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