In-situ Tracer Diffusion in Solids

实验测定固体中的扩散系数、活化能和扩散机制的最重要的分析方法之一是基于稳定示踪剂的使用。一种元素的稀有稳定同位素沉积在被研究材料的表面。在高温下扩散退火后，示踪剂重新分布。经过冷却和随后的二次离子质谱仪等同位素深度剖面分析，可以确定示踪剂的自扩散系数。在过去的几年里，中子反射测量方法在这一研究领域建立起来。该方法可测定薄膜和块体材料的扩散长度和扩散系数，扩散系数可达10~26m~2/S。这种方法和所有其他示踪剂方法的缺点是不能在退火期内直接对扩散过程进行现场监测。样品总是需要冷却到室温。对于各种现代亚稳态和纳米材料系统来说，在实际扩散过程中原位检测扩散系数具有很大的优势。由于中子反射方法的实际进步，在样品上可以产生更高的中子强度，这将使在不到5分钟的时间内记录到反射率图案。这将首次实现对固体物在退火过程中自扩散的原位检测。作为时间函数的扩散系数的准连续检测成为可能。此外，大大减小了单次测量的误差，可以更准确地测定活化能。本研究项目的目的是以“无定形锗”为模型系统，用中子反射仪原位定量测量退火过程中的自扩散系数。应该进行实验，以便在广泛的基础上测试该方法，并为未来的发展提供必要的条件。将进行准连续检测结构松弛和结晶过程中随时间变化的扩散系数的实验。这项工作也应该有助于在扩散科学领域建立这种方法。在这里提出的延长建议的框架内，应该测试和改进在第一阶段开发的测定单一退火热实验中活化能的方法(等时法)。此外，还应对非晶态Si1-xGex中的扩散进行系统的研究，同时将利用原位方法的优点(样品吞吐量高，扩散系数随时间变化)。

One of the most important analytical methods for the experimental determination of diffusivities, activation energies and diffusion mechanisms in solids is based on the use of stable tracers. A rare stable isotope of an element is deposited on the surface of the material under investigation. After diffusion annealing at elevated temperature the tracer is redistributed. After cooling down and subsequent isotope depth profile analysis by e. g. Secondary Ion Mass Spectrometry tracer self-diffusivities can be determined.During the last years the method of neutron reflectometry was established in this field of research. The method allows to determine diffusion length < 1 nm and diffusivities down to 10-26 m2/s on thin films and bulk materials. The drawback of this - and all other tracer methods – is that the diffusion processes cannot be monitored in-situ, directly during annealing. There is always a cool down of the sample to room temperature necessary. For various modern metastable and nanoscale materials systems an in-situ detection of diffusivities during the actual diffusion process is of big advantage.Due to the actual advancement of the Neutron Reflectometry method in form of the “Focusing Reflectometry”, higher neutron intensities can be produced at the sample, which will allow to record a reflectivity pattern in less than 5 min. This will enable an in-situ detection of self-diffusion in solids during annealing for the first time. A quasi-continuous detection of diffusivities as a function of time becomes possible. In addition, the error of a single measurement is significantly reduced and activation energies can be determined more precisely. The aim of the present research project is the quantification of self-diffusivities in-situ during annealing with neutron reflectometry using “amorphous Germanium” as a model system. The experiments should be done to test the method on a broad base and to give the necessary conditions for future development. Experiments for a quasi-continuous detection of time-dependent diffusivities during structural relaxation and crystallization will be done. The work should also help to establish this method in the field of diffusion science.In the framework of the prolongation proposal present here, a method for the determination of activation energies in a single annealing experiment (isochronic method) developed during the first period should be tested and refined. In addition, systematic investigations on the diffusion in amorphous Si1-xGex should be carried out, while the advantages of the in-situ method (high sample throughput, time dependence of diffusivities) will be used.