Pulsed Laser Deposition of Bi2Te3 based thermoelectric thin films

Abstract

Thin film thermoelectric coolers offer several advantages that include reliability and integration with device processing. Successful thin film thermoelectric cooling requires integration with thin film diamond or aluminum nitride heat spreaders and the device wafers. Numerous deposition techniques have been attempted previously including evaporation, flash evaporation, molecular beam epitaxy, chemical vapor deposition, and sputtering. In the case of thermoelectric thin films, a primary difficulty is maintaining stoichiometry. In the present effort, thin films of p-type Bi[subscript 0.5]Sb[subscript 1.5]Te₃, n-type Bi₂Te[subscript 2.7]Se[subscript 0.3] and n-type (Bi₂]Te₃)₉₀(Sb₂Te₃)₅(Sb₂Se₃)₅ (with 0.13 wt.% SbI₃) were deposited on mica and aluminum nitride substrates using Nd-YAG pulsed laser deposition.

The film quality in terms of composition and crystal perfection was studied as a function of growth temperature. The films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) for crystalline quality, and by scanning electron microscopy (SEM) for surface morphology. The films showed uniform thickness and high crystalline quality with a preferred (0 0 n) alignment with the substrates. The Seebeck coefficient, electrical resistivity and Hall mobility were measured and compared with the bulk properties. An improvement in the thermoelectric properties by reduction in laser induced particulates has been demonstrated by the use of lower incident laser energy. The thermoelectric characteristics of the films deposited on AlN⁄ Si substrates were found to be superior to those deposited on mica substrates. X-ray mapping and energy-dispersive-spectroscopy were performed to determine the composition and homogeneity of the thin films. The results showed that pulsed laser deposition has the ability to produce congruent transfer of the target composition to the thin films under the suitable conditions of lower laser fluence, low density plume and ideal substrate temperature. The present work has illustrated the use of AlN⁄ Si (an efficient heat spreader) as a favorable substrate material for thin film deposition. Thus, it has been shown that there exists great potential for producing efficient thermoelectric thin films by means of pulsed laser deposition. In addition, the ability to deposit nanocrystalline thermoelectric thin films has also been demonstrated.