Absence of confinement in (SrTiO3)/(SrTi0:8Nb0:2O3) superlattices

TL;DR

This study investigates (SrTiO3)/(SrTi0.8Nb0.2O3) superlattices and finds that the expected quantum confinement of carriers is not present, with electrons remaining delocalized, challenging previous assumptions about their thermoelectric properties.

Contribution

The paper provides a detailed analysis showing that electrons in these superlattices are delocalized, contradicting the common confinement-based explanation for their high Seebeck coefficients.

Findings

Electrons are delocalized in both in-plane and growth directions.

Quantum confinement is unlikely in these superlattices.

High Seebeck coefficients are not due to confinement.

Abstract

The reduction of dimensionality is an efficient pathway to boost the performances of thermoelectric materials, it leads to the quantum confinement of the carriers and thus to large Seebeck coefficients (S) and it also suppresses the thermal conductivity by increasing the phonon scattering processes. However, quantum confinement in superlattices is not always easy to achieve and needs to be carefully validated. In the past decade, large values of S have been measured in (SrTiO3)/(SrTi0:8Nb0:2O3) superlattices (Nat. Mater. 6, 129 (2007) and Appl. Phys. Lett. 91, 192105 (2007)). In the $\delta$-doped compound, the measured S was almost 6 times larger than that of the bulk material. This huge increase has been attributed to the two dimensional confinement of the carriers in the doped regions. In this work, we demonstrate that the experimental data can be well explained quantitatively within the scenario in which electrons are delocalized in both in-plane and growth directions, hence strongly suggesting that the confinement picture in these superlattices may be unlikely.