Electronic structure of SrTi1−xVxO3 films studied by in situ photoemission spectroscopy: Screening for a transparent electrode

Abstract

This study investigated the electronic structure of SrTi1-xVxO3 (STVO) thin films, which are solid solutions of strongly correlated transparent conductive oxide (TCO) SrVO3 and oxide semiconductor SrTiO3, using in situ photoemission spectroscopy. STVO is one of the most promising candidates for correlated-metal TCO because it has the capability of optimizing the performance of transparent electrodes by varying x. Systematic and significant spectral changes were found near the Fermi level (EF) as a function of x, while the overall electronic structure of STVO is in good agreement with the prediction of band structure calculations. As x decreases from 1.0, spectral weight transfer occurs from the coherent band near EF to the incoherent states (lower Hubbard band) around 1.0-1.5 eV.Simultaneously, a pseudogap is formed at EF, indicating a significant reduction in quasiparticle spectral weight within close vicinity of EF. This pseudogap seems to evolve into an energy gap at x = 0.4, suggesting the occurrence of a composition-driven metal-insulator transition. From angleresolved photoemission spectroscopic results, the carrier concentration n changes proportionally as a function of x in the metallic range of x = 0.6-1.0. In contrast, the mass enhancement factor, which is proportional to the effective mass (m*), does not change significantly with varying x. These results suggest that the key factor of n/m* in optimizing the performance of correlated-metal TCO is tuned by x, highlighting the potential of STVO to achieve the desired TCO performance in the metallic region.

Composition-driven Mott transition within SrTi 1−x V x O3

“…At a critical doping, x c ∼ 0.5, the system undergoes a MIT as the V t 2g quasiparticle spectral weight is completely diminished at the Fermi level. This is in very good agreement with [32]. Similar results are obtained with a different value of U and for different atomic arrangements of the supercell (see appendix A and following discussion).…”

“…For example, substitutional doping has been shown to be an effective way to introduce filling-controlled MIT in the aliovalent A-site substitution of the intensely studied La 1−x Sr x VO 3 [29,30] (where x represents doping concentration). Recent studies of B-site substitutionally doped SrTi 1−x V x O 3 show that there is a temperature-dependent MIT which is composition driven [31][32][33][34]. SrTi 1−x V x O 3 is of particular interest as it is gives access to a tuned electronic structure between SrVO 3 and SrTiO 3 , which gives an insight into the rise of interesting correlated electron behaviour.…”

“…SrTi 1−x V x O 3 is of particular interest as it is gives access to a tuned electronic structure between SrVO 3 and SrTiO 3 , which gives an insight into the rise of interesting correlated electron behaviour. Beyond this composition-driven MIT, other properties of this doped system have been investigated such as its impedance spectroscopy and electrical conduction mechanism for certain dopings [35] and it may also be classified as a correlated transparent conductive oxide [32] which has the potential for use in optoelectronic devices. Several studies have measured the critical doping for the MIT to be at different dopings, ranging from between 0.4 [32], 0.6 [34], 0.67 [33] and 0.7 [31].…”

“…Beyond this composition-driven MIT, other properties of this doped system have been investigated such as its impedance spectroscopy and electrical conduction mechanism for certain dopings [35] and it may also be classified as a correlated transparent conductive oxide [32] which has the potential for use in optoelectronic devices. Several studies have measured the critical doping for the MIT to be at different dopings, ranging from between 0.4 [32], 0.6 [34], 0.67 [33] and 0.7 [31]. The critical doping was shown to be restrained between 0.5 and 1 when introducing oxygen vacancies [36].…”

“…From the conductivity and magnetic susceptibility measurements by Hong et al, they proposed that this MIT is controlled by the Anderson localized states [38] in which a MIT occurs where the mobility edge crosses the Fermi level [31]. The authors of the subsequent studies of the conductivity by Gu et al [33] and x-ray absorption spectroscopy and angle-resolved photoemission spectroscopy (ARPES) experiments by Kanda et al [32] proposed that the mechanism behind this composition drived MIT is due to a combination of both electron correlation effects (Mott physics) and disorder-induced (Anderson) localisation [32,33], where the Ti 4+ ion substitution introduces Anderson-localized states along with lattice distortions which result in a reduction in the effective bandwidth W. Therefore, it is clearly necessary to understand how the electronic structure transitions in SrTi 1−x V x O 3 between the band-insulator to the correlated metal from a theoretical perspective in order for this doped system to be effectively manipulated. We note that recent DFT+U calculations of SrTi 0.75 V 0.25 O 3 in a (low-temperature) distorted pseudo-cubic orthorhombic structure of SrTiO 3 were shown to be insulating due to distortion-induced crystal-field effects [39].…”