Masato Oda

Electron Transport in a π-Stacking Molecular Chain

We have investigated the electronic structure and transport properties of a pi-stacking molecular chain which is covalently bonded to a H/Si(100) surface, using the first-principles density functional theory approach combined with Greens function method. The highest occupied molecular orbital (HOMO) dispersion is remarkably reduced, but remains noticeable ( approximately 0.1 eV), when a short pi-stacking styrene wire is cut from an infinitely long wire and sandwiched between metal electrodes. We find that the styrene chains HOMO and lowest unoccupied molecular orbital (LUMO) states are not separated from Si, indicating that it does not work as a wire. By substituting -NO2 or -NH2 for the top -H of styrene, we are able to shift the position of the HOMO and LUMO with respect to the Fermi level. More importantly, we find that the HOMO of styrene-NH2 falls into the band gap of the substrate and is localized in the pi-stacking chain, which is what we need for a wire to be electrically separated from the substrate. The conductance of such an assembly is comparable to that of Au/benzene dithiolate/Au wire based on chemical bonding, and its tunability makes it a promising system for a molecular device.

Interacting quasi-band model for electronic states in compound semiconductor alloys: Zincblende structure

The interacting quasi-band model proposed for electronic states in simple alloys is extended for compound semiconductor alloys with general lattice structures containing several atoms per unit cell. Using a tight-binding model, a variational electronic wave function for quasi-Bloch states yields a non-Hermitian Hamiltonian matrix characterized by matrix elements of constituent crystals and concentration of constituents. Solving secular equations for each k-state yields the alloys energy spectrum for any type of randomness and arbitrary concentration. The theory is used to address III–V (II–VI) alloys with a zincblende lattice with crystal band structures well represented by the sp3s* model. Using the resulting 15 × 15 matrix, the concentration dependence of valence and conduction bands is calculated in a unified scheme for typical alloys: Al1−xGaxAs, GaAs1−xPx, and GaSb1−xPx. Results agree well with experiments and are discussed with respect to the concentration dependence, direct–indirect gap transition, and band-gap-bowing origin.

Energy-Level Alignment, Ionization, and Stability of Bio-Amino Acids at Amino Acid/Si Junctions

The electronic structures of 20 bio-amino acids and amino acid/Si junctions are studied using ab initio calculations. It is shown that the amino acids can be classified into two groups depending on where the highest occupied molecular orbital (HOMO) state is localized. This classification is possible owing to the molecular structural geometry and the constituent atoms in the residue part of amino acids. Moreover, we found that, owing to the hybridization of electronic states between amino acids and Si substrate, the optical transition from the HOMO state of the amino acid to the conduction band states of Si becomes possible at the amino acid/Si interface. This result indicates the possibility of the optical ionization of amino acid by producing amino acid/semiconductor junctions. The present results provide basic data not only for the microscopic understanding of protein electronic structures but also for the electronic design of new protein functions.

Interacting quasi-band theory for electronic states in compound semiconductor alloys: Wurtzite structure

This paper reports on the electronic states of compound semiconductor alloys of wurtzite structure calculated by the recently proposed interacting quasi-band (IQB) theory combined with empirical sp3 tight-binding models. Solving derived quasi-Hamiltonian 24 × 24 matrix that is characterized by the crystal parameters of the constituents facilitates the calculation of the conduction and valence bands of wurtzite alloys for arbitrary concentrations under a unified scheme. The theory is applied to III–V and II–VI wurtzite alloys: cation-substituted Al1− x Ga x N and Ga1− x In x N and anion-substituted CdS1− x Se x and ZnO1− x S x . The obtained results agree well with the experimental data, and are discussed in terms of mutual mixing between the quasi-localized states (QLS) and quasi-average bands (QAB): the latter bands are approximately given by the virtual crystal approximation (VCA). The changes in the valence and conduction bands, and the origin of the band gap bowing are discussed on the basis of mixing character.

First-principles calculation of electron?phonon coupling at a Ga vacancy in GaN

We investigate the dependence of the electronic band structure on the localized phonon mode at a Ga vacancy in GaN. The electronic states and phonon modes are both calculated using a first-principles method based on the density functional theory. Comparing the calculated electronic band structures and phonon-frequency densities of states of GaN without and with a Ga vacancy, we find that 1) there are localized electronic midgap states closely above the top of the valence band, 2) localized phonon modes appear above the acoustic and optical phonon bands, 3) one of these localized phonon modes contains asymmetric distortions in which one of the four N atoms around a Ga vacancy strongly oscillates, and 4) the localized electronic midgap states strongly couple with the localized asymmetric mode. These results indicate that a Ga vacancy can act as a nonradiative recombination center and may trigger defect reactions in GaN-based devices.

Electronic structures calculation of Si 1−x Sn x compound alloy using interacting quasi-band model

We investigate energy band structures of Si_1-xSn_x compound alloy in zincblende structure using interacting qasi-band (IQB) model. The previous IQB model has been developed for three element compound semiconductors such as A_1-xB_xD. To apply IQB for Si_1-xSn_x, we here extend the IQB for four element compounds and calculate the electronic structures of virtual alloy as Si_1-xSn_xSi_1-ySn_y, where x=y. Diagonalizing a 20 × 20 non-Hermitian Hamiltonian matrix using sp3s* tight binding theory, we obtain quasi-band structures for several x. Comparing the band structures, we reveal that indirect-direct gap crossover in Si_1-xSn_x occurs around x = 0.39.

Electronic states of III–V and II–VI alloys calculated by IQB theory

The electronic states of compound semiconductor (III-V and II-VI) alloys are calculated by the recently proposed interacting quasi-band (IQB) theory. Combining with the sp3(s*) empirical tight-binding model, quasi-Hamiltonian matrix facilitates the calculation of the conduction and valence bands of general alloys, A1-xBxD (AD1-yFy) for arbitrary concentration x (y) under a unified scheme. The concentration dependence of the electronic bands, including midgap states, is discussed in particular attention to constituent materials, lattice structure (zincblend or wurtzite), and substitution type (anion or cation).

First-principles study of initial oxidation process of Ge(100) surfaces

Stable structures of oxygen atoms inserted into Ge(100) surfaces are investigated by first-principles calculations based on the density functional theory. Comparing the total energies of several models, the most stable structure is realized when oxygen atoms are inserted into the backbond of a lower dimer atom and the next bond along the (100) direction. We calculate the electronic density of states to reveal the origin of the stability. The structure is stable because a dangling bond of the lower dimer atom disappeared to form a four-coordinated structure. We also reveal that the dangling bond disappears from equal-amplitude plots of wave functions. These results are due to the strong electronegativity of the oxygen atom.

Origin of electronic transport of lithium phthalocyanine iodine crystal

The electronic structures of Lithium Phthalocyanine Iodine are investigated using density functional theory. Comparing the band structures of several model crystals, the metallic conductivity of highly doped LiPcIx can be explained by the band of doped iodine. These results reveal that there is a new mechanism for electronic transport of doped organic semiconductors that the dopant band plays the main role.

Re-Examination of Performance and Reliability Degradation in Metal?Oxide?Nitride?Oxide?Semiconductor Memory with Ultrathin SiN Charge Trap Layers

We demonstrated that the degradation of program characteristics in metal–oxide–nitride–oxide–semiconductor (MONOS) devices consisting of an ultrathin (~2 nm) SiN charge trap layer is due to a decrease in the electron capture efficiency, instead of a reduction in the number of available trap sites. From the data retention properties with applied gate bias voltage, we clarified that charge loss through the tunnel layer during data retention becomes more significant with decreasing SiN thickness. These results indicate that to improve the performance and reliability of MONOS devices with an ultrathin SiN charge trap layer, measures must be taken to enhance the capture cross section of the traps and to inhibit carrier motion in the SiN layer simultaneously.