Surface Electronic Structure of Epi Ge(001)-2×1
T.W. Pi/皮敦文1*, Y. T. Cheng/鄭伊婷2, W. S. Chen/陳婉馨2, Y. H. Lin/林延勳2, K. Y. Lin/林耕雍2, H.W. Wan/萬獻文2, C.-P. Cheng/鄭秋平3, J. Kwo/郭瑞年4, M. Hong/洪銘輝2
1Science Department, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
2Graduate Institue of Applied Physics and Department of Physics, National Taiwan University, Taipei, Taiwan
3Department of Electrophysics, National Chiayi University, Chiayi, Taiwan
4Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
* Presenter:T.W. Pi/皮敦文
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. The high-κ community has so far overlooked the Ge surface electronic structure; in general, most published reports have not discussed the effect of surface conditions on the electric performance. In the specific studies of Ge surfaces, most work has concentrated on the interpretation of the shallow Ge 3d core-level spectrum obtained using synchrotron radiation photoelectron spectroscopy (SRPES), which allows the acquisition of highly surface-sensitive data. However, a consensus has yet to be reached, despite similar line shapes of these reports. This is rather unusual, considering that the surface construction of Ge(001)-2×1 is similar to that of Si(001)-2×1 and that the contributed line components in Ge’s 3d states are fewer than those of the Si 2p core-level spectrum.

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. Up to now, no reports have existed regarding as-grown epi surfaces without any treatment. In this work, for the first time, the electronic structure of an epi Ge(001)-2×1 surface is presented using SRPES as a probe. The present study not only clarifies the peak assignments of the Ge 3d line spectrum, but also makes a justifiable hypothesis explaining the poor reliability in the electric performance of Ge MOS devices. In short, in contrast with the single Ge atoms or small Ge droplets, which appear on the chemically treated/ultra-high vacuum (UHV) annealed Ge surfaces, the epi Ge(001)-2×1 surface exhibits solely buckled dimers, an unconventional phenomenon. The former Ge surfaces could render the high- interface unstable, accounting in part for the poor device reliability issues.

This study reports electronic structure of an epi Ge(001)-2×1 surface using synchrotron radiation as a probe. The topmost surface atoms are buckled with the up-dimer and down-dimer atoms exhibiting surface core-level shifts (SCLSs) of −0.492 and −0.178 eV, respectively. The subsurface layer shows a +0.083 eV SCLS. The final-state effect suffices to explain the sign of the shift. The electron affinity and ionization potential for the epi Ge surface are 4.36 and 5.09 eV, respectively. An argument contrasting the current results with those of existing reports with non-epi surfaces is also given. Non-epi surfaces possess Ge surfaces with isolated single atoms or small droplets that affect Ge’s contact with dielectric layers and the electric performances of Ge metal-oxide-semiconductor structure. If time is permitted, the electronic structure of epi Ge(001)-2×1interfaces will also be given with representative examples.

Keywords: Ge, high k, synchrotron radiation photoemission