Surface Modification of Silicon at the Nanometer Scale

为了提高激光微纳结构硅的效率和适用性，采用时间分辨测量和理论模拟相结合的方法，对硅与单脉冲和多脉冲辐射的相互作用进行了全面的研究。我们发展了一个混合原子-连续介质多尺度模式，它在一个单一的介观计算方法中结合了几种数值技术。该模型至少包括五个物理现象：第一，分子动力学部分，用于在原子水平上描述物质在激光诱导下的瞬态态。第二，激光产生的自由载流子(电子-空穴对)的影响是连续的。第三，由于新发展的原子间势而产生的非热相变。第四，SPP激发对激光能量沉积的影响。最后，用CGMD方法模拟了低激发下的大体积物质和局域平衡条件下光滑的激光诱导相变。此外，所获得的构成最终周期图案的激光诱导结构可以根据它们的新的光学性质在从头计算中分别进行研究。理论模拟得到了光谱成像泵浦-探测椭偏测量的支持，这使得能够全面地确定辐照时的复折射率。因此，对硅表面重构过程及其瞬时光学性质的研究可以看作是以下理论和实验目标的成功实现和相互联系：1)发展超短激光-脉冲与半导体相互作用的理论描述。2)用实验测量验证模型预测。3)研究非热过程在硅激光诱导相变中的作用。4)发展IT技术中具有所需光学性质的硅表面功能化实验的设计方法。实验目标：1)扩展光谱泵浦探头设置在紫外光中的范围。2)综合研究瞬态复折射率ñ(t，H，λ)。3)通过实验和模拟的互补考虑，建立了光学梯度折射率模型。4)确定了多脉冲辐照形成LIPSS的暂态复折射率ñ(t，H，λ)。本项目研究了以硅为代表的一大类半导体的暂态。因此，在这个项目中获得的许多成果可以扩展到In、As、Ge和其他微电子和纳米技术中高度需要的材料。对激光诱导过程的深入理解有助于利用硅的瞬变光学性质进行操纵，从而开发出新一代纳米级的电子器件。

In order to improve the efficiency and applicability of laser micro- and nano-strucutring of silicon, the interaction of silicon with single- and multi-pulsed radiation is comprehensively investigated by time-resolved metrology and theoretical simulations. We develop a hybrid atomistic-continuum multiscale model that combines several numerical techniques in a single mesoscopic computational approach. The model includes at least five physical phenomena: First, the MD part for describing the laser-induced transient states of matter at the atomic level. Second, the effect of laser-generated free carriers (the electron-hole pairs) accounted in continuum. Third, non-thermal phase transitions due to newly developed interatomic potential. Fourth, the effect of SPP excitation on the laser-energy deposition. And finally, the CGMD method for modeling of large volumes of matter under low excitation and smooth laser-induced phase transitions under local equilibrium conditions. Moreover, the obtained laser-induced structures constituting the final periodic patterns can be separately studied in ab-initio calculations subjects to their new optical properties. The theoretical simulations are supported by spectroscopic imaging pump-probe ellipsometry measurements, which enable the comprehensive determination of the complex refractive index upon irradiation. Therefore, the investigation of the Si surface restructuring processes and its transient optical properties can be seen as a successful implementation and interconnection of the following theoretical and experimental objectives: Theoretical objectives:1) Development of the theoretical description of the ultrashort laser-pulse interaction with semiconductors on the example with Si.2) Verification of the model predictions with the experimental measurements.3) Investigation of the role of non-thermal processes involved into the laser-induced phase transitions in Si.4) Development of the methodology of designing the experiment for functionalization of Si surface with demanded optical properties in IT-technologies.Experimental objectives:1) extend spectral range of pump probe setup into the UV.2) comprehensive investigation of transient complex refractive index n ̃(t,H,λ).3) develop optical gradient index model by complementary consideration of experiment and simulation.4) determine transient complex refractive index n ̃(t,H,λ) for multipulse irradiation for LIPSS formation.The project studies the transient state of Si as a representative of a large group of semiconductors. A number of results obtained in this project, therefore, can be extended to In, As, Ge, and other materials, highly demanded in microelectronics and nanotechnologies. The deep understanding of the laser-induced processes can result in ability of manipulation with the transient optical properties of Si and development of new generation of electronic devices at the nanoscale.