(Invited) Seebeck Coefficient Measurement of Pbsete / PbTe Nanolaminate Structures

Thermoelectric (TE) materials can be applied for solid state power generation or heating/cooling systems that convert heat into electricity and vice-versa. The green renewable technology related to thermoelectrics could play an important role to decrease the energy consumption from fossil fuels and decrease greenhouse gas emission. The main challenge for thermoelectric devices produced currently is their low efficiency. Therefore, it is essential to develop high efficiency TE materials and devices with superior performance. The efficiency of TE materials 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 has demonstrated two main TE materials have higher ZT values. The first TE material with a complex crystal structure called a phonon glass electron crystal in which the voids, defects, and rattlers could scatter phonons to decrease the lattice thermal conductivity, and thus to increase ZT. Another one is called nanomaterials: including quantum wells, quantum dots, superlattice, quantum wires and nanolaminates, which introduce a large density of interfaces to enhance phonon scattering. The thermal conductivity of these nanomaterials was effectively reduced, and the ZT of TE materials was improved significantly. The IV-VI semiconducting lead chalcogenides, such as PbTe and PbSe, have been investigated 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.

There are different techniques that can be used to synthesis PbTe and PbSe, including pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MO-CVD), magnetron sputtering deposition, molecular beam epitaxy (MBE) and atomic layer deposition (ALD). The ALD is a self-limiting technique that allows atomic layer growth each time.
ALD can precisely control the film layer thickness, stoichiometry, composition, uniformity, and sharp interface. ALD also shows perfect conformal coverage when it is deposited on complex surface structures. Therefore, ALD is considered as a novel and competitive method to deposit PbTe/PbSe nanolaminate structures. It is possible to generate the reproducible and well-defined nanolaminate structures. In addition, the ALD deposition can be operated at rather low temperature compared to other techniques.

In this work we report on the Seebeck coefficient measurements in PbSeTe / PbSe nanolaminate structures. The multiple PbTe/PbSeTe nanolaminates were deposited on silicon substrates by a thermal ALD system. This new nanolaminate structure was achieved by first depositing a very thin layer of the PbSeTe ternary compound with binary PbTe layers. The PbSeTe ternary compound is formed by nucleation of PbSe quantum dots between binary PbTe layers, and followed by an annealing process step. It is noted that the formation of the PbSe quantum dots is facilitated by 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). The Seebeck coefficient of the sample was also measured in order to calculate ZT value using Seebeck coefficient measurement equipment of MMR SB100.

Figure 1 shows an FE-SEM image of surface morphplogy of PbTe/PbSe nanolaminate films on silicon substrate.  

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).