Structural relaxation of E' gamma centers in amorphous silica

We report experimental evidence of the existence of two variants of the E' gamma centers induced in silica by gamma rays at room temperature. The two variants are distinguishable by the fine features of their line shapes in paramagnetic resonance spectra. These features suggest that the two E' gamma differ for their topology. We find a thermally induced interconversion between the centers with an activation energy of about 34 meV. Hints are also found for the existence of a structural configuration of minimum energy and of a metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let

Energetic and spatial distribution of grain boundary states in insb-bicrystals

The spatial and energetic distribution of grain boundary interface states in p-doped InSb-bicrystals with an n-inversion layer adjacent to the grain boundary is determined by fitting the calculated two-dimensional subband structure of the inversion layer to the experimentally obtained data. The total density of inversion electrons ns and the subband occupation nsi are established by magnetotransport measurements. A rapid decrease of ns is observed when hydrostatic pressure is applied. By analysing the pressure dependence of ns an energetic density of grain boundary states of 1.5 x 10^13 cm -2 eV-1 was evaluated for the energy range from the conduction band edge up to 90 meV above it. Calculations performed within a numerically simple model indicate the existence of two types of ionized grain boundary states : positively charged donorlike ones localized in the grain boundary core with a density of 4.22 x 10^12 cm-2 and negatively charged ones spatially more extended with 2.27 x 10^12 cm-2 . Some possible reasons for these interface states are discussed.Nous étudions les états liés aux joints de grain dans les bicristaux de InSb de type p où les couches d'inversion adjacentes aux joints de grain contiennent un gaz d'électrons à deux dimensions. La distribution énergétique et spatiale de ces états est obtenue en ajustant la structure des sous-bandes à deux dimensions aux résultats expérimentaux. La densité totale n s de la couche d'inversion et celle de la sous-bande n = 1 sont obtenues à partir de mesures de magnéto-transport. ns décroît rapidement quand on applique une pression hydrostatique. En analysant cette variation on évalue la densité des joints de grains à 1,5 × 10^13 cm -2 eV-1 dans une gamme d'énergie comprise entre le bas de la bande de conduction et 90 meV au-dessus de celle-ci. Des calculs numériques réalisés à partir d'un modèle théorique relativement simple indiquent l'existence de deux types d'états ionisés liés aux joints. Des états chargés positivement de caractère donneur sont localisés au centre du joint avec une densité de 4,22 × 10^12 cm-2 ; des états chargés négativement ont une plus grande extension spatiale avec une densité de 2,27 x 10^12 cm-2. On discute l'origine possible de ces états

Optical properties of wetting layers in stacked InAs/GaAs quantum dot structures

The optical transitions of the wetting layers in two-fold self- assembled InAs/GaAs quantum dot samples are studied as a function of GaAs spacer thickness by various methods. The absorption related studies by photoreflectance and selective photoluminescence excitation spectroscopy reveal already for thick barriers, for which coupling effects can be excluded, two energetically separated heavy-hole transitions. This splitting indicates the formation of two wetting layers during growth with a 10% difference in width and reflects strain field interaction between the island layers. Thin spacer layer samples show in addition the expected wetting layer coupling as confirmed by subband calculations. (C) 2002 Elsevier Science B.V. All rights reserved

Structural and optical properties of both pure poly (3-octylthiophene) (P3OT) and P3OT/fullerene films

We have investigated the structural and optical properties of P3OT and P3OT/fullerene thin films in view of their application as active layer in plastic solar cells. Films of these materials were prepared by spin coating from toluene solutions onto silicon substrates. Their optical properties were studied by spectroscopic ellipsometry, which provides the anisotropic dielectric function of the films. Moreover, structural properties were studied using X-ray diffraction. A close correlation between the results obtained by both methods could be found. Especially, the strong optical anisotropy of the films can be explained in terms of a preferable orientation of the polymer chains parallel to the substrate. The effect of the optical anisotropy on the performance of optoelectronic devices is discussed. (C) 2003 Elsevier B.V. All rights reserved

Investigation of Laser Irradiated Areas with Electron Backscatter Diffraction

AbstractIn this work, two silicon nitride (SiNx) layers with two different refraction indices, deposited on polished or damage-etched silicon wafers were locally irradiated by laser pulses. The focus was set on the investigation of the ablation mechanisms. Thereby, an ultra-short laser source (pulse duration 10 ps, wavelength 532nm, Gaussian profile) was used. The irradiated areas were investigated by electron backscatter diffraction (EBSD) in order to analyze the near-surface crystallographic orientation and crystallinity.In this work an indirect ablation was observed for SiNx (n = 1.9). Further, a change from an indirect ablation to a partial lift-off for SiNx (n = 2.1) was determined to be fluence dependent. At low fluences, the SiNx was completely removed. However, at higher fluences, SiNx was not completely removed, due to direct ablation. The two-photon-absorption coefficient of SiNx (n = 2.1) was estimated to be 2.105cm/TW