Synthesis, structural, and electrical characterization of RuO₂ sol–gel spin-coating nano-films

Ruthenium dioxide, RuO₂, is a conductive ceramics that belongs to the family of transition metal oxides with tetragonal rutile-type structure. Among many other conductive oxides, RuO₂ exhibits distinct physical and chemical properties and has been used in various applications such as diffusion barriers [1], thin-film resistors [2], bottom electrodes of ferroelectric thin films in dynamic random access memories (DRAMs) [3], and electrochemical capacitors [4]. As one of the most promising electronic ceramics, nano-scale films of RuO₂ can be used for a wide variety of applications because of its semi-metallic conductivity (i.e., 35 μΩ cm: bulk single crystal) as well as good thermal stability [5], excellent diffusion barrier properties [6, 7], and high chemical corrosion resistance [8‐10]. One of the most promising and important use of RuO₂ is for the applications in organic light emitting devices (OLED), flat panel display and organic solar cells. This is because the ultrathin films of RuO₂ are transparent and has high enough work function (5.2 eV) which realizes excellent hole conduction into the active layer of the device. The primary requirement here is the sufficiently high electrical conductance, high optical transparency, and smoothness as well as the closed nature of the film to avoid current leakage in the fabricated devices that also promotes smooth growth of the organic layers. Especially the latter two features are of particular importance to make low-wattage high-performance light emitting device or flat panel displays.

Up to now, most of the reported RuO₂ thin films are prepared by physical vapor deposition (PVD) and subsequent oxidation of Ru films, or by direct deposition of RuO₂ by magnetron sputtering [11], or reactive sputtering of Ru [12]. More recently, metal organic chemical vapor deposition (MOCVD) [13, 14], pulsed laser deposition (PLD) [15], electrodeposition from aqueous solution [16] or cyclo voltammetry [17], atomic layer deposition (ALD) [18], and chemical vapor deposition(CVD) [19], are also used for the fabrication of RuO₂ films. Polycrystalline RuO₂ thin films have been routinely deposited on different substrates by the techniques mentioned above. However, a serious drawback in the processing of materials based on RuO₂ is the well-established chemical reactivity above ~700 °C where RuO₂ becomes volatile and oxidizes to RuO₃/RuO₄ gases (in air) or reduces to Ru metal (in a vacuum) [20].

Compared with commonly used vacuum-based techniques, the sol–gel method is a low- temperature “soft” synthesis process which is relatively simple and cost effective method. This method allows a soft chemical tailoring of the composition and texture of the final product by a suitable choice of the synthesis conditions and of the starting precursors [21]. The sol–gel method is often adopted with spin coating and subsequent mild annealing to yield uniform nano-scale layer across the substrate surface [22, 23]. The film thickness can be controlled by the rotational frequency and accurate controlling of the films thickness can be done. However, up to date, there are very few reports yet available in the literature for the synthesis of RuO₂ thin films by using the sol–gel process [21, 24].

In the present work we report on the sol–gel synthesis of smooth ultrathin films of RuO₂ with precisely controlled thickness in the nanometer region. The films were wet-chemically synthesized and coated through sol–gel spin coating processes. Subsequently their crystal structure, surface morphology, thickness, roughness, optical transmittance, functional groups, electrical resistivity, carrier concentration, and mobility are studied. The fabricated film showed good electrical transport and high optical transparency together with high surface smoothness suitable for the use in organic electronics. The fabrication method reported here can open a way to readily fabricate smooth and optically transparent conductive nano-films with cost effective and relatively mild synthesis route, which fits nicely to the application for organic optoelectronics as well as for solar energy harvesting devices.

This work is aimed at obtaining smooth and ultrathin coatings of pure RuO₂ on Silicon and glass substrates with controlled uniformity in both morphology and chemical composition. We examined the SEM images (not shown here) of the as-coated and dried at 60 °C thin films. Before annealing, the RuO₂ films of various compositions all exhibited smooth and continuous morphology when observed by SEM. To determine the phases present in the samples after annealing at 400–700 °C, we analyzed the samples by XRD. Figure 1a, b show the XRD patterns of the RuO₂ films annealed at 450 °C and 0.5, and 1.0 mmol RuO₂ films at different annealing temperatures. From Fig. 1 one can observe that after annealing, diffraction peaks of the RuO₂ phase are present (i.e., RuO₂ phase precipitates.) in the samples of 0.5 mmol annealed at 450, 600 and 700 °C including for 1.0 mmol RuO₂ annealed at 400–700 °C. The (1 1 0), (1 0 1), (2 0 0), and (2 1 1) characteristic peaks of RuO₂ phase [9] are identified from the XRD profiles. For the annealed 0.25 mmol RuO₂ films at 400–700 °C we didn’t notice any RuO₂ phase through our XRD study. This could be due to formation of very thin film (below 10 nm thickness) for 0.25 mmol RuO₂ concentration due to the low viscosity of the sol solution. Simultaneously, the intensity of RuO₂ peak for all composite films increases with increasing the annealing temperature up to 600 °C. It should be noted that the peak position of RuO₂ did not shift with annealing temperature up to 600 °C. For 0.5, and 1.0 mmol RuO₂ films annealed at 700 °C, the intensity of diffraction peaks is decreased compared with 600 °C due to desorption of more RuO₃ and RuO₄ gases from the RuO₂ films at this high temperature [21, 25].

The similarity of the appearance of the rutile reflections in both silicon wafers and glass substrates indicate that the physical properties of the coating, such as crystallite size and composition, were preserved independently of the substrate.

As an example, we have investigated the cross-section of the 0.25, and 0.5 mmol RuO₂ films annealed at 450 °C (not shown here) by SEM measurement. For 0.25 mmol RuO₂ film, we find it difficult to observe clear cross section of the film coated on Silicon wafer as the thickness of the film is in the range of a few nm. For 0.5 mmol RuO₂ film we measured the thickness and the value is ~13 nm. Using AFM, the topography of the sample surface can be examined. An indication of the mechanical integrity of the coating is also given by the AFM technique. Figure 2 shows the AFM images of the 0.5, and 1.0 mmol RuO₂ films annealed at 450 °C. The surface roughness values of the films evaluated from these images are 1.2, and 0.9 nm for 0.5, and 1.0 mmol RuO₂ films, respectively. Figure 3 shows the SEM images of the 1.0 mmol RuO₂ films annealed at 400–700 °C including the cross sectional view of the annealed film at 400 °C. From these images we noticed that as the annealing temperature increases from 400 to 700 °C, the surface gradually changed from smooth and denser to granular and finally porous like due to desorption of more RuO₃ and RuO₄ gaseous phases. For 1.0 mmol RuO₂ film the measured thickness value is around 20 nm. Figure 4 shows the optical transmission spectra for the 450 °C annealed samples coated on glass substrate. With the increase of Ru molar ratio the optical transparency of the films decreases in the UV–Vis-IR range. This is possibly due to the increase in the viscosity of the sol solution which subsequently results in the thicker films.

The transmittance spectra of RuO₂ films observed at wavelengths shorter than 585 nm are related to the p–d interband transitions [9]. However, the spectra at wavelengths longer than 585 nm are primarily due to free-carrier absorption and d-electron intraband transitions. Figure 5a, b show the Mid-IR and Far-IR spectra of the films annealed at 450 °C; and 450, 600, and 700 °C annealed 1.0 mmol RuO₂ films (Fig. 5c, d), respectively. A vertical displacement of some of the FTIR spectra presented was necessary for easy visualization and comparison of different samples profiles. In this case, the y-axis may indicate greater than 100 % transmission for some samples. For all the films, the crystalline RuO₂ showed a main feature centered at 648 cm⁻¹. Generally for RuO₂ based thin films, two broad bands that appear at 800 and 880 cm⁻¹ can be attributed to higher oxidation states of RuO₃ and RuO₄ species, respectively. These species of higher oxidation state in thin films would decompose readily and develop to thermodynamically stable RuO₂ species if left at ambient temperature [26, 27]. For our synthesized films, these 800 and 880 cm⁻¹ bands from the measured FTIR spectra are not recognizable. Table 1 shows the experimental values derived from the hall measurement for 0.25, 0.5, and 1.0 mmol RuO₂ films annealed at 450 °C, using four-probe method at room temperature. The P-type conductivity of RuO₂ films is confirmed by the Hall Effect measurement. The resistivity of the RuO₂ films decreases gradually from 3.63 × 10⁻³ to 8.93 × 10⁻⁴ Ω cm as the RuO₂ concentration increased from 0.25 to 1.0 mmol (Table 1). We also measure the resistivity value for the 1.0 mmol RuO₂ film annealed at 600 °C and Table 2 shows the respective experimental values derived from the hall measurement for 1.0 mmol RuO₂ film. The resistivity of 1.0 mmol RuO₂ film annealed at 600 °C acquired by Hall measurement was 2.9 × 10⁻⁴ Ω cm. With the annealing temperature increment from 450 to 600 °C, carrier concentration increased from 1.92 × 10²¹ (1/cm³) to 1.183 × 10²² (1/cm³) and resistivities including sheet resistivity values are decreased for 1.0 mmol RuO₂ film. Thus for better conductivity of the synthesized sol–gel films, here concentration and annealing temperature both play a crucial role. It is well known that the conductivity depends on the carrier concentration as well as the mobility of carriers. Thus, one can say that the sol–gel process combined with the spin-coating technique provides a feasible way to fabricate RuO₂ films with controlled compositions, carrier concentration, mobility, resistivity, and transmittance.

Ultrathin films of RuO₂ were wet-chemically synthesized and coated through sol–gel spin coating processes. Diffraction peaks of the RuO₂ phase are confirmed in the annealed thin films. For the 1.0 mmol RuO₂ films, as the annealing temperature increases from 400 to 700 °C, the surface gradually changed from smooth and denser to granular and finally porous like due to desorption of more RuO₃ and RuO₄ gaseous phases. The surface morphology and texture of the deposited RuO₂ films was also measured using atomic force microscopy. The resistivity of the RuO₂ films decreases gradually from 3.63 × 10⁻³ to 8.93 × 10⁻⁴ Ω cm as the RuO₂ concentration increased from 0.25 to 1.0 mmol. With the annealing temperature increment from 450 to 600 °C, carrier concentration increased from 1.92 × 10²¹ (1/cm³) to 1.183 × 10²² (1/cm³) and resistivities including sheet resistivity values are decreased for 1.0 mmol RuO₂ film. The fabricated film showed good electrical transport and high optical transparency together with high surface smoothness suitable for the use in organic electronics.