Thermally induced metal-to-insulator transition in $\mathrm{Nb}{\mathrm{O}}_{2}$ thin films: Modulation of the transition temperature by epitaxial strain

Thermally induced metal-to-insulator transition in $Nb O_{2}$ thin films: Modulation of the transition temperature by epitaxial strain

Toyanath Joshi, Eli Cirino, Sophie A. Morley, and David Lederman

Phys. Rev. Materials 3, 124602 – Published 16 December 2019

Abstract

Modification of the carrier dynamics in correlated oxide systems via epitaxial strain is a promising pathway for the practical realization of energy-efficient electronic devices. Here we report on the thermally induced metal-to-insulator transition (MIT) of $Nb O_{2}$ films grown on $A l_{2} O_{3}$ substrates and the modulation of the MIT temperature via epitaxial strain from the substrate. The metal-insulator transition temperature ( $T_{MIT}$ ) increased from 910 to 1066 K with increasing strain. An ultrathin 3.9-nm film consisting of a single strained layer with minimal structural defects yielded a bulklike sharp transition. The substrate-induced strain offers another degree of freedom to improve device functionality of MIT materials.

Received 25 August 2019
Revised 28 October 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.124602

Physics Subject Headings (PhySH)

Electrical conductivity

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Toyanath Joshi ^*, Eli Cirino, Sophie A. Morley , and David Lederman

Department of Physics, University of California, Santa Cruz, California 95064, USA

^*tojoshi@ucsc.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 3, Iss. 12 — December 2019

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Structural analysis. (a) X-ray diffraction (XRD) measured on the as-grown films. Scans around $Nb O_{2} {(440)}_{T}$ only are presented, for clarity. The vertical solid and dashed lines represent the bulk-peak position and the peak position of the ultrathin film, respectively. (b) The crystal structure of the tetragonal $Nb O_{2}$ phase. The growth direction is represented by an arrow labeled with [110]. (c) Rocking curves of $Nb O_{2} {(440)}_{T}$ peaks. Films thicker than 15 nm showed a clear two-peak structure. (d) Ratio of the fitted broad- to sharp-peak area vs film thickness with a linear fit to all samples grown under similar conditions (solid line).
Reuse & Permissions
Figure 2
In-plane x-ray diffraction and epitaxial strain model. (a) A schematic of the epitaxial compressing strain acting in the ${(110)}_{T}$ plane and resulting tensile strain along the ${[001]}_{T}$ direction of the $Nb O_{2}$ crystal (not drawn to scale). (b) An estimation of the band structure in the strained and unstrained films (not drawn to scale). (d) In-plane θ-2θ scans from ${(400)}_{T}$ and ${(222)}_{T}$ reflections measured from a thin (6.4-nm) and 63.6-nm film showing a shift in the (222) peak position of the thinner film by 0.2° with respect to the thicker film. (e) In-plane $Φ$ scans of $Nb O_{2} {(400)}_{T}$ and $A l_{2} O_{3}$ (113) reflections measured from a 6.4-nm film. The film showed sixfold rotation symmetry due to the three mutually rotated in-plane twin domains as illustrated in (c).
Reuse & Permissions
Figure 3
Temperature-dependent resistivity. (a) The current-voltage characteristics of the Ohmic contacts. (b) The resistivity vs T measured using a four-probe technique. Inset: the sample image and measurement schematics. (c) A plot of resistivity vs ${(10^{3} / T)}^{1 / (1 + p)}$ of the 15.6-nm-thick film for thermal activation ( $p = 0$ ), Efros-Shklovskii ( $p = 1$ ), 2D Mott ( $p = 2$ ), and 3D Mott ( $p = 4$ ) conductivity models. Resistivity vs $T$ data near $T_{MIT}$ for (d) 3.9-, (e) 6.5-, (f) 15.6-, and (g) 46.8-nm films.
Reuse & Permissions
Figure 4
Variation of transition temperature. (a) The metal-insulator transition temperature ( $T_{MIT}$ ) as a function of out-of-plane spacing ( $d_{110}$ ) showing a general increase in $T_{MIT}$ with a decreasing $d_{110}$ spacing. Horizontal and vertical dotted lines represent the bulk $T_{MIT}$ and bulk $d_{110}$ , respectively. Inset: SEM cross-sectional image of the 63.6-nm film. The interface between strained and relaxed layer is indicated by the yellow arrow. (b) $T_{Infl}$ plotted as a function of $d_{110}$ . Inset: On the left, a high-resolution cross-sectional SEM image of the interface, with the yellow arrow showing the boundary between the relaxed and strained layers. On the right, the conductive probe atomic force microscopy (CPAFM) image of the same film cross section showing a less conducting interface and the bulk layer with larger conductivity.
Reuse & Permissions
Figure 5
Metal-to-insulator transition in an ultrathin film. Metal-to-insulator transition in a 3.9-nm film during heating and cooling, indicated by the arrowhead directions. The shaded region represents the sharp change in the resistivity due to the structural change and the dotted line represents the $T_{MIT}$ . Inset: The figure replotted with a larger temperature range scale where the solid vertical lines represent the $T_{Infl}$ during heating and cooling, showing a hysteresis of 70 K.
Reuse & Permissions

Physical Review Materials

Thermally induced metal-to-insulator transition in NbO2 thin films: Modulation of the transition temperature by epitaxial strain

Toyanath Joshi, Eli Cirino, Sophie A. Morley, and David Lederman

Phys. Rev. Materials 3, 124602 – Published 16 December 2019

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

Supplemental Material (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Thermally induced metal-to-insulator transition in $Nb O_{2}$ thin films: Modulation of the transition temperature by epitaxial strain