D. G. J. Sutherland

Characterization of nanocrystalline diamond films by core‐level photoabsorption

Core‐level photoabsorption has been used to determine the sp2 and sp3 bonding content of nanocrystalline diamond thin films grown using C60 or CH4 precursors. The C(1s) absorption spectra show clear bulk diamond excitonic and sp3 features with little evidence of sp2 bonding, while the Raman spectra measured from these same films are ambiguous and indeterminate. This result can be attributed to the local structure (near‐neighbor bonding) sensitivity of core‐level photoabsorption that is insensitive to domain size, unlike Raman spectroscopy.

Photo‐oxidation of electroluminescent polymers studied by core‐level photoabsorption spectroscopy

The C 1s and O 1s core‐level photoabsorption spectra of poly[2‐methoxy,5‐(2′‐ethyl‐ hexoxy)‐1,4‐phenylene vinylene] (MEH‐PPV) before and after exposure O2 and broadband visible light were recorded to determine the degradation pathway for this polymer. The change in the O 1s spectra as a function of exposure demonstrates that the O adds to the polymer chain to form a carbonyl group. Exposure to only O2 or only light causes no change in the C 1s or O 1s spectra. In the C 1s spectra, the change in the dependence on the photon angle of incidence after exposure demonstrates that O attacks the polymer at the double bond in the vinyl group thereby altering the extended conjugation of the polymer.

Core-level photoabsorption study of defects and metastable bonding configurations in boron nitride

A comprehensive study of different local bonding environments in boron nitride -bulk and thin filmshas been performed by core level photoabsorption. Several new features not present in crystalline reference samples are found in the absorption spectra of the thin films. These are identified as nitrogen vacancies in the hexagonal bonding of BN, nitrogen interstitials, boron clustering, sp -like metastable phases and sp phases. Quantitative information on the concentration and distribution of point defects is easily extracted from the photoabsorption data and is discussed with regard to formation of riew phases, the B :N ratio in the films, and compared with a random model of defect formation. Information on the stability of the new bonding environments is gained by annealing the thin films. Modification of the orientation of the sp hexagonal planes is attained by ion bombardment and annealing, and is monitored by angle resolved photoabsorption.

Near‐edge x‐ray absorption fine structure study of bonding modifications in BN thin films by ion implantation

Near‐edge x‐ray absorption fine structure (NEXAFS) has been used to study the defect content and the bonding modifications induced in BN thin films by ion implantation. The initial films were hexagonal‐like BN grown on Si(100) by pulsed laser deposition. Subsequent ion implantation with N2+ at 180 keV induces the formation of a significant proportion of sp3 bonding (cubic‐like), and the formation of nitrogen void defects in the remaining sp2 BN. These modifications in the bonding of a film lacking long range order can only be distinguished with a local order technique like NEXAFS.

Synthesis and characterization of amorphous carbon nitride films

Abstract We report the high-pressure, 1066 Pa (8 Torr), chemical vapor deposition (CVD) synthesis of amorphous carbon nitride films using a d.c. glow discharge technique. X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure were used to study the film stoichiometry and bonding as a function of substrate temperature and flux of hydrogen to the growth surface. Experiments show that the film stoichiometry is constant with substrate temperature up to 600 °C, above which film growth was not observed. The addition of small amounts (1.5 at.%) of molecular hydrogen causes poisoning of film growth. Multiple sp 2 bonding states, with little sp 3 bonding, were present in the films. Scanning electron microscopy of the films reveals an unusual filamentary growth phase.

The chemisorption of H2C[Si(CH3)3]2 and Si6(CH3)12 on Si(100) surfaces

The chemisorption of bis(trimethylsilyl)methane (BTM, CH2[Si(CH3)3]2) and dodecamethylcyclohexasilane (DCS, Si6(CH3)12) on clean Si(100) surfaces has been studied by C 1s core-level and valence-band photoemission spectroscopy. Our model for the deposition of carbon by BTM involves decomposition into a –CH2Si(CH3)3 surface moiety for room-temperature adsorption, which further decomposes upon annealing to 550 °C to form a surface terminated primarily by CHx units. DCS deposits almost three times as much C on the surface as BTM. The data are consistent with DCS undergoing a ring opening and bonding to the surface as polydimethylsilane chains. Annealing both adsorbates to 950 °C causes a large decrease in the C 1s signal due to the fact that Si segregates to the surface at temperatures above 900 °C. The valence-band photoemission of Si(100) dosed with DCS at 950 °C is in good agreement with that of β-SiC, whereas the analogous BTM spectrum deviates significantly.

Observation of core-level binding energy shifts between (100) surface and bulk atoms of epitaxial CuInSe2

Synchrotron radiation soft x-ray photoemission spectroscopy was used to directly observe Se 3d core-level binding energy shifts from surface atoms of the (100) face of epitaxial CuInSe2/ GaAs(100). High-resolution spectra show two sets of Se 3d5/2,3/2 spin-orbit components separated by 0.6 eV, with the low-binding-energy peaks being associated with the surface atoms. However, surface state emission from Se p states in the valence band was not observed due to the low photoionization cross sections. Cation bonding-antibonding states were observed in the valence band and are centered at about 1.0 and 3.1 eV below the valence band edge.

Core level spectroscopy for surface analysis

This is a review of the applications that various core level spectroscopies have in surface analysis. Three methods are highlighted, i.e., photoelectron spectroscopy of core level shifts (XPS or ESCA), absorption spectroscopy, and soft X-ray fluorescence. These techniques provide not only elemental analysis at surfaces, but also the chemical state of atoms and molecules in the outermost atomic layers, such as oxidation state, hybridization, and nearest neighbor bonding information. The probing depth can be adjusted in non-destructive fashion from 3 A to 300 A by detecting electrons or photons of variable energies. Advances in detectors and light sources, such as synchrotron radiation, are breaking ground for new applications, such as chemically-resolved microscopy.