Thickness-driven structural evolution and vacancy-linked superconductivity in epitaxial NbN thin films

Epitaxial -NbN thin films (9–205 nm) were grown on MgO(001) and MgO(110) to examine how thickness-driven structural evolution controls normal-state transport and superconducting performance, and how these changes are linked to vacancy-mediated lattice evolution. Structural characterization shows that increasing thickness generally improves the microstructural coherence length. The room-temperature resistivity generally decreases with thickness when a single epitaxial registry is preserved. However, the 205-nm NbN(110) film, which exhibits a weak secondary (111) component, shows a higher resistivity despite its larger microstructural coherence lengths, indicating that resistivity is not governed by structural coherence alone. All measured films exhibit a negative temperature coefficient of resistivity, and the broad minimum in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi mathvariant="normal">d</a:mi> <a:msub> <a:mi>ρ</a:mi> <a:mrow> <a:mi>x</a:mi> <a:mi>x</a:mi> </a:mrow> </a:msub> <a:mo>/</a:mo> <a:mi mathvariant="normal">d</a:mi> <a:mi>T</a:mi> </a:mrow> </a:math> appears for all samples around 50–80 K, which correlates more closely with the disorder parameter <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:msub> <d:mi>k</d:mi> <d:mi>F</d:mi> </d:msub> <d:mi mathvariant="script">l</d:mi> </d:mrow> </d:math> than with microstructural coherence lengths. Hall measurements indicate that the effective carrier density <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msub> <f:mi>n</f:mi> <f:mi>eff</f:mi> </f:msub> </f:math> increases with thickness and correlates with the evolution of unit-cell volume. Together with density functional theory calculations showing that percent-level vacancy disorder can produce comparable changes in lattice volume, this supports a thickness-driven vacancy-linked lattice evolution accompanied by strain relaxation, coinciding with an increase of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msub> <g:mi>T</g:mi> <g:mi>c</g:mi> </g:msub> </g:math> (8.5–12.3 K). The largest estimated upper critical field is 44.9 T for the 205-nm NbN(110) film, corresponding to a superconducting coherence length of 2.7 nm.