Synchrotron radiation photoemission study of SiGe(001)-2×1: Clean surface, oxidation, and hydrogenation

Yi-Ting Cheng¹, Hsien-Wen Wan¹, Jueinai Kwo², Minghwei Hong¹, Tun-Wen Pi^3*

¹Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei, Taiwan
²Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
³Science 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 κ dielectrics, where the interfacial quality plays a critically important role. We have investigated microscopically how the surface Ge atoms orient themselves, using the high-resolution synchrotron radiation photoelectron spectroscopy (SRPES) as a probe. For the SRPES measurements, SiGe(001)-2×1 samples that were grown via molecular-beam epitaxy on epi Ge(001) and Si(001) substrates were always kept under an ultra-high vacuum without exposure to air. The present study covers Ge content from 10% to 90%. The low-energy electron diffraction equipped in the SRPES chamber showed 2×1 double domain reconstruction. Analyses of the Ge 3d core-level spectra acquired using different photon energies and emission angles consistently reveal the ordered spots to be in a Ge–Ge tilted configuration. It was further found that the subsurface layer was actually dominated by Ge, which supported the buckled surface configuration. The Si atoms were first found in the second subsurface, or the third surface, layer. The corresponding electronic structures of these Si atoms were different from Si atoms mixing with Ge in the bulk. These Si atoms further divided into two parts, one underneath the Ge–Ge dimer and one between the dimer row. The distinct energy positions of the Si 2p core-level spectrum were caused by stresses, not by charge alternations.

The exposure of atomic oxygen started from as low as 0.1 L up to 500 L, although the interfacial reaction practically stopped at 10 L of exposure. Atomic oxygen bonded only with the up-dimer atoms, while the atomic hydrogen passivated both dimerized atoms. Upon annealing, the GeOx oxides and GeHx hydrides disappeared completely, and the Ge region became similar to that of the clean surface. However, over half of the Ge–Ge dimers were removed from the surface after annealing in both interfaces. Heat caused the oxygen atoms in GeOx to move over to the Si region, thereby giving rise to 1+ to 4+ charge states of SiOx, which was similar to the case of the oxidation of c-Si(001)-2×1. The catalytic effect of Ge decreased the elevated temperature to, at most, 500 °C.

Keywords: SiGe, oxidation, hydrogenation , Synchrotron radiation photoemission