Record Conductivity for P-Type Transparent Conductors in Polycrystalline Thin Films of a Sulfide-Fluoride
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Record Conductivity for P-Type Transparent Conductors in Polycrystalline Thin Films of a Sulfide-Fluoride Jesse A. Frantz1, Jasbinder S. Sanghera2, Syed B. Qadri2, and Ishwar D. Aggarwal2 1 SFA, Inc., 2200 Defense Hwy., Suite 405, Crofton, MD, 21114 2 Code 5606, U.S. Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC, 20375 ABSTRACT Barium copper sulfur fluoride thin films with a face-centered cubic phase in the Fm3m space group were synthesized via RF magnetron sputtering. The results of a detailed optical and electronic characterization of the films are presented. As-deposited, they exhibit degenerate ptype conductivity at room temperature of approximately 260 S/cm – higher than that of any previously reported p-TC. Their conductivity after post-deposition processing increases to as high as 800 S/cm. The films exhibit bandgaps ranging from 1.45-1.75 eV. They are typically deposited with a substrate temperature between room temperature and 100 °C, making them suitable for deposition on plastic as well as glass or crystalline substrates. It was found that a silica protective layer reduces degradation in film transparency that is caused by exposure to air. INTRODUCTION Transparent conducting films with p-type charge carriers are desirable for several applications. These include multijunction solar cells, transparent electronics, low operating voltage light-emitting diodes, and electro-magnetic interference (EMI) shielding. In the last ten years, there has been considerable interest in the development of new p-type transparent conductors (p-TCs) for such applications. A variety of materials – most of them oxides – have been studied. One non-oxide material that has been the subject of recent research is barium copper sulfur fluoride (BCSF). Previous reports showed that this material exhibits a tetragonal crystal structure in space group P4/nmm [1]. When doped with potassium, it has been shown to possess p-type conductivity as high as 100 S/cm in selected bulk crystals [2]. Yanagi et al. also prepared polycrystalline thin films of BCSF by co-evaporating Cu metal and BaF2 onto silica and MgO substrates followed by treatment under flowing H2S gas [1]. They demonstrated with X-ray diffraction (XRD) data that the films are of the same phase as the bulk. The films exhibited a conductivity of ~1 S/cm and a bandgap of 3.2 eV. The authors recently reported a newly synthesized face-centered cubic phase of BCSF in the Fm3m space group that, like the P4/nmm phase, exhibits p-TC behavior [3]. The films were deposited by RF magnetron sputtering while maintaining a substrate temperature of < 100 ºC. In this proceeding, we describe a detailed optical and electronic characterization of these films. The optical transmittance of the film was characterized, and the bandgap was determined. The conductivities of the films were measured at room temperature with the 4-point probe technique. Hall bar structures were fabricated in order to characterize the films’ electronic properties with varying temperature, and th
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