Structural stability, elemental ordering, and transport properties of layered ScTaN2

Ternary transition metal (TM) nitrides have gained significant attention in thin film research due to their promising properties for a broad range of applications. Particularly, some of the ternary TM nitrides have been predicted to adopt layered structures that make them interesting for thermoelectric conversion and quantum materials applications. Unfortunately, synthesis of TM ternary nitride films by physical vapor deposition often favors disordered 3D structures rather than the predicted 2D-like layered structure. In this study, we investigate the structural interplay in the Sc-Ta-N ternary system using a combinatorial approach. Combinatorial libraries <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi mathvariant="normal">S</a:mi><a:msub><a:mi mathvariant="normal">c</a:mi><a:mi>x</a:mi></a:msub><a:mi mathvariant="normal">T</a:mi><a:msub><a:mi mathvariant="normal">a</a:mi><a:mrow><a:mn>1</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub><a:mi mathvariant="normal">N</a:mi></a:mrow></a:math> are synthesized following a two-step method: First, deposit film precursors by cosputtering and then process the resulting 3D-structured samples with rapid thermal annealing. Synchrotron grazing-incidence wide-angle x-ray scattering on films annealed at <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mn>1200</g:mn><g:msup><g:mspace width="0.16em"/><g:mo>∘</g:mo></g:msup><g:mi mathvariant="normal">C</g:mi></g:mrow></g:math> for 20 min leads to the nucleation of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi>ScTaN</j:mi><j:mn>2</j:mn></j:msub></j:math> layered structure (<k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mi>P</k:mi><k:msub><k:mn>6</k:mn><k:mn>3</k:mn></k:msub><k:mo>/</k:mo><k:mi>m</k:mi><k:mi>m</k:mi><k:mi>c</k:mi></k:mrow></k:math>) near stoichiometry. We find that the layered structure can accommodate large off-stoichiometry in the Ta-rich region (<l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:mi>x</l:mi><l:mo><</l:mo><l:mn>0.5</l:mn></l:mrow></l:math>), facilitated by the alloying with quasi-isostructural <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:msub><m:mi>Ta</m:mi><m:mn>5</m:mn></m:msub><m:msub><m:mi mathvariant="normal">N</m:mi><m:mn>6</m:mn></m:msub></m:mrow></m:math> compound that exists on a composition tie line at <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mi>x</o:mi><o:mo>=</o:mo><o:mn>0</o:mn></o:mrow></o:math>. While focusing on <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:msub><p:mi>ScTaN</p:mi><p:mn>2</p:mn></p:msub></p:math>, we estimate the long-range order parameter in near-stoichiometric films to be 0.86, corresponding to a fraction of Sc/Ta antisites of 7%. Transport measurements on <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:msub><q:mi>ScTaN</q:mi><q:mn>2</q:mn></q:msub></q:math> reveal a nearly temperature-independent high carrier density <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mrow><r:mo>(</r:mo><r:msup><r:mrow><r:mn>10</r:mn></r:mrow><r:mn>21</r:mn></r:msup><r:mspace width="0.16em"/><r:mi mathvariant="normal">c</r:mi><r:msup><r:mrow><r:mi mathvariant="normal">m</r:mi></r:mrow><r:mrow><r:mo>−</r:mo><r:mn>3</r:mn></r:mrow></r:msup><r:mo>)</r:mo></r:mrow></r:math>, suggesting a heavily doped semiconductor or semimetallic character, consistent with a small positive Seebeck coefficient of +19 µV/K. The carrier mobility at 2 K is relatively small (<v:math xmlns:v="http://www.w3.org/1998/Math/MathML"><v:mrow><v:mn>9.5</v:mn><v:mspace width="0.16em"/><v:mi mathvariant="normal">c</v:mi><v:msup><v:mrow><v:mi mathvariant="normal">m</v:mi></v:mrow><v:mn>2</v:mn></v:msup><v:mspace width="0.16em"/><v:msup><v:mrow><v:mi mathvariant="normal">V</v:mi></v:mrow><v:mrow><v:mo>−</v:mo><v:mn>1</v:mn></v:mrow></v:msup><v:mspace width="0.16em"/><v:msup><v:mrow><v:mi mathvariant="normal">s</v:mi></v:mrow><v:mrow><v:mo>−</v:mo><v:mn>1</v:mn></v:mrow></v:msup></v:mrow></v:math>) and the residual-resistivity ratio is minor, suggesting that electrical conduction is dominated by defects or disorder. Measured magnetoresistance suggests possible weak antilocalization at 2 K. This paper highlights the interplay between <db:math xmlns:db="http://www.w3.org/1998/Math/MathML"><db:msub><db:mi>ScTaN</db:mi><db:mn>2</db:mn></db:msub></db:math> and <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML"><eb:mrow><eb:msub><eb:mi>Ta</eb:mi><eb:mn>5</eb:mn></eb:msub><eb:msub><eb:mi mathvariant="normal">N</eb:mi><eb:mn>6</eb:mn></eb:msub></eb:mrow></eb:math> crystal structures in stabilizing layered materials, emphasizes the importance of cation order/disorder for potential tunable alloys, and suggests that <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML"><gb:msub><gb:mi>ScTaN</gb:mi><gb:mn>2</gb:mn></gb:msub></gb:math> is a promising platform for exploring electronic properties.