A systematic experimental and theoretical study was performed to determine the causes of oxide-induced Fermi level pinning and unpinning on GaAs(001)-c(2 x 8)/(2 x 4). Scanning tunneling spectroscopy (STS) and density functional theory (DFT) were used to study four different adsorbates' (O(2), In(2)O, Ga(2)O, and SiO) bonding to the GaAs(001)-c(2 x 8)/(2 x 4) surface. The STS results revealed that out of the four adsorbates studied, only one left the Fermi level unpinned, Ga(2)O. DFT calculations were used to elucidate the causes of the Fermi level pinning. Two distinct pinning mechanisms were identified: direct (adsorbate induced states in the band gap region) and indirect pinnings (generation of undimerized As atoms). For O(2) dissociative chemisorption onto GaAs(001)-c(2 x 8)/(2 x 4), the Fermi level pinning was only indirect, while direct Fermi level pinning was observed when In(2)O was deposited on GaAs(001)-c(2 x 8)/(2 x 4). In the case of SiO on GaAs(001)-c(2 x 8)/(2 x 4), the Fermi level pinning was a combination of the two mechanisms.

The correlation between atomic bonding sites and the electronic structure of SiO on GaAs(001)-c(2x8)/(2x4) was investigated using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT). At low coverage, STM images reveal that SiO molecules bond Si end down; this is consistent with Si being undercoordinated and O being fully coordinated in molecular SiO. At approximately 5% ML (monolayer) coverage, multiple bonding geometries were observed. To confirm the site assignments from STM images, DFT calculations were used to estimate the total adsorption energies of the different bonding geometries as a function of SiO coverage. STS measurements indicated that SiO pins the Fermi level midgap at approximately 5% ML coverage. DFT calculations reveal that the direct causes of Fermi level pinning at the SiO GaAs(001)-(2x4) interface are a result of either local charge buildups or the generation of partially filled dangling bonds on Si atoms.

Spatial hole burning near-field scanning optical microscopy ͑SHB-NSOM͒ is used to locally photopattern three species of organic thin films, poly͑2-methoxy, 5-͑2Ј-ethyl hexyloxy͒p-phenylene vinylene͒ ͑MEH-PPV͒, tris-8-hydroxyquinoline aluminum (Alq 3 ) and dye-functionalized polyelectrolyte self-assembled layers, on a 100 nm length scale. In SHB-NSOM the film is illuminated with light from a stationary NSOM tip to induce photo-oxidation. The reduction in the fluorescence yield resulting from this exposure is then mapped using fluorescence NSOM ͑FL-NSOM͒. We have examined the localized photo-oxidation as a function of time, position, and environment free from the limits of far-field spatial averaging. In all of the thin film materials studied we find that the long-time diameter of the dark spot is much larger than the tip diameter and is a signature of energy migration. Characteristic lengths of the energy migration are extracted from this data by a simple diffusion model and are found to be of the order of a few hundred nanometers for each of the films studied.

We use near-field scanning optical microscopy (NSOM) to probe the local optical and morphological properties in annealed, vacuum-deposited Alq3 films (10−480-nm thick) with 10−100-nm resolution. We use concurrent shear force microscopy (an analogue to atomic force microscopy, AFM) to correlate the morphology of different regions to intensity variations in our fluorescence images as well as variations in the localized fluorescence spectra. We have observed nanoscale effects of annealing temperature on film morphology and fluorescence emission. Our studies show that Alq3 films annealed below the glass transition temperature of Alq3 (T < T g = 172 °C) were very similar to unannealed films, except in very thin films (<50 nm). Films annealed above T g (T = 200 °C) for the same amount of time exhibit increased surface morphology and decreased fluorescence. In sufficiently thick films (≈500 nm) annealed at 200 °C, we observe the formation of microcrystalline domains.

The surface reconstructions of decapped In 0.53 Ga 0.47 As(001) have been studied using scanning tunneling microscopy (STM). It is shown that the As-rich 2(2×4) and 2(2×4) reconstructions, predicted by density function theory (DFT) (1-3) for GaAs(001)-(2×4), InAs(001)-(2×4) and InGaAs(001)-(2×4) surfaces, were observed to coexist on In 0.53 Ga 0.47 As (001). In contrast to molecular beam epitaxy (MBE) grown In 0.53 Ga 0.47 As(001), the STM results on decapped In 0.53 Ga 0.47 As(001) do not show the existence of the heterodimer In 0.53 Ga 0.47 As(001)-(4×3) structure (4-5). At the intermediate annealing temperature ranges of 400-440°C, a (2×4)-(4×2) mixed surface reconstruction was observed. When In 0.53 Ga 0.47 As(001)/InP sample was annealed between 440°C and 470°C, a pure In/Ga-rich (4×2) surface reconstruction was observed.