Advances in Atomic Layer Deposition (ALD) Nanolaminate Synthesis of Thermoelectric Films in Porous Templates for Improved Seebeck Coefficient

Abstract

Thermoelectrics is a green renewable energy technology which can significantly contribute to power generation due to its potential in generating electricity out of waste heat. The main challenge for the development of thermoelectrics is its low conversion efficiency. One key strategy to improve conversion efficiency is reducing the thermal conductivity of thermoelectric materials. In this paper, the state-of-the-art progresses made in improving thermoelectric materials are reviewed and discussed, focusing on phononic engineering via applying porous templates and ALD deposited nanolaminates structure. The effect of nanolaminates structure and porous templates on Seebeck coefficient, electrical conductivity and thermal conductivity, and hence in figure of merit zT of different types of materials system, including PnCs, lead chalcogenide-based nanostructured films on planar and porous templates, ZnO-based superlattice, and hybrid organic-inorganic superlattices, will be reviewed and discussed.

Keywords: PbSe; PbTe; Seebeck coefficient; atomic layer deposition; lead chalcogenide; nanolaminates; thermoelectric.

Figure 1

Simulation results of the dependence of efficiency on figure of merit ZT of thermoelectric (TE) device and temperature difference [1].

Figure 2

Major milestones achieved for zT as a function of temperature. p-type Bi₂Te₃ [5], n-type Bi₂Te₃ [6], GeSi [7], Pb_1-xEu_xTe/PbTe quantum well [8], CsBi₄Te₆ [9], p-type Bi₂Te₃/Sb₂Te₃ SL [10], n-type Bi₂Te₃/Bi₂Te_2.83Se_0.17 SL [10], n-type (Bi,Sb)₂(Se,Te)₃ Quantum Dot SL [11], AgPb_mSbTe_2+m (m = 10,18) [12], Na_0.95Pb₂₀SbTe₂₂ [13], Ba₈Ga₁₆Ge₃₀ [14], Si nanowire [15,16], Zn₄Sb₃ [17], Pb_1-xSn_xTe /PbS (x = 0.08) [18], Ti doped PbTe [19], Na doped PbTe_0.85Se_0.15 [20], TiS₂/Organic (ip) [21], Mg₂Sn_0.75Ge_0.25 [22], Bi₂Te₃ nanoplates [23].

Figure 3

Electronic density of states as a function of energy for (a) 3-D bulk semiconductors, (b) 2-D quantum well or superlattices structures, (c) 1-D nanowire or nano-tube structures, and (d) 0-D quantum dots structures.

Figure 4

Schematic of phonon scattering mechanisms and electron transport within thermoelectric phonon glass electron crystal (PGEC) model material [44].

Figure 5

(a) Thermal conductivity of a Phononic Crystal (PnC) vs. the radius of the holes in porous templates. The inserted image is a schematic diagram of a PnC with hole-center distance of 300 nm [63]. (b) Theoretical simulation of the dependence of thermal conductivity on the configurations of variously shaped pores in porous membranes [67].

Figure 6

FE-SEM micrographs of (a) 1000 ALD deposition cycles of PbTe film without pre-treating the Si substrate resulting not in a dense film, (b) compact PbTe/PbSe (10/10 nm) nanolaminate film grown on hydroxyl OH^- terminated Si substrate [1], (c) TEM cross-sectional images highlighting the onset of the early stages of nucleation of an ALD deposited PbTe film of 700 ALD deposition cycles grown at 170 °C on Si substrate [72], (d) featuring thicker PbTe/PbSe (10/10 nm) nanolaminate layers, when all the initial nuclei have finally coalesced into continuous compact layers grown on a Si substrate at 150 °C [73].

Figure 7

(a) FE-SEM images of PbTe/PbSe (10/10 nm) nanolaminates grown on porous Si substrate with misaligned staggered square pores of 700 nm × 700 nm size, which are horizontally spaced 1.72 μm apart, and 0.6 μm vertically apart [1]. (b) cross-sectional images of PbTe/PbSe nanolaminates with period of 10 nm [73]. (c) and (d) SEM image of ALD deposited PbTe/PbSe on porous Si templates [68]. (e) and (f) are ALD deposited Sb₂Te₃ film on porous Si templates with square shaped pore structures from the team at MicroXact Inc. [71].

Figure 8

Plot of Seebeck coefficient as a function of temperature comparing ALD PbTe/PbSe (10/10 nm) nanolaminates deposited on planar Si substrates, versus ALD deposition on microporous silicon templates with small square shaped pores, and versus porous silicon templates with thinner pore wall and larger pore diameter, measured by MMR Seebeck measurement system. The data were extracted from previous published works of Old Dominion University [1,73].

Figure 9

The cumulative thermal conductivity of the PbTe-PbSe, with respect to mean free path of phonons at 300 K, shown as dotted lines. The data were extracted from ref [77] and are based on theoretical calculations. The superimposed symbols with error bars were individual experimental thermal conductivity measurements of our ALD PbTe, PbSe, PbTe-PbSe nanolaminates, grown on planar and porous silicon substrates with respect to their total thickness. The thermal conductivity data were measured by time domain thermo-reflectance (TDTR) at University of Virginia (UVA). The data were extracted from ref [78] and [73].