**6. Conclusion**

Koren et al. observed the local superconductivity in Bi2Te2Se and Bi2Se3 films below 2–3 K, which was naturally induced by small amounts of superconducting Bi inclusions or precipitations on the surface [78]. Moreover, Le et al. reported superconductivity at an onset critical temperature of approximately 3.1 K in a topological insulator 200-nm-thick Bi2Te3 thin film grown by pulsed laser deposition [53]. Indeed, **Figure 12a** shows the normalized resistivity *ρ/ ρ*300 K of a 46-nm-thick Bi2Te3 film (S1) and a 200-nm-thick Bi2Te3 film (S2) as functions of temperatures (*T*) from 1.8 to 300 K. Both films show a decrease in resistivity (*ρ*) with decreasing *T* in the range of 20–300 K, implying that the films exhibit weak metallic properties commonly seen in narrow band-gap semiconductors with high carrier concentrations [53]. Below 20 K (**Figure 12a**, the lower inset), the *ρ/ρ*300 K of S1 shows a gentle upturn because of the weak localization of electrons [7], whereas the *ρ/ρ*300 K of S2 reaches a plateau before dropping slightly at *Tc1* ≈ 5.8 K and then sharply at *Tc2* ≈ 3.1 K. **Figure 12b** further shows the *H*||c-dependent *ρ*(*T*) of S2 in the low *T* regime, where *H*||c is the applied magnetic field along the *c*-axis of the

76 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

*T* = 1.8 K. This nonzero *ρ* at low *T* indicates that the superconducting volume ratio is not 100%.

(1

with increasing *H||c* from 0 to 0.2 T, strongly indicating that both transitions are supercon-

The detailed investigations of S2 strongly suggest the existence of superconducting Bi nanoclusters on the surface that induce the *Tc1* ~ 5.8 K. EDS lateral elemental mapping revealed that the distributions of Te and Bi were not uniform, and many Bi-rich (47–54 at.%) clusters were visible (green color), as shown in the upper inset in **Figure 12a**, differing substantially from the uniform distribution and cluster-free surface observed in film S1. The size distribution and the most probable size of Bi-rich clusters are in the range of 400–2400 nm and 560–772 nm, respectively. Intriguingly, a closer inspection reveals that Bi-rich clusters are composed by some

The Bi-rich environment on the film surface is confirmed by AES analysis (**Figure 12c**) [53]. This is because the vapor pressure of Te (at *T*<sup>S</sup> = 300°C) is approximately 105 times higher than that of Bi, and therefore, more Te atoms are re-evaporated from the 300°C substrates [79]. The loss of Te is more severe in film S2 than in film S1 (14.3% in S2 and 4.5% in S1 at depth *Z* = 0, **Figure 12c**) because of the six times longer deposition time of S2 (60 min) compared to S1 (10 min). In addition, the nonstoichiometric effect is strongly depth-dependent (**Figure 12c**). The Te/Bi ratio gradually increases toward the stoichiometric ratio of 3/2 in 200-nm-thick films or slightly exceeds it (Te-rich) in 46-nm-thick films when the depth (*Z*) of the films increases. Under such a sufficiently high surface concentration of Bi atoms, the Bi clusters precipitate and segregate readily on the S2 surface to minimize overall free energy, as long as the substrate temperature of 300°C is higher than the 271°C melting point of Bi, as demonstrated in **Figure 12f**. Notably, the Bi clusters can only be observed in highly Bi-rich (14.3% at *Z* = 0) films (S2) and not in low Bi-rich (4.5% at *Z* = 0) films (S1), suggesting a critical Bi-rich concentration for Bi precipitation (separating a Bi phase) in a Bi2Te3 film [53]. The *Tc1* at ~5.8 K found in our samples should be induced by the superconducting transition of the Bi nanoclusters, which is

but does not go down to zero, even at

) decreases from 3.1 (5.8) to 1.8 (5.4) K

film. At *H*||c = 0, *<sup>ρ</sup>* drops abruptly by 8% below 2

Bi nanoclusters (or nanograins) with a size of 20–62 nm.

The inset in **Figure 12b** shows that the 2

ducting in nature.

This chapter provides the effects of ambient pressures and substrate temperatures in PLD growths on the structural-morphology, thermoelectric, nanomechanical, and magnetoresistance properties of bismuth chalcogenide thin films. The thermoelectric power factor of the stoichiometric Bi2Te3films grown in the range of 220–340°C and *PAr* of 80 Pa reached remarkably high values, ranging between 18.2 ± 0.25 and 24.3 ± 0.44 μW cm−1 K−2. The optimal *PF* values were 5.54 ± 0.34 μW cm−1 K−2 for the layered hexagonal platelet Bi2Se3 films deposited at 300°C and *P*He of 40 Pa and 8.3 μW cm−1 K−2 for the nanocrystalline Bi3Se2Te films deposited at 250°C and *P*He of 40 Pa. We also reported the effects of *P*He in PLD on nanomechanical properties (i.e., hardness and Young's modulus) of Bi2Te3 and Bi3Se2Te thin films. It was observed that the hardness and Young's modulus increased with increasing *P*He, depending on the grain sizes following the inverse Hall-Petch effect for Bi2Te3 films grown at *P*He ≤ 2.0 × 10−3 Torr and following the Hall-Petch relationship for Bi3Se2Te grown at *P*He of 2.0 × 10−5 to 6.5 × 10−1 Torr. PLD has been successfully employed to grow epitaxially bismuth chalcogenide thin films on large-misfit substrates, for example, Bi2Te3/SrTiO3 (1 0 0), Bi2Se3/Al2O3 (0 0 0 1), and Bi3Se2Te/ Al2O3 (0 0 0 1). The magnetotransport studies show that the bismuth chalcogenide thin films such as Bi2Te3, Bi2Se3, and Bi3Se2Te films present a two-dimensional weak antilocalization effect in a low magnetic field (*B*) regime and linear magnetoresistance in a high *B* regime, which could be attributed to the topological insulator surface states. Furthermore, proximity-induced superconductivities in Bi2Te3 thin films have an onset *Tc* of approximately 3.1 K, evidently induced by Bi inclusions (nanoclusters with onset *Tc* at 5.8 K) segregated on the surface of films.
