Optical memory based on a metal–insulator–semiconductor–insulator–metal structure

An analysis is made of the properties of a metal–insulator–semiconductor–insulator–metal structure which can be used in memory devices with optical write and read stages. It is shown that optically recorded information can be stored in the form of free-charge layers which screen the external field for several days. The capture of free carriers by surface centers makes it possible to dispense the division of a memory into separate cells. The information can be erased by the application of a voltage pulse. The time of drift of free carriers (~10^–9 sec) into the surface layers governs the response in the write stage. The resolution is determined by the edge effect of the electric field in microcapacitors formed in such a structure and by the diffusion of carriers: The resolution may amount to 300 lines/mm. The information is read out using the Franz–Keldysh effect or the electrooptic effect. The main disadvantage of the first method is the change in the information during the readout process, which reduces considerably the information storage time. The electrooptic readout method requires higher electric fields, and consequently, materials of high electric strength. It is shown that ZnS can be used (its electric strength permits the use of fields of ~10⁵ V/cm). Other suitable materials include ZnTe and CdTe, which have the highest known electrooptic coefficients.