Thermoelectrical Properties Enhancement By ALD Synthesis of Lead Chalcogenide on Stripe Patterned Silicon Substrates
Authors: Chen Xin , Lin Pengtao , Zhang Kai , Baumgart Helmut 
Published: February 2020

As a green renewable technology, thermoelectric contributes to save energy by recovering waste heat into electricity based on Seebeck effect, and  the inverse process based on the Peltier effect can be used for cooling. The primary challenge is to increase the efficiency of thermoelectric materials, which is expressed by the dimensionless thermoelectric figure of merit, ZT = S2σT/
k, where S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity.  A promising approach is to use low dimensional structured materials in order to improve the ZT value, such as 2-D superlattice structure, 1-D nanowire, and 0-D quantum dots. Initially it was assumed that the quantum confinement effect in lower dimensional structure results in an increase of the density of state near the Fermi level and consequently in an enhancement of the thermal power (S2σ). However, most experimental works found reduction in thermal conductivity plays a dominate role in enhancing ZT value, resulting from phonon scattering by numerous interfaces in low dimensional structures.

Very recently, researchers focused their attention on phononic crystal (PnC) nanostructures in thermoelectric materials. The thermal conductivity of PnC samples is  lower compared to  non-patterned thermoelectric samples  [1] due to phonon-boundary scattering. It has been demonstrated  that porous silicon has lower thermal conductivity up to two orders of magnitude than bulk silicon [2, 3]. Our previous work indicates PbTe/PbSe nanolaminates grown on porous silicon membranes have higher Seebeck coefficients than the ones grown on regular silicon wafers. The higher Seebeck values result from the lower thermal conductivity k
in porous structures. Consequently thermal conductivity of the film could be reduced and in term ZT could be enhanced by adjusting the size and periodicity of the pattern and the thickness of the thermoelectric film in relation to the mean free path (MFP) of the phonons of the TE material.

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. This TE material can be synthesized by pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MO-CVD), magnetron sputtering, molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Since 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.

In this work we report on the successful synthesis of PbTe and PbSe on patterned silicon substrates by a thermal ALD system. The stripe patterned silicon substrates were fabricated by etching silicon wafers in KOH solution followed by photolithography process with a stripe mask. Figure 1 provides schematic diagram of the stripe patterned silicon substrate. Different sizes of the stripe pattern were fabricated to investigate size effect of the pattern on thermoelectrical properties of the films. 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 150 ˚C. The solid lead precursor was volatilized at a temperature of 170 ˚C, the liquid Te precursor required heating to 40 ˚C, and the liquid Se precursor was kept at room temperature. The chamber base pressure was kept at 30 mTorr. Since the MFP of PbTe or PbSe is around 10~20 nm. The film thickness was controlled around 10~20 nm, aiming to confine thermal transport along film coating direction and be scattered by closed patterns.

Several physical characterization techniques have been employed to determine the surface characteristics of the sample. The samples were characterizated by X-ray diffraction (XRD) for film crystal structure, and by field emission scanning electron microscopy (FE-SEM) for film morphology and structure. 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). For thermoelectrical properties of the sample, Seebeck coefficient, electrical conductivity and thermal conductivity of the films were measured to determine the ZT value of the samples as a function of pattern size.

 References:

1. Nakagawa, J., et al., Applied Physics Letters, 2015. 107(2): p. 023104.

2. Romano, G. and J.C. Grossman, Applied Physics Letters, 2014. 105(3): p. 033116.

3. Song, D. and G. Chen, Applied physics letters, 2004. 84(5): p. 687-689.

Figure 1