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We report the effect of Ga substitution on magnetization, magnetoresistance, and magnetostriction in polycrystalline Pr0.6Sr0.4Co1-yGayO3 (y=0.0-0.3) samples. Upon substitution of the non-magnetic Ga3+ cation for magnetic Co3+, the low-temperature ground state transforms from ferromagnetic metallic for y = 0 to cluster glass semiconductor for y = 0.2. Magnetoresistance at 7T is negative and its magnitude increases from 2 percentage for y = 0 to 30 percentage for y = 0.3 at 10 K. On the other hand, magnetostriction at 10 K is positive and its value decreases with increasing y. Interestingly, the field-dependent magnetization, magnetoresistance, and magnetostriction for y greater than or equal to 0.2 and at T less than or equal to 3 K show reversible abrupt steps for both positive and negative magnetic field whereas all these quantities vary smoothly with the magnetic field above 4 K. Such steps in all three distinct physical quantities were never reported earlier in perovskite cobaltites and they differ from observations made in manganites and intermetallic alloys. It is suggested that field-induced avalanche flipping of ferromagnetic clusters could be the origin of observed steps in all these three quantities., 11 pages, 5 figures

We demonstrate a method to detect the spin resonance of Mn electrons at room temperature in paramagnetic samples of La1−xCaxMnO3 with x = 0.35, 0.40, and 0.45 using a simple magnetoimpedance (MI) measurement. The magnetic field dependent “ac” resistance and reactance were simultaneously measured at 300 K using a single-port impedance analyzer by passing a current of frequency f = 500 MHz–3 GHz directly through the sample. The magnetoresistance of each sample increased abruptly at a critical value of the applied magnetic field while the magnetoreactance displayed a dip. Both these features shifted linearly toward higher “dc” magnetic fields with increasing frequency of the current. We suggest that these anomalies in MI are imprints of spin resonance from the Mn electrons. Our results were also verified using a conventional broadband spectrometer (f = 2–10 GHz). Thus, the electrical detection of paramagnetic resonance using a low-cost impedance analyzer, as demonstrated here, can provide an alternate method to investigate magnetic resonances in various materials.

We report the electrical detection of multiple ferromagnetic (FM) resonance modes in multilayers of Pt/Fe20Ni80/Ru/Fe20Ni80 using microwave spin pumping and the inverse spin Hall effect (ISHE) as a function of Ru layer thickness tRu. The electrically detected voltage signals were analyzed for the anti-parallel magnetization state as well as the “spin-flop” magnetization state of the two Fe20Ni80 layers. When the two FM layers are antiferromagnetically (AFM) coupled (tRu = 1 nm), both the optic and acoustic modes were observed. The amplitude of the acoustic mode voltage signals obtained increases with frequency. However, when the two magnetic layers are ferromagnetically coupled (tRu = 1.4 nm), only acoustic mode was observed whose voltage signals decreases as frequency increases. We demonstrate the tunability of the sign of the ISHE voltage in the AFM coupled system by controlling the polarization of the spin currents.

The chemical and electronic structures of industrial chalcopyrite photovoltaic absorbers after KF post-deposition treatment (KF-PDT) are investigated using electron spectroscopies to probe the occupied and unoccupied electronic states. In contrast to a variety of recent publications on the impact of KF-PDT, this study focuses on industrial Cu(In,Ga)(S,Se)2 absorbers that also contain sulfur at the surface. We find that the KF-PDT removes surface adsorbates and oxides and also observe a change in the S/Se ratio. Furthermore, the KF-PDT leads to a Cu reduction at the surface but to a much lower degree than the strongly Cu-depleted or even Cu-free surfaces reported for (non-industrial) sulfur-free Cu(In,Ga)Se2 absorbers. The valence band maximum at the surface is found at a lower energy compared to the untreated absorber, and the conduction band minimum is found at a higher energy, overall revealing a widening of the bandgap in the surface region.

In a previous paper [C.-S. Jiang et al., Appl. Phys. Lett. 84, 3477 (2004)], we reported the existence of a local built-in potential on grain boundaries (GBs) of photovoltaic Cu(In,Ga)Se2 (CIGS) thin films. However, whether the built-in potential benefits photovoltaic properties of the device has not been proven. Using a scanning Kelvin probe microscope, we found that, with increasing Ga content in the CIGS film, the built-in potential on the GB drops sharply in a Ga range of 28%–38%. Comparing the changes in the built-in potential, the device efficiency, and the CIGS band gap, we conclude that the built-in potential on the GB plays a significant role in the device conversion efficiency of NREL’s three-stage CIGS device.

We report on a direct measurement of two-dimensional potential distribution on the surface of photovoltaic Cu(In,Ga)Se2 thin films using a nanoscale electrical characterization of scanning Kelvin probe microscopy. The potential measurement reveals a higher surface potential or a smaller work function on grain boundaries of the film than on the grain surfaces. This demonstrates the existence of a local built-in potential on grain boundaries, and the grain boundary is positively charged. The local built-in potential on the grain boundary is expected to increase the minority-carrier collection area from one to three dimensional. In addition, a work function decrease induced by Na on the film surface was observed.

We report detailed charge trapping reliability characteristics of ultrathin ZrO2 and HfO2 dielectrics grown by the ultraviolet ozone oxidation (UVO) method. We also discuss the dependence of charge trapping on the top electrode deposition process, specifically in situ versus ex situ processed gates. Electrical data show that the charge trapping characteristics depend on the deposition method for the top electrode, likely due to exposure of the high-k material to ambient prior to deposition of the electrode. It is also shown that C–V hysteresis is not a complete measure of the charge trapping characteristics of the dielectrics.

We show that the polycrystalline perovskite antiferromagnet Pr0.5Sr0.5MnO3 exhibits a giant anisotropic magnetostriction (λt=1.5×10−3 at T=25 K and H=14.2 T) contrary to much smaller λt (

The kinetics of the interfacial layer (IL) growth between Hf aluminates and the Si substrate during high-temperature rapid thermal annealing (RTA) in either N2 (∼10 Torr) or high vacuum (∼2×10−5 Torr) is studied by high-resolution x-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy. The significant difference of the IL growth observed between high vacuum and relatively oxygen-rich N2 annealing (both at 1000 °C) is shown to be caused by the oxygen species from the annealing ambient. Our results also show that Hf aluminates exhibit much stronger resistance to oxygen diffusion than pure HfO2 during RTA in N2 ambient, and the resistance becomes stronger with more Al incorporated into HfO2. This observation is explained by the combined effects of (i) smaller oxygen diffusion coefficient of Al2O3 than HfO2, and (ii) higher crystallization temperature of the Hf aluminates.

High-resolution x-ray photoelectron spectroscopy (XPS) was applied to characterize the electronic structures for a series of high-k materials (HfO2)x(Al2O3)1−x grown on (100) Si substrate with different HfO2 mole fraction x. Al 2p, Hf 4f, O 1s core levels spectra, valence band spectra, and O 1s energy loss all show continuous changes with x in (HfO2)x(Al2O3)1−x. These data are used to estimate the energy gap (Eg) for (HfO2)x(Al2O3)1−x, the valence band offset (ΔEν) and the conduction band offset (ΔEc) between (HfO2)x(Al2O3)1−x and the (100) Si substrate. Our XPS results demonstrate that the values of Eg, ΔEν, and ΔEc for (HfO2)x(Al2O3)1−x change linearly with x.

High-dielectric-constant materials such as zirconia are currently being investigated to replace SiO2 as the gate dielectric in future complementary metal–oxide–semiconductor devices. In this letter, we present the electrical properties of ultrathin zirconia/SiO2 dielectric stacks in which both layers are grown in situ at room temperature by the ultraviolet–ozone oxidation technique. A capacitance-based equivalent oxide thickness of 15 A (without accounting for quantum-mechanical corrections), leakage current of 1.8×10−4 A/cm2 at 1 V from flatband, and negligible hysteresis has been obtained from these films. It is shown that in order to grow dielectric stacks of high electrical quality, it is important to understand the oxidation kinetics of these films.

In this letter, we report on electrical and microstructural properties of ultrathin zirconia dielectrics grown on SiO2 by ultraviolet (UV) ozone oxidation and natural oxidation (no UV light). Capacitance–voltage (C–V) measurements were performed at multiple frequencies on capacitors fabricated from a ZrO2–SiO2 stack. It was found that the C–V curves from samples grown by natural oxidation were distorted and showed severe frequency dependence while samples grown with UV light exposure under otherwise identical conditions had superior electrical behavior. Loss tangent measurements and detailed electron energy loss spectroscopy studies performed on the two samples revealed that the sample grown by natural oxidation was highly oxygen deficient, and this led to its poor electrical properties.

In this letter, we report on the growth of ultrathin films of zirconia on silicon oxide and silicon oxynitride passivated (001) Si by ultraviolet ozone oxidation of Zr metal precursor layers. The oxidation kinetics has been measured using an accelerator-based nuclear reaction analysis. It was found that oxide films up to 55 A could be grown at room temperature by oxidation at 600 Torr while oxidation at 80 mTorr is self-limiting at 20 A. The interfaces between the dielectric and the substrate have been characterized by scanning transmission electron microscopy. The ZrO2 films were found to be crystalline as grown. Electrical measurements on capacitor structures with 30-A-thick ZrO2 films grown on native oxide on silicon show a capacitance–voltage hysteresis of 15 mV and a capacitance-based equivalent oxide thickness of 17 A at 100 kHz.

Lasers are well suited for micromanufacturing by ablating nearly all the material within the focal spot before any significant heat conduction or mass flow takes place, thus enabling precise machining with little thermal damage. In this letter, the results of an idea of using nonlinear liquids as lenses to fabricate micron-sized features are reported. Our research showed that transmitting the laser pulse through a transparent condensed medium such as carbon disulfide that has very high intensity-dependent nonlinear refractive index creates a lens and focuses the beam. Results on 316 stainless steel showed a two-fold decrease in the hole size and reduced taper when liquid optics were used over traditional solid optics. Polarization effects were also substantially reduced. The beneficial effects of liquid optics are explored.

Electron doping by hydrogenation can reversibly modify the electrical properties of complex oxides. We show that in order to realize large, fast, and reversible response to hydrogen, it is important to consider both the electron configuration on the transition metal 3d orbitals, as well as the thermodynamic stability in nickelates. Specifically, large doping-induced resistivity modulations ranging several orders of magnitude change are only observed for rare earth nickelates with small ionic radii on the A-site, in which case both electron correlation effects and the meta-stability of Ni3+ are important considerations. Charge doping via metastable incorporation of ionic dopants is of relevance to correlated oxide-based devices where advancing approaches to modify the ground state electronic properties is an important problem.

We compare the impact effect of external magnetic fields on the magnetic entropy (Sm), electrical resistivity, and thermopower in two metallic oxides R0.7Sr0.3CoO3 that become ferromagnetic below TC = 240 K and 117 K for R = La3+(4f0) and Gd3+(4f7), respectively. While Gd0.7Sr0.3CoO3 shows unusually high values of negative magnetothermopower (≈−35.5%), magnetic entropy change (ΔSm ≈ −12 J/kg K), and negative magnetoresistance (≈−3.7%) at 10 K for a field change of 5 T, these effects are absent in the La sample at 10 K. We suggest that the large ΔSm arises from the field-induced suppression of spin fluctuations associated with localized 4f7 moments of Gd3+ ions, but magnetic interaction among Co ions dictates magnetoresistance and magnetothermopower. Magnetization data indicate that while Co3+ ions adopt intermediate spin state (S = 1) in the La compound, major fraction of them transforms into low spin state (S = 0) in the Gd sample. Hence, the Gd sample is microscopically heterogeneous with a mixture of f...

Bi or Sb doping has been used to make better material properties of polycrystalline Cu2(In,Ga)Se2 as solar cell absorbers, including the experimentally observed improved electrical properties. However, the mechanism is still not clear. Using first-principles method, we investigate the stability and electronic structure of Bi- and Sb-related defects in CuInSe2 and study their effects on the doping efficiency. Contrary to previous thinking that Bi or Sb substituted on the anion site, we find that under anion-rich conditions, the impurities can substitute on cation sites and are isovalent to In because of the formation of the impurity lone pair s states. When the impurities substitute for Cu, the defects act as shallow double donors and help remove the deep InCu level, thus resulting in the improved carrier life time. On the other hand, under anion-poor conditions, impurities at the Se site create amphoteric deep levels that are detrimental to the device performance.

Vanadium dioxide (VO2) is a correlated electron material which undergoes an insulator-metal transition proximal to room temperature. The large change of optical properties across this phase transition is promising for tunable optical and optoelectronic devices especially at infrared frequencies. We demonstrate the ability to locally tune the optical properties on the micron scale through a simple design consisting of two electrodes patterned on a VO2 thin film. By current injection between the electrodes, a localized conducting path (metallic phase) can be formed within the insulating background. The width of the conducting path can be controlled by varying the applied current. Fourier transform infrared imaging shows that this current-modulated reflectance changes significantly over a distance on the order of the wavelength in the mid-infrared spectral range.

Effect of particle size on the electron transport and magnetic properties of La0.7Ca0.3MnO3 has been investigated. While the ferromagnetic Tc, low field magnetic susceptibility, and insulator‐metal transition are markedly affected by the particle size, the maximum magnetoresistance exhibited by the samples near Tc is not sensitive to the particle size. However, the magnetoresistance at 4.2 K increases with decrease in particle size, suggesting a substantial contribution by the grain boundaries. Preliminary measurements on La0.7Sr0.3MnO3 samples of different particle sizes also corroborate the above conclusions.

In this letter we report large magnetoresistance (MR) in ceramic samples of La1?xSrxMnO3 (0.1?x?0.4) in the temperature range 4.2 K?T?350 K in fields up to 6 T. We find that a large negative and isotopic MR exists for the whole composition range studied and the absolute value of resistivity change on application of magnetic field is more for samples with lower x which have higher resistivity. We find that the large MR occurs in the ferromagnetic state only and MR has a close relation with the magnetization M. © 1995 American Institute of Physics.

Epitaxial LaNiO3(LNO) thin films on LaAlO3(LAO), SrTiO3(STO), and YSZ are grown by pulsed laser deposition method at 350 mTorr oxygen partial pressure and 700 °C substrate temperature. As‐deposited LNO films are metallic down to 10 K. c‐axis oriented YBa2Cu3O7 (YBCO) films were grown on LNO/LAO as well as LNO/STO surfaces without affecting superconducting transition temperature of YBCO. Textured LNO thin films were grown on c‐axis oriented YBCO/STO and YBCO/YSZ . Transport measurements of these bilayer films showed that LNO is a good metallic contact material for YBCO.

We investigated the dependence of absolute SnS band-edge energies on surface orientation using density functional theory and GW method for all surfaces with Miller indices −3≤h,k,l≤3 and found variations as large as 0.9 eV as a function of (hkl). Variations of this magnitude may affect significantly the performance of photovoltaic devices based on polycrystalline SnS thin-films and, in particular, may contribute to the relatively low measured open circuit voltage of SnS solar cells. X-ray diffraction measurements confirm that our thermally evaporated SnS films exhibit a wide distribution of different grain orientations, and the results of Kelvin force microscopy support the theoretically predicted variations of the absolute band-edge energies.

We report on direct imaging of current collection by performing conductive atomic force microscopy (C-AFM) measurement on a complete Cu(In,Ga)Se2 solar cell. The localized current was imaged by milling away the top conductive layer of the device by repeated C-AFM scans. The result exhibits enhanced photocurrent collection on grain boundaries (GBs) of CIGS films, consistent with the argument for electric-field-assisted carrier collection on the GBs.

We report the magnetic entropy change (Delta Sm) in magnetoelectric Eu1-xBaxTiO3 for x = 0.1- 0.9. We find - delta Sm = 11 (40) J/kg.K in x = 0.1 for a field change of 1 (5) Tesla respectively, which is the largest value among all Eu-based oxides. Delta Sm arises from the field-induced suppression of the spin entropy of Eu2+:4f7 localized moments. While -delta Sm decreases with increasing x, -DeltaSm = 6.58 J/kg.K observed in the high spin diluted composition x = 0.9 is larger than that in many manganites. Our results indicate that these magnetoelectrics are potential candidates for cryogenic magnetic refrigeration., 13 pages, 6 figures

We characterize effects of dimensionality reduction and electron confinement on thermopower in La-doped SrTiO3 (LSTO) thin films using hetero-epitaxial capping layers with known conduction band offsets. Hall measurements on SrZrO3/LSTO/(La0.3Sr0.7)(Al0.65Ta0.35)O3 quantum well structures indicate carriers are homogeneously confined in the LSTO film, leading to an unambiguous well width. We do not observe a substantial enhancement of the Seebeck coefficient (S) for confinement widths as thin as 8 A. Recent work towards elucidating the role of thickness scaling on S in complex oxides and implications of our results for using 2D electron confinement to develop advanced thermoelectric materials are discussed.

We report on a direct nm-resolution resistance mapping on the Cu(In,Ga)Se2 photovoltaic thin films, using scanning spreading resistance microcopy. We found a conductance channel along the grain boundaries (GBs) of the polycrystalline materials, which is consistent with the argument that carrier polarity of the GB and the space charge region around it is inverted. To minimize the probe/film contact resistance, so that the local spreading resistance beneath the probe is measured, the probe must be adequately indented to the film and a bias voltage larger than the onset value of the probe/film barrier should be applied.

We report the effects of tunnel coupling on the Quantum-Confined Stark Effect (QCSE) for excitons in InAs/GaAs coupled quantum dots (CQDs). As the barrier separating the dots is reduced, the zero-field dipole moment and the polarizability are both found to increase. This systematic variation as a function of barrier thickness is due to factors including the formation of molecular wavefunctions, the electron/hole effective masses, and the CQD structural properties. The dipole moment for the interdot exciton is found to be up to 100 times larger than that of the intradot exciton resulting in a predominantly linear shift with field. The ability to control the QCSE of the exciton in a single CQD could be useful for a new class of single photon optical switches and tunable emitters.