Electronic characteristics of (001), (110) and (111) epi-Ge surfaces and their oxidation using in-situ high-resolution synchrotron radiation photoemission

Yi-Ting Cheng^1*, Hsien-Wen Wan¹, Chiu-Ping Cheng², Tun-Wen Pi³, Jueinai Kwo⁴, Minghwei Hong¹

¹Graduat Inst. of Appl. Phys. and Dept., National Taiwan University, Taipei, Taiwan
²Department of Electrophysics, National Chiayi University, Chiayi, Taiwan
³National Synchrotron radiation Research Center, Hsinchu, Taiwan
⁴Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Yi-Ting Cheng, email:d06245005@ntu.edu.tw

Germanium (Ge) is a promising channel material to replace Si for its high hole mobility in the sub-5nm node complementary metal-oxide-semiconductor (CMOS) technology. The precise knowledge of the high-κ/Ge interface is based on the fundamental understanding of the adsorption of oxygen on Ge surfaces in the (001), (110) and (111) orientations. We have used in-situ high-resolution synchrotron radiation photoemission spectroscopy (SREPS) to investigate the initial O2 exposure to these epi surfaces at room temperature. The Ge(001)-2×1 surface is renowned of the buckled dimers in the topmost surface, having the up- and down- dimer atoms, respectively (denoting as S1u and S1d). For Ge(111)-c(2×8), the reconstructed surface is dominated by adatoms (A), rest atoms (R), and the subsurface atoms in the second layer (S2). Charge transfer occur between atoms A and R, thereby rendering the former a positive sign and the latter the negative sign of the shift. Note the core-level shift of the S2 atoms is negative, which is in contrast to that of the S2 atoms in Ge(001)-2×1, despite of the common fact of the charge transfer in the topmost surface layer. For the Ge(110) reconstructed surface, the Ge 3d core level spectrum is composed of the bulk and two surface components, S and SS.
The three samples were simultaneously exposed to high purity O₂ at dosages as small as 0.5 L (Langmuir) and up to 20 L. The high-resolution SRPES has enabled the observation of the development of the O-bonded Ge 3d components below the bulk component under low oxidation rates. The Ge-O components clearly emerge in the high-energy regions. By examining the Ge-O intensity, we found that the Ge(110) surface is less susceptible to molecular oxygen than the (001) and (111) surfaces at room temperature. An analytical fit to the 20L Ge 3d core-level spectrum of epi Ge(001)-2×1. The result is that one of the dissociated O atoms removes the Ge up-dimer atom to form the GeO_x species, being denoted as GeO(II). The other dissociated O atom inclines to be positioned at the site of the removed Ge up-dimer atom, and bonds with the underneath Ge atom in the subsurface layer to form the Ge-O bonding, which is denoted as GeO(I). Unexpectedly, the down-dimer atom and its back-bonded subsurface Ge atom are inert to O₂. For Ge(111)-c(2×8), it is surprised to find that the topmost surface A and R atoms are inert to initial O₂ exposure, which instead affects the S2 atoms in the subsurface layer. As to the Ge(110) surface, the atoms are lightly oxidized without being removed from the surface. The present oxidation study would assist in improving the growth of Ge MOS and the device performance.

Keywords: Ge, epi surfaces, Synchrotron Radiation Photoemission, molecular oxygen, oxidation