Molecular Assembly At Bare Semiconductor Surfaces
Open Access
- Author:
- McGuiness, Christine Lara
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 23, 2006
- Committee Members:
- David Lawrence Allara, Committee Chair/Co-Chair
Nicholas Winograd, Committee Member
Thomas E Mallouk, Committee Member
Carlo G Pantano, Committee Member - Keywords:
- GaAs surface
self assembled monolayers
organic thin films
alkanethiolate monolayers
chemical passivation
bare semiconductors - Abstract:
- The formation of well- organized monolayers with highly reproducible structures by solution self assembly of octadecanethiol (ODT) on GaAs (001) and (011) surfaces at ambient temperature through rigorous control of assembly conditions is demonstrated. Using TOF-SIMs and HRXPS measurements, it is shown that ODT molecules form a direct S-GaAs attachment with little to no substrate oxidation and exhibit an overall thermal stability up to 100 C, lower than the same monolayers on Au{111} surfaces. Detailed characterization using a combination of ellipsometry, liquid drop contact angles, atomic force microscopy, near edge x-ray absorption fine structure, infrared spectroscopy and x-ray photoemission spectroscopy probes reveal a structure with conformationally ordered alkyl chains tilted 14 ±1 from the surface normal with a 43 ±5 twist, a highly oleophobic and hydrophobic ambient surface. On GaAs (111A) surfaces, the ODT monolayers also form a direct S-GaAs attachment with little to no substrate oxidation but the final monolayer structure is more disordered, with the alkyl chains tilted 25 ±5 from surface normal with a 45 ±5 twist, resulting in a less oleophobic and hydrophobic ambient surface. Further, in contrast to previously reported results with Na2S·9H2O treatments, Raman scattering measurements reveal that the ODT monolayers do not significantly modify the near-surface electronic structure of GaAs. Analysis of the tilt angle and film thickness data in conjunction with grazing incidence x-ray diffraction measurements show that the monolayers form structures with a significant mismatch of the average adsorbate molecule spacings with the spacings of intrinsic GaAs crystal lattices. For the square surface lattices of GaAs (001) and (011), ODT monolayers are observed to form pseudo-hcp structures. On the hcp surface lattice of GaAs (111A) however, the monolayers form a true hcp structure. This suggests that formation of the monolayers is driven significantly by the molecular packing and is also highly dependent on the crystal termination of the GaAs surface. The structure of the monolayers is highly dependent on chain length, moving from well ordered monolayers of long chain ODT monolayers to disorganized structures for short chain dodecanethiol monolayers and aromatic molecules, such as 4’ 4-biphenylthiol. Increased oxidation of the substrate is observed as the disorder of the monolayer increases.