Chalcogen-based transparent conductors
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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2016, Computational Materials ScienceCitation Excerpt :The family of ternary compounds named sulvanites Cu3TMX4 (TM = V, Nb, Ta; X = S, Se) have gained attention due to their blend of interesting properties such as good ionic conductivity, p-type transparent conductivity, lattice constants similar to silicon making possible the growth of hetero-epitaxial thin films on Si substrates and band gap range [1–4].
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