Formation of secondary iron-sulphur phases during the growth of polycrystalline iron pyrite (FeS₂) thin films by MOCVD

Thin films of iron pyrite (FeS₂) have been prepared on glass and glassy carbon substrates by low pressure metal organic chemical vapour deposition (LP-MOCVD) using iron pentacarbonyl (Fe(CO)₅) and di-tert.-butyldisulphide (TBDS) as precursors. The TBDS partial pressure was varied from 1 to 100 Pa for different iron pentacarbonyl partial pressures (0.25, 0.5 and 1 Pa) while all other parameters were maintained constant. It was found that there is a critical TBDS-partial pressure of about 30 Pa for a deposition temperature of 475°C, where a drastic change in the layer properties occurs. Below this TBDS partial pressure pyrrhotite type phases (Fe_1-xS) will be formed although there is a sulphur precursor excess in the gas phase. If the layers contain pyrrhotite, the electrical properties of the FeS_x-films are changed significantly. The occurrence of the pyrrhotite phases does not depend on the growth rate, hence it is not controlled kinetically. Therefore, the sulphur pressure above the growing pyrite film is the important parameter controlling the solid phase. The present investigation shows that, in order to prepare pyrite thin films of good electronic quality, one has to take care to avoid the secondary sulphur-iron phases even in very small concentrations.