Mechanism of ultimate recording density in nano-scale phase-change memories

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Performance of rewritable digital-versatile-disk (DVD) memories is steady progressing. Then, a main object of the present study is to give an answer to a question "How high can we increase the recording density? ", which can be paraphrased to "How small can we reduce the mark size in phase change memories? ", since a smaller mark can give a higher recording density. It is known that, in the devices presently commercialized, the minimal mark size is 150 nm. However, this size is governed by the wavelength of the recording light, and we expect that a recording film Ge_2Sb_2Te_5 may be capable to produce much smaller marks.To produce the smallest mark, I have adopted two notions. One is that, instead of the optical phase change, the electrical phase change should be used, since smaller marks will be written using small electrodes. The other is, as the electrode, to employ the scanning probes in a scanning tunneling microscope (STM) and an atomic force microscope (AFM). These machines will produce the ultimately small mark even in a common laboratory room without dust controlling.The present study has demonstrated that the minimal size can be reduced to 〜10 nm. Interestingly, both of the minimal marks, i.e. a crystallized mark in an amorphous film and an amorphous mark in a crystalline film, the latter being the method presently used in DVDs, are around 10 nm. Consequently, it can be assumed that the recording density can surpass 1 TB / layer in a near future.The origins of the ultimate mark size of 〜10 nm have been investigated. The minimal size of crystallized marks can be understood thermodynamically. On the other hand, that of the amorphous mark is governed by the size of crystallites in crystalline recording films.In addition, un-stability of Ge_2Sb_2Te_5 films has been pointed out through studies on the electrical phase change. For electrical phase-change random access memories (PRAM), newer materials will be needed.