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

可重写数字多功能盘(DVD)存储器的性能正在稳步发展。然后，本研究的一个主要目的是给出一个问题的答案：我们可以将记录密度增加到多高？，该问题可以被解释为“我们可以将相变存储器中的标记尺寸减小到多小？”，因为更小的标记可以提供更高的记录密度。众所周知，在目前商业化的器件中，最小标记尺寸为150 nm。然而，这个尺寸是由记录光的波长决定的，我们预计记录膜Ge_2Sb_2Te_5可能能够产生更小的标记。为了产生最小的标记，我采用了两个概念。一种是，应该使用电相变而不是光学相变，因为将使用小电极写入较小的标记。另一种是将扫描探针用作扫描隧道显微镜(STM)和原子力显微镜(AFM)的电极。即使在没有粉尘控制的普通实验室里，这些机器也会产生最终很小的印记。目前的研究表明，最小尺寸可以减少到大约10纳米。有趣的是，这两个最小标记，即非晶膜中的结晶标记和结晶膜中的非晶标记，后者是目前在DVD中使用的方法，都在10 nm左右。因此，可以假设在不久的将来，记录密度可以超过1Tb/层。结晶痕迹的最小尺寸可以从热力学上理解。另一方面，非晶态标记的稳定性取决于晶态记录薄膜中的微晶尺寸。此外，通过对电相变的研究，指出了Ge_2Sb_2Te_5薄膜的不稳定性。对于电相变随机存取存储器(PRAM)，将需要更新的材料。

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.