Exploring electronic and thermoelectric properties in Nb-doped SrTiO₃ thin films

The Internet of Things (IoT) represents a transformative network of interconnected sensors and actuators that has profoundly impacted both our everyday life as well as industrial processes. By enabling extensive data sharing and guiding decision making, IoT devices have been seamlessly integrated into diverse applications, ranging from wearable technology and health monitoring to advanced home automation systems.

With IoT devices projected to reach trillions by 2040 [1], managing their substantial energy demands [2–4] becomes increasingly urgent. Energy harvesting techniques—such as those converting light, vibrations, or waste heat into electrical power—have have emerged as promising renewable alternatives [5–7]. Among these, thermal energy harvesting, which leverages the thermoelectric effect with available waste heat, stands out as a promising solution for applications due to its long lifetimes, absence of movable parts, and capability to operate without sunlight [5,6,8–10], despite current challenges related to material efficiency and availability [11, 12].

In this thesis, an overview is first provided of the properties and maximum efficiencies of thermoelectric materials using well-performing bulk compounds at high temperatures. A large set of simulations evaluating the efficiencies of segmented thermoelectric materials is discussed, where several high-performing thermoelectric materials are joined together to enhance overall efficiency [13]. Optimal segmented pairs of materials with significant efficiency gains were identified at relatively high temperature gradients (ΔT = 500 − 600 K), describing combinations that could potentially reach efficiencies of 15-24%, as long as tolerable contact resistances are not exceeded. Notable combinations included transition metal tellurides with lead tellurides, magnesium antimonides with various compositions, and metallic tellurides with selenide alloys.

Following these simulations, the experimental work focuses on exploring the electronic and thermoelectric properties of thin films composed of the inorganic perovskite system: doped strontium titanate (SrTiO₃). The large reported thermoelectric power factors [14–20] indicate that n-type doped-SrTiO₃ can not only be an excellent thermoelectric energy harvester but also host fascinating physics typical of complex oxides. Nb-doped SrTiO₃ (Nb:SrTiO₃) was synthesized using pulsed laser deposition, and the growth conditions were experimentally modified to alter both the electrical and thermoelectric properties while evidencing property-defect relationships in the perovskite material. Moreover, low-temperature investigations of the electrical transport in the thin films revealed a variety of phenomena ranging from metallic-to-insulating transitions to possible signs of Kondo physics. However, the thermoelectric effect measured in Nb:SrTiO₃ Hall bar devices showed relatively small power factors at room temperature, PF = 1−2 μW cm⁻¹ K⁻², compared to those of other reported stoichiometric thin films or bulk crystals.

Electrolyte gating with an ionic liquid was used to tune and enhance the thermoelectric properties in the Nb:SrTiO₃ Hall bars. Thinner films and lower doping contributed to the largest ionic gating response due to their lower number of carriers. This approach allowed for up to a nearly 18.5-fold enhancement of the power factor in Nb:SrTiO₃ devices when the gate voltage was swept from -2 V to 3 V, a result that cannot be achieved by conventional electrostatic gating.

Finally, we focused on integrating SrTiO₃ onto silicon and sapphire substrates, a crucial step for large-area SrTiO₃ growth, semiconductor integration, and scalable device fabrication. Despite the different crystal structures, we achieved epitaxial thinfilm deposition on silicon and sapphire substrates using a commercial SrTiO₃-buffered silicon substrate and a custom-made gamma-Al₂O₃-buffered sapphire substrate, respectively. For the SrTiO₃-buffered silicon substrate, we found well-conductive epitaxial thin films with structural and electronic properties similar to those of epitaxial Nb:SrTiO₃ films deposited on single-crystal perovskite substrates. The conductivities of these epitaxial films were superior to their polycrystalline counterparts obtained by deposition on silicon wafers terminated with Si₃N₄ or SiO₂. For Nb:SrTiO₃ thin films deposited on sapphire, the intermediate layer of spinel gamma-Al₂O₃ formed at the sapphire surface using solid-phase epitaxy was essential for enabling the epitaxial growth of Nb:SrTiO₃. However, these films showed lower conductivity and thermoelectric response compared to those on single-crystal perovskite substrates, although a significant enhancement was achieved compared to polycrystalline SrTiO₃ films deposited directly on sapphire.

In summary, this thesis explores the potential of doped SrTiO₃ for thermoelectric energy harvesting, emphasizing both simulation and experimental approaches. Key findings include the substantial enhancement of thermoelectric properties through electrolyte gating and successful integration of SrTiO₃ on silicon and sapphire substrates, demonstrating promising pathways for scalable device fabrication. These advancements contribute to the broader goal of developing efficient and versatile thermoelectric materials for future IoT applications.