Structural and stoichiometric modifications in ultrathin epitaxial $\mathrm{BaBiO}{}_3$ films

${\mathrm{BaBiO}}_3$ (BBO) is well known as the parent material for the high-$T_c$ superconducting compounds ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{BiO}}_3$ and ${\mathrm{BaPb}}_{1-x}{\mathrm{Bi}}_x{\mathrm{O}}_3$. In its pristine state, BBO is a charge-ordered (CO) insulator, resulting from a static breathing distortion of the ${\mathrm{BiO}}_6$ octahedra with alternating long and short bond lengths. Recently, it has been reported that the CO state is suppressed for BBO films grown on ${\mathrm{SrTiO}}_3$ (STO) below a thickness of approximately 4 nm, possibly resulting in a metallic phase. While we do confirm structural modifications in our BBO/Nb:STO samples in this thickness range by Raman spectroscopy and electron diffraction, in situ photoemission evidences that these changes are accompanied by a Bi deficit and that the films remain insulating. We hence conclude that, in line with previous findings for the BBO/STO interface, the thickness-controlled suppression of the CO state is not purely driven by the two-dimensional confinement but rather originates from modifications of the composition and structure inherent to the epitaxial growth of BBO on ${\mathrm{SrTiO}}_3$(001).

Figure 1

(a) Raman response at a wavelength $\lambda =633$ nm of BBO films grown on Nb:STO(001) substrates with thicknesses $d_{\text{BBO}}$ ranging from 0.4 to 17.5 nm. (b) Relative intensity (left axis) and width (right axis) of the Raman response of the breathing phonon mode at about 565 ${\mathrm{cm}}^{-1}$ dependent on the BBO film thickness. Both quantities indicate the presence of a dead layer, not hosting the breathing phonon, below a critical thickness of 3 nm. (c) Model of the heterostructure with a layer of thickness $d_c$ at the interface, which does not contribute to the Raman response of the film. (d) Evolution of the low-energy electron diffraction pattern of the samples dependent on the BBO thickness. For the 0.4- and 1-nm-thick samples a $(1\times 1)$ symmetry of the surface lattice is observed, while thicker films exhibit a characteristic ${\mathrm{R}45}^{\circ }(3\sqrt{2}\times \sqrt{2})$ reconstruction. In the upper-left corner of each image the kinetic energy of the electrons is denoted.

Figure 2

(a) O $1s$, (b) Ba $4d$, and (c) Bi $4f$ spectra of BBO/Nb:STO samples with different film thicknesses $d_{\text{BBO}}$. (d) Comparison of the Bi $4f_{7/2}$ line shape of BBO films with $d_{\text{BBO}}=1$, 2, and 17.5 nm. *The binding energy is corrected for charging as described in Appendix pp3.

Figure 3

(a) Evaluation of the integrated intensities of the Bi $4f$ and Ba $4d$ core levels for determination of the cation stoichiometry. (b) Upper graph: Cation off-stoichiometry $x$ dependent on the film thickness. $x$ is obtained by a calibration of the ratio of the Bi $4f$ and Ba $4d$ core-level area intensities to that of a reference measurement on the stoichiometric BBO target material ($x=0$). Experimental uncertainties are indicated by error bars. Lower graph: Binding energy of Bi $4f$ (left axis) and energy shift relative to the reference position (right axis) dependent on the film thickness. Dotted lines are guides for the eye. *The binding energy is corrected for charging as described in Appendix pp3.

Figure 4

(a) XPS valence band spectra of samples with different BBO thicknesses. The transition from the STO-dominated valence band for thin films to the BBO valence band at larger film thicknesses is illustrated. (b) Exemplary decomposition of the valence band of the 1-nm-thick BBO sample into the valence band spectra of a bare Nb:STO substrate and a 10-nm-thick BBO film. The typical steplike valence band onset (arrow) is missing for samples with $d_{\text{BBO}}<3$ nm. *The binding energy is corrected for charging as described in Appendix pp3.

Figure 5

Properties of BBO thin films deposited in different oxygen background pressures between $p_{{\mathrm{O}}_2}=2\times {10}^{-2}$ mbar and $p_{{\mathrm{O}}_2}=2\times {10}^{-1}$ mbar. (a) $\omega \text{–}2\theta$ scans around the BBO(002) diffraction peak. (b) Corresponding rocking curves of the BBO(002) peak. (c) Cation off-stoichiometry $x$ evaluated in the same way as in Fig. 3.

Figure 6

(a) Background-subtracted Raman spectra of the breathing phonon excitation for different film thicknesses. (b–f) Lorentzian fits of the breathing phonon response of samples with $d_{\text{BBO}}\ge 3$ nm. Each ordinate scaling is normalized to the intensity of the peak maximum.

Figure 7

(a) O $1s$ core-level spectra of a BBO film on Nb:STO dependent on the emission current of the Al $K_{\alpha }$ x-ray tube. (b) Decomposition of the valence band of a sample with a 4-nm-thick BBO film. (c) Band alignment at the ${\mathrm{BaBiO}}_3/\mathrm{Nb}$:${\mathrm{SrTiO}}_3$ interface as derived from (b). (d) Upward band bending towards the interface in the Nb:STO substrate, hampering electron transfer across the interface into the film.