Elsevier

Applied Surface Science

Volume 258, Issue 11, 15 March 2012, Pages 4844-4847
Applied Surface Science

Transparent conductive CuFeO2 thin films prepared by sol–gel processing

https://doi.org/10.1016/j.apsusc.2012.01.022Get rights and content

Abstract

In this study, transparent conductive CuFeO2 thin films were deposited onto a quartz substrate using a low-cost sol–gel process and sequential annealing in N2. The sol–gel derived films were annealed at 500 °C for 1 h in air and then annealed at 700 °C in N2 for 2 h. The CuO and CuFe2O4 phases appeared as the film annealed in air, and a single CuFeO2 phase (delafossite, R3m) appeared as the film annealed in N2. X-ray photoelectron spectroscopy showed that the chemical composition of the CuFeO2 thin films was similar to the stoichiometry. The optical bandgap of the CuFeO2 thin films was 3.1 eV. The p-type characteristics of the films were verified by Hall-effect measurements. The electrical conductivity and carrier concentration of the CuFeO2 thin films were 0.358 S cm−1 and 5.34 × 1018 cm−3, respectively. These results show that the proposed low-cost sol–gel process provides a feasible method of depositing transparent CuFeO2 thin films.

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)2radical dotH2O and 0.02 mol Fe(NO3)3radical dot9H2O 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.

References (10)

  • M.A. Marquardt et al.

    Thin Solid Films

    (2006)
  • J. Tate et al.

    Thin Solid Films

    (2002)
  • A.N. Banerjee et al.

    Prog. Cryst. Growth Charact. Mater.

    (2005)
  • D.H. Choi et al.

    Thin Solid Films

    (2009)
  • A. Barnabé et al.

    Mater. Lett.

    (2006)
There are more references available in the full text version of this article.

Cited by (76)

View all citing articles on Scopus
View full text