Control of the metal-insulator transition in ${\mathrm{NdNiO}}_3$ thin films through the interplay between structural and electronic properties

Heteroepitaxy offers a new type of control mechanism for the crystal structure, the electronic correlations, and thus the functional properties of transition-metal oxides. Here we combine electrical transport measurements, high-resolution scanning transmission electron microscopy (STEM), and density functional theory (DFT) to investigate the evolution of the metal-to-insulator transition (MIT) in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ films as a function of film thickness and $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ substrate crystallographic orientation. We find that for two different substrate facets, orthorhombic (101) and (011), modifications of the $\mathrm{Ni}{\mathrm{O}}_6$ octahedral network are key for tuning the transition temperature $T_{\text{MIT}}$ over a wide temperature range. A comparison of films of identical thickness reveals that growth on [101]-oriented substrates generally results in a higher $T_{\text{MIT}}$, which can be attributed to an enhanced bond disproportionation as revealed by the $\text{DFT}+U$ calculations, and a tendency of [011]-oriented films to formation of structural defects and stabilization of nonequilibrium phases. Our results provide insights into the structure-property relationship of a correlated electron system and its evolution at microscopic length scales and give new perspectives for the epitaxial control of macroscopic phases in metal-oxide heterostructures.

Figure 1

(a) Schematic of the orthorhombic $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ unit cell, with the (101) and (011) crystallographic planes highlighted in red and blue, respectively. Additionally, one $\mathrm{Ni}{\mathrm{O}}_6$ octahedron is shown in gray. Note that $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ exhibits a similar unit cell (not shown here) with slightly different lattice parameters (see Methods). (b) and (c) Projections of the $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ crystal structure along the [101] and [011] direction, respectively. The unit cell from (a) is indicated by black lines, and the (101) and (011) crystallographic planes by red and blue lines, respectively. The orange lines indicate characteristic straight and zigzag patterns of the Ni and Nd cation positions in the two projections. These patterns can be identified in STEM-HAADF images.

Figure 2

(a) and (b) Electrical resistivity $\rho$ as a function of temperature $T$ measured for $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ films of different thickness grown on [101]- and [011]-oriented $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ substrates, respectively. Solid lines show $\rho \left(T\right)$ measured upon cooling, while additional dashed curves show $\rho \left(T\right)$ upon warming for selected samples. The filled symbols indicate the transition temperature $T_{\text{MIT}}$, determined as described in the text. Note that the 14.4 Å film is capped by $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ and is therefore marked by $(*)$ in the legend of (a). This sample is semiconducting without MIT. All curves, except for the one of the 14.4 Å film, are offset in vertical direction by multiples of $250\mu \mathrm{\Omega }\text{cm}$ for clarity. (c) and (d) Enlarged views of $\rho$ around the metal-insulator transition temperatures for (101) and (011) samples, respectively. Gray boxes of $\pm 5\mu \mathrm{\Omega }\text{cm}$ height were used to determine the error bars of $T_{\text{MIT}}$. Dotted, vertical lines mark the zero crossing of $-\partial \left[\text{ln}\right(\rho \left)\right]/\partial T$. (e) $T_{\text{MIT}}$ as a function of $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ film thickness $d$. The shaded areas indicate the thickness-dependent trends, but serve only as a guide for the eye.

Figure 3

(a) STEM-HAADF image of the ultrathin 14.4 Å $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ film, including the $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3-\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ substrate film and the $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3-\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ film-capping layer interfaces, indicated by dashed orange lines. Purple and orange rectangles correspond to the regions presented in (b) and (c), respectively. (b) Atomic-resolution two-dimensional elemental maps of Ga (blue) and Ni (red). Interfacial intermixing occurs within one monolayer at most for both interfaces. (c) The STEM-ABF image provides information about the oxygen positions. The atomic column within the region marked by the white dashed-dotted lines corresponds to $B$-O-$B$ bonds, with $B$ = Ga, Ni, which are analyzed and quantified in (d). The dashed orange lines mark the nominal interfaces. (d) Bond angle measurement across the $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3-\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3-\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ structure, indicating that the $B$-O-$B$ bond angle remains constant within the experimental error. Error bars indicate the 95% confidence interval, corresponding to two times the standard error. The blue and orange colored areas are guides for the eye indicating the $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ and $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ regions, respectively. Note that the $B$-O-$B$ bond angle corresponds to the angle of the projection between a $B$ column, two columns of oxygen, and another $B$ column, as illustrated in the corresponding projection [Fig. 1].

Figure 4

(a) and (b) STEM-HAADF images of the 70 Å $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ films on [101]- and [011]-oriented $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ substrates, respectively. The insets illustrate the presence of characteristic straight and zigzag patterns of the Ni and Nd cation positions, which are indicative of the [101] and [011] orientations, respectively. While the $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ film on (101) $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ is entirely [101] oriented and of high crystalline quality (a), a more defective structure together with reoriented (101) patches is observed for the film on (011) $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$.

Figure 5

Top panels: Schematic projections of the dimensions of the bulk $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ and $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ unit cells for different film orientations on (a) (101) $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ and (b) (011) $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ substrates. The crystallographic in-plane directions ($a^{*}, b^{*}, a, b$) of $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ and $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ are indicated, together with the normalized, directional lattice mismatch $(l_{{\mathrm{NdGaO}}_3}-l_{{\mathrm{NdNiO}}_3})/l_{{\mathrm{NdNiO}}_3}$ in percent. Bottom panels: Ground state energy of $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ as a function of the normalized out-of-plane lattice spacing as obtained from $\text{DFT}+U$ calculations for the four different substrate-film orientation configurations depicted in the top panels. Keeping the in-plane lattice parameters fixed to those of [101]-oriented $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ in (a) and [011]-oriented $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ in (b), the ground state energies have been calculated for a range of values for the out-of-plane lattice spacing. Since the $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ substrate induces tensile strain in all configurations, a reduced $c^{*}$ spacing is expected with respect to the bulk (for the normalization we used bulk $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ [21], that is $c_{\left(101\right)}^{*}=4.397$ Å for the (101) phase and $c_{\left(011\right)}^{*}=4.394$ Å for the (011) phase). Furthermore, the $\text{DFT}+U$ derived structures reveal an enhanced bond disproportionation in [101]- compared to [011]-oriented films on both substrate orientations, as illustrated by the insets.