2.3.2. Magnetotransport

DC resistivity ρ, magnetoresistance and Hall e ffect measurements were done in the temperature interval from -270 to 27 ◦C and in magnetic fields up to 5 T. Thin film samples were cut for the Hall-bar geometry with typical dimensions (10 mm × 2 mm × 500 nm). Two current contacts and three pairs of Hall contacts were made by applying silver paint directly to the surface of the films. Magnetoresistance and Hall e ffect were measured simultaneously at fixed temperatures and magnetic field, B, going from −5 to 5 Tesla. Magnetic field was oriented perpendicular to the current through the sample and perpendicular to the surface of the films. Magnetoresistance data were symmetrized, Vxx = Vxx(+B)+Vxx(−<sup>B</sup>) 2 (Vxx is the measured voltage), in order to eliminate the possible mixing of the Hall component and the Hall signal was antisymmetrized, Vyx = Vyx(+B)−Vyx(−<sup>B</sup>) 2 , in order to eliminate the possible mixing of the magnetoresistance component. Hall resistance Ryx = Vyx/I was linear in magnetic field B throughout the whole temperature range for all samples, and the Hall coe fficient was obtained as R H = Vyx t I B , where I is the current and t the sample thickness. The magnetoresistance was determined as standard as Δρ ρ0= <sup>ρ</sup>(B)−<sup>ρ</sup>(0) ρ(0).

#### **3. Results and Discussion**

#### *3.1. Structural Properties*

#### 3.1.1. Scanning Electron Microscopy

In Figure 1, SEM images of SnO2 thin film samples are presented. Average grain size was calculated from SEM image line profiles using the so-called "linear intercept technique" as follows [14]:

$$\overline{D} = 1.56 \frac{\text{C}}{\text{MN}} \tag{1}$$

where C is the total length of test line used, N is the number of intercepts and M is the SEM image magnification.

There is a significant di fference in surface morphology (grain size and shape) for samples deposited in the one-step process—S-590 and S-610 (Figure 1a,b)—and samples deposited a in the two-step process—B-590 and B-610 (Figure 1c,d). Samples B-590 and B-610 have larger grains with sharp edges (pyramidal shape), while samples S-590 and S-610 have smaller grains with a more rounded shape. The samples deposited in the one-step process were deposited with addition of methanol, just as was done in the experiments reported in Refs. [10,15,16], which resulted in smoother films with smaller grains and a higher nucleation density, as reported in Ref. [15,16]. This effect was attributed to the removal of absorbed HCl, which is produced during deposition in reaction of SnCl4 with H2O. Removal of absorbed HCl leads to an increase in the number of adsorption sites for SnCl4 and H2O, and consequently to an increase in the micro-grain density and simultaneous decrease of grain size.

**Figure 1.** SEM images of SnO2 samples: (**a**) S-590, (**b**) S-610, (**c**) B-590 and (**d**) B-610. Average grain size calculated using Equation (1) are: 104 nm (S-590), 84 nm (S-610), 190 nm (B-590), 99 nm (B-610).

Higher deposition temperature produces samples with smaller grains (surface roughness) for both single-layer and bi-layer samples. This effect could be a result of competing gas-phase and/or surface reactions, which are a complex function of temperature and composition of reactants [17].

#### 3.1.2. Grazing Incidence X-Ray Diffraction

Figure 2 shows GIXRD patterns of the single-layer and bi-layer SnO2 thin film samples recorded at the fixed value of incident angle 2.0◦. All observed reflections are unambiguously assigned to the tetragonal structure of SnO2 (space group *P*42/*mnm*, SnO2 COD database ID: 9009082 [18]), mineralogical name cassiterite). XRD peaks (Figure 2) are fitted to PseudoVoight peak profiles.

**Figure 2.** GIXRD diffractograms of SnO2 thin film samples. All visible peaks are indexed and labelled.

Lattice parameters calculated from peak positions (labelled in Figure 2) are presented. Compared to values from the literature (a = 0.4737(3) nm and c = 0.3186(4) nm), all samples have larger values of the lattice parameter a, and equal values for the lattice parameter c. Variations of lattice parameters are due to the presence of intentional (fluor) and non-intentional (chlorine) dopant atoms with different ionic radii which substitute oxygen atoms in the lattice.

There is also a significant variation in XRD peak intensity ratio from sample to sample, which is also different from the values expected based on the literature. This suggests the presence of preferred orientation (texture) in films. For quantitative analysis the texture coefficient for a selected XRD peak was calculated as intensity ratio of the selected XRD peak and the total sum of all fitted XRD peaks using the following formula [19,20]:

$$\text{TC}(\text{h}, \text{k}, \text{l}) = \frac{\text{I}(\text{h}\text{kl}) / \text{I}\_0(\text{h}\text{kl})}{\frac{1}{N} \sum\_{1}^{N} \text{I}(\text{h}\text{kl}) / \text{I}\_0(\text{h}\text{kl})} \tag{2}$$

where I(h,k,l) is the measured relative intensity of the plane (h k l), I0(h k l) is the standard intensity of the plane (h k l) taken from literature (COD database, SnO2 COD ID: 9009082 [18]), and N is the number of reflections included in the calculations. The results for the four most intense XRD peaks are presented in Table 2. It is interesting that samples deposited at the lower temperature (S-590 and B-590) have significantly larger texture coefficients for (110), while B-590 and B-610 samples have larger (200) texture coefficients.

**Table 2.** Results of GIXRD analysis for SnO2 samples: lattice parameters (a, c), texture coefficients for four most intense peaks (calculated using Equation (2)), and average crystallite size (calculated using Equation (3)).


Average crystallite size was estimated using the standard Scherrer Equation [21]:

$$\mathbf{D} = \frac{\mathbf{K}\lambda}{\beta\_{\text{hkl}}\cos\Theta} \tag{3}$$

where βhkl is XRD line width, D is crystallite size, K is shape factor (0.94) and λ (= 0.154 nm) is the wavelength of Cu Kα radiation. The mean values of average crystallite size obtained by Scherrer equation for the four most intense di ffraction peaks are presented in Table 2. The much smaller values for crystallite size compared to grain size estimated from SEM images indicate that grains consist of several crystallites.

Samples deposited in the one-step process have smaller crystallite size calculated from GIXRD and grain size clearly seen from SEM images. Two possible e ffects/reasons could be responsible for smaller crystallite and grain size in the single-layer samples. First is the smaller thickness of single-layer samples, due to which it can be assumed that there are smaller crystallites and grains at the surface according to the standard thin film growth model [22]. The second one is variation in the deposition temperature. Samples deposited at the lower temperature have larger average crystallite size. Temperature a ffects a complicated set of chemical reaction mechanisms during the mixing of reactants in vapour phase and at the substrate surface.
