Effects of in situ thermal annealing on the structural, optical, and electrical properties in Hg0.7Cd0.3Te epilayers grown on CdTe buffer layers

Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) transmission, and Hall effect measurements were performed to investigate the structural, optical, and electrical properties of as-grown and in situ-annealed Hg_0.7Cd_0.3Te epilayers grown on CdTe buffer layers by using molecular beam epitaxy. After the Hg_0.7Cd_0.3Te epilayers had been annealed in a Hg-cell flux atmosphere, the SEM images showed that the surface morphologies of the Hg_0.7Cd_0.3Te thin films were mirror-like with no indication of pinholes or defects, and the FTIR spectra showed that the transmission intensities had increased in comparison to that of the as-grown Hg_0.7Cd_0.3Te epilayer. Hall-effect measurements showed that n-Hg_0.7Cd_0.3Te epilayers were converted to p-Hg_0.7Cd_0.3Te epilayers. These results indicate that the surface, optical, and electrical properties of the Hg_1 − xCd_xTe epilayers are improved by annealing and that as-grown n-Hg_1 − xCd_xTe epilayers can be converted to p-Hg_1 − xCd_xTe epilayers by in situ annealing.     

Annealing effect on soft magnetic properties and magnetoimpedance of Finemet Fe73.5Si13.5B9Nb3Au1 alloy

Effect of annealing on the soft magnetic properties of Fe_73.5Si_13.5B₉Nb₃Au₁ amorphous ribbon has been investigated by means of structure examination, magnetoimpedance ratio (MIR) and incremental permeability ratio (PR) spectra measured in the frequency range of 1–10 MHz at a fixed current of 10 mA X-ray diffraction analysis showed that the as-cast sample was amorphous and it became nanocrystalline under a proper heat treatment. When annealing amorphous alloy at 530 °C for 30, 60, 90 min, soft magnetic properties have been improved drastically. Among the samples investigated, the sample annealed at 530 °C for 90 min showed the softest magnetic behavior. The MIR and PR curves revealed the desirable changes in anisotropy field depending upon annealing.     

Flexible a‐Si:H/nc‐Si:H tandem thin film silicon solar cells on plastic substrates with i ‐layers made by hot‐wire CVD

In this letter we report the result of an a‐Si:H/nc‐Si:H tandem thin film silicon solar mini‐module fabricated on plastic foil containing intrinsic silicon layers made by hot‐wire CVD (efficiency 7.4%, monolithically series‐connected, aperture area 25 cm²). We used the Helianthos cell transfer process. The cells were first deposited on a temporary aluminum foil carrier, which allows the use of the optimal processing temperatures, and then transferred to a plastic foil. This letter reports the characteristics of the flexible solar cells obtained in this manner, and compares the results with those obtained on reference glass substrates. The research focus for implementation of the hot‐wire CVD technique for the roll‐to‐roll process is also discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Coherence transition in granular YBa2Cu3O7-δ, YBa2Cu2.95Zn0.05O7-δ, and YBa1.75Sr0.25Cu3O7-δ superconductors

We have studied experimentally the electrical magneto-conductivity near the superconducting transition of YBa₂Cu₃O_7-δ, YBa₂ (Cu_2.95Zn_0.05)O_7-δ and Y(Ba_1.75Sr_0.25)Cu₃O_7-δ polycrystalline samples. The measurements were performed in magnetic fields ranging from 0 to 400 Oe applied parallel to the current orientation. The results show that the resistive transition of these systems proceeds in two stages. The pairing transition occurs at the bulk critical temperature T_c, where superconductivity is stabilized within small and homogeneous regions of the sample generically called grains. The regime of approach to the zero resistance state reveals the occurrence of a coherence transition at a lower temperature T_c0. This transition is related to the connective nature of the granular samples and is controlled by fluctuations of the order-parameter phase of individual grains. Our experiments show that the Zn-doping, besides depressing the pairing critical temperature, strongly enlarges the temperature range dominated by effects related to the coherence transition. The substitution of Ba by Sr causes only a small reduction of T_c, but also enhances significantly the effects related to the grain coupling phenomenology. In general, our results indicate that these impurity substitutions in YBa₂Cu₃O_7-δ produce or magnify the granularity at a microscopic level, enhancing the effects of phase fluctuations in the conductivity near the transition.     

Optimization on Synchrotron Radiation Lattice of BEPCII

The Beijing Electron and Positron Collider II （BEPCII） is a double ring electron positron collider, which can also be used as a synchrotron radiation （SR） light source. Since the BEPCII will start commissioning with SR mode in November 2006, it is essential to have a satisfying SR lattice. Here we first perform frequency map analysis on the original BEPCII SR lattice. The result shows that the resonance is really severe in the lattice and the transverse motion of particles is unstable. Then a new SR lattice is designed to obtain a large dynamic aperture and much more stable transverse motion. The dynamic aperture of the new lattice including nonlinear wiggler effect is calculated to be very good. This new SR lattice can be used in the commissioning stage of the BEPCII.     

Effects of nearest and next-nearest neighbour interaction parameters on atomic correlation functions of stepped surfaces

We extend the range of the interaction between the surface scatterers to the next-nearest neighbours and thus treat the stepped surface as a second-order Markov chain. We employ two interaction parameters to describe the behaviour of the atomic pair correlation function for a two-level system and derive an exact expression for this function. This expression is then used to calculate the profile of the diffracted intensity. The effects of the interaction parameters on the intensity profiles as well as on the terrace width distributions are investigated in detail and compared to the molecular beam epitaxy measurement of Si on a Si(111) surface. The physical meaning of the interaction parameters is also interpreted in terms of the Ising model.     

Effects of ZnS:Mn/AlN multilayer structure on luminescent properties of nanostructured thin-film EL device

Effects of ZnS:Mn/AlN multilayer structure on luminescent properties of nanostructured (NS) thin-film electroluminescent (TFEL) device of which emission layer is a multilayer composed with ZnS:Mn layers and 0.7-nm-thick AlN interlayers were studied. The bandgap widening and the increased PL efficiency of Mn²⁺ 3d-3d transitions with a decrease in the ZnS:Mn single-layer thickness down to 5 nm were observed, which is ascribed to quantum confinement effects. Meanwhile, the multilayer with 2-nm-thick ZnS:Mn single-layers shows a drop of PL efficiency, indicating the presence of defective region just on AlN. The tendency of the luminous efficiency of the NS-TFEL device against the ZnS:Mn single-layer thickness is similar to the tendency found in the PL efficiency, indicating the importance of the ZnS:Mn/AlN interface for the device performance.     

On the enhancement of the figure of merit in bulk nanocomposites

In this paper we make a detailed comparison of the thermoelectric properties of quantum dot superlattices with those of equivalently doped bulk material and show that a major contribution to the enhancement of the figure of merit comes from the increase of the thermoelectric power over that of bulk, in addition to the lattice thermal conductivity reduction in quantum dot superlattices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dopant distribution, oxygen stoichiometry and magnetism of nanoscale Sn0.99Co0.01O2

In recent work, we have shown that chemically synthesized Sn_1−xCo_xO₂ nanoscale powders with x≤0.01 are ferromagnetic at room temperature when prepared by annealing the reaction precipitate in the narrow temperature window of 350-600 ^°C. Combined high resolution x-ray photoelectron spectroscopy (on as-prepared and Ar⁺ ion sputtered samples), x-ray diffraction and magnetometry measurements showed that the Co distribution is more uniform throughout the individual Sn_0.99Co_0.01O₂ particles when prepared at lower annealing temperatures of 350-600 ^°C and this uniform dopant distribution is essential to produce stable high temperature ferromagnetism. However, surface segregation of the dopant atoms in samples annealed at >600 ^°C destroys the room-temperature ferromagnetic behavior and reduces the Curie temperature to <300 K.     

STM investigations of PTCDA and PTCDI on graphite and MoS2. A systematic study of epitaxy and STM image contrast

Monolayers of the organic molecules perylene-3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and diimide (PTCDI) on graphite and MoS₂ have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the intermolecular interaction but nearly independent of the substrate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aromatic perylene system whereas the polar oxygen and nitrogen groups are invisible. The correlation of the observed inner structure of the molecules to their molecular structure allows us to compare our results with theoretical considerations.