Analysis of surface properties of semiconducting (Ti,Pd,Eu)O_x thin films

1. Introduction

In recent years the applicability of various metal oxides with unique properties constantly increases, because they play an important role in the development of modern technologies. Titanium dioxide is a material frequently reported in the literature. This fact stems from a number of its advantages like, e.g., high transparency, high stability (thermal, chemical and mechanical) or photocatalytic activity [1–3]. The properties of titanium dioxide can be modified by the change of deposition process parameters [2,4], doping [5,6], or by the post-process treatment (e.g., annealing) [7,8]. With regards to the doping process, it is possible to obtain in this way coatings with desired structural, optical and electrical properties. In the case of thin-film coatings for so called ‘transparent electronics’ a low resistivity (i.e., < 10⁶ Ω cm – for the semiconducting films or even < 10^–3 Ω cm – for the conductive one) and high transparency in the visible light range must be obtained. One of the elements which advantageously affect the electrical parameters of TiO₂ is palladium. It is a well-known catalyst (used for years) [9–11]. The addition of Pd decreases the resistivity of TiO₂, but unfortunately, it involves a large reduction of its transparency [11]. The solution of this problem may be co-doping with other elements. Lanthanides are the dopants that significantly modify the optical properties of TiO₂. They allow receiving the photoluminescence and can be used for preparation of coatings for optoelectronics [6,12]. As it was shown in our previous works [13–15], especially europium is an element that small amount can significantly change the structure, optical and surface properties of TiO₂. For this reason in this paper, the properties of transparent and semiconducting (Ti,Pd,Eu)O_x thin-film coatings have been described. These films were prepared by magnetron sputtering. Analysis of their properties revealed that such nanocrystalline materials could be used in the construction of electronic components for the purpose of transparent electronics.

2. Experimental details

Thin films were prepared by a magnetron sputtering method [16]. During the process Ti-Pd-Eu target in the form of metallic, mosaic disc with diameter of 30 mm (99.999% purity) was sputtered by 180 min. in pure oxygen atmosphere with the gas flow of 40 sccm. The pressure in the chamber was equal to 2·10^–2 mbar. The magnetron with the target was powered by a MSS2 power supply (Dora Power System). The distance between the target and the substrates was 160 mm. Apparatus was also equipped in vacuum gauges and gas-flow control system (with mass-flow controllers). The coatings were as-deposited on Si and Corning 7059 glass substrates in size of 2×2 cm² and 2×3 cm², respectively. The thickness of as-deposited films was 400 nm and 380 nm for (Ti,Pd,Eu)O_x and TiO₂ (reference sample), respectively.

The structure of (Ti,Pd,Eu)O_x films was determined by the X-Ray Diffraction (XRD) method. For the measurements, Siemens 5005 powder diffractometer with Co Kα X-ray (λ = 1.78897 Å) was used. The studies were per formed using Co lamp filtered by Fe (30 mV, 25 mA), step size was equal to 0.02° in a 2θ range, while time-per-step was 5 s.

The surface morphology of the films was investigated by FESEM FEI Nova NanoSEM 230 scanning electron microscope (SEM) with 30 kV of acceleration voltage. The system was equipped with EDAX EDS micro-analyser for studies on material composition. To determine the surface topography, the AFM measurements were performed with the use of a UHV VT AFM/STM Omicron atomic force microscope operating in ultra-high vacuum conditions. A 3×3 μm2 sample area was scanned in the contact mode. For an analysis of experimental data WS×M ver. 5.0 software was used [17]. To determine the oxidation states of each element on the sample surface X-Ray Photoelectron Spectroscopy (XPS) measurements were done using Specs Phoibos 100 MCD-5 (5 single channel electron multiplier) hemispherical analyser with a Specs XR-50 X-ray source with a Mg Kα (1253.6 eV) beam. The results were analysed using CasaXPS software. All spectra were calibrated with respect to the binding energy of adventitious C1s peak at 284.8 eV.

Optical properties were evaluated on the basis of transmission measurements. The experimental system was equipped in Ocean Optics QE 65000 spectrophotometer and a coupled deuterium-halogen light source. Based on experimental results such parameters like transmission level, cut-off wavelength and optical band gap were evaluated.

Electrical properties of the manufactured thin films were investigated with the aid of Jandel four-probe system and Keithley type 2611 source measure unit. Measurements of surface resistivity were carried out at room temperature.

Additionally, Optical Beam Induced Current (OBIC) measurements were performed in order to study photoelectrical properties of a prepared thin TOSs (Ti,Pd,Eu)O_x film deposited on a silicon (p-type) substrate. The I_ph(x) photoelectrical characteristics vs. light beam displacement in line were recorded at λ_exc = 527 nm excitation. Applied light beam had diameter of ca. 30 μm in size and the measurement step was 10 μm. The experiment was carried out at room temperature, without any additional voltage bias applied to the measured heterostructure. The light beam (laser diode) was modulated by a reference square wave signal at 300 Hz frequency and for the photocurrent detection a lock-in technique was used.

3. Results and discussion

The EDS analysis of material composition showed that as-deposited (Ti,Pd,Eu)O_x thin film was composed of 78.4 at. % of Ti, 17.5 at. % of Pd and 4.1 at. % of Eu (without taking into consideration a signal from oxygen). Almost identical participation of these elements was found at different areas of the sample which shows a high content uniformity. Moreover, structural studies have revealed that the film was nanocrystalline and the TiO₂-rutile structure was received after deposition. According to the XRD results, the film was built from crystallites in a size of 7.8 nm (Fig. 1). It is noteworthy that usually titanium dioxide thin films prepared using magnetron sputtering, directly after deposition have the anatase structure and in order to receive the rutile form an additional annealing at a temperature above 700°C is required. Typically annealing, except phase transition, causes an increase of the crystallites size. According to Gribb et al. [18]. the most important factor for preparation of TiO₂ thin films with the rutile structure (without additional annealing) is the initial size of the particles deposited on the substrate surface. In the case of the applied magnetron sputtering method [16]. application of oxygen enables not only chemical reaction required for formation of the oxide thin film but also the impingement of the substrate with heavy oxygen ions provides an extra energy in the place of the film formation and promotes shredding of nucleation centres. Moreover, due to application of low oxygen pressure in the working chamber the increase of free-path mean of sputtered particles was received what had a positive effect on increase of their energy. The deposition parameters had also an impact on receiving of homogenous structure and dense packing of the grains, about what testifies both SEM [Fig. 2(a)] and AFM images [Figs. 2(b)–2(d)]. Moreover, the particles with nanometric sizes, with similar shape and dimensions over the entire surface of the (Ti,Pd, Eu)O_x film can be observed.

Analysis of the surface state was made using the photoelectron spectroscopy. Figure 3 contains XPS spectra which were measured in the range corresponding to the occurrence of the emission of photoelectrons for titanium (state Ti2p), palladium (state Pd3d), europium (Eu4d states) and oxygen (O1s state). In the case of titanium, XPS results showed that the location of Ti2p_1/2 – Ti2p_3/2 doublet peak and the mutual difference of binding energy between them [E_wΔ (Ti2p)] was 5.76 eV [Fig. 3(a)]. This shows that titanium occurred at 4+ oxidation state and was present in the form of TiO₂. In turn, with regards to the palladium the peak for Pd3d_5/2 appears in the spectrum at the energy of 338.15 eV and E_wΔ for Pd3d_3/2 – Pd3d_5/2 was 5.36 eV [Fig 3(b)]. This means that the palladium was in the form of PdO2 – also at 4+ oxidation state. The spectrum recorded for Eu4d state has revealed presence of Eud43/2 and Eu4d5/2 doublet peak [Fig. 3(c)]. Position of both peaks, as well as the difference of their binding energy (E_wΔ (Eu4d)) was 5.61 eV. This indicates on presence of europium at 3+ oxidation state (in Eu₂O₃ form). The experimental results were also analysed to identify components associated with the oxygen O^2– and hydroxyl groups (OH^–) adsorbed on the thin film surface. It was found that the relative percentage amount of OH^– to the total amount of oxygen was 53% [Fig. 3(d)].

Fig. 1

XRD pattern of (Ti,Pd,Eu)O_x thin film with the rutile structure, as-deposited on Si substrate. Designations: D – average crystallites size, d – interplanar distance.

Fig. 2

SEM (a) and AFM (b–d) images of a nanocrystalline (Ti,Pd,Eu)O_x thin film.

Fig. 3

XPS spectra of (Ti,Pd,Eu)O_x thin film recorded for: (a) Ti2p, (b) Pd3d, (c) Eu4d, (d) O1s states.

Fig. 4

Transmission characteristics (a) and Tauc plots for indirect transitions (b) for (Ti,Pd,Eu)O_x and TiO₂ thin films as-deposited on a SiO₂ substrate. Designations: λ_cutoff – optical fundamental absorption edge, E_g^opt – width of optical band gap.

As a part of the research transmission characteristics of (Ti,Pd,Eu)O_x were measured. Obtained results were compared to an undoped TiO₂ – with the rutile structure after deposition (Fig. 4). It was found that the addition of palladium and europium reduces the value of transmission coefficient from about 86% to 47% (for λ = 600 nm). Besides, an effect of optical absorption edge (λ_cutoff) displacement from 345 nm to 452 nm [Fig. 4(a)]. Based on the optical transmission measurements the width of optical band gap (E_g^opt) with the use of Tauc plot (for indirect transitions) was determined [19]. As it can be seen the reduction of the optical band gap E_g^opt from 3.04 eV to 1.82 eV was observed [Fig. 4(b)].

In turn the results of electrical studies have shown that a (Ti,Pd,Eu)O_x thin film had semiconducting properties. The resistivity at room temperature was about 1·10³ Ωcm and the majority electrical charge carriers were electrons (n-type conduction) as given by Seebeck coefficient measurements. This means that the coating can be classified as a TOS film (transparent oxide semiconductor). Therefore, the next step was to examine the sample response to excitation by the light beam (Fig. 5). The photocurrent was recorded in the areas between Au-contacts deposited by thermal evaporation. The I_ph(x) amplitude was dependent from the distance between the contacts, but for too large distance the recombination phenomenon begins to dominate. However, as-deposited coating was optically active and transparent in visible light range.

Fig. 5

Characteristics of photocurrent I_ph(x) obtained as an effect of a single scan (in line) of a light beam (λ = 527 nm) across the area between the electrodes on (Ti,Pd,Eu)O_x/Si (p type).

4. Conclusions

In this paper structural, electrical and surface state studies of transparent and semiconducting (Ti,Pd,Eu)O_x thin films have been described. It was found that as-prepared films were nanocrystalline. They had TiO₂-rutile structure composed of crystallites in an average size of 7.8 nm. Besides, they were homogeneous and had densely packed grains. The results of XPS have revealed that titanium and palladium were present at 4+ state in a separate form of TiO₂ and PdO₂, respectively. Europium occurred at 3+ state – in Eu₂O₃ form. In turn the research of the optical properties showed that the (Ti,Pd,Eu)O_x coating was characterized by transmission factor of approx. 47% at λ = 600 nm. Taking into account the results of the resistivity measurements (1·10³ Ωcm) and the generation of photocurrent should be noted that the prepared films meet the requirements for TOS-type coatings (transparent oxide semiconductor) that are necessary for the further development of transparent electronics.