Tuning the electronic properties at the surface of BaBiO3 thin films

The presence of 2D electron gases at surfaces or interfaces in oxide thin films remains a hot topic in condensed matter physics. In particular, BaBiO3 appears as a very interesting system as it was theoretically proposed that its (001) surface should become metallic if a Bi-termination is achieved (Vildosola et al., PRL 110, 206805 (2013)). Here we report on the preparation by pulsed laser deposition and characterization of BaBiO3 thin films on silicon. We show that the texture of the films can be tuned by controlling the growth conditions, being possible to stabilize strongly (100)-textured films. We find significant differences on the spectroscopic and transport properties between (100)-textured and non-textured films. We rationalize these experimental results by performing first principles calculations, which indicate the existence of electron doping at the (100) surface. This stabilizes Bi ions in a 3+ state, shortens Bi-O bonds and reduces the electronic band gap, increasing the surface conductivity. Our results emphasize the importance of surface effects on the electronic properties of perovskites, and provide strategies to design novel oxide heterostructures with potential interface-related 2D electron gases.

The perovskite BaBiO 3 has generated a great deal of attention since the discovery of superconductivity upon doping with lead or potassium [1,2,3]. BaBiO 3 is a semiconducting oxide in contrast to the metallic behaviour expected for Bi 4+ ions half-filling the 6s band (this configuration implies only one Bi site per unit cell). Based on neutron diffraction experiments, the presence of two different Bi sites was established [4], related to the charge disproportionation of Bi ions (2Bi 4+ Bi 3+ +Bi 5+ ). This charge disproportionation, together with the breathing distortions of the BiO 6 octahedra [4], originates the splitting of the conduction band. Therefore, the metallic behaviour of the cubic structure at high temperatures leads to a semiconducting Peierls-like phase and a monoclinic distorted structure at room temperature. Besides the fascinating properties of this material in bulk, recently, Vildosola et al., have theoretically proposed a new mechanism for the formation of a two-dimensional electron gas (2DEG) at the surface of (001) Bi-terminated BaBiO 3 [5] . Based on first-principle calculations, they propose that this mechanism is related to the breaking of charge ordering at the surface due to the incomplete oxygen environment of the surface ions, which renders a cubic-like metallic behaviour confined to a few monolayers close to the surface. The experimental confirmation of this effect remains very challenging and has not been achieved so far. Only a few reports dealing with BaBiO 3 thin films can be found on the literature. showing that the orientation of their films can be controlled by growing a BaO buffer layer between the film and the substrate [6]. Later, Gozar et al. fabricated BaBiO 3 films by the same technique and identified their termination layer as BiO 2 [7]. Finally, Inumaru el at. reported on BaBiO 3 thin films deposited by pulsed laser deposition on MgO, and claim that, in spite that the tilting of BiO 6 octahedra is suppressed, their films remained "insulating" [8].
In the present work, the surface electronic and transport properties of (100)-oriented BaBiO 3 thin films are studied. We have grown BaBiO 3 thin films on Si (001) substrates and show that, although not epitaxial, the films can be highly textured in the (100) direction given the right growth conditions. X-ray photoemission experiments show the presence of an anomalous Bi contribution, which is maximized in (100) textured films. Based on new theoretical calculations, we identify that this contribution is related to electron doping at the (100) surface, which stabilizes the Bi valence in +3 states, shortens Bi-O bonds and reduces the band gap of the material when compared to the bulk case. This implies an enhanced conductivity for the (100)-surface, which is consistent with our transport measurements.
These results demonstrate the importance of surface effects on the electronic properties of perovskites and suggest that combining BaBiO 3 with other oxides in heterostructures could be a feasible a route to produce 2DEG systems. BaBiO 3 thin films were grown on top of (001) silicon by pulsed laser deposition. A Nd:YAG solid state laser, operating at =266nm with a repetition frequency of 10Hz, was used. The deposition temperature and oxygen pressure were 500ºC and 0.01 mbar, respectively. The used fluencies ranged between 1.9 and 3.4 J/cm 2 . X-ray diffraction experiments were performed by means of an Empyrean (Panalytical) diffractometer with a Pixcel 3D detector.
Profiles matching of the XRD spectra were done by using the FullProf software and cell parameters and volume were extracted. The film thickness was estimated by focused ion beam cross-sectioning and scanning electron microscopy imaging as 160nm. XPS measurements were performed under UHV conditions (base pressure < 5x10 -10 mbar) using a SPECS UHV spectro-photometer system equipped with a 150 mm mean radius hemispherical electron energy analyser and a nine channeltron detector. Spectra were acquired at a constant pass energy of 20 eV using an un- Figure 1 displays X-ray diffraction patterns corresponding to two BaBiO 3 films grown at fluencies of 1.9 and 3.4 J/cm 2 , respectively. We recall that both fluences are above the minimum threshold necessary for congruent growth (<1.8J/cm 2 for Bi-based perovskites [9]), which indicates that the target stochiometry is fully transferred to the films. Besides a small amount of segregated BaO, produced during the first growth stages and therefore localized close to the substrate/film interface in both cases (to be reported elsewhere), no other secondary phases were found. A strong change in the BaBiO 3 -texture is found for different fluences. The film grown at lower fluency is strongly textured in the (100) direction, while the film grown at the higher fluency is more polycrystalline-like. In the case of the textured film, we checked the absence of epitaxy by performing -scans around Si (202) and BaBiO 3 (220) reflections, to be reported elsewhere. The presence of the native amorphous SiO x layer at the Si surface prevents the epitaxial growth, in spite of the possible structural matching between Si and BaBiO 3 [10]. The analysis of the XRD patterns of Figure 1 indicates that our oriented film presents an out of plane cell parameter of a=(6.180.01)Å, in excellent agreement with the bulk crystalline structure (a=6.181Å). A similar procedure can be done for the nonoriented film; for example, from the (00l) reflections we get c=(8.660.01)Å which is also consistent with the bulk value (c=8.669Å). It is also worth comparing the extracted cell volumes of bulk BBO and the non-oriented film, both values being consistent within the error of the technique ((318±2)Å 3 and (321±2)Å 3 , respectively). We recall that in the case of the oriented films the lack of in-plane ordering (as determined by XRD phi-scans) does not allow obtaining in-plane cell parameters by means of 4-circle diffraction. In consequence, a direct comparison between oriented and non-oriented cell volumes is not possible. The similarity between our films cell parameters and volume (the latter only possible in the case of the nonoriented film) and bulk values suggests that both families of films (oriented and non-oriented) are strain-free and there are no significant off-stochiometries, which, in the case of existing, should necessarily impact on the films structure. The inset of Figure 1  intense, one at 158.1eV and 163.3eV. It is worth pointing out that photoemission experiments in bulk samples [11,12] show the presence of a single, although broad, 4f doublet, which could not be unambiguously resolved into the two components expected from the Bi charge disproportionation. In our case, it is therefore reasonable to attribute the dominant doublet to a bulk-like contribution, and the second one to the presence at the surface of Bi ions with different chemical environment. Interestingly, it is found that the integrated intensities ratio I secondary /I dominant between both doublets changes from 0.21 for the non-textured film to 0.30 for the textured film. This clearly shows that the presence of the modified Bi chemical environment is maximized in the case of the (100) orientation.
In order to shed light on the observed experimental behaviour, we have performed ab-initio calculations of the BaBiO 3 (100) surface, as shown in Figure 3 We perform first-principles Density Functional Theory (DFT) calculations using the generalized gradient approximation for the exchange and correlation potential (GGA) [13].
Standard DFT calculations using local or semilocal functionals predict a semimetallic behaviour for bulk BaBiO 3 , however, it is well known that it presents an indirect gap whose experimental reported value goes from 0.2 eV to 1.1 eV [14]. We take care of the gap problem using the modified Becke-Johnson potential (MBJ) [15] (see supplementary information of Ref. [5]). The relaxation of the supercell is performed within the VASP code [16]. The MBJ correction is done within the WIEN2K code [17].
In Figure 3 subsurface Bi 3+ ions, this bandwidth increases due to the shorter Bi-O distance. This larger bandwidth, in turn, produces a decrease in the band-gap that should induce a decrease of the surface resistance. In consequence, it can be expected for (100)-textured films a more conducting behaviour in comparison with non-textured films. This is consistent with data plotted in Figure 5(a), which shows the evolution of the resistance as a function of the temperature for both textured and non-textured films. Although both cases display a semiconducting behaviour (the resistance increases as the temperature is decreased), the textured film displays a lower resistance (around one order of magnitude at room temperature) with respect to the non-textured case. On the other hand, we recall in the case of an (100)surface enhanced conductivity, the longitudinal (in plane) resistance of films with different thicknesses should not display significant variations. Figure 5(b) shows, for oriented films with different thicknesses, the evolution of R  =R 4p x (t/L), where R 4p is the four-points resistance, t is the width of the sample and L is the distance between the voltage contacts. In the case of bulk conductivity, it would be expected a R  vs. thickness dependence as that depicted with a full red line in Figure 5(b) (R  scaling with 1/thickness). We experimentally find, instead, that R  remains roughly constant for different thicknesses, consistently with the increased surface conductivity that our theoretical calculation suggests for (100)-oriented films. We recall that transport experiments were repeated on BBO films grown on (highly insulating) SiO 2 buffered Si, with similar results to what was reported above. This allows to safely discard the semiconducting substrate or the substrate/BBO interface contributing to the measured transport properties. Further experimental work involving other techniques such as conductive-atomic force microscopy are needed to get microscopic evidence on the increase of electrical conductivity at the (100) surface.
In summary, we have shown that it is possible to control the out-of-plane texture of nonepitaxial BaBiO 3 thin films by controlling the growth conditions. XPS spectra display an anomalous Bi doublet which is maximized for (100)-texture. The origin of this doublet is linked to our ab-initio calculations results, which suggest that electron doping stabilizes Bi 3+ species and shortens Bi-O bonds at the (100)-surface. This also implies a reduction of the band gap and an increase of the surface conductivity, consistently with our transport measurements. We conclude that as the texture of the films can be easily controlled by the growth conditions, it is possible to tune the electronic properties at the surface of these films.
Finally, we would like to stress out that as this surface related effects come from the incomplete oxygen environment at surface Bi ions, a stronger effect can be anticipated if BaBiO 3 is combined in heterostructures with an oxide with a high oxygen affinity such as yttrium-stabilized zirconia (YSZ), which could favour oxygen transfer between both layers. In particular, the possibility of developing a metallic behaviour at such interface should be explored in the future.   (b), (c) Bi-4f X-ray photoemission spectra corresponding to textured and non-textured BBO thin films, respectively.