**1. Introduction**

Transparent conductive oxides (TCO) are binary or ternary compounds containing one or two metallic elements. A very good balance of optical and electrical properties characterizes TCO materials. Widely used TCO include oxides such as ZnO, SnO2, In2O3 doped with metallic elements: Al-doped ZnO (AZO), Sn-doped In2O3 (ITO) and F-doped SnO2 (FTO) [1–3].

Among others, tin oxide (SnO2), also known as stannic oxide, is an n-type semiconductor (due to oxygen vacancies) with high optical transparency in visible spectral range (>85%) and a wide energy band gap (3.6 eV). It can be found in nature as a mineral known as cassiterite, and it is the main ore of tin [4]. It has a rutile-like crystal structure. The SnO2 thin films are chemically inert, scratch resistant, and can withstand high temperatures [5].

There are various chemical and physical methods for preparation of pure and doped SnO2 thin films: chemical vapour deposition, sol gel, spray pyrolysis, electron beam evaporation, vapour deposition, pulsed laser deposition, molecular beam epitaxy, thermal evaporation, reactive evaporation and magnetron sputtering, reactive magnetron sputtering, ion beam deposition [5].

Atmospheric Pressure Chemical Vapour Deposition (APCVD) is a process that enables deposition of vapour species in the form of thin solid films via suitable chemical reactions at atmospheric pressure. By careful choice of the deposition parameters the film properties can be systematically targeted. APCVD is often used in industry for thin film coatings deposition because of reduced costs due to low material consumption, high deposition rates and running costs (compared to low pressure systems) [6].

SnO2 thin films (pure and doped) have various applications in devices used in daily life: solar energy conversion, flat panel displays, electro-chromic devices, invisible security circuits, LEDs, transparent electrical conductors and non-colouring electrodes, in smart windows, for energy and illumination control, in anti-dazzling rear view windows, and non-emissive displays, low-emittance coatings for energy e fficient windows, anti-frost coatings on car windows and transparent electrode for solar cells. In these applications, the film thickness normally lies in the 100–1000 nm range [5].

For photovoltaic application (as transparent front electrode), it is important that SnO2 is transparent for UV-VIS light while at the same time having very good conductivity. Electrical conductivity can be improved by increasing charge carrier density (doping with foreign atoms) or charge carrier mobility. The most favourable dopants are antimony (SnO2:Sb) and fluorine (SnO2:F). Fluorine-doped tin oxide (SnO2:F, FTO) exhibits good visible transparency owing to its wide band gap, while retaining a low electrical resistivity due to the high carrier concentration caused by the oxygen vacancies and the fluorine dopant. Highers numbers of charge carriers cause lower film transparency. Therefore, better conductivity should be achieved by optimizing charge carrier mobility.

Several di fferent approaches have been reported with respect to how to improve charge carrier mobility of SnO2, including post-deposition heat treatment, deposition technique, substrate, doping control [7]. For example, charge carrier mobility can be improved by use of highly oriented substrate, use of tin tetrachloride as Sn precursor, higher methanol content.

In our previous publication [8], we reported in detail on the structural properties, examined by XRD, of undoped and doped SnO2 thin film samples deposited by APCVD with a short discussion about the influence on average transmittance in VIS part of the spectrum and specific surface resistivity at room temperature. In this work, we investigate in more detail the magnetotransport properties of such APCVD deposited SnO2 thin films in a wide temperature range and relate them with the structural properties. Transport properties were examined in a wide temperature range by impedance spectroscopy, DC resistivity, Hall e ffect and magnetoresistance. Structure and composition of SnO2 thin films were examined by Scanning Electron Microscopy (SEM), Grazing Incidence X-Ray Di ffraction (GIXRD) and Time-Of-Flight Elastic Recoil Detection Analysis (TOF-ERDA). The analysis of the obtained experimental data shows that the scattering at grain boundaries is the dominant scattering process in this type of SnO2 samples and the small variation in the charge carrier mobility between the samples most likely stems from the di fference in preferred orientation of crystallites (texture coe fficient).

#### **2. Materials and Methods**

#### *2.1. Thin Film Deposition by APCVD*

SnO2 thin film samples were prepared by the APCVD on soda-lime glass substrates, in an industrial moving belt reactor [9] with constant-temperature zones. The reactor oven is interrupted with two slots perpendicular to the moving direction of the belt, supplied with a setup with a nozzle that enables the vapour or reactants to flow onto the heated glass surface. The reacting gas mixture was SnCl4, H2O, methanol and oxygen for undoped films, while for doped films, methanol was omitted and the freon gas was added into the vapour mixture. The vapour of reactants was produced in a "bubbler" by passing a carrier gas through the precursors at room temperature (methanol), or moderately heated to 50 ◦C (SnCl4) or 40 ◦C (H2O). The carrier gas for SnCl4, ethanol and H2O was nitrogen. The temperature of the glass substrate before and after deposition was 590 ◦C for samples S-590 and B-590, and 610 ◦C for samples S-610 and B-610 (see Table 1). The glass substrates were loaded by a belt into the furnace for SnO2 layer deposition. The single-layer deposition duration of 1.5 min and the post-deposition thermal treatment of some 30 min were adjusted by the belt speed. SnO2 film is formed by a very fast reaction of tetrachloride with water vapour in which methanol is often added as a moderator. For that reason, in the doped samples where the methanol was omitted, the growth rate was about 30% faster, which is similar to the results reported in Ref. [10].

**Table 1.** Parameters for SnO2 thin film samples: sample labels, deposition process type, deposition temperature and layer thickness.


As a result, two types of films were prepared. The first type (S-590 and S-610) was deposited in a one-step process, and the produced samples were intrinsic. The second type (B-590 and B-610) was prepared by depositing the first layer on the glass substrate in the same way as for the first type, while the second (top) layer, which was formed from the solution without methanol and with the addition of fluorine atoms in the form of freon (Chlorofluorocarbon, CFC), was deposited on the already-formed first layer.

#### *2.2. Structural Characterization*

#### 2.2.1. Scanning Electron Microscopy (SEM)

Sample surface morphology was analysed using JEOL, JSM 7000F field emission scanning electron microscope (FE-SEM, Zagreb, Croatia). The image acquisition conditions used were: 5 kV, 10 mm working distance, magnification 25k.

#### 2.2.2. Grazing Incidence X-Ray Diffraction (GIXRD)

As-deposited films were thoroughly studied by GIXRD, using the synchrotron radiation source. GIXRD was carried out at the MCX beamline [11] (Elettra synchrotron, Trieste, Italy) with a wavelength of the incident beam of 1.5498 Å (8 keV). The angle of incidence was set to 2.0◦ (a value much higher than the critical angle for total external reflection for SnO2 αc = √2δ ≈ 0.37◦, calculated from the SnO2 index of refraction, real part δ [12]). For the critical angle, the beam penetrates 10–20 nm below the surface and gives information about the surface morphology. For the angle of incidence αi = 2.0◦ the beam penetrates much deeper and the GIXRD pattern contains the morphological information for the entire SnO2 layer.

#### 2.2.3. Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA)

Atomic content and depth profiles of the elements in the samples were determined using TOF-ERDA. TOF-ERDA measurements were done by 20 MeV 127I 6+ ions with 20◦ incidence angle toward the sample surface, and TOF-ERDA spectrometer positioned at the angle of 37.5◦ toward the beam direction. More details about TOF-ERDA setup used in this work can be found in Ref. [13].

#### *2.3. Transport Characterization*

#### 2.3.1. Impedance Spectroscopy

Sheet conductivity of SnO2 thin film samples was measured by impedance spectroscopy (Novocontrol Alpha-N dielectric analyser, Zagreb, Croatia) in the frequency range from 0.01 Hz to 1 MHz, in three subsequent temperature cycles: (i) cooling from 20 ◦C to −100 ◦C, (ii) heating from −100 ◦C to 220 ◦C, (iii) cooling from 220 ◦C to 20 ◦C. The temperature step for all cycles was 40 ◦C. Sheet conductivity data were normalized to the film thickness (Table 1) to obtain specific conductivity (S/cm).
