Fabrication of Nanocrystal Memories by Position Controlled Deposition of Ge Nanocrystals

在这个研究项目中，我们尝试用硅衬底来制作纳米晶存储器，在硅衬底上沉积了Ge纳米晶，并且很好地控制了定位和大小。法国马赛CRMC2的Berbezier博士为硅衬底提供了分子束外延沉积的Ge纳米晶，我们在SiO_2中嵌入Ge纳米晶制作了MOS电容器。该项目最重要但也是最困难的部分是沉积定位和尺寸控制良好的Ge纳米晶。Berbezier博士和她的团队成员开发了一种在FIB图案的硅衬底上生长Ge纳米晶体的技术。在这种技术中，通过聚焦离子束(FIB)将Ga注入到硅衬底上，并在局部耕作缺陷区。发现在Ge分子束外延生长中，Ge选择性地生长在损伤区。利用这种Ge的选择性外延，可以排列Ge纳米晶。在优化的生长条件下，Berbezier博士成功地在硅上实现了Ge纳米晶体的阵列。然而，…更多的是，阵列不是一个单分子层，一些Ge纳米晶堆积在一起。这样的样品没有通过在Ge纳米晶体中注入电子而显示出显著的平带电压漂移。Nozaki和他的团队提出了一种在单层中形成高密度Ge纳米晶体的技术。在这项技术中，通过超音速喷嘴的气体蒸发在隧道氧化物上沉积了Ge纳米晶。采用这种沉积方法，可以得到尺寸均匀的Ge纳米晶。然而，它们是堆积如山的。对Ge纳米晶进行了退火处理，以去除单层上多余的纳米晶。由于Ge纳米晶与SiO_2之间的结合强度很强，在完全去除多余的纳米晶后，在SiO_2上只保留了一层Ge纳米晶而没有失去任何纳米晶。然后，将Ge纳米晶暴露在紫外光下进行光氧化，通过氧化Ge纳米晶来电隔离Ge纳米晶。在Ge纳米晶上沉积控制氧化物后，制备了以Ge纳米晶为浮栅的MOS电容器。C-V曲线表现出滞后现象，证实了电子在纳米晶中的注入。虽然含Ge纳米晶的栅电极在纳米晶存储器中是有用的，但实际应用还需要进一步提高电荷保持能力。较少

In this research project, we made an attempt to make a nanocrystal memory using a silicon substrate on which Ge nanocrystalls are deposited with a good control of the positioning and size. Dr. Berbezier at CRMC2, Marseille, France provided the Si substrates with Ge nanocrystals deposited by MBE, and we made MOS capacitors by embedding Ge nanocrsytals in SiO_2. The most important but difficult part of the project is to deposit Ge nanocrstals with a good control of positioning and size. Dr. Berbezier and her group members developed a technique to grow the Ge nanocrstals on the FIB-patterned Si substrates. In this technique, the Ga is implanted in the Si substrates by focused ion beam (FIB) and farms the defective area locally. It was found the Ge selectively grew on the damaged area in the MBE growth of Ge. Using this selective epitaxy of Ge, Ge nanocrystals can be arranged. With the optimized growth condition, Dr. Berbezier successfully achieved an array of Ge nanocrystals on Si. Howeve … More r, the array was not one monolayer, and some Ge nanocrystals were stacked. Such a sample did not show a significant flatband voltage shift by injecting electrons in Ge nanocrystals.Nozaki and his group proposed a technique to form a high-density of Ge nanocrystals in a monolayer. In this technique, the Ge nanocrystals were deposited on the tunnel oxide by the gas evaporation with a supersonic jet nozzle. Using this deposition method, the Ge nanocrystals with a good uniformity in the size were obtained. They are, however, stacked. The Ge nanocrystals were annealed to remove the extra naocrystals on the monolayer. Because of strength of bonding between Ge nanocrystals and SiO_2, one monolayer of Ge nanocrystals remained without losing any nanocrystals on SiO_2 after complete removal of the extra nanocrystals. Then, the Ge nanocrystals were exposed to UV light for photo-oxidation, which electrically isolates the Ge nanocrystals by oxidizing the Ge nanocrystals. After depositing the control oxide on the Ge nanocrystals, the MOS capacitors with the Ge nanocrystals as a floating gate were fabricated. The C-V showed the hysteresis to confirm the electron injection in the nanocrystals. Although the gate electrode with the Ge nanocrystals proved to be useful in the nanocrystal memories, the further improvement of charge retention is required for practical application. Less