ALD Synthesis of PbSe Thin Films inside Porous Si Templates for Innovative Thermoelectric Applications

The IV-VI lead chalcogenides are considered as prospective thermoelectric materials which have their potential applications in green renewable energy by waste heat recovery and infrared devices. Among these materials, PbSe demonstrates a remarkable thermoelectric performance, The thermoelectric figure of merit ZT  exceeds 1 for both n-type and p-type PbSe at high temperature. ZT is defined as ZT=σS²T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity, respectively. In addition, Se element is more abandant and less costly than Te which is a scarce element [1].

PbSe thin films on Si substrates have been synthesized by various methods. However, a careful  literature search reveals, there are few if any reports on the synthesis of PbSe thin films by atomic layer deposition (ALD). The ALD technique exhibits self-limiting surface reactions in each ALD cycle which consequently results in precise layer thickness control, stoichiometry, composition, uniformity of large area on the substrate. ALD also can be used to deposit conformal film onto very complex substrates or nano-porous template surfaces. This is the one advantageous property of ALD which renders it more competitive and promising in porous template nano-structures. In contrast, magnetron sputtering and PLD cannot be used to coat complex 3D structures with PbSe. Furthermore, the growth temperature of ALD is rather low in comparison with other fabrication processes.

In this work we report on the successful synthesis of multiple PbSe thin films on silicon substrates and inside  microporous Si templates by  thermal ALD, using lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C₁₁H₁₉O₂)₂) and (trimethylsilyl) selenide ((Me₃Si)₂Se) as the chemical  ALD precursors for lead and selenide, respectively. The repetitive structure of nano-porous Si templates offers an additional degree of periodicity in the quest for high ZT thermoelectric films.

The XRD and FE-SEM results reveal that the Lead selenide thin films  are polycrystalline at deposition temperatures of 170 °C based on Volmer Weber Island growth mode and have characteristic FCC  rock-salt crystal structure as shown in Figure (1), (2) and (3).

References:

H. Wang, Z. M. Gibbs, Y. Takigawa, G. J. Snyder, Energy Environ. Sci., 7, 804 (2014).