With the goal of producing high-density terabits/inch2 storage, we are studying a method in which nanometer-sized marks are recorded on Ge2Sb2Te5 (GST) phase-change media by current beams injected from sharpened probes. The current beams can inject powers with lower transmission losses into nanometer-sized local regions that are much smaller than the refraction limit of light. Moreover, the difference in the conductivity between amorphous and crystalline GST is orders larger than the difference in their refraction index. Therefore, the current beams are advantageous for such high-density data storage with nanometer-sized marks.

We have experimentally and numerically studied phase changes in GST using probe current injections. The structures of the media are composed of GST and a metal electrode stacked on a polycarbonate substrate. We simulated temperatures, crystallization rates, and mark sizes of GST as a function of power injection conditions. The results clarify that a small crystalline mark can be stably made in an amorphous region by injecting probe current pulses with the same beam sizes. For supplying higher injection powers to produce phase change, metal oxide, such as indium-tin- oxide (ITO), is more robust at high powers than metals. We have developed conductive transparent (CT) probes consisting of ITO coated on a fiber tapered to a point with a nanometer-scale radius. The CT probe can inject current in a nanometer-sized region and simultaneously illuminate the same local region. We also propose a new method of optical rewritable phase-change storage using the CT probes.