极薄栅极绝缘层锗基MOS器件的平带电压漂移机理及其抑制方法的研究

Further enhancement of the Si metal-oxide-semiconductor field-effect transistor (MOSFET) performance is becoming increasing difficult due to the limitation of conventional device scaling. Germanium has been attracting a lot of interest as the most promising candidate as channel material of MOSFET to replace Si, because of not only its high bulk hole and electron mobility but also the compatibility with Si technology. In order to realize high performance Ge MOSFET, advanced Ge gate stacks with high quality and thin thickness simultaneously are mondatory. Recently, many researches have been carried out to improve the properties of Ge gate stacks. However, the flat band voltage shift, which is one of the major problems in Ge gate stacks, is still not solved yet. Additionally, the physical mechanism resulting in the flat band voltage shift of Ge gate stacks is also not clear, in spite of importance. In this research, the ultrathin Al2O3/GeOx/Ge gate stacks, which has been demonstrated to be one of the most promising gate stack structures for Ge MOSFETs, will be invesgated intensively. The defects dominating the flat band voltage shift will be characterized, and the physical mechanism of their generation will be studied. The impact of MOS (metal-oxide-semiconductor) interface structure on the flat band voltage shift will be discussed. Through the control of MOS interface structure, the flat band voltage of these Ge gate stacks will be modulated. On the other hand, an in-situ nitridation will be also carried out for the Al2O3/GeOx/Ge gate stacks to further suppress the generation of fix charges and dipoles in the gate stacks, revealing an AlON/GeOx/Ge gate stack with ultrathin thickness, low defect density and minimized flat band voltage shift.

传统的硅器件微细化方法已接近物理极限，因此进一步提高硅沟道场效应晶体管的性能已比较困难。锗由于具有非常高的电子、空穴迁移率，以及和当前硅器件工艺的良好兼容性，有希望作为新一代场效应晶体管的沟道材料。高质量的极薄栅绝缘层是实现高性能锗沟道场效应晶体管的关键问题，受到了广泛关注。尽管近年来锗基栅绝缘层的研究已取得较大进展，但是目前仍然无法解决其平带电压漂移的问题。本项目将从极薄Al2O3/GeOx/Ge栅绝缘层的平带电压漂移机理入手，对造成这一问题的关键原因进行分析并澄清其物理起因。通过对锗MOS界面的成分和结构的控制，实现对锗基MOS器件平带电压的调控。在此基础上，还将通过对Al2O3/GeOx/Ge栅绝缘层在表面极薄范围内进行原位氮化的方法，在保持高质量GeOx/Ge界面的情况下制备AlON/GeOx/Ge栅绝缘层，减少栅绝缘层中固定电荷、偶极子等缺陷，实现对锗基MOS器件平带电压的调控。

本项目的研究从极薄Ge基栅极中平带电压漂移的机制出发，首先探讨了极薄Ge基栅极中的缺陷种类及其表征方法。研究发现，极薄Ge基栅极中造成平带电压漂移的重要因素是能带内的界面态。针对这一特点，开发了原子状重氢退火技术，有效地抑制了Ge基栅极中的能带内界面态，并提升了Ge MOSFET器件的电学特性（迁移率）。.基于这些发现，本项目进一步开发了如何在更薄的Ge基栅极中获得更低的界面态密度。我们发现利用Ge原子掺杂的手段可以诱导HfO2薄膜晶化为立方相，形成具有更高相对介电常数的higher-k栅氧。从而突破了传统工艺中减薄栅极等效氧化层厚度导致界面态密度增大的瓶颈，同时实现了栅极等效氧化层厚度的减薄和界面态密度的减小。但是，实验发现，在充分减薄栅极厚度和抑制界面态密度（提升迁移率）后，器件的输出特性仍然较差。通过对器件的分析，确定了这是由于Ge MOSFET中源漏电阻较大导致的。因此，本项目中进一步开发了微波退火技术，实现了低寄生电阻和高肖特基势垒高度的NiGe/Ge肖特基结。利用这一结构作为金属源漏，实现了高性能Ge MOSFET器件。为了充分理解Ge MOSFET器件的电学特性，尤其是在高场下器件中的迁移率与理论不完全吻合的现象，本项目利用舒伯尼科夫-德哈斯法从实验上测定了各种晶向的Ge沟道中反型层载流子的有效质量。实验发现Ge沟道中的反型层载流子有效质量远大于价带顶和导带底的理论值，同时载流子有效质量随电场增强而显著增大，表明当载流子占据能带中的较高能级后，抛物线近似已无法充分描述载流子的E-k关系。这些结果对正确理解Ge MOSFET器件的电学特性具有重要意义。