Anomalously large measured thermoelectric power factor in Sr1−xLaxTiO3 thin films due to SrTiO3 substrate reduction

Abstract

We report the observation that thermoelectric thin-films of La-doped SrTiO 3 grown on SrTiO 3 substrates yield anomalously high values of thermopower to give extraordinary values of power factor at 300K. Thin-films of Sr 0.98 La 0.02 TiO 3 , grown via pulsed laser deposition at low temperature and low pressure (450°C, 10 -7 Torr), do not yield similarly high values when grown on other substrates. The thin-film growth induces oxygen reduction in the SrTiO 3 crystals, doping the substrate n-type. It is found that the backside resistance of the SrTiO 3 substrates is as low (~12Ω/ ) as it is on the film-side after film growth.

Thermal conductivity as a metric for the crystalline quality of SrTiO3 epitaxial layers

“…Extended defects and residual strain created by growth on a lattice mismatched substrate, e.g., LSAT with a lattice mismatch of 0.95%, 11 do not have a significant effect on thermal conductivity ⌳ at temperatures T Ͼ 150 K. The lack of sensitivity to residual strain is expected. The effect of strain on ⌳ can be estimated based on Leibfried-Schlomann equation which states that ⌳ should scale with the cube of the Debye temperature.…”

“…Furthermore, stray electrical conductivity in substrates can confound electrical transport measurements on films. 11 Here, we describe the use of thermal conductivity as a tool for evaluating the quality of epitaxial layers of a prototypical perovskite oxide, SrTiO 3 ͑STO͒. The foundations of our approach have a long history: thermal conductivity has frequently been used in the past to evaluate the perfection of bulk nonmetallic crystals.…”

Enhanced effective mass in doped SrTiO3 and related perovskites

“…[5][6][7][8][9] Further improvement requires the increase of the effective mass as the most important factor 10 for the Seebeck coefficient and conductivity. This paper compares the experimentally determined values for the effective electron mass with those obtained from ab-initio calculations.…”

Large Seebeck coefficients and thermoelectric power factor of La-doped SrTiO3 thin films

“…It is important to ensure oxygen stoichiometry and avoid substrate conduction, which can lead to erroneous estimates of the three-dimensional ͑3D͒ carrier concentration ͑n 3D ͒ from the measured 2D carrier concentration ͑n 2D ͒. 18,19 Samples were oxygen annealed after growth and substrates were confirmed to be insulating. Furthermore, the carrier concentration in the films closely corresponded to the Laconcentration, indicating that all La donors are ionized and donate one electron to the conduction band of SrTiO 3 over several orders of magnitude in doping concentration.…”