Z. H. Ming

X-ray excited luminescence and local structures in Tb-doped Y2O3 nanocrystals

Pronounced structure in x-ray excited luminescence (XEL) has been observed in dilute Tb-doped Y2O3 (Y2O3:Tb) nanocrystals. This effect affords a means to assess different energy transfer mechanisms in the nanocrystals and also an opportunity for novel device applications. Sharp jumps and oscillations are found in the XEL output with the incident x-ray energy around the absorption edges of Y and Tb. When compared with a bulk Y2O3:Tb sample, these effects are attributed to some unique electronic and optical properties of doped nanocrystals related to quantum confinement of charge carriers, and the main features can be explained by a proposed model of multichannel energy transfer. Extended x-ray absorption fine structure techniques have also been employed to study the effect of size variation and chemical doping on the local structures in Y2O3 and Y2O3:Tb nanocrystals. The local environment surrounding Y and Tb in the nanocrystals is compared with that in the respective bulk material. The results indicate th...

Thickness modulation of InGaAs/GaAs superlattices studied by large angle x‐ray scattering

Superlattices of 100‐period InxGa1−xAs (15 A)/GaAs(100 A) grown on GaAs (100) substrates by molecular beam epitaxy were studied by using large angle x‐ray scattering techniques. In contrast to the usual superlattice satellite peaks corresponding to structural periodicity along the growth direction, unusual satellite peaks in the lateral direction parallel to the sample surface were observed in a sample with x=0.535 grown at 480 °C, indicating an in‐plane ordering. This result is confirmed by high resolution transmission electron microscopy observations that thickness modulation in the InxGa1−xAs layers gives rise to long‐range lateral periodic arrays of clusterlike microstructures with spacing on the order of a few hundred angstroms. This thickness modulation is found to occur only in the [110] direction, thus the material can be viewed as a somewhat disordered array of grown‐in parallel quantum wires.

INTERFACIAL ROUGHNESS SCALING AND STRAIN IN LATTICE MISMATCHED SI0.4GE0.6 THIN FILMS ON SI

Interfacial roughness parameters and lattice strain of Si0.4Ge0.6 films with varying thickness epitaxially grown on Si(100) were determined using the techniques of grazing‐incidence x‐ray scattering and diffraction. The roughness of both the buried interface and sample surface follows a similar power‐law scaling behavior with an exponent β around 0.71 for films below the critical thickness, and it undergoes a large change above the critical thickness. Observation of such a scaling law thus establishes a quantitative correlation between the interfacial roughness and lattice strain, and also allows the prediction of interfacial roughness as a function of film thickness of this compound.

X-RAY SCATTERING AND ABSORPTION STUDIES OF MNAS/GAAS HETEROSTRUCTURES

Ferromagnetic MnAs thin films grown on GaAs (001) substrates by molecular‐beam epitaxy have been studied by the methods of grazing incidence x‐ray scattering, x‐ray diffraction, and extended x‐ray‐absorption fine structure. Microstructures in two films prepared with different first‐layer growth conditions (template effects) are compared in terms of the interfacial roughness in the layer structure, lattice constants, epilayer thickness, local environment surrounding the Mn atoms, coordination number, and local disorder. Our results indicate that the template effects can cause significant differences in the local structures and crystallinity of the MnAs epitaxial layers.

Local Mn structures in III-V diluted magnetic semiconductor (In,Mn)As

We describe the local Mn structures obtained by the X-ray absorption fine structure (XAFS) measurements for MBE-grown (In,Mn)As epilayers with high Mn compositions (Mn>0.1). Both homogeneous and inhomogeneous epilayers were investigated. For the inhomogeneous layers grown at T s =280 o C, the entire shell structures are similar to those of NiAs-type bulk MnAs. For the homogeneous layers grown at T s =210 o C, the first whell around Mn was found to consist of disordered structure of six As atoms. This shell is then surrounded by twelve In atoms from the host InAs

INTERFACIAL MICROSTRUCTURES OF ULTRATHIN GE LAYERS ON SI PROBED BY X-RAY SCATTERING AND FLUORESCENCE YIELD

Angular dependence of grazing‐incidence x‐ray scattering and Ge Kα fluorescence yield were measured for buried ultrathin Ge layers grown on bulk Si by molecular beam epitaxy. Results obtained for samples with different Ge layer thickness are compared. The data reveal information on microstructures in these layered materials in terms of the average interfacial roughness, correlation lengths of height fluctuations, and Ge density profile. Structural parameters are obtained by comparison of experimental data with theoretical models.The results also indicate that the interfacial roughness at neighboring interfaces is highly correlated. Significant changes of microstructures in the Ge epilayer are found as the layer thickness approaches the critical thickness. The x‐ray scattering techniques are demonstrated to be capable of detecting a precursor of lattice relaxation in multilayers of lattice‐mismatched compound semiconductors.

Structural ordering in InGaAs/GaAs superlattices

Various x‐ray techniques have been applied to a study of semiconductor superlattices consisting of 100‐period of InxGa1−xAs (15 A)/GaAs (100 A) grown on GaAs(100) substrates by molecular beam epitaxy. Structural parameters pertaining to the morphology of interfaces and thickness variations were obtained. The interfaces in these superlattices are found to be highly correlated, and the layers all show a high degree of crystallinity. Splittings in the x‐ray reflectivity and diffraction patterns in one of the samples provide clear evidence for pronounced thickness modulation, and direct comparison of the diffraction satellite peaks with results of high resolution transmission electron microscopy indicates that there exists a lateral structural ordering in the [110] direction during epitaxial growth.

Investigation of Local Structures Around Luminescent Centers in Doped Nanocrystal Phosphors

Extended x-ray absorption fine structure (EXAFS) technique has been employed to investigate the local structures around luminescent centers in nanocrystals of Mn-doped ZnS and Tb-doped Y 2 O 3 . Size-dependent local structural changes around Mn luminescent centers have been found in Mn-doped nanocrystals of ZnS by using Mn K-edge EXAFS. Local structures around Tb investigated by Tb Li-edge EXAFS also show substantial differences between bulk and nanocrystal samples. This structural information is useful for understanding the novel optical properties of doped nanocrystals.

O 1s x‐ray absorption spectroscopy of Tl2Ba2Ca3O10 high‐TC superconductors

X‐ray absorption around oxygen K‐edge of Tl2Ba2Ca2Cu3O10 high‐TC superconductors was measured by means of a bulk sensitive x‐ray fluorescence yield detection method. Three distinct pre‐edge peaks are revealed. They are ascribed to core‐level excitations of oxygen 1s electrons to empty states at Fermi level which have predominantly oxygen 2p character. These oxygen holes are located in CuO2, BaO and TlO planes. The strong dependence of TC on oxygen holes concenration on O(1) site in CuO2 layer is found.

Local environment about O atoms in Pr‐Y‐Ba‐Cu‐O films studied by O K‐edge XAFS

The local environment about oxygen atoms in PrxY1−xBa2Cu3O7 films obtained by laser ablation has been investigated by means of the x‐ray absorption fine structure (XAFS) spectroscopy at the O K‐edge. It has been found that the Pr atoms do not subsitute for the Cu atoms nor form interstitial defects in the CuO2 planes and that the presence of Pr dopant does not disturb the local environment of oxygen.