D. Teehan

Chemical nature of the luminescent centre in fresh and aged porous silicon layers

Abstract In this study we have used high resolution parallel electron energy loss spectroscopy (PEELS) and X-ray excited optical luminescence (XEOL) to investigate the chemical nature of the luminescence centre in fresh and aged porous silicon. We find that regardless of the non-stoichiometric oxides which were observed by PEELS in our fresh porous silicon layers, SiSi bonded material is involved in the luminescence process. However, in the case of aged porous silicon both SiSi and SiO bonded material are involved.

Surface electronic structure of the SrSn1−xSbxO3 perovskite system

Abstract Pure and antimony-doped strontium stannate (SrSn 1− x Sb x O 3 ) samples have been prepared by a high temperature (1360°C) solid-state synthetic procedure over the composition range 0 ⩽ x ⩽ 0.40. The bulk properties have been determined by powder X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX) and scanning electron microscopy (SEM), showing the SrSn 1− x Sb x O 3 system to be single phase over the doping range 0 ⩽ x ⩽ 0.085. Surface analysis of SrSn 1− x Sb x O 3 ceramic discs by X-ray photoemission (XPS), Auger electron spectroscopy (AES) and synchrotron excited valence band photoemission has probed the occupied electronic states, whilst the unoccupied states have been investigated by inverse photoemission spectroscopy (IPES). Considerable segregation of the Sb-dopant to the surface of the sample has been demonstrated. We show that this system exhibits strong similarities to the well studied Sb-doped SnO 2 system and the less well characterised Sb-doped BaSnO 3 perovskite.

The interaction of bismuth with the GaAs(111)B surface

Abstract We present a core level photoemission and low energy electron diffraction study of the interaction of Bi with the GaAs(111)B-(2×2) surface. Results for room temperature growth are consistent with islanding where the islands contain Bi in two distinct chemical forms. Some 30% of the clean surface As trimers are removed by the deposition of Bi. A c(4×2) structure is stable in the 110–350°C temperature range and its formation coincides with the complete removal of the clean surface As trimers and with all of the Bi being bonded to As of the bulk-terminating layer. A structural model for the Bi terminated surface is proposed.

Core-level photoemission study of the BiGaAs(111)A interface

Abstract Core-level photoemission spectra of the As 3d, Ga 3d and Bi 5d core levels were recorded for the BiGaAs (111)A-(2×2) surface. From the early stages of Bi growth, there are two distinct chemical environments for Bi. Surface Bi is bonded to both As and Ga atoms. The deposition of approximately 1 ML of Bi removes the dangling bonds from the surface As and Ga atoms, which are associated with the (2×2) vacancy-buckling structure. Annealing to 350°C reverses this process and surface dangling bonds for Ga reappear. When the Bi had fully desorbed at 425°C, photoemission and LEED results showed that the surface recovers the (2×2) vacancy structure but with a larger degree of surface disorder than that found before the deposition of Bi.

Higher-order suppression in diffraction-grating monochromators using thin films

Although a continuously tuneable source of photons is a very desirable feature of synchrotron radiation it has one main drawback: the contamination of the photon beam by higher-order diffracted light. Several elements have absorption edges which lie between 10 and 200 eV, a range prone to high second- and third-order content in XUV monochromators. They can, therefore, be used as transmission filters to reduce this higher-order content. This paper describes the use of thin filters to reduce the higher-order content in diffraction-grating monochromators. Their suppression efficiency, transmission and ageing have been characterized using photoelectron spectroscopy and compared with calculated values. The effect of oxide contamination on their performance has been assessed. Filters are now installed on eight XUV beamlines and have been in routine use for several years.

Adsorption of Sb on GaAs(111)B studied by photoemission and low energy electron diffraction

Abstract The surface structures resulting from the deposition of Sb on the GaAs(111)B-(2 × 2) surface at room temperature followed by annealing, have been studied by high-resolution soft X-ray photoemission (SXPS) and low energy electron diffraction (LEED). For depositions at room temperature with no subsequent anneal and for annealing temperatures up to 300°C, Sb islands are formed between which the As trimer-based (2 × 2) substrate reconstruction of the clean GaAs surface is observed. Annealing to temperatures between 350 and 475°C leads to the creation of Sb chain pairs coexisting with regions of Sb trimers. At 500°C an ordered surface is produced, associated with Sb trimers and an As vacancy.

The electronic structure of Ti4O7 studied by resonant photoemission

We present and discuss resonant photoemission (PES) spectra of Ti4O7 taken at 50 and 300 K. The PES spectra taken in the metallic phase at 300 K show Ti 3d bands at the Fermi level and O 2p bands at higher binding energies. The intensity of the Ti 3d bands increases for photon energies above 40 eV. This effect is attributed to a constructive interference with the Ti 3p → 3d resonance which is confirmed by the corresponding constant-initial-state (CIS) spectra. The Ti 3d bands shift approximately 0.25 eV towards higher binding energies in the low temperature semiconducting phase at 50 K. This shift reflects the opening of the band gap and provides a rough estimate of the electronic contribution to the formation energy of the Ti3+-Ti3+ pairs in the low-temperature phase.

Surface electronic structure of complex B-metal perovskites

We present photoemission results for two systems, SrSn1−xSbxO3 and its row 6 analogue, BaPb1−xBix03. The variation in electronic structure as a function of composition is explored, particularly in the region of the metal-to-nonmetal transitions. In the case of the Sr system, dramatic surface segregation of Sb is revealed. This is analogous to that observed in Sn1−xSbx02. Resonant photoemission at oxygen and metal thresholds is used to explore the character of the valence band and gap states in these materials.

Low temperature photoemission study of La2CuO4+x single crystals

New angle-resolved photoemission results obtained using synchrotron radiation of 15–40 eV from cleaved La2CuO4+x single crystals at a temperature of about 30 K are presented. We have observed spectral enhancements at photon energies around 17 eV that are consistent with autoionization resonance excitations of the hybridized La 5p-O 2s states to O 2p empty states. The crystal surface was found to be unstable. A broad feature around 5 eV binding energy that could be caused by surface reconstruction was observed.