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We investigate the magnetic and ferroelectric behaviors of 5d Ir4+ substituted Mn1−xIrxWO4 + δ samples. The electric polarization is enhanced to a certain extent due to the Ir substitution of Mn. Furthermore, the non-polar AF1 phase is partially suppressed by the ferroelectric AF2 phase, resulting in the coexistence of the two phases below TAF1. It is suggested that the lattice distortion, the suppressed magnetic moment, and the increased Dzyaloshinskii–Moriya interaction are the key reasons for the enhanced polarization and the stabilized ferroelectric AF2 phase.

Hubnerite MnWO4 is a highly frustrated magnetic compound that has been known for its multiferroic properties. The intrinsic connection of ferroelectric polarization and magnetically frustrated structure allows an opportunity to probe the stability of magnetic structures against perturbations by means of measuring ferroelectric polarization. In this work, we investigate the ferroelectric polarization of Mn1 − 2xIrxWO4 to probe the stability of the low-temperature (T) collinear antiferromagnetic (AF1) phase against the Ir substitution, considering the strong spin-orbital coupling of Ir4+ that would enhance the single-ion anisotropy, on the one hand, and would favor the noncollinear spin alignment, on the other hand. Different from Mn1 − xRux/2WO4, it is suggested that the AF1 phase is only partially suppressed by the Ir substitution, allowing the emergence of the noncollinear antiferromagnetic (AF2) phase in coexistence with the collinear AF1 phase. Proper Ir substitution may promote both the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction, thus making the modulation of the magnetic structure more complicated.

Thin films of copper, with thickness between 0.1 and 3 μm, were vapor-deposited on 12.7 or 7.6-μm-thick polyimide (Kapton) substrates. They were tested in a microtensile tester in which the strain is measured by optical diffraction from a microlithographically applied grid. The Young modulus is independent of film thickness and is about 20% below the value calculated from single-crystal elastic constants. The yield stress depends strongly on the film thickness and is fit by σy=116+355(t)−0.473, where t is the thickness in μm and σy is in MPa. The microstructure of the films was studied by focused ion-beam microscopy. The grains are heavily twinned and the microstructural lengths (grain size, twin spacing, twin width) depend only weakly on the film thickness. A substantial part of the yield stress is therefore attributable to an effect of the film thickness, such as that predicted by strain gradient plasticity theory. The lower limit and some estimates of the thickness contribution to the yield stress are ...

Adhesive contact between an axisymmetric punch and a piezoelectric half space in the presence of a liquid bridge formed by capillary condensation is studied. The coupling between liquid-bridge force and piezoelectric elastic deformation is taken into account. The solution is obtained by using the integral transform method with the aid of the dual integral equation technique. Combined with the thermodynamics equilibrium of capillary condensation, two competitive factors closely related to the relative humidity are shown to affect the capillary adhesive force. Two dimensionless parameters, which represent the material properties, and geometric relative relationships between liquid-bridge and contact systems, respectively, are defined to investigate the role of the capillary bridge on adhesive contact. Due to the electromechanical coupling, the applied electric load is shown to evidently affect adhesive contact. This may provide a possible means by which to weaken the influence of the capillary bridge in ato...

Effects of growth interruption on the optical and the structural properties of InGaN/GaN quantum wells were investigated by using photoluminescence, transmission electron microscopy, optical microscopy, and high resolution x-ray diffraction. The InxGa1−xN/GaN (x>0.2) quantum wells used in this study were grown on c-plane sapphire by using metalorganic chemical vapor deposition. The interruption was carried out by closing the group-III metalorganic sources before and after the growths of the InGaN quantum well layers. The transmission electron microscopy images show that with increasing interruption time, the quantum-dot-like regions and well thickness decreased due to indium reevaporation or the thermal etching effect. As a result the photoluminescence peak position was blueshifted and the intensity was reduced. Temperature- and excitation-power-dependent photoluminescence spectra support the results of transmission electron microscopy measurements. The sizes and the number of V defects did not differ with the interruption time. The interruption time is not directly related to the formation of defects. The V defect originates at threading dislocations and inversion domain boundaries due to higher misfit strain.

Photoreflectance (PR) spectroscopy, x-ray diffraction (XRD) method, and secondary ion mass spectroscopy (SIMS) were used to observe the radial distribution of the band-edge transition and modification of the band structure due to the variation of indium in 3 in. indium-alloyed semi-insulating GaAs (InxGa1−xAs) grown by the liquid-encapsulated Czochralski method. The data from room temperature PR measurements showed the variation of the transition energy with positions indicating the radial distribution of the indium content across the wafers; indium content being higher around the edge region than the central area. The splitting of the degenerate valence band around the edge region of the wafers was also shown in PR data due to the different indium content in adjacent regions where indium content varies rapidly. The XRD measurements showed the drastic change in the distribution of lattice constant on where the splitting of the heavy and light holes happened and the SIMS analysis was adopted to confirm the...

Infrared oxygen‐related photoluminescence measurements of GaAs epilayers grown by organometallic vapor phase epitaxy (OMVPE) and intentionally doped with dimethylaluminum methoxide (DMALMO) are reported. The photoluminescence emissions at 1.08, 0.95, 0.81, and 0.62 eV are detected. The centers responsible for the emissions at 0.81 and 0.62 eV are tentatively attributed to Al–O–Ga and Al–O–Al, respectively, based on correlation with layer growth conditions. The growth parameters lead to information on the number of nearest‐neighbor aluminum associated with oxygen. The Al–O–Ga center responsible for the 0.81 emission is also studied for AlxGa1−xAs(x=0.05–0.9) layers grown by OMVPE without DMALMO. The 0.81 eV emission is attributed to the transition from the conduction band to the Al–O–Ga center. The center shows the lattice relaxation exemplified by the Franck–Condon shift of 0.15 eV. The thermal ionization energy of this acceptor is determined to be 0.56 eV above the valence band.

The effective response of a class of weakly nonlinear conducting composite media is investigated. We combine the previously developed perturbation expansion method for solving electrostatic boundary‐value problems with the well‐known Rayleigh method for treating periodic composite media to compute the effective conductivities. We derive general expressions for the effective nonlinear response functions and show that the results agree with the generalized Landau’s formula at low inclusion concentrations. Numerical calculations are also obtained for a square array of cylindrical inclusions suspended in a host medium in two dimensions. Possible applications and extensions of the method to more complicated problems are discussed.

We report the temperature‐dependent characteristics of photoluminescence emission in modulation δ‐doped AlxGa1−xAs/InyGa1−yAs/GaAs structures. Transition energies are analyzed using a self‐consistent solution of the coupled k⋅P Hamiltonian–Poisson equation. At low temperatures, dominant emissions are due to the transitions from the first electron subband to the first heavy‐hole subband and from the second electron subband to the first heavy‐hole subband irrespective of the location of modulation doping. The second hole subband related transitions associated with the first electron subband or the second electron subband emerges with increasing temperature depending on the location of doping. The relative intensities of the transitions from the first and second electron subbands to the first heavy‐hole subband transitions are analyzed as a function of the Fermi energy position. An excellent agreement is found between the measurements and calculations.

We report the excitation intensity dependent photoluminescence properties of GaAs1−xSbx layers grown by molecular beam epitaxy on InP substrates. Photoluminescence consists of the bound exciton and the quasi‐donor‐acceptor pair transitions for the layers in the range of 0.26≤x≤0.94. The concentration modulation produced by the relaxation of the misfit strain between the epitaxial GaAs1−xSbx layer and InP substrate is responsible for the quasi‐donor‐acceptor pair transition. A large Stokes shift between the photoluminescence transition of the bound exciton and the band gap determined by the optical absorption measurements is also consistent with our model of concentration modulation.

A detailed surface chemistry mechanism is proposed for chemical vapor deposition of diamond films, which extends the growth‐by‐methyl mechanism proposed by Harris to treat any CHm radical, m=0–3, as a growth monomer. Numerical computations were performed in which the mechanism was coupled to a model for the boundary layer above the substrate, for conditions typical of diamond deposition in an atmospheric‐pressure thermal plasma. The predicted linear growth rate increases strongly as the boundary layer thickness δ is decreased, and the results indicate a strong dependence of the diamond growth chemistry on δ. For relatively thick boundary layers (modest velocities of the reactant jet) growth is dominated by CH3. For very thin boundary layers (high velocities) the model predicts that growth is dominated by C. For the transition region where C and CH3 each contribute about 40% to growth, CH2 also contributes about 17%. The carbon conversion efficiency is also predicted to peak in the transition region, and drops sharply for very thin boundary layers.

Photoluminescence properties of Mg‐ion implanted layers of low‐temperature (179–400 °C) grown molecular beam epitaxial GaAs are reported. The Mg incorporation into the Ga site mainly produces the Mg‐related donor‐acceptor pair transition. The Mg incorporation depends strongly on the growth temperature of the layers. Mg trapping at defect centers formed by the excess As increases exponentially with the increase of the reciprocal growth temperature and is the dominant mechanism for the failure of Mg to incorporate into the Ga site. The possibility of the trapping center for Mg being the Asin‐related defect originating from As‐rich growth of low temperature GaAs is discussed.

The free electron distribution in δ‐doped high electron mobility transistors with pseudomorphic InGaAs wells is calculated self‐consistently. The electron distribution is calculated from the single‐particle wave functions obtained from a four‐band k⋅p theory. The Hartree part of the Coulomb interaction is obtained from the Poisson equation and the exchange‐correlation part from density functional theory within the local‐density approximation. The calculated energy separations between the electron and hole subbands agree well with observed peak positions in photoluminescence data. In addition, it is found that for spacer layers thicker than about 40 A and a δ‐doping density of about 5×1012 cm−2, the δ layer can form a channel as deep as the p well and can draw electrons from the latter.

We investigate the influence of interaction between dispersed particles to ac conductivity of suspension of charged particles immersed in electrolyte solution. Using the Rayleigh technique, we establish an identity for charged suspensions, and derive a formula for effective complex conductivity of the systems. A useful scheme for computation of effective constants of composite media is proposed.

A photoluminescence study has been made, at temperatures 2–300 K, on indium‐alloyed semi‐insulating GaAs subjected to bulk heat treatments. Heat treatments were made by slow and fast cooling following a 15 h, 950 °C soak. Two intrinsic acceptors at Ev + 0.069 eV and Ev + 0.17 eV are produced after heat treatments. The acceptor at Ev + 0.069 eV is attributed to the intrinsic pair defect GaAs−VGa and is produced more in the fast‐cooled materials than in the slow‐cooled materials. The deficiency of intermediate deep donors causes no drastic variation of the electron concentration after the heat treatments.

In modeling inductively coupled plasmas, the boundary condition for the electromagnetic field equations can be treated by specifying either the current in the induction coil or the total power dissipated in the plasma. This paper presents a method for using the coil current boundary condition. An advantage of using the coil current boundary condition is that coil current, unlike plasma power dissipation, is easily measured; in this approach the plasma power dissipation is an outcome of the calculation. The results of sample calculations are presented, covering a range of coil currents from 59 to 110 A. The conditions of the calculations correspond to experimental argon plasmas at atmospheric pressure and at 3.0 MHz frequency. The calculated isotherms are in good qualitative agreement with photographs of the laboratory plasmas.

Room‐temperature photoreflectance measurements were used to determine the radial and axial distribution of low levels of indium in 3‐in.‐diameter semi‐insulating bulk GaAs materials grown by the liquid‐encapsulated Czochralski method. These results were compared with 4.2‐K photoluminescence data and found to be accurate and more convenient for this application. Room‐temperature photoreflectance allows an accurate determination of the indium content in the range of mole fraction 0.1%–2.0% with standard deviation of 0.03%. Two types of radial inhomogeneity were found in commercially available GaAs wafers. This is discussed in terms of indium segregation and shape of solid and liquid interface during the crystal growth.

In this paper, a combined experimental, computational, and analytical approach is used to provide new insights into the lamination of organic solar cells and light emitting devices at macro- and micro-scales. First, the effects of applied lamination force (on contact between the laminated layers) are studied. The crack driving forces associated with the interfacial cracks (at the bi-material interfaces) are estimated along with the critical interfacial crack driving forces associated with the separation of thin films, after layer transfer. The conditions for successful lamination are predicted using a combination of experiments and computational models. Guidelines are developed for the lamination of low-cost organic electronic structures.

This paper presents the results of experimental and theoretical/computational micro-wrinkles and buckling on the surfaces of stretchable poly-dimethylsiloxane (PDMS) coated with nano-scale Gold (Au) layers. The wrinkles and buckles are formed by the unloading of pre-stretched PDMS/Au structure after the evaporation of nano-scale Au layers. They are then characterized using atomic force microscopy and scanning electron microscopy. The critical stresses required for wrinkling and buckling are analyzed using analytical models. The possible interfacial cracking that can occur along with film buckling is also studied using finite element simulations of the interfacial crack growth. The implications of the results are discussed for potential applications of micro-wrinkles and micro-buckles in stretchable electronic structures and biomedical devices.

A selective area chemical vapour deposition technique has been used to fabricate continuous and patterned epitaxial CrO2 thin films on (100)-oriented TiO2 substrates. Precessional magnetization dynamics were stimulated both electrically and optically, and probed by means of time-resolved Kerr microscopy and vector network analyser ferromagnetic resonance techniques. The dependence of the precession frequency and the effective damping parameter upon the static applied magnetic field were investigated. All films exhibited a large in-plane uniaxial anisotropy. The effective damping parameter was found to exhibit strong field dependence in the vicinity of the hard axis saturation field. However, continuous and patterned films were found to possess generally similar dynamic properties, confirming the suitability of the deposition technique for fabrication of future spintronic devices.

This paper presents the results of an experimental study of the adhesion between bi-material pairs that are relevant to organic light emitting devices, hybrid organic/inorganic light emitting devices, organic bulk heterojunction solar cells, and hybrid organic/inorganic solar cells on flexible substrates. Adhesion between the possible bi-material pairs is measured using force microscopy (AFM) techniques. These include: interfaces that are relevant to organic light emitting devices, hybrid organic/inorganic light emitting devices, bulk heterojunction solar cells, and hybrid combinations of titanium dioxide (TiO2) and poly(3-hexylthiophene). The results of AFM measurements are incorporated into the Derjaguin-Muller-Toporov model for the determination of adhesion energies. The implications of the results are then discussed for the design of robust organic and hybrid organic/inorganic electronic devices.

Time-resolved scanning Kerr microscopy has been used to directly observe magnetostatically coupled transverse domain walls (TDWs) in a pair of closely spaced, curved nanowires (NWs). Kerr images of the precessional response of the magnetic domain to either side of the TDW revealed the TDW as a minimum in the Kerr signal in the region of closest NW separation. When the TDWs were ejected from the NW pair, the minimum in the Kerr signal was no longer observed. By imaging this transition, the static de-coupling field was estimated to be in the range from 38 to 48 Oe in good agreement with a simple micromagnetic model. This work provides a novel technique by which DC and microwave assisted decoupling fields of TDWs may be explored in NW pairs of different width, separation, and curvature.

A partially built hard disk writer structure with a NiFe/CoFe/Ru/NiFe/CoFe synthetic antiferromagnetic (SAF) yoke was studied by time and vector resolved scanning Kerr microscopy. All three time dependent components of the magnetization were recorded simultaneously as a bipolar current pulse with 1 MHz repetition rate was delivered to the coil. The component of magnetization parallel to the symmetry axis of the yoke was compared at the pole and above a coil winding in the centre of the yoke. The two responses are in phase as the pulse rises, but the pole piece lags the yoke as the pulse falls. The Kerr signal is smaller within the yoke than within the confluence region during pulse cycling. This suggests funneling of flux into the confluence region. Dynamic images acquired at different time delays showed that the relaxation is faster in the centre of the yoke than in the confluence region, perhaps due to the different magnetic anisotropy in these regions. Although the SAF yoke is designed to support a single domain to aid flux conduction, no obvious flux beaming was observed, suggesting the presence of a more complicated domain structure. The SAF yoke writer hence provides relatively poor flux conduction but good control of rise time compared to single layer and multi-layered yokes studied previously.

In2O3 is a promising partner of Cu2O to form a Cu2O/In2O3 heterojunction system. We used x-ray photoelectron spectroscopy to determine the valence band offset (VBO) of a Cu2O/In2O3 heterojunction. The valence band offset is found to be 1.43 ± 0.2 eV. Given the experimental bandgaps of 2.0 eV and 2.6 eV for Cu2O and In2O3, respectively, we calculate the band alignment of a Cu2O/In2O3 heterojunction with a conduction band offset (CBO) of 0.83 ± 0.2 eV. To apply Cu2O/In2O3 bilayers in electronic devices, it is important to determine the band alignment accurately based on the VBO and CBO.

Thermal conductivity and mechanical effects of silicon carbide nanoparticles uniformly dispersed in water were investigated. Mean size of SiC particles was 170 nm with a polydispersity of 30% as determined from small-angle x-ray scattering and dynamic light scattering techniques. Room temperature viscosity of the nanofluids ranged from 2 to 3 cP for nominal nanoparticle loadings 4 – 7 vol %. On a normalized basis with water, viscosity of the nanofluids did not significantly change with the test temperature up to 85 °C. Optical microscopy of diluted nanofluid showed no agglomeration of the nanoparticles. Thermal conductivity of the fluid was measured as a function of the nominal nanoparticle loading ranging from 1 to 7 vol %. Enhancement in thermal conductivity was approximately 28% over that of water at 7 vol % particle loadings under ambient conditions. Enhancements in thermal conductivities for the nanofluids with varying nanoparticle loadings were maintained at test temperatures up to 70 °C. Results of thermal conductivity have been rationalized based on the existing theories of heat transfer in fluids. Implications of using this nanofluid for engineering cooling applications are discussed. Universidad de Chicago Argonne LLC (EE. UU.)-DE-AC02-06CH11357

We report a study on the coupling induced subband structures and the collective single electron behavior in a single layer Si quantum dot (Si-QD) array, which is fabricated by a layer-by-layer technique using hydrogen diluted silane gas in plasma enhanced chemical vapor deposition system. Unique peak structures are observed in both the I-V and the capacitance-voltage (C-V) characteristics. The total number of electrons charged into the Si-QD array is found to be the same as the number of coupled quantum dots under the electrode. This phenomenon originates from a collective charging behavior of electrons into the subband structures in the Si-QD array, which evolved from the discrete energy levels in the individual Si QDs due to the weak interdot coupling. The different coupling and retention properties for the s-state and p-state subbands as well as the mechanisms for the charging and redistribution of electrons among the subbands are analyzed and discussed.

The alternating current (ac) response has been investigated theoretically in colloidal suspensions consisting of suspended radially inhomogeneous graded particles having complex permittivity profiles under an external ac electric field. The gradation in the colloidal particles is modeled by physically motivated graded profiles as the dielectric function may only vary slightly along the radius, while the conductivity profile may vary rapidly along the radius. More precisely, the dielectric function is assumed to be a constant, while the conductivity has a power-law dependence on the radius variable r, namely, epsilon(i)(r)=A+cr(k)/(i omega). In previous attempts, this model was solved numerically via the differential effective dipole approximation. In this work, we will demonstrate the existence of exact analytical solutions of the local potentials in the graded particles in terms of the hypergeometric functions, and hence the effective ac response is calculated in the dilute limit. Our exact results will be applied to graded biological cell suspensions. Extensions to nonlinear ac response will be discussed.

The effective nondegenerate four-wave mixing (NDFM) susceptibility of graded metallic films with weak nonlinearity is exactly derived by invoking the local-field effects. It is found that the presence of gradation in metallic films can yield a broad resonant plasmon band in the optical region, which results in a large enhancement in the NDFM response and thus a large figure of merit.

Photoluminescence of gallium nitride (GaN) layers 7.5–870 μm thick was studied with changes in temperature and excitation intensity. These layers were grown by hydride vapor phase epitaxy on a buffer layer of aluminum nitride (AlN). The photoluminescence emission consists of the shallow-donor bound exciton at 3.471 eV and the free hole-to-electron bound to a donor (possibly a nitrogen vacancy VN or oxygen) transition at 3.40–3.433 eV. The peak position varies depending on the thickness of the GaN and AlN layers. The localized donor due to donor concentration fluctuation is attributed to the variable peak position. The observed 3.269 eV emission is attributed to a donor–acceptor pair transition. The relationship between the peak and the excitation intensity is described accurately by a theoretical description which yields Ed=32 meV and Ea=230 meV, which originate, respectively, from a silicon donor and carbon acceptor.

We have studied anomalous diffusion in the random superconducting network (RSN) with a wide distribution of conductivity. We consider a composite medium of superconducting and normal conducting regions in which the normal conducting component obeys a transfer‐rate distribution of the form W−(1+α) (0

Temperature‐dependent Hall‐effect and photoluminescence measurements have been performed on a series of antimony‐doped bulk GaAs samples that were otherwise undoped. A new donor level located 0.48 eV below the conduction‐band edge has been detected by both experiments in all antimony‐doped samples studied. This level reduces the resistivity of antimony‐doped material below the semi‐insulating limit. Comparison with known intrinsic levels in undoped material have been made and it is shown that the 0.48‐eV donor is distinct from any of these. It is concluded that the defect responsible for the 0.48‐eV donor involves an impurity antisite SbGa either isolated or in a complex with intrinsic defects.

The low‐temperature properties of the excitation‐dependent photoluminescence emission, in nominally undoped n‐type GaxIn1−x As (0.44≤x≤0.48) layers grown on InP by molecular‐beam epitaxy, are investigated with changes of temperature and excitation intensity. The excitation‐dependent emission is attributed to the quasi‐donor‐acceptor pair transition in impure compensated crystals. The random impurity potential arising from residual impurities causes a larger energy shift than expected for the usual donor‐acceptor pair transition.

The behavior of Fe in semi‐insulating InP substrates during growth by liquid‐phase epitaxy (LPE) was studied using secondary‐ion mass spectrometry and photoluminescence measurements in conjunction with ion implantation. The redistribution of Fe at the interface between the substrate and the layer follows diffusion theory. The experimentally determined diffusion coefficient D is 6.8×105 exp[‐3.4 (eV)/kT]. Fe accumulates near the surface of the substrate during preheating in the H2 ambient. This accumulation cannot be explained by simple diffusion theory and could be influenced by thermal degradation of the surface.

We have studied heavily doped GaAs:Sb and GaAs:In by temperature‐dependent Hall effect and photoluminescence spectroscopy. Both samples had good mobilities and the GaAs:In was semi‐insulating. We have found a new deep level in GaAs:Sb at 0.47 eV below the conduction band. Both samples showed new weak luminescence bands near the intrinsic edge which are as yet unidentified.