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Grazing incidence x-ray scattering and x-ray diffraction techniques have been employed to investigate the microstructures in various ZnSe-based semiconductor thin films grown on GaAs substrates by molecular beam epitaxy and metalorganic chemical vapor deposition methods. The results are also used for a comparison of the interfacial roughness and overall quality of the II–VI thin films prepared by these two different growth methods. Structural parameters such as the interfacial roughness and layer thickness obtained from the scattering measurements and lattice constants obtained from the x-ray diffraction pattern around the GaAs(004) peak can be correlated with the film deposition rate, compound composition, and lattice strain in the epilayers. We thus demonstrate that x-ray scattering techniques in conjunction with diffraction measurements are useful tools for nondestructive characterization of buried interfaces in semiconductor layer materials.

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...

A series of type-II GaAs/AlAs superlattices epitaxially grown with different interrupts have been investigated using the techniques of grazing incidence X-ray scattering and diffraction. The interrupts are specifically designed to alter the interfacial roughness in the superlattices for the present study. Various structural parameters including the layer thickness, interfacial roughness, and intra-layer correlation lengths of fluctuations in the quantum-well widths have been determined. These results are compared with measurements made on the same set of samples using photoluminescence and optical imaging techniques.

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.

X-ray-absorption fine-structure measurements were carried out to probe the local environment surrounding Mn ions in Mn-doped nanocrystals of ZnS with different size distributions ranging from 30--35 \AA{} to 50--55 \AA{}. The interatomic distances between Mn and neighboring atoms, the coordination number, local disorder, and effective valency determined for the nanocrystals are compared with those in bulk Mn-doped ZnS. The Mn ions are found to substitute for the Zn sites in the host ZnS but with significant size-dependent local structural changes. The questions of Mn-cluster formation and the presence of Mn impurities on the surface of the nanocrystals are addressed. Near-edge x-ray-absorption fine structures indicate that the effective valency of Mn ions in the nanocrystals is close to +2 with a weak size dependence. These local structures are believed to be closely related to the novel optical properties observed in this new class of semiconductors.

The angular dependences of grazing-incidence x-ray scattering and Ge K\ensuremath{\alpha} fluorescence yield were measured for ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si and its inverted Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ heterostructures. The results reveal useful information on microstructures in these layered materials and show similar interfacial structures in terms of the rms interfacial roughness, correlation length of height fluctuations, and Ge density profile. Two ten-period superlattices with different thickness and Ge concentration were also investigated; correlation between height fluctuations of different interfaces is clearly demonstrated in the data of x-ray-diffuse scattering. These results show that x-ray scattering and fluorescence techniques can be employed as convenient tools for nondestructive characterization of epilayer thickness, interfacial roughness, density profile of selected atomic species, and correlations between microstructures of different interfaces in layered materials.

X-ray-absorption fine-structure measurements were performed at the Mn [ital K] edge on In[sub 0.88]Mn[sub 0.12]As diluted magnetic semiconductors prepared by molecular-beam epitaxy. It has been found that in the high-growth-temperature samples ([ital T][sub [ital s]]=280 [degree]C), Mn atoms are primarily incorporated in the form of MnAs clusters with NiAs structure. No significant disorder is observed. In the low-growth-temperature samples ([ital T][sub [ital s]]=210 [degree]C), the majority of Mn atoms form small ([ital r][similar to]3 A), disordered, sixfold-coordinated centers with As. The presence of disorder in MnAs centers for the latter case is established using the method of cumulants. Only a very small fraction of Mn atoms may substitute for In in the zinc-blende InAs structure. Effective valency and coordination of Mn atoms deduced from the near-edge structure are the same for both the high- and low-growth-temperature In[sub 1[minus][ital x]]Mn[sub [ital x]]As films. The formal valency is lower than +3. The local structures established in the present work are consistent with the observed difference in magnetic behavior for samples prepared at different substrate temperatures.

Soft-x-ray-absorption fine-structure (XAFS) measurements were performed at the F {ital K} edge on Nd{sub 2}CuO{sub 4{minus}{ital x}}F{sub {ital x}} compounds with {ital x}=0.15 and 0.30. It is found that a majority of F atoms substitute for the O(2) sites in fluorite-type NdO layers between CuO{sub 2} sheets. For the compositions {ital x}=0.15 and 0.30, a fraction of 0.33{plus minus}0.15 and 0.39{plus minus}0.14 of F dopants, respectively, substitute for the O(1) site in the CuO{sub 2} planes. Fluorine anions placed at the O(2) sites in the NdO layers form a reservoir of extra electrons for the superconducting CuO{sub 2} sheets in Nd{sub 2}CuO{sub 4{minus}{ital x}}F{sub {ital x}} superconductors, analogous to the substitution of Ce or Th cations for Nd. This observation thus provides evidence for a symmetry of anionic (F) and cationic (Ce and Th) substitution in the NdO layers in connection with superconductivity.

X-ray absorption around the oxygen {ital K} and the Cu {ital L}{sub III} edge of Tl{sub 2}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10{minus}{delta}} high-{ital T}{sub {ital c}} superconductors with different oxygen content {delta}, was measured. Near the O 1{ital s} edge, three distinct pre-edge peaks with maxima at 528.3, 529.4, and 530.6 eV, respectively, are revealed. They are ascribed to core-level excitations of oxygen 1{ital s} electrons to empty states which have predominantly oxygen 2{ital p} character, located in CuO{sub 2}, BaO, and TlO planes, respectively. A strong dependence of {ital T}{sub {ital c}} on oxygen hole concentration at O(1) and O(1){prime} sites in the CuO{sub 2} layer is found. Near the Cu 2{ital p}{sub 3/2} edge, two main peaks are observed. The peak at 931.2 eV is ascribed to a 2{ital p}3{ital d}{sup 10} final state. This implies a predominately Cu{sup 2+} ground state which has mainly 3{ital d}{sup 9} character. The peak at 933 eV is ascribed to a 2{ital p}3{ital d}{sup 10}{ital L} final state. This peak shows an intensity increase with {ital T}{sub {ital c}}, i.e., with ligand hole concentration, and is believed to arise from a formally Cu{sup 3+} ground state which has mainly 3{ital d}{sup 9}{italmore » L} character. No evidence of a monovalent (Cu{sup 1+}) 3{ital d}{sup 10} ground state was found.« less

The symmetry and density of unoccupied states of twin-free YBa{sub 2}Cu{sub 3}O{sub 7} and YBa{sub 2}Cu{sub 4}O{sub 8} single crystals have been investigated by orientation-dependent x-ray absorption spectroscopy at the O 1{ital s} edge, using a bulk-sensitive fluoresence-yield-detection method. In conjunction with earlier results obtained by electron-energy-loss spectroscopy, the relative density of unoccupied states for different oxygen sites has been derived. Comparison with local-density energy-band-structure calculations shows reasonable agreement with the experimental results.

Oxygen K-edge absorption spectra of carefully characterized ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ samples were measured using a bulk-sensitive fluoresence-yield-detection method. They reveal two distinct pre-edge peaks which evolve systematically as a function of Sr concentration. The measured spectra are quantitatively described by calculations based on the Hubbard model, including local Coulomb interactions and core-hole excitonic correlations. The absorption data are consistent with a description of electronic states based on a doped charge-transfer insulator.

L'etude de Nd 2 CuO 4 dope par Ce par absorption de rayons-X au seuil K de l'oxygene en utilisant aussi bien le rendement electronique totale que l'emission de fluorescence, presente des structures similaires a celles de Nd 2 CuO 4 non dope. Ceci est en contradiction avec celles observees pour La 2 CuO 4 dope par Sr et YBa 2 Cu 3 O 7 . En plus, les spectres d'absorption du seuil-M de Ce et Nd montrent que les ions Ce sont dans un etat de valence intermediaire et que Nd reste dans l'etat trivalent. Presentation des resultats et discussion, en parallele avec ceux des autres supraconducteurs a base d'oxydes de cuivre

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.

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.

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.

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

Semiconductor superlattices have been studied by grazing incidence x-ray scattering and x-ray diffraction. For superlattices of 100-period InGaAs/GaAs, lateral structural ordering has been found to occur in the material. For one particular sample (M1400), periodic thickness modulations have been observed in the InGaAs layers. X-ray results also provide evidence for an improvement of interface quality by using interrupt-growth method for 55-period AlAs/GaAs superlattices grown by MBE.

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 Y2O3. 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.

Roughness parameters of sample surface and buried interfaces in a series of thin layers of Si0.4 GeO.6 grown on Si(100) by molecular beam epitaxy (MBE) were measured by using the technique of grazing-incidence x-ray scattering (GIXS). The strain in the layer and the critical thickness of the film were determined from x-ray diffraction of the Si(004) peak. The roughness parameters can be described by a scaling-law with an exponent β = 0.71 for both the surface and interfacial roughness. Establishment of a scaling law thus allows a possibility of predicting the interfacial roughness as a function of the epilayer thickness.

Extended x-ray absorption fine structure (EXAFS) techniques have been used to investigate the local structures in Inl-xMnxAs films grown by molecular beam epitaxy (MBE) under different processing conditions. For samples grown at low substrate temperatures (near 200°C) or with a low Mn concentration (about 1 atomic%), the Mn atoms can substitute for In in the InAs host, thus indicating that III-V diluted magnetic semiconductors (DMS) can indeed be prepared by substitutional doping of magnetic impurities. On the other hand, substitution dose not take place in high Mn concentration (above 10%) samples grown at high substrate temperatures (around 300°C); these samples contain a large amount of MnAs clusters and become ferromagnetic

Angular dependence of grazing incidence x-ray scattering and Ge fluorescence yield were measured for the heterostructures of Si1-xGex/Si and the inverted bilayer Si/Si1-xGex as well as two 10-period superlattices. Interfacial roughness, correlation of height fluctuations between interfaces and Ge density profiles in the multilayers were investigated.

the local environment about oxygen atoms in the CuO2 plane in YBa2Cu3O7 single crystals has been investigated by orientation‐dependent x‐ray absorption fine structure (XAFS) spectroscopy at the O 1s edge using bulk‐sensitive fluorescence‐yield‐detection method. Within experimental accuracy (±0.03 A) no structural change in local ordering about oxygen atoms in the CuO2 plane at 90 K has been detected.

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.

XAFS spectra at the Mn K-edge were obtained for films of In1-xMnxAs (0.0014 ≤x ≤ 0.12) grown by MBE method at two different substrate temperatures Ts, = 200–210 °C and Ts, = 280–300 ° C. It has been found that Mn-As complexes which consist of a central Mn atom surrounded by six neighboring As most likely arranged in the form of a distorted trigonal antiprism with one or two additional Mn atoms placed on the long axis of the antiprism, can substitute for the In-As tetrahedron in the undistorted zincblende structure. For a composition of x = 0.12 we have found the formation of MnAs clusters with NiAs-like structure in the high-growth-temperature samples. We thus conclude that the magnetic properties of the In1-xMnx As semiconductors are mainly determined by the formation and local structure ol'the Mn-As complexes.

The symmetry and density of unoccupied states of YBa{sub 2}Cu{sub 3}O{sub 7} YBa{sub 2}Cu{sub 4}O{sub 8} have been investigated by orientation dependent x-ray absorption spectroscopy on the O 1s edge using a bulk-sensitive fluorescence-yield-detection method. It has been found that the O 2p holes are distributed equally between the CuO{sub 2} planes and CuO chains and that the partial density of unoccupied O 2p states in the CuO{sub 2} planes are identical in both systems investigated. The upper Hubbard band has been observed in the planes but not in the chains in both systems. 18 refs.

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.

X-ray fluorescence and total electron yield around the oxygen {ital K} edge of high-{Tc} Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O superconductor films were measured. The oxygen fluorescence spectra in these materials were obtained by using a new parallel-plate avalanche detector and a high-resolution soft-x-ray spectrometer. Comparison of the fluorescence results with total electron yield and with other data indicates that the fluorescence method is free from spurious features caused by contaminants and oxygen-depletion layers, as usually found in the electron-spectroscopy measurements, and that meaningful information on photoexcitation of core electrons at the oxygen sites can be readily obtained by these bulk-sensitive measurements.

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.

Superconducting films of YBa2Cu3O7−6 obtained by laser ablation on MgO and ZrO2 substrates were investigated by soft x-ray absorption fine structure technique (XAFS) at O K-edge. Local environment around oxygen atoms was probed and information on the distribution of the nearestneighbors was obtained. Much higher degree of local disorder than that in a bulk YBa2Cu3O7−δ sample was observed.

Local structure around the constituent atoms in the compound system Y1-xPrxBa2CU3O7-y has been investigated by means of x-ray absorption fine structure (XAFS) techniques. By comparing the local structure in the compound x=1 with its counterpart x=0, the XAFS results provide a direct evidence that Pr has replaced Y in the material. The nearest-neighbor structure in the CuO2 planes seems to remain intact as × varies from 0 to 1, indicating that substitution of Pr for Y does not disturb the local distribution of holes in the CuO2 planes. We suggest that local disorder and distortions in the second-neighbor bonding configuration could be responsible for suppression of superconductivity with increasing Pr content in the system.