1. Introduction

Transparent conducting oxides (TCOs) are widely used as the electrodes or coatings of many optoelectronic devices such as solar cells, light emitting diodes and flat panel displays. The performance of TCOs is usually evaluated by the figure of merit FOM, defined as FOM = −1/(R_sh × lnT), where R_sh is the sheet resistance and T is the average optical transmittance, respectively [1]. So far most widely used TCOs, such as ZnO, In₂O₃, SnO₂ and TiO₂ are all n-type semiconductors with large band-gap [2–4]. Compared to n-type TCOs, most p-type TCOs exhibited much smaller FOM due to their low carrier mobility and have not yet be largely commercialized. Therefore, searching for high-performance p-type TCOs is critical to develop some new optoelectronic devices that require both p- and n-type TCO materials [5]. In the past decade, delafossite compound CuMO₂ (M = Cr, Al, Ga, Fe, Sc, Y), NiO, Cr₂O₃, MoO₃ and etc. have been extensively studied as novel p-type TCOs for optoelectronic devices applications [1,6–10].

Layered cobalt oxides are newly discovered p-type oxide thermoelectric materials with very high in-plane electrical conductivity [11–13]. Very recently, both Zhu et al and Robinson et al reported that solution-processed layered cobalt oxide thin films exhibited very high FOM despite their Seebeck coefficient or/and electrical conductivity were much lower than those reported for the corresponding single crystal samples [14, 15]. This fact indicates that layered cobalt oxides could be a new promising candidate for p-type TCOs and there is still much room to improve its performance by further optimizing the sample properties (for example, carrier mobility). Therefore, it is desirable to grow high-quality epitaxial thin films with excellent properties. Bi₂Sr₂Co₂O_y (BSCO) is one of the prime examples of layered cobalt oxides and its bulk form has been extensively studied as the high-temperature thermoelectric material. This material has a misfit structure with two subcells consisting of an alternated stack of the CdI₂-type CoO₂ conducting layer and the rock salt-type Bi₂Sr₂O₄ insulating layer along the c-axis [13]. In this letter, we report the fabrication and transparent conductive properties of epitaxial BSCO thin films by pulsed laser deposition (PLD) technique. A very high FOM of 1350 MΩ⁻¹ is achieved in the epitaxial films, which is one of the highest values so far reported for all p-type TCOs.

2. Experiments

Epitaxial BSCO thin films with thickness of about 50 nm were grown on LaAlO₃ (LAO) single crystal substrates ((001)-oriented, double-polished) by PLD technique. A 308 nm excimer laser with a laser energy density of 1.5 J/cm² and a repetition rate of 3 Hz was used for film deposition. An oxygen pressure of 60 Pa was maintained during the film growth at substrate temperatures of 968 K. The distance between the substrate and the target was about 50 mm. After the deposition, the films were cooled to room temperature under the oxygen pressure of about 0.5 × 10⁵ Pa.

The oxygen content (i.e. the value of y) in the films was determined by Energy dispersive spectroscopy (EDS) and it was very close to the nominal value of 8. The x-ray diffraction (XRD) of the films was measured using a Philips X'Pert 4-circle diffractometer with Cu Kα radiation. Since the standard reference pattern for Bi₂Sr₂Co₂O_y phase is not available in literature, we estimated the subcell parameters using the powder sample that was used to prepare the target for the films: subcell I (a = 4.911 (4) Ǻ, b = 5.111 (3) Ǻ, c = 29.91(2) Ǻ, β = 93.43 (6)°) and subcell II (a = 4.913 (4) Ǻ, b = 2.805 (2) Ǻ, c = 29.90(2) Ǻ, β = 93.45(5)°). The index of the hkl reflections of the films were derived from the subcell II. The transmission electron microscopy (TEM) was performed by using a JEOL 2010F microscope. The Seebeck coefficient was measured by a Quantum Design Physical Property Measurement Systems (PPMS). The resistivity measurement was performed on a 10 mm × 10 mm film sample using a Van Der Pauw geometry over the temperature range of 10 K to 300 K, while the optical transmission of the same sample was measured by using TU-1901 UV-Vis and Bio-Rad IR spectrometers.

3. Results and discussion

Figure 1(a) presents the XRD θ-2θ scan of the PLD-processed BSCO thin film on LAO substrate. Besides the peaks of LAO substrate, only BSCO (00l) peaks can be detected, indicating that the film is c-axis oriented without any second phase. The rocking curve of BSCO (0 0 12) peak has a full width at half maximum (FWHM) of 0.11°, suggesting good crystallinity of the film. The epitaxial nature of the BSCO film on LAO substrate was confirmed by the φ scan measurement, as shown in Fig. 1(b). The presence of four peaks of the BSCO film reveals that the four-fold symmetry of BSCO and the film is epitaxial grown on LAO substrate. The epitaxial relationship between the BSCO film and the LAO substrate was determined to be (001) BSCO||(001) LAO and [100] BSCO||[100] LAO.

 

Fig. 1 XRD (a) θ-2θ scan of the PLD-processed BSCO film on LAO substrate and (b) φ scans of the BSCO film and LAO substrate.

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The crystallinity and epitaxial quality of the PLD-processed BSCO thin films were further investigated by TEM. A bright field cross sectional TEM image displayed in Fig. 2(a) shows that the film is about 50 nm, which is consistent well with the measurement by the Step Profiler (Dektak 150). The selected area electron diffraction (SAED) pattern taken from the film part is shown in the inset of Fig. 2(a). The sharp electron diffraction spots with no satellites suggest that the films have good single crystallinity. Figure 2(b) shows the high resolution cross sectional TEM image of the area in the vicinity of the film/substrate interface. It can be clearly seen that the BSCO film is epitaxial grown on the LAO substrate along its c-axis direction. Moreover, the interface is very sharp without any obvious interdiffusion between the film and substrate. We also measured the film surface morphology by scanning electron microscopy (SEM), as displayed in Fig. 3.The PLD-processed film looks much denser than those of solution-processed films. Moreover, the density of grain boundaries in the epitaxial film is also much less than that of the solution-processed films [14, 15].

 

Fig. 2 (a) TEM and (b) high resolution TEM of BSCO/LAO interface. The inset of Fig. 2 (a) is the SEAD pattern taken from the film part.

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Fig. 3 (a) low and (b) high magnification surface SEM images of the PLD-processed BSCO film.

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The carrier type of the PLD-processed epitaxial film was determined by the Seebeck coefficient measurement. The inset of Fig. 4(a) is the temperature dependence of the ab-plane Seebck coefficient (S_ab) of a 50 nm-thick BSCO thin film. The value of S_ab is positive in the whole temperature range, indicating a p-type conducting. The room temperature S_ab (~120 μV/K) is close to that reported for the single crystals and is much higher than that of the CSD-processed films, suggesting the high-quality of the PLD-processed film [13–15]. Figure 4(a) presents the temperature dependence of the sheet resistance R_sh of a 50 nm-thick BSCO film sample with the size of 10 mm × 10 mm. The resistivity of the film shows a metalliclike behavior as temperature decreases down to about 140 K and diverges with further decreasing the temperature. The detailed discussions about this ρ-T behavior for BSCO can be found in literatures of [13], [16] and etc. The room temperature resistivity ρ of the PLD-processed BSCO film is about 5.5 mΩ·cm (i.e. R_sh~1.1 KΩ), which is similar to the best solution-processed thin film samples [14] and is higher than that reported for other p-type TCOs [1, 6–9]. We also measured the room temperature carrier concentration n and carrier mobility μ of this film by using Van Der Pauw method, which is about 5.17 × 10²⁰ cm⁻³ and 2.20 cm² V⁻¹ s⁻¹, respectively. The value of carrier mobility μ of the PLD-processed epitaxial film is almost two times with respect to the solution-processed film [14]. The higher carrier mobility in the present epitaxial film is suggested to be related with its denser morphology and lower grain boundary density.

 

Fig. 4 (a) Temperature dependence of resistivity of the PLD-processed BSCO thin film, the inset is the Seebeck coefficient of the film; (b) photograph of the BSCO thin film on a labeled paper; (c) transmittance spectra of the BSCO thin film after abstracting the substrate contribution. The inset shows the transmittance spectra of bare LAO substrate as well as BSCO thin film on the LAO substrate; (d) (αhν)² versus hν plot of BSCO thin film.

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Despite the excellent electrical conductivity, the PLD-processed epitaxial film shows good transparency. Figure 4(b) shows the photograph of the same BSCO thin film used for R_sh measurement, and the symbol on the paper under the film can be clearly observed. Figure 4(c) shows the transmittance spectra of this PLD-processed BSCO thin film sample. Here, the data are presented by subtracting the contribution of LAO substrate. It can be clearly seen that the optical transmittance of the film increases with the increase of photo wavelength and is greatly enhanced in the infrared region. For example, the optical transmittance of the film is about 20.2%, 73.2% and 94.4% at the photo wavelength of 400, 700 and 1500 nm, respectively. The average optical transmittance (T) of the epitaxial BSCO film in the visible range is about 51%, which is estimated by using the same photo energy of 1.77, 2, 2.25, 2.5, 2.75 and 3eV as in refs 14 and 15. Moreover, we estimated the optical band gap of BSCO thin films by using the equation of αhν = A(hν-E_g)^m, here α is the optical absorption coefficient, hν is the photo energy, A is a constant, m is 1/2 for a direct band transition and m is 2 for an indirect band transition. A linear relationship between (αhν)² and hν, as shown in Fig. 4(d), indicates a direct energy band gap of the epitaxial BSCO film. The optical band gap width of BSCO is estimated to be about 3.14 eV according to the fitting results, which is very close to the reported value in Ref. 14.

To evaluate the transparent conducting performance of the epitaxial film, we calculated its figure of merit FOM according to the equation of FOM = −1/(R_sh × lnT), where the room temperature R_sh of the epitaxial film is 1.1 KΩ and the average optical transmittance T in the visible range is about 51%. The resulting FOM of the PLD-processed BSCO epitaxial thin film is as high as 1350 MΩ⁻¹, which is about 1.7 times larger than that reported for the best CSD-processed film. Compared with BSCO thin film developed on STO substrate by CSD method, the PLD-processed epitaxial film presented in this work has less grain boundaries and denser morphology. This will lead to an increase in charge carrier mobility μ due to the reduced carrier scatterings at grain boundaries or nanopoles in films. Therefore, BSCO thin films in this work showed a significant high figure of merit in comparison to CSD-processed similar films.

4. Conclusion

Epitaxial Bi₂Sr₂Co₂O_y thin films were grown on LaAlO₃ substrate by pulsed laser deposition. The films showed a p-type conductivity with the room temperature resistivity of about 5.5 mΩ cm and the average optical transmittance of about 51% in the visible range. The calculated figure of merit is about 1350 MΩ⁻¹, which is one of the highest values reported for p-type TCOs. This work demonstrated that epitaxial BSCO thin films are a new promising candidate for p-type TCOs and have great potential applications in optoelectronic devices.