Effect of oxygen vacancy distribution on the thermoelectric properties of La-doped SrTiO3 epitaxial thin films

Abstract

A detailed study of the role of oxygen vacancies in determining the effective mass and high temperature (300–1000 K) thermoelectric properties of La-doped epitaxial SrTiO3 thin films is presented. It is observed that at intermediate temperatures, a transition from degenerate to non-degenerate behavior is observed in the Seebeck coefficient, but not electrical conductivity, which is attributed to heterogeneous oxygen non-stoichiometry. Heikes formula is found to be invalid for the films with oxygen vacancies. By fitting the spectroscopic ellipsometry (SE) data, obtained in the range 300–2100 nm, using a Drude-Lorentz dispersion relation with two Lorentz oscillators, the electrical and optical properties of the films are extracted. Using the excellent agreement between the transport properties extracted from SE modeling and direct electrical measurements, we demonstrate that an increase in concentration of oxygen vacancies results in a simultaneous increase of both carrier concentration and electron effective mass, resulting in a higher power factor.

Polaron Transport and Thermoelectric Behavior in La‐Doped SrTiO3 Thin Films with Elemental Vacancies

“…The values have been obtained from the Drude–Lorentz fitting, using m * / m e = 6 . n D values of our thin films range between ≈4 × 10 20 and ≈1 × 10 21 cm −3 , which is similar to other LSTO samples . The decreasing trend of n D with increasing P (O 2 ) was consistently observed.…”

“…A thermo‐electromotive force (Δ V ) is generated between the two ends of the LSTO films by introducing a temperature difference (Δ T ). The overall S values of our LSTO films are comparable to other doped STO bulk, thin films, and heterostructures . However, the LSTO samples grown at different P (O 2 ) did not show the usual increasing trend of S with decreasing n D , which has been observed in most of the previous studies .…”

“…We finally note that the polaron mass enhancement is different from the electronic band mass, conventionally determined by the curvature of the electronic band . Therefore, our discussion on the polaronic conduction cannot be directly applied to the argument regarding the P (O 2 )‐dependent change in the effective band mass in doped STO . However, as we have shown in this paper, the polaron transport in LSTO films provide critical understanding on the system, which should be carefully considered in discussing the thermoelectric properties …”

“…In ruthenate superlattices, spin channel‐dependent thermopower and large thermopower anisotropy have been theoretically predicted . Many studies have also focused on the 3 d perovskite SrTiO 3 (STO)‐based material system, which is an n ‐type thermoelectric material . A dimensional crossover of thermoelectric phenomena has been observed in Nb‐doped STO‐based heterostructure, where the 3 d 1 electrons in Ti strongly couple with the lattice phonons to exhibit a large thermopower .…”

Grain Boundary Interfaces Controlled by Reduced Graphene Oxide in Nonstoichiometric SrTiO3-δ Thermoelectrics

“…In recent years, strontium titanium oxide (STO) is particularly considered an interesting n -type material in thermoelectric research due to its high absolute Seebeck coefficient 9 . Thus, many efforts have been made to enhance its electrical conductivity, such as cationic and anionic substitution, but ZT is still not competitive because of the high lattice thermal conductivity ( κ L ) 10–14 . Nano-structuring, doping, and creating oxygen vacancies in STO provide an opportunity to decrease the lattice thermal conductivity through phonon scattering without noticeably affecting their electrical conductivity 15–19 .…”