Transparent conductive CuFeO2 thin films prepared by sol–gel processing
Highlights
► Delafossite CuFeO2 thin films are successfully deposited by low-cost sol–gel processing. ► The bandgap of delafossite CuFeO2 thin films is 3.1 eV. ► The electrical conductivity of delafossite CuFeO2thin films is 0.358 S cm−1.
Introduction
Transparent conducting oxides (TCOs) are functional oxide materials with a wide optical bandgap and semiconductive behavior. The optoelectronic industry and research community have long used these materials in flat panel displays, touch panels, and solar cells. Currently, most of the popular TCOs exhibit n-type characteristics. Though p-type TCOs are not yet well established, researchers have successfully prepared a series of delafossites in recent years [1], [2], [3], [4]. Delafossites are ternary oxides with the basic formula AIBIIIO2, where A represents monovalent cations such as Cu or Ag, and B represents trivalent metals ranging from Al to La [2]. The layered crystal structure of cuprous delafossites consists of small bandgap Cu2O layers and large bandgap B2IIIO3. These B2IIIO3 layers widen the energy bandgap and result in a relative large optical bandgap. Both n- and p-type TCOs make it possible to fabricate all-oxide transparent semiconductive pn-heterojunctions [3], [4].
CuFeO2 was the first of the delafossites to be discovered, and has a relative higher conductivity than other delafossites, except for CuCrO2 [2]. A few researchers have attempted to prepare CuFeO2 thin films using techniques such as pulsed laser deposition and rf-sputtering [5], [6], [7]. Choi et al. [5] used the pulsed laser deposition method to deposit CuFeO2 thin films on amorphous glass substrates, but at a growth temperature of 750 °C. Their results indicate that CuFeO2 film has a conductivity of 2.21 × 10−5 S cm−1 with a Hall coefficient of 1.84 × 106 m2/C, suggesting that it is an insulator. Barnabé et al. [6], [7] used rf-sputtering to deposit CuFeO2 thin films on glass substrates. The as-deposited films in their study were electrical insulators with a direct optical bandgap of 2 eV. The conductivity of the films reached 1.03 × 10−3 S cm−1 after annealing in air at 450 °C for 6 h. Barnabé et al. examined the p-type semiconducting properties of these films using thermopower measurements because Hall measurements were insufficient.
However, these vacuum-based processes are complex and time consuming. In contrast, the chemical solution method of preparing TCOs films has many advantages, including low cost, easy set-up, large area coating, and mass production. Previous research has demonstrated that the sol–gel method is a powerful technique for growing delafossite thin films [8]. This study reports the deposition of transparent conductive p-type CuFeO2 thin films on quartz substrates through sol–gel processing. This is the first study proposing the preparation of transparent conductive CuFeO2 thin films using this low-cost process.
Section snippets
Experimental details
Transparent conductive delafossite-CuFeO2 thin films were prepared on quartz substrate by single spin-coating and sequential annealing. Specifically, 0.02 mol Cu(CH3COOH)2H2O and 0.02 mol Fe(NO3)39H2O were first dissolved in 70 mL ethanol, and 4.5 g triethanolamine was then added to the solution. This precursor, with the desired stoichiometric ratio, was then spin coated onto quartz substrates at 2500 rpm for 15 s. The specimens were then annealed at 500 °C in air for 1 h at a ramp rate of 5 °C/min. To
X-ray diffraction analysis
Fig. 1 shows the grazing incident X-ray diffraction (GIXD) results of the CuFeO2 thin films measuring 90 nm in thickness. This figure shows that CuO (JCPDS# 89-2530) and CuFe2O4 (JCPDS# 77-0010) are the predominating phases in a specimen annealed at 500 °C in air. The single delafossite-CuFeO2 phase (JCPDS #75-2146) is well-defined and the high intensity reflections at (0 0 6), (0 1 2), and (1 1 0) are the peaks in the scanned range from the CuFeO2 thin film. GIXD results indicate that the CuFeO2 film
Conclusions
In summary, single-phase high conductivity transparent p-type delafossite CuFeO2 thin films were successfully deposited on quartz substrates using a sol–gel process. The desired delafossite phase and chemical composition were confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The optical bandgap for the direct band transition was estimated to be 3.1 eV. The conductivity and carrier concentration of the resulting films were 0.358 S cm−1 and 5.34 × 1018 cm−3, respectively. The
Acknowledgements
We thank the National Science Council of R.O.C. for financial assistance under grant numbers NSC 97-2221-E-151-004 and NSC 98-2221-E-151-024-MY2. We also thank Dr. R.-S. Yu of Asia University for helping the Hall-effect measurement.
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