Pressure-controlled preparation of nanocrystalline complex oxides using pulsed-laser ablation at room temperature

Nanocrystalline thin films of complex oxides such as BaTiO₃ and LaFeO₃ were prepared by pulsed laser ablation without substrate heating. Targets under various Ar pressures were irradiated using an ArF excimer laser. The off-axis configuration of targets and substrates was used to synthesize the films. The crystallinity and chemical composition of the deposited films were strongly dependent on the processing Ar gas pressure. In case of BaTiO₃, the film deposited at 10 Pa was a single phase of BaTiO₃ with a crystallite size around 7.2 nm. With increasing Ar pressure to 200 Pa, XRD peaks of BaTiO₃ as well as BaCO₃ were observed. The by-products could be due to reaction with carbon dioxide in air after taking the sample out of the chamber. For LaFeO₃, the films deposited under 50 to 200 Pa had a single phase with a crystallite size below 10 nm. When the Ar pressure exceeded 100 Pa, the crystallite size tended to decrease for both BaTiO₃ and LaFeO₃, which could be due to formation of aggregated nanoparticles. Below 10 Pa, oxygen deficiency was observed. Over 50 Pa, the atomic concentration of all the constituent elements was almost constant, especially the [Ba]/[Ti] and [La]/[Fe] ratios, which were nearly unity. Received: 19 June 2002 / Accepted: 24 June 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6355, E-mail: yoon-jw@aist.go.jp     

Microcrystalline silicon‐carbon alloys as anti‐reflection window layers in high efficiency thin film silicon solar cells

Microcrystalline silicon‐carbide (μc‐SiC:H) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The μc‐SiC:H films were employed as window layers in microcrystalline silicon (μc‐Si:H) solar cells. The short‐circuit current density (J_SC) in these n‐side illuminated n–i–p cells increases with increasing the deposition time t_W of the μc‐SiC:H window layer from 5 min to 60 min. The enhanced J_SC is attributed to both the high transparency and an anti‐reflection effect of the μc‐SiC:H window layer. Using these favourable optical properties of the μc‐SiC:H window layer in μc‐Si:H solar cells, a J_SC value of 23.8 mA/cm² and cell efficiencies above 8.0% were achieved with an absorber layer thickness of 1 μm and a Ag back reflector. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of an external magnetic field on the <Emphasis Type="Italic">L</Emphasis><Subscript>3</Subscript> subshell fluorescence yields and level widths for Ta,W, Tl,Th and U at 59.54 keV photons

The effect of an external magnetic field on the L₃ subshell fluorescence yields (ω₃ ) and level widths ($\Gamma_{L_3}$) for paramagnetic Ta, W, Tl, Th and U have been investigated using the 59.54 keV incident photon energy in the external magnetic field of intensities ±0.60 T. L₃ X-ray fluorescence cross sections ($\sigma_{L_3}^{X}$) have been measured for the same elements. The measured ω ₃, $\Gamma_{L_3} $ and $ \sigma_{L_3}^X$ values for B = 0 are in good agreement with the theoretical values. It was observed that the values of $\sigma_{L_3}^X$ and ω₃ with the applied magnitude of the magnetic field in both directions show a decreasing trend for paramagnetic Ta, W, Tl, Th and U. Furthermore, in the presence of an external magnetic field, the values of $\Gamma_{L_3}$ show an increasing trend for the same elements. The results show that the atomic parameters such as spectral linewidth, radiation rates, photoionization cross section and fluorescence yield can change when the irradiation is conducted in a magnetic field.     

Synthesis and low temperature transport properties of Mg2Ge1–y Sby

We report on the synthesis and low temperature transport of Mg₂Ge_1–y Sb_y with 0 ≤ y ≤ 0.33. In these materials Sb substitutes for Ge in the antifluorite structure. Electrical and thermal transport measurements indicate that as the Sb content increases, vacancies are formed on the Mg sites thereby contributing to variations in the transport properties. With increasing Sb content both the absolute Seebeck coefficient and electrical resistivity first decrease and then increase, while the thermal conductivity decreases monotonically. Hall measurements indicate this tendency is associated with vacancy formation at higher Sb concentrations. The lattice thermal conductivity is fitted using the Debye approximation in order to elucidate the effect of alloying. We discuss these results in terms of potential for thermoelectric applications. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lifetime vibrational interference during the NO 1s-1π* resonant excitation studied by the NO+(A 1Π → X 1Σ+) fluorescence

Dispersed fluorescence from fragments formed after the de-excitation of the 1s^-1π^* resonances of N^*O and NO^* has been measured in the spectral range of 118–142 nm. This range is dominated by lines of atomic nitrogen and oxygen fragments and by the bands in the NO⁺ ion which result from the participator Auger decay of the 1s^-1π^* resonances. Ab-initio calculations of the transition probabilities between vibrational levels during the reaction NO N^*O ⇒ NO were used to explain the observed intensity dependence for the fluorescence bands on the exciting-photon energy across the resonances and on both v^′ and v^′′ vibrational quantum numbers. The multiplet structure of the 1s^-1π^* resonance and lifetime vibrational interference explain the observed exciting-photon energy dependence of the fluorescence intensity. A strong spin-orbit coupling between singlet and triplet states of NO⁺ is proposed to reduce additional cascade population of the state via radiative transitions from the and states and to explain remaining differences between measured and calculated integral fluorescence intensities.     

Atomic layer deposition coating of ZnO shell for GaN-ZnO core-sheath heteronanowires

We have synthesized GaN-core/ZnO-shell nanowires and investigated effects of the ZnO coating. The X-ray diffraction pattern showed that as-synthesized samples are composed of GaN and ZnO. Transmission electron microscopy indicated that the deposited ZnO shell layer is poly-crystalline. The photoluminescence (PL) spectrum of GaN has been changed by the ZnO coating, where emission bands centered at roughly 1.9 eV, 2.5 eV, and 3.3 eV were newly added to the emissions from core GaN nanowires. We found that overall PL intensity has been significantly increased by coating the ZnO shell layers.     

Theoretical Investigation of Decay Process in a Doubly Excited 2s^2 ^1S0 State of He-Like Ions

In the framework of multi-configuration Dirac-Fock theory, a detailed calculation is performed for the decay rates and the energies of the doubly excited 2s^2 ^1So state of He-like ions, of which atomic number Z ranges from 6 to 92. The 2s^2 ^1So - ls ^2S1/2 Auger decay is predominant at low Z regime, whereas the 2s^2 ^1So - 1s2p ^1,3P1 two-electron one-photon transitions become quite important in moderate and high Z regimes. For heavy ions with Z ≥ 72, the contribution of 2s^2 ^1So - 1s2s ^3S1 M1 transition is significant. The Breit interaction considerably enhances the 2s^2 ^1So - 1s ^2S1/2 Auger rate at high Z regime.     

Direct synthesis of beta gallium oxide nanowires, nanobelts, nanosheets and nanograsses by microwave plasma

In this study, beta-gallium oxide (β-Ga₂O₃) nanowires, nanobelts, nanosheets, and nanograsses were synthesized through microwave plasma of liquid phase gallium containing H₂O in Ar atmosphere using silicon as the substrate. The nanowires with diameters of about 20-30 nm were several tens of microns long and the nanobelts with thickness of about 20-30 nm were tens to hundreds of microns long. The morphology and structure of products were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and X-ray diffraction (XRD). These results showed that multiple nucleation and growth of β-Ga₂O₃ nanostructures could easily occur directly out of liquid gallium exposed to appropriate H₂O and Ar in the gas phase. The growth process of β-Ga₂O₃ nanostructures may be dominated by VS (vapor-solid) mechanism.     

An alternative procedure for the determination of the optical band gap and thickness of amorphous carbon nitride thin films

In this work, we propose an alternative procedure to obtain the optical band gap and the thickness of amorphous carbon nitride thin films that requires only the measurement of the absorbance spectrum of the samples. The method is based on an absorbance spectrum fitting (ASF) procedure using the Tauc model, which is widely applied to the study of amorphous semiconductors. With the aim of evaluating the proposed method two sets of carbon nitride samples deposited on glass substrates were analyzed; one prepared by pulsed laser ablation (PLA) and the second by magnetron sputtering. The obtained results using different conventional methods were compared with the results of the ASF method and a good agreement between the values and the tendencies with the experimental conditions used to prepare the films were observed.