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Monolayer passivation of semiconductor surfaces


Germanium is a promising channel material for the next generation MOSFET devices since it has superior electronic properties when compared to silicon. However, high interface trap densities between a Ge surface and a Ge native oxide have been a challenging issue when fabricating practical devices, which demands a proper passivation of the Ge surface. Among various passivation methods, nitridation and oxidation have shown the most promising results. In this study, the monolayer passivation of Ge(100) surface via formation of Ge-N and Ge-O surface species was investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Direct nitridation was performed on a Ge(100) surface using an electron cyclotron resonance (ECR) plasma source with pure N₂ gas, and a Ge-N ordered structure was formed at 500°C. However, STS showed that the Fermi level of the n-type Ge(100) surface was pinned near the valence band edge after the nitridation process. Theoretical modeling using DFT calculations showed that the bandgap states are produced from the ordered nitride structure, which is consistent with the Fermi level pinning of the surface. It is predicted that a further passivation process using hydrogen will unpin the Fermi level by reducing the dangling bonds and bond strain of the ordered structure. Using a differentially-pumped H₂O dosing system, a monolayer of H₂O chemisorption sites on a Ge(100) surface was obtained with a low density of unreacted dangling bonds at room temperature. By annealing up to 250°C, the coverage of H₂O sites decreased significantly. This is consistent with desorption of H₂ or H₂O, and the multiple prepulsing of H₂O at 250°C reported by other groups. The H₂O chemisorbed Ge surface is an ideal monolayer passivation of a Ge serving as a great template for the ALD process, since it contains a half monolayer of -OH which induces the formation of Al-O bonds with the introduction of tri-methyl aluminum (TMA). The ability of the H₂O passivated Ge(100) to react with TMA even at 300K was verified with X-ray photoelectron spectroscopy (XPS) experiments which showed thermally unstable Ge-OH bonds have been converted to thermally stable Al-O bonds

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