C. Bandis

Desorption of positive ions from ionic crystals accompanying 248 nm laser irradiation

We present a study of the energy distributions of positive ions (Na+, Li+, Ca+, and Mg+) photodesorbed from cleaved NaCl, LiF, MgO, NaNO3, and CaCO3 surfaces during 248 nm excimer laser irradiation at fluences well below the damage thresholds. The observed ion energies are significantly higher than those predicted by already existing models that allow ion rearrangement and relaxation during the electrostatic ion repulsion by the nearby photoionized sites. In contrast to what one would expect, we find that treating the ions as fixed charges and neglecting any ion rearrangement during the emission of the adions describes best the experimentally observed ion energies from all five ionic crystals.

Characterization of electron emission from N-doped diamond using simultaneous field emission and photoemission technique

Abstract Nitrogen (N)-doped diamond films have been grown by hot filament chemical vapor deposition (CVD) technique using urea (NH2)2CO) as a dopant. The bulk and surface of the film are identified as diamond from the results of reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), reflective electron energy loss spectroscopy (REELS) and Raman spectroscopy, while the N concentration is confirmed to be of the order of 1020 cm−3 from Rutherford backscattering (RBS) and X-ray photoelectron spectroscopy (XPS) measurements. The threshold field for field emission from the films has been reported to be about 0.5 V/μm. The origin of the electronic states and the related characteristics of field emission from heavily N-doped diamond using the technique of simultaneous field emission and photoemission (FEPES) has been discussed. The FEPES result for the N-doped diamond suggests that after an activation procedure, there is a decrease in the required applied field together with a change in the field emission from the valence band edge to the Fermi level, indicating that metallic states at the surface dominate field emission.

Consequences of Combining Laser Irradiation with Other Stimuli on Laser Desorption and Ablation from Wide Bandgap Insulators

When a transparent, wide bandgap insulator is exposed to sub-bandgap laser irradiation, defects at and near the surface often dominate the response in terms of particle emission and eventual ablation of material. We explored the consequences of applying a variety of stimuli that can generate defects on single crystal surfaces of inorganic ionic crystals. The stimuli include: electron beam irradiation, a second laser beam, mechanical treatment, and thermal treatment. We found that a common theme evolves where these stimuli generate sites for strong interactions of the probing laser beam, leading to a dramatic decrease in the laser intensities needed for ejection of ions, neutrals, and eventual plume formation.

Electron Emission Properties of the Negative Electron Affinity (111)2×1 Diamond-Tio Interface

We report photoemission measurements from the (111)2×1 diamond-titanium monoxide (TiO) interface. Submonolayer deposition of TiO on the (111)2×1 diamond surface modifies the electron affinity of the surface from positive to negative. No change in the band bending was observed as a result of the TiO deposition. Total electron yield measurements from the diamond-TiO interface were also performed. The yield spectra, as expected for a negative electron affinity (NEA) surface, have emission thresholds that are in good agreement with the absorption coefficient of diamond. To further understand the emission properties of NEA (111) diamond surfaces we also compare the electron yield photo-excitation spectra of the diamond-TiO interface with the yield spectra of hydrogenated (111)1×1:H diamond surfaces.

Fundamental studies of laser desorption from modified surfaces of ionic single crystals

Abstract The observation of photoinduced electron and ion emissions from a variety of wide bandgap insulators under UV and/or IR irradiation suggest that these materials typically possess occupied electronic defect states in the band gap. We have investigated the consequences of a variety of defect-generating stimuli (electron irradiation, laser irradiation, mechanical treatments and heating) on electron and ion emission from inorganic ionic crystals. These stimuli generate defects that strongly interact with the probe laser on a wide variety of ionic crystals. These stimuli dramatically decrease the probe laser intensities required for ion and neutral emissions, laser damage, and plume formation. We reference a number of these effects and illustrate a selected few on one material namely single crystal NaNO3.

When A Mild Mannered 1-5 eV Photon Meets A Big 10 eV Bandgap: Studies Of Laser Desorption From Modified Surfaces of Ionic Single Crystals

Exposing wide-bandgap ionic materials to UV and IR photons can produce ion emissions with kinetic energies of several eV, well in excess of the photon energy. Electron emissions are also observed. This implies that these materials possess occupied electronic defect states within the band gap. We have investigated the consequences of a variety of defect-generating stimuli (electron irradiation, laser irradiation, mechanical treatments, and heating) on electron and ion emission from inorganic ionic crystals. These stimuli generate defects that strongly interact with the probe laser on a wide variety of ionic crystals, and dramatically decrease the probe laser intensities required for ion and neutral emissions, laser damage, and plume formation.

Investigations of laser desorption from modified surfaces of ionic single crystals

Wide band gap insulators irradiated with UV and/or IR pulsed lasers have been shown to yield neutrals, photoinduced electrons, and energetic positive ions suggesting occupied electronic defect states in the band gap. We explore the consequences of applying a variety of stimuli, which can generate defects on single crystal surfaces of inorganic ionic crystals. The stimuli include electron beam irradiation, a second laser beam, mechanical treatment, and thermal treatment. Our experiments on a wide variety of ionic crystals find that a common theme evolves where these stimuli generate sites that strongly interact with the probing laser beam. Such interactions lead to dramatic decrease in the laser intensities needed for ejection of ions, neutrals, as well as eventual plume formation, and result in increased vulnerability of the materials to the laser radiation.

Surface Photovoltage Effects in Photoemission from Diamond Surfaces

Photovoltaic effects in ultraviolet photoemission spectroscopy of the in-situ “rehydrogenated”, and reconstructed (111) diamond surfaces are evaluated. We show that photovoltaic charging effects during photoemission studies of the in-situ “re-hydrogenated” (111)-(l×l):H diamond surface are significant at temperatures as high as room temperature. In contrast, experiments on the reconstructed (111 )-(2×l) diamond surface find no photovoltaic charging which suggests that the surface exhibits relatively high conductivity (effectively grounded surface). These results demonstrate that extra care should be taken in determining the Fermi level pinning position relative to the bands at diamond surfaces and interfaces. The assumption that the UPS photoelectron spectra reflect the equilibrium energy band arrangement should be experimentally confirmed in each case, especially when wide band gap materials are involved.

Interaction of Hyperthermal Hydrogen with the Diamond Surface

A mass-selected low kinetic energy (1–50 eV) ion source is used to expose the diamond (111) surface to ionized atomic hydrogen (H + ) at controlled impact kinetic energy. We report the result of 20 eV and 50 eV kinetic energy exposures as measured by ultraviolet photoelectron spectroscopy (UPS). UPS is found to be a useful probe of hydrogen adsorption on diamond.