C. F. McConville

The structure of the formate species on copper surfaces: new photoelectron diffraction results and sexafs data reassessed

Abstract Photoelectron diffraction from the C 1s and O 1s levels of the surface formate species (HCOO) on Cu{100} and Cu{110} has been measured and shows almost identical modulation structure indicating that the adsorption site is the same on both surfaces. Calculations show that the molecule adsorbs on the (short) bridge site in each case with the oxygen atoms close to atop positions and with a CuO nearest-neighbour distance of 1.98±0.04A and also indicate that the OCO bond angle may be increased to 134°. The bond lengthsagree with previous SEXAFS results but the sites differ and the reason for the discrepancies are discussed by reference to the results from model calculations of the SEXAFS data.

Origin of the n-type conductivity of InN: The role of positively charged dislocations

As-grown InN is known to exhibit high unintentional n-type conductivity. Hall measurements from a range of high-quality single-crystalline epitaxially grown InN films reveal a dramatic reduction in the electron density (from low 1019 to low 1017cm−3) with increasing film thickness (from 50to12000nm). The combination of background donors from impurities and the extreme electron accumulation at InN surfaces is shown to be insufficient to reproduce the measured film thickness dependence of the free-electron density. When positively charged nitrogen vacancies (VN+) along dislocations are also included, agreement is obtained between the calculated and experimental thickness dependence of the free-electron concentration.

An XPS study of iron sodium silicate glass surfaces

Abstract A series of (SiO2)0.7−x(Na2O)0.3(Fe2O3)x glasses (0.0 ≤ x ≤ 0.18) were prepared and investigated by means of X-ray photoelectron spectroscopy (XPS). The quantitative ratio [Fe2+]/[Fetotal], for each glass has been determined from an analysis of the Fe 3p spectra. For low Fe2O3 content both iron valencies are present, however, it was found that Fe3+ is the dominant species for high Fe2O3. From an analysis of the O 1s spectra, it was possible to discriminate between bridging and non-bridging oxygen atoms. It was found that the ratio of the non-bridging oxygen content to the total oxygen content increases with increasing iron concentration. It has also been shown that the non-bridging oxygen contribution to the O 1s spectra can be simulated by summing the contributions from the SiONa, SiOFe(II) and SiOFe(III) components present in the glass.

Surface alloying at InAsGaAs interfaces grown on (001) surfaces by molecular beam epitaxy

Abstract In situ scanning tunnelling microscopy and reflection high energy electron diffraction have been used to study InAsGaAs interfaces grown on (001) surfaces by molecular beam epitaxy. For the deposition of InAs on GaAs(001) a number of different surface reconstructions can be identified, depending on substrate temperature and InAs coverage, and there is significant evidence for alloying in the surface layer. An (In,Ga)As wetting layer is formed and the complete range of surface reconstructions are discussed, together with the tendency of indium atoms to segregate following the deposition of a further GaAs capping layer and the formation of GaAsInAsGaAs(001) interfaces. Finally, a comparison is made with the surface structure of GaAs layers deposited on InAs(001) substrates.

A simple X-ray standing wave technique for surface structure determination - theory and an application

Abstract A modification of the usual X-ray standing wave technique for adsorbate structure determination is described which is applicable not only to highly perfect semiconductor surfaces but also to normal metal crystals having substantial mosaicity. The method, which relies on normal incidence Bragg reflection, also places weak demands on the energy resolution and degree of collimation of the X-ray beam and is appropriate for existing synchroton radiation beam lines designed for surface EXAFS and photoelectron diffraction. Experimental data show the Auger detection mode is to be preferred to photoelectron or total yield, and an analysis of the Cu(111)(√3 × √3) R30°-Cl structure yields a Cl to bulk Cu layer spacing of 1.81 ± 0.05 A, consistent with a previous SEXAFS and photoelectron diffraction study of this surface.

Universality of electron accumulation at wurtzite c- and a-plane and zinc-blende InN surfaces

Electron accumulation is found to occur at the surface of wurtzite (112¯0), (0001), and (0001¯) and zinc-blende (001) InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low Г-point conduction band minimum lying significantly below the charge neutrality level.

An X-ray absorption and photoelectron diffraction study of the Cu{100} c(2 × 2) CO structure

Abstract The structure of CO adsorbed on Cu{100} has been investigated using near edge X-ray absorption fine structure at the oxygen K-edge to determine the molecular orientation, and normal emission photoelectron energy-intensity spectra for the C 1s and O 1s core levels to determine the adsorption site. The photoelectron diffraction data have been interpreted by comparison with the results of single scattering model calculations. Good agreement is found for CO molecules adsorbed normal to the surface on on-top sites with a CuC distance of 1.92±0.05 A , in accordance with a previous LEED study. Calculations show that intramolecular scattering can be rather unimportant for this technique and that the individual atomic sites may then be obtained from the photoelectron diffraction spectrum by studying the emission from the relevant atom and scattering from the substrate. The method should therefore be applicable to more complex molecular adsorbates.

Synchrotron radiation core level photoemission investigation of the initial stages of oxidation of Al(111)

Synchrotron radiation photoemission from the Al 2p levels shows that oxygen adsorption on Al(111) leads to at least four different chemically shifted peaks in addition to the metallic substrate peak. Three of these, associated with the chemisorption stage, are discussed in terms of a local bonding model and are attributed to emission from surface Al atoms bonded to one, two or three chemisorbed O atoms occupying 3-fold coordinated hollow sites. The fourth peak is the usual surface oxide peak, although its exact energy is found to be dependent on the stage of oxidation and whether this is achieved by exposure to molecular oxygen or water. Much lower occupation of the chemisorption states is found if the clean Al(111) surface is exposed to water instead of oxygen.

Heteroepitaxial growth of InSb on (100)GaAs using molecular beam epitaxy

Molecular beam epitaxy has been used to grow thin (0.5 μm<t<10 μm) InSb epilayers on (100) GaAs substrates. Reflection high‐energy electron diffraction studies indicate that the early stages of layer growth involve three‐dimensional nucleation and the formation of a nonpseudomorphic structure. High‐resolution electron microscopy studies of the interface are reported for the first time and directly confirm that the large lattice mismatch (14.6% at room temperature) is accommodated by the generation of misfit dislocations. Nevertheless, the structural quality of the InSb is observed to improve dramatically with increasing thickness. Detailed secondary‐ion mass spectrometry measurements also demonstrate that there is no large‐scale interdiffusion of constituent elements at the interface. Finally, electrical measurements show the InSb to be p type and comparable with homoepitaxial material.

Transition from electron accumulation to depletion at InGaN surfaces

The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0⩽x⩽1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x=0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.The composition dependence of the Fermi-level pinning at the oxidized (0001) surfaces of n-type InxGa1−xN films (0⩽x⩽1) is investigated using x-ray photoemission spectroscopy. The surface Fermi-level position varies from high above the conduction band minimum (CBM) at InN surfaces to significantly below the CBM at GaN surfaces, with the transition from electron accumulation to depletion occurring at approximately x=0.3. The results are consistent with the composition dependence of the band edges with respect to the charge neutrality level.