Structural, optical and electrical properties of RuS2 ± x films, prepared by reactive magnetron sputtering

Abstract

Thin layers of ruthenium sulfide (RuS2±x, laurite) were deposited on a heated substrate by reactive magnetron sputtering using a metallic ruthenium target and H2S as reactive gas. The balanced magnetron plasmas were excited by a direct-current (DC) or a radio-frequency (RF, 27MHz) power supply, respectively. To comprehensively study the influence of substrate temperature, H2S partial pressure and total pressure of Ar–H2S mixtures of different ratio on film composition, density, film structure, morphology and optical as well as electrical properties series of layers were deposited on different substrates such as titanium foil, glassy carbon and oxidized silicon wafers. Highest density of 5.5 to 5.6gcm−3 in films of 600nm thickness deposited on glassy carbon was found using a total sputtering pressure in the sputter chamber of 0.5–1.5Pa, a DC sputter power of 100W and an Ar:H2S ratio=3:1 at a substrate temperature Tsub=360°C.Layers deposited on glassy carbon at a pressure>1Pa were sulfur rich with a S-to-Ru ratio of 2.1. To elucidate phase formation during film growth in-situ energy dispersive X-ray diffraction experiments were performed employing Ti foil and thermally oxidized Si wafers as substrates. In both cases ruthenium rich (S:Ru=1.7) seed layers were formed prior to the growth of nearly stoichiometric laurite crystallites. Evaluating the width of the diffraction peaks 111 and 200 a growth of laurite grains from 20 to 40nm was inferred when increasing the substrate temperature from 150 to 450°C. The formation of larger grains is correlated with an increase in resistivity. Absorption spectra obtained from laurite layers deposited under RF sputter conditions at substrate temperatures of 360 and 430°C indicate a direct band gap of 1.8eV and a high number of defect states in the band gap.