Microcharacterisation of bulk, ion beam processed and nanoparticle cadmium sulphide (CdS)

摘要

Cadmium sulphide (CdS) is a II-VI semi-conductor with a direct band gap of 2.42 eV for the hexagonal crystal structure at room temperature. CdS is an important electronic and optoelectronic material with applications in light-emitting diodes, lasers, flat panel displays, etc. Bulk CdS crystals at three different orientations were modified using a Ga+ focused ion beam (FIB). The effects of this modification were investigated using Atomic Force Microscopy (AFM), Kelvin Probe Microscopy (KPM) and Cathodoluminescence (CL) techniques. These techniques provide information about the morphology, contact surface potential and crystal structure (including defect structure, impurities etc) of the semiconductor crystal. All of these factors are important in any subsequent application of the CdS. The bulk crystals were prepared either by mechanical polishing or by Ga+ ion beam polishing. It was found that the Ga+ ion beam sputtering depths increased linearly with exposure to the beam and at the same rate for each crystal orientation. The contact surface potential in the mechanically polished crystals increased quickly with dose until an equilibrium was reached, but in the ion beam polished samples the increase in contact surface potential was linear. The UV-visible CL spectra (1.3 – 6 eV) for the bulk crystals consisted of two peaks – the intrinsic peak at ~ 2.42 eV and a defect peak at ~ 1.7 eV. The CL emission was highly sensitive to the scratch damage caused by the mechanical polishing. Ion beam polishing removed most of the scratch damage and allowed the effect of ion beam sputtering to be investigated. The intensity of the defect peak did not change significantly with dose. The intrinsic peak was initially reduced in intensity and then increased to an equilibrium point.The CL characteristics of cadmium sulphide powder crystals were also investigated and compared to the characteristics of CdS microstructures of similar size patterned using the FIB. The characteristics of the powder and the patterned structures were similar.This study shows that AFM, KPM and CL are all useful techniques for the characterization of FIB modified CdS.