Microstructure, optical and electrical properties of thin films of gallium- phosphorus-titanium alloys synthesized by asymmetric bipolar pulsed direct current magnetron sputtering

摘要

Thin films of gallium-phosphorus-titanium (Ga-P-Ti) alloys were prepared on glass substrates at 573 K by an asymmetric bipolar pulsed direct current sputtering technique using an argon atmosphere and targets made from gallium phosphide (GaP) powder and metallic titanium (Ti), at the surface ratios of 8:1, 5:1, 2:1 and 1:1 GaP to Ti (GaP:Ti) on the sputtered area. Examination by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscope indicated that the as-deposited films from the sputtering targets having GaP:Ti ratios of 8:1, 5:1, and 2:1 were polycrystalline with the cubic zinc-blende crystal structure having GaP as the host material, i.e., Ti-doped GaP. Elemental compositions of the film obtained from the target at a GaP:Ti ratio of 5:1 closely resembled the theoretically predicted intermediate band compound Ga4P3Ti. It was projected that the Ga4P3Ti compound could be fabricated by co-sputtering of GaP and Ti from a single target having the surface area ratio GaP:Ti of 3.5:1. Optical transmission and reflection spectra, temperature dependence of electrical resistivity, and light response of the electrical resistivity showed semiconductor-like behavior for the films obtained from the targets with the GaP:Ti of 8:1 and 5:1, and were metal-like for those deposited from the other targets. Optical band gaps determined from the transmission spectrum of the semiconducting films by Tauc's expression for indirect transition were 1.2-1.5 eV. The results of the study could provide an alternative route for fabricating the intermediate band material based on the Ga-P-Ti system.