Site-selective X-ray Absorption Fine Structure Research Articles

Capacitance XAFS method: a new site-selective and microscopic x-ray absorption spectroscopy

For local structure and electronic state analyses of electron trapping centers, we developed a site-selective x-ray absorption fine structure XAFS measurement, i.e., the capacitance XAFS method. The concept of the capacitance XAFS measurement is based on the fact that the x-ray absorption of trapping centers, not the bulk, can be evaluated from the capacitance change due to x-ray induced photoemission of a localized electron. With this method, micro spectroscopy can be achieved by scanning probe detection of the capacitance. Moreover, trapping centers with a specific eigen-energy can be selectively observed by controlling the sample bias voltage. We tested these advantages in detail for two typical metal and semiconductor systems. (1) The capacitance XAFS spectrum of Cu indicated twodimensional (2D) electronic states at a hetero interface between native oxide (Cu2O) and bulk Cu; the Cu 4pπ states of the electron trap were confined into a 2D plane. (2) The capacitance XAFS spectrum of GaAs depends on the sample bias voltage and on selectively indicated electronic states of surface trapping center due to Ga oxide, which captures electrons at a positive bias voltage.

Selective X-ray absorption spectroscopy of self-assembled atom in InAs quantum dot

To discuss the local structure of an atom in the self-organized InAs quantum dot (QD), we perform a site-selective X-ray absorption fine structure (XAFS) measurement, employing the capacitance XAFS method. Since the X-ray-induced photoemission of confined electrons in a QD via inner-shell absorption can be detected by capacitor, the photon energy dependence of the capacitance provides the XAFS spectrum of the atom in the QD. It is found that when the bias voltage applied to the capacitor aligned the Fermi energy with the quantum level, the site-selectivity is enhanced. The peak energy shift of ∼1.5 eV is observed in the site-selective XAFS spectrum of an As atom in QD. The molecular orbital calculation indicates that the energy shift originates from the point defect at the boundary between an InAs QD and a GaAs barrier layer.

Capacitance X-ray absorption fine structure measurement using scanning probe A new method for local structure analysis of surface defects

A capacitance X-ray absorption fine structure (XAFS) method using a scanning probe is developed for the selective analysis of surface defects in selected local regions. Since capacitance is sensitive to localized electrons in defects, capacitance change owing to X-ray absorption will provide site-selective XAFS spectra of the defects. The capacitance detection by the scanning probe under an X-ray beam gives XAFS spectra dependent on the surface state. An electron transition model, in which the surface density of states is taken into account, reproduces the spectral shape differences between the normal surface and defect surface.

Site-selective x-ray absorption fine structure analysis of an optically active center in Er-doped semiconductor thin film using x-ray-excited optical luminescence

In order to discuss the local structure of an optically active center in Er-doped Si thin film, site-selective x-ray absorption fine structure (XAFS) analysis using x-ray-excited optical luminescence was performed. The XAFS spectrum at the Er LIII edge was obtained from the x-ray photon energy dependence of the peak intensity of infrared luminescence due to Er intra-4f transition. Although conventional XAFS measurement analyzes the average structure of all of the Er, this method intrinsically provides structural information for only optically active Er. A broad 2p–5d resonant peak in the site-selective XAFS spectrum is reproduced by a density-of-state calculation of a distorted ErO6 cluster, assuming an Er transformation from an octahedral center of 0.25 Å.

Site-Selective X-Ray Absorption Fine Structure (XAFS) Spectroscopy. (1) Design of Fluorescence Spectrometer and Emission Spectra

Abstract A Rowland-type fluorescence spectrometer was designed to measure site-selective XAFS spectra by utilizing the chemical shift of emission peak for each site. The positions of Johansson-type cylindrically-bent Ge(111) crystal and scintillation counter were controlled independently by five-channel stepping motors, thereby satisfying precise Rowland condition. Thus, measurements of many fluorescence lines over wide ranges of Bragg angle 55.6—83.9° and Rowland radius 127.7—240.9 mm are made feasible. The Cu Kα peak shifted by +1.6 eV on going from Cu to CuCl and by -0.6 eV on going from CuCl to CuCl2. The Cr Kβ1 peak shifted by +0.8 eV on going from Cr to Cr2O3 and by -1.6 eV on going from Cr2O3 to K2CrO4. The FWHM of Cu Kα peaks was as small as 3.0 eV. The major factor to control the energy resolution of this spectrometer was found to be geometrical angle width (slit size). The effects of Rowland radius and X-ray penetration depth into the bent crystal were smaller in the range of slit size in this paper [slit length (horizontal) in front of I0 ion chamber was 0.5—2.0 mm and two slit lengths (vertical) in Rowland circle were 0.5—5.5 mm].

Site-Selective X-Ray Absorption Fine Structure (XAFS) Spectroscopy (2). XAFS Spectra Tuned to Surface Active Sites of Cu/ZnO and Cr/SiO2 Catalysts

Abstract Site-selective XAFS spectra were measured by tuning the Rowland-type fluorescence spectrometer to each site of Cu/ZnO and Cr/SiO2 catalysts. The chemical shifts between Cu0 and CuI sites and between CrIII and CrVI sites were relatively large for model compounds (ΔE = 1.6 eV), and were utilized as the tune energies of site-selective XAFS. Site-selective XANES tuned to Cu0, CuI, and CrIII sites were successfully obtained. Based on the correlation between the chemical shift and the peak width (energy resolution of the fluorescence spectrometer), site-selective XANES spectra were analyzed. The population ratio of Cu metal, Cu2O, and CuI atomically dispersed on ZnO was estimated to be 70(±23) : 22(±14) : 8(±5). The contribution of the CrIII site to site-selective XANES tuned to the CrIII site was 94(±3)%.

Site-selective XAFS: a new tool for catalysis research

Site-selective X-ray absorption fine structure (XAFS) experiments will become an important tool in the study of active sites and metal–support interactions in catalysis. Site-selective XAFS is possible with the detection of specific X-ray emission channels. Selectivity can be obtained for the valence or for the nature of the neighbouring atoms. This opens a wide range of new experiments, for example with valence-selective XAFS the difference in local structure of an element present with two valences can be studied. With site-selective XAFS the local surroundings of a metal in contact with oxygen or carbon can be differentiated from the local surroundings of all the other metal atoms.

Site-selective x-ray absorption fine structure: Selective observation of Ga local structure in DX center of Al0.33Ga0.67As:Se

In order to discuss the local structure of deep level carrier traps, the site-selective x-ray absorption fine structure (XAFS) by measuring the x-ray photon-energy dependence of the capacitance of a Schottky barrier diode is proposed. Dropping of the localized electron into a core hole arising from x-ray absorption of the atom in the carrier trap, not in bulk, increases the capacitance. The site-selective XAFS is adopted for the local structure analysis of DX center in Al0.33Ga0.67As:Se. The Ga K-edge site-selective XAFS is different from the conventional XAFS, suggesting that Ga in the DX center with a large lattice relaxation is selectively observed.