High velocity vortex channeling in vicinal YBCO thin films

We report on electrical transport measurements at high current densities on optimally doped YBa 2Cu 3O 7-δ thin films grown on vicinal SrTiO 3 substrates. Data were collected by using a pulsed-current technique in a four-probe arrangement, allowing to extend the current-voltage characteristics to high supercritical current densities (up to 24 MA cm -2) and high electric fields (more than 20 V/cm), in the superconducting state at temperatures between 30 and 80 K. The electric measurements were performed on tracks perpendicular to the vicinal step direction, such that the current crossed between ab planes, under magnetic field rotated in the plane defined by the crystallographic c axis and the current density. At magnetic field orientation parallel to the cuprate layers, evidence for the sliding motion along the ab planes (vortex channeling) was found. The signature of vortex channeling appeared to get enhanced with increasing electric field, due to the peculiar depinning features in the kinked vortex range. They give rise to a current-voltage characteristics steeper than in the more off-plane rectilinear vortex orientations, in the electric field range below approximately 1 V/cm. Roughly above this value, the high vortex channeling velocities (up to 8.6 km/s) could be ascribed to the flux flow, although the signature of ohmic transport appeared to be altered by unavoidable macroscopic self-heating and hot-electron-like effects. © 2012 Elsevier B.V. All rights reserved.

Synthesis and luminescence properties of erbium silicate thin films

We have studied the structure and the room temperature luminescence of erbium silicate thin films deposited by rf magnetron sputtering. Films deposited on silicon oxide layers are characterized by good structural properties and excellent stability. The optical properties of these films are strongly improved by rapid thermal annealing processes performed in the range of temperature 800-1250 °C. In fact through the reduction of the defect density of the material, a very efficient room temperature photoluminescence at 1535 nm is obtained. We have also investigated the influence of the annealing ambient, by finding that treatments in O2 atmosphere are significantly more efficient in improving the optical properties of the material with respect to processes in N2. Upconversion effects become effective only when erbium silicate is excited with high pump powers. The evidence that all Er atoms (about 1022 cm-3) in erbium silicate films are optically active suggests interesting perspectives for optoelectronic applications of this material. © 2007 Elsevier B.V. All rights reserved.

Co-catalytic absorption layers for controlled laser-induced chemical vapor deposition of carbon nanotubes.

The concept of co-catalytic layer structures for controlled laser-induced chemical vapor deposition of carbon nanotubes is established, in which a thin Ta support layer chemically aids the initial Fe catalyst reduction. This enables a significant reduction in laser power, preventing detrimental positive optical feedback and allowing improved growth control. Systematic study of experimental parameters combined with simple thermostatic modeling establishes general guidelines for the effective design of such catalyst/absorption layer combinations. Local growth of vertically aligned carbon nanotube forests directly on flexible polyimide substrates is demonstrated, opening up new routes for nanodevice design and fabrication.

Effect of process parameters on the morphology and nanostructure of roll-to-roll printed P3HT:PCBM thin films for organic photovoltaics

Roll-to-roll (R2R) gravure exhibits significant advantages such as high precision and throughput for the printing of photoactive and conductive materials and the fabrication of flexible organic electronics such as organic photovoltaics (OPVs). Since the photoactive layer is the core of the OPV, it is important to investigate and finally control the process parameters and mechanisms that define the film morphology in a R2R process. The scope of this work is to study the effect of the R2R gravure printing and drying process on the nanomorphology and nanostructure of the photoactive P3HT:PCBM thin films printed on PEDOT:PSS electrodes towards the fabrication of indium tin oxide (ITO)-free flexible OPVs. In order to achieve this, P3HT:PCBM blends of different concentration were R2R printed under various speeds on the PEDOT:PSS layers. Due to the limited drying time during the rolling, an amount of solvent remains in the P3HT:PCBM films and the slow-drying process takes place which leads to the vertical and lateral phase separation, according to the Spectroscopic Ellipsometry and Atomic Force Microscopy analysis. The enhanced slow-drying leads to stronger phase separation, larger P3HT crystallites according to the Grazing Incidence X-Ray Diffraction data and to weaker mechanical response as it was shown by the nanoindentation creep. However, in the surface of the films the P3HT crystallization is controlled by the impinged hot air during the drying, where the more the drying time the larger the surface P3HT crystallites. The integration of the printed P3HT:PCBM and PEDOT:PSS layers in an OPV device underlined the feasibility of fabricating ITO-free flexible OPVs by R2R gravure processes. © 2013 Elsevier B.V.

In situ intercalation dynamics in inorganic-organic layered perovskite thin films.

The properties of layered inorganic semiconductors can be manipulated by the insertion of foreign molecular species via a process known as intercalation. In the present study, we investigate the phenomenon of organic moiety (R-NH3I) intercalation in layered metal-halide (PbI2)-based inorganic semiconductors, leading to the formation of inorganic-organic (IO) perovskites [(R-NH3)2PbI4]. During this intercalation strong resonant exciton optical transitions are created, enabling study of the dynamics of this process. Simultaneous in situ photoluminescence (PL) and transmission measurements are used to track the structural and exciton evolution. On the basis of the experimental observations, a model is proposed which explains the process of IO perovskite formation during intercalation of the organic moiety through the inorganic semiconductor layers. The interplay between precursor film thickness and organic solution concentration/solvent highlights the role of van der Waals interactions between the layers, as well as the need for maintaining stoichiometry during intercalation. Nucleation and growth occurring during intercalation matches a Johnson-Mehl-Avrami-Kolmogorov model, with results fitting both ideal and nonideal cases.

Performance of hybrid buffer Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) layers doped with plasmonic silver nanoparticles

We compare the performance of a typical hole transport layer for organic photovoltaics (OPVs), Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) thin film with a series of PEDOT:PSS layers doped with silver (Ag) nanoparticles (NPs) of various size distributions. These hybrid layers have attracted great attention as buffer layers in plasmonic OPVs, although there is no report up to date on their isolated performance. In the present study we prepared a series of PEDOT:PSS layers sandwiched between indium tin oxide (ITO) and gold (Au) electrodes. Ag NPs were deposited on top of the ITO by electron beam evaporation followed by spin coating of PEDOT:PSS. Electrical characterization performed in the dark showed linear resistive behavior for all the samples; lower resistance was observed for the hybrid ones. It was found that the resistivity of the samples decreases with increasing the particle's size. A substantial increase of the electric field between the ITO and the Au electrodes was seen through the formation of current paths through the Ag NPs. A striking observation is the slight increase in the slope of the current density versus voltage curves when measured under illumination for the case of the plasmonic layers, indicating that changes in the electric field in the vicinity of the NP due to plasmonic excitation is a non-vanishing factor. © 2014 Published by Elsevier B.V. All rights reserved.

Fracture Properties of LPCVD Silicon Nitride and Thermally Grown Silicon Oxide Thin Films From the Load-Deflection of Long Si3N4 and SiO2/Si3N4 Diaphragms

The bulge test is successfully extended to the determination of the fracture properties of silicon nitride and oxide thin films. This is achieved by using long diaphragms made of silicon nitride single layers and oxide/nitride bilayers, and applying comprehensive mechanical model that describes the mechanical response of the diaphragms under uniform differential pressure. The model is valid for thin films with arbitrary z-dependent plane-strain modulus and prestress, where z denotes the coordinate perpendicular to the diaphragm. It takes into account the bending rigidity and stretching stiffness of the layered materials and the compliance of the supporting edges. This enables the accurate computation of the load-deflection response and stress distribution throughout the composite diaphragm as a function of the load, in particular at the critical pressure leading to the fracture of the diaphragms. The method is applied to diaphragms made of single layers of 300-nm-thick silicon nitride deposited by low-pressure chemical vapor deposition and composite diaphragms of silicon nitride grown on top of thermal silicon oxide films produced by wet thermal oxidation at 950 degrees C and 1050 degrees C with target thicknesses of 500, 750, and 1000 mn. All films characterized have an amorphous structure. Plane-strain moduli E-ps and prestress levels sigma(0) of 304.8 +/- 12.2 GPa and 1132.3 +/- 34.4 MPa, respectively, are extracted for Si3N4, whereas E-ps = 49.1 +/- 7.4 GPa and sigma(0) = -258.6 +/- 23.1 MPa are obtained for SiO2 films. The fracture data are analyzed using the standardized form of the Weibull distribution. The Si3N4 films present relatively high values of maximum stress at fracture and Weibull moduli, i.e., sigma(max) = 7.89 +/- 0.23 GPa and m = 50.0 +/- 3.6, respectively, when compared to the thermal oxides (sigma(max) = 0.89 +/- 0.07 GPa and m = 12.1 +/- 0.5 for 507-nm-thick 950 degrees C layers). A marginal decrease of sigma(max) with thickness is observed for SiO2, with no significant differences between the films grown at 950 degrees C and 1050 degrees C. Weibull moduli of oxide thin films are found to lie between 4.5 +/- 1.2 and 19.8 +/- 4.2, depending on the oxidation temperature and film thickness.

Raman scattering characterization of Mn composition and strain in Ga1-xMnxSb/GaSb epitaxial layers

Raman scattering (RS) experiments have been performed for simultaneous determination of Mn composition and strain in Ga1-xMnxSb thin films grown on GaSb substrate by liquid phase epitaxy technique. The Raman spectra obtained from various Ga1-xMnxSb samples show only GaSb-like phonon modes whose frequency positions are found to have Mn compositional dependence. With the combination of epilayer strain model, RS and energy dispersive x-ray (EDX) experiments, the compositional dependence of GaSb-like LO phonon frequency is proposed both in strained and unstrained conditions. The proposed relationships are used to evaluate Mn composition and strain from the Ga1-xMnxSb samples. The results obtained from the RS data are found to be in good agreement with those determined independently by the EDX analysis. Furthermore, the frequency positions of MnSb-like phonon modes are suggested by reduced-mass model. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Study of GaN growth on ultra-thin Si membranes

10 mu m-thick ultra-thin Si (111) membranes for GaN epi-layers growth were successfully fabricated on silicon-on-insulator (SOI) substrate by backside etching the handle Si and buried oxide (BOX) layer. Then 1 mu m-thick GaN layers were deposited on these Si membranes by metal-organic chemical vapor deposition (MOCVD). The crack-free areas of 250 mu m, x 250 mu m were obtained on the GaN layers due to the reduction of thermal stress by using these ultra-thin Si membranes, which was further confirmed by the photoluminescence (PL) spectra and the simulation results from the finite element method calculation by using the software of ANSYS. In this paper, a newly developed approach was demonstrated to utilize micromechanical structures for GaN growth, which would improve the material quality of the epi-layers and facilitate GaN-based micro electro-mechanical system (MEMS) fabrication, especially the pressure sensor, in the future applications. (C) 2008 Elsevier Ltd. All rights reserved.

Effect of silver growth temperature on the contacts between Ag and ZnO thin films

Highly c-axis oriented ZnO thin films were deposited on Si substrates by the pulsed laser deposition (PLD) method. At different growth temperatures, 200 nm silver films as the contact metal were deposited on the ZnO thin films. The growth temperatures have great influence on the crystal quality of Ag films. Current-voltage characteristics were measured at room temperature. The Schottky contacts between Ag and ZnO thin films were successfully obtained when silver electrodes were deposited at 150A degrees C and 200A degrees C. Ohmic contacts were formed while the growth temperatures were lower than 150A degrees C or higher than 200A degrees C. After analysis, the forming of Ag/ZnO Schottky contacts was shown to be dependent on the appearance of the p-type inversion layer at the interface between Ag and ZnO layers.

Determination of the tilt and twist angles of curved GaN layers by high-resolution x-ray diffraction

The full-width at half-maximum (FWHM) of an x-ray rocking curve (XRC) has been used as a parameter to determine the tilt and twist angles of GaN layers. Nevertheless, when the thickness of GaN epilayer reaches several microns, the peak broadening due to curvature becomes non-negligible. In this paper, using the (0 0 l), l = 2, 4, 6, XRC to minimize the effects of wafer curvature was studied systematically. Also the method to determine the tilt angle of a curved GaN layer was proposed while the Williamson-Hall plot was unsuitable. It was found that the (0 0 6) XRC-FWHM had a significant advantage for high-quality GaN layers with the radius curvature of r less than 3.5 m. Furthermore, an extrapolating method of gaining a reliable tilt angle has also been proposed, with which the calculated error can be improved by 10% for r < 2 m crystals compared with the (0 0 6) XRC-FWHM. In skew geometry, we have demonstrated that the twist angles deriving from the (2 0 4) XRC-FWHM are in accord with those from the grazing incidence in-plane diffraction (IP-GID) method for significantly curved samples.

Silicon-on-insulating multi-layers for total-dose irradiation hardness

Silicon-on-insulating multi-layer (SOIM) materials were fabricated by co-implantation of oxygen and nitrogen ions with different energies and doses. The multilayer microstructure was investigated by cross-sectional transmission electron microscopy. P-channel metal-oxide-semiconductor (PMOS) transistors and metal-semiconductor-insulator-semiconductor (MSIS) capacitors were produced by these materials. After the irradiated total dose reaches 3 x 10(5) rad (Si), the threshold voltage of the SOIM-based PMOS transistor only shifts 0.07 V, while thin silicon-on-insulating buried-oxide SIMOX-based PMOS transistors have a shift of 1.2V, where SIMOX represents the separated by implanted oxygen. The difference of capacitance of the SOIM-based MSIS capacitors before and after irradiation is less than that of the thin-box SIMOX-based MSIS capacitor. The results suggest that the SOIM materials have a more remarkable irradiation tolerance of total dose effect, compared to the thin-buried-oxide SIMOX materials.

Growth of crack-free AlGaN film on thin AlN interlayer by MOCVD

We have studied the effect of low-temperature-deposited (LT) and high-temperature-deposited (FIT) AlN interlayer with various thickness on AlGaN film grown on GaN using c-plane sapphire as substrate. All the Al0.25Ga0.75N films thicker than 1 mum with LT-AlN interlayer or with HT-AlN interlayer were free of cracks, however, their surfaces were different: the Al0.25Ga0.75N films with LT-AlN interlayer showed smooth surface, while those with HT-AlN interlayer exhibit rough surface morphology. The results of X-ray double crystal diffraction and Rutherford backscattering showed that all of the AlGaN films were under compressive strain in the parallel direction. The compressive strain resulted from the effect of interlayer-induced stress relieving and the thermal mismatch for the samples with LT-AlN interlayer, and it was due to the thermal mismatch between AlGaN and the underlying layers for those with HT-AlN interlayer. (C) 2004 Elsevier B.V. All rights reserved.

Ordering growth of InAs quantum dots on ultra-thin InGaAs strained layer

InAs quantum dots (QDs) were grown On Ultra-thin In0.15Ga0.85As strained layers by molecular beam epitaxy on GaAs (00 1) substrates. Combining reflection high-energy electron diffraction, atomic force microscopy and transmission electron microscopy, we analyzed the stress field of dislocations in the strained layer/substrate interface. Specially, we revealed the relative position of QDs and dislocations. We found that the difference of the stress field around dislocations is prominent when the strained layer is ultra-thin and the stress field will directly affect the following growth. On the strained layer surface, In0.15Ga0.85As ridges will form at the inclined upside of dislocations. Then, InAs QDs will prefer nucleating on the ridges, there is relatively small stress between InAs and In0.15Ga0.85As. By selecting ultra-thin In0.15Ga0.85As layer (50 nm) and controlling the QD layer at just form QDs, we obtained ordered InAs QDs. (C) 2004 Elsevier B.V. All rights reserved.

Strain-induced anodization of SiGe/Si multiple layers to form high density SiGe/Si heterogeneous nanorods

Nation Natural Science Foundation of China 50672079 60676027 60837001 60776007; National Basic Research Program of China (973 Program) 2007CB613404; China-MOST International Sci & Tech Cooperation and Exchange 2008DFA51230