Growth of polycrystalline zinc phosphide thin films by reactive radio frequency magnetron sputtering

• Polycrystalline α-Zn3P2 films were grown on glass by r.f. sputtering of Zn and PH3.
• Films had crystallite size of 100–150 nm and low surface roughness (< 10 nm).
• Films exhibit bulk-like optoelectronic properties (Egdirect ~ 1.58 eV; n ~ 1016 cm− 3).
• Schottky diodes were fabricated using Mg.

Polycrystalline, tetragonal (P42/nmc) zinc phosphide (Zn3P2) thin films were deposited on borosilicate glass substrates by reactive radio frequency (r.f.) sputtering of zinc (Zn) in the presence of phosphine gas (PH3). Various sputtering parameters, including substrate temperature (Tsub), gun power, gas flow rates of PH3 and argon (Ar), and total sputtering pressure (PTot) were varied to optimize crystalline growth. Under optimal growth conditions [Tsub ~ 200–230 °C, PH3:(PH3 + Ar) < 0.1, and PTot: 1.30–2 Pa], films were polycrystalline in nature, with crystallite sizes up to 100–150 nm and orientation along the (004) direction. Lowering the Tsub or increasing the PH3 content resulted in amorphous growth. The stoichiometry of the films was measured by top-down energy dispersive X-ray spectroscopy (EDS), and cross-sectional EDS mapping indicated compositional homogeneity throughout the film depth. The average surface roughness under optimal growth conditions was less than 10 nm. As-grown films were p-type, with high carrier concentrations (~ 1016 cm− 3) but low mobilities (< 1 cm2/V·s). Ex situ annealing of films in nitrogen or argon/hydrogen (Ar/H2) led to an order of magnitude decrease in the film's resistivity. Films exhibited a direct and an indirect optical bandgap of 1.58 and 1.44 eV, respectively, as measured by ellipsometry. Equipment limitations defined the reactive sputtering conditions explored here, but the data suggest that film properties may be further improved with deposition at higher substrate temperatures (> 250 °C) and higher sputtering pressures (> 2 Pa).