T. Yoshino

Ce- and Yb-based Kondo semiconductors

Abstract Kondo semiconductors are a class of strongly correlated f-electron materials whose low-energy excitations exhibit a (pseudo)gap at low temperatures. The significant difference between the pseudogap in orthorhombic CeNiSn and CeRhSb and the real gap in cubic Ce 3 Bi 4 Pt 3 and YbB 12 is highlighted by a comparison of transport properties. Low-temparature measurements of the magnetoresistance, Hall coefficient and specific heat of CeNiSn have revealed a field-induced excitation of the coherent low-carrier state. A systematic study of CeNi 1− x T x Sn ( T = Co, Cu and Pt ) has shown that the residual carriers in CeNiSn are immobilized by 1% substitution irrespective of the substitutes. Further substitution with 5% Cu induces a magnetic instability at T = 0.

Magnetic-field- and temperature-dependent Fermi surface of CeBiPt

The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.

Localization effects of kondo semimetals CeNiSn and CeRhSb

Abstract We have found that the semiconductor-like increase in resistivity previously reported on CeNiSn and CeRhSb below about 8 K is suppressed with decreasing impurity concentration in single-crystalline samples. Nevertheless, the absolute values of the Hall coefficient strongly increase as temperature decreases. Therefore, we interpret the metallic behaviour found in the a -axis resistivity of a purified crystal of CeNiSn as a consequence of the significant increase of relaxation time of carriers. Even for the purified crystal, the residual γ value is 40 mJ/K 2 mol at 0.03 K. These results suggest that the energy gap is closed along the a -axis and that residual carriers are strongly localized in the presence of impurities.

Magnetic-field-induced band-structure change in CeBiPt.

We report on a field-induced change of the electronic band structure of CeBiPt as evidenced by electrical-transport measurements in pulsed magnetic fields. Above approximately 25 T, the charge-carrier concentration increases nearly 30% with a concomitant disappearance of the Shubnikov-de Haas signal. These features are intimately related to the Ce 4f electrons since for the non-4f compound LaBiPt the Fermi surface remains unaffected. Electronic band-structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.

Electrical Characteristics of Mesoporous Pure-Silica–Zeolite Film

The dependence of the electrical characteristics of hydrothermally crystallized pure-silica–zeolite films on the water concentration in the precursor was investigated. Zeolite was crystallized in a silica–zeolite composite film after spin-coating the precursor, which was composed of tetraethylorthosilicate, tetrabutylammonium hydroxide, ethylalcohol, and deionized (DI) water, followed by calcinations at 400 °C. The results of Fourier-transform infrared (FT-IR) spectroscopy indicated that the number of Si–OH and O–H bonds decreased with an increase in the water concentration in the precursor. The dielectric constant of the film decreased with an increase in water concentration, while the leakage current increased.

Novel self-assembled ultra-low-k porous silica films with high mechanical strength for 45 nm BEOL technology

Novel ultra-low-k porous silica films were developed by use of a self-assembly technology. The mechanical properties of the porous silica films could be reinforced independently of the dielectric constant by introducing a tetramethyl-cyclo-tetra-siloxane (TMCTS) treatment. High modulus porous silica films, with an elastic modulus of 8 GPa and dielectric constant of 2, can be achieved simultaneously. Ultra-low-k/Cu damascene with sufficient mechanical strength was demonstrated for 45 nm BEOL (back-end-of-line) technology.

Determination of Mechanical Properties of Porous Silica Low-k Films on Si Substrates Using Orientation Dependence of Surface Acoustic Wave

The measurement accuracy of Youngs modulus E and Poissons ratio σ of thin low-dielectric-constant (low-k) films is improved by the simultaneous analysis of the laser-pulse-generated surface-acoustic wave (SAW) propagating along two different crystalline Si orientations. Frequency ( f)-dependent phase velocities v110( f) and v100( f) of SAW propagating along [110] and [100] directions of the Si substrate were obtained by analyzing the SAW waveforms measured using a piezoelectric transducer. The mass density ρ and the thickness d of low-k films were determined by X-ray reflectance and spectroscopic ellipsometry, and were then used to determine the values of E and σ that fit the dispersion curves v110( f) and v100( f) best. Different dependencies of v110( f) and v100( f) on E and σ were the key for the accurate determination of the values. This method was employed to study a series of porous organosilica films in which methyl content was varied. The results showed that E and σ decrease with methyl content from 9.5 to 4.3 GPa and from 0.36 to 0.25, respectively, in the studied range of methyl content. It is concluded that the reported method is an accurate nondestructive technique for the simultaneous determination of E and σ of low-k films on Si.

Effect of Phosphorus Atom in Self-Assembled Monolayer as a Drift Barrier for Advanced Copper Interconnects

An excellent barrier effect against Cu ion drift is demonstrated using self-assembled monolayer (SAM) barrier dielectrics in which phosphorous (P) atom is contained. Three kinds of SAMs, such as P-, carbon (C-), and nitrogen (N-) containing SAMs with the identical molecular structure, were wet-chemically formed on a thermal SiO2. The X-ray reflectance and infrared absorption spectroscopy revealed that nearly 1-nm-thick monolayers were successfully formed. The barrier effects of SAMs were investigated by the time-dependent dielectric breakdown measurements of Cu/SAM/SiO2/Si metal–insulator–semiconductor structures under applying a bias-temperature stress. The time-to-breakdown tBD of P-SAM was approximately 10-fold compared to SiO2 without SAM formation. The tBD of C-SAM and N-SAM were, on the other hand, well agreed with that of SiO2, indicating that the barrier effects of C- and N-SAMs are much weaker than that of P-SAM. From these results, we concluded that the existence of P atom in the SAM molecule to form Cu-P complex is the key for the barrier mechanism of the SAM studied in this report.

High-Field Magnetization and Magnetoresistance of Single-Crystal CeRhSb.

We report the high-field magnetization and longitudinal magnetoresistance measurements in magnetic fields of up to 55 T, and Hall resistivity and transverse magnetoresistance measurements in fields of up to 15 T for single crystals of CeRhSb with a pseudogap in the density of states. The magnetization at 1.3 K shows rather weak anisotropy along the orthorhombic principal axes, M a ≃ M b > M c , and the initial straight lines deviate upward near magnetic field values of 20, 30 and 25 T, respectively. The longitudinal magnetoresistance at 1.3 K is largely negative for both B // a and B // b and saturates near 30 T, while that for B // c is positive. The transverse magnetoresistance is positive and increases in proportion to B 1.5 for B // c , while those for B // a and B // b saturate at around 10 T. The Hall coefficient for B // a changes sign from positive to negative below 4 K, while that for B // c progressively increases with decreasing temperature. The Hall resistivity for B // a and B // b possesses,...

Influence of Synthesis Process on Mechanical and Electrical Characteristics of Mesoporous Pure Silica-Zeolite

The dielectric constant, elastic modulus and reliability of the pure silica-zeolite composite film which was formed by self-assembly of porous silica having hydrothermally crystallized zeolite nanoparticles. Fourier-transform-infrared spectroscopy indicated that Si-OH and O-H bonds decreased by zeolite formation, resulting in the decrease of the dielectric constant. Silylation hardening by 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor treatment could decrease the dielectric constant due to the decrease of Si-OH and O-H bonds. The elastic modulus of 7.03 GPa and the dielectric constant of 1.94 were achieved for the meso-porous silica-zeolite film by silylation hardening. Furthermore, mean-time-to-failure (MTF) lifetime of time-dependent dielectric breakdown (TDDB) is longer than ten years at 125°C under the electric field of 3.4 MV/cm.