Surface electronic structure of Si1-xGex(001)-2×1: A synchrotron radiation photoemission study

Yi-Ting Cheng (鄭伊婷)¹, Hsien-Wen Wan (萬獻文)¹, Chao-Kai Cheng (鄭兆凱)¹, Chiu-Ping Cheng (鄭秋平)², Jueinai Kwo (郭瑞年)³, Minghwei Hong (洪銘輝)¹, Tun-Wen Pi^4*

¹Graduate Institute of Applied Physics and Dept. of Physics, National Taiwan University, Taipei, Taiwan
²Department of Electrophysics, National Chiayi University, Chiayi, Taiwan
³Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁴Science Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan

* Presenter:Tun-Wen Pi, email:pi@nsrrc.org.tw

SiGe, with high carrier mobility, has become a viable material to replace Si as the channel layer for the aggressively scaled complementary metal-oxide-semiconductor (CMOS) technology in the sub 3-nm node. Research efforts on the SiGe MOS gate stacks have involved high  dielectrics, where the interfacial quality plays a critically important role. However, there has been little discussion about the electronic structure of a wide range of Ge contents in the strained Si1−xGex grown on Ge or on Si. A basic understanding of the electronic structure of the SiGe surface in an atomic scale helps in elucidating the atom-to-atom interaction at the interfaces of the gate stacks.
In this study, we used high-resolution core-level photoemission to probe the electronic structure of Si1−xGex(001)-2×1 with x ranging from 10% to 90%. SiGe(001) strained epi-layers on an epi Ge(001)-2×1 surface were grown by MBE in a multi-chamber UHV system. The 2×1 surface reconstruction was monitored in situ by reflection high-energy electron diffraction (RHEED), where the 2× pattern clearly shows streaky diffraction spots and distinct Kikuchi arcs. The sharp and intense diffraction patterns indicate that the surface is an atomically flat and ordered surface. Five samples with Ge contents of 10%, 30%, 51%, 70%, and 90% were prepared for photoemission measurements, where the sample composition was determined by high-resolution synchrotron X-ray diffraction (XRD). An X-ray photoelectron spectroscopy (XPS) chamber equipped with an Al K monochromatic X-ray source with 1486.71-eV photon energy was connected in situ with the SiGe MBE growth chamber.
The Ge 3d core-level spectra of Si1−xGex(001)-2×1 are essentially the same as that of epi Ge(001)-2×1,1 terminating with tilted Ge-Ge dimers and Ge atoms in the subsurface layer. The strain effect is manifest only for the Si atoms, for which three types are revealed, namely two near the surface region and one in the bulk. The electronic structure of the Ge content, which ranges from 10% to 90%, shows similar surface behaviors.
The work was published in Applied Physics Express, Vol. 13, issue 9, page 095503 (2020).
[1] Y.T. Cheng, Y.H. Lin, W.S. Chen, K.Y. Lin, H.W. Wan, C.-P. Cheng, H.H. Cheng, J. Kwo, M. Hong, T.W. Pi, Surface electronic structure of epi Germanium (001)-2×1, Appl. Phys. Express vol. 10, p075701 (2017).

Keywords: SiGe, surface electronic structure, photoemission