Y. H. Kao

Lattice relaxation of InAs heteroepitaxy on GaAs

Abstract Single layers of InAs are grown on GaAs at different orientations by molecular beam epitaxy. They are evaluated by electron diffraction, supplemented by X-ray absorption and reflection measurements. The threshold in layer thickness for lattice relaxation is similar for both (100) and (100). The former, however, generally exhibits an extended transition region with details depending on the growth conditions, while such a transition is invariably abrupt in the latter situation. The (100) growth, in addition, retains the narrowly streaked diffraction patterns throughout the deposition in contrast to the (100) growth. These observations are attributed to different microscopic local atomic environments.

EXAFS Studies of Yttria Stabilized Zirconia

Yttria stabilized zirconia is a well known solid electrolyte which exhibits fast anion conduction at elevated temperatures. Below 1200° C pure zirconia (ZrO2) is monoclinic, but forms a stabilized cubic phase with the fluorite structure in a solid solution with 9.4 to 24 mole percent Y2O3.

Soft x-ray investigation of the effect of growth conditions on InAs/GaAs heterostructures

Abstract The effect of growth conditions on interfacial roughness in InAs/GaAs hetcrostructures prepared by molecular beam epitaxy has been investigated using synchrotron radiation. Measurement of grazing incidence X-ray reflectivity (GIXR) and total electron yield (TEY) in the soft X-ray wavelengths are used for nondestructive characterization of interfacial roughness in these semiconductor heterojunctions. Our results indicate that a smoothing process takes place during epitaxial growth, and that the measured interfacial roughness is lowest for In-stabilized deposition on a GaAs substrate at 2° off (100), and highest with As-stabilized overgrowth on GaAs(100). The surface and interfacial roughness are practically independent of the layer thickness in the range investigated (70–600 A).

Probing the structure of semiconductor superlattices and heterostructures by EXAFS

Abstract We have applied the technique of extended X-ray absorption fine structure (EXAFS) to the study of semiconductor superlattices and heterostructures. The data provide structural information on interatomic distance and local order in several different materials. The results of direct measurement of lattice distortion in the thin films suggest that EXAFS can be employed as a useful method for studying the strained-layer superlattices.

Local Environment Around Barium Atoms in the Superconductor Y-Ba-Cu-O

The local structure around Ba atoms in the compound superconductor Y-Ba-Cu-O has been studied by measuring the extended x-ray absorption fine structure (EXAFS) near the Ba K edge. This technique is used to investigate the changes in the short range order structure when the system is doped with impurities.

Studies of Microstructures in High-Tc Superconductors by X-ray Absorption Techniques

Fluorescence emission and total-electron-yield vs. grazing angle of incidence at fixed energy of incoming soft x-ray radiation, around O is absorption edge, for YBa2Cu3O7−δ and Bi2Sr2 CaCu2O8+x thin films on MgO and ZrO2substrates were investigated. Surface rms roughness and oxygen depth distribution were estimated. We have found that in these materials the information depth of microstructures derived from fluorescence yield and total-electron-yield is around 2500 A and 100 A, respectively.

Soft X‐Ray Studies of Y‐Ba‐Cu‐O Thin Films Prepared by Laser Ablation

Angular variation of x‐ray fluorescence due to oxygen atoms in high‐T Y‐Ba‐Cu‐O thin films is measured for the first time by using a new parallel plate avalanche chamber. This technique allows the possibility of nondestructive probing of the depth‐profile of oxygen atoms in the superconducting materials. Our preliminary results indicate that the near surface region of the Y‐Ba‐Cu‐O film may contain an oxygen‐depleted layer of thickness around 20 nm.

Short Range Order Structure Around Oxygen Atoms in the Superconductor Y-Ba-Cu-O

Spectra of the oxygen K-edge extended x-ray absorption fine structure (EXAFS) of the superconducting compound Y-Ba-Cu-O were obtained by measuring the total electron yield as a function of incident x-ray energy at temperatures between 77K and 300K. This technique affords a convenient way for probing the local environment around the oxygen atoms and soft modes in the lattice.

X-Ray Absorption and Resistivity Studies of the Quinary-Compound Superconductor Y-Ba-Cu-O-S

A quinary-compound system consisting of Y, Ba, Cu, O, and S has been prepared in our laboratory. The nominal composition of Y, Ba, Cu, and O is similar to the well known material YBa 2 Cu 3 O 7-y ; the addition of S is intended to partially substitute the O sites. X-ray absorption measurements show that the lattice is distorted with the presence of S atoms. The transition temperature is found to decrease from 93K to about 77K with an increasing S content up to a S/Cu concentration ratio near 0.25; further increase of S results in unstable superconducting behavior and very broad resistive transition.

Extended X-ray absorption fine structure investigations of Nb3Ge0.26Al0.74

Abstract We have performed Extended X-ray Absorption Fine Structure (EXAFS) studies of the high-Tc ternary superconducting alloy Nb3Ge0.26Al0.74 above the Ge K-edge. The EXAFS indicates that the local environment surrounding the Ge atoms is characterized by a high degree of structural order, similar to that found in previous investigations of high-Tc Nb3Ge thin films. We also present direct “microscopic” structural evidence that this material is a solid solution of Nb3Ge and Nb3Al. No evidence of Nb3Ge microcrystals is observed.