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

  • G. LakshminarayanaEmail author
  • I. V. KitykEmail author
  • T. Nagao
Open Access
Article

Abstract

In this work a series of RuO2 thin films were synthesized through sol–gel spin coating processes and their structural and electrical properties are studied. The (1 1 0), (1 0 1), (2 0 0), and (2 1 1) characteristic peaks of RuO2 phase are identified from the annealed RuO2 films X-ray diffraction profiles. Through the Atomic Force Microscopy, the surface roughness values of the films evaluated as 1.2 nm, and 0.9 nm for 0.5, and 1.0 mmol RuO2 films, respectively. With Ru molar ratio increment the optical transparency of the synthesized films decreases in the UV–Vis-IR range. The P-type conductivity of RuO2 film is confirmed by Hall Effect measurement and the resistivity of 1.0 mmol RuO2 film annealed at 600 °C acquired by Hall measurement was 2.9 × 10−4 Ω cm. The synthesized RuO2 nano-thin films characterization demonstrates that an optically transparent conductive material can be reliably and efficiently created using simple sol–gel spin-coating methods.

Keywords

RuO2 Hall Measurement Ultrathin Film Hall Effect Measurement Dynamic Random Access Memory 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

1 Introduction

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, 9, 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.

2 Experimental

2.1 Synthesis

It was reported that [21], RuO2–SiO2 nano-composite films were obtained by dip-coating from alcoholic solutions of Ru(OEt)3 and subsequent thermal treatments in air or N2 between 100 and 400 °C, and mixed RuO2–SiO2 coatings were synthesized starting from solutions of RuCl3 and [NH2(CH2)2NH(CH2)3Si(OMe)3]. The choice of suitable precursors and moderate temperature processing has proved to be effective to obtain RuO2–based nanostructured coatings, whose purity increased with the annealing temperature from 100 to 400 °C [21]. Following the Ref. [24], ruthenium oxide films were prepared by sol–gel spin coating technique using aqueous solution of ruthenium (III)-nitrosylnitrate Ru(NO)(NO3)3 in 2-methoxyethanol and typical film thicknesses achieved are several 100 nm after calcinations at 1000 K for 2 h with high crystallinity. In this work, we have synthesized three RuO2 thin films using sol solutions with Ru concentrations of 0.25, 0.5, and 1.0 mmol; spin coated on both p type Si (1 0 0) substrates and silica glass substrates. For RuO2 sol solution (light to deep purple), stoichiometric amounts of RuCl3·nH2O (Wako Chem. Com., Japan; we speculated “n” should be 3.5 here) were first mixed with 10 ml absolute ethanol (Wako Chem. Com., Japan), and then stirred with a magnetic stirrer for 2 h at room temperature. The sols were allowed to age for a minimum of 48 h in a sealed container prior to spin coating. After spin coating (conditions: 3000 rpm/30 s. with 50 μl solution) on the substrates, the films were dried at 60 °C for 12 h under vacuum. Finally, the films were annealed in vacuum (c.a.1 mm Hg) in a horizontal quartz-tube furnace at 400, 450, 500, 600, and 700 °C for 30 min.

2.2 Characterization

The X-ray diffraction (XRD) profiles were obtained on a Rigaku X-ray diffractometer (model Rint 2000) with Cu–Kα (λ = 1.542 Å) radiation using an applied voltage of 40 kV and 40 mA anode current, calibrated with Si at a rate of 2 deg/min. The surface morphology of the prepared samples was observed by scanning electron microscopy (SEM; S-4800 HITACHI Company) at 1 nm resolution and an acceleration voltage of 5 kV. The surface morphology and texture of the deposited RuO2 films was also measured using atomic force microscopy (AFM) (SII E-Sweep) using tapping mode. Si cantilever (SI-DF20) with spring constant of 14 N/m and frequency of 134 kHz was used. The Fourier transform infrared (FTIR) spectra of the thin films were measured in the transmission mode over the 100–6000 cm−1 range by a Thermo Nicolet NEXUS-670 FTIR machine using solid substrate beam splitter and DTGS polyethylene detector with a spectral resolution of ~4 cm−1. The optical transmission spectra of the thin films coated on glass substrates were measured by using JASCO V-7200 absorption spectrophotometer in the range 300–3500 nm. The mobility and carrier concentration of the fabricated RuO2 nano-films were examined by using a Hall measurement system (Model 8403 AC/DC Hall Effect Measurement System, Toyo Corporation) based on the Van der Pauw four-probe technique, at room temperature in air.

3 Results and discussion

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].
Fig. 1

XRD profiles of the RuO2 films annealed at a 450 °C, b at different temperatures

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.
Fig. 2

AFM images of a 0.5 mmol, and b 1.0 mmol RuO2 films annealed at 450 °C

Fig. 3

SEM images of 1.0 mmol RuO2 film annealed at a 400 °C, b 450 °C, c 500 °C, d 600 °C, and e 700 °C for 30 min. f Cross sectional view of 1.0 mmol RuO2 film annealed at 400 °C

Fig. 4

The optical transmittance spectra of the samples annealed at 450 °C

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.
Fig. 5

FTIR spectra of some of the thin film samples

Table 1

Experimental values derived from the hall measurement for (a) 0.25 (b) 0.5, and (c) 1.0 mmol RuO2 films annealed at 450 °C, using four-probe method

(a)

Hall mobility (cm2/V s)

10.1

Carrier type

P

Carrier concentration (1/cm3)

1.71 × 1020

Sheet carrier concentration (1/cm2)

7.35 × 1014

Hall coefficient (cm3/C)

0.0365

Sheet Hall coefficient (cm2/C)

8490

Resistivity(Ω cm)

0.00363

Sheet resistivity (Ω/sq.)

845

Hall voltage (V)

0.000004244

(b)

Hall mobility (cm2/V∙s)

1.97

Carrier type

P

Carrier concentration (1/cm3)

1.64 × 1021

Sheet carrier concentration (1/cm2)

7.05 × 1015

Hall coefficient (cm3/C)

0.00381

Sheet Hall coefficient (cm2/C)

885

Resistivity(Ω cm)

0.00193

Sheet resistivity (Ω/sq.)

449

Hall voltage (V)

0.0000008852

(c)

Hall mobility (cm2/V s)

3.64

Carrier type

P

Carrier concentration (1/cm3)

1.92 × 1021

Sheet carrier concentration (1/cm2)

8.26 × 1015

Hall coefficient (cm3/C)

0.00325

Sheet Hall coefficient (cm2/C)

756

Resistivity(Ω cm)

0.000893

Sheet resistivity (Ω/sq.)

208

Hall voltage (V)

0.00000378

Table 2

Experimental values derived from the hall measurement for 1.0 mmol RuO2 film annealed at 600 °C, using four-probe method

Hall mobility (cm2/V s)

1.8

Carrier type

P

Carrier concentration (1/cm3)

1.183 × 1022

Sheet carrier concentration (1/cm2)

2.25 × 1016

Hall coefficient (cm3/C)

5.3 × 10−4

Sheet Hall coefficient (cm2/C)

277.74

Resistivity(Ω cm)

2.9 × 10−4

Sheet resistivity (Ω/sq.)

154.2

Hall voltage (V)

0.00013886

4 Conclusions

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.

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Authors and Affiliations

  1. 1.International Center for Materials Nanoarchitectonics (MANA)National Institute for Materials ScienceTsukubaJapan
  2. 2.CREST, Japan Science and Technology AgencySaitamaJapan
  3. 3.Wireless and Photonic Networks Research Centre, Faculty of EngineeringUniversiti Putra MalaysiaSelangorMalaysia
  4. 4.Faculty of Electrical EngineeringCzestochowa University of TechnologyCzestochowaPoland
  5. 5.Department of Condensed Matter Physics, Graduate School of ScienceHokkaido UniversitySapporoJapan

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