抄録
The Slow Positron Facility (SPF) at the Institute of Materials Structure Science (IMSS) in KEK recently developed a system of low-energy positron diffraction (LEPD), which is the positron counterpart of low-energy electron diffraction (LEED), for the surface atomic structure determination [1]. It is considered that positron diffraction is an ideal method for the study of surface atomic arrangements [2].
The LEPD experimental station at SPF has been significantly developed lately to enable the execution of the so-called quantitative I-V analysis for the determination of precise surface structures. In case of I-V analysis the intensities of the diffracted spots are measured with the change in beam energy and analyzed. Recently we have conducted the experiment on the clean Cu(001) and Pb adsorbed Cu(001)-c(2x2) surface. The LEPD diffraction pattern of both the surface structures are obtained in 2eV steps for a span of 100 seconds each using the dedicated HEX-MCP delay line detector. The experimental data are still being analyzed but its recent progress and the I-V analysis will be discussed during the presentation. In addition to this, we have carried out the first trial of LEPD-ARPES multiprobe research where the clean Cu(001) sample prepared at the LEPD station has been transported by a UHV transfer vessel to BL-13B in the Photon Factory (PF) at IMSS, and its electronic band structure has been observed by angle-resolved photoemission spectroscopy (ARPES).
The successful LEPD experiment has been performed after the recent upgradation of the LEPD experimental station which now consists of two dedicated chambers for LEPD measurement and sample preparation. The sample manipulators at both the sample preparation chamber and LEPD measurement chamber have been upgraded which allows precise and highly reproducible adjustment of sample position and orientation. This helps to attain the normal beam incidence onto the sample to equalize the intensities between symmetrical diffraction spots necessary for I-V analysis. The manipulator of the LEPD chamber along with the upgraded sample holder can rapidly cool the sample to -181 °C within 30 minutes which is crucial as the intensities of LEPD spots are affected by the Debye-Waller factor as in in LEED. Moreover, the manipulator of the preparation chamber has the access to electrically heat the sample up to ~1200 °C, besides cooling. The sample holders are specially designed to accommodate efficient temperature change as well as compatible with Omicron-type sample holder which is being used in several stations in PF. The preparation chamber is also equipped with an Ar+ sputtering, a triple-pocket electron beam evaporator for thin film growth on the surface, three different gas introduction systems, and additional ports available on request. The prepared sample surfaces can be evaluated using an LEED-AES spectrometer. The preparation chamber is also attached to the adjacent load lock chamber for additional sample storage, and it can be connected to the UHV portable transfer vessel for the transfer of sample to other stations such as for ARPES measurement. The successful implementation of the new LEPD experimental station for LEPD I-V analysis and the ability to conduct multiprobe analysis on the same sample is an important step towards the development and understanding of new functional surface materials.
Reference
[1] K. Wada et al., e-J. Surf. Sci. Nanotech. 16, 313 (2018). [2] S. Y. Tong, surface science 457, L432 (2000).
