Atom-to-atom interaction of O₂ with epi Ge(001)-2×1 in elucidating GeO_x formation

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

¹Grad. Inst. of Appl. Phys. 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, National Synchrotron Radiation Research Center, Hsinchu, Taiwan

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

In the era of quantum-scale devices, germanium (Ge) is becoming a viable channel material to replace silicon in p-type metal-oxide-semiconductor field-effect transistors (p-MOSFETs) due to Ge’s high carrier mobility. On the researches for improving the Ge MOS device performances, perfecting the dielectric/Ge interfaces would be required before such a replacement becomes reality. A pristine Ge surface may be a key to build a high-quality high-κ/Ge interface. Both the high-κ and surface-science communities use similar cleaning methods to obtain a Ge surface free from contamination; that is, acid treatments in air followed by annealing or sputtering of the Ge surface in vacuum. The subsequent acts of adsorption with atoms and dielectric oxides were performed on this treated surface. However, we have previously shown that a treated Ge(001) surface would introduce single Ge atoms [1], which become reaction sites to render the high-k interface unstable. Along the line of scrutinization of the established literature, furthermore, we found no reports that used an as-grown epi Ge(001)-2×1 surface to form an interface. Needless to say, researchers incline to take advantage of the existed Si-based knowledge to rationalize the Ge(001) interfaces.

This study reports a microscopic view of O₂ adsorption on an epi Ge(001)-2×1 surface at room temperature as studied by synchrotron radiation photoemission. We found that the oxidation path of the epi Ge(001)-2×1 surface is entirely different from that of Si(001)-2×1 counterpart. The Si surface is ready to give rise to 1+ to 4+ Si oxidization states. Moreover, the four Si oxidization states developed locally, meaning that they appear with the presence of the surface dimerized atoms. As to the epi Ge(001)-2×1 surface, full passivation by O₂ is out of the question, and the O₂/Ge interface would never achieve 1+ to 4+ Ge oxidization states. To be specific, one of the dissociated O atoms etches off the up-dimer atom to form the GeO species. The other dissociated O atom fills the vacancy and bonds with the underneath Ge atom in the subsurface layer. The down-dimer atom and its backbond with the subsurface atom remain inert to O₂. The present investigation reports the notorious effect of O₂ on the Ge surface, which should not be overlooked in Ge metal-oxide-semiconductor (MOS) device growth.

The work was published in Applied Physics Express, Vol. 11, issue 11, page 115701 (2018).

[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 10, 075701 (2017).

Keywords: Ge, oxidation, synchrotron radiation photoemission