Takeo Ohta

Tellurium suboxide thin films TeO_x were found to change in refractive index and extinction coefficient on thermal or optical heating, with accompanying changes in the reflectivity and transmission. The preparation method and thermal or optical properties of the TeO_x thin films were investigated to obtain a stable and highly sensitive optical disc memory. A two‐source evaporation method using Te and TeO₂ provided uniform and any desired composition. The properties of the film depended on the x value; increasing x to as large as x=1.2, which represents a Te‐poor composition, produced an excellent humidity and heat stability, and decreasing x to 0.8 made the film more sensitive to a laser diode, but susceptible to humidity. The TeO_1.1 thin film was found to sufficiently satisfy all requirements for practical disc applications. A reflective optical disc was prepared using the TeO_1.1 thin film deposited on a polymethylmethacrylate substrate with grooves for optical tracking. This disc is capable of recording video signals at real time by laser diodes. It showed no reflectivity changes on recorded or unrecorded areas and no sensitivity change to laser light after long term storage in high humidity and temperature.

Thin films of SbOx, GeOx, TeOx, MoOx, InOx and PbOx ( x is smaller than the stoichiometric value for each component ) were found to have a feasibility to be applied to an optical recording disc. Upon heating, these thin films are critically changed in their optical constants at 150°, 280°, 120°, 150°, 120° and 100°C, respectively. The contrast ratios before to after heat treatment of the GeOx, TeOx and MoOx films of 1000 - 2000 Å thick are very high, they are 50, 20 and 15, respectively. Among them, the TeOx (x ≈1.1) film was excellent in both the sensitivity and the contrast ratio, and an optical recording disc was prepared by depositing the TeOx on a PMMA disc, substrate. Video-signals can be recorded and played back using a laser diode (λ =8300 Å, N.A.= 0.5) at the laser power of 6 - 8 mW incident. The carrier to noise ratio at 5 MHz was in excess of 55 dB.