Field emission properties of lanthanum sulfide thin films deposited on Si and InP substrates by pulsed laser deposition

Thin films of lanthanum monosulfide (LaS) have been successfully deposited on Si and InP substrates by pulsed laser deposition. The values of deposition parameters (chamber pressure, substrate temperature, substrate-to-target separation, laser energy, repetition rate and spot size on the target) leading to a successful growth of cubic rocksalt structure films are identified. For the films grown on Si substrates, the root-mean-square (RMS) variation of surface roughness is found to be 1.74 nm by atomic force microscopy. XRD scans suggest the presence of amorphous regions in the film. This is confirmed by cross-section transmission electron microscopy analysis. For films grown on InP substrates, a thin crystalline film about 500 nm thick followed by an abrupt transition to an amorphous layer about 120 nm thick is observed. The work function of the LaS thin films deposited on Si substrates deduced from Fowler-Nordheim plots at room temperature has a mean value of 0.65 eV, when measured using the scanning field emission microscope technique (SAFEM) at very different locations across the sample. A strong dependence of the field emission current with temperature is also observed. At very large external electric field, the field emission increases dramatically and is no longer of the Fowler-Nordheim type.