Experiment using SPring-8
The X-ray is ideal for exploring atomic structure and electron structure. Since the X-ray is an electromagnetic wave, it interacts with electrons. In addition, most electrons gather around the atomic nucleus. By close analysis of the energy and scatter directions of X-rays that are radiated to and reflected from a material, we can determine its atomic structure (position of atomic nucleus) and electronic structure in detail.
We mounted our system on the synchrotron radiation facility named ôSPring-8üEin Nishi-Harima, Hyogo Prefecture, to study the atomic and electronic structures of high-temperature liquids. The SPring-8 is an experimental facility that allows use of the most powerful X-rays in the world. Many international researchers come to use it. We conducted a variety of experiments on ultra-high-temperature liquids using SPring-8. In this article, we introduce an experiment on liquid silicon.
Silicon is an indispensable material for the electronic devices supporting society as it is used in computer CPUs, solar cells, etc. In particular, single-crystal growth technology from silicon melt is fundamental to the silicon semiconductor industry. If we could elucidate the flow and coagulation processes of liquid silicon, it would open the way to improve control of single-crystal growth. Eventually, we can expect large technical ripple effects üEsuch as larger-diameter single-crystal silicon. To this end, the accurate identification of the basic properties of liquid silicon is essential.
Silicon is a semiconductor in its solid state and, accordingly, conducts almost no electricity. When melted, however, its electrical conductivity (ease of electrical conduction) increases greatly, reaching almost the same level as that of liquid aluminum, a typical metal that enables free electron approximation. Because of this nature, it has been long believed that, once crystalline silicon with covalent bonds melts, and can be considered a typical semiconductor, it dramatically transforms to a type of simple metal like aluminum. A simple metal has an isotropic structure where valence electrons behave as free electrons.
Considering silicon crystal growth based on the above assumption, it is noticeable that it is very different from the conditions of other crystal growth, such as melted metal-to-metal crystals like sodium and melted salt-to-ion crystal like sodium chloride. In the case of sodium or sodium chloride, there is almost no difference in their ion arrangement and electronic nature between liquid and solid phases. With silicon, however, its arrangement and nature are completely different, as though silicon was at the other end of the spectrum. The question is then, how does silicon crystal with its diamond structure featuring typical covalent bonds grow from metallic liquid silicon with its homogeneous structure? To answer the question at the atomic or electron level is important from both academic and industrial standpoints.
Recently, theoretical research using the first-principles calculation (see postscript to this article) was made for liquid silicon. The research provided detailed information on the valence electron state. The findings suggested the possibility that, contrary to traditional suppositions (i.e., free electron image), covalent bonds repeatedly form and disappear in a femtosecond (10-15 sec) in liquid silicon. Moreover, these bonds are present in a very large number. Compared to those in solid silicon, atoms in liquid silicon move very actively, approaching and moving away from each other. The research revealed that covalent bonds form when the distance between atoms becomes less than approx. 2.5 angstrom. On the other hand, covalent bonds are severed due to atomic movement when the distance between the atoms becomes greater than approx. 2.5 angstrom. The critical distance of 2.5 angstrom is a little longer than the length of a crystal-silicon bond, 2.35 angstrom. The research concluded that, in solid silicon, all the silicon atoms extend their four bonds to connect with each other whereas, in liquid silicon, the bonds are formed or severed over time in response to changes in the distance between the atoms according to their movement.
First-principles calculation: calculation based on the most fundamental principles. For material science, the calculation starts with the Coulomb interaction between electrons, between atomic nuclei, and between electron and atomic nuclei. Furthermore, electronic distribution is determined using the electronic-state theory based on the basic principles of quantum mechanics. Various characteristics of materials are obtained by the calculation. This calculation is useful to understand experimental results by adding information at the micro level that is not provided by experiments. Recently, the calculation has also contributed to anticipation of new materials not yet synthesized, as well as material-science research under extreme conditions where experimentation is difficult. Due to recent advances in computer science and methodology derived from these computer advances, first-principles calculation has become simpler and more reliable/practical than ever before. Thus, the calculation is spreading steadily for various applications.