ALD Synthesis of Ternary PbSeTe Layers Alternating with the Binary PbTe in Nanolaminate Structures for High Zt Thermoelectric Materials

Thermoelectric materials have attracted much attention for their potential in conversion between heat and electricity and consequently could be used for heating and cooling systems. The efficiency of a thermoelectric material is expressed by the dimensionless thermoelectric figure of merit, ZT = S²σT/k, where S is the Seebeck coefficient, σ is the electrical conductivity, and kis the thermal conductivity [1].  Previous work demonstrated alloying was used to decrease the lattice thermal conductivity and thus to increase ZT owning to mass difference enhanced phonon scattering.  In order to further progress in the development and commercialization of the next generation of thermoelectric devices it is necessary to enhance the ZT value. Most recently research focused on low dimensional nanostructures, including quantum wells, quantum dots, superlattice, quantum wires and nanolaminates to improve ZT. In superlattice structures, reduced lattice thermal conductivity is mainly due to the phonon scattering   at multiple interface boundaries. Significant efforts have been made to investigate the IV-VI semiconducting lead chalcogenides, such as PbTe and PbSe due to their high figure of merit, good chemical stability, low vapor pressure and high melting point.  Therefore they are promising thermoelectric materials for intermediate temperature applications ranging from 600 to 800 K.

PbTe and PbSe films have been deposited by different techniques, such as pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MO-CVD), magnetron sputtering, molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Among these, ALD is considered as a novel and competitive method to deposit PbTe/PbSe nanolaminate structures. The ALD is a self-limiting atomic layer reaction in each ALD cycle introduce. It can precisely control the film layer thickness, stoichiometry, composition, uniformity, and sharp interface. ALD also can be used to deposit conformal films onto very complex structures. It is possible to generate the reproducible and well-defined nanolaminate structures. Furthermore, the deposition temperature of ALD is rather low in comparison with other growth techniques.

In this work we report on the successful synthesis of multiple PbTe/PbSeTe nanolaminates on silicon substrates by a thermal ALD system. In this new nanolaminate structure a very thin layer of the PbSeTe ternary compound is formed by nucleation of PbSe quantum dots between binary PbTe layers, which is followed by an annealing process step. It is noted that the formation of the PbSe quantum dots is facilitated the prevailing Volmer-Weber island growth mode.  Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C₁₁H₁₉O₂)₂), (trimethylsilyl) telluride ((Me₃Si)₂Te)  and (trimethylsilyl) selenide ((Me₃Si)₂Se) were employed as the chemical  ALD precursors for lead, telluride and selenide, respectively. 20 sccm N₂ was used as a carrier gas to transport the chemical precursors into the ALD reaction chamber. The ALD growth temperature was 170 ^oC. The solid lead precursor was volatilized at a temperature of 140 ^oC, the liquid Te precursor required heating to 40 ^oC, and the liquid Se precursor was kept at room temperature. The chamber base pressure was kept at 30 mTorr.

Several physical characterization techniques have been employed to determine the ALD nanolaminate formation. The crystal structure was analyzed by X-ray diffraction (XRD). The film morphology and structure of the products were determined by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM).The surface roughness was analyzed by atomic force microscopy (AFM). The analysis of the composition and stoichiometry of the ternary and binary layers were carried out by X-ray photoelectron spectroscopy (XPS) and Energy dispersive X-ray spectroscopy (EDS).

References:

1. K. Zhang, A. D. Ramalingom Pillai, D. Nminibapiel, M. Tangirala, V. S Chakravadhanula, C. Kübelc, H. Baumgart, V. Kochergin, ECS Trans., 58, 10, 131 (2013).