Electronic properties of epitaxial ${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\mathrm{Rh}{\mathrm{O}}_3$ thin films

We report on the synthesis and electronic properties of epitaxial perovskite ${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\mathrm{Rh}{\mathrm{O}}_3$ thin films. Thin films with a Sr content ranging from $x=0$ to 0.5 have been grown using molecular beam epitaxy. Transport and x-ray photoemission spectroscopy data reveal an insulator-metal-insulator transition, accompanied by a $p$- to $n$-type carrier change observed in Hall measurements. Combined with theoretical calculations, we find that the addition of Sr does not directly dope carriers into the conduction band, but rather induces localized Rh $4d$ states within the $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_3$ band gap. The bandwidth of the impurity band increases with Sr content, eventually causing the valence band (VB) and the localized Rh $4d$ band to overlap, which explains the first insulator-to-metal transition occurring at $x=0.35$. For Sr content $x>0.4$, possible cation ordering results in an increase of the gap between the VB and the Rh $4d$ band, leading to the second metal-to-insulator transition. We map out the electronic phase diagram of the Sr-doped $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_3$ system and suggest strategies to engineer the electronic states in rhodate systems via delocalizing the ${\mathrm{Rh}}^{3+}$ states.

Figure 1

Characterization of LSRO films grown on STO and LAO substrates. (a) Atomic force microscopy (AFM) topographic image (left) and reflection high-energy electron diffraction (RHEED) pattern (right) of a 30-uc-thick LRO film on STO. (b) AFM topographic image (left) and RHEED pattern (right) of a 30-uc-thick LRO film on LAO. (c) X-ray diffraction (XRD) $\theta -2\theta$ scan of 30-uc LRO (top) and $x=0.5$ (bottom) LSRO thin films on STO. (d) XRD $\theta \text{−}2\theta$ scan of 30-uc LRO (top) and $x=0.5$ (bottom) LSRO thin films on LAO. (e) Reciprocal space map (RSM) around (103) reflections of $x=0$ and 0.5 thin films on STO (left column) and on LAO (right column).

Figure 2

Photoemission measurements as a function of Sr content. (a) Wide energy range x-ray photoemission (XPS) data of $x=0.05$, 0.3, and 0.5 thin films. The characteristic core levels for La, Rh, Sr, and O are indicated. Note that a C $1s$ peak is also observed due to air exposure of the samples before the measurements. (b) Valence band XPS data of $x=0$, 0.3, and 0.5 LSRO films, which indicate a band gap of 0.54, 0.05, and 1.1 eV, respectively.

Figure 3

Temperature-dependent resistivity (RT) as a function of Sr-doping. (a) RT curves of samples grown on LAO from $x=0.08$ to 0.4. An insulator-to-metal transition occurs at $x=0.35$. (b) RT curves of samples grown on LAO from $x=0.4$ to 0.5. A metal-to-insulator transition takes place at $x=0.45$. (c) RT curves of samples grown on STO from $x=0.08$ to 0.4. An insulator-to-metal transition occurs at $x=0.4$. (b) RT curves of samples grown on STO from $x=0.4$ to 0.5. A metal-to-insulator transition takes place at $x=0.45$. (e) Three-dimensional (3D) carrier densities of the LSRO thin films on both LAO and STO substrates. (f) Phase diagram of the LSRO thin films, showing the insulator-metal-insulator transition and a $p$-to-$n$-type carrier transition. The data plotted are extracted from the room temperature RT of the thin films grow on LAO.

Figure 4

(a)–(d) Theoretical calculations of the band structures of $x=0$, 0.0625, 0.25, and 0.5 LSRO thin films, respectively. (e) Schematic of a C-type ordering of the Sr and La cations. (f) Schematic of the change in band structures and electronic states with different doping levels.