Sarah R. Bishop

Dynamics of analyte binding onto a metallophthalocyanine: NO∕FePc

The gas-surface reaction dynamics of NO impinging on an iron(II) phthalocyanine (FePc) monolayer were investigated using King and Wells sticking measurements. The initial sticking probability was measured as a function of both incident molecular beam energy (0.09-0.4 eV) and surface temperature (100-300 K). NO adsorption onto FePc saturates at 3% of a monolayer for all incident beam energies and surface temperatures, suggesting that the final chemisorption site is confined to the Fe metal centers. At low surface temperature and low incident beam energy, the initial sticking probability is 40% and decreases linearly with increasing beam energy and surface temperature. The results are consistent with the NO molecule sticking onto the FePc molecules via physisorption to the aromatics followed by diffusion to the Fe metal center, or precursor-mediated chemisorption. The adsorption mechanism of NO onto FePc was confirmed by control studies of NO sticking onto metal-free H2Pc, inert Au111, and reactive Al111.

Initiation of a passivated interface between hafnium oxide and In(Ga)As(0 0 1)-(4x2).

Hafnium oxide interfaces were studied on two related group III rich semiconductor surfaces, InAs(0 0 1)-(4x2) and In(0.53)Ga(0.47)As(0 0 1)-(4x2), via two different methods: reactive oxidation of deposited Hf metal and electron beam deposition of HfO(2). The interfaces were investigated with scanning tunneling microscopy and spectroscopy (STS). Single Hf atom chemisorption sites were identified that are resistant to oxidation by O(2), but Hf islands are reactive to O(2). After e(-) beam deposition of <<1 ML of HfO(2), single chemisorption sites were identified. At low coverage (<1 ML), the n-type and p-type HfO(2)/InGaAs(0 0 1)-(4x2) interfaces show p-type character in STS, which is typical of clean InGaAs(0 0 1)-(4x2). After annealing below 200 degrees C, full coverage HfO(2)/InGaAs(0 0 1)-(4x2) (1-3 ML) has the surface Fermi level shifted toward the conduction band minimum for n-type InGaAs, but near the valence band maximum for p-type InGaAs. This is consistent with the HfO(2)/InGaAs(0 0 1)-(4x2) interface being at least partially unpinned, i.e., a low density of states in the band gap. The partially unpinned interface results from the modest strength of the bonding between HfO(2) and InGaAs(0 0 1)-(4x2) that prevents substrate atom disruption. The fortuitous structure of HfO(2) on InAs(0 0 1)-(4x2) and InGaAs(0 0 1)-(4x2) allows for the elimination of the partially filled dangling bonds on the surface, which are usually responsible for Fermi level pinning.

Monolayer Passivation of Ge(100) Surface via Nitridation and Oxidation

The monolayer passivation of Ge(100) surface via formation of Ge-N and Ge-O surface species was studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Direct nitridation using an electron cyclotron resonance (ECR) plasma source formed an ordered Ge-N structure on a Ge(100) surface at 500C. DFT calculations found the hydrogen passivation on this Ge-N ordered structure could reduce the bandgap states by decreasing the dangling bonds and the bond strain. Oxidation of Ge(100) using H2O produced an –OH and –H terminated surface with very few Ge ad-atoms, while e-beam evaporation of GeO2 formed semi-ordered Ge-O structures and Ge ad-species at room temperature. Annealing above 300C formed suboxide rows on both H2O and GeO2 dosed surfaces, and the scanning tunneling spectroscopy (STS) showed that the Fermi level was pinned near the valence band edge on the n-type Ge surfaces covered by suboxides.

NO chemisorption dynamics on thick FePc and ttbu-FePc films

The NO chemisorption dynamics on ordered multilayer iron phthalocyanine (FePc) and quasiamorphous multilayer tetra-t-butyl FePc (ttbu-FePc) films on a Au(111) substrate was investigated using the King and Wells reflection technique. The NO zero coverage or initial sticking probabilities (S(0)) were measured as a function of sample temperature (T(s)) and beam energy (E(i)). The experimental results for both films show a monotonic decrease in S(0) with increasing T(s) and E(i) consistent with NO adsorption occurring via a multiple pathway precursor-mediated mechanism in which the adsorbate initially physisorbs to the FePc organics, diffuses, and chemisorbs to the Fe metal center. The saturation coverage is 3% for the multilayer FePc surface and only 2% for the multilayer ttbu-FePc surface consistent with NO chemisorption occurring only on the Fe metal, where NO chemisorbs to 100% of the surface Fe metal centers. The reduced saturation coverage in the ttbu-FePc film is attributed to fewer Fe metal centers in the less dense ttbu-FePc films. A comparison of NO sticking on a multilayer FePc/Au(111) film with NO sticking on a monolayer FePc/Au(111) film shows that S(0) is greater on the multilayer FePc film for all T(s) and E(i), consistent with an increase in collision inelasticity for NO/multilayer FePc/Au(111).

Theoretical analysis of initial adsorption of high-κ metal oxides on InxGa1−xAs(0 0 1)-(4×2) surfaces

Ordered, low coverage to monolayer, high-κ oxide adsorption on group III rich InAs(0 0 1)-(4×2) and In(0.53)Ga(0.47)As(0 0 1)-(4×2) was modeled via density functional theory (DFT). Initial adsorption of HfO(2) and ZrO(2) was found to remove dangling bonds on the clean surface. At full monolayer coverage, the oxide-semiconductor bonds restore the substrate surface atoms to a more bulklike bonding structure via covalent bonding, with the potential for an unpinned interface. DFT models of ordered HfO(2)/In(0.53)Ga(0.47)As(0 0 1)-(4×2) show it fully unpins the Fermi level.