Optical recording characteristics of WO3 films grown by pulsed laser deposition method

Abstract

W O 3 films were deposited on the glass substrate (Corning No. 7059 with an area of 26×38mm) by the pulsed laser deposition method using an ArF excimer laser. It was found that after annealing at 500°C for 10min, the film thickness became 1.8 times compared with that (approximately 40nm) in the as deposited state. At this time, the difference in the transmittance, ΔT, between the annealed state and the as deposited state was about 40% at the wavelength of 400nm. From x-ray diffraction spectra and x-ray photoelectron spectroscopy spectra, it was considered that the ratio of the peak values of W6+ 4f5∕2 (tungsten oxide) versus W 4f5∕2 (metal tungsten) increased steeply after the annealing process. From this, it was considered that oxygen was absorbed into the WO3 films through the annealing process. From the revolution test for the sample without the protection layer in which the WO3 films were deposited upon the digital versatile disk disk substrate, a write peak-power dependence of carrier to noise ratio (CNR) (at λ=406nm, NA=0.65) of 3T signal (58.5MHz) was measured at a linear velocity of 5m∕s and a read power of 0.6mW. It was confirmed that the values of CNR obtained at the write peak-power 5–6mW were near 50dB (the region A) and the ones obtained at the write power 7–10mW were more than 60dB (the region B). From scanning electron microscopy observation, it was recognized that bits with 0.16–0.25μm size, having the fine-shaped dots with clear-cut edge, were made in the region A. This corresponded to the maximal storage capacity of 25GB in the “Blu-ray disk” specification. However, it was also identified that holes were formed in the region B and the film materials were scattered by receiving a train of high write power impulses. Next, it was found that the write power corresponding to region A for the structures with the WO3 layer sandwiched between a Al2O3 or ZnS–SiO2 protection layer increased or decreased, respectively. Larger values of the CNR will be obtained if the film thickness of each layer including the active layer were optimized.