**2. Experimental**

Using Molecular Beam Epitaxy (MBE), the thin films of Bi<sup>2</sup> Te3 were grown on 7 × 7 mm dimension Al<sup>2</sup> O3 (0001) substrate. Prior to the deposition, the base pressure was maintained at ~ 8 × 10−10 mbar. To evaporate Bi (99%) and Te (99.9%) sources, the standard Knudsen diffusion cells were used and the Te and Bi were heated to 205 and 630°C respectively. At last at Te/Bi flux ratio of ~10 with a growth rate of 8 Å per minute the Bi<sup>2</sup> Te3 Thin films were prepared at a substrate temperature of 230°C. By defining the pattern, using standard lithographic techniques like electron-beam lithography or photolithography, numerous methods in the recent literatures [19–22] were reported in making thin film hall bar geometry but in our work, without using any resists and lithography techniques, we applied a clean method of making a topological insulator thin film hall bar device. In our device fabrication, we employed two physical masks, for the same sample holder. One for using in reactive ion etching (RIE), called etching mask and other for depositing metal electrodes, called metal mask. For defining the dimension of the thin film hall Bar, the etching mask served the purpose and the metal mask served the purpose of depositing the metal electrodes. After the Observation of the Weak Antilocalization and Linear Magnetoresistance in Topological… http://dx.doi.org/10.5772/intechopen.76900 55

**Figure 1.** Methodology of fabrication of topological insulator Bi<sup>2</sup> Te3 thin film hall bar device.

thin film sample synthesized from the MBE, with the help of RIE the thin film hall Bar was made, using the etching mask placed over the sample on the sample holder. With the aid of CF<sup>4</sup> gas for 30 s, the etching was done for getting the required Hall Bar structure. Finally, on the same sample holder, Au (40 nm)/Cr (40 nm) metal ohmic contacts were made with the help of thermal evaporation, using the metal mask over the thin film Hall Bar sample. With dimension of 2 mm long and 1.5 mm wide we obtained our fabricated Topological Insulator Bi2 Te3 thin film Hall Bar device. The **Figure 1a** shows the Schematic diagram of the device fabrication with image of Bi<sup>2</sup> Te3 topological insulator thin film hall bar device and the 7 × 7 mm dimension of the thin film hall bar device in **Figure 1b** and **c**, respectively.

### **3. Characterizations**

attention [1–4]. The surface states which has been observed, are believed to be protected against time-reversal-symmetry, owing to the fact that, electrons in the surface state behave as dirac electrons as in case of 3D TI [5], which can be applied to spintronics devices [6], quantum computing [7] and it is necessary to investigate TI from the transport point of view in order to address the electronic properties of Dirac electrons. In terms of weak antilocalization (WAL) effect in thin films [8, 9], nanoribbons [10] and 3D TI crystals [11, 12], several research groups have already analyzed this transport behavior, by making small scale devices using conventional lithography techniques, but no paper has yet reported to observe this magneto-

Also in terms of potential applications in magnetic sensors and magnetic random access memory [13], materials exhibiting linear magnetoresistance (LMR) are found to be promising

recent literatures and these TIs provide an ideal platform to study the origin of LMR because

It is necessary to grow a high quality TI thin film, in order to observe this magnetotransport

demonstrated and found to be suitable in producing samples with carrier mobilities higher than the bulk crystals with precise control on the growth rate, out of modern thin film growth techniques. In order to realize layer by layer growth and obtaining the right stoichiometry [18] this technology is very important. Here, with respect to weak antilocalization (WAL) and magnetoresistance (MR), we report on the magnetotransport measurement. After fabrication, the thin film Hall Bar device is subjected to Physical Property Measurement System (PPMS), where the magnetic field is applied perpendicular to the plane of Hall Bar device. At programmed temperatures, by sweeping the magnetic field between −9 T and + 9 T, the

[14, 15] and Bi<sup>2</sup>

Se3

of the unique surface states that are naturally zero band gaps with linear dispersion.

Te3

thin film Hall Bar device without using lithography techniques.

Te3

. Molecular beam epitaxy (MBE) in this respect has

Te3

(0001) substrate. Prior to the deposition, the base pressure was maintained

at ~ 8 × 10−10 mbar. To evaporate Bi (99%) and Te (99.9%) sources, the standard Knudsen diffusion cells were used and the Te and Bi were heated to 205 and 630°C respectively. At

prepared at a substrate temperature of 230°C. By defining the pattern, using standard lithographic techniques like electron-beam lithography or photolithography, numerous methods in the recent literatures [19–22] were reported in making thin film hall bar geometry but in our work, without using any resists and lithography techniques, we applied a clean method of making a topological insulator thin film hall bar device. In our device fabrication, we employed two physical masks, for the same sample holder. One for using in reactive ion etching (RIE), called etching mask and other for depositing metal electrodes, called metal mask. For defining the dimension of the thin film hall Bar, the etching mask served the purpose and the metal mask served the purpose of depositing the metal electrodes. After the

[16, 17] has been revealed in

were grown on 7 × 7 mm

Thin films were

Te3

transport behavior in Bi<sup>2</sup>

54 Heterojunctions and Nanostructures

Te3

candidates. The linear MR behavior in Bi<sup>2</sup>

behavior in topological insulator Bi<sup>2</sup>

Longitudinal Resistance is measured.

O3

Using Molecular Beam Epitaxy (MBE), the thin films of Bi<sup>2</sup>

last at Te/Bi flux ratio of ~10 with a growth rate of 8 Å per minute the Bi<sup>2</sup>

**2. Experimental**

dimension Al<sup>2</sup>

Using AFM in a tapping mode, the topography of thin films was evaluated and by scanning a scratch deliberately made on as-grown thin films, the thickness was reliably determined. With the help of Siemens D-500 X-ray Diffractometer (XRD) further structural analyses were also carried out. In **Figure 2b** and **c** representative topographic AFM images of Bi2Te3 thin films are shown respectively, suggesting a layer-by-layer growth mode, revealing the ultra-smooth

electronic waves propagating in opposite directions along the same closed path, in the absence of spin-orbit interaction, which gives rise to Weak-Localization (WL) effect. This effect gives results in increase of resistance or decrease of conductance. But the constructive interference is broken as a result of a phase difference between the two electronic waves, when the magnetic field is applied perpendicular to the plane of the system. By increasing the magnetic field, the increase of resistance can gradually be removed and consequently negative magnetoresistivity occurs. It finally gives rise to increase of the resistance or reduction in the conductance around zero magnetic field as the resistance correction influenced by this localization. In the presence of the spin-orbit interaction, there is a significant enhancement in the resistance, which is known as weak antilocalization effect [23, 24]. As far as the quality of the grown thin film is concerned, it has significant impact on the studies of transport properties of charge carriers and the weak antilocalization (WAL) behavior is an evaluation and indication for such improvement in the quality of the thin film, which manifests both the Dirac nature of the surface states in the bulk of Topological Insulators [8, 25] and strong spin-orbit interaction. If we Compare the 2D electron system, the 2D surface state of the three-dimensional topological insulator is different [26] as an odd number of Dirac points are considered to be encircled by Fermi arc [27, 28]. If we evaluate the topological insulator, its surface remains metallic and cannot be localized by disorder [29]. By Hikami et al. [30], the surface state of the topological insulator is well described. The **Figure 3a** shows the results of the magnetotransport measurement for the temperature ranges from T = 4 to 100 K and fields up to 9 T. The pronounced effect of WAL cusps are marked at low temperature between 4 and 10 K and in the low field regions as shown in the **Figure 3a**. Also we observe the enhancement of peak and the dip structure behaviors in the magnetoresistance, which is quite remarkable with decreasing temperatures. We have defined the normalized magnetoresistance (MR) as a function of magnetic field as MR = [[R (B) − R (0)]/R (0)] × 100%., where R (B) and R (0) are the resistances at field B and at zero field, respectively. The WAL cusps disappear, as the temperature is increased from 10 K onwards. We observe the WAL characteristic behaviors in the temperature ranges from 4 to 10 K as shown in the **Figure 3a**, and disappearance of WAL cusp from 10 K onwards. The MR curves seems to be quadratic like B dependence at low fields between 2 and 6 T and at higher fields from 6 T onwards, the MR follows linear like behavior and does not saturate. The quadratic growth here can be well explained and analyzed by semi-classical model, where the magnetic field drifts the conduction electrons and these conduction electrons are deflected by the Lorentz force. At T = 4 K, the thickness dependent WAL behavior is shown in the **Figure 3b** showing the WAL cusps for 10, 20, 50 nm thickness film, where as there is no observable WAL effect in ultra-thin films like 4 and 2 nm and the magnetoresistance (MR) curve attains to be flat w.r.t. magnetic field (B). The Fermi level is not in the gap but crosses the Surface State, as the film is thinned enough. Hence, the observed drastic suppression of the surface transport is likely due to an enhanced scattering of the carriers. The phase breaking length (Lϕ), which is of temperature dependence is extracted from the Hikami-Larkin-Nagaoka (HLN) model fit [30] for 10 nm thickness film is shown in the **Figure 3c**, which reveals the relatively large phase coherent length of 155.8 nm at 4 K, by fitting to the HLN model. The **Figure 3d** shows the Conductance change with respect to low magnetic field region with the HLN model fit for

Observation of the Weak Antilocalization and Linear Magnetoresistance in Topological…

http://dx.doi.org/10.5772/intechopen.76900

57

10 nm thickness film.

**Figure 2.** Characterization evidences of topological insulator Bi<sup>2</sup> Te3 thin film.

surfaces with large area terraces. In **Figure 2b**, the dashed line drawn, reveals 10 nm thickness film and **Figure 2c** illustrates 10 nm thin film, by the formation of 4–5 stacked layer comprising of domains of triangular terraces. This suggests a favorable growth dynamics accounting for the high crystalline quality of Topological Insulator thin film on Al<sup>2</sup> O3 substrate with the absence of spirals on the terraces together with the shape of the terrace. The XRD experiments were further conducted to investigate the crystalline quality and orientations. With the diffraction peak from Al<sup>2</sup> O3 substrate, **Figure 2d** displays (003) family diffraction peak from Bi<sup>2</sup> Te3 thin film. The result of the Angle Resolved Photo Emission Spectroscopy (ARPES), implying strong surface states is shown in **Figure 2a**.
