Crystallize It before It Diffuses: Kinetic Stabilization of Thin-Film Phosphorus-Rich Semiconductor CuP₂

Abstract

Numerous phosphorus-rich metal phosphides containing both P–P bonds and metal–P bonds are known from the solid-state chemistry literature. A method to grow these materials in thin-film form would be desirable, as thin films are required in many applications and they are an ideal platform for high-throughput studies. In addition, the high density and smooth surfaces achievable in thin films are a significant advantage for characterization of transport and optical properties. Despite these benefits, there is hardly any published work on even the simplest binary phosphorus-rich phosphide films. Here, we demonstrate growth of single-phase CuP2 films by a two-step process involving reactive sputtering of amorphous CuP2+x and rapid annealing in an inert atmosphere. At the crystallization temperature, CuP2 is thermodynamically unstable with respect to Cu3P and P4. However, CuP2 can be stabilized if the amorphous precursors are mixed on the atomic scale and are sufficiently close to the desired composition (neither too P poor nor too P rich). Fast formation of polycrystalline CuP2, combined with a short annealing time, makes it possible to bypass the diffusion processes responsible for decomposition. We find that thin-film CuP2 is a 1.5 eV band gap semiconductor with interesting properties, such as a high optical absorption coefficient (above 105 cm–1), low thermal conductivity (1.1 W/(K m)), and composition-insensitive electrical conductivity (around 1 S/cm). We anticipate that our processing route can be extended to other phosphorus-rich phosphides that are still awaiting thin-film synthesis and will lead to a more complete understanding of these materials and of their potential applications.

Indirect Band Gap Semiconductors for Thin-Film Photovoltaics: High-Throughput Calculation of Phonon-Assisted Absorption

“…In general, phosphorus-rich phosphides have not been studied as potential PV materials before. They had hardly been grown in the thin-film form until a very recent report of single-phase CuP 2 films in the P 2 1 / c structure by reactive sputtering and soft annealing in an inert atmosphere …”

Is Cu3–xP a Semiconductor, a Metal, or a Semimetal?

“…This possibility has allowed us to grow polycrystalline CuP 2 films by a two-step process. 12 Fourth, PH 3 assists the crystallization of Cu 3−x P even at room temperature, where films sputtered in pure Ar are amorphous (Figure 2). This effect is presumably caused by bombardment of the growing film by energetic species formed from the dissociation of PH 3 in an RF plasma.…”

“…The steep increase in resistivity on the left side of the minimum is due to the much higher resistivity of CuP 2 (around 1 Ω cm) with respect to Cu 3−x P, as shown elsewhere. 12 On the right side of the minimum, the resistivity increases until the Cu 3.8 P composition is reached (Figure 5a). Since metallic Cu peaks begin to appear in XRD at a similar composition (Figure 1b), we assume that the resistivity increase is caused by changes in concentration of point defects up to the Cu precipitation threshold.…”

“…11 CuP 2 is an unconventional semiconductor containing P−P bonds, with optimal band gap and doping density for photovoltaics. 12 Cu 3 P is another copper(I) phosphide, which has been synthesized in the form of single crystals, 13,14 powders, 15−18 thin films, 19−23 and especially nanoparticles and other nanostructured forms. 24−30 Cu 3 P is usually found to be strongly p-type due to substantial Cu deficiency 13,17 and is therefore often referred to as Cu 3−x P. In recent years, Cu 3−x P has been the subject of intense research as an electro-and photocatalyst for various reactions, 20,21,25,27,28,30 as well as a battery anode.…”

“…Cu 3 N is a defect-tolerant semiconductor with an ideal band gap for photovoltaics . CuP 2 is an unconventional semiconductor containing P–P bonds, with optimal band gap and doping density for photovoltaics …”