Sensitivity enhancement of EPR spectroscopy on single crystalline surfaces under UHV conditions by employing an electrical detection scheme

在超高真空(UHV)条件下，传统的单晶表面电子顺磁共振(EPR)谱的灵敏度往往不足以检测高质量表面上有限数量的缺陷位。被称为EDMR光谱的信号的电检测提供了将传统的CW-EPR光谱的灵敏度提高几个数量级的可能性，并且被证明能够检测到不到1000个自旋。在目前的项目中，这种检测方案将被实施到超高电压设备中，以表征半导体表面的顺磁缺陷。顺磁缺陷对许多硅基器件(如太阳能电池)的功能特性起着至关重要的作用，这是因为这些缺陷在材料的带隙中产生状态，作为电荷载流子的复合位置。通过对系统进行化学和热修改，可以显著改善缺陷的性质，从而提高器件的性能。由于缺乏足够灵敏的现场EPR分析，这些修饰对表面缺陷的影响变得非常难以评估。一方面，本项目致力于研究超高真空条件下低指数硅表面(001)和(111)表面的缺陷。原始表面的顺磁性缺陷将被表征，并随后在超高真空条件下进行修饰，这提供了将所观察到的缺陷结构的变化与随后的热处理或化学处理(如氢化或氧化)改性相关联的可能性。另一方面，该策略将被应用于表征单晶立方碳化硅(001)表面的顺磁性缺陷。除了表征原始表面上的缺陷作为终止和简单气体如氧和氢的吸附的函数外，特别感兴趣的是探索在吸附后缺陷结构的改变，例如用于有机/无机混合太阳能电池的低聚(3-己基噻吩基)(P3HT)分子。

The sensitivity of conventional electron paramagnetic resonance (EPR) spectroscopy on single crystal surfaces under ultrahigh vacuum (UHV) conditions is often insufficient to detect the limited number of defect sites available on high quality surfaces. Electrical detection of the signal known as EDMR spectroscopy offers the possibility to enhance the sensitivity of conventional cw-EPR spectroscopy by several orders of magnitude and was shown to be able to detect less than a thousand spins. In the current project this detection scheme will be implemented into an UHV apparatus to characterize paramagnetic defects at semiconductor surfaces. Paramagnetic defects play a crucial role for the functional properties many silicon-based devices such as solar cells, due to the fact that these defects create states in the band gap of the material, which act as recombination sites for charge carriers. The properties of the defect and hence the performance of the device can be significantly improved by chemical and thermal modifications of the system. Due to a lack of in-situ EPR analytics with sufficient sensitivity the impact of these modifications on the surface defects become very difficult to assess. On the one hand, this project focuses on the investigation of defects on the low index silicon surfaces (001) and (111) under ultrahigh vacuum conditions. The paramagnetic defects of the pristine surfaces will be characterized and subsequently modified under UHV conditions, which offers the possibility to correlate the observed changes in the defect structure to modification by subsequent thermal or chemical treatments such as hydrogenation or oxidation.On the other hand this strategy will be applied to characterize paramagnetic defects on single crystalline cubic SiC(001) surfaces. Apart from the characterization of defects on the pristine surface as a function of the termination and the adsorption of simple gases such as oxygen and hydrogen it will be of particular interest to explore the modification of the defect structure upon adsorption of molecules such as oligo(3-hexylthiophene) (P3HT), which are used in hybrid organic/inorganic solar cells.