Chemistry and Electronics of the Ge(111) Surface

Citation

Knapp, David (2011) Chemistry and Electronics of the Ge(111) Surface. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/CGA1-QJ35. https://resolver.caltech.edu/CaltechTHESIS:12152010-142322737

Abstract

The halogenation/alkylation procedure that has been proven to chemically and electrically passivate the Si(111) surface has been adapted for application to Ge(111). Removal of the Ge(111) surface oxide with 6–9 M HF(aq), followed by exposure to Br₂ vapor, then alkylmagnesium or alkyllithium reagents yields air stable surfaces with surface recombination velocities (SRVs) as low as 40 cm/sec⁻¹ at flat-band conditions. Surface charges with a density on the order of 10¹² cm⁻² cause a negative surface potential of almost 300 mV in n-type CH3 -Ge(111) samples prepared with this method. The oxidized surface shows a strongly positive surface potential in atmospheric conditions. A negative surface potential is also present in CH₃ -Si(111), but the wider bandgap prevents this from causing inversion conditions in extrinsic samples. Ge(111) surfaces alkylated with a larger organic group, such as ethyl or decyl, displayed a weaker surface potential and higher surface recombination velocity as the surface was brought near flat-band. Mercury contacts to alkylated n-type substrates form rectifying junctions with barrier heights of 0.6 ± 0.1 eV. Contacts to p-type substrates or to oxidized n-type substrates show no measurable rectification. X-ray photoelectron spectroscopy (XPS) confirms that the area concentration of surface-bound carbon on CH₃ -Ge(111) surfaces is equal to that of CH₃ -Si(111) surfaces. Other passivation methods were less successful.

Every atop Ge atom of an ideal CH₃-Ge(111) should be capped and the Ge-C bonds should be directed normal to the surface plane. Infrared absorption spectroscopy (IRAS) of methyl-terminated surfaces prepared from HF-etched precursors did not display distinguishable absorption peaks, but if the Ge substrate is first treated with an anisotropic etch before the HF etch, IRAS confirms the methyl group orientation with the polarization-dependent “umbrella” mode absorption at 1232 cm⁻¹ and a polarization-independent rocking mode at 755 cm⁻¹. Well-ordered CH₃-Ge(111) surfaces displayed less surface charging while maintaining the low SRVs, indicating that such surfaces are successfully passivated.

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