In an attempt to find new substrate material for CMOS, Ge has recently regained tremendous attention due to its higher hole and electron mobility than Si (m(electrons) 2.5 times higher, m(holes) 4 times higher). However unlike SiO2, the Ge/oxide interface tends to feature a large density of interface states which compromise transistor performance, justifying the search for Ge passivation layers. For Si(100) surfaces, chemical and electrical passivation by submonolayer Sr amounts has been predicted by ab initio calculations [1], and has also been confirmed experimentally [2]. There is limited evidence in the published literature that Ba could act as an electronic passivant of Ge [3,4].
In this paper we present a detailed study on the effect of Ba termination layers on chemical passivation of Ge(100). Crystallinity and oxidation behavior of clean and Ba terminated Ge (100) surfaces were monitored as a function of oxygen pressure and temperature by LEED, XPS and RHEED in real time for oxygen exposures varying from 10 -7 to 150 Torr.
The RHEED and LEED analysis during oxidation of a clean, 2x1 reconstructed Ge (100) surface indicates that the Ge(100) surface remains crystalline (1x1 reconstructed) up to oxygen pressure of 0.1 Torr. Ba forms ordered termination layers on Ge(100), but causes a massive increase in Ge oxidation rate and amorphization tendency compared to clean Ge: Ge (100) terminated with ½ ML and 1 ML Ba amorphizes for an oxygen pressure of 10-2 and 10-5 Torr, respectively.
In conclusion, we find that Ba termination layers promote oxidation of Ge(100). Meanwhile, the 1x1 reconstructed, O terminated Ge(100) surface has surprisingly large chemical and structural stability, making it an interesting candidate interface for Ge MOS.
[1] C.J.Foerst et al. Nature 427, 53 (2004).
[2] Liang et al. Appl. Phys. Lett. 79, 3591 (2001).
[3] R.A.McKee et al. Science 293, 468 (2001).
[4] R.A.McKee et al. US Patent 6,143,072 (www.uspto.gov).
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