Photoelectric Properties of SnO2/Ti/GZO Multilayer Thin Film after Annealing at Different Temperatures for Photosensor Applications

In this study, RF magnetron sputtering is employed to deposit a SnO2/Ti/GZO (TTG) transparent conductive film on a Corning glass substrate to investigate the effect of the annealing temperature on the electrical properties, transmittance characteristics, surface properties, and structural properties of a TTG conductive film. The results show that upon annealing at 300 °C, the minimum resistance was achieved by the TTG multilayer film, i.e., 1.57 × 10−2 Ω∙cm. Regarding the transmittance characteristics of all multilayer films, it is evident that the higher the annealing temperature, the higher the mean transmittance, but above an annealing temperature of 300 °C, the transmittance decreases slightly.


Introduction
In the visible light and near-IR regions, a transparent conductive oxide (TCO) can exhibit excellent conductivity and high optical transmittance. Owing to these characteristics, TCO films can be widely applied in a variety of photoelectric components such as solar cells, (1,2) organic light-emitting appliances, (3,4) thin-film transistors, (5,6) photovoltaic batteries, (7)(8)(9) electrochromic devices, (10) and tablet displays. (11)(12)(13)(14) Normally, metallic films are opaque to visible light. When the thickness of a metallic film is below 100 Å, it allows visible light to be transmitted while exhibiting greater reflectivity in the IR region. Materials with energy gaps larger than 3 eV should be used to form semiconductors, for example, In 2 O 3 , ZnO, SnO 2 , TiO 2 , and CdO, (15) which can be used as semiconductor materials.
Research to achieve better conductivity and transmittance has been conducted in recent years. (16)(17)(18) It has been proposed that oxide and metallic films can be stacked together to form a three-layer transparent conductive film of the form oxide/metal/oxide or metallic oxide/metal/ metallic oxide, (19) which is called a sandwiched film or multilayer film. Such films can not only prohibit the reflection of visible light by the metallic layer but can also transmit visible light. (20)(21)(22)(23) In industry, SnO 2 was commercialized earlier than Ga-doped ZnO (GZO). Despite the inferior electrical properties of SnO 2 to those of GZO, its photoelectric properties are superior to those of GZO in the IR region above a wavelength of 600 nm. In addition to having high chemical stability and heat stability, SnO 2 can also be used to configure the tissue structure on a film surface to enlarge the working wavelength range. Because of these characteristics, SnO 2 conductive films are now widely applied in gas sensors or as the electrodes of solar energy batteries; therefore, they are extensively used in modern buildings. Transparent conductive SnO 2 films have also been widely used in low-radiation glass. However, it is still extremely difficult to apply these films in tablet displays owing to their high resistance and poor resistance to erosion.
In this research, Ti element is selected to constitute the intermediate metallic layer when using RF magnetron sputtering to deposit a SnO 2 sandwiched multilayer film. Ti is selected because its maximum chemical valence is +4. (24) The radius of Ti 4+ is 0.0605 nm and that of Sn 4+ is 0.069 nm. Because of the similar radii, Ti 4+ can replace Sn 4+ at the same position in the crystal lattice of SnO 2 . As such, the interface between the Ti layer and the SnO 2 layer will not exhibit significant lattice twisting, thus reducing the scattering of light in the interface between the two layers. Finally, SnO 2 /Ti/GZO (TTG) films are annealed in order to find a sandwiched multilayer film that can exhibit optimal electrical and optical properties. (25)

Experimental Methodology
In the experimental process, a Corning glass substrate was used. To begin with, the glass substrate was cut into pieces of the same size [i.e., 25 mm (length) × 25 mm (width) × 7 mm (thickness)]. Next, the glass substrates were cleaned sequentially in deionized (DI) water, acetone, isopropanol, then DI water again to wash off the non-organic compounds. Finally, the DI water was used to remove the residual solvent and then the specimens were loaded into a 90 ℃ oven until the water was completely vaporized. An RF sputtering system was employed for film deposition, and the sputtering was then executed after adjusting the required process parameters. During the sputtering process, we did not heat the substrate or charge additional gas. The purpose of this experiment was to study the impact of different annealing temperatures on the characteristics of transparent conductive films. The films were also annealed and the properties of the films were analyzed before annealing and after annealing at 200, 300, 400, and 500 ℃. We used X-ray diffraction (XRD) to investigate the crystal growth of these multilayer thin films. We used a TENCOR (Alpha-Step 500) surface profilometer to measure the thickness of the films, scanning electron microscopy (SEM) to analyze the surface features, and atomic force microscopy (AFM) to investigate the roughness and grain size.

Results and Discussion
After the multilayer film was deposited by sputtering at a low power, annealing was conducted in a vacuum environment for 20 min at 200, 300, 400, or 500 ℃.

Thickness comparison
As shown in Table 1, the vacuum annealing temperature slightly affected the thickness of the TTG three-layer film. This means that the total thickness of the multilayer film was maintained regardless of the annealing temperature. Figure 1 shows the structural properties of the TTG multilayer films obtained by XRD. Peaks appear in the spectra of the TTG multilayer films annealed at 300 to 500 ℃ at angles of 26.6, 34.5, and 51.7°. These values correspond to (110), (002), and (211) phases, respectively. The (002) phase corresponds to ZnO. This suggests that Sn or Ti atoms can be replaced by a diffusion process following the annealing process. (26,27) The ZnO peak intensity was lowest after annealing at 300 ℃. This means that the crystallized phase SnO·SnO 2 (Sn 2 O 3 ) existed inside the structure of the TTG multilayer film. After annealing at 400 and 500 ℃, the SnO·SnO 2 (Sn 2 O 3 ) peak was observed at 26.6°, which corresponds to the (110) phase. As such, the Sn 2 O 3 phase will affect the subsequent electrical properties.

Analysis of electrical properties
The annealing was performed after the sputtering deposition of the TTG multilayer film. Table 2 shows the change in the electrical properties of the films with the annealing   temperature. The resistivity was a minimum of 1.57 × 10 −2 Ω•cm after annealing at 300 ℃, which was mainly due to the highest carrier concentration. The resistivity was higher after annealing at a higher temperature. The main reason for this was probably the doping of Sn 4+ ions, which formed a Sn 2 O 3 phase with higher resistivity, as indicated by the XRD data.

Transmittance of TTG multilayer film
From Tables 3 and 4 and Figs. 2 and 3, we can see the transmittance properties of the TTG multilayer films. Before annealing, the mean transmittance of the TTG multilayer film was 63.85%, and the low transmittance was mainly caused by the reflection from the mirror effect of the intermediate Ti layer. The transmittance properties significantly improved with increasing annealing temperature. The optical energy gap of a conventional GZO film tends to increase as soon as the carrier concentration increases. This is mainly because the Fermi level moves into the conduction band and the electrons on the valence band are forced to jump to the conduction band, thereby requiring more energy, and the so-called Burstein-Moss effect is therefore induced. (28) However, the opposite phenomenon was seen during the experimental process, where the optical energy gap increased with decreasing carrier concentration. This phenomenon could be due to the interaction between ionic compounds. For example, Zn 2+ and Sn 4+ ions coexisted in the GZO films, which may have generated donor-acceptor pairs, thus   shrinking the energy gap and offsetting the Burstein-Moss effect. (29) Although the carrier concentration decreased with increasing annealing temperature, the energy gap and the mean transmittance increased. In particular, after annealing at 200 ℃, the crystallinity of the thin film was increased and the mean optical transmittance was increased to about 70%. The TTG film annealed at 300 ℃ was crystalline and its optical transmittance reached nearly 90%. The large increase in the transmittance should be due to the lattice disorder or surface-interfacerelated defects found in the crystalline film. (29)

Surface feature analysis
From Table 5 and Figs. 4 and 5, we can see the impact of the increased annealing temperature of the TTG multilayer film on the surface topography and the distribution of crystal grains. The surface roughness Ra before and after annealing has an average value of about 0.2-0.4. After annealing at 300 ℃, Ra remains at about 0.375 nm. The crystal grains started to crystallize from 300 ℃ and the surface formed a crystallized pattern and Ra therefore increased. From SEM and AFM images, we can see that the TTG multilayer film has a flatter surface before the annealing and when subjected to annealing at 200 ℃. After annealing at 300 ℃, the surface forms a phase-island valley structure with higher roughness due to greater fluctuation.

Analysis of grain size
From the XRD peak value, the full width at half maximum (FWHM) can be calculated using Scherrer's formula (30) as where D is the grain size, β is the XRD peak FWHM, λ is the wavelength of the incident X-ray, and cosθ is the diffraction angle of incident light. λ and cosθ are constant values, so β becomes the most important factor, as shown in Fig. 6. Because λ and θ are fixed values and D and β have a reciprocal relation, the smaller the FWHM, the larger the crystal grains. (29) The crystal grain size increases with the annealing temperature up to 300 ℃ but then decreases. When the annealing temperature is 300 ℃, the crystal grain size has a maximum value of 14.9 nm.

Conclusions
The experimental results indicated that the thickness of the TTG multilayer film is not affected by the annealing temperature. According to the results of XRD structure analysis, the TTG multilayer film has (110), (002), and (211) phases. Furthermore, the TTG multilayer film has a minimum resistance of 1.57 × 10 −2 Ω•cm after annealing at 300 ℃. The TTG multilayer film has a maximum mean transmittance of 90.14% after annealing at 300 ℃, and the energy gap is 3.21 eV. According to surface feature observation, the Ra value increased with increasing annealing temperature up to 300 °C, above which it decreased. The crystal grain size also increased with increasing annealing temperature up to 300 °C and then decreased. Our experimental results show that TTG multilayer thin films can be used as a TCO and as the photoabsorbing layer in photosensors. Furthermore, they can inhibit the transmission of light of some wavelengths in photosensors.