**2.4. Preparation of bismuth chalcogenide films by PLD**

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 then deposit thin films onto a substrate. **Figure 3** shows a typical PLD system for preparing TE and TI thin films. A substrate is heated and maintained at a desired substrate temperature (*T*S) using a thermocouple and a proportional-integral-derivative temperature controller. The thermocouple was buried inside a stainless steel substrate holder, which is heated by a tungsten lamp just behind the holder. The pressure of ambient gas (He/Ar) can be fine-tuned by the needle valve. A KrF excimer laser beam (*λ* = 248 nm, pulsed duration of 15–20 ns, repetition rate in the range of 1– 15 Hz, and fluence of 1–10 J/cm2 ) is guided by several UV mirrors and focused on a stoichiometric polycrystalline or a single crystal target (e.g., Bi2Te3, Bi2Se3, Bi2Se2Te) within the vacuum chamber by a UV lens. The target-to-substrate distance was 40 mm. During the deposition of Bi2Se3 films, pure (6N) He/Ar gas was introduced into the vacuum chamber, which was evacuated to a base pressure of 4 × 10−4 Pa (or 3 × 10−6 Torr) and maintained at a certain constant pressure (*P*), using a differential evacuation system.

After an initially random nucleation of islands on the surface of the substrates, the deposition on the top of the islands is faster than that in the valleys due to the oblique incident flux (the so-called **shadowing effect**) [12, 13]. Isolated columns are therefore formed on these islands

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

Epitaxy refers to the growth of a single crystal film on top of a single crystal substrate. The deposited film is denoted as an epitaxial film or epitaxial layer. The growth is called homoepitaxy if the film and the substrate are the same material, and it is called heteroepitaxy if they are different materials. Epitaxial relationship is determined as: (*HKL*) || (*hkl*); [*UVW*] || [*uvw*], where (*hkl*) and (*HKL*) are the Miller indices of the overgrowth plane and substrate at the common interface. The corresponding parallel directions in the overgrowth and substrate

The key factors governing epitaxy growths are structural compatibility, chemical compatibility,

**- Structural compatibility:** The structures of a film and a substrate should have good lattice matching in terms of crystal structures (*a*0, sub) and lattice constants (*a*0, film), that is, small lattice

*aa aa <sup>f</sup> a a* (1)

0, 0, 0, 0, 0, 0, 2. - - <sup>=</sup> » *sub film sub film film sub*

**- Chemical compatibility:** This includes chemical bonding and chemical diffusion.

**2.4. Preparation of bismuth chalcogenide films by PLD**

**- Growth temperatures:** Good epitaxy growth is obtained at above or around the well-defined elevated substrate temperature (*Te*). *Te* depends on the deposition rate, particle energy, and surface contamination. Generally, a higher temperature is recommended to reduce surface contamination by desorption (or enhance surface mobility) of atoms to reach the favorable sites and also enhance the diffusivity in deposition for favoring re-crystallization and defect

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 then deposit thin films onto a substrate. **Figure 3** shows a typical PLD system for preparing TE and TI thin films. A substrate is heated and maintained at a desired substrate temperature (*T*S) using a thermocouple and a proportional-integral-derivative temperature controller. The thermocouple was

planes, denoted by [*wuw*] and [*UVW*], respectively, must also be specified.

during subsequent growths (**Figure 2d**) [13].

**2.3. Factors governing the epitaxy growth**

and growth temperatures.

misfit.

Lattice misfit *f*:

annihilation.

**2.2. Epitaxy growth**

**Figure 3.** Schematic illustration of a pulsed laser deposition (PLD) system. G: gauge.

The surface of substrates should be atomically clean and free from impurities because the contaminants can interact with the thin films being deposited and substantially degrade its quality and adhesion to the substrates. The presence of unwanted surface contaminants can also influence the growth and orientation of the films in an undesired manner. In the depositions for TE thin films, an approximately 300-nm-thick SiO2 layer was thermally grown on the Si wafers (thickness 525 μm) for electrical isolation purpose. The wafers were cut into 1.5 cm × 1.5 cm substrates. The substrates were cleaned with acetone to dissolve any contaminants adhering to the surface of substrates such as grease and oils. This was followed by rinsing with methanol to remove any residues left behind after cleaning with acetone. Afterward, the substrates were rinsed in distilled water and dried with nitrogen flow. The substrates were then used for the deposition of TE thin films.

Here are some examples of PLD growth of TE films. For Bi2Se3 thin films, the depositions were at *T*<sup>S</sup> of 200–350°C and helium ambient pressure (*P*) of 0.7–173 Pa. The number of laser pulses was 9000 and the deposition took 30 min. The average growth rate was approximately 0.46 Å/ pulse [14]. For the growth of Bi2Te3 thin films, *T*<sup>S</sup> was varied from room temperature (30°C) to 380°C and the Ar ambient pressure (*PAr*) was at 80 Pa. The number of laser pulses was 12,000 and the deposition took 40 min. The average growth rate was approximately 0.52 Å/pulse [15]. For the growth of Bi-Se-Te thin films, the depositions were at *T*S of 200–350°C and a helium ambient pressure (*P*He) of 0.027–86.7 Pa. The number of laser pulses was 9000 and the deposition took 15 min. The average growth rate was approximately 0.6 Å/pulse [16].
