Hole-induced electronic and optical transitions in $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ epitaxial thin films

We have investigated the electronic and optical properties of epitaxial $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ for $0\le x\le 1$ prepared by molecular-beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing $x$, indicating downward movement of the Fermi level toward the valence-band maximum. Combining valence-band photoemission and O $K$-edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level as a result of hole doping. In-plane transport measurements confirm that the material becomes a $p$-type semiconductor at lower doping levels and exhibits a conversion from semiconducting to metallic behavior at $x=1$. Low-energy optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.

Figure 1

(a) RHEED patterns for the as-grown $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ films on STO(001) viewed along the [100]-zone axis. (b) Representative AFM images for the as-grown LSFO films with $x=0$, 0.5, and 1 on STO(001). The scan image size is $2\mu \mathrm{m}\times 2\mu \mathrm{m}$. (c) XRD $\theta -2\theta$ scans of oxygen plasma-annealed films near the (002) peak for the LSFO/STO(001) film set.

Figure 2

(a) $\mathrm{Fe}2p$, O $1s$, Sr 3d, and La $4d$ XPS spectra for $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ with various $x$ values. The binding-energy scales were calibrated using the Au $4f_{7/2}$ peak binding energy (84.00 eV). (b) Average chemical potential shift deduced from O $1s$, Sr 3d, and La $4d$ binding energies. (c) Room-temperature resistivity vs $x$.

Figure 3

(a) $\mathrm{Fe}2p$ core-level XPS spectra for $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ with various $x$ values. All spectra are shifted so the associated O $1s$ peaks fall at 530.0 eV. As a guide to the eye, we mark the core-level peaks of the LFO (black) and SFO (magenta) films with dashed lines. For comparison with $x=1$, we replot the $\mathrm{Fe}2p$ spectrum of LFO $(x=0)$ as a dotted line. The satellite peaks for $\mathrm{F}{\mathrm{e}}^{3+}$ (indicated by arrows) are absent in $\mathrm{SrFe}{\mathrm{O}}_3$ ($\mathrm{F}{\mathrm{e}}^{4+}$). (b) $\mathrm{Fe}K$-edge NEXAFS for $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ with various $x$ values. The dashed arrow indicates the peak shift.

Figure 4

(a) Absorption spectra for $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ films deposited on STO(001). (b) Tauc plots of ${\left(\alpha E\right)}^{2/3}$ versus photon energy $E$ for the direct dipole-forbidden excitations. The resulting onsets of transitions $A$ and $B$, shown as extrapolations to the energy axis, are summarized in Table 1. (c) XPS-VB (left, filled symbols) and O $K$-edge XAS (right, open symbols) for selected LSFO films. (d) Schematic of the electronic structure and optical transitions for $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ ($x>0$). Red and blue colors refer to neighboring Fe atoms (labeled as $\mathrm{F}{\mathrm{e}}_1$ and $\mathrm{F}{\mathrm{e}}_2$ in the text) that carry ground-state spin-density up $(\uparrow )$ and down $(\downarrow )$, respectively. Solid and dashed areas correspond to the occupied and virtual one-electron states. Hybridization of O $2p$ and $\mathrm{Fe}3d{e}_g$ forms a distinct feature at the top of the valence band, which becomes partially depopulated for $x>0$. Transitions $A$ (interatomic) and $B$ (intra-atomic) as indicated with arrows. (e) $\mathrm{L}{\mathrm{a}}_{8-n}\mathrm{S}{\mathrm{r}}_n\mathrm{F}{\mathrm{e}}_8{\mathrm{O}}_{24}$ one-electron DOS calculated using B3LYP with an embedded cluster. Here, the highest-occupied (black) and the lowest-unoccupied (red) one-electron states are separated by a few tenths of an electron volt for $n>0$. A small overlap between the tails of the occupied and unoccupied DOS (where it exists) is a result of Gaussian broadening, rather than a metallic electronic structure. (f) Low-energy region of the optical absorption spectra calculated using TDDFT and B3LYP; green, red, and gray colors indicate contributions due to $A$- and $B$-type transitions, and their sum, respectively.