Yun-Liang Soo

Local environment surrounding S and Cd in CdS:O thin film photovoltaic materials probed by x-ray absorption fine structures

Local environments surrounding Cd and S in CdS:O thin films have been determined using extended x-ray absorption fine structure (EXAFS) and near-edge x-ray absorption fine structure (NEXAFS). As indicated by the Cd EXAFS, Cd atoms remain predominantly bonded with S. The S EXAFS and NEXAFS clearly demonstrate the presence of S–O bonds. The oxygen atoms actually combine with S to form SO3 and SO4 complexes. Combined with the transmission electron micrograph, these x-ray results suggest formation of oxygen-free CdS nanocrystals and provide an unambiguous explanation for the mystery of increased band gap that appears to violate the band anticrossing model.

Thermal annealing and grain boundary effects on ferromagnetism in Y2O3:Co diluted magnetic oxide nanocrystals

Structures and magnetic properties of Y2O3:Co nanocrystals were investigated using high resolution transmission electron microscopy, x-ray diffraction, x-ray absorption fine structures, and superconducting quantum interference device. Cobalt atoms were found to be driven by thermal annealing from interstitial locations inside the Y2O3 nanoparticles toward particle surface, where increased O vacancies surrounding Co atoms led to increased saturation magnetization. Our results strongly support a bound magnetic polaron model for ferromagnetism in diluted magnetic oxides mostly due to magnetic dopants located on the grain boundaries.

Room temperature ferromagnetic behavior in cluster free, Co doped Y2O3 dilute magnetic oxide films

Extensive structural analysis and magnetic properties are reported on cluster free, dilute magnetic oxide of Co doped Y2O3 (Co:Y2O3) film by low temperature deposition and characterized by extended x-ray absorption fine structure, x-ray absorption near edge structure, magnetometer, and x-ray magnetic circular dichroism. Room temperature ferromagnetism was observed, and the saturation magnetic moment was modulated by oxygen vacancy concentration through post annealing process. Oxygen vacancies are shown to play a crucial role in ferromagnetic ordering, as defect centers in the bound magnetic polaron model to account for this dilute magnetic oxide of medium band gap with low carrier concentration.

Correlation between oxygen vacancies and magnetism in Mn-doped Y2O3 nanocrystals investigated by defect engineering techniques

Defect engineering techniques have been employed to generate and remove oxygen vacancy defects in nanoparticles of Y2O3:Mn diluted magnetic oxide (DMO). These samples were prepared by thermal decomposition method followed by a series of thermal annealing in oxygen and forming gas. The x-ray absorption analysis reveals that O vacancies surrounding Mn and Y atoms were appreciably increased by forming-gas-annealing and decreased by oxygen-annealing, accompanied by enhanced and reduced saturation magnetization as demonstrated by magnetic measurements, respectively. Our results demonstrate strong correlation between magnetism and O vacancies and therefore strongly support the bound magnetic polaron model for these high-k DMOs.

Ferromagnetism in cluster free, transition metal doped high κ dilute magnetic oxides: Films and nanocrystals

The structural and magnetic properties of high κ dilute magnetic oxides (DMOs) have been studied in two types of samples: thin films and nanocrystals, including Co doped HfO2 films, Co doped Y2O3 films, Co doped Y2O3 nanocrystals, and Mn doped Y2O3 nanocrystals. The characterizations were conducted by extended x-ray absorption fine structure, x-ray absorption near edge structure, and superconducting quantum interference device vibrating sample magnetometer. Oxygen vacancies are shown to play a crucial role in ferromagnetic ordering, as defect centers in the bound magnetic polaron model to account for DMOs with medium band gap and low carrier concentration. The observation of room temperature ferromagnetism in tri-valence oxide of Y2O3 as well as in tetra-valence oxide of HfO2 suggests a generic feature of transition metal doped high κ oxides as good candidates for DMO. The ability to modulate the magnetic behavior of DMO via oxygen vacancy concentration by means of post anneals can be exploited for potent...

Substitutional incorporation of Sn in compressively strained thin films of heavily-alloyed Ge1 − xSn x /Ge semiconductor probed by x-ray absorption and diffraction methods

Short-range-order and long-range-order structures in Ge1 − xSn x /Ge thin films grown by molecular beam epitaxy (MBE) were investigated by using extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) techniques, respectively. These materials are of great potential for constructing efficient optoelectronic devices. The EXAFS analysis demonstrates that Sn atoms occupy Ge sites in these thin-film samples with Sn concentration up to 20 at.%. The Ge-Sn bonds expected in the substitutional model were also observed in Raman spectra of these samples. The XRD results show that, in the out-of-plane direction, the lattice constants of the films are distinctly larger than that of the Ge substrates. However, such increased lattice parameters were not observed in the in-plane direction. Our x-ray and Raman results have clearly revealed substitutional incorporation of Sn with high concentration in dislocation-free MBE-grown Ge films of practical-device thickness.

Revisiting La0.5Sr1.5MnO4 lattice distortion and charge ordering with multi-beam resonant diffraction

Sinusoidal wave type distortions of La0.5Sr1.5MnO4 in the low-temperature orthorhombic phase were observed using multi-beam resonant X-ray diffraction (MRXD) with (7/4 7/4 0) fractional primary diffraction. Two four-beam diffractions with opposite asymmetry were measured at 6.5545 keV and compared with the curves simulated by the dynamical X-ray diffraction theory. This approach provides the possibility of resolving the distortion modes which are perpendicular to the momentum transfer by a single azimuthal scan. The paper also demonstrates the sensitivity of MRXD profiles versus incident X-ray energy in the vicinity of the Mn K edge to the charge disproportion between the two manganese sites, reconfirming the small charge disproportion feature.