductivity observed in the samples grown in Ni-deficient conditions is more likely to result from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>Ni</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo>+</mml:mo> <mml:mrow> <mml:msub> <mml:mi>O</mml:mi> <mml:mi>I</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> defects than Ni-vacancies ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>V</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>Ni</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ).