**2.1 PLD growths of nanostructured Bi2Te3-based thin films**

PLD is one of the most convenient thin film growth techniques that uses a high intensity pulsed laser beam as an external energy source to ablate a target, form a plume, and deposit thin films onto a substrate. In practice, a large number of variables affect the properties of the film, such as substrate temperature (*Ts*), background gas pressure (*P*) and laser fluence. **Figure 8** shows a PLD system for preparing thermoelectric thin films [38, 39]. The substrate was heated and maintained at desired *Ts* using a thermocouple and a proportional-integral-derivative temperature controller. The thermocouple was buried inside a substrate holder which was heated by a tungsten lamp or electrical resistance heating. The pressure of ambient gas (He, Ar) could be fine-tuned by the needle valve. Laser source can

• Small refrigerator devices are used for camping and outdoor activities. For example, the cooler/warmer TE device (Engel Thermo 8) has volume 8 L and weighing just over 3 kg. Its features include cooling performance up to 22°C

• Gentherm designed and developed Automotive Climate Control Seat [36], which has TE heat pumps in the back and bottom cushions. The TE system makes conditioned flowing air through channels to the occupant for providing on-demand cooling or heating. As shown in the first panel in **Figure 6**, the seat has the heat pump consisting of a TE module (green box) and a fan (orange).

• Thermal management of tiny laser diodes is used in fiber optic telecom, datacom backhaul networks. TEC can also be used for contact cooling of semiconductor lasers, infrared detectors, CCD- matrix, and miniconditioners

• Localized cooling at hot spots of chips was created. For example, the Intel group is the first to demonstrate both concepts of applying the TE material only to a chip's hottest spots (**Figure 6**) [33, 37]. On the substrate, the researchers grew a 100-μm-thick layered structure, called a superlattice, containing bismuth, tellurium, antimony, and selenium. The structure can pump 1300W/cm2 heat from the back side of the chip to the heat spreader. The superlattice induced an approximately 6°C temperature drop at the hot spot even before the device was powered up, because it conducts heat better than the grease that bonds the rest of the heat spreader to the chip. Yet, when a 3 Acurrent went through the thermoelectric cooler, the total temperature change was only of 15°C. Managing heat in electronics is a common issue, and TE

below ambient temperature and warming up to +65°C.

*Overview of potential thermoelectric cooling (TEC) applications [33, 34].*

for photomultipliers.

**Figure 6.**

*Materials at the Nanoscale*

**48**

**Figure 8.** *A schematic illustration of a PLD system.*

be KrF excimer laser beam (λ = 248 nm) and Q-switched Nd:YAG laser (λ = 355 nm) with properly selected laser fluence (e.g., 3.8, 6.2, or 8.3 J/cm<sup>2</sup> ) pulsed duration of 5–20 ns, repetition rate of 5–10 Hz [38, 40–42]. The laser beam was guided by several UV mirrors and focused on a stoichiometric polycrystalline target (e.g., Bi2Se3, Bi2Te3, Bi0.5Sb1.5Te3, etc.) inside the vacuum chamber by the UV lens. The deposition chamber was evacuated to a base pressure of <sup>10</sup><sup>6</sup> Torr, and highpurity ambient gas (He or Ar) was then introduced until obtaining a target pressure (e.g., usually 10<sup>5</sup> – <sup>3</sup><sup>10</sup><sup>1</sup> Torr).

The enhancement of the PF of Bi2Te3-based thin films is challenging due to the coupling among TE material properties [3], and the difficulty in growing stoichiometric films [38]. Indeed, stoichiometry is a key factor for obtaining better TE properties [5, 38, 43–45]. Yet, both tendency for re-evaporation of volatile elements (i.e., Te, Se) at elevated *Ts* [45–48] and the low sticking coefficient Te (< 0.6 for Bi2Te3) at *Ts* beyond 300°C [49, 50] constrain to grow stoichiometric Bi2Te3-based films (**Figure 9a** and **b**).

horizontally stacked for 600°C-film (**Figure 10a–d**). Moreover, the bottom of each of the deposited super-assemblies has a relatively continuous and dense layer, and this layer thickness increases with increasing substrate temperature from 350–450° C (**Figure 10a–d**). The top-view SEM images confirm for the high uniformity and presents the unique super-assembling features of the repetitively and regularly assembled nano-flakes (**Figure 10e–h**). These four films are uniformly composed of

*(a)–(d) The cross-sectional and (e)–(l) the corresponding top-view SEM images of the Bi2Te3 superassemblies*

*(a) Vapor pressures of Bi, Sb,Te, Se, Bi2Se3, and Bi2Te3 as a function of temperature [46]. (b) The variation of sticking coefficient Ks (Bi,Te) as a function of substrate temperature Ts at fixed flux ratio FR = 4.5 [49].*

*Nanostructuring Bi2Te3-Based Thermoelectric Thin-Films Grown Using Pulsed Laser Deposition*

*DOI: http://dx.doi.org/10.5772/intechopen.99469*

spindle-like (**Figure 10e**), worm-like (**Figure 10f** and **g**) and island-like (**Figure 10h**) hierarchical nanostructures. Magnified top-view SEM images (**Figure 10i–k**) further show that the nanoflakes are composed of oriented and regular assemblies of numerous rice-like and elongated primitive nanoparticles [41]. At a higher substrate temperature, thin- and large-size nanoflakes are formed from packing of dense rice-like nanoparticles, driving by the relatively sufficient thermal energy for diffusion. In addition, the out-of- plane superassembly structure (600°C) has a limited column width, which is not always consistent along the outof-plane direction (**Figure 10d**). Also, the parallel nano-flakes (at 600°C) are

*deposited at 350°C, 400°C, 450°C, and 600°C, respectively [41].*

**Figure 10.**

**51**

**Figure 9.**
