G. Ciatto

Treatment of EXAFS data taken in the fluorescence mode in non-linear conditions.

The aim of this work is to investigate the possibility of extracting correct structural parameters from fluorescence EXAFS data taken at high count rates with an energy-resolving detector. This situation is often encountered on third-generation synchrotron radiation sources which provide a high flux on the sample. Errors caused by pulse pile-up in the extraction of structural information have been quantified in a real experiment, and different approaches to the problem of data correction have been elaborated. The different approaches are discussed in a comparison of the ability of each kind of correction to recover the correct structural parameters. The result of our analysis is that it is possible to work in non-linear conditions and correct the data, if the response of the acquisition system is known. Reliable structural information can be obtained with data acquired up to a count rate equal to approximately 60% of the inverse of the dead time.

Determination of lattice parameter and of N lattice location in InxGa1−xNyAs1−y/GaAs and GaNyAs1−y/GaAs epilayers

We have used an experimental strategy that, combining nuclear reaction analysis and Rutherford backscattering spectrometry both in random and channeling geometry, allowed an accurate quantification of the total amount of N in InxGa1−xNyAs1−y/GaAs and GaNyAs1−y/GaAs epitaxial systems (0.038<x<0.044, 0.015<y<0.045), and a precise localization of nitrogen atoms into the lattice. All N atoms were found on substitutional positions. This information was then exploited to correlate the relaxed lattice parameter of the epilayers obtained by high-resolution x-ray diffraction to the N concentration, by taking into account the elasticity theory, allowing a verification of the validity of Vegard’s rule in the whole range of investigated N concentrations for both alloys. The effect of N incorporation on the lattice parameter has been found to be the same both for ternary and quaternary alloys.

How much room for BiGa heteroantisites in GaAs1−xBix?

We addressed the issue of bismuth heteroantisite defects (BiGa) in GaAs1−xBix/GaAs epilayers by coupling x-ray absorption spectroscopy at the bismuth edge with density functional theory calculations of the defect structure. Calculations predict a large relaxation of the Bi-As interatomic distances when Bi atoms substitute Ga, however we found no experimental evidence of it. Quantitative analysis of the x-ray absorption spectra allows us to establish a maximum concentration limit for BiGa, which corresponds to about 5% of the total Bi atoms. BiGa do not account for the modifications in the spectra previously attributed to short range ordering.

Atomic ordering in (InGa)(AsN) quantum wells: An In K-edge X-ray absorption investigation

Abstract (InGa)(AsN) quantum wells are particularly interesting for applications in fiber optics based communications, since they permit to make lasers with emission at 1.3 and 1.55 μm, with good temperature stability. The aim of this work is to study the local environment of In by means of X-ray absorption fine structure. In particular, the site ordering around In will be investigated in order to understand the structural origin of the non-linear optical and electronic proprieties which characterize this alloy. Despite the low N concentration (⩽5.2%) a significant contribution of In–N bonds to the signal is detected. The data provide evidence of a preferential distribution of N atoms around indium, as recently predicted via Monte Carlo simulations. Hydrogenation further enhances the increase in the number of In–N over In–As bonds.

The effect of Co doping on the conductive properties of ferromagnetic ZnxCo1−xO films

The investigation of the electrical properties of Co-doped ZnO thin films provides two unexpected results: a decrease of the electrical conductivity and the contemporary occurrence of a reduction of conductivity and of an enhancement of ferromagnetic order. The former result is surprising since Zn atoms are replaced with iso-valent Co atoms. The latter finding questions previously suggested beneficial effects of n-type doping on the ZnO:Co magnetic behavior. While morphological and structural characterization permits us to exclude an influence of the morphology and of the presence of Co metal or Co-oxide phases on present experimental findings, with the aid of first-principles electronic structure calculations, we propose a qualitative picture which can explain in a coherent way both the changes in conductivity and ferromagnetic behavior ensuing from the Co doping and the above-mentioned, beneficial effects of n-type doping.

Ferromagnetism and Conductivity in Hydrogen Irradiated Co-Doped ZnO Thin Films.

Impressive changes in the transport and ferromagnetic properties of Co-doped ZnO thin films have been obtained by postgrowth hydrogen irradiation at temperatures of 400 °C. Hydrogen incorporation increases the saturation magnetization by one order of magnitude (up to ∼1.50 μB/Co) and increases the carrier density and mobility by about a factor of two. In addition to the magnetic characterization, the transport and structural properties of hydrogenated ZnO:Co have been investigated by Hall effect, local probe conductivity measurements, micro-Raman, and X-ray absorption spectroscopy. Particular care has been given to the detection of Co oxides and metal Co nanophases, whose influence on the increase in the transport and ferromagnetic properties can be excluded on the ground of the achieved results. The enhancement in ferromagnetism is directly related to the dose of H introduced in the samples. On the contrary, despite the shallow donor character of H atoms, the increase in carrier density n is not related to the H dose. These apparently contradictory effects of H are fully accounted for by a mechanism based on a theoretical model involving Co-VO (Co-O vacancy) pairs.

X-ray absorption and diffraction study of II?VI dilute oxide semiconductor alloy epilayers

Dilute oxide semiconductor alloys obtained by adding oxygen to a II–VI binary compound are of potential applicative interest for blue-light emitters in which the oxygen content could be used to tune the band gap. Moreover, their properties can be usefully compared to the more thoroughly studied dilute nitrides in order to gain insight into the common mechanisms which give rise to their highly non-linear physical properties. Recently, it has been possible to deposit ZnSeO and ZnSeOS epilayers on GaAs(001), which exhibit a red-shift of the band gap and giant optical bowing. In order to provide a structural basis for an understanding of their physical properties, we have performed a study of a set of ZnSeO and ZnSeOS epilayers on GaAs by high resolution x-ray diffraction and x-ray absorption fine structure. We have found that the strain goes from compressive to tensile with increasing O and S concentration and that, while all epilayers are never found to be pseudomorphic, the ternary ones exhibit a low relaxed fraction if compared to the ZnSe/GaAs sample. O K-edge x-ray absorption near edge spectra and corresponding simulations within the full multiple-scattering regime show that O is substitutionally incorporated in the host lattice. Zn and Se K-edge extended x-ray absorption fine structure detect the formation of Zn–O and Zn–S bonds; the analysis of these spectra within multiple-scattering theory has allowed us to measure the local structural parameters. The value of Zn–Se bond length is found to be in agreement with estimates based on models of local distortions in strained and relaxed epilayers; an increase of the mean-square relative displacement is detected at high O and S concentration and is related to both intrinsic and extrinsic factors.

Synchrotron x-ray diffraction study of micro-patterns obtained by spatially selective hydrogenation of GaAsN

We report a comparative synchrotron radiation x-ray diffraction study of GaAs1−yNy micro-structures obtained by two different patterning methods: spatially selective H incorporation achieved by using H-opaque masks and spatially selective H removal attained by laser writing. These methods are emerging as original routes for fabrication of micro- and nano-structures with in-plane modulation of the bandgap energy. By measuring the out-of-plane and in-plane lattice parameters, we find that for both patterning approaches the largest part of the micro-structure volume remains tensile-strained and pseudomorphic to the substrate, regardless of the compressive-strained hydrogenated barriers. However, a larger lattice disorder is probed in the laser-written micro-structures and attributed to partial removal of H and/or strain changes at the micro-structure boundaries. This larger lattice disorder is confirmed by photoluminescence studies.

Quantitative determination of In clustering in In-rich InxGa1−xN thin films

We investigated atomic ordering in In-rich InxGa1−xN epilayers in order to obtain an understanding of whether a deviation from a random distribution of In atoms in the group-III sublattice could be the origin of the strong carrier localization and defect-insensitive emission of these semiconductor alloys. This phenomenon can be exploited for application in optoelectronics. By coupling In K-edge x-ray absorption spectroscopy and high resolution x-ray diffraction, we were able to discard the hypothesis of significant phase separation into InN + GaN, in agreement with previous N K-edge absorption spectroscopy. However, we found an enrichment of In neighbours in the second atomic shell of In as compared to random statistics (clustering) for x = 0.82, while this is not the case for x = 0.46. This result, which is also supported by optical spectroscopy, is likely to stimulate new theoretical studies on InxGa1−xN alloys with a very high In concentration.

Defect-induced Magnetism in Cobalt-doped ZnO Epilayers

We used a synergic Co-edge X-ray absorption spectroscopy (XAS) and density functional theory calculations approach to perform a study of defects which could account for the room temperature ferromagnetism of ZnCoO, an oxide of great potential interest in semiconductor spintronics. Our results suggest that a key role is played by specific defect complexes in which O vacancies are located close to the Co atoms. Extended defects such as Co clusters have a marginal function, although we observe their formation at the epilayer surface under certain growth conditions. We also show preliminary results of the study of hydrogen-induced defects in ZnCoO epilayers deliberately hydrogen irradiated via a Kaufman source. Hydrogen was in fact predicted to mediate a ferromagnetic spin-spin interaction between neighboring magnetic impurities.