Electron Tunneling Current Research Articles

Passivation analysis of silicon surfaces via sol—gel derived Al-rich ZnO film

Electronic recombination losses can be reduced via passivation of silicon surfaces. Most techniques available in the literature are either not cost effective or not applicable for solar cell applications. We investigate low cost sol–gel derived Al-rich zinc oxide (ZnO:Al) film and its effective passivation of p-type silicon surfaces. Herein, we present the elemental composition of the film and interfacial structure of ZnO:Al/Si using FTIR, XPS, TEM, and SIMS characterizations. ZnO:Al is polycrystalline and contains some very small amorphous regions of Al2O3. At the ZnO:Al/c-Si interface, a thin SiOx layer with a thickness of ∼6 nm is formed. The XPS analyses reveal that the Al/Zn molar ratio in the ZnO:Al increases from ∼10% at the surface to ∼80% at the ZnO:Al/c-Si interface. The hydrogen content also gradually increases from the surface to the interface. The FTIR absorption area corresponding to the Si–H bonding is ∼2.89 au. The obtained hydrogen concentration is ∼3.93 × 1022 atoms cm−3. A fixed negative charge is created by ZnO:Al on ZnO//SiOx interface. The thermal equilibrium was established between Si and ZnO:Al through SiOx by electron tunneling current. Here, the c-Si may be passivated for two reasons: (i) Al creates defects on the ZnO:Al/c-Si interface and H is attached to the defects (dangling bonds) and (ii) due to the field effect passivation via the negative charged ZnO:Al film.

The Effect of Molecules Adsorbed to the Cathode Surface on the Characteristics of a High-Voltage Gas Discharge

Relationships inherent in the processes that occur at the initial stage of a high-voltage (electric field strength E > 1 MV/cm) gas discharge with participation of nonmetal nanofilms adsorbed to the cathode are considered. It is shown that the current-voltage characteristic of the prebreakdown current obeys the Fowler-Nordheim equation corrected for the quantum character of image forces. Based on electronic polaron theory, a general expression for the field emission current at the interface between two media is obtained. It is shown that for the abovementioned fields, the field dependence of the electron tunneling current is appreciably different from the well-known classical relations.

Phonon effects on the current noise spectra and the ac conductance of a single molecular junction

By using nonequilibrium Green’s functions and the equation of motion method, we formulate a self-consistent field theory for the electron transport through a single-molecule junction (SMJ) coupled with a vibrational mode. We show that the nonequilibrium dynamics of the phonons in a strong electron–phonon coupling regime can be taken into account appropriately in this self-consistent perturbation theory, and the self-energy of the phonons is connected with the current fluctuations in the molecular junction. We calculate the finite-frequency nonsymmetrized noise spectra and the ac conductance, which reveal a wealth of inelastic electron tunneling characteristics on the absorption and emission properties of this SMJ. In the presence of a finite bias voltage and the electron tunneling current, the vibration mode of the molecular junction is heated and driven to an unequilibrated state. The influences of unequilibrated phonons on the current and the noise spectra are investigated.

BOEMDET-Band Offsets and Effective Mass Determination Technique applied to MIS devices on silicon to obtain the unknown bandgap of insulators

The Fowler-Nordheim carrier tunneling slope constants for electron and hole conduction through MOS devices fabricated on silicon substrate can be utilized to determine the conduction and valence band offsets, carrier effective masses in the SiO2 and its unknown bandgap, independent of photoemission spectroscopic measurements of band offsets on SiO2/Si samples. The slope constants can be obtained from the electron and hole tunneling currents versus oxide voltage characteristics on a pair of n-MOS and p-MOS devices in accumulation. This characterization technique called BOEMDET, can be applied to other insulating materials grown or deposited on silicon, such as jet vapor deposited (JVD) nitride, HfSiON, SiOxNy, AlN, BN, high-K oxides, and transparent conducting oxides (TCO).

BOEMDET-Band Offsets and Effective Mass Determination Technique utilizing Fowler-Nordheim tunneling slope constants in MIS devices on silicon

The Fowler-Nordheim carrier tunneling slope constants for electron and hole conduction through MOS devices fabricated on silicon substrate are utilized to determine the conduction and valence band offsets, carrier effective masses in the SiO2 and its unknown bandgap, independent of photoemission spectroscopic measurements of band offsets on SiO2/Si samples. The slope constants are obtained from the electron and hole tunneling currents versus voltage characteristics on a pair of n-MOS and p-MOS devices in accumulation or an n-channel MOSFET device in inversion that allows carrier separation. This characterization technique called BOEMDET, can be applied to other insulating materials grown or deposited on silicon, such as JVD nitride, HfSiON, SiOxNy, and high-K oxides found viable for future MOS technology. To an accuracy of one decimal place, the conduction band offset for the SiO2/Si MIS structure is determined to be 3.2 eV, the valence band offset obtained is 4.6 eV and the bandgap of SiO2 is found to be 8.9 eV. The electron and hole effective masses in the SiO2 are calculated to be 0.42m and 0.58m, where m is the free electron mass. In case of quantum confinement at the SiO2/Si interface, the electron and hole masses correct to 0.51m and 0.65m. Also, the carrier effective masses in thermal SiO2 are found to be the same for all thickness of the oxide. Keywords: effective mass, FN-tunneling, band offsets, metal-insulator-semiconductor

The Effects of Nitrogen Concentration in TiN<sub>x</sub> and the Thickness of HfSiO<sub>x</sub>N to the Tunneling Currents in Isotropic TiN<sub>x</sub>/HfSiO<sub>x</sub>N/SiO<sub>2</sub>/Si(100) Capacitors

In this paper, we have computed electron tunneling currents in TiNx/HfSiOxN/SiO2/Si (100) MOS capacitors by including a coupling term between transverse and longitudinal kinetic energies which is represented by an electron phase velocity in the gate. The effective mass of the substrate is considered as an isotropic mass. The transmittance was analytically calculated by employing an Airy-wavefunction approach, and the obtained transmittance was then utilized to calculate the tunneling current for different nitrogen compositions in the TiNx metal gate and the equivalent oxide thicknesses (EOTs) of HfSiOxN. It was shown that the tunneling current reduces considerably as the nitrogen composition of the TiNx metal gate decreases. It was also shown that the increase in the EOT reduces the tunneling current. In addition, the tunneling current shows an oscillatory behavior at high oxide voltages.

Modeling of tunneling current in ultrathin MOS structure with interface trap charge and fixed oxide charge

A model based on analysis of the self-consistent Poisson—Schrodinger equation is proposed to investigate the tunneling current of electrons in the inversion layer of a p-type metal-oxide-semiconductor (MOS) structure. In this model, the influences of interface trap charge (ITC) at the Si—SiO2 interface and fixed oxide charge (FOC) in the oxide region are taken into account, and one-band effective mass approximation is used. The tunneling probability is obtained by employing the transfer matrix method. Further, the effects of in-plane momentum on the quantization in the electron motion perpendicular to the Si—SiO2 interface of a MOS device are investigated. Theoretical simulation results indicate that both ITC and FOC have great influence on the tunneling current through a MOS structure when their densities are larger than 1012 cm−2, which results from the great change of bound electrons near the Si—SiO2 interface and the oxide region. Therefore, for real ultrathin MOS structures with ITC and FOC, this model can give a more accurate description for the tunneling current in the inversion layer.

Reduced Gate-Leakage Current and Charge Trapping Characteristics of Dysprosium-Incorporated $\hbox{HfO}_{2}$ Gate-Oxide n-MOS Devices

The higher effective barrier height of Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , which is around 2.32 eV calculated from the Fowler-Nordheim plot, accounts for the reduced leakage current in Dy-incorporated HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> n-type metal-oxide-semiconductor devices. The lower barrier height of HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> characterizes the increasing electron-tunneling currents enhanced by the buildup of hole charges trapped in oxide, which causes a severe increase in the stress-induced leakage current (SILC), leading to oxide breakdown. However, the increased barrier height in Dy-incorporated HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> inhibits a further increase in the electron tunneling from the TaN gate, and trapped holes lessen the hole-tunneling currents, resulting in a negligible SILC. The lower trap generation rate by the reduced hole trap density and the reduced hole tunneling of the Dy-doped HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dielectric demonstrate the high dielectric-breakdown strength by weakening the charge trapping and the defect generation during the stress.

Comparison of electron transmittances and tunneling currents in an anisotropic TiNx/HfO2/SiO2/p-Si(100) metal—oxide—semiconductor (MOS) capacitor calculated using exponential- and Airy-wavefunction approaches and a transfer matrix method

Analytical expressions of electron transmittance and tunneling current in an anisotropic TiNx/HfO2/SiO2/p-Si(100) metal—oxide—semiconductor (MOS) capacitor were derived by considering the coupling of transverse and longitudinal energies of an electron. Exponential and Airy wavefunctions were utilized to obtain the electron transmittance and the electron tunneling current. A transfer matrix method, as a numerical approach, was used as a benchmark to assess the analytical approaches. It was found that there is a similarity in the transmittances calculated among exponential- and Airy-wavefunction approaches and the TMM at low electron energies. However, for high energies, only the transmittance calculated by using the Airy-wavefunction approach is the same as that evaluated by the TMM. It was also found that only the tunneling currents calculated by using the Airy-wavefunction approach are the same as those obtained under the TMM for all range of oxide voltages. Therefore, a better analytical description for the tunneling phenomenon in the MOS capacitor is given by the Airy-wavefunction approach. Moreover, the tunneling current density decreases as the titanium concentration of the TiNx metal gate increases because the electron effective mass of TiNx decreases with increasing nitrogen concentration. In addition, the mass anisotropy cannot be neglected because the tunneling currents obtained under the isotropic and anisotropic masses are very different.

Electron transport through asymmetric ferroelectric tunnel junctions: Current-voltage characteristics

We have carried out calculations of current-voltage characteristics for the electron tunnel current through a junction with a thin insulating ferroelectric barrier assuming that interface transmissions for the left and right interfaces noticeably differ due to dissimilarity of the interfaces. Obtained conductance versus voltage and current versus voltage curves exhibit well distinguishable asymmetric hysteresis. We show that the asymmetry in the hysteretic effects could originate from the asymmetric bias voltage profile inside the junction. In particular, we analyze the hysteresis asymmetries occurring when the bias voltage distribution is low sensitive to the spontaneous polarization reversal.

Theory of tunneling across hydrogen-bonded base pairs for DNA recognition and sequencing

We present the results of first-principles calculations for the electron tunnel current through hydrogen-bonded DNA base pairs and for (deoxy)nucleoside-nucleobase pairs. Electron current signals either through a base pair or through a deoxynucleoside-nucleobase pair are a potential mechanism for recognition or identification of the DNA base on a single-stranded DNA polymer. Four hydrogen-bonded complexes are considered: guanine-cytosine, diaminoadenine-thymine, adenine-thymine, and guanine-thymine. First, the electron tunneling properties are examined through their complex band structure (CBS) and the metal contact's Fermi-level alignment. For gold contacts, the metal Fermi level lies near the highest occupied molecular orbital for all DNA base pairs. The decay constant determined by the complex band structure at the gold Fermi level shows that tunnel current decays more slowly for base pairs with three hydrogen bonds (guanine-cytosine and diaminoadenine-thymine) than for base pairs with two hydrogen bonds (adenine-thymine and guanine-thymine). The decay length and its dependence on hydrogen-bond length are examined. Second, the conductance is computed using density functional theory Green's-function scattering methods and these results agree with estimates made from the tunneling decay constant obtained from the CBS. Changing from a base pair to a deoxynucleoside-nucleobase complex shows a significant decrease in conductance. It also becomes difficult to distinguish the current signal by only the number of hydrogen bonds.

Defect Size Effect and Defect Band Conduction of Ultrathin Oxides After Degradation and Breakdown

In this letter, the relationship between gate oxide degradation and defect band formation is investigated quantitatively based on the 1-D linear chain model of defects. Results show that degradation as a defect band occurs when the local defect distance reaches a critical value to make the bound-electron wave function overlap within adjacent defects; that the degradation oxide energy band is composition; that there is a defect band in the oxide band gap, besides the ordinary conduction band, which contributes to the electron tunneling current; and that both Weibull slope and defect current increase with decreasing defect spacing. Therefore, the variation of defect size and leakage current after degradation and breakdown of ultrathin SiO2 can be explained by the change of defect distance ratio.

Investigation of the Correlation Between Temperature and Enhancement of Electron Tunneling Current Through $\hbox{HfO}_{\bf 2}$ Gate Stacks

A model is established to describe the temperature dependence of the electron tunneling current through HfO2 gate stacks based on analyzing the coupling between the longitudinal and transverse components of electron thermal energy caused by the difference of the effective electron mass between the HfO2 gate stacks and silicon. By analyzing the three-dimensional Schrodinger equation for a MOS structure with HfO2 gate stacks, a reduction in the barrier height is resulted from the large effective electron mass mismatch between the gate oxide and the gate (substrate). The calculated electron tunneling currents agree well with the experimental data over a wide temperature range. This coupling model can explain the temperature dependence of the electron tunneling current through HfO2 gate stacks very well. The numerical results also demonstrate that the temperature dependence of the electron tunneling current strongly depends on the effective electron mass of HfO2. This temperature sensitivity of the electron tunneling current can be proposed as a novel method to determine the effective electron mass of the gate oxide.

Discrimination of Class I Cyclobutane Pyrimidine Dimer Photolyase from Blue Light Photoreceptors by Single Methionine Residue

DNA photolyase recognizes ultraviolet-damaged DNA and breaks improperly formed covalent bonds within the cyclobutane pyrimidine dimer by a light-activated electron transfer reaction between the flavin adenine dinucleotide, the electron donor, and cyclobutane pyrimidine dimer, the electron acceptor. Theoretical analysis of the electron-tunneling pathways of the DNA photolyase derived from Anacystis nidulans can reveal the active role of the protein environment in the electron transfer reaction. Here, we report the unexpectedly important role of the single methionine residue, Met-353, where busy trafficking of electron-tunneling currents is observed. The amino acid conservation pattern of Met-353 in the homologous sequences perfectly correlates with experimentally verified annotation as photolyases. The bioinformatics sequence analysis also suggests that the residue plays a pivotal role in biological function. Consistent findings from different disciplines of computational biology strongly suggest the pivotal role of Met-353 in the biological function of DNA photolyase.

Design Considerations for Nonmagnetic Semiconductor-Based Spin Filters

In this paper we have analyzed the possibility of enhancing spin-polarization performance of conventional nonmagnetic semiconductor heterostructures which rely on the resonant tunneling mechanism. Both the bulk inversion asymmetry (BIA) and the structural inversion asymmetry (SIA) effects are taken into account in the presented model. The aim is to engineer nanostructures with maximal degree of spin separation in the electron tunneling current, which might be useful in studying various spin-related phenomena in semiconductor materials. Spin-polarization status of the current, in the devices under consideration, should be controllable by moderate emitter-collector voltages. Additionally, the spin orbit-interactions affect the dwell times of electrons in spin-up and spin-down states, therefore the prospects of spin-filtering in the time domain may be considered as well.

First-principles study of the effects of oxygen vacancy on hole tunneling current

The effects of oxygen vacancies on the electronic structure of silicon dioxide and the hole tunneling current were investigated using first-principles calculations. A level related to oxygen vacancy was obtained to be nearly 2.0 eV from the top of valence band within the bandgap of the α-quartz supercell with one oxygen vacancy. And therefore the defect assisted hole (electron) tunneling currents were calculated. The results shows that the hole tunneling current will be dominant for a thinner oxide thickness at low oxide field and the contribution of trap assisted hole tunneling to the total tunneling current decreases with oxide thickness and oxide field increasing. It is concluded that the effects of the oxygen vacancies on the hole tunneling current become smaller with larger oxide thickness and higher electric field.

Electronic structure mechanism of spin-polarized electron transport in a Ni–C 60–Ni system

The nature of chemical bonding and its effect on spin-polarized electron transport in Ni–C 60–Ni are studied using density functional theory in conjunction with the Landauer–Büttiker formalism. The binding site on the C 60 cage surface appears to have a strong influence on the electron tunneling current between Ni leads. The tunnel current has a much higher magnitude when Ni is bonded to hole sites (H6, H5) than at bridge sites (B66, B56) of the fullerene cage. Furthermore, the magnitude of junction magnetoresistance is predicted to be significantly high for the molecular Ni–C 60–Ni system.

Parameters extraction of hafnium based gate oxide capacitors

From quantum simulations of both capacitance and current measurements, the main physical parameters (dielectric thickness and permittivity, doping levels) of hafnium based (HfSiO x and HfO 2) gate oxide capacitors have been extracted. Three kinds of gates (n +-polysilicon, totally silicided (TOSI) NiSi and metal TiN gates) have been studied. In the case of thick (EOT between 11.1 and 12.3 nm) HfSiO x gate oxides or thin (EOT inferior to 2 nm) HfO 2 stacks with n +-polysilicon or TiN gates, a good agreement between simulations and experimental data is obtained. Electron tunneling currents are prevalent in these stacks except for the specific case of TiN/HfO 2 stacks in p-substrate accumulation mode. In this case, electron and hole tunneling transparencies become of the same order of magnitude. Hole transport contribution can no more be neglected and should be taken into account in simulations.

2P321 蛋白質中電子トンネリングカレントの揺らぎの起源(光生物学(光合成・視覚と光受容),口頭発表,第45回日本生物物理学会年会)

2P321 蛋白質中電子トンネリングカレントの揺らぎの起源(光生物学(光合成・視覚と光受容),口頭発表,第45回日本生物物理学会年会)

Optimization of spin-filtering properties in diluted magnetic semiconductor heterostructures

We have performed structural parameter optimizations of asymmetric ZnSe∕Zn1−xMnxSe multilayer structures in magnetic and electric fields to maximize the spin polarization of the electron tunnel current. The optimization procedure was carried out by simulated annealing, with target functions set to obtain the best possible spin-polarization properties within the chosen range of bias voltages. The performance of the optimized structure is predicted to exceed that of the existing spin-diode designs.