Quantum transport through single-molecule junctions with orbital degeneracies

We consider electronic transport through a single-molecule junction where the molecule has a degenerate spectrum. Unlike previous transport models, and theories a rate-equations description is no longer possible, and the quantum coherences between degenerate states have to be taken into account. We present the derivation and application of a master equation that describes the system in the weak-coupling limit and give an in-depth discussion of the parameter regimes and the new phenomena due to coherent on-site dynamics

Negative oxygen vacancies in HfO2_2 as charge traps in high-k stacks

We calculated the optical excitation and thermal ionization energies of oxygen vacancies in m-HfO$_2$ using atomic basis sets, a non-local density functional and periodic supercell. The thermal ionization energies of negatively charged V$^-$ and V$^{2-}$ centres are consistent with values obtained by the electrical measurements. The results suggest that negative oxygen vacancies are the likely candidates for intrinsic electron traps in the hafnum-based gate stack devices.Comment: 3 pages, 2 figure

Numerical adiabatic potentials of orthorhombic Jahn-Teller effects retrieved from ultrasound attenuation experiments. Application to the SrF2:Cr crystal

A methodology is worked out to retrieve the numerical values of all the main parameters of the six-dimensional adiabatic potential energy surface (APES) of a polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) from ultrasound experiments. The method is based on a verified assumption that ultrasound attenuation and speed encounter anomalies when the direction of propa- gation and polarization of its wave of strain coincides with the characteristic directions of symmetry breaking in the JTE. For the SrF2:Cr crystal, employed as a basic example, we observed anomaly peaks in the temperature dependence of attenuation of ultrasound at frequencies of 50-160 MHz in the temperature interval of 40-60 K for the wave propagating along the [110] direction, for both the longitudinal and shear modes, the latter with two polarizations along the [001] and [110] axes, respectively. We show that these anomalies are due to the ultrasound relaxation by the system of non-interacting Cr2+ JT centers with orthorhombic local distortions. The interpretation of the ex- perimental findings is based on the T2g (eg +t2g) JTE problem including the linear and quadratic terms of vibronic interactions in the Hamiltonian and the same-symmetry modes reduced to one interaction mode. Combining the experimental results with a theoretical analysis we show that on the complicated six-dimensional APES of this system with three tetragonal, four trigonal, and six orthorhombic extrema points, the latter are global minima, while the former are saddle points, and we estimate numerically all the main parameters of this surface, including the linear and quadratic vibronic coupling constants, the primary force constants, the coordinates of all the extrema points and their energies, the energy barrier between the orthorhombic minima, and the tunneling splitting of the ground vibrational states.Comment: 8 pages, 3 figure

Giant isotope effect and spin state transition induced by oxygen isotope exchange in (Pr1−xSmx)0.7Ca0.3CoO3Pr_{1-x}Sm_x)_{0.7}Ca_{0.3}CoO_3

We systematically investigate effect of oxygen isotope in $(Pr_{1-x}Sm_x)_{0.7}Ca_{0.3}CoO_3$ which shows a crossover with x from ferromagnetic metal to the insulator with spin-state transition. A striking feature is that effect of oxygen isotope on the ferromagnetic transition is negligible in the metallic phase, while replacing $^{16}O$ with $^{18}O$ leads to a giant up-shift of the spin-state transition temperature ($T_s$) in the insulating phase, especially $T_s$ shifts from 36 to 54 K with isotope component $\alpha_S=-4.7$ for the sample with x=0.175. A metal-insulator transition is induced by oxygen isotope exchange in the sample x=0.172 being close to the insulating phase. The contrasting behaviors observed in the two phases can be well explained by occurrence of static Jahn-Teller distortions in the insulating phase, while absence of them in the metallic phase.Comment: 4 pages, 5 figure

Degradation of polycrystalline HfO2-based gate dielectrics under nanoscale electrical stress

The evolution of the electrical properties of HfO2/SiO2/Si dielectric stacks under electrical stress has been investigated using atomic force microscope-based techniques. The current through the grain boundaries (GBs), which is found to be higher than thorough the grains, is correlated to a higher density of positively charged defects at the GBs. Electrical stress produces different degradation kinetics in the grains and GBs, with a much shorter time to breakdown in the latter, indicating that GBs facilitate dielectric breakdown in high-k gate stacks

The role of nitrogen-related defects in high-k dielectric oxides: Density-functional studies

Using ab initio density-functional total energy and molecular-dynamics simulations, we study the effects of various forms of nitrogen postdeposition anneal(PDA) on the electric properties of hafnia in the context of its application as a gate dielectric in field-effect transistors. We consider the atomic structure and energetics of nitrogen-containing defects which can be formed during PDA in various N-based ambients: N2, N2+, N, NH3, NO, and N2O. We analyze the role of such defects in fixed charge accumulation, electron trapping, and in the growth of the interface SiO2 layer. We find that nitrogen anneal of the oxides leads to an effective immobilization of native defects such as oxygen vacancies and interstitial oxygen ions, which may inhibit the growth of a silica layer. However, nitrogen in any form is unlikely to significantly reduce the fixed charge in the dielectric.Peer reviewe

Temperature (5.6-300K) Dependence Comparison of Carrier Transport Mechanisms in HfO2/SiO2 and SiO2 MOS Gate Stacks

Temperature dependent measurements have been used to examine transport mechanisms and energy band structure in MOS devices. In this study, a comparison between high-k HfO2 dielectrics and conventional SiO2 dielectrics is made to investigate dielectric specific thermally activated mechanisms. Temperature dependent measurements on large area n/pMOSFETs composed of SiO2 and HfO2/SiO2 gate dielectrics were performed from 5.6 K to 300 K. A large increase in the gate leakage current is observed at the formation of the minority carrier channel. The data indicate that gate leakage current prior to the formation of the minority channel is carrier rate limited while gate leakage current is tunneling rate limited above the threshold voltage. Gate leakage current measurements show two distinct Arrhenius transport regimes for both SiO2 and HfO2 gate dielectrics. The Arrhenius behavior of the gate leakage current is characterized by a strong temperature dependent regime and a weak temperature dependent regime. The activation energy of the strong temperature regime is found to vary with the applied gate voltage. Frenkel-Poole or other electric field models are able to explain the gate voltage dependence of the gate leakage current for the low-temperature/voltage regime investigated. The data suggest that the variation of the activation energy for the Arrhenius behavior is weakly electric-field driven and strongly voltage, or Fermi energy level, driven

Low-energy excitations of a linearly Jahn-Teller coupled orbital quintet

The low-energy spectra of the single-mode h x (G+H) linear Jahn-Teller model is studied by means of exact diagonalization. Both eigenenergies and photoemission spectral intensities are computed. These spectra are useful to understand the vibronic dynamics of icosahedral clusters with partly filled orbital quintet molecular shells, for example C60 positive ions.Comment: 14 pages revte

Analysis of effective mobility and hall effect mobility in high-k based In0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistors

We report an In0.75Ga0.25As metal-oxide-semiconductor high-electron-mobility transistor with a peak Hall mobility of 8300 cm(2)/Vs at a carrier density of 2 x 10(12) cm(-2). Comparison of split capacitance-voltage (CV) and Hall Effect measurements for the extracted electron mobility have shown that the split-CV can lead to an overestimation of the channel carrier concentration and a corresponding underestimation of electron mobility. An analysis of the electron density dependence versus gate voltage allows quantifying the inaccuracy of the split-CV technique. Finally, the analysis supported by multi-channel conduction simulations indicates presence of carriers spill over into the top InP barrier layer at high gate voltages. (C) 2011 American Institute of Physics. (doi: 10.1063/1.3665033