Elsevier

Thin Solid Films

Volume 516, Issue 17, 1 July 2008, Pages 5795-5799
Thin Solid Films

Chalcogen-based transparent conductors

https://doi.org/10.1016/j.tsf.2007.10.073Get rights and content

Abstract

Chalcogenide-oxides and chalcogenide-fluorides exhibit p-type conductivity and transparency, and in many cases, these properties exist simultaneously. We present the properties of several of these materials, BaCuQF (Q = S, Se, Te), BiCuOSe, and Cu3TaS4. BiCuOSe is a low band gap degenerate semiconductor with a high conductivity, and oriented films can be easily prepared. We show that epitaxial thin films of BaCuTeF and highly c-axis oriented films of BaCuSeF can be prepared by pulsed laser deposition on MgO substrates. Undoped BaCuTeF thin films exhibit a maximum conductivity of 167 S/cm, mobility of 8 cm2/Vs and band gap of 3 eV, while the selenide and sulfide versions are progressively less conducting and exhibit lower mobility. Strong excitonic features are observed in BaCuSeF and BaCuSF. Cu3TaQ4 powders and films have been prepared and these show p-type conductivity, with absorption onsets in the range 2.8–3.0 eV.

Introduction

Transparent semiconductors are of interest for their use as active and passive elements in transparent opto-electronic devices. Appropriate and increasing dopant concentrations can change a transparent wide band gap semiconductor from an insulator to a lightly doped semiconductor suitable for a channel in a thin-film transistor to a degenerate semiconductor used for transparent contacts and conductor lines. To date, only n-type transparent conductors have been successfully employed in useful transparent circuits [1]; development of complementary p-type materials for improved circuit performance is highly desirable. Oxides are most commonly used for n-type transparent electronics, combining the appropriate ranges of conductivity and mobility (> 10 cm2/Vs) with excellent transparency in the visible range [2]. p-type conductivity in wide band gap oxide semiconductors has been demonstrated, mostly in Cu-based oxides with the delafossite structure [3], [4], [5], [6]. However, the mobility is lower than 0.5 cm2/Vs, and high conductivity comes at the expense of transparency. Chalcogenide-oxide [8] and chalcogenide-fluoride [9] materials offer the prospect of higher p-type conductivity and mobility since the valence band contains a stronger mix of Cu 3d and chalcogenide np orbitals [10], [11]. In this paper, we present properties of BiCuOSe, BaCuQF (Q = S, Se, Te) and Cu3TaS4 films. All are p-type semiconductors with band gaps ranging from 1.5 eV to above 3.0 eV. Conductivity ranges from insulating to about 180 S/cm, and mobility is from 2–8 cm2/Vs. Like many Cu-based p-type semiconductors, BiCuOSe and BaCuQF are structurally anisotropic, while Cu3TaS4 is unusual in that it has a cubic structure, which offers advantages for device processing.

Section snippets

Experiment

We have used pulsed laser deposition (PLD) to produce several types of p-type transparent semiconductors. PLD technology and expertise developed rapidly after the discovery of high temperature superconductors and the method has proven particularly useful for oxides [12]. PLD has also been used to produce high-quality oxide-chalcogenides [13], chalcogenide-fluorides [14] and sulfides. Our systems and procedures are described elsewhere [14], [15], but typical parameters for the materials

BiCuOSe

BiCuOSe consists of alternating layers of [Cu2Se2]2− tetrahedra and anti-fluorite [Bi2O2]2+ distorted tetrahedra in a tetragonal P4/nmm structure [16]. Its p-type conductivity can be enhanced by substitution of Ca for Bi. It is the Bi-based analog of LaCuOSe, which, when doped with Mg, exhibits degenerate conductivity (p  2 × 1020 cm−3) at about 140 S/cm with a Hall mobility of about 4 cm2/Vs [17]. La-based chalcogenide-oxides require ex-situ processing of up to 1000 °C to produce epitaxial films

Summary

We have reported the first thin films of BiCuOSe, a p-type chalcogenide-oxide semiconductor with a band gap of 1.5 eV. Textured, c-axis oriented films form in-situ on MgO (100) substrates heated to 450 °C. Ca-doped films have a conductivity of 176 S/cm and a mobility of 2 cm2/Vs. Undoped films have a high reflectivity across the visible and near IR spectrum. Films of the related material family BaCuQF (Q = S, Se, F), are also p-type semiconductors, but with a wide band gap, rendering them

Acknowledgements

This work is supported by the National Science Foundation under DMR 0245386 and IGERT DGE 0549503. We thank Dr. Hiroshi Yanagi and Dr. Cheol-Hee Park for contributions to early work on BaCuQF.

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